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  • 52451.
    Zhang, Wenjie
    KTH, School of Industrial Engineering and Management (ITM), Industrial Ecology.
    Gap Analysis of ISO 26000 in Two Atlas Copco Companies in China:: Atlas Copco (Nanjing) Construction and Mining Equipment Co., Ltd. and Wuxi Pneumatech Air/ Gas Purity Equipment Co., Ltd. 2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Social Responsibility(SR)is gathering accumulative attentionrecently.It has become a key criterion of business campaign among companies.This thesis studiesthe current SR performancein two branch companies inAtlas Copco (China), Atlas Copco (Nanjing) Construction and Mining Equipment Ltd. andWuxi Pneumatech Air/Gas Purity Equipment Ltd.Interviews were made separately with company staff, including employees from management level and staff from workshop. ISO 26000 was adoptedas aguiding standard when conducting SR investigation and following analysing work.Core subjects of ‘Labour Practices’, ‘the Environment’ and ‘Community involvement and development’ in ISO 26000 were thethreeresearching aspects in the work. For each detailed actions and expectations in the subject, particular comments and acorresponding rating score aregiven according to its status quo. These rating scoresare utilized in the gap analysing process to illustrate the differences between the current SR situation and theexpectations illustrated in ISO 26000.Moreover, the disparity between twoselected companies is alsodefined by illustrating theirfinal scores on each issueina radar chart.As a result from the assessment,bothcompanies have a far way to go to entirely accomplish ISO 26000’s requirements. For the core subject which is strong at, their current measures need to be kept and finely turned; for the weak core subjects, more attentions should be attracted and concrete actions should be conducted.

    Social Responsibility(SR)is gathering accumulative attentionrecently.It has become a key criterion of business campaign among companies.This thesis studiesthe current SR performancein two branch companies inAtlas Copco (China), Atlas Copco (Nanjing) Construction and Mining Equipment Ltd. andWuxi Pneumatech Air/Gas Purity Equipment Ltd.Interviews were made separately with company staff, including employees from management level and staff from workshop. ISO 26000 was adoptedas aguiding standard when conducting SR investigation and following analysing work.Core subjects of ‘Labour Practices’, ‘the Environment’ and ‘Community involvement and development’ in ISO 26000 were thethreeresearching aspects in the work. For each detailed actions and expectations in the subject, particular comments and acorresponding rating score aregiven according to its status quo. These rating scoresare utilized in the gap analysing process to illustrate the differences between the current SR situation and theexpectations illustrated in ISO 26000.Moreover, the disparity between twoselected companies is alsodefined by illustrating theirfinal scores on each issueina radar chart.As a result from the assessment,bothcompanies have a far way to go to entirely accomplish ISO 26000’s requirements. For the core subject which is strong at, their current measures need to be kept and finely turned; for the weak core subjects, more attentions should be attracted and concrete actions should be conducted.

     

  • 52452.
    Zhang, Wenxin
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Environmental Physics.
    Long-term Trend of Evapotranspiration in Sweden Affected by Climate Change or Land-use Change.2011Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Evapotranspiration (ET) is an essential component of water cycle as it is an interlinkage between atmosphere, vegetation and soil surface in terms of energy and water balance. However, whether potential ET has the same tendency to change as actual ET and how ET trend (based on the difference between precipitation and runoff) is directly driven by dominant meteorological factors alone or combined with ecosystem‘s feedbacks to climate change (like land-use change) is still under the discussion. In this report, five ET parameterizations within two rainfall-runoff models [Coupled Heat and Mass Transfer Model (CoupModel) and Hydrologiska Byråns Vattenbalansavdelning (HBV)] have been set up based on six subcatchments of Sweden. The scenario derived from CoupModel shows that the trend of ET is affected by the change of land-use, where soil evaporation tends to shift to transpiration and interception evaporation. However, HBV model produces the other scenario: the trend of ET is merely the consequence of meteorological factors. Increased ET is contributed by increased interception evaporation due to the increased precipitation. After identifying the time split of changing ET trends, a dynamic simulation constructed both from HBV and CoupModel indicate that the increased total ET is primarily from increased ET in winter time. More and more interceptive water loss and transpiration resulted from land-use change due to more vegetation. On the other hand, land-use change is also a feed back to climate change. Transpiration controlled by the mechanism of stomata and water uptake controlled by reduction of soil moisture is highly related to variations of climatic conditions.

  • 52453. Zhang, X.
    et al.
    Ebrahimi, M.
    Huang, L.
    Li, G.
    Jantsch, Axel
    KTH, School of Information and Communication Technology (ICT), Electronics and Embedded Systems.
    A routing-level solution for fault Detection, masking, and Tolerance in NoCs2015In: Proceedings - 23rd Euromicro International Conference on Parallel, Distributed, and Network-Based Processing, PDP 2015, IEEE conference proceedings, 2015, 365-369 p.Conference paper (Refereed)
    Abstract [en]

    Faults may occur in numerous locations of a router in a NoC platform. Compared with the faults in the data path, faults in the control path may cause more severe effects which may result in crashing the entire system. Most of the current efforts in literature focus on disabling a router when a fault is detected. Considering this level of coarse-granularity, the functioning parts of a router have to be unnecessarily disabled which may severely affect the performance or functionality of the on-chip network. To cope with this problem, in this paper we propose a mechanism to tolerate faults in the control path which largely avoid disabling a router as long as the fault is not severe. This mechanism is called DMT, standing for three distinguishing characteristics of the proposed method as fault Detection, fault Masking and fault Tolerance. The proposed mechanism can efficiently detect the faults expressed as illegal turns while it has the capability to tolerate faults without a prior knowledge on where and why a fault has happened.

  • 52454. Zhang, X. L.
    et al.
    Shao, L. Y.
    Yu, Z. W.
    Cheng, D. R.
    Jin, R. X.
    He, Sailing
    KTH, Superseded Departments, Electromagnetic Theory.
    Automatic real-time control for gain-flattened fiber Raman amplifiers2004In: Optics Communications, ISSN 0030-4018, E-ISSN 1873-0310, Vol. 239, no 01-3, 79-84 p.Article in journal (Refereed)
    Abstract [en]

    An automatic control algorithm for flattening the gain of a fiber Raman amplifier is derived from the Raman scattering equations. A pseudo-inverse gain matrix is introduced to adjust the powers of the pump lasers. The algorithm is simple, fast, effective, robust and insensitive to the length and type of the fiber. It is demonstrated experimentally that the gain flattening for Raman amplifiers comprising various types and lengths of fiber and with different target gains can be achieved automatically with this algorithm. The algorithm converges for both small and large input signal. Flattened Raman gain with a fluctuation less than 0.40 dB over a 50-nm bandwidth for a 50-km SMF or less than 0.20 dB over a 40-nm bandwidth for a 6.6-km dispersion compensated fiber is realized in the experiment.

  • 52455. Zhang, X. -M
    et al.
    Cao, W. -H
    Yu, X. -X
    Wu, Sihong
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering (moved 20130630).
    Effect of LUCC on runoff regulation in watershed in loess gullied-hilly region of China2009In: Shuili Xuebao/Journal of Hydraulic Engineering, ISSN 0559-9350, Vol. 40, no 6, 641-650 p.Article in journal (Refereed)
    Abstract [en]

    By using the measurement technique of dynamic hydrological process and the estimation method of landscape ecology, the effect of LUCC (Land Use-Cover Change) on runoff regulation in the Luoyugou watershed, located at loess gullied-hilly region of China, in the period from 1986 to 2004 was studied. The results show that the impact of LUCC on runoff regulation is significant due to the increase of forest vegetation coverage and terracing of the land on the slopes. The capacity of runoff yield will be reduced due to the optimal adjustment of land use structures and the increase of forest vegetation in any time scales, including single precipitation event, the period of rainfall season and in the whole year. The reduction percentages of average annual runoff coefficient in rich rainfall year, normal rainfall year and poor rainfall year in the latter land use period from 1995 to 2004 are approximately 51%, 85%, 84% respectively comparing with those in the former land use period from 1986 to 1994. Besides, if neglecting the influence of precipitation amount, the predicted runoff coefficient for latter period decreases by about 63% comparing with those in former period. In addition, the impact of LUCC on runoff exhibits seasonal characteristics and the peak flood amount decreases with the optimal adjustment of land use structures. Although the impact of increasing forest vegetation coverage on the capacity of runoff amount is lager than that of changing land use type from slopping farmland to terrace, both measures must be adopted for fulfilling the requirements of land resources sustainable utilization in Loess gullied-hilly region.

  • 52456. Zhang, X.
    et al.
    Matthaiou, M.
    Coldrey, M.
    Björnson, Emil
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. SUPELEC, France .
    Impact of residual transmit RF impairments on training-based MIMO systems2014In: 2014 IEEE International Conference on Communications, ICC 2014, 2014, 4741-4746 p.Conference paper (Refereed)
    Abstract [en]

    Radio-frequency (RF) impairments, that exist intimately in wireless communications systems, can severely degrade the performance of traditional multiple-input multiple-output (MIMO) systems. Although compensation schemes can cancel out part of these RF impairments, there still remains a certain amount of impairments. These residual impairments have fundamental impact on the MIMO system performance. However, most of the previous works have neglected this factor. In this paper, a training-based MIMO system with residual transmit RF impairments (RTRI) is considered. In particular, we derive a new channel estimator for the proposed model, and find that RTRI can create an irreducible estimation error floor. Moreover, we show that, in the presence of RTRI, the optimal training sequence length can be larger than the number of transmit antennas, especially in the low and high signal-to-noise ratio (SNR) regimes. An increase in the proposed approximated achievable rate is also observed by adopting the optimal training sequence length. When the training and data symbol powers are required to be equal, we demonstrate that, at high SNRs, systems with RTRI demand more training, whereas at low SNRs, such demands are nearly the same for all practical levels of RTRI.

  • 52457. Zhang, X.
    et al.
    Yan, J.
    Li, H.
    Chekani, Shabnam
    KTH.
    Liu, L.
    KTH.
    Energy saving for biogas production and upgrading - Thermal integration2014In: Energy Procedia, 2014, Vol. 61, 121-125 p.Conference paper (Refereed)
    Abstract [en]

    Thermal integration of anaerobic digestion (AD) biogas production with amine based biogas upgrading has been studied for improving the overall thermal efficiencies of the two systems. The thermal characteristics have been investigated for typical AD raw biogas generation and MEA absorption biogas upgrading. The investigation provides a basic understanding of energy saving for both industrial scale biogas production and upgrading processes. The thermal integration is carried out based on the thermal characteristics of the two systems by well-defined case studies, which take the following factors into account such as important thermal conditions of sub-systems, material and energy balances, the efficiencies of heat exchange and heat transfer, necessary integration optimization and ambient conditions. The results show that the thermal integration is achievable with very positive effects for overall energy efficiency and water usage.

  • 52458.
    Zhang, Xi
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Transceiver Design for Multiple Antenna Communication Systems with Imperfect Channel State Information2008Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    Wireless communication links with multiple antennas at both the transmitter and the receiver sides, so-called multiple-input-multiple-output (MIMO)systems, are attracting much interest since they can significantly increase the capacity of band-limited wireless channels to meet the requirements of the future high data rate wireless communications. The treatment of channel state information (CSI) is critical in the design of MIMO systems. Accurate CSI at the transmitter is often not possible or may require high feedback rates, especially in multi-user scenarios. Herein, we consider the robust design of linear transceivers with imperfect CSI either at the transmitter or at both sides of the link. The framework considers the design problem where the imperfect CSI consists of a channel mean and an channel covariance matrix or, equivalently, a channel estimate and an estimation error covariance matrix.

    For single-user systems, the proposed robust transceiver designs are based on a general cost function of the average mean square errors. Under different CSI conditions, our robust designs exhibit a similar structure to the transceiver designs for perfect CSI, but with a different equivalent channel and/or noise covariance matrix. Utilizing majorization theory, the robust linear transceiver design can be readily solved by convex optimization approaches in practice.

    For multi-user systems, we consider both the communication link from the users to the access point (up-link) as well as the reverse link from the access point to the users (down-link). For the up-link channel, it is possible to optimally design robust linear transceivers minimizing the average sum mean square errors of all the data streams for the users. Our robust linear transceivers are designed either by reformulating the optimization problem as a semidefinite program or by extending the design of a single-user system in an iterative manner. Under certain channel conditions, we show that the up-link design problem can even be solved partly in a distributed fashion.

    For the down-link channel, a system with one receive antenna per user is considered. A robust system design is obtained by reducing the feedback load from all users to allow only a few selected users to feed back accurate CSI to the access point. We study the properties of four typical user selection algorithms in conjunction with beamforming that guarantee certain signal-to-interference-plus-noise ratio (SINR) requirements under transmit power minimization. Specifically, we show

    that norm-based user selection is asymptotically optimal in the number of transmitter antennas and close-to-optimal in the number of users. Rooted in the practical significance of this result, a simpler down-link system design with reduced feedback requirements is proposed.

  • 52459. Zhang, Xi
    et al.
    Gao, S.
    He, Sailing
    Optimal design of a silicon-on-insulator nanowire waveguide for broadband wavelength conversion2009In: Progress In Electromagnetics Research, ISSN 1070-4698, E-ISSN 1559-8985, Vol. 89, 183-198 p.Article in journal (Refereed)
    Abstract [en]

    The broadband wavelength conversion based on four-wave mixing in a silicon nanowire waveguide is theoretically investigated by taking into account the influence of the waveguide loss and free-carrier absorption on the phase-matched condition. The lossy wavelength conversion is compared with the lossless one in terms of conversion efficiency and bandwidth. The size of the silicon-on-insulator nanowire waveguide is optimized to be 400nm x 269nm for broadband wavelength conversion by realizing a flattened dispersion. The pump wavelength is also optimized to 1538.7nm in order to further enhance the conversion bandwidth. A 3-dB conversion bandwidth of over 280 nm is achieved in the optimized waveguide with the optimized pump wavelength.

  • 52460.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Jorswieck, Eduard A.
    Communications Laboratory, Faculty of Electrical Engineering and Information Technology,Dresden University of Technology (TUD), Germany.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Paulraj, Arogyasvsami
    Information Systems Laboratory, Stanford University, US.
    On the Asymptotic Optimality of Opportunistic Norm-Based User Selection with Hard SINR Constraint2009In: EURASIP Journal on Advances in Signal Processing, ISSN 1687-6172, E-ISSN 1687-6180, ISSN 1687-6172Article in journal (Refereed)
    Abstract [en]

    Recently, user selection algorithms in combination with linear precoding have been proposed that achieve the same scaling as the sum capacity of the MIMO broadcast channel. Robust opportunistic beamforming, which only requires partial channel state information for user selection, further reduces feedback requirements. In this work, we study the optimality of the opportunistic norm-based user selection system in conjunction with hard SINR requirements under max-min fair beamforming transmit power minimization. It is shown that opportunistic norm-based user selection is asymptotically optimal, as the number of transmit antennas goes to infinity when only two users are selected in high SNR regime. The asymptotic performance of opportunistic norm-based user selection is also studied when the number of users goes to infinity. When a limited number of transmit antennas and/or median range of users are available, only insignificant performance degradation is observed in simulations with an ideal channel model or based on measurement data.

  • 52461.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Jorswieck, Eduard A.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Paulraj, Arogyaswami
    Information Systems Laboratory, Stanford University, USA.
    MSE based optimization of multiuser MIMO MAC with partial CSI2006In: 2006 Fortieth Asilomar Conference on Signals, Systems and Computers, 2006, 374-378 p.Conference paper (Refereed)
    Abstract [en]

    This paper studies the optimal transmission design for multiple antenna multiple access channels with linear MMSE receiver at the base station and partial channel state information at the mobiles. The performance criterium for optimization is based on a Schur-concave function that works on the average individual MSEs of all users. The optimal beamforming matrix of each user corresponds to the eigenvector matrix of his channel correlation matrix. The remaining power allocation problem can be solved at reduced complexity. It is also direct to extend the results to Schur-convex functions. The results are illustrated by numerical simulations.

  • 52462.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Jorswieck, Eduard A.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Paulraj, Arogyaswami
    Information Systems Laboratory, Stanford University, USA.
    User selection schemes in multiple antenna broadcast channels with guaranteed performance2007In: 2007 IEEE 8TH WORKSHOP ON SIGNAL PROCESSING ADVANCES IN WIRELESS COMMUNICATIONS, NEW YORK: IEEE , 2007, 1-5 p.Conference paper (Refereed)
    Abstract [en]

    Recently, user selection algorithms in combination with linear precoding have been proposed that achieve the same scaling as the sum capacity of the MIMO BC. In this work, we study the properties of four user selection algorithms in conjunction with beamforming that guarantee certain SINR requirements under transmit power minimization. It is shown that for a large number of transmit antennas, the norm-based user selection performs close to the optimum. The results are illustrated by various numerical simulations.

  • 52463.
    Zhang, Xi
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Joint Space-Frequency Optimization in Multi User MIMO System2003In: IEEE International Symposium on Signal Processing and Information Technology, IEEE , 2003, 170-173 p.Conference paper (Refereed)
    Abstract [en]

    Herein, joint space-time multiplexing in a multi carrier multiple input multiple output (MIMO) system is considered. Previous results concerning the spatial multiplexing optimization are used to address the joint problem of carrier assignment and choice of beamforming weights in a multi user MIMO system. For practical reasons, the joint optimization is divided into two parts: subcarrier assignment based on beamformer characteristics to adjust the carrier load, followed by beamforming optimization with subcarrier reassignment. The goal is to minimize total downlink transmit power while keeping the fixed user data rates. The proposed multiplexing scheme is evaluated on a number of numerical examples.

  • 52464.
    Zhang, Xi
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Joint space-frequency optimization in multi user MIMO system2003In: PROCEEDINGS OF THE 3RD IEEE INTERNATIONAL SYMPOSIUM ON SIGNAL PROCESSING AND INFORMATION TECHNOLOGY, NEW YORK: IEEE , 2003, 170-173 p.Conference paper (Refereed)
    Abstract [en]

    Herein, joint space-time multiplexing in a multi carrier multiple input multiple output (MIMO) system is considered. Previous results concerning spatial multiplexing optimization are used to address the joint problem of carrier assignment and choice of beamforming weights in a multi user MIMO system. For practical reasons, the joint optimization is divided into two parts: subcarrier assignment based on beamformer characteristics to adjust the carrier load, followed by beamforming optimization with subcarrier reassignment. The goal is to minimize total downlink transmit power while keeping fixed user data rates. The proposed multiplexing scheme is evaluated on a number of numerical examples.

  • 52465.
    Zhang, Xi
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Performance analysis of V-BLAST structure with channel estimation errors2004In: 2003 4TH IEEE WORKSHOP ON SIGNAL PROCESSING ADVANCES IN WIRELESS COMMUNICATIONS - SPAWC 2003, NEW YORK: IEEE , 2004, 487-491 p.Conference paper (Refereed)
    Abstract [en]

    The V-BLAST structure is a promising MIMO transmission scheme that attempts to utilize the high capacity of a MIMO system while maintaining low complexity. While no channel knowledge is required at the transmitter, accurate estimates of the channel are necessary at the receiver. The performance of V-BLAST under channel estimation errors is analyzed in this paper and a very tight error floor is derived. This error floor is due to the equivalent system noise which is a combination of the channel estimation errors and the additive white Gaussian receiver noise.

  • 52466.
    Zhang, Xi
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Power Allocation and Bit Loading for Spatial Multiplexing in MIMO Systems2003In: Proceedings IEEE International Conference on Acoustics, Speech, and Signal Processing, 2003, 53-56 p.Conference paper (Refereed)
    Abstract [en]

    The power assignment problem is important for Multiple-Input-Multiple-Output (MIMO) systems to achieve high capacity. Although this problem is solved by well-known water filling algorithms, this does not provide an optimal solution if the system is constrained to a fixed raw bit error rate threshold and to discrete modulation orders. In this work an approximate approach, called QoS based WF, is proposed to solve the power assignment problem with such constrains. It is shown to outperform quantization of the conventional water filling solution and a well known bit loading algorithm (Chow’s algorithm) used in the Digital Subscriber Lines (DSL).

  • 52467.
    Zhang, Xi
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Speech, Music and Hearing.
    Power allocation and bit loading for spatial multiplexing in MIMO systems2003In: 2003 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, VOL V, PROCEEDINGS - SENSOR ARRAY & MULTICHANNEL SIGNAL PROCESSING AUDIO AND ELECTROACOUSTICS MULTIMEDIA SIGNAL PROCESSING, NEW YORK: IEEE , 2003, 53-56 p.Conference paper (Refereed)
    Abstract [en]

    The power assignment problem is important for Multiple-Input-Multiple-Output (MIMO) systems to achieve high capacity. Although this problem is solved by well-known water filling algorithms, this does not provide an optimal solution if the system is constrained to a fixed raw bit error rate threshold and to discrete modulation orders. In this work an approximate approach, called QoS based WF, is proposed to solve the power assignment problem with such constrains. It is shown to outperform quantization of the conventional water filling solution and a well known bit loading algorithm (Chow's algorithm) used in the Digital Subscriber Lines (DSL).

  • 52468.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Palomar, Daniel P.
    Dept. of Electrical Engineering, Princeton University Princeton, NJ 08544, USA.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Robust design of linear MIMO transceiver for low SNR2005In: 2005 39th Asilomar Conference on Signals, Systems and Computers, Vols 1 and 2, NEW YORK: IEEE , 2005, 398-402 p.Conference paper (Refereed)
    Abstract [en]

    In practical systems, the design of linear MIMO transceivers should be robust to partial or imperfect channel state information (CSI). This paper considers the case in which only the second-order statistics of the channel is known at the transmitter while the receiver has a perfect CSI. In such a case, it is possible to optimally design robust MIMO transceivers based on a general cost function covering several well known performance criteria. In particular, two families are considered in detail: Schur-convex and Schur-concave functions. Approximations are used in the low SNR regime to obtain simple optimization problems that can be readily solved. Numerical examples show substantial gains compared to other methods.

  • 52469.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Palomar, Daniel P.
    Dept. of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Robust design of linear MIMO transceivers under channel uncertainty2006In: 2006 IEEE International Conference on Acoustics, Speech, and Signal Processing: signal processing for communication, sensor array and multichannel signal processing, 2006, 77-80 p.Conference paper (Refereed)
    Abstract [en]

    This paper considers the robust design of a linear transceiver with imperfect channel state information (CSI) at the transmitter of a MIMO link. The framework embraces the design problem when CSI at the transmitter consists of the channel mean and covariance matrix or, equivalently, the channel estimate and the estimation error covariance matrix. The design of the linear MIMO transceiver is based on a general cost function covering several well known performance criteria. In particular, two families are considered in detail: Schur-convex and Schur-concave functions. Approximations are used in the low SNR and high SNR regimes separately to obtain simple optimization problems that can be readily solved. Numerical examples show gains compared to other suboptimal methods.

  • 52470.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Palomar, Daniel P.
    Dept. of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Robust MAC MIMO transceiver design with partial CSIT and CSIR2007In: CONFERENCE RECORD OF THE FORTY-FIRST ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS & COMPUTERS, VOLS 1-5, NEW YORK: IEEE , 2007, 324-328 p.Conference paper (Refereed)
    Abstract [en]

    In practical wireless systems, the design of linear MIMO transceivers should be robust to imperfect channel state information (CSI), especially in multiuser systems where accurate CSI is difficult to obtain. This paper considers the case in which the CSI is perturbed at both sides of the link. In such a case, it is possible to optimally design robust MIMO transceivers to minimize the average sum MSE of all the data streams and the users. For systems with a sum power constraint, our robust design can be shown to be a convex SDP problem; for systems with individual power constraints, our robust design is based on an underlying SDP problem.

  • 52471.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Palomar, Daniel P.
    Electrical Engineering, Princeton University, Princeton.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Statistically robust design of linear MIMO transceivers2008In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 56, no 8, 3678-3689 p.Article in journal (Refereed)
    Abstract [en]

    The treatment of channel state information (CSI) is critical in the design of MIMO systems. Accurate CSI at the transmitter is often not possible or may require high feedback rates. Herein, we consider the robust design of linear MIMO transceivers with perfect CSI either at the transmitter or at both sides of the link. The framework considers the design problem where the imperfect CSI consists of the channel mean and covariance matrix or, equivalently, the channel estimate and the estimation error covariance matrix. The robust transceiver design is based on a general cost function of the average MSEs as well as a design with individual MSE based constraints. In particular, a lower bound of the average MSE matrix is explored for the design when only the CSI at the transmitter is imperfect. Under different CSI conditions, the proposed robust transceivers exhibit a similar structure to the transceiver designs for perfect CSI, but with a different equivalent channel and/or noise covariance matrix.

  • 52472.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Svedman, Patrick
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Lundin, Henrik
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Implementation of a smart antenna multiuser algorithm on a DSP-based wireless MIMO test-bed2005In: IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC, 2005, Vol. 1, 126-130 p.Conference paper (Refereed)
    Abstract [en]

    This paper describes the implementation and performance of three communication schemes on a DSP-based MIMO test-bed. A multicell scenario with two basestations and two users in the same room is evaluated. Eigenbeamforming with and without interference pre-whitening at both transmitter and receiver is compared with SISO multicell communication. Experimental results show that the gain from using interference pre-whitening is significant. The BER performance of beamforming without intercell interference suppression is only marginally better than single antenna communication. The paper also presents the implementation of synchronization, equalization, frequency offset estimation and decision directed beamforming mismatch compensation.

  • 52473.
    Zhang, Xian
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Leygraf, Christofer
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Odnevall Wallinder, Inger
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Atmospheric corrosion of Galfan coatings on steel in chloride-rich environments2013In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 73, 62-71 p.Article in journal (Refereed)
    Abstract [en]

    Galfan coatings on steel in laboratory exposures with predeposited NaCl and cyclic wet/dry conditions exhibit nearly the same corrosion products as after 5 years of marine exposure. A general scenario for corrosion product evolution on Galfan in chloride-rich atmospheres is proposed. It includes the initial formation of ZnO, ZnAl2O4 and Al2O3 and subsequent formation of Zn6Al2(OH)(16)CO3 center dot 4H(2)O, and Zn2Al(OH)(6)Cl center dot 2H(2)O and/or Zn5Cl2(OH)(8)center dot H2O. An important phase is Zn6Al2(OH)(16)CO3 center dot 4H(2)O, which largely governs the reduced long-term zinc runoff from Galfan. A clear influence of microstructure could be observed on corrosion initiation in the slightly zinc-richer eta-Zn phase adjacent to the beta-Al phase.

  • 52474.
    Zhang, Xian
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Liu, Xiaoyan
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Odnevall Wallinder, Inger
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Leygraf, Christofer
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    The protective role of hydrozincite during initial corrosion of a Cu40Zn alloy in chloride-containing laboratory atmosphere2016In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 103, 20-29 p.Article in journal (Refereed)
    Abstract [en]

    Hydrozincite, Zn-5(CO3)(2)(OH)(6), was recently found to play a key role in reducing corrosion product flaking on Cu-Zn alloys. A fundamental study was undertaken to explore the underlying mechanisms, in particular why hydrozincite can suppress the interaction between chlorides and the alloy surface. Hydrozincite could be formed by exposure of Cu40Zn to air at 70% relative humidity and 1000 ppm of CO2 resulting in a surface of decreased wettability. Its presence reduces the initial spreading ability of NaCl-containing droplets and lowers the overall initial corrosion rate when the alloy is exposed to pre-deposited NaCl and wet/dry cycles.

  • 52475.
    Zhang, Xian
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Odnevall Wallinder, Inger
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Leygraf, Christofer
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Mechanistic studies of corrosion product flaking on copper and copper-based alloys in marine environments2014In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 85, 15-25 p.Article in journal (Refereed)
    Abstract [en]

    The mechanism of corrosion product flaking on bare copper sheet and three copper-based alloys in chloride rich environments has been explored through field and laboratory exposures. The tendency for flaking is much more pronounced on Cu and Cu-4 wt%Sn than on Cu-15 wt%Zn and Cu-5 wt%Al-5 wt%Zn. This difference is explained by the initial formation of zinc and zinc-aluminum hydroxycarbonates on Cu15Zn and Cu5Al5Zn, which delays the formation of CuCl, a precursor of Cu-2(OH)(3)Cl. As a result, the observed volume expansion during transformation of CuCl to Cu-2(OH)(3)Cl, and concomitant corrosion product flaking, is less severe on Cu15Zn and Cu5Al5Zn than on Cu and Cu4Sn.

  • 52476.
    Zhang, Xiaobo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Influence of drying pressure on interfibre bond strength.2012Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In this thesis the influence of the drying pressure on interfibre bond strength was investigated. Five different drying pressures, 0.7, 2.9, 4.5, 6.7, and 15 kPa, were applied during the preparation of fiber-fiber cross test pieces. The nominal overlap area of each fibre-fibre cross was measured in a transmission light microscope. A tensile tester was used to record the loaddeformation behavior of each fiber-fiber cross. The final results of the interfibre bond strength were defined by both the overlap area and the maximum force of each bond. The results showed that the influence of drying pressure to the average strength were very weak, although a maximum could be seen at 2.9 kPa of drying pressure. Moreover, the results suggested the overall trend of decreasing strengths at very high drying pressures. Finally, a statistical significance study of the results was presented. In addition, the influences of fiber type (spring wood vs. summer wood) and press type (steel vs. steel or steel vs. rubber) on interfibre bond strength were also discussed.

  • 52477.
    Zhang, Xiaodong
    KTH, School of Information and Communication Technology (ICT).
    Multi-Cloud simulation environment for WebRTC streams2017Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Real-time streaming is becoming popular nowadays on the internet. WebRTC is a promising web technology that enables media stream transmission between two browsers without other third-party plugins. However, traditional WebRTC applications can only establish peer-to-peer (P2P) communications, which cannot be directly used in one-to-more streaming scenarios such as a multi-party video conference.

    This thesis project presents a development of a multi-cloud simulation framework to implement software multicast of WebRTC streams to enable oneto-more real-time streaming. The framework can generate a cloud network topology with a simple script, and provides flexible ways to model the network links with parameters such as bandwidth, packet loss, and link latency. By using selective forwarding units (SFUs), a stream publisher only needs to send a single copy of the data to the cloud, the data packets are duplicated and forwarded to multiple subscribers. In addition, three resource allocation algorithms are included in the framework to determine the data center for a task.

    To evaluate the performance, this framework enables people to monitor the throughputs and running streams on the data centers during the experiments. We develop benchmark applications as substitutes for real WebRTC traffic. These applications can generate UDP stream packets with the same dimension of WebRTC packets and provide the customization of stream parameters. In this report, we present a comparison of the stream performances under different allocation algorithms. Finally, as an outcome of this project, we design an integrated functional test to simulate a real-world scenario. The result shows that the framework is able to work well on complex real cases and simulate most of the multi-cloud networks.

  • 52478.
    Zhang, Xiaofan
    et al.
    KTH, School of Information and Communication Technology (ICT), Industrial and Medical Electronics.
    Ebrahimi, Masoumeh
    KTH, School of Information and Communication Technology (ICT), Industrial and Medical Electronics.
    Huang, Letian
    KTH, School of Information and Communication Technology (ICT), Industrial and Medical Electronics.
    Li, Guangjun
    Jantsch, Axel
    KTH, School of Information and Communication Technology (ICT), Industrial and Medical Electronics.
    A Network-Level Solution for Fault Detection, Masking, and Tolerance in NoCs2015Conference paper (Refereed)
  • 52479.
    Zhang, Xiaohu
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Control Strategy of Cascaded H-Bridge Multilevel Inverter with PV System as Separate DC Source2011Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The integration of solar energy to the power grid is a challenging topic nowadays. Many topologies and control methods have already been proposed. In this thesis, the cascaded H-bridge (CHB) multilevel inverter is adopted for the photovoltaic (PV) energy integration. The purpose of this thesis is to develop a control strategy for the CHB multilevel inverter with PV array as separate DC source. The thesis first gives a short review of the three commonly used multilevel inverter topologies. Then the PV array with maximum power point tracker (MPPT) is modeled. The selection mechanism with some improvements is chosen as the control and modulation method for the CHB inverter with PV system. Real and reactive power exchange between the inverter and grid are also discussed. Finally, the simulation results are presented to verify the control methods. The software for the simulation is PSCAD/EMTDC and all the figures are plotted in MATLAB.

  • 52480. Zhang, Xiaojing
    et al.
    Yan, Jinying
    Li, Hailong
    Chekani, Shabnam
    KTH.
    Liu, Longcheng
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Investigation of thermal integration between biogas production and upgrading2015In: Energy Conversion and Management, ISSN 0196-8904, E-ISSN 1879-2227, Vol. 102, 131-139 p.Article in journal (Refereed)
    Abstract [en]

    Thermal integration of anaerobic digestion (AD) biogas production with amine-based chemical absorption biogas upgrading has been studied to improve the overall efficiency of the intergraded system. The thermal characteristics have been investigated for industrial AD raw biogas production and amine-based chemical absorption biogas upgrading. The investigation provides a basic understanding for the possibilities of energy saving through thermal integration. The thermal integration is carried out through well-defined cases based on the thermal characteristics of the biogas production and the biogas upgrading. The following factors are taken into account in the case study: thermal conditions of sub-systems, material and energy balances, cost issues and main benefits. The potential of heat recovery has been evaluated to utilise the waste heat from amine-based upgrading process for the use in the AD biogas production. The results show that the thermal integration has positive effects on improving the overall energy efficiency of the integrated biogas plant. Cost analysis shows that the thermal integration is economically feasible.

  • 52481.
    Zhang, Xiaolei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Micro-reaction Mechanism Study of the Biomass Thermal Conversion Process using Density Functional Theory2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Biomass, or bio-energy, is one of the most important alternative energies because of environmental concerns and the future shortage of fossil fuels. Multi-scaled bioenergy studies have been performed in the division of Energy and Furnace Technology, which included studies of macroscopic systems such as systems and reactors, modeling of computational fluid dynamics (CFD), and atomic/molecular level studies. The present thesis focus on the atomic/molecular level that based on quantum chemistry methods.

    The microscopic structure study of biomass is the first and an important step for the investigation of the biomass thermal conversion mechanism. Cellulose, hemicellulose, and lignin are the three most important components for biomass. The atomic interactions among these three main components were studied, including the hydrogen bond linkages between cellulose and hemicellulose, and the covalent bond linkages between hemicellulose and lignin.

    The decomposition of biomass is complicated and includes cellulose decomposition, hemicellulose decomposition, and lignin decomposition. As the main component of biomass, the mechanism of cellulose pyrolysis mechanism was focused on in this thesis. The study of this mechanism included an investigation of the pathways from cellulose to levoglucosan then to lower-molecular-weight species. Three different pathways were studied for the formation of levoglucosan from cellulose, and three different pathways were studied for the levoglucosan decomposition. The thermal properties for every reactant, intermediate, and product were obtained. The kinetics parameters (rate constant, pre-exponential factor, and activation energy) for every elementary step and pathway were calculated. For the formation of levoglucosan, the levoglucosan chain-end mechanism is the favored pathway due to the lower energy barrier; for the subsequent levoglucosan decomposition process, dehydration is a preferred first step and C-C bond scission is the most difficult pathway due to the strength of the C-C bonds.

    The biomass gasification process includes pyrolysis, char gasification, and a gas-phase reaction; Char gasification is considered to be the rate-controlling step because of its slower reaction rate. Char steam gasification can be described as the adsorption of steam on the char surface to form a surface complex, which may transfer to another surface complex, which then desorbs to give the gaseous products (CO and H2) and the solid product of the remaining char. The influences of several radicals (O, H, and OH) and molecules (H2 and O2) on steam adsorption were investigated. It was concluded that the reactivity order for these particles adsorbed onto both zigzag and armchair surfaces is O > H2 > H > OH > O2. For water adsorbs on both zigzag and armchair carbon surfaces, O and OH radicals accelerate water adsorption, but H, O2, and H2 have no significant influence on water adsorption.

    It was also shown that quantum chemistry (also known as molecular modeling) can be used to investigate the reaction mechanism of a macroscopic system. Detailed atomic/molecular descriptions can provide further understanding of the reaction process and possible products.

  • 52482.
    Zhang, Xiaolei
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Möjlilgheter till överbelastning av högspända AC-kablar vid havsförläggning2006Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 52483.
    Zhang, Xiaolei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Li, Jun
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Yang, Weihong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Blasiak, Wlodzimierz
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Formation Mechanism of Levoglucosan and Formaldehyde during Cellulose Pyrolysis2011In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 25, no 8, 3739-3746 p.Article in journal (Refereed)
    Abstract [en]

    Biomass pyrolysis is an efficient way to transform raw biomass or organic waste materials into useable energy, including liquid, solid, and gaseous materials. Levoglucosan (1,6-anhydro-beta-D-glucopyranose) and formaldehyde are two important products in biomass pyrolysis. The formation mechanism of these two products was investigated using the density functional theory (DFT) method based on quantum mechanics. It was found that active anhydroglucose can be obtained from a cellulose homolytic reaction during high-temperature steam gasification of the biomass process. Anhydroglucose undergoes a hydrogen-donor reaction and forms an intermediate, which can transform into the products via three pathways, one (path 1) for the formation of levoglucosan and two (paths 2 and 3) for formaldehyde. A total of six elementary reactions are involved. At a pressure of 1 atm, levoglucosan can be formed at all of the temperatures (450-750 K) considered in this simulation, whereas formaldehyde can be formed only when the temperature is higher than 475 K Moreover, the energy barrier of levoglucosan formation is lower than that of formaldehyde, which is in agreement with the mechanism proposed in the experiments.

  • 52484.
    Zhang, Xiaolei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Yang, Weihong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Blasiak, Wlodzimierz
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Density functional study on levoglucosan decomposition during cellulose pyrolysis2012In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 243Article in journal (Other academic)
  • 52485.
    Zhang, Xiaolei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Yang, Weihong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Blasiak, Wlodzimierz
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Formation and Characterization of Carbon-Radical Precursors in Char Steam Gasification2010In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 24, 6513-6521 p.Article in journal (Refereed)
    Abstract [en]

    Highly reactive radicals play an important role in high-temperature gasification processes. However, the effect of radicals on gasification has not been systematically investigated. In the present study, the formation of carbon-radical precursors using atomic radicals such as OH, O, and H and molecules such as H-2 and O-2 was characterized, and the effect of the precursors on the adsorption step of steam char gasification was studied using quantum chemistry methods. The results revealed that the radicals can be chemisorbed exothermically on char active sites, and the following order of reactivity was observed: O > H-2 > H > OH > O-2. Moreover, hydrogen bonds are formed between steam molecules and carbon-radical complexes. Steam molecule adsorption onto carbon-O and carbon-OH complexes is easier than adsorption onto clean carbon surfaces. Alternatively, adsorption on carbon-O-2, carbon-H-2, and carbon-H complexes is at the same level with that of clean carbon surfaces; thus, OH and O radicals accelerate the physical adsorption of steam onto the char surface, H radical and O-2 and H-2 molecules do not have a significant effect on adsorption.

  • 52486.
    Zhang, Xiaolei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Yang, Weihong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Blasiak, Wlodzimierz
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Kinetics of levoglucosan and formaldehyde formation during cellulose pyrolysis process2012In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 96, no 1, 383-391 p.Article in journal (Refereed)
    Abstract [en]

    The mechanisms and kinetics studies of the formation of levoglucosan and formaldehyde from anhydroglucose radical have been carried out theoretically in this paper. The geometries and frequencies of all the stationary points are calculated at the B3LYP/6-31+G(D,P) level based on quantum mechanics, Six elementary reactions are found, and three global reactions are involved. The variational transition-state rate constants for the elementary reactions are calculated within 450-1500 K. The global rate constants for every pathway are evaluated from the sum of the individual elementary reaction rate constants. The first-order Arrhenius expressions for these six elementary reactions and the three pathways are suggested. By comparing with the experimental data, computational methods without tunneling correction give good description for Path1 (the formation of levoglucosan); while methods with tunneling correction (zero-curvature tunneling and small-curvature tunneling correction) give good results for Path2 (the first possibility for the formation of formaldehyde), all the test methods give similar results for Path3 (the second possibility for the formation of formaldehyde), all the modeling results for Path3 are in good agreement with the experimental data, verifying that it is the most possible way for the formation of formaldehyde during cellulose pyrolysis.

  • 52487.
    Zhang, Xiaolei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Yang, Weihong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Blasiak, Wlodzimierz
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Kinetics study on thermal dissociation of levoglucosan during cellulose pyrolysis2013In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 109, 476-483 p.Article in journal (Refereed)
    Abstract [en]

    The mechanisms and kinetics studies of the levoglucosan (LG) primary decomposition during cellulose pyrolysis have been carried out theoretically in this paper. Three decomposition mechanisms (C-O bond scission, C-C bond scission, and LG dehydration) including nine pathways and 16 elementary reactions were studied at the B3LYP/6-31 + G(D, P) level based on quantum mechanics. The variational transition-state rate constants for every elementary reaction and every pathway were calculated within 298-1550 K. The first-order Arrhenius expressions for these 16 elementary reactions and nine pathways were suggested. It was concluded that computational method using transition state theory (TST) without tunneling correction gives good description for LG decomposition by comparing with the experimental result. With the temperature range of 667-1327 K, one dehydration pathway, with one water molecule composed of a hydrogen atom from C3 and a hydroxyl group from C2, is a preferred LG decomposition pathway by fitting well with the experimental results. The calculated Arrhenius plot of C-O bond scission mechanism is better agreed with the experimental Arrhenius plot than that of C-C bond scission. This C-O bond scission mechanism starts with breaking of C1-O5 and C6-O1 bonds with formation of CO molecule (C1-O1) simultaneously. C-C bond scission mechanism is the highest energetic barrier pathway for LG decomposition.

  • 52488.
    Zhang, Xiaolei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Yang, Weihong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Blasiak, Wlodzimierz
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Modeling Study of Woody Biomass: Interactions of Cellulose, Hemicellulose, and Lignin2011In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 25, no 10, 4786-4795 p.Article in journal (Refereed)
    Abstract [en]

    Lignocellulosic biomass pretreatment and the subsequent thermal conversion processes to produce solid, liquid, and gas biofuels are attractive solutions for today's energy challenges. The structural study of the main components in biomass and their macromolecular complexes is an active and ongoing research topic worldwide. The interactions among the three main components, cellulose, hemicellulose, and lignin, are studied in this paper using electronic structure methods, and the study includes examining the hydrogen bond network of cellulose-hemicellulose systems and the covalent bond linkages of hemicellulose-lignin systems. Several methods (semiempirical, Hartree-Fock, and density functional theory) using different basis sets were evaluated. It was shown that theoretical calculations can be used to simulate small model structures representing wood components. By comparing calculation results with experimental data, it was concluded that B3LYP/6-31G is the most suitable basis set to describe the hydrogen bond system and B3LYP/6-31G(d,p) is the most suitable basis set to describe the covalent system of woody biomass. The choice of unit model has a much larger effect on hydrogen bonding within cellulose-hemicellulose system, whereas the model choice has a minimal effect on the covalent linkage in the hemicellulose-lignin system.

  • 52489.
    Zhang, Xiaolei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Yang, Weihong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Dong, Changqing
    Levoglucosan Formation Mechanism during Cellulose PyrolysisArticle in journal (Other academic)
  • 52490.
    Zhang, Xiaolei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Yang, Weihong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Dong, Changqing
    Levoglucosan formation mechanisms during cellulose pyrolysis2013In: Journal of Analytical and Applied Pyrolysis, ISSN 0165-2370, E-ISSN 1873-250X, Vol. 104, 19-27 p.Article in journal (Refereed)
    Abstract [en]

    Levoglucosan is one important primary product during cellulose pyrolysis either as an intermediate or as a product. Three available mechanisms for levoglucosan formation have been studied theoretically in this paper, which are free-radical mechanism; glucose intermediate mechanism; and levoglucosan chain-end mechanism. All the elementary reactions included in the pathway of every mechanism were investigated; thermal properties including activation energy. Gibbs free energy, and enthalpy for every pathway were also calculated. It was concluded that free-radical mechanism has the highest energy barrier during the three levoglucosan formation mechanisms, glucose intermediate mechanism has lower energy barrier than free-radical mechanism, and levoglucosan chain-end mechanism is the most reasonable pathway because of the lowest energy barrier. By comparing with the activation energy obtained from the experimental results, it was also concluded that levoglucosan chain-end mechanism fits better with the experimental data for the formation of levoglucosan.

  • 52491.
    Zhang, Xiaoxiang
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Numerical Study on Combustion Features of Gasified Biomass Gas2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    There is a great interest to develop biomass combustion systems for industrial and utility applications. Improved biomass energy conversion systems are designed to provide better combustion efficiencies and environmental friendly conditions, as well as the fuel flexibility options in various applications. The gas derived from the gasification process of biomass is considered as one of the potential candidates to substitute traditional fuels in a combustion process. However, the gascomposition from the gasification process may have a wide range of variation depending on the methods and fuel sources. The better understanding of the combustion features for the Gasified Biomass Gas(GBG) is essential for the development of combustion devices to be operated efficiently and safely at the user-end.

    The objective of the current study is therefore aiming to achieve data associated with the combustion features of GBG fuel for improving the efficiency and stability of combustion process. The numerical result is achieved from the kinetic models of premixed combustion with a wide range of operating ranges and variety of gas compositions. The numerical result is compared with experimental data to provide a better understanding of the combustion process for GBG fuel.

    In this thesis the laminar flame speed and ignition delay time of the GBG fuel are analyzed, using 1-D premixed flame model and constant volume model respectively. The result from different kinetics are evaluated and compared with experimental data. The influences of initial temperature, pressure and equivalence ratio are considered, as well as the variation of gas compositions. While the general agreement is reached between the numerical result and experimental data for laminarflame speed prediction, deviations are discovered at fuel-rich region and increased initial temperature. For the ignition delay time, deviations are found in the low-temperature and low pressure regime. The empirical equations considering the influence of initial temperature,pressure and equivalence ratio are developed for laminar flame speed and ignition delay times. The influence of major compositions such as CO, H2 and hydrocarbons are discussed in details in the thesis. Furthermore, a simplified kinetic model is developed and optimized based on the evaluation of existing kinetics for GBG fuel combustion. The simplified kinetic model is expected to be used for simulating the complexc ombustion process of GBG fuel in future studies.

  • 52492.
    Zhang, Xiaoxiang
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Jayasuriya, Jeevan
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Fakhraie, Reza
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Fransson, Torsten
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Evaluation of reduced kinetics in simulation of gasified biomass gas combustion2013In: ASME Turbo Expo 2013: Turbine Technical Conference and Exposition: Volume 1B: Combustion, Fuels and Emissions, ASME Press, 2013, Vol. 1B, V01BT04A045- p.Conference paper (Refereed)
    Abstract [en]

    It is essentially important to use appropriate chemical kinetic models in the simulation process of gas turbine combustion. To integrate the detailed kinetics into complex combustion simulations has proven to be a computationally expensive task with tens to thousands of elementary reaction steps. It has been suggested that an appropriate simplified kinetics which are computationally efficient could be used instead. Therefore reduced kinetics are often used in CFD simulation of gas turbine combustion. At the same time, simplified kinetics for specific fuels and operation conditions need to be carefully selected to fulfill the accuracy requirements. The applicability of several simplified kinetics for premixed Gasified Biomass Gas (GBG) and air combustion are evaluated in this paper. The current work is motivated by the growing demand of gasified biomass gas (GBG) fueled combustion. Even though simplified kinetic schemes developed for hydrocarbon combustions are published by various researchers, there is little research has been found in literature to evaluate the ability of the simplified chemical kinetics for the GBG combustion. The numerical Simulation tool "CANTERA" is used in the current study for the comparison of both detailed and simplified chemical kinetics. A simulated gas mixture of CO/H2/CH4/CO2/N2 is used for the current evaluation, since the fluctuation of GBG components may have an unpredictable influence on the simulation results. The laminar flame speed has an important influence with flame stability, extinction limits and turbulent flame speed, here it is chosen as an indicator for validation. The simulation results are compared with the experimental data from the previous study [1] which is done by our colleagues. Water vapour which has shown a dilution effect in the experimental study are also put into concern for further validation. As the results indicate, the reduced kinetics which are developed for hydrocarbon or hydrogen combustion need to be highly optimized before using them for GBG combustion. Further optimization of the reduced kinetics is done for GBG and moderate results are achieved using the optimized kinetics compared with the detailed combustion kinetics.

  • 52493.
    Zhang, Xiaoxiang
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Jayasuriya, Jeevan
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Fransson, Torsten
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Kinetic Evaluation of the Laminar Flame Speed for Biomass Derived Gas CombustionManuscript (preprint) (Other academic)
    Abstract [en]

    The gas composition derived from gasification of biomass has been used in gasturbine combustion to achieve higher energy efficiency. However, there is an essential requirement to better understand the combustion characteristics of biomass derived gas before it can be used in the existing combustion facilities. A quantified study of the laminar flame speed of biomass derived gas combustionis presented in this paper. The study was carried out based on the kinetic model of the biomass derived gas flame and the results are compared with the experimental data from the our laboratory and various literatures. The laminarflame speed of the biomass derived gas was evaluated through a range of initial temperature (298 K - 398 K) and pressure (1 atm - 10 atm), as well as with various gas compositions. An empirical relationship for estimating the laminarflame speed has been derived for a composition of typical biomass derived gas. Furthermore, the evaluation of laminar flame speeds with various compositions have been carried out through numerical calculations and results were compared with experimental data from previous studies. The hydrogen concentration in gas composition has shown an essential importance for the laminarflame speed variation.

  • 52494.
    Zhang, Xiaoxiang
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Jayasuriya, Jeevan
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Fransson, Torsten
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
    Kinetic Study on Ignition Delay Time of Biomass Derived Gas CombustionManuscript (preprint) (Other academic)
    Abstract [en]

    The ignition delay time is one of the fundamental characteristics of a combustionprocess and has an essential effect on the performance of the combustion process. In the current study, a kinetic study on auto iginition delaytime is carried out for biomass derived gas combustion. A gas mixture of CO/H2/CO2/CH4/N2 is used to represent the typical composition from a biomass gasification process. The gas mixture is mixed with air under a certain range of operating conditions. A pressure range from 1 – 32 atm and an initial temperature range from 900 K to 1250 K were considered in the current study.The correlation between the ignition delay time and the operating conditions (pressures, initial temperatures and equivalence ratio) was derived for the biomass derived gas based on the kinetic calculations and published experimental data. The empirical correlation was obtained for the gas mixture ofCO/H2/CO2/CH4/N2/air and the gas mixture of CO/H2/O2/Ar. The influence of fuel compositions of the ignition delay time has also been discussed within this study. However, the influence of composition variation shown in the current study was not significant and was difficult to be cross-validated by various experimental data.

  • 52495. Zhang, Xiaoyu
    et al.
    Guo, Fuling
    Li, Xin
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    He, Jinxiang
    Wu, Wenjun
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Hua, Jianli
    Effect of thiophene in bithiazole-bridged sensitizers on the performance of dye-sensitized solar cells2014In: Nano, ISSN 1793-2920, Vol. 9, no 5, 1440009- p.Article in journal (Refereed)
    Abstract [en]

    In this paper, we have designed and synthesized four bithiazole-bridged sensitizers (BT-T2, TBT-T2, BT-T3 and TBT-T3) with triphenylamine and indoline as the donor segment and applied them to dye-sensitized solar cells (DSSCs). For triphenylamine-based sensitizers as BT-T2 and TBT-T2, adding one thiophene unit between triphenylamine donor and bithiazole moiety not only led to bathochromic shift of the maximum absorption and increase of molar extinction coefficient, but also enhanced the photovoltaic conversion efficiency from 7.12% of BT-T2 to 7.51% of TBT-T2. But for indoline-based sensitizers as BT-T3 and TBT-T3, adding one thiophene unit between indoline donor and bithiazole moiety resulted in hypochromatic shift instead of bathochromic shift. We employed the density functional theory (DFT) calculations to further investigate the influence of the thiophene unit on their optical and electronic properties and photovoltaic performance of corresponding DSSC devices. Given the results, a reasonable explanation is the introduction of thiophene unit suppressed the intramolecular charge transfer and charge separation in the conjugation system of indoline-based sensitizer, which led to the hypochromatic shift of the maximum absorption wavelength and finally the low J(sc). Since the J(sc) dropped sharply from 15.26mAcm(-2) to 4.52mAcm(-2), the photovoltaic conversion efficiency decreased dramatically from 7.86% to 1.93%.

  • 52496. Zhang, Xiaoyu
    et al.
    Mao, Jiangyi
    Wang, Dan
    Li, Xin
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Yang, Jiabao
    Shen, Zhongjin
    Wu, Wenjun
    Li, Jing
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Hua, Jianli
    Comparative Study on Pyrido[3,4-b]pyrazine-Based Sensitizers by Tuning Bulky Donors for Dye-Sensitized Solar Cells2015In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 4, 2760-2771 p.Article in journal (Refereed)
    Abstract [en]

    Dye-sensitized solar cells (DSSCs) with cobalt electrolytes have gained increasing attention. In this Research Article, two new pyrido[3,4-b]pyrazine-based sensitizers with different cores of bulky donors (indoline for DT-1 and triphenylamine for DT-2) were designed and synthesized for a comparative study of their photophysical and electrochemical properties and device performance and were also analyzed through density functional theory calculations. The results of density function theory calculations reveal the limited electronic communication between the biphenyl branch at the cis-position of N-phenylindoline and the indoline core, which could act as an insulating blocking group and inhibit the dye aggregation and charge recombination at the interface of TiO2/dye/electrolyte. As expected, DSSCs based on DT-1 with cobalt redox electrolyte gained a higher photoelectric conversion efficiency of 8.57% under standard AM 1.5 G simulated sunlight, with J(sc) = 16.08 mA cm(-2), V-oc = 802 mV, and FF = 0.66. Both electrochemical impedance spectroscopy (EIS) and intensity-modulated photovoltage spectroscopy (IMVS) suggest that charge recombination in DSSCs based on DT-1 is much less than that in their counterparts of DT-2, owing to the bigger donor size and the insulating blocking branch in the donor of DT-1.

  • 52497.
    Zhang, Xin
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering (moved 20130630), Water, Sewage and Waste technology.
    Application of Partial Nitritation/Anammox Process for Treatment of Wastewater with High Salinity.2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The combination of partial nitritation and anaerobic ammonium oxidation (Anammox) is a composting way to remove the nitrogen in the wastewater. In this article the analysis was made to investigate how the salinity in the wastewater affects the process. Two strategies of salt concentration increase were tested in two reactors. The physical, chemical parameters and the activity of the bacteria in the reactors were monitored. The results of two strategies were compared and the reactor with less salt in each period showed higher bacteria activities and efficiency. Finally the outlook for the future research was made.

  • 52498.
    Zhang, Xinhai
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
    Chen, DeJiu
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Mechatronics.
    Törngren, Martin
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Mechatronics.
    Towards Design Space Exploration through EAST-ADL and AUTOSAR Modeling Frameworks2014Conference paper (Refereed)
  • 52499.
    Zhang, Xinhai
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
    Feng, Lei
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Mechatronics.
    Chen, De-Jiu
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Mechatronics.
    Törngren, Martin
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Mechatronics.
    Design-Space Reduction for Architectural Optimization of Automotive Embedded Systems2015In: High Performance Computing and Communications (HPCC), 2015 IEEE 7th International Symposium on Cyberspace Safety and Security (CSS), 2015 IEEE 12th International Conferen on Embedded Software and Systems (ICESS), 2015 IEEE 17th International Conference on, IEEE Computer Society, 2015, , 7 p.1103-1109 p.Conference paper (Refereed)
    Abstract [en]

    A key decision for the synthesis of automotiveembedded systems is the allocation of application softwarecomponents to ECUs. Design Space Exploration (DSE) supportsthe decision by automatically characterizing and evaluating alarge number of possible design alternatives, and thereby suggestingthe optimal ones. A primary challenge for applying DSEmethods to support this decision is to reduce the computationtime of the DSE process while maintaining the generality andoptimality. This paper exploits legacy system architectures andthe AUTOSAR standard to preemptively reduce the design space,because both artifacts limit the flexibility of certain designvariables. A new DES formulation incorporating the constraintsof the legacy system architectures and the AUTOSAR standardis proposed in this paper. Computation result shows a largereduction of the computation time comparing to traditionalmodeling and formulations. The scalability of our method is alsoanalyzed by testing it on a set of random problem instances.

  • 52500.
    Zhang, Xinhai
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
    Mohan, Naveen
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Mechatronics.
    Törngren, Martin
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Mechatronics.
    Axelsson, J.
    Chen, DeJiu
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Mechatronics.
    Architecture exploration for distributed embedded systems: A gap analysis in automotive domain2017In: 2017 12th IEEE International Symposium on Industrial Embedded Systems, SIES 2017 - Proceedings, Institute of Electrical and Electronics Engineers (IEEE), 2017, 7993377Conference paper (Refereed)
    Abstract [en]

    A large body of work can be found in literature on Design Space Exploration (DSE) methods for distributed embedded system architecting (DESA). However, almost none of these methods is successfully adopted in automotive industry. To clarify the reasons, this paper 1) analyzes the current state of the art (SOTA) on DSE methods for DESA through a systematic literature study, focusing on the assumed architecting process and concerns; 2) investigates the state of practice (SOP) on DESA in the automotive industry through a literature study and interviews with experienced system architects from five different automotive manufacturers; and 3) analyzes the gap between SOTA and SOP, and thereby discusses potential improvements of DSE methods.

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