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  • 1.
    Andrae, Johan
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Björnbom, Pehr
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Edsberg, Lennart
    KTH, Superseded Departments, Numerical Analysis and Computer Science, NADA.
    Numerical studies of wall effects with laminar methane flames2002In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 128, no 1-2, p. 165-180Article in journal (Refereed)
    Abstract [en]

    Wall effects in the combustion of lean methane mixtures have been studied numerically using the CHEMKIN software. To gain a deeper understanding of the flame-wall interaction in lean burn combustion, and in particular the kinetic and thermal effects, we have simulated lean and steady methane/air flames in a boundary layer flow. The gas-phase chemistry is modeled with the GRI mechanism version 1.2. Boundary conditions include an inert wall, a recombination wall and catalytic combustion of methane. Different pressures, wall temperatures and fuel-air ratios are used to address questions such as which part of the wall effects is most important at a given set of conditions. As the results are analyzed it can be seen that the thermal wall effects are more significant at the lower wall temperature (600 K) and the wall can essentially be modeled as chemical inert for the lean mixtures used. At the higher wall temperature (1,200 K), the chemical wall effects become more significant and at the higher pressure (10 atm) the catalytic surface retards homogeneous combustion of methane more than the recombination wall because of product inhibition. This may explain the increased emissions of unburned fuel observed in engine studies, when using catalytic coatings on the cylinder walls. The overall wall effects were more pronounced for the leaner combustion case (phi = 0.2). When the position of the reaction zone obtained from the boundary layer calculations is compared with the results from a one-dimensional premixed flame model, there is a small but significant difference except at the richer combustion case (phi = 0.4) at atmospheric pressure, where the boundary layer model may not predict the flame position for the given initial conditions.

  • 2.
    Andrae, Johan
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Björnbom, Pehr
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Edsberg, Lennart
    Eriksson, L-E
    Kinetic and Transport Effects of Pressurized Methane Flames in a Boundary Layer2003In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 133, no 4, p. 503-506Article in journal (Refereed)
    Abstract [en]

    We have recently modeled combustion of lean methane-air [1] mixtures in a boundary layer flow using the program CRESLAF [2, 3 and 4]. A uniform fuel-air mixture above the auto-ignition temperature was introduced at the inlet edge of the wall, which gave an arrested flame zone, propagating in the upstream direction with a local flame speed that is equal and opposite to the local flow velocity. We compared the interaction of this flame with three model wall materials representing three idealized cases from a chemical point of view, a completely inert wall, a radical recombining wall, and a wall supporting catalytic combustion.

    Here we report on an analogous study of a flame geometry that may be considered a combination of a one-dimensional flame propagating towards a wall and the combustion of a uniform fuel-air mixture in a boundary layer flow. In contrast to our previous work [1] where we had a uniform inlet flow composition consisting of unburnt gas, here there is only unburnt gas close to the walls while there is burnt gas in the center of the channel. The present study concerns lean pressurized methane flames propagating toward hot isothermal walls where chemistry on the wall is considered important.

    The main purpose is to compare the results with those obtained in Ref. [1], which enables us, for the same flow field (boundary layer flow), to compare the effect of flame geometry on the wall effects. We have found and have been able to explain theoretically that such subtle changes of the flame geometry, which would be rather difficult to study experimentally, may have surprisingly significant effects on the combustion process.

  • 3. Andrae, Johan C. G.
    et al.
    Bjornbom, P.
    Cracknell, R. F.
    Kalghatgi, G. T.
    Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics2007In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 149, no 02-jan, p. 2-24Article in journal (Refereed)
    Abstract [en]

    A detailed chemical kinetic model for the autoignition of toluene reference fuels (TRF) is presented. The toluene submechanism added to the Lawrence Livermore Primary Reference Fuel (PRF) mechanism was developed using recent shock tube autoignition delay time data under conditions relevant to HCCI combustion. For two-component fuels the model was validated against recent high-pressure shock tube autoignition delay time data for a mixture consisting of 35% n-heptane and 65% toluene by liquid volume. Important features of the autoignition of the mixture proved to be cross-acceleration effects, where hydroperoxy radicals produced during n-heptane oxidation dramatically increased the oxidation rate of toluene compared to the case when toluene alone was oxidized. Rate constants for the reaction of benzyl and hydroperoxyl radicals previously used in the modeling of the oxidation of toluene alone were untenably high for modeling of the mixture. To model both systems it was found necessary to use a lower rate and introduce an additional branching route in the reaction between benzyl radicals and O-2. Good agreement between experiments and predictions was found when the model was validated against shock tube autoignition delay data for gasoline surrogate fuels consisting of mixtures of 63-69% isooctane, 14-20% toluene, and 17% n-heptane by liquid volume. Cross reactions such as hydrogen abstractions between toluene and alkyl and alkylperoxy radicals and between the PRF were introduced for completion of chemical description. They were only of small importance for modeling autoignition delays from shock tube experiments, even at low temperatures. A single-zone engine model was used to evaluate how well the validated mechanism could capture autoignition behavior of toluene reference fuels in a homogeneous charge compression ignition (HCCI) engine. The model could qualitatively predict the experiments, except in the case with boosted intake pressure, where the initial temperature had to be increased significantly in order to predict the point of autoignition.

  • 4. Andrae, Johan C. G.
    et al.
    Brinck, Tore
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Physical Chemistry (closed 20110630).
    Kalghatgi, G. T.
    HCCI experiments with toluene reference fuels modeled by a semidetailed chemical kinetic model2008In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 155, no 4, p. 696-712Article in journal (Refereed)
    Abstract [en]

    A semidetailed mechanism (137 species and 633 reactions) and new experiments in a homogeneous charge conic pression ignition (HCCI) engine on the autoignition of toluene reference fuels are presented. Skeletal mechanisms for isooctane and n-heptane were added to a detailed toluene submechanism. The model shows generally good agreement with ignition delay times measured in a shock tube and a rapid compression machine and is sensitive to changes in temperature, pressure, and mixture strength. The addition of reactions involving the formation and destruction of benzylperoxide radical was crucial to modeling toluene shock tube data. Laminar burning velocities for benzene and toluene were well predicted by the model after some revision of the high-temperature chemistry. Moreover, laminar burning velocities of a real gasoline at 353 and 500 K Could be predicted by the model using a toluene reference fuel as a surrogate. The model also captures the experimentally observed differences in combustion phasing of toluene/n-heptane mixtures, compared to a primary reference fuel of the same research octane number, in HCCI engines as the intake pressure and temperature are changed. For high intake pressures and low intake temperatures, a sensitivity analysis at the moment of maximum heat release rate shows that the consumption of phenoxy radicals is rate-limiting when a toluene/n-heptane fuel is used, which makes this fuel more resistant to autoignition than the primary reference fuel. Typical CPU times encountered in zero-dimensional calculations were on the order of seconds and minutes in laminar flame speed calculations. Cross reactions between benzylperoxy radicals and n-heptane improved the model prediction,,; of shock tube experiments for phi = 1.0 and temperatures lower than 800 K for an n-heptane/toluene fuel mixture, but cross reactions had no influence on HCCI Simulations.

  • 5.
    Andrae, Johan C. G.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Head, R. A.
    HCCl experiments with gasoline surrogate fuels modeled by a semidetailed chemical kinetic model2009In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 156, no 4, p. 842-851Article in journal (Refereed)
    Abstract [en]

    Experiments in a homogeneous charge compression ignition (HCCI) engine have been conducted with four gasoline surrogate fuel blends. The pure components in the Surrogate fuels consisted of n-heptane, isooctane, toluene, ethanol and diisobutylene and fuel sensitivities (RON-MON) in the fuel blends ranged from two to nine. The operating conditions for the engine were p(in) = 0.1 and 0.2 MPa, T-in = 80 and 250 degrees C, phi = 0.25 in air and engine speed 1200 rpm. A semidetailed chemical kinetic model (142 species and 672 reactions) for gasoline surrogate fuels, validated against ignition data from experiments conducted in shock tubes for gasoline Surrogate fuel blends at 1.0 <= p <= 5.0 MPa, 700 <= T <= 1200 K and 0 = 1.0, was successfully used to qualitatively predict the HCCI experiments using a single zone modeling approach. The fuel blends that had higher fuel sensitivity were more resistant to autoignition for low intake temperature and high intake pressure and less resistant to autoignition for high intake temperature and low intake pressure. A sensitivity analysis shows that at high intake temperature the chemistry of the fuels ethanol, toluene and diisobutylene helps to advance ignition. This is consistent with the trend that fuels with the least Negative Temperature Coefficient (NTC) behavior show the highest octane sensitivity, and become less resistant to autoignition at high intake temperatures. For high intake pressure the sensitivity analysis shows that fuels in the fuel blend with no NTC behavior consume OH radicals and acts as a radical scavenger for the fuels with NTC behavior. This is consistent with the observed trend of an increase in RON and fuel sensitivity. With data from shock tube experiments in the literature and HCCI modeling in this work, a correlation between the reciprocal pressure exponent oil the ignition delay to the fuel sensitivity and volume percentage of single-stage ignition fuel in the fuel blend was found. Higher fuel sensitivity and single-stage fuel content generally gives a lower value of the pressure exponent. This helps to explain the results obtained while boosting the intake pressure in the HCCI engine.

  • 6.
    Andrae, Johan
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Reaction Engineering.
    Johansson, David
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Reaction Engineering.
    Björnbom, Pehr
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Reaction Engineering.
    Risberg, Per
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Kalghatgi, Gautam
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Cooxidation in the auto-ignition of primary reference fuels and n-heptane/toluene blends2005In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 140, no 4, p. 267-286Article in journal (Refereed)
    Abstract [en]

    Auto-ignition of fuel mixtures was investigated both theoretically and experimentally to gain further understanding of the fuel chemistry. A homogeneous charge compression ignition (HCCI) engine was run under different operating conditions with fuels of different RON and MON and different chemistries. Fuels considered were primary reference fuels and toluene/n-heptane blends. The experiments were modeled with a single-zone adiabatic model together with detailed chemical kinetic models. In the model validation, co-oxidation reactions between the individual fuel components were found to be important in order to predict HCCI experiments, shock-tube ignition delay time data, and ignition delay times in rapid compression machines. The kinetic models with added co-oxidation reactions further predicted that an n-heptane/toluene fuel with the same RON as the corresponding primary reference fuel had higher resistance to auto-ignition in HCCI combustion for lower intake temperatures and higher intake pressures. However, for higher intake temperatures and lower intake pressures the n-heptane/toluene fuel and the PRF fuel had similar combustion phasing.

  • 7. Bai, X. S.
    et al.
    Fuchs, Laszlo
    Mauss, F.
    Laminar flamelet structure at low and vanishing scalar dissipation rate2000In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 120, no 3, p. 285-300Article in journal (Refereed)
    Abstract [en]

    The laminar flamelet structures of methane/air, propane/air, and hydrogen/air nonpremixed combustion at low and vanishing scalar dissipation rates are investigated, by numerical calculations of a system of conservation equations in a counterflow diffusion flame configuration, together with a transport equation defining the mixture fraction and scalar dissipation rate. The chemical reaction mechanisms consist of 82 elementary reactions up to C-3 species. In the limit of vanishing scalar dissipation rate, two types of structures are shown to appear. In one structure fuel and oxygen are consumed in a thin layer located near the stoichiometric mixture fraction, Z(st), where the temperature and the major products reach their peaks. This is similar to the so-called Burke-Schumann single layer flame sheet structure. One example is the hydrogen/air diffusion flame. The second structure consists of multilayers. Fuel and oxygen are consumed at different locations. Oxygen is consumed at Z(l) (near Z(st)), where the temperature and the major products reach their peaks. Fuel is consumed at Z(r) (> Z(st)). Between Z(l) and Z(r) some intermediate and radical species are found in high concentrations. Hydrocarbon/air nonpremixed flames are of this type. It is shown that for methane/air diffusion flames, some chemical reactions which are negligible at large scalar dissipation rate near flame quenching conditions, play essential roles for the existence of the multilayer structure. Examples of such reactions are, CH4 --> CH3 + H, H2O + O-2 --> HO2 + OH, H2O + M --> H + OH + M and CHO + H-2 --> O + H. The sensitivity of the species distributions in the flamelet to the scalar dissipation rate varies for different species. The most sensitive species are the intermediates and radicals at the fuel-rich side. At low scalar dissipation rate the radiative heat transfer can significantly move the fuel consumption layer to the oxygen consumption layer, increase the oxygen leakage to fuel side, and even quench the flame. Differential diffusion modifies the species and temperature profiles in the flamelet, but does not affect the multilayer nature of the flamelet. This result is used to successfully explain the high CO emissions in a turbulent methane/air diffusion flame.

  • 8.
    Blasiak, Wlodzimierz
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Yang, Weihong
    KTH, Superseded Departments, Materials Science and Engineering.
    Rafidi, Nabil
    KTH, Superseded Departments, Materials Science and Engineering.
    Physical properties of a LPG flame with high-temperature air on a regenerative burner2004In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 136, no 4, p. 567-569Article in journal (Refereed)
  • 9. Cracknell, R. F.
    et al.
    Andrae, Johan C. G.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    McAllister, L. J.
    Norton, M.
    Walmsley, H. L.
    The chemical origin of octane sensitivity in gasoline fuels containing nitroalkanes2009In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 156, no 5, p. 1046-1052Article in journal (Refereed)
    Abstract [en]

    Experimental octane measurements are presented for a standard gasoline to which has been added various quantities of nitromethane, nitroethane and 1-nitropropane. The addition of nitroalkanes was found to suppress the Motor Octane Number to a much greater extent than the Research Octane Number. in other words addition of nitroalkanes increases the octane sensitivity of gasoline. Density Functional Theory was used to model the equilibrium thermodynamics and the barrier heights for reactions leading to the break-up of nitroethane. These results were used to develop a chemical kinetic scheme for nitroalkanes combined with a surrogate gasoline (for which a mechanism has been developed previously). Finally the chemical kinetic simulations were combined with a quasi-dimensional engine model in order to predict autoignition in octane rating tests. Our results suggest that the chemical origin of octane sensitivity in gasoline/nitroalkane blends cannot be fully explained on the conventional basis of the extent to which NTC behaviour is absent. Instead we have shown that the contribution of the two pathways leading to autoignition in gasoline containing nitroalkanes becomes much more significant under the more severe conditions of the Motor Octane method than the Research Octane method.

  • 10.
    Fiorina, B.
    et al.
    CNRS, Laboratoire EM2C, CentraleSupelec, France.
    Mercier, R.
    CNRS, Laboratoire EM2C, CentraleSupelec, France.
    Kuenne, G.
    TU Darmstadt, Institute of Energy and Power Plant Technology, Germany. TU Darmstadt, Darmstadt Graduate School of Energy Science and Engineering, Germany.
    Ketelheun, A.
    TU Darmstadt, Institute of Energy and Power Plant Technology, Germany. TU Darmstadt, Darmstadt Graduate School of Energy Science and Engineering, Germany.
    Avdić, A.
    TU Darmstadt, Institute of Energy and Power Plant Technology, Germany. TU Darmstadt, Graduate School of Computational Engineering, Germany.
    Janicka, J.
    TU Darmstadt, Institute of Energy and Power Plant Technology, Germany. TU Darmstadt, Darmstadt Graduate School of Energy Science and Engineering, Germany.
    Geyer, D.
    Darmstadt University of Applied Sciences, Thermodynamik, Germany.
    Dreizler, A.
    TU Darmstadt, Institute of Energy and Power Plant Technology, Germany. TU Darmstadt, Darmstadt Graduate School of Energy Science and Engineering, Germany.
    Alenius, Emma
    Lund University, Sweden.
    Duwig, Christophe
    KTH, School of Engineering Sciences (SCI), Mechanics. Lund University, Sweden.
    Trisjono, P.
    RWTH Aachen University, Institute for Combustion Technology, Germany.
    Kleinheinz, K.
    RWTH Aachen University, Institute for Combustion Technology, Germany.
    Kang, S.
    Sogang University, Department of Mechanical Engineering, Republic of Korea.
    Pitsch, H.
    RWTH Aachen University, Institute for Combustion Technology, Germany.
    Proch, F.
    University of Duisburg-Essen, Institute for Combustion and Gasdynamics (IVG), Chair for Fluid Dynamics, Germany.
    Cavallo Marincola, F.
    University of Duisburg-Essen, Institute for Combustion and Gasdynamics (IVG), Chair for Fluid Dynamics, Germany.
    Kempf, A.
    University of Duisburg-Essen, Institute for Combustion and Gasdynamics (IVG), Chair for Fluid Dynamics, Germany.
    Challenging modeling strategies for LES of non-adiabatic turbulent stratified combustion2015In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921Article in journal (Refereed)
    Abstract [en]

    Five different low-Mach large eddy simulations are compared to the turbulent stratified flame experiments conducted at the Technical University of Darmstadt (TUD). The simulations were contributed by TUD, the Institute for Combustion Technology (ITV) at Aachen, Lund University (LUND), the EM2C laboratory at Ecole Centrale Paris, and the University of Duisburg-Essen (UDE). Combustion is modeled by a premixed flamelet tabulation with local flame thickening (TUD), a premixed flamelet progress variable approach coupled to a level set method (ITV), a 4-steps mechanism combined with implicit LES (LUND), the F-TACLES model that is based on filtered premixed flamelet tabulation (EM2C), and a flame surface density approach (UDE). An extensive comparison of simulation and experimental data is presented for the first two moments of velocity, temperature, mixture fraction, and major species mass fractions. The importance of heat-losses was assessed by comparing simulations for adiabatic and isothermal boundary conditions at the burner walls. The adiabatic computations predict a flame anchored on the burner lip, while the non-adiabatic simulations show a flame lift-off of one half pilot diameter and a better agreement with experimental evidence for temperature and species concentrations. Most simulations agree on the mean flame brush position, but it is evident that subgrid turbulence must be considered to achieve the correct turbulent flame speed. Qualitative comparisons of instantaneous snapshots of the flame show differences in the size of the resolved flame wrinkling patterns. These differences are (a) caused by the influence of the LES combustion model on the flame dynamics and (b) by the different simulation strategies in terms of grid, inlet condition and numerics. The simulations were conducted with approaches optimized for different objectives, for example low computational cost, or in another case, short turn around.

  • 11.
    Fooladgar, Ehsan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Duwig, Christophe
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    A new post-processing technique for analyzing high-dimensional combustion data2018In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 191, p. 226-238Article in journal (Refereed)
    Abstract [en]

    This paper introduces a novel post-processing technique for analyzing high dimensional combustion data. In this technique, t-Distributed Stochastic Neighbor Embedding (t-SNE) is used to reduce the dimensionality of the combustion data with no prior knowledge while preserving the similarity of the original data. Multidimensional combustion datasets are from premixed and non-premixed laminar flame simulations and measurements of a series of well documented piloted flames with inhomogeneous inlets. The resulting reduced manifold is visualized by scatter plots to reveal the global and local structure of the data (manual labeling). Unsupervised clustering algorithms are then utilized for post-processing the t-SNE manifold in order to develop an automatic labeling process. 

  • 12.
    Fooladgar, Ehsan
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tóth, P.
    Duwig, Christophe
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Characterization of flameless combustion in a model gas turbine combustor using a novel post-processing tool2019In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, p. 356-367Article in journal (Refereed)
    Abstract [en]

    Flameless combustion is a very promising technology for the future gas turbines. It is clean and stable—without large oscillations, noise and flashback. To facilitate the adoption of this technology in gas turbines, advanced design tools are needed. In this paper, a recently developed unsupervised post-processing tool is used to analyze the large amount of high-dimensional data produced in a series of Large Eddy Simulations (LES) of a model gas turbine operating in flameless mode. Simulations are performed using Finite Rate Chemistry (FRC) combustion modeling and a detailed description of chemistry. The automatic post-processing reveals important features of the combustion process that are not easily recognizable by other methods, making it a complementary step for the already established FRC–LES approach, and a potential design tool for advanced combustion systems.

  • 13. Hoeijmakers, Maarten
    et al.
    Kornilov, Viktor
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Arteaga, Ines Lopez
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    de Goey, Philip
    Nijmeijer, Henk
    Flame dominated thermoacoustic instabilities in a system with high acoustic losses2016In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 169, p. 209-215Article in journal (Refereed)
    Abstract [en]

    The thermoacoustic stability behaviour of a flame is experimentally investigated in the presence of large acoustic losses. Recently it has become clear that under such conditions an instability can occur due to an intrinsic local feedbackloop at the heat source. The experimental results confirm that despite significant acoustic losses, thermoacoustic instabilities can still be present. These findings imply that the effectiveness of passive thermoacoustic damping devices is limited by the intrinsic stability properties of the flame. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

  • 14. Hoeijmakers, Maarten
    et al.
    Kornilov, Viktor
    Lopez Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Eindhoven University of Technology, Netherlands.
    de Goey, Philip
    Nijmeijer, Henk
    Intrinsic instability of flame-acoustic coupling2014In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 161, no 11, p. 2860-2867Article in journal (Refereed)
    Abstract [en]

    This paper shows that a flame can be an intrinsically unstable acoustic element. The finding is clarified in the framework of an acoustic network model, where the flame is described by an acoustic scattering matrix. The instability of the flame acoustic coupling is shown to become dominating in the limit of no acoustic reflections. This is in contrast to classical standing-wave thermoacoustic modes, which originate from the positive feedback loop between system acoustics and the flame. These findings imply that the effectiveness of passive thermoacoustic damping devices is limited by the intrinsic stability properties of the flame.

  • 15. Hosseini, N.
    et al.
    Kornilov, V. N.
    Teerling, O. J.
    Lopez Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. Eindhoven Univ Technol, Dept Mech Engn, POB 513, NL-5600 MB Eindhoven, Netherlands.
    de Goey, P.
    Evaluating thermoacoustic properties of heating appliances considering the burner and heat exchanger as acoustically active elements2018In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 191, p. 486-495Article in journal (Refereed)
    Abstract [en]

    Heat exchangers are an essential constituent part of many combustion systems. The thermoacoustic instability in such systems is a common problem and it has been studied extensively. However, the heat exchanger has not gained much attention in the field of combustion thermoacoustics, leading to a lack of knowledge about the thermoacoustic interactions between the burner and the heat exchanger. In this paper, a modeling approach is introduced to study these interactions in an academic representation of a heating appliance, comprised of a perforated slit burner and a tube heat exchanger. Both elements are considered thermally and acoustically active. A CFD model is used in a two-dimensional domain to simulate the response of the system to small amplitude broadband velocity perturbations. The thermochemical and acoustic coupling between the burner and the heat exchanger is investigated and a method is introduced to decouple their effects and study them separately. The extents to which this method is valid are addressed by varying the distance between the elements. Results show that as long as the flames do not impinge on the heat exchanger surface, a linear network modeling approach can be applied to construct the acoustic response of the composed configuration from the responses of its constituting elements. This approach requires registering the average velocity on a properly chosen intermediate plane between the burner and heat exchanger. Choosing this plane may be to some point difficult, i.e. when the burner and heat exchanger are close and cannot be considered independent. Moreover, when flame impingement occurs, the interactions between the flame and heat exchanger affect their individual thermoacoustic behaviors and the burner plus heat exchanger assembly needs to be considered as one coupled acoustic element. Particularly, flame impingement changes the phase of the heat absorption response of the heat exchanger and it may significantly alter the acoustic properties of the coupled assembly. The physics lying behind the effects of such interactions on the thermoacoustics of the system is discussed. The obtained results signify that a correct stability prediction of an appliance with burner and heat exchangers requires considering active thermoacoustic behavior of both elements as well as their interactions.

  • 16. Ivanov, Mikhail F.
    et al.
    Kiverin, Alexey D.
    Liberman, Mikhail A.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Ignition of deflagration and detonation ahead of the flame due to radiative preheating of suspended micro particles2015In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 162, no 10, p. 3612-3621Article in journal (Refereed)
    Abstract [en]

    We study a flame propagating in the gaseous combustible mixture with suspended inert solid micro particles. The gaseous mixture is assumed to be transparent for thermal radiation emitted by the hot combustion products, while particles absorb and reemit the radiation. Thermal radiation heats the particles, which in turn transfer the heat to the surrounding unburned gaseous mixture by means of thermal heat transfer, so that the gas phase temperature lags that of the particles. We consider different scenarios depending on the spatial distribution of the particles, their size and the number density. In the case of uniform spatial distribution of the particles the radiation causes a modest increase of the temperature ahead of the flame and corresponding modest increase of the combustion velocity. In the case of non-uniform distribution of the particles (layered dust cloud), such that the particles number density is relatively small in the region just ahead of the flame front and increases in the distant regions ahead of the flame, the preheating caused by the thermal radiation may trigger additional independent source of ignition. Far ahead of the flame, where number density of particles increases forming a dense cloud of particles, the radiative preheating results in the formation of a temperature gradient with the maximum temperature sufficient for ignition. Depending on the steepness of the temperature gradient formed in the unburned mixture, either deflagration or detonation can be initiated via the Zel'dovich's gradient mechanism. The ignition and the resulting combustion regimes depend on the number density profile and, correspondingly, on the temperature profile (temperature gradient), which is formed in effect of radiation absorption and gas-dynamic expansion. The effect of radiation preheating as stronger as smaller is the normal flame velocity. The effect of radiation heat transfer in the case of coal dust flames propagating in layered particle-gas deposits cloud can result in the spread of combustion wave with velocity up to 1000 m/s and it is a plausible explanation of the origin of dust explosion in coal mines.

  • 17.
    Li, Jun
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Biagini, Enrico
    Yang, Weihong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Tognotti, Leonardo
    Blasiak, Wlodzimierz
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Flame characteristics of pulverized torrefied-biomass combusted with high-temperature air2013In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 160, no 11, p. 2585-2594Article in journal (Refereed)
    Abstract [en]

    In this work, the flame characteristics of torrefied biomass were studied numerically under high-temperature air conditions to further understand the combustion performances of biomass. Three torrefied biomasses were prepared with different torrefaction degrees after by releasing 10%, 20%, and 30% of volatile matter on a dry basis and characterized in laboratory with standard and high heating rate analyses. The effects of the torrefaction degree, oxygen concentration, transport air velocity, and particle size on the flame position, flame shape, and peak temperature are discussed based on both direct measurements in a laboratory-scale furnace and CFD simulations. The results primarily showed that the enhanced drag force on the biomass particles caused a late release of volatile matter and resulted in a delay in the ignition of the fuel-air mixture, and the maximum flame diameter was mainly affected by the volatile content of the biomass materials. Furthermore, oxidizers with lower oxygen concentrations always resulted in a larger flame volume, a lower peak flame temperature and a lower NO emission. Finally, a longer flame was found when the transport air velocity was lower, and the flame front gradually moved to the furnace exit as the particle size increased. The results could be used as references for designing a new biomass combustion chamber or switching an existing coal-fired boiler to the combustion of biomass.

  • 18. Mattsson, Roger
    et al.
    Kupiainen, Marco
    KTH, Superseded Departments, Numerical Analysis and Computer Science, NADA.
    Gren, Per
    Wahlin, Anders
    Carlsson, Torgny E
    Fureby, Christer
    Pulsed TV holography and schlieren studies, and large eddy simulations of a turbulent jet diffusion flame2004In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 139, no 1-2, p. 1-15Article in journal (Refereed)
    Abstract [en]

    In the search for an improved understanding of jet-flame dynamics we here compare predictions from large-eddy simulations (LES) and measurements using schlieren and holographic interferometry of a round turbulent jet diffusion flame. The studies concern a turbulent propane-air (C3H8-O-2/N-2) diffusion flame under ambient conditions at a Reynolds number of Re = 10(4). The interferometric measurements were performed with an all-electronic method, pulsed TV holography, using a pulsed laser and a fast charge coupled device (CCD) camera. The LES calculations use the probability density function (PDF) flamelet approach with a beta function as the probability density function, whereas the subgrid turbulence is modeled with a one-equation eddy viscosity model. In order to validate the LES model quantitative comparisons of first-order statistical moments of the velocity were first made with available data for nonreactive jets. The LES model captures the statistics well. The next step in the validation process concerns comparing the jet-flame development between LES and the schlieren and pulsed TV holography data. To this end the results of the LES calculations were used to simulate instantaneous interference patterns using ray tracing. The LES model describes the overall behavior of the flame successfully.

  • 19.
    Robert, Etienne
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Monkewitz, Peter A.
    Experimental realization and characterization of unstretched planar one-dimensional diffusion flames2013In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 160, no 3, p. 546-556Article in journal (Refereed)
    Abstract [en]

    A burner configuration that allows the creation of nearly unstretched one-dimensional diffusion flames has recently been introduced. This paper presents the first detailed characterization of flames produced in such a burner to assess how well they approach the idealized one-dimensional configuration, which is used extensively for the development of theoretical models for diffusion flame stability. Particular emphasis is directed at quantifying the remaining inhomogeneities in the burner, such as residual flame stretch or variations in the boundary conditions, and identifying possible means to minimize them. Measurements made include the velocity field, stable species concentration profiles and temperature distribution throughout the burning chamber. Additionally, the flame position in the burner is determined as a function of mixture strength, reactant transport properties and bulk flow velocity. All the measurements are found to be in good agreement with a simplified one-dimensional flame model. However, the boundary conditions and their physical location have to be determined experimentally on both sides of the flame, which complicates comparison with results from theoretical and numerical models. Despite these limitations, we conclude that this new burner type provides a good experimental approximation of the one-dimensional idealized construct.

  • 20.
    Robert, Etienne
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Monkewitz, Peter A.
    Thermal-diffusive instabilities in unstretched, planar diffusion flames2012In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 159, no 3, p. 1228-1238Article in journal (Refereed)
    Abstract [en]

    The recent development of a novel research burner at EPFL has opened the way for experimental investigations of essentially unstretched planar diffusion flames. In particular, it has become feasible to experimentally validate theoretical models for thermal-diffusive instabilities in idealized one-dimensional diffusion flames. In this paper, the instabilities observed close to the lean extinction limit are mapped in parameter space, notably as function of the two reactant Lewis numbers. Cellular and pulsating instabilities are found at low and high Lewis numbers, respectively, as predicted by linear stability analyses. The detailed investigation of these two types of instabilities reveals the dependence of cell size and pulsation frequency on the transport properties of the reactants and on flow conditions. The experimental scaling of the cell size is found in good agreement with linear stability. The comparison between experimental and theoretical pulsation frequencies, on the other hand, was hampered by the impossibility of experimentally reproducing the parameters of the stability calculations. Hence, a heuristic correlation between pulsation frequency and flow parameters, transport properties, in particular the Damkohler number, and oscillation amplitude has been developed and awaits theoretical interpretation.

  • 21. Troiani, G.
    et al.
    Battista, F.
    Picano, Francesco
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent consumption speed via local dilatation rate measurements in a premixed bunsen jet2013In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 160, no 10, p. 2029-2037Article in journal (Refereed)
    Abstract [en]

    The mean local reaction rate related to the average expansion across the front and computed from the mean velocity divergence is evaluated in this work. Measurements are carried out in a air/methane premixed jet flame by combined PIV/LIF acquisitions. The procedure serves the purpose of obtaining values of a turbulent flame speed, namely the local turbulent consumption speed SLC, as a function of the position along the bunsen flame. With the further position that the flamelet assumption provides a proportionality between turbulent burning speed normalized with the laminar unstretched one and the turbulent to average flame surface ratio, the proportionality constant, i.e., the stretching factor becomes available. The results achieved so far show the existence of a wide region along which the bunsen flame front has a constant stretching factor which apparently depends only on the ratio between turbulent fluctuations and laminar flame speed and on the jet Reynolds number.

1 - 21 of 21
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