Due to the reciprocating engine, a pulsating flow occurs in the turbine turbocharger, which experiences conditions far from the continuous flow scenario. In this work, the effects of the characteristics of the mass flow pulse, parameterized through amplitude, frequency and temporal gradient, are decoupled and studied via unsteady Computational Fluid Dynamics calculations under on-engine operating conditions. Firstly, the model is validated based on comparisons with experimental data in steady flow conditions. Then, the effect of each parameter on exergy budget is assessed by considering a +/-10% variation with respect to a baseline pulse. The other factors defining the operating conditions (e.g. mass flow, shaft speed and inflow exergy) are kept the same as the baseline. The adopted approach enables to completely isolate the effects of each parameter in contrast with previous literature studies. Based on the results observed, pulse amplitude is identified as the primary parameter affecting the hot-side system response in terms of turbine performance, heat transfer and entropy generation, while frequency and temporal gradient show a smaller influence compared to it. As the pulse amplitude increases, the turbine work is reported to improve up to 9.4%. Smaller variations are observed for the frequency and temporal gradient analysis. With a 10% increase of the pulse frequency the turbine work is registered to improve by 5.0%, while the same percentage reduction of the temporal gradient leads to an increase of turbine work equal to 3.6%.

Under on-engine operating conditions, a turbocharger turbine is subject to a pulsating flow and, consequently, experiences deviations from the performance measured in gas-stand flow conditions. Furthermore, due to the high exhaust gases temperatures, heat transfer further deteriorates the turbine performance. The complex interaction of the aerothermodynamic mechanisms occurring inside the hot-side, and consequently the turbine behavior, is largely affected by the shape of the pulse, which can be parameterized through three parameters: pulse amplitude, frequency, and temporal gradient. This paper investigates the hot-side system response to the pulse amplitude via detached eddy simulations (DES) of a turbocharger radial turbine system including the exhaust manifold. First, the computational model is validated against experimental data obtained in gas-stand flow conditions. Then, two different mass flow pulses, characterized by a pulse amplitude difference of ≈5%≈5%⁠, are compared. An exergy-based post-processing approach shows the beneficial effects of increasing the pulse amplitude. An improvement of the turbine power by 1.3%1.3%⁠, despite the increment of the heat transfer and total internal irreversibilities by 5.8%5.8% and 3.4%3.4%⁠, respectively, is reported. As a result of the higher maximum speeds, internal losses caused by viscous friction are responsible for the growth of the total internal irreversibilities as pulse amplitude increases.

The pulsating flow conditions which a turbocharger turbine is exposed cause important deviations of the turbine aerodynamic performance when compared to steady flow conditions. Indeed, the secondary flows developing in the turbine are determined by the inflow aerodynamic conditions, which largely vary during the pulse cycle. In this paper, a high-resolved Large Eddy Simulation is performed to investigate and characterize the flow field evolution in a turbocharger radial turbine over the pulse cycle. At first, the model is validated against experimental results obtained in gas-stand flow conditions. Then, the instantaneous flow field at the rotor mid-span section is compared to the one given by the equivalent cycle-averaged steady flow conditions. The results highlight five distinct flow features. At low mass flow rates, when the relative inflow angle assumes large negative values, the flow separates at the blade pressure side, causing a secondary flow consisting in two counter-rotating vortices characterized by a diameter comparable to the blade passage. As the mass flow rate increases, the first vortex persists at the blade tip while the second one moves closer to the blade trailing edge. This corresponds to the second characteristic flow field. With increasing relative inflow angle, for the third characteristic flow feature, only the recirculation at the blade leading edge is displayed and its size gradually reduces. For the fourth characteristic flow feature, at moderate negative values of the relative inflow angle, the flow field is well aligned with the blade profile and free of secondary flows. Then, as the relative inflow angle gradually grows towards large positive values, the flow separates on the blade suction side causing the mixing of the flow with the stream flowing on the pressure side of the previous blade.

Under on-engine operating conditions, a turbocharger turbine is subject to a pulsating flow and, consequently, experiences deviations from the performance measured under continuous flow. Furthermore, due to the high exhaust gas temperatures, heat transfer further deteriorates the turbine performance. The complex interaction of the aerothermodynamic mechanisms occurring inside the hot-side, and consequently the turbine behavior, is largely affected by the shape of the pulse, which can be parameterized through three parameters: pulse amplitude, frequency, and temporal gradient. This paper investigates the hot-side system response to the pulse amplitude via a Detached Eddy Simulation (DES) approach of a radial turbocharger turbine system including exhaust manifold. Firstly, the computational model is validated against experimental data obtained under gas stand continuous flow conditions. Then, two different mass flow pulses, characterized by a pulse amplitude difference of 5%, are compared. An exergy-based post-processing approach shows the beneficial effects of increasing pulse amplitude. An improvement of the turbine power by 1:3%, despite the increment of the heat transfer and total internal irreversibilities by 5:8% and 3:4%, respectively, is reported. As a result of the higher maximum speed, internal losses by viscous friction are responsible for the growth of the total internal irreversibilities as pulse amplitude increases.

Heat transfer in a turbocharger plays a crucial role in the optimization of turbocharger-engine matching process. Due to high temperature gradients between the hot exhaust gas compared to the compressor as well as the environment, it is well-known, that the heat loss from a turbocharger turbine is significant. Investigations of turbocharger performance are commonly done by quantifying the performance parameters under adiabatic conditions, following the paradigm of the first law of thermodynamics, based on the energy balance method. It turns out that an adiabatic assumption and the energy balance method is insufficient to provide a deep understanding about the aerothermodynamic effects on the turbine performance due to heat transfer. Based on the current state-of-the-art, this study aims to improve the characterization methods for passenger car turbocharger turbines, considering the impacts of heat transfer. Firstly, the turbocharger is measured on a hot gas test bench. Based on this experimental data, the turbine heat transfer is being quantified through implementing a new innovative power-based approach. Consequently, a heat loss free turbine performance map can be derived. Secondly, a CFD analysis is performed on selected operating points, taking turbine housing temperature measurements as boundary conditions. CFD results are verified and validated by using the experimental data, both at adiabatic and diabatic test conditions. Finally, a flow exergy-based method is being applied to the predicted 3D flow field from the CFD simulation. This approach allows to identify and quantify the aerothermodynamic impacts of heat transfer on the turbine performance for cases with and without heat loss, considering both first and second laws of thermodynamics. This study aims to enhance our understanding of the underlying thermo-fluid physics in a turbocharger turbine associated with heat loss. It will also demonstrate the potential of the application of flow exergy method to 3D CFD data, rather than limited to 1D adiabatic models in current engine research and development. 

Understanding how energy-conversion mechanisms, i.e. turbine power, heat transfer and total internal irreversibilities are affected by mass flow rate pulse characteristics, i.e. amplitude, frequency and temporal gradient, is a key technological factor to improve turbocharger turbine efficiency under pulsating flow conditions. In this work, the turbocharger turbine response to pulse shape is studied by comparing two different mass flow profiles, characterized by a 5% difference in amplitude, via a Detached Eddy Simulation (DES). With increasing amplitude, an exergy-based post-processing analysis shows a 1.5% increase in turbine power, and the growth of total internal irreversibilities by 7%, 8% and 3% inside the exhaust manifold, volute and turbine, respectively. The Bejan number distribution demonstrates that viscous dissipation, due to higher velocity gradients, is responsible for the increase of total internal irreversibilities. Furthermore, pulse amplitude effect on heat transfer is larger at the exhaust manifold compared to the volute. Under pulsating flow conditions, isentropic efficiency is demonstrated to provide inconsistent results rather than an exergy-based efficiency, which requires no phase correction. The analysis of secondary flows inside the volute highlights the formation of pairs of counter-rotating vortices, which interfere with the correct alignment of velocity triangles by adding a vertical component to the velocity field at the rotor inlet in the direction of the turbine axis of rotation.

This research was primary motivated by limited efforts to understand the effects of secondary flow and flow unsteadiness on the heat transfer and the performance of a turbocharger turbine subjected to pulsatile flow. In this study, we aimed to investigate the influence of exhaust manifold on the flow physics and the performance of its downstream components, including the effects on heat transfer, under engine-like pulsatile flow conditions. Based on the predicted results by detached eddy simulation (DES), qualitative and quantitative flow fields analyses in the scroll and the rotor's inlet were performed, in addition to the quantification of turbine performance by using the flow exergy methodology. With the specified geometry configuration and exhaust valve strategy, our study showed that (1) the exhaust manifold influences the flow field and the heat transfer in the scroll significantly and (2) although the exhaust gas blow-down disturbs the relative flow angle at rotor inlet, the consequence on the turbine power is relatively small.

This study was motivated by the difficulties to assess the aerothermodynamic effects of heat transfer on the performance of turbocharger turbine by only looking at the global performance parameters, and by the lack of efforts to quantify the physical mechanisms associated with heat transfer. In this study, we aimed to investigate the sensitivity of performance to heat loss, to quantify the aerothermodynamic mechanisms associated with heat transfer and to study the available energy utilization by a turbocharger turbine. Exergy analysis was performed based on the predicted three-dimensional flow field by detached eddy simulation (DES). Our study showed that at a specified mass flow rate, (1) pressure ratio drop is less sensitive to heat loss as compared to turbine power reduction, (2) turbine power drop due to heat loss is relatively insignificant as compared to the exergy lost via heat transfer and thermal irreversibilities, and (3) a single-stage turbine is not an effective machine to harvest all the available exhaust energy in the system.

Coupling of turbomachine to reciprocating automotive engine in turbocharging leads to complex fluid flow and thermal characteristics in the turbine. Some undesirable characteristics include heat transfer, flow pulsation and secondary flow due to the complex geometry of the upstream exhaust manifold. The performed literature review exposed that there is a need for an enhanced understanding of the thermo-fluid physics of a turbocharger turbine operating under realistic on-engine conditions, and on quantifying the impact on the performance. Often, simplified set-ups and geometries are employed, neglecting the heat transfer.

This dissertation aimed to improve the quality of heat transfer analysis in a turbocharger turbine, and to enhance the understanding of aerothermodynamic effects due to heat transfer on the performance under engine-like pulsatile flow scenarios. Firstly, a flow exergy based analysis was developed to be used with the input provided by three-dimensional flow field data predicted by Detached Eddy Simulation (DES). Its applicability to identify and to quantify the aerothermodynamic related losses due to heat transfer was thoroughly investigated with a set-up replicating a hot gas stand continuous flow scenario. Next, the developed methodology was applied to engine-like pulsatile flow scenarios, to investigate the effects of flow pulsation and the influences of upstream exhaust manifold on the heat transfer and turbine performance. For the investigated geometry and specified boundary conditions, this dissertation mainly concluded that 1) The most sensitive measures associated with heat loss are the flow exergy lost via heat transfer and the thermal irreversibilities. The influence of heat loss on turbine power reduction is small in a relative sense, and 2) Although the exhaust manifold characteristics govern the fundamental flow physics and heat transfer in the scroll, its impact on the turbine power seems to be small relatively. 

The contributions with this dissertation were mainly twofold. Firstly, it contributes to a deeper understanding of the thermo-fluid physics of a turbocharger turbine operating under engine-like pulsating flow scenario. This knowledge might be useful for industrial product development in the future. Secondly, from academic perspective, the flow exergy budget analysis could potentially serve as a practical example to students in connecting the dots between classroom theory and real life engineering application.

Large temperature gradients are associated with automotive turbocharger and thus the turbine experiences significant heat loss. Currently, the investigation of aerothermodynamic effects as a result of heat loss in turbine is commonly done by looking at the global performance parameters, i.e. pressure ratio and efficiency. This study aims to investigate the aerothermodynamic effects of heat transfer on a radial turbine operating under engine-like pulsating flow condition by identifying and quantifying the loss mechanisms via an exergy-based method using Detached Eddy Simulation data. Major findings with this study are: 1) Although exergy lost via heat transfer and internal irreversibilities could be as much as the turbine power, the drop of turbine power is only 4% as compared to an adiabatic turbine;2) Only about 12% of the available exhaust energy is extracted by the investigated turbine.