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Inverted Brayton Cycles for Exhaust Gas Energy Recovery University of Bath PHD Inverted Brayton Cycles for Exhaust Gas Energy Recovery Chen, Zhihang Award date: 2019 Link to publication Alternative formats If you require this document in an alternative format, please contact: [email protected] General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Download date: 08. Oct. 2021 Inverted Brayton Cycles for Exhaust Gas Energy Recovery Zhihang Chen A thesis submitted for the degree of Doctor of Philosophy University of Bath Department of Mechanical Engineering April 2019 COPYRIGHT Attention is drawn to the fact that the copyright of this thesis rests with its author and copyright of any previously published materials included may rest with third parties. A copy of this thesis has been supplied on condition that anyone who consults it understands that they must not copy it or use material from it except as licenced, permitted by law or with the consent of the author or other copyright owners, as applicable. This thesis may be made available for consultation within the University Library and may be photocopied or lent to other libraries for the purposes of consultation. I declare that the work in this thesis is my own and relevant citations are included to acknowledge the work of others. A as Zhihang Chen Abstract Abstract The exhaust gas from an internal combustion engine contains approximately 30% of the thermal energy of combustion. Waste heat recovery (WHR) systems aim to reclaim a proportion of this energy in a bottoming thermodynamic cycle to raise the overall system thermal efficiency. One of promising heat recovery approaches is to employ an inverted Brayton cycle (IBC) immediately downstream of the primary cycle. However, it is a little-studied approach as a potential exhaust-gas heat-recovery system, especially when applied to small automotive power-plants. Thus, this thesis presents comprehensive study of IBC as wasted heat recovery (WHR) system for a 2-litre turbocharged gasoline internal combustion (IC) engine. The basic IBC system consists of a radial turbine, a heat exchanger, and a centrifugal compressor in sequence. The use of the radial turbine is to further expand the exhaust gases from the turbocharged engine down to subatmospheric, so that the high turbine expansion ratio can be achieved. At the given turbine efficiency, the high turbine expansion ratio leads to the high specific power that generates by the turbine. Then, the residual heat in the expanded exhaust gas is rejected by the downstream heat exchanger in order to reduce the temperature of the exhaust gases. By doing so, the lower exhaust gas temperature leads to the higher gas density, thereby decreasing the compressor power consumption for the given compression ratio. The use of the centrifugal compressor is to pressurize the cooled gas back up to ambient. The IBC net power is the power differential between the turbine power generation and compressor power consumption. The main advantage of the IBC system is that the exhaust gases can be expanded below atmospheric pressure in the IBC turbine, thereby increasing the potential to recover thermal energy from the exhaust gases. The heat exchanger implemented between the turbine and compressor aims to lower the temperature i Abstract of the exhaust gases prior to the compression process. Since the role of this heat exchanger is to reject as much heat as possible, a liquid coolant loop with inexpensive materials offers a relatively light-weight, cost-effective solution. In addition, although the IBC system is installed immediately downstream of the top cycle and utilizes the exhaust gases as the operating fluid, it is still possible to leave the top cycle unaffected by adjusting its working pressure, that is, there is no back pressure caused by the employment of the IBC system. Finally, the characteristics of the IBC system is the simple configuration and relatively light weight. The key components – centrifugal compressor and radial turbine are quite mature technologies which has been widely used as turbochargers for automotive applications. The core of this thesis contains four main divisions. First, the IBC thermodynamic model was created by using finite-time thermodynamics (FTT) to perform the parametric study. The simulation results show that the increase of IBC inlet temperature, pressure, and turbomachinery efficiencies are beneficial to the IBC systemic performance at the given IBC expansion ratio. Moreover, there exists an optimum IBC expansion ratio that delivers the maximum specific power. Thus, the IBC system should be optimised according to the design conditions. In the second section, the correlated gasoline engine model was coupled with the high-fidelity IBC 1D model, in order to demonstrate the IBC heat-recovery capability as a bottoming WHR system for a commercial engine. The moderate improvement in the systematic performance was expected at engine high-load and high-speed conditions, due to the employment of an IBC system. Later, engine mini-map points for the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) driving cycle were selected as the design conditions for the IBC prototype, as such mini-map points can fully represented the engine operating conditions during real-world driving. IBC 1D simulations were conducted to give the guidance of the turbomachinery selection. However, due to the limited access to the commercial turbomachinery, the selected compressor and turbine suffer the suboptimal performance at the design point. ii Abstract Thus, the third section presented the performance optimisation of the selected compressor and turbine. The 3D modelling method was employed to evaluate the performance of all compressor and turbine design candidates. Due to the limited time, the compressor and turbine blades were trimmed to achieve the optimal performance. It is a common practice in the turbomachinery industry to optimise the existing design at the new design points. The optimal trimmed compressor and turbine, delivering the T-S efficiency of 72.32% and 77.38% respectively, were manufactured and employed in the IBC prototype. In parallel to the experiments of the IBC prototype, the compressor in-house design and optimisation procedure was created in order to achieve a high-performance compressor at the design points. By integrating Generic Algorithm (GA) optimisation method with the compressor design process, the final compressor design was able to reach at the T-S efficiency of 77.67%, which is 5.35 percentage points higher than that of the trimmed commercial compressor. The experiments of the IBC prototype were conducted in the gas stand in University of Bath. This is the first experimental demonstration of the IBC application for the automotive use. The test results show that the IBC prototype is able to generate the net power when the selected IC engine works at motorway cruise conditions. The parametric study of the IBC prototype was also conducted in tests. Finally, an IBC 1D model was correlated to the test data, and then utilized to predict the corresponding power generation over all engine mini-map points. Besides, the correlated IBC model can be utilized for the further development of the IBC system. iii Acknowledgement Acknowledgement First, I would like to express my great gratitude to Dr. Colin Copeland for his continuous support throughout my PhD. Without his guidance and persistent help this dissertation would not have been possible. Additionally, his patience, encouragement, and vision let me successfully transfer from a manufacturing engineer to a vehicle engineer. I also would like to express my sincere thanks to all the colleagues at PVRC for their emotional and technical support throughout my research, Dr. Dian Liu, Dr. Huayin Tang, Dr. Ian Kennedy, Dr. Lupeng Fei, Dr. Liu Dai, Dr. Pin Lu, Dr. Pavlos Dimitriou, Dr. Qiyou Deng, Dr. Tomasz Duda, Dr. Zhangqing Ning, Stefan Tüchler, and Linzhen Tan. I really enjoyed in working with all of you. Very special and sincere thanks to my girlfriend, Danni He, for her support during the writing-up phase in my research. It was a tough time, but she made it wonderful. Last but not the least, I would like to thank my parents, Jun Chen and Yangwan Gui, for their unflagging support and encouragement. 谢谢爸爸和妈妈的鼓励和 支持，我爱你们！ iv Nomenclature Nomenclature 퐴 area 휌 density 푚̇ mass flow rate 훼1 discharge flow angle 푊̇ power extracted from the working fluid 푈 turbine blade tip speed 푊 gas velocity relative to the rotor blades 퐶 absolute gas velocity 퐶푝 specific isobaric heat capacity 푅푔 gas constant of the working fluid 퐶푣 specific isochoric heat capacity 푘 ratio of the specific heats 푝 pressure 푇 temperature 푇0 average temperature 푟 pressure ratio ℎ enthalpy 퐶푟 ratio of heat capacity rates 푊 specific work 푄̇ heat rate 퐵 the constant related to heat-transfer
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