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Title: Inverted Brayton cycles for exhaust gas energy recovery
Author: Chen, Zhihang
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2019
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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 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. 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.
Supervisor: Copeland, Colin ; Burke, Richard ; Brace, Christian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available