Design optimization of heat exchangers of an engine waste heat recovery system for a gasoline vehicle based on combined-dimensional thermal flow analysis approach

Abstract

Strict regulations on emission and fuel consumption are expanding worldwide in response to the global warming and the depletion of natural resources. The improvement of fuel efficiency by development of power-train technology has reached its limit. In this study, a waste heat recovery system based on Rankine cycle is constructed to improve the fuel efficiency of gasoline vehicles. A commercial CFD analysis tool is capable of predicting detailed flow distribution for complex geometries, but it cannot adequately simulate the phase change processes yet. On the other hand, the performance prediction code is one-dimensional, but it can take phase change into account, and enables a quick verification of the effect of change in design factors. In this study, an optimized design process has been established by harmonizing the advantages of these two design techniques. The design of the waste heat recovery system have been performed through combined-dimensional analytical processes. First of all, the system layout has been decided by the one-dimensional analysis. The cycle performance prediction program has been developed to select working fluids through the system performance prediction, design the cycle performance, and decide the design specification of core components. As a result, the system layout containing dual-loop cycles has been designed. The waste heat recovery system consists of an HT (high temperature) loop, in which water, as the HT working fluid, recovers waste heat from the exhaust gas, and an LT (low temperature) loop, in which a refrigerant, as the LT working fluid, recovers heat dissipation from the HT loop, and waste heat from the engine coolant of relatively low temperature. A new design of HT boiler, which recovers waste heat from exhaust gas, has been conducted. The working fluid in the HT boiler experiences a phase change from liquid state to saturated state. At a liquid state, the specific volume of the working fluid is so small that the cross-sectional area must be small, otherwise it would not recover sufficiently the waste heat. On the other hand, at a saturated state, the specific volume of the working fluid grows with quality, i.e., the cross-sectional area of the heat exchanger need to be considerably large compared to that of liquid state. Focusing on this point, three structural concepts have been established, designed via 1D and 3D analytical design process, embodied as prototypes, and assessed by experiments. A novel design process model for an LT condenser has been built, so that the pressure drop is reduced, while the heat transfer performance is maintained close to a target value. The refrigerant has low enough evaporation temperature to recover the waste heat from engine coolant of about 100 ℃, but has small saturation enthalpy. Thus, excessive mass flow rate of the LT working fluid, e.g. over 150 g/s, causes a significant pressure drop to maintain the heat dissipation performance of more than 20 kW. An investigation for multi-pass structural design has been conducted by inspecting the number of passes, and the arrangement of the numbers of tubes, in order to enhance the flow uniformity and reduce the pressure drop of working fluid. The cycle design technology and the combined-dimensional optimization design process for the core heat exchangers are expected to play a role of bridgehead to secure technological competitiveness in the future automotive waste heat recovery field.