Micro-Channel Heat Exchanger: An Exhaust Waste Heat Recovery Device 2018-01-0052
Almost one-third of the fuel energy is wasted through the exhaust of a vehicle. An efficient waste heat recovery (WHR) process will undoubtedly lead to improved fuel efficiency and reduced greenhouse gases (GHG) emission. Currently, there are multiple WHR technologies that are being investigated by various entities in the auto industry. One relatively simple device to extract heat energy from the exhaust is a heat exchanger. Heat exchangers are used in some automotive applications to transfer heat from the hot exhaust gas to the colder coolant fluid to raise the coolant temperature. The warmer coolant fluid can be used for several purposes such as; faster heating of the engine’s lubrication oil and transmission fluids during cold starts, and faster cabin heating, which in turn, can potentially improve the overall engine efficiency and reduce exhaust emissions. Currently, in the US market place, hybrid vehicles, such as the Toyota Prius and Chevy Malibu, use heat exchangers as an integral part of their WHR systems. While traditional heat exchangers have been studied and employed in some automotive applications, the use of a micro-channel heat exchanger (MCHX) is a relatively new concept. Especially, in the realm of automobiles, use of micro-channel heat exchanger is unprecedented. As the name suggests, the micro-channel heat exchanger may provide a significant advantage in terms of packaging compared to a traditional heat exchanger when higher thermal performance is desired. However, the MCHX design has to be optimized for a particular application to avoid creating high backpressure in the exhaust system. In this paper, the design and performance of a micro-channel heat exchanger is described for a range of exhaust mass flows and temperatures. Predicted results from two 1-D models developed using the GT SUITE and Excel are compared and validated against preliminary experimental results from a prototype MCHX design. Finally, the thermal performance of the MCHX is compared to other traditional heat exchangers.