The Performance Analysis And Optimism Of The Exhaust Heat Exchanger Used For Waste Heat Recovery Bottom Cycle Of Diesel Engine

Only 30-40% of combustion energy is transformed to useful power when engines are working. Most of the energy is dissipated to the environment through exhaust gas and cooling water. The energy taken away by the exhaust takes up 30-40% of the total energy. It also has higher temperature and better quality than others. So recover the heat in the exhaust has important significance. Organic rankine cycle(ORC) is an effectively technology to recover waste heat from engine. In the ORC waste heat recovery system, the exhaust heat exchanger is a key component, which can decide the amount of energy that will be recovered. It is necessary to research the performance of exhaust heat exchanger.In this paper, a shell and tube exhaust heat exchanger in an ORC waste heat recovery experiment system was studied. It was used to exchange heat from the exhaust of an engine to thermal oil. The heat will be transformed to mechanical work in bottom cycle. Experimental research on the heat exchanger was done first, which will help to verify the calculation models. The results indicate that the exhaust heat exchanger can recover up to 74.2kW energy and the maximum of heat recover efficiency is 0.63.A calculation model was built with mean temperature difference method. The difference between calculation results and test results was less than 10%. Research work on the performance of heat exchanger on main working conditions of engine was done using this model. The results indicate that the performance of heat exchanger will be better and better with both the increasing of temperature and mass flow rate of exhaust.A simulation model of the heat exchanger was built with the help of commercial software Fluent and some computational fluid dynamics(CFD) work was finished. The difference between CFD results and test results is less than 8%. The results indicate that the temperature of exhaust gas and thermal oil varies gradually in heat exchanger. The velocity of exhaust gas in the heat exchange tube is stable. There is vortex in the head of the heat exchanger and the whole flow in the shell side is a vortex.Some optimism work was also done. The measures, such as increasing the length of the tube, decreasing the diameter of the tube and adding baffles in the shell side, were tried. The results show that they can all improve the performance of the heat exchanger. A plate and fin heat exchanger was designed as a competitor with shell and tube heat exchanger. It has quite better performance than shell and tube heat exchanger. The convective heat transfer coefficient can be 10 times larger than the one in the shell and tube heat exchanger and the heat recover efficiency can be up to 0.8.