Performance Analysis Of Heat Exchanger For An ORC System And Influence Of Evaporator On The Engine Operating Characteristics

Based on the energy balance of engine, about 30% of the fuel combustion energy is released into the air in the form of waste heat in exhaust gas. Therefore, recovering the waste heat from internal combustion engine is an effective method for reducing pollutant emission, and saving fuel. In this study, organic Rankine cycle(ORC) system was used to recover the waste heat from an engine. As the heat transfer components of the ORC system, the evaporator and the condenser have an important influence on the thermal power conversion performance of the ORC system. Therefore, the flow and heat transfer characteristics of the fluid in the evaporator and the condenser were analyzed. In addition, the influence of the evaporator of the ORC system on engine performance was studied. These researches play an important role in designing the heat exchanger structure and improving the performance of the waste heat recovery system.Based on the fin-and-tube evaporator of the ORC system, the simulation model of shell side and tube side of the fin-and-tube evaporator was established by using CFD method. Then, the flow and heat transfer characteristics of the exhaust at the shell side and the working fluid at the tube side of the evaporator were obtained, and then the performance of the fin-and-tube evaporator of the ORC system was analyzed based on the field synergy principle. Subsequently, the influences of the engine speed, the evaporation pressure and the velocity of working fluid on the gas volume fraction of working fluid, the pressure drop, the average surface heat transfer coefficient and Nusselt number at the tube side were studied. Results show that it benefits evaporation of working fluid that the engine is working at high speed region. By increasing the evaporation pressure, the flow resistance of working fluid in the tube of evaporator can be decreased, but the gas volume fraction of working fluid is decreased. By increasing the velocity of working fluid, the average surface heat transfer coefficient and the average surface Nusselt number can be improved, but the gas volume fraction of working fluid is decreased and the flow resistance of working fluid in the tube of evaporator is increased.Based on the herringbone corrugated plate condenser of the ORC system, a simulation model of the single-channel of herringbone corrugated plate condenser was established by using CFD method. The influences of the velocity of working fluid on the pressure drop and the average surface heat transfer coefficient of corrugated plate were studied. Results show that by increasing the velocity of working fluid, the heat transfer performance can be improved, but the flow resistance of working fluid in the condenser is increased.Furthermore, the influence of the evaporator of the ORC system on engine performance was studied. A simulation model of the engine was built by using GT–Power software under various operating conditions, and the variation tendency of engine power, torque, and brake specific fuel consumption(BSFC) were obtained. According to the theoretical calculation, the variation tendency of the power increase rate and the BSFC decrease rate of engine–ORC combined system were obtained. As a result of the pressure drop caused by the evaporator of the ORC system, the engine power and torque decreases slightly, whereas the BSFC increases slightly with the increase of exhaust back pressure. Compared with the engine itself, the power output of the engine–ORC combined system is increased and the BSFC is decreased.