Evaporator Boiling Heat Transfer Analysis for Engine Exhaust Heat Recovery

In the Rankine cycle, the pressure differential generated by the phase change of the working fluid produces turbine output power, which enables the recovery of waste heat from the internal combustion engine. The heat transfer ability of the evaporator is the key factor that determines the quality of turbine's mechanical work. In this paper, the performance of the evaporator with two-phase zone and preheated zone is studied. After obtaining the thermal characteristics of diesel engine exhaust from the experimental data, the mathematical model of the evaporator is built according to the specific working conditions of ORC and geometrical parameters of the evaporator. Three typical engine operating conditions are used to estimate the heat transfer characteristics of the evaporator. The result shows that, in the evaporator, the heat transfer coefficient of the Rankine working fluid is much greater than the exhaust side of the engine. The heat transfer rate of preheated zone is larger than the two-phase zone, which is almost 67% of the overall heat transfer rate. The heat recovery efficiency reaches 72% at the rated power point. In addition, the heat transfer area in the preheated zone dominates the total heat transfer area. The outlet temperature of the Rankine working fluid in the evaporator is not greatly affected by the torque and speed of the engine, and the mass flow of the R245fa is relatively stable. For the certain pressure at both ends of the turbine, increasing the organic working fluid mass flow rate could obtain more effective power from the turbine.