液氮发动机循环可用能及效率分析

针对液氮发动机开始朗肯循环过程可用能损失较大的问题,提出分析方法,研究液氮发动机开式朗肯循环过程中可用能损失分布；研究并设计液氮二级再热系统,分析液氮发动机开式朗肯循环与内燃机混合加热循环的联合循环.结果表明,排气是可用能损失的主要环节之一,约占系统输入可用能的20%；且初始膨胀温度受环境温度限制是导致排气可用能损失的主要原因.再热系统可以消除环境温度因素对循环效率提高的限制；采用二级再热膨胀系统,选取合理的再热温度和二级膨胀压力可以得到更高的效率.采用液氮发动机开式朗肯循环与内燃机混合加热循环的联合循环,可以将无法直接做功的内燃机常压高温排气的温度转化为液氮发动机进气可用能,等质量工质的联合循环将整个系统的平均效率提高了8.06%.

Theoretical analysis of available energy and efficiency in liquid nitrogen engine cycle

In order to solve the problem of energy loss during the open Rankin cycle of liquid nitrogen(LN2) engine, exergy thermodynamics was used to research the energy loss distribution. A two-stage reheat system was researched and designed, and a combined cycle of open Rankin cycle and internal combustion engine(ICE) was analyzed. The result showed that the exhaust is one of the main parts of available energy loss, which occupies 20% of the input exergy. The main reason for the exhaust exergy loss is that the initial expansion temperature is limited by ambient temperature. The reheat system can eliminate the limitation of ambient temperature on the efficiency improvement. With rationally selected reheat temperature and expansion pressure, a two stage reheat system can reach higher exergy efficiency. Finally, the combined cycle could convert the exhaust exergy of ICE to the intake exergy of LN2 engine, which cannot be converted to work directly. The mean efficiency of the whole system is increased by 8.06% provided equal mass working substance．