有机朗肯循环中的工质热源转折温度及其特征

摘要

有机朗肯循环中存在有极值功循环与无极值功循环,即存在一种净功的转折线与所对应的工质热源转折温度,它对工质的选择与循环性能评价具有指标性的作用.基于梯形循环及其理论模型,本文建立工质饱和线形状(饱和液线斜率、饱和气线斜率及其斜率比与临界温度)及物性为自变量的数学关系,并提出和定义线性饱和线工质模型,建立循环性能与工质物性之间的数学关系.分别推导出有机朗肯循环中工质热源转折温度的经验公式,以及梯形循环下线性饱和线工质的热源转折温度理论公式及其修正.研究热源转折温度及其对应的优化工况的特征与一般性规律,得到21种工质热源转折温度的有机朗肯循环数值计算与经验公式之间的偏差小于1.97％.线性饱和线工质模型下,工质的热源转折温度为临界温度和蒸发器窄点温差之和;热源转折温度所对应的最高优化工况(最高优化蒸发温度与最大净功极值)随斜率比单调增加,至干工质时,最高优化工况接近工质的临界点.%The existence of the maximum power cycle and no maximum power cycle in organic Rankine cycle(ORC)means that there is a kind of shift-curve of net power output and its corresponding shift-temperature of heating fluid for the working fluids; which played an index function in the selection of the working fluids and the evaluation of the cycle performance. Based on trapezoidal cycle and its theoretical model,the relation and formulas of the saturated line shape(saturated liquid line slope,saturated gas line slope and its ratio and critical temperature)and thermal properties of the working fluids as variables were established. The model of working fluid with linear saturated line was proposed and defined. The mathematical model and relation between the cycle performance and properties of the working fluids were built. The empirical equations of the shift-temperature of heating fluid for the working fluid in ORC and the theoretical equations of the shift-temperature of heating fluid in working fluid with linear saturated line were derived,respectively. The characteristics and general principles of the shift-temperature of heating fluid and its corresponding optimum condition were investigated. Results showed that the deviation of shift-temperature of heating fluid with 21 working fluids between empirical equations and numerical simulations in ORC was less than 1.97％. The shift-temperature of heating fluid in model of working fluid with linear saturated line is the addition of the critical temperature of working fluid and the pinch point temperature difference of evaporator. The maximum optimal conditions(maximum optimal evaporation temperature and maximum net power output)corresponding to the shift-temperature of heating fluid increases with the ratio of the saturated line slope, which will be close to the critical point of the working fluid when it becomes the dry working fluid.