| Our country has a variety of low and medium grade energy with huge amount,organic Rankine cycle(ORC),which is characterized by simple facilities and low operation and maintenance cost,has made rapid development in recent years as the focus of research in technology of exploration and evaluation of low and medium grade energy.In terms of working fluids of ORC,the “temperature glide” of zeotropic mixtures in two-phase region can achieve the better temperature matching between the circulating working fluid and cold or heat source in the heat exchanger,thereby reducing the irreversible loss in the heat exchanger caused by heat transfer temperature difference.In the ORC system using zeotropic mixtures,the ratio of mixed working fluid is an important parameter that affects the thermodynamic performance of ORC system.According to the theory of two-phase flow,the velocity of the gas phase is not the same as that of the liquid phase,thus the circulating ratio that is the composition ratio of circulating working fluid in thermodynamic analysis is deviated from the local composition ratio in the two-phase region.Therefore,there is a difference between circulating concentration and charge concentration in the system,called “composition shift”.In order to solve this problem,this paper aims to determine the charge concentration in the system to get a desired concentration(mass fraction)and proposes three methods,namely segmentation,integration and empirical fitting,to calculate the average density and average concentration in the two-phase region.The main contents of this paper are as follows:(1)Based on the Smith two-phase flow model and the thermodynamic equilibrium assumption,a segmentation calculation model of the average density and average concentration of the working fluid in the two-phase region was established.The effects of the pressure and circulating concentration on composition shift in two-phase region were analyzed,the results show that the composition shift decreases with the increase of the pressure,with the increase of the circulating concentration,the composition shift increases first and then decreases,and the circulating concentration is close to 0.5 when the maximum value is reached.(2)The average density and the average concentration in the two-phase region were deduced by the integral method.According to the Smith two-phase flow model,the void fraction was fitted as a rational or power function of the vapor quality in a certain scale of the ratio of liquid phase density to gas phase density.Based on thermodynamic equilibrium,the density and concentration of gas and liquid in different vapor qualities were obtained and they were fitted as functions of the vapor quality.(3)According to the results of segmentation calculation in two-phase region,the fitting relation of the average density and composition shift in two-phase region with the reduced pressure and the circulating concentration were obtained.(4)The ORC system for recovering the waste heat of 150 ? flue gas using R600 a / R601 a and R13I1 / R601 a as working fluids was chosen as the research object.The thermal efficiency was chosen as the optimal objection function,given the optimal circulating concentration obtained by optimization,the optimal charge concentration of the system was obtained by the method of segmentation,integration and empirical fitting and the equation for calculating the charge concentration in ORC system was proposed. |
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