Experimental And Simulation Study On Flow Boiling Characteristics Of R245fa In Horizontal Tubes With Non-uniform Heat Flux Distribution

Solar energy is a renewable energy,which is environmentally friendly and sustainable.Therefore,it is one of the important options for people to deal with energy shortages and climate change.Direct vapor generation(DVG)in parabolic trough solar collectors,featured by solar radiation as heat source and vapor generated directly from organic working fluids in the absorber tube,is advantageous with a better performance of power cycle,a smaller system scale,lower operation temperature and pressure and a higher collector efficiency.As a result,it is applicable to distributed energy systems.At present,there are few related researches and its technical feasibility needs to be verified.Therefore,this paper established a R245fa flow boiling experimental apparatus under non-uniform heat flux and studied the boiling heat transfer performance of R245fa,which can provide reference for the application of direct vapor generation system,and has certain engineering application value.In this paper,experimental and numerical simulation methods are used to study the boiling characteristics of R245fa in a horizontal tube under non-uniform heat flux.The main work and conclusions are as follows:1.The R245fa flow boiling heat transfer experiment apparatus was designed and established.It mainly consists of preheating section,experimental testing section,condensing section and working fluid pump.The experimental section has an inner diameter of 29 mm and experimental conditions of mass flow rate of 80 to 160 kg/(m~2·s),heat flux density of 8 to 14 kW/m~2,saturation temperature of 55 to 75?and quality of 0 to 0.35.2.Through the analysis of experimental data,effects of parameters such as quality,mass flow rate,heat flux density and temperature on the boiling heat transfer coefficient were studied.The experimental results were compared with those of four commonly used heat exchange correlations and a new experiment correlation was developed.The results show:1)Under the conditions of circumferentially non-uniform heat flux and low quality,the heat transfer coefficient of R245fa in the stratified flow section decreases with the increase of quality;2)The boiling heat transfer is mainly affected by the formation and motion of bubble,and the change of the mass flow rate has almost no influence on the heat transfer coefficient of R245fa;3)Higher heat flux density easily forms more nucleation sites and nucleate boiling is significantly enhanced;4)As the saturation temperature increases,the superheat of the wall required to generate bubbles decreases.Boiling heat transfer coefficient increases with the increase of saturation temperature;5)Results calculated by existing correlations are generally small compared with the experimental data;6)The experimental data have been used to develop a new correlation of heat transfer coefficient.The new correlation has a predicted deviation of±20%for 95.4%of data sets and the mean and absolute mean deviation is-4.56%and 8.84%for all experimental data respectively.3.According to experimental conditions,numerical simulation software Fluent was used to simulate the flow boiling heat transfer of R245fa in horizontal tubes under non-uniform heat flux.It is found that the bubble motion destroys the secondary flow generated by the non-uniform heat flux inside the fluid,which enhances the boiling heat transfer.The Lee model which is one kind of multi-phase flow model does not consider the enhancement effect of non-uniform heat flow on bubble generation.Therefore,it is necessary to improve existing source terms,especially in the region of low quality.