| The parabolic trough solar collectors is considered to be one of the most reliable and sustainable technologies to utilize solar energy.Organic working fluids in the absorber tube vapor generated directly.Direct vapor generation(DVG)in parabolic trough solar collectors is advantageous with a higher collector efficiency,a lower investment cost and a better performance of power cycle.However,the harm done by non-uniform heat flux distribution would be more serious in this kind of system because of the existence of gas-liquid two-phase flow.At present,there are few studies on two-phase flow and boiling heat transfer during phase transition of organic working fluids in circular tubes under non-uniform heat flux.This paper mainly through theoretical analysis and experimental research methods.A quantitative index of the uniformity of heat flow distribution outside the parabolic trough collector is proposed theoretically.The flow pattern transformation and heat transfer characteristics of R245fa in circular tubes under non-uniform heat flux were studied experimentally.The main work and conclusions are as follows:1.Based on the characteristic of non-uniform heat flow distribution outside the PTC.A new quantitative index of evaluation evenness is proposed.In this study,"uniformity index"was decomposed into two sub-indexes,"uniformity index of value"and"uniformity index of spatial position".The uniformity of heat flow distribution in the section of collecting tube is described quantitatively.The rationality and validity of the new index are verified by case analysis.The new index is strictly bounded and dimensionless,which plays an important role in the optimal design of collector.2.The typical flow pattern distribution of R245fa in circular tube under non-uniform heat flow was observed by using high speed camera recording and post-processing technology.Based on the experimental study,the main conclusions are as follows:(1)Under the condition of non-uniform heat flux,four typical flow patterns were observed:bubble flow,bubble-plug flow,stratified-wave flow,annular flow,and typical secondary flow.(2)Under the same heating condition,the comparison between non-uniform heating and non-uniform two-phase flow shows that the initial dryness of non-uniform flow I/A transformation curve is low.(3)By comparison with the existing flow diagram,it is found that the conversion curve of Ong and Thome(2011)is in the best agreement with the experiment.3.The experimental conditions were set as follows:heat flux:9.95,22.4,35.9k W/m~2;mass velocity:192.9,275.6,385.8 kg/(m~2·s);saturation temperature:40,60℃.Based on the analysis of experimental data,firstly,the influence of variation of operating parameters(vapor quality,mass velocity,heat flux density and saturation temperature)on boiling heat transfer coefficient and section temperature uniformity was studied.Secondly,the results calculated by the existing typical two-phase heat transfer correlation formula under the same working condition are compared with the heat transfer coefficient obtained in the experiment.Finally,a new heat transfer correlation is developed by fitting experimental data.The results show that:(1)For the pure working fluids R245fa,the heat transfer coefficient first increases and then decreases with the increase of vapor quality.(2)The heat transfer coefficient of R245fa increases with the saturation temperature of mass velocity and heat flux.(3)By comparing the experimental data with the calculated results of the existing correlation formula for the heat transfer coefficient of two-phase boiling,it is found that the prediction results of the existing correlation formula for the experimental data are generally small.(4)The new correlation was fitted according to the calculated results of experimental data.The prediction deviation of the newly fitted heat transfer correlation formula for 94%of the data was within±20%,the absolute average deviation was 10.98%,and the average prediction deviation was-4.75%. |
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