Heat Transfer Mechanism And Experimental Study Of Direct Contact Heat Exchange Between R245fa And Thermal Oi

The direct contact heat exchangers have a good application prospect because of its simple structure and small heat transfer temperature difference.However,the commercial application of the direct contact heat exchangers faces great challenges due to the unknown mechanism heat transfer process between immiscible fluids,the difficult measure of heat transfer area,and the high retention rate of dispersed phase.In order to solve the current problems of the direct contact heat exchanger,it is of practical significance to establish a visual experimental system and build accurate heat transfer models.In view of this,this article has studied R245 fa and Thermol under different operating conditions.The growth model of dispersed phase bubbles was established and validated.The empirical correlation for direct contact boiling heat transfer was modified,and its effectiveness was experimentally verified.The evolution of dispersed phase bubble growth and convective heat transfer coefficient under different operating conditions was analyzed,revealing the mechanism of phase change heat transfer in immiscible fluids.Based on the two-phase flow drift flow model,an empirical correlation of void fraction is proposed in direct contact heat transfer,it provides a new idea for improving the efficiency of waste heat utilization and optimizing the design of heat exchangers.First,the direct contact heat exchange visual platform is built,the bubble growth process is recorded by high-speed camera,the bubble characteristic attributes are obtained by image processing technology,the bubble growth empirical correlation is established and corrected based on the bubble heat balance formula,and the influence of different continuous phase temperature,flow ratio and liquid level height on bubble growth was analyzed.It is found that the maximum error between the corrected bubble diameter and the actual bubble diameter is 7.2%,and the bubble diameter is positively correlated with the flow rate ratio,continuous phase temperature and liquid level height.The establishment of bubble growth empirical correlation solves the determination of heat transfer area between two phases,and provides effective evidence and theoretical support for the determination of convective heat transfer coefficient.Secondly,in order to accurately describe the heat transfer performance of the direct contact heat exchanger and describe the heat transfer process,the empirical correlation of the local convective heat transfer coefficient between R245 fa and Therminol?66 is derived in the heat transfer process,and the derivation and correction of empirical correlations for local convective heat transfer coefficients are introduced.The accuracy of empirical correlations of local convective heat transfer coefficients between R245 fa and Therminol?66 was verified.The experimental data shows that the maximum error between the theoretical heat exchange and the actual heat exchange under different working conditions is 9.4%,so the error of empirical correlation of local convective heat transfer coefficient is within 10%.It is found that the local convective heat transfer coefficient is negatively correlated with the liquid level height,continuous phase temperature and flow rate ratio.Finally,based on the drift flow model of gas-liquid two-phase flow,an empirical correlation of void fraction was derived and modified based on experimental conditions.The accuracy of the correlation was verified,and the effects of continuous phase temperature and flow rate ratio on void fraction were analyzed.The relationship between void fraction and local convective heat transfer coefficient was investigated.The results show that the maximum error between the corrected theoretical section void fraction and the actual section void fraction is 9.8%.The void fraction is positively correlated with the height,continuous phase temperature and flow rate ratio,and the void fraction is negatively correlated with the local convective heat transfer coefficient.The void fraction is introduced to evaluate the heat transfer rate,which provides a new idea for the optimization of the direct contact heat exchanger.