Numerical Analysis And Optimization Investigation On Condensation Flow Of Non-azeotropic Medium In Plate-Fin Heat Exchanger

In the air separation and natural gas liquefaction industries,the most commonly used cryogenic heat exchangers were tubular heat exchanger and plate-fin heat exchanger up to now.Plate-fin heat exchanger was used extensively due to its large heat exchange area per unit volume and assembly modularization of cold box.In this paper,the condensation heat transfer and flow performance of non azeotropic mixtures in the channel of plate-fin heat exchanger were analyzed by the numerical simulation method.Upon numerical simulation research,the channel geometry parameters of plate-fin heat exchanger with multi-objective genetic algorithm were optimized.The main work and results were as follows:A wall condensation model of methane-propane mixed medium in laminar and turbulent flow was established for the numerical simulation analyze based on the similarity principle.The flow mechanism of condensation heat transfer of mixed medium in the channel of straight fin.?serrated fin and corrugated fin heat exchangers were numerical simulated by the wall condensation model.The changes of non-condensable medium methane in the aggregation state,heat transfer factor j,resistance factor f and comprehensive performance factor FTEF during the propane condensation process in different finned channels were compared and analyzed when the Re numbers of the three types of finned channels were the same(about 330).Compared to straight fin and corrugated fin,serrated frn heat transfer factor of j increased by 20.65%and 12.25%,respectively.The resistance factor increased by 57.07%and 40.38%.Comprehensive performance factor FTEF increased by 3.79%and 0.25%.The results explained that in the process of condensation heat transfer of gas non azeotropic mixtures fraction(high boiling meditum mole fraction content of 5%),serrated fin did a better comprehensive performance.The main reason for the phenomenon was that with the condensation of the flat fin surface would form a stable growth of methane "gas film".The corrugated fin weakened the "gas film layer" on the windward side by changing the flow direction of the medium.However,non-condensable methane on the leeward side accumulated faster.As the serrated fin staggered,the "air film layer" attached on the fin was destroyed,the staggered discontinuous distribution of the serrated fin led to the periodic flow diversion of the fluid and the erosion of the discontinuous fin front,which inhibited the development of "air film layer" and improved the heat transfer performance significantly.It was found that the serrated fin had a better comprehensive performance in the recovery process of natural gas light hydrocarbon.The influence on the condensation heat transfer and flow performance of mixed media caused by the structural parameters of serrated fins,including fin height,fin spacing,fin length,fin thickness and fin staggered ratio was studied by using the control variable method.The results showed that at the channel Re=600,the heat exchanger with serrated fins had the best performance when the fin height h was 7.1 mm,the fin spacing s was 2.2 mm,the fin length l was 3 mm,the fin thickness t was 0.2 mm,and the fin staggered ratio s0 was 0.4.At the channel Re=2000,the heat exchanger with serrated fins had the best performance when the fin height h was 4.9 mm,the fin spacing s was 1.8 mm,the fin length l was 3 mm,the fin thickness t was 0.35 mm,and the fin staggered ratio s0 was 0.4.The structural parameters of serrated fins were quantified with the help of response surface design.The target parameters j factor,f factor and FTEF factor were functionalized,and an appropriate response surface model was selected to approximate the relationship between variables and functions.On this basis.multi-objective genetic algorithm(MOGA)was adopted to optimize the structure of the serrated fins.The results showed that jfactor was the most sensitive to the change of fin length,f factor was the most sensitive to the change of fin thickness and FTEF factor was the most sensitive to the change of fin staggered ratio.J factor and f factor were affected strongly by the interaction between fin space and fin thickness.FTEF factor was affected strongly by the interaction between fin staggered ratio and fin thickness.Compared with common structures(47JC2003),j factor,f factor and FTEF factor were improved by 25%,23.9%and 16.1%,respectively.