Experimental Study On R245fa In Tube Flow Boiling Characteristics And Heat Transfer Enhancement On Sintered Electroplating Composite Surface

Organic Rankine cycle（ORC）is a Rankine cycle in which low boiling point organic working medium is used instead of water to do work.It is a key technology to realize efficient utilization of medium and low temperature heat source.At present,R245fa is a better working medium considering its thermal properties,environmental protection and safety,but its flow boiling characteristics in conventional size channels need to be further studied.In addition,the existing enhanced heat transfer technology in tube still adopts the idea of single-phase heat transfer,ignoring the wetting characteristics of boiling surface.Therefore,in this paper,the flow boiling characteristics of organic working medium R245fa in evaporator,the construction of a new efficient evaporator and its heat transfer performance were studied by means of parameter experimental study.A R245fa visualized rotating experimental platform for flow boiling in tube was built.The saturated flow boiling experiments of R245fa in horizontal flow(θ_fd=0°),vertical upward flow(θ_fd=90°)and vertical downward flow(θ_fd=-90°)were carried out on a smooth stainless steel tube（BTE）and sintered electroplating strengthened stainless steel tube（SEE）with an inner diameter of 10mm and the effective heating length of 820mm.Experimental conditions are as follows:saturation pressure 0.6Mpa,mass flux 200-700kg/（m~2·s）,heat flux 5-75k W/m~2,quality 0-1.The flow patterns in the experiment were stratified flow（S）,intermittent flow（I）,stirred flow（C）,annular flow（A）and dry flow（D）.Forθ_fd=0°,the heat transfer coefficient increases first and then decreases with the increase of quality at low heat flux,and basically decreases with the increase of quality at high heat flux.Forθ_fd=±90°,the heat transfer coefficient increases first and then decreases with the increase of quality.In the three flow directions,the heat transfer coefficient increases with the increase of heat flux,while the mass flux has a weak effect on the heat transfer coefficient.The heat transfer coefficient changes with the flow direction（-90°,0°,90°）in"U"shape,and the heat transfer coefficient ofθ_fd=-90°is better due to buoyancy.Comparative analysis was made between the experimental heat transfer coefficient value and different types of predictive correlations,among which Fang et al.had the best prediction effect.For friction pressure dropΔP_f,Lockhart-Martinelli model is used to verify the two phase friction pressure drop ofθ_fd=0°,which has good consistency.In addition,the frictional pressure drop increases first and then decreases with the increase of quality,and is weakly affected by heat flux,but increases with the increase of mass flux.A coating was prepared on the inner wall of BTE tube to form SEE by means of high-temperature sintering/electroplating.The heat transfer enhancement ratio EF value of SEE ranged from 1.078 to 2.333,and the performance evaluation parameter PEC value ranged from 1.059 to 2.287,indicating a good enhancement effect.For BTE and SEE,the drop of friction pressure is the smallest in horizontal flow direction,and the maximumΔP_falways occurs atθ_fd=90°.Forθ_fd=±90°,the changes of EF and PEC are relatively gentle with the increase of mass flux and heat flux.Forθ_fd=0°,EF and PEC increase with the increase of mass flux,and the increase amplitude of EF and PEC is abrupt.With the increase of heat flux,the abrupt change point of EF and PEC is delayed.According to the flow pattern visualization analysis,the annular flow pattern transformation line of SEE is earlier than that of BTE.In addition,SEE has strong wettability,which can effectively reduce liquid level height,promote bubble consolidation and reduce liquid film thickness to improve boiling heat transfer.