Experimental Study On Condensate Water Formation Characteristics Of Heat Transfer Channels Under Wet Conditions

Compact fin-and-tube heat exchangers are widely used in air conditioners as a typical heat exchanger.When the surface temperature of the fin-fin heat exchanger fins is lower than the dew point temperature of the wet air,the water vapor in the wet air will decompose on the surface of the fins.As the condensation process continues and the condensate merges,the condensate will gradually grow until the combined force of gravity and airflow drag is greater than the adhesion of the condensate on the surface of the fin,and the condensate begins to move on the surface of the fin.Finally drained off the surface of the fins.The formation,growth and movement behavior of condensed water on the surface of the fins will affect the pressure drop and heat and mass transfer performance of the air-side of the tube-fin heat exchanger.This paper focuses on the formation and kinetic characteristics of condensate in the heat exchange channel and the mechanism by which condensate affects heat transfer and drag.Firstly,based on the visualization technology,the test pieces of the three kinds of fin materials used in this experiment are designed and processed for the better parameters of the compact and high-performance heat exchanger structure.From the two aspects of the formation and movement characteristics of condensate in the heat exchange channel and the influence of condensed water on heat transfer and drag,the experimental scheme required for this paper is developed.And the evaluation indicators and data processing methods of the experiment are determined.Secondly,according to the experimental scheme,a visualized thermal characteristic test bench was built,by analyzing the reliability of the test bench,we can know that: The maximum error of heat exchanger heat exchange is 10.45%,the minimum error is 3.92%;The maximum error of the air side heat transfer coefficient is 15.05%,and the minimum error is 8.52%;The maximum error of the air side drag coefficient is 7.6% with a minimum error of 3.57%.The error is basically controlled within ±15%,which satisfies the reliability of the experiment.Thirdly,through the experimental analysis of a large number of condensed water pictures and experimental data analysis,the precipitation of condensed water on the surface of fins of three kinds of tube-fin heat exchangers was obtained.It was found that the condensed water on the surface of copper fins and aluminum fins appeared as beads.Condensation,the condensed water on the surface of the copper fin with the hydrophilic layer appears as a film-like condensation.The inlet air flow rate increases,and the detached diameters of the condensed water on the surface of the copper fins,the aluminum fins,and the copper fins with the hydrophilic layer are gradually reduced from 0.402 mm,0.411 mm,and 0.443 mm at 0.5m/s to 4.0m/s,respectively.0.18 mm,0.2mm and 0.239 mm,the flushing period is gradually increased from 241 s,250s and 269 s at 0.5m/s to 366 s,461s and 502 s at 4.0m/s.The relative humidity of the inlet increased,and the diameter of the condensed water on the fin surface of the tube-fin heat exchanger was almost unchanged,and the flushing period is gradually reduced from 399 s,442s and 455 s at 40% relative humidity to 49 s,66s and 101 s at 80%.Fourthly,the critical air flow rate of the condensed water on the fin surface of the tube-fin heat exchanger in the form of three fin materials was obtained.It is found that the inlet air flow rate and the relative humidity increase will increase the critical air flow rate.Under the same inlet air temperature and relative humidity conditions,the critical air flow rate of the copper fin with the hydrophilic layer more than the critical air flow rate of the aluminum fin more than the copper fin criticality Air flow rate.The maximum critical air flow rate occurs at the inlet air temperature of 35°C,the inlet relative humidity of 80% with the hydrophilic layer of the copper fin surface,the maximum critical air flow rate is 3.87m/s,that is,the critical air flow rate under given conditions Not more than 4.00m/s.The result can provide a reference for the heat exchanger in the dehumidification condition to prevent the condensate entrainment caused by the air flow.Fifthly,the effects of different inlet air velocity,different inlet relative humidity,different inlet air temperatures,different inlet water temperatures and different fin materials on heat transfer and drag characteristics of tube-fin heat exchangers were analyzed.It is found that increasing the air flow rate,inlet air temperature and tube side inlet water temperature can improve the heat transfer performance of the tube-fin heat exchanger,f increases with the increase of the inlet air temperature,with the inlet air flow rate and the inlet temperature of the tube side.Increase and decrease.For example,if the inlet air temperature is 27°C,the tube side inlet water temperature is 15°C,and the inlet relative humidity is 50%,the copper fins,aluminum fins and the tube-fin heat exchanger with the hydrophilic layer at the inlet air flow rate of 4.0m/s the f is reduced by 29.32%,41.80% and 47.98%,respectively,compared to 0.5m/s.It has been found that the hydrophilic layer on the surface of the fin reduces the pressure drop on the air side of the tube-fin heat exchanger,but reduces the heat transfer performance on the air side.