| Heat exchangers are widely used in various household electrical products such as air conditioners and refrigerators,and the heat transfer resistance of heat exchangers is mainly on the air side.In order to improve the performance of heat exchangers,it is necessary to optimize the structure of fin on the air side.The heat exchangers used in the air conditioners usually operate under complicated conditions,including the dehumidifying condition,the dust deposition condition and the frosting-defrosting condition.When the heat exchanger is operating under the dehumidifying condition,the temperature of the fin surface is lower than the dew point temperature of the moist air,and the condensing droplets form on the fin surface.The droplets on the adjacent fins contact to each other and form the water bridge.When the heat exchanger is working under the dust deposition condition,the water bridges between fins block the flow channel of dusty air,and some particles are captured by the water bridges,leading to the performance deterioration of the heat exchanger.When the heat exchanger is operating under the frosting-defrosting condition,the defrosting water flows away from the fin surface forced by gravity,and the flow velocity directly affects the defrosting time of the air conditioner.Besides,the deposited particles on the fin surface can be removed by the condensate and defrosting water,leading to the self-clean process of the heat exchanger.In order to know the influence of operating conditions on the performance of heat exchangers,the simulation and experimental method are carried out in this thesis,including the experimental research and mathematical model of water bridge shape between fins under dehumidifying condition,the numerical model and experimental validation of dust particles captured by water bridge between fins,the experimental research of dust removal by condensate on fin surface,the experimental research of dust removal by frosting-defrosting process on fin surface,and the simulation and experimental validation of water drainage performance of microchannel heat exchangers.Based on the researches in this thesis,the results are shown as below:1)Based on the experimental observation results,the contact line and contact angle of the water bridge between fins are established,and the three-dimensional shape prediction model and volume calculation method of the water bridge are obtained.The visualized experimental apparatus is set up to observe the shape of water bridge from different angles.The formula for describing the three-phase contact line of the water bridge is developed to fit the experimental correlation of the aspect ratio of the elliptic contact line of the water bridge.The contact angle of the water bridge between the vertical fin samples at various observation angle is obtained,and the correlation of the contact angle of the water bridge is established.Based on the experimental correlation of water bridge contact line and contact angle,the model of water bridge shape between vertical fins are established.The meniscus curve of water bridge is developed according to the force equilibrium between fins,and the four-degree polynomial is used to describe the meniscus curve of the water bridge.The equation of contact line of the water bridge is established by an ellipse equation and the volume calculation method of water bridge is developed.The experimental validation setup is designed and built,and the predicted shapes of water bridge agree well with the experimental images for both front view and side view.The calculation results of the water bridge volume agree with 89% of the experimental data within the deviation of ±15%,and the mean deviation is 9.4%.2)The numerical model for the capture of dust particles by water bridge between fins is established and validated by experiments.The deformation model of water bridge interface between vertical fins under the action of air drag force is established,and the deformed water bridge shape is determined.The simulation method of the motion trail of dust particles under the action of inertia force is developed,and the mass of dust particles captured by the water bridge surface are calculated.The experimental validation apparatus for capturing the dust particles by water bridge is built.The validation results show that the numerical model numerical model for the capture of dust particles by water bridge between fins can predict the deformation of water bridge under the action of airflow,and the simulation results of captured particle mass agree with 92% of the experimental data within the deviation range of ± 20%,and the average error is 14.8%.The influence of various particle diameter and air velocity on the captured particle mass and the distribution of captured particles on the water bridge surface are both analyzed.3)The dust removal process by condensate and defrosting water on fin surfaces with various surface wettability is experimentally studied,and the influence of the surface wettability on the dust removal rate and the remnant dust weight is obtained.The fin samples with various surface wettability are prepared to study the effect of surface wettability on dust removal characteristics.The dust deposition apparatus is designed to realize the uniform deposition of dust layer on the samples,and the morphology and thickness of pre-dust layer on experimental samples are compared.The visualization experimental setup of dust removal with semiconductor cooling device is established to realize the experimental condition of water condensation and defrosting,and the visualization photos of the dust removal process are obtained.The dust removal characteristics,including the dust removal rate and the remnant dust mass of fins with various surface wettability during dehumidifying and defrosting conditions are compared.The results show that the condensing water and defrosting water on the hydrophobic surface can almost clean the whole dust layer on samples,and the dust removal efficiency of hydrophobic surface is more than 90%.4)The numerical simulation method for predicting the water drainage process of microchannel heat exchanger is developed and validated by experiment,and the influence of fin structures and heat exchanger position on the discharge rate and residual mass of condensate is obtained.The force analysis of the water droplet on the microchannel heat exchanger is produced,and the droplet movement is established and applied by numerical simulation.The experimental setup is built for validating the water drainage process,including the water drainage velocity and the residual water mass.The influence of the louver angle of fins,the louver number of fins and the installation angle of the microchannel heat exchanger on the water drainage performance are analyzed by the simulation method.The simulation results of water drainage velocity and the residual water mass are compared to the experimental results with the experimental validation deviation less than 15%. |
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