Experimental And Numerical Investigations Of Frost Formation On Cold Surface

Frost formation is widespread in air conditioning,refrigeration,aerospace industries and coal mane and so on.It is a common phenomenon in wet air environment.During the process of frost growth,the frosting process will show different morphologies under different conditions and different growth stages.It is of guiding significance for the design of the heat exchanger working under the condition of frosting to correctly understand the frosting law and the changing characteristics of frost-layer morphology during frosting process,and accurate simulation.In this paper,the growth process of forst formation under low temperature is studied experimentally,and the distribution of frost layer on cold surface and tube fin heat exchanger are studied by numerical simulation.The main contents are as follows:In low temperature environment,experimental study on frosting characteristics of cold surface under different operating conditions.Experimental equipment of frost formation on cold surface was set up.By changing the temperature of the wet air,the flow rate and the cold surface temperature,analyzed the effects of these factors within frost layer morphology,frost layer height,frost layer quality,ice volume fraction and average thermal conductivity.The results show that:In a certain range,the lower cold surface temperature,the higher wet air temperature and flow velocity,the greater of frost volume in the same time,the faster frost rate.At the same time,the ice volume fraction and the average thermal conductivity increase with the increase of the frost layer height and the quality of the frost layer.According to the observation of the growth process of the frost crystal morphology,it is found that the growth process of a single frost crystal is similar to the dynamic growth process of natural trees.On the basis of this,a new growth model of tree shaped frost layer was proposed.Numerical simulation of frosting process on cold surface.Combined with the experimental data of this paper,the simulation method of Euler multiphase flow coupled with a mass transfer model.The frosting process of cold surface is simulated,and calculated the average height of frost layer and quality of frost layer,then compared with the experimental data.Discussed the applicability and accuracy of the model with the simulation.The results show that:In the comparison of simulated and experimental data,in terms of the average height of frost layer,at the temperature of the wet air at T_in=5?,the difference between the experimental and simulated data is 15%.When the temperature of wet air is T_in=-5?,the difference between the simulation and the experimental data is larger gap.In the quality of frost layer,the simulated data of the two conditions are in good agreement with the experiment.Study on frost simulation of tube-fin heat exchanger.The results show that the fin near the base tube has lower temperature,greater frost density and greater frost formation.But away from the tube,the temperature of the fin is higher and the density of the frost layer decreases.At the same time,the upwind surface of the base tube begins to frost first,and with the increasing of frosting amount,the frosting layer gradually begins to form on the leeward surface of the base tube.After the heat exchanger frosts,frosting between fin and base tube will hinder the flow of wet air in the heat exchanger,and the pressure difference will increase before and after heat exchanger,and the air flow will weaken.