Experimental And Numerical Investigations Of Frost Formation On Complex Surfaces

Frosting is widespread in air conditioning, refrigeration and aerospace industries,which can have negative impacts on the heat exchangers under frosting condition. Forexample, in air conditioning systems, frosting on the evaporator surface would reducethe heat transfer due to both the additional thermal resistance on the heat exchanger andthe increasing airside pressure drop. Therefore, investigations of frost formation andprecise simulation are quite essential for the design of heat exchangers under frostingconditions, which would effectively improve their efficiency, like the achievements onthe exchanger fins. This study aims at exploring a new and more reasonable frostformation predicting method and combines numerical simulation with experiment toconduct investigations of frost formation on complex cold surfaces. The main works aresummarized as follows.With improved experimental system, frost formation is investigated on coldsurfaces with fins. Tests conditions are as follows: the air inlet temperature between2and7oC, the air relative humidity60%and80%, the air inlet velocity between0.3and1.1m/s, the cold surface temperature between10and19oC. The experimentalresults show that the frost weight per unit fin area is found to be greater than per unitbase plate area; the frost deposits more on the front and back parts of the fins rather thanin middle of the fins in the flow direction, with more on the bottom of the fin than thetop along the fin height; for five columns of angled fin, the largest amount of frost is onthe fins in the first row, which suggests that the arrangement of sparser front row andcloser back row may improve the heat exchanger performance with severe frostingconditions. A dimensionless empirical correlation of frost weight on finned surface isproposed, with well prediction than less than15%of the data exceeding the error of30%.A new model based on the water vapor concentration potential is proposed topredict the frost growth and densification process, which added the frosting masstransfer rate into frosting criterion, derived referring to the water vapor concentrationdifference and humid air velocity. This model could effectively reveal the mass transferprocess of water vapor from humid air on and within the frost layer and moving boundary between humid air and frost layer, which results in excellent agreement ofmorphology and frost weight with experimental data.Using the validated frost growth predicting model, two-dimensional numericalinvestigation on frost formation on a cold plate with local cooling of the experimentalobject in literature is carried out. The numerical result shows that the frost layer growthprocess is actually the result of the water vapor concentration difference combined withthe velocity field. With the time passing by, the frost average thickness and the frostaverage density both increase, but the increasing rate of the frost average thicknessslows down, while the increasing rate of the frost average density accelerates.Three-dimensional numerical simulation on frost formation on complex coldsurface with single aligned fin is carried out using the proposed frost predicting model.The calculated frost morphology and frost weight are both in good agreement with theexperimental results. Through the three-dimensional simulation, the applicability of thenew frost formation predicting model is verified well. The new model is useful forsimulating the frost formation at different conditions with different structures, whichcan provide theoretical direction for the design of heat exchanger working underfrosting condition.