Figure 5) compares the airflow velocity ( and mass flow rate) reduction and frost mass accumulation on the channel for different cases. The reference time ( ) is when, in the reference case, the velocity drops to 23% of its initial value ( ). As shown in Figure 5), the black line (case No.3) reports less velocity reduction and frost mass (in one section) than the blue line (case Ref.). It means that for the reference case frost grows faster and the airflow blockage is significantly more for a given time. It could be due to the less mass flow rate through a single channel that carries less water vapour in case 3. However, for the total frost mass, accumulated on the surface, the case 3 is slightly lower than case 1 and higher than case 2.
Furthermore, Figure 6) displays a higher pressure drop, more than 2 times for the case 3 comparing to the reference case. It means that more power for the fan is needed to provide the same velocity at the inlet. Another interesting outcome of the currentstudy emerges from the simulation of case 2. As Figure 5) shows, the frost growth rate for higherFPI (case 2) is lower than the reference case with the same face area while theaccumulated frost mass on the surface is slightly lower. In other words, as Figure 5) displays, by decreasing the fin´s bottomwidth (X2) and increasing the fin´s height (H) the frosting time issignificantly prolonged. In accordance with the present results, a previousexperimental study for louvered fin evaporator 18 has demonstratedthat in some circumstances the increase in height could result in longer frostingtime. In this way, case 2 would be a better choice with higher COP when theevaporator faces the undesirable frosting phenomenon.
In the present work, a numerical study wasperformed to predict frost growth on a three-dimensional plate-fin evaporator.The numerical approach proposed by 13 was used while thefrosting criteria were modified by separating the velocity and super-saturationconditions. First, the model was validated by comparing the numerical resultswith experimental data obtained under various operating conditions for thefrost thickness and density.
These results agreed well with availableexperimental data. Then, the Eulerian-granular multiphase model was used tosimulate the frost formation growth on a three-dimensional plate-fin evaporatorand the numerical results were in keeping with previous experimentalobservations. Finally, a parametric analysis was carried out and different geometrieswere investigated. One interesting outcome emerging from this study is that thefrosting rate is not just affected by the FPI parameter and the distance betweenrefrigerant tubes can play an important role in the frosting time.