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 current
study emerges from the simulation of case 2. As Figure 5) shows, the frost growth rate for higher
FPI (case 2) is lower than the reference case with the same face area while the
accumulated frost mass on the surface is slightly lower. In other words, as Figure 5) displays, by decreasing the fin´s bottom
width (X2) and increasing the fin´s height (H) the frosting time is
significantly prolonged. In accordance with the present results, a previous
experimental study for louvered fin evaporator 18 has demonstrated
that in some circumstances the increase in height could result in longer frosting
time. In this way, case 2 would be a better choice with higher COP when the
evaporator faces the undesirable frosting phenomenon.In the present work, a numerical study was
performed to predict frost growth on a three-dimensional plate-fin evaporator.
The numerical approach proposed by 13 was used while the
frosting criteria were modified by separating the velocity and super-saturation
conditions. First, the model was validated by comparing the numerical results
with experimental data obtained under various operating conditions for the
frost thickness and density. These results agreed well with available
experimental data. Then, the Eulerian-granular multiphase model was used to
simulate the frost formation growth on a three-dimensional plate-fin evaporator
and the numerical results were in keeping with previous experimental
observations. Finally, a parametric analysis was carried out and different geometries
were investigated. One interesting outcome emerging from this study is that the
frosting rate is not just affected by the FPI parameter and the distance between
refrigerant tubes can play an important role in the frosting time.