This paper investigates frost formation on a flat horizontal surface, with humid air flowing over the surface and a cold liquid desiccant flowing below the surface. Two different surfaces, a semipermeable membrane and an impermeable plate, are tested. The condensation/frosting limit, that is, the lowest air humidity ratio, Wair, at a constant liquid temperature, Tliq, or the highest Tliq at a constant Wair that leads to condensation/frosting, is determined for each surface. The main aim of this study is to find the effect of moisture transfer through the semipermeable membrane on the condensation/frosting limit. It is found that the semipermeable membrane has a lower condensation/frosting limit, due to the moisture transfer through the semipermeable membrane, which dehumidifies the air flow. For a given Wair, the surface temperature can be approximately 5 to 8 °C lower when using a semipermeable membrane, compared to an impermeable plate, before condensation/frosting occurs. Furthermore, it is shown that at some operating conditions, frost appears on the semipermeable membrane only at the air flow entrance of the test section, while the impermeable plate was fully covered with frost at the same operating conditions. Moreover, it is shown that increasing the moisture transfer rate through the semipermeable membrane decreases the frosting limit and delays frost formation.
The mechanisms of frost formation on a semipermeable membrane
