Page 8 - Defrosting for Air Source Heat Pump

4 Defrosting for Air Source Heat Pump

After reaching a critical time, t c , all coalescent droplets turn into ice particles or crystals.
The length of this period is determined by the ambient and surface conditions, including
the surrounding air temperature/relative humidity, the natural or forced convection
affected by air velocity, the cold surface temperature, and its roughness. The second
period, the solidified liquid tip-growth period, starts from the critical time to a transi-
tional time, t t , when a relatively uniform porous layer of frost is formed. At this period,
nonuniform tip growth occurs on individual droplets.
The entire process of frost branch formation is referred to as the third period, the
frost layer growth period, when frost branches would form at the top of ice crystals.
These frost branches grow in three dimensions and connect to the neighboring frost
branches, thus forming a flat frost layer. During the fourth period, the frost layer full
growth period, the interface between the frost layer and the ambient air is at a tem-
perature of 0°C, owing to the thermal resistance of the porous frost layer and the upper
part of the frost layer being melted into water. The melted water penetrates the frost
layer and freezes, thus forming a new, considerably thicker ice layer. When ice layers
form inside the frost layer, the density of the frost layer increases, and the conduction
thermal resistance decreases. As a result, the surface temperature of the frost layer is
lower than 0°C, causing the frost layer to grow on the surface again. After ice crystals
form on the surface, the water vapor on the surface that surrounds the humid air
becomes frost, which branches on the top of the ice crystals and no further mass trans-
fer take places between the surface and the humid air. In fact, all previously mentioned
ambient and surface conditions may affect a frost formation process. Such has been
extensively studied on the surfaces of plates and heat exchangers. In addition, frosting
characteristics on hydrophobic and superhydrophobic surfaces are different from
those on ordinary surfaces. The changes in surface properties affect the frosting
behaviors at the early stage, from a dry surface to the formation of ice crystals.
The surface properties are hardly possible to influence the growth of the frost layer,
and the change in the surface properties is considered only prior to the crystal growth
period or the droplet condensation period.
Frosting has caused severe negative effects in various application fields. Therefore,
numerous theoretical and experimental studies have been conducted, aiming at the
mitigation and control of frost formation on various cold surfaces. For example, in
the field of aerospace technology, the phenomenon of frost deposition is harmful.
For the airplanes that fly at night, frost deposits may occur on their wings owing to
low air temperature. The frost deposition increases the surface friction drag during
take off or navigation, thus affecting safety [4]. In launching rockets, there are similar
frosting problems. Similar to that on the surface of airplane wings, frost may deposit
on the rocket surface, thus causing satellites to fail to enter a correct synchronous
orbit [5]. For liquid-fueled rockets, frost may also deposit on the surface of cryogenic
oxidizer tanks of very low temperature, as rockets fly through the atmosphere. The
frost deposition may change both the shape and the weight of the oxidizer tanks, thus
influencing the aviation performance of the rocket. In addition, in the field of LNG
production and application, frost may deposit on the surfaces of LNG evaporators,
and thus affect the so-called passive-evaporation technology that uses air as a heat
source [6]. Harmful frosting phenomena mainly occur in various industry processes