Kar oluşumunun değişik çevresel koşullar ve yüzey konfigrasyonları üzrindeki etkileri üzerine deneysel araştırma

Frost formation on finned surfaces is observed on heat exchangers in cooling systems such as refrigerators and air conditioners. Accumulated frost on finned surfaces blocks the air flow in between fins and reduces the effective surface area and it creates an additional resistance for heat transfer. The frost accumulation on finned surfaces increases the energy consumption of a cooling system and needs to be mitigated by the design of the finned surfaces. In this experimental study, the effects of air velocity, relative air humidity, surface temperature, fin length, fin space, surface orientation and surface coating on the heat transfer performance of the finned surface have been investigated. The measurements were conducted in a horizontal wind tunnel open to atmosphere. Air at 18 °C was used over temperature controlled cold finned surfaces positioned inside the test section of the tunnel. The heat transfer rate through the finned surface was measured during the experiment using an in-house built Heat Flux Measurement System (HFMS). Experiments showed that frost accumulation on finned surfaces blocks the air penetration into fin gaps and results in a sharp drop in the heat transfer rate. It has been found that high relative humidity, high incoming air velocity, high fin length increases the heat transfer rate from the finned surface by reducing the negative impact of frosting on the heat transfer rate. Condensed water droplets occur on the surfaces since higher parameter values provides higher heat transfer rates. Condensed water droplets turn to ice with the beginning of the frost formation. High thermal conductivity of ice increases the effective thermal conductivity of frost and provides higher heat transfer rates. Narrow fin space is found to be disadvantageous under the frosting conditions. The small gap between fins is easily blocked by the frost and the effective surface area of the finned surface is heavily reduced. Fin gaps larger than 4 mm are required if the surface is to be used under frosting conditions. The effect of different fin shapes has also been studied. Knife edge shaped fins are found to be better in terms of heat transfer rate as compared to staggered and inline type fins. Regarding the orientation of the finned surface with respect to air flow, it was found that the parallel flow arrangement results in the highest heat flow rate compared to the impinging flow arrangement. This is due to the fact the finned surfaces under parallel flow arrangement is subjected to less frost accumulation. Finally, the effect of the superhydrophobic coating on the heat transfer performance of the finned surface was studied. The coated surfaces do not have a significant amount of water condensation in the vertical orientation and also in the parallel flow arrangement. On the other hand, over the uncoated surface considerable amount of water condensation was found. It could be that the water droplets could not adhere to the coated surface because of the superhydrophobic coating. The absence of the water droplets on the coated surface resulted in an earlier frost growth which resulted in an earlier blockage of the fins and an earlier drop in the heat transfer rate. Finally, the measurements have shown that hydrophobic coatings show a negative impact on the heat transfer rate of the finned surfaces under impinging flow conditions.