Theoretical analysis and experimental investigation of a combined compact evaporative cooler

Abstract

Evaporative cooling utilizes the fact that water can absorb a relatively large amount of heat during the evaporation process. When this evaporation process takes place in ambient air, the ambient air temperature can be dropped significantly resulting in the cooling effect. In the current study, theoretical analysis and experimental investigation of an innovative compact combined evaporative cooler (CCEC) have been conducted. The innovative CCEC mainly consists of an innovative compact heat exchanger, a blower, a water distribution system, and a water reservoir. The compact heat exchanger has integrated two sets of orthogonally orientated air-air channels, which are made of aluminum sheet with a thickness of 0.3 mm and a fin gap of 3 mm. Aluminum surfaces are coated with nanolayer coatings using a vapor deposition process, which can form thin liquid film for high evaporating heat transfer coefficient and meniscus of liquid-vapor interface for low local saturation pressure. This aluminum surface can significantly modify the wetting characteristics and reduce the local saturation pressure. The contact angle can be reduced from 50 degrees to almost zero degrees. With this wetting condition, water can be readily spread, allowing a thin water film to be easily formed on the surface of the compact heat exchanger to produce thin-film evaporation resulting in an extra high evaporating heat transfer coefficient. More importantly, when the curved liquid-vapor interface exists, the liquid saturation pressure can be reduced which can be predicted by the Kelvin equation, i.e., pk=p[infinity]e^-2[sigma][nu]1/rRT resulting in a decrease of evaporating temperature. In order to predict the heat transfer performance occurring in this innovative CCEC, a mathematical model is developed. The model can be used to predict the effects of relative humidity, dry bulb temperature, channel spacing, air flow rate, thin-film evaporation, mixing process, and saturation pressure on the production air temperature and relative humidity. The prediction is compared with experimental data and shows that the prediction agrees with the experimental data. In addition, the investigated cooler does not use an evaporative pad as media, so no replacement of pad or maintenance is needed, furthermore, the investigated cooler eliminates corrosion, mineral deposits, mold, and bacteria that could potentially accumulate on the media pads, particularly during the off-season or in the presence of dust.