A simplified numerical model for evaporative cooling by water spray over roof surfaces

Evaporative cooling is a well-known passive technique adopted especially in hot and dry climatic regions. This paper presents a simplified numerical model to assess the impact of water spray over roof surfaces on the building thermal load. First, we performed a series of long-term outdoor measurements of heat fluxes from an asphalt pavement and roof top of a laboratory building waterproofed by asphalt sheet. Based on the measured data, a correlation is proposed to estimate the evaporation rate of the concerned surfaces after rain showers. A simplified numerical model is then developed using this correlation based on the unsteady heat transfer to analyze the effect of precipitation and artificial water spray on the roof of the laboratory building. Simulation is performed with the help MATLAB using the hourly weather data from Fukuoka, Japan, and the simulated results are found to be in correspondence with the measured data. Finally, the cooling effect achieved by varying the amount and rate of water spray over the roof is compared to find out the optimum spraying conditions. The novelty of this study is that, in this model, the evaporation rate can be expressed in terms of the evaporation ratio, which is defined as a function of the water content of the surface layer of the roof. The water content of the surface layer can be obtained by the water balance equation, which is coupled dynamically with conductive equations. This procedure can drastically reduce the calculation load compared to the conventionally applied simultaneous hygrothermal transfer equation.