Analysis of the influence of outdoor surface heat flux on the inlet water and the exhaust air temperature of the wetting pad of a direct evaporative cooling system

Abstract

The study investigates the interconnectivity between the inlet water temperature, wind flow rate, and storage water heat flux with the performance of biomass wetting pads in direct evaporative cooling under the external ambient condition of Sub-Saharan Africa. Thus, a standalone direct, evaporative cooling system with an upper water storage tank exposed to wind flow was locally developed and evaluated with jute, palm fruit mesocarp and wood charcoal as biomass cooling pad at three air velocities and constant pad thickness and three different air flow rates. The results indicated that increasing the heat flux around the water tank and decreasing the relative humidity of the inlet air through the wetting pad will lower both the inlet water and pad exhaust temperatures. The water demand was higher in palm fruit mesocarp fibre at airflow rates of 3 m/s, while at 4 and 4.5 m/s, it was higher in wood charcoal, and the value ranged from 9.64 × 10 −4 to 1.46 × 10 −3 kg/s. Except for jute fibre at 4 m/s, higher humidity difference or low cold room temperature did not translate to higher evaporative cooling effectiveness or efficiency. However, the lower inlet water temperature significantly affected the evaporative effectiveness. This shows the possibility of free moisture transfer into the cold room from the pad materials at increased air flow rates that helped boost the exhaust air's humidity. The average evaporative efficiency for the three pads ranged from 56.4 % to 80.96 %. The values for the enlargement coefficient ranged from 5 to 6.82, while the temperature thermal stress ranged from 24.37 to 28.66 °C.