Experimental and numerical investigation on the performance of a closed-wet cooling tower (CWCT) integrated into the underground exhaust air channel

Abstract

A closed-wet cooling tower (CWCT) integrated into an underground exhaust air channel is proposed in this paper. This design aims to harness the cooling capacity of exhaust air from underground buildings, thereby conserving space and enhancing overall space utilization in underground space. To elucidate the fundamental thermal characteristics of the CWCT, a numerical method was developed using COMSOL Multiphysics software to predict the heat and mass transfer processes within the tower. The reliability of this method was validated through experimental testing and also corroborated by two existing studies in the literature. Compared to conventional closed-wet cooling towers (CWCTs), the cooling performance of the CWCT is not significantly affected by the heat dissipation from nearby rocks. However, it demonstrates greater thermal efficiency than conventional ones. The thermal efficiency can be enhanced by adjusting the operating conditions, such as increasing the flow rate and inlet temperature of the cooling water, as well as the water spray density. However, it is important to note that the cooling efficiency only improves with an increase in water spray density. The sensitive analysis shows that the thermal efficiency and heat exchange capacity are most affected by water spray density, while the cooling efficiency is more affected by the cooling water mass flow rate. Therefore, adjusting the spray density might be the most recommended method for enhancing thermal performance. Under the current calculation conditions, adjusting the spray density can achieve a thermal efficiency of 30.77 % and a cooling efficiency of 48.57 %. Additionally, it can provide approximately 22.7 % of the cooling demand for the case building when the water spray density is set at 2.924 kg/(m²·s). Moreover, connecting multiple heat exchanger units can enhance thermal efficiency while simultaneously reducing cooling efficiency. Under the current calculation conditions, it is advisable to connect two units in series within the air channel. The heat exchange capacity can reach 14.8 kW, with a cooling efficiency of 32.2 %, thereby meeting more than 29 % of the cooling demand for the case building.