Numerical investigation of the impact of coating thickness and air gaps on cooling adsorption performance

Adsorption cooling is a promising alternative to conventional refrigeration systems, as it utilizes environmentally friendly refrigerants and can be driven by solar energy or industrial waste heat. However, its performance is strongly governed by heat and mass transfer limitations within the adsorbent bed, which are closely related to bed configuration and packing quality. This study numerically investigates two key yet often overlooked design parameters: the coating thickness of the adsorbent layer applied directly onto the cooling surface and the presence of air gaps in packed adsorber beds. A computational fluid dynamics (CFD) model was developed in ANSYS Fluent, incorporating a user-defined function (UDF) to describe adsorption kinetics for silica gel/water and MOF 801/water working pairs, and was validated against published experimental data. The results show that coating thicknesses between 1 and 8 mm achieve a maximum uptake of 0.23 g_v g_s⁻¹ within approximately 500 s, whereas a thickness of 12 mm yields a slightly lower uptake of 0.22 g_v g_s⁻¹ with a significantly longer cycle time of around 1500 s. A threshold coating thickness of 16 mm is identified, beyond which the uptake decreases sharply to 0.12 g_v g_s⁻¹ and further to 0.05 g_v g_s⁻¹ at 20 mm due to insufficient heat removal. The presence of air gaps is found to significantly deteriorate adsorption performance, particularly for a 12 mm packed bed with a 3-mm air gap, which exhibits the poorest uptake behavior among all configurations. Increasing the cooling water mass flow rate from 0.0001 to 0.01 kg s⁻¹partially compensates for this degradation. These findings provide practical design guidelines for optimizing both coated and packed adsorber beds.