Effects of Air Velocity, Air Gap Thickness and Configuration on Heat Transfer of a Wearable Convective Cooling System

Abstract

The effects of air velocity, air-gap thickness and configuration on heat transfer of a wearable convective cooling system were modeled using a 2-D Computational Fluid Dynamics (CFD) simulation. Three different configurations, one with eight 1 cm fans (8 × 1 model); one with four 2 cm fans (4 × 2 model); and one with four 1 cm fans (4 × 1 model) were studied at different inlet air velocities of 0.25 m/s, 0.5 m/s, 0.75 m/s and 1.0 m/s and with air gaps of 3 mm, 7 mm and 11 mm. The simulations showed that the convective and evaporative heat transfer coefficients varied with a power function of inlet air velocity, but the exponent was larger than reported in the literature. At lower air velocities, the heat transfer coefficients increased as the gap between the skin and the undergarment widened. At higher inlet air velocities, 3 mm and 11 mm air gaps showed the highest heat transfer values. The ratios of the heat transfer coefficients of the 8 × 1 and 4 × 2 models to the 4 × 1 models were both less than 2 indicating that doubling the inlet airflow does not double the heat transfer capability. The ratios of convective to evaporative heat transfer coefficients were around 8.63 K/kPa (SD 0.25 K/kPa), and were independent on the air velocity and configurations of the cooling system, suggesting that convective and evaporative heat transfer are fully coupled. This ratio also shows that the vapor permeability index of our system is around 0.52. Using slightly larger fans should be more effective in enhancing heat transfer than using more fans.