Numerical evaluation of air gap effect on double-glazed solar air heater performance

Abstract

Double glazing is one of the effective ways to improve the thermal performance of flat-plate solar air heaters. In this regard, by considering this type of heat exchanger, the free convection flow between two glass covers and the forced convection flow inside the main air channel were analyzed in the present study. The exact implementation of this idea was numerically investigated using COMSOL Multiphysics commercial software. Due to the occurrence of Bénard convective flows in the space between two glasses, the air gap thickness must be optimized so that the positive effect of the presence of the air layer as a suitable thermal insulation is not affected by this phenomenon and does not lead to thermal dissipation. By choosing air gaps of 1 cm to 5 cm and sun radiative heat fluxes of 600, 800, and 1000 W/m2, it was found that the higher rate of heat transfer from the absorber plate into laminar forced convection airflow took place in the air heater with 3 cm air gap. Under the solar irradiation of 1000 W/m2, computations indicated the values of 11.2 K, 9.7 K, and 8.8 K for air temperature increase along the air heaters with the air gap thicknesses of 3 cm, 5 cm, and 1 cm, respectively. Comparisons between the present numerical findings and experimental data reported in the literature revealed an excellent consistency.