An experimental study on the frost formation over a flat plate: Effect of frosting on heat transfer

Abstract

Frost accumulation on cooled surfaces generates thermal barrier between the surface and air. Apart from the frost layer thickness, the thermal properties of the frost also affect its heat transfer characteristics. In this experimental study, the effect of the frosting on heat transfer is investigated at different cold surface temperatures and at different air velocities in a closed loop wind tunnel. A large number of experiments is performed by altering the parameters of free stream air velocity, relative humidity, and cold plate temperature. Frost layer impact on heat transfer rate is measured through a high precision thin film heat flux sensor for 60 min. Accordingly, heat flow was significantly obstructed due to frosting. The rate of diminishment in the heat flux was not found to be constant over time but shows a decreasing trend. Furthermore, an enhancement on the heat transfer was reported during the early stage of frost formation under certain environmental conditions, majorly due to the increase in effective heat transfer area induced by ice crystals and corresponding increase in latent heat generation. The densification of the frost layer and its effect on the total thermal resistance was discussed. Based on the convective heat and mass transfer balance over the frost layer, frost surface temperature was predicted. Lastly, a correlation for estimating the heat transfer decrease during frosting is proposed via non dimensional numbers.