Abstract
Heat pump systems utilizing ambient air as a heat source are widely used; however, in the case of using conventional tube banks as an evaporator, moisture in humid air freezes and forms the frost layer on tube surface, which produces a decrease in heat transfer, an increase in pressure loss, and finally choking of the evaporators. High performance heat exchanger can be expected by applying an impinging jet having high heat-transfer characteristics for the conventional tube-bank heat exchanger. In order to utilizing the impinging-jet tube-bank heat exchanger in an environment where the above-mentioned frost formation is unavoidable, the heat-transfer and frosting characteristics of a cooled tube in an impinging jet must be clarified. In the present report, a comparison is made between heat transfer of a tube in an uniform flow and that in an impinging jet, and then the possibility of improving the performance of tube-bank heat exchanger by utilizing the impinging jet is shown. After that the local heat-transfer characteristics of a cooled tube in an air impinging jet and the influence of frosting on them are analyzed in an intensive experiment. The results obtained in the present report are as follows; (1) The heat transfer of a heated tube in an impinging jet is much higher than that in a uniform flow when they are compared at the same superficial velocity. (2) The transient frost-layer thickness, local heat-transfer coefficient, and the circumferential distribution of overall local Nusselt number were clarified. (3) In the test conditions applied in the present study, the heat transfer of a frosted tube is less than that of the heated tubes because the effect of the increase of thermal resistance due to the frost layer is dominant as compared with the effects of the latent-heat release and increase of heat-transfer area due to frosting. (4) Finally the ratio of the convection thermal resistance between the surface of the frost layer and air flow to the total thermal resistance between tube surface and air flow including that of frost layer was obtained and shown by an empirical formula as a function of time and temperature difference between inlet air and tube surface.
