Cooling Characteristics of Hot Surface by Air-Water Mixture

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Measurement of the flow characteristics of nozzles and the heat transfer experiment have been performed on fog cooling in the region of low mass velocity of water. Distance between the nozzle and the heat transfer surface was fixed at 520 [mm] throughout the experiments. The mass velocity of water lies in the range of W=0.02-0.14 [kg/m^2s], the droplet diameter d=15-36 [<micrn>m], the impinging velocity V=4-7.2 [m/s], and temperature of the surface 100-600 [゜C]. Theoretical analysis has been also carried out on the heat transfer in the region equivalent to film boiling with the aid of experimental data. Total heat flux by fog cooling is assumed to be expressed by the sum of radiation Q., forced convection by air qa and evaporation of the droplets q_w which are estimated independently. Heat flux by radiation can be calculated by q_e=<epsilon><alpha> (T_w^4-T ^4). The emissivity <epsilon> was determined by the experiment by air convection. Assuming that the air flow can be considered to be an axsymetric laminar stagnation flow, the expression of the heat flux q_a=5.4Pr^<0.4><lambda> a(D8vi<nu>D8(T_w-T_a) was obtained. For the heat flux by the evaporation of droplets, the model which the droplet is dragged by the air flow and moves in the radial direction was employed and the expression of q_w=2.15X10^3<lambda>_vW(T_w-T _<sat>) was obtained. In the analysis, it is assumed that each droplet floats on the vapor film without contact with the surface and the heat is transfered to the droplet only by conduction through the vapor film. It was made clear by the analysis that the heat flux by the droplet evaporation is proportional to the mass velocity of water, and inversely to d^<0.5> and V^<0.5>. The total heat flux calculated by this analysis agrees well with that by the experiment in the range of the surface temperature of 250-600 [゜C].