The average heat losses are evaluated at every level of the capsule temperature, using experimental data on the cooling rate, correlated according to: The cooling rate is the at switched off lamp condition. Its value is measured experimentally. The results of calculation according to the previous correlation for various temperature levels are given in Figure 6. These results comprise a heat flux surface density obtained by dividing the average value of heat losses by the capsule surface area. Figure 6. Overall heat loses as function of capsule temperature. The data at temperatures over 900 K are not given in this figure because in this case we have the heat effect of the fusion process of the PCM in the surface temperature. The temperature level presented in Figure 6 was taken as a mean value between the readings of the thermocouples installed on the outer and inner surfaces. The rate of heating was calculated in the same way. The analyses of the data, given in Figure 6, shows that the uniformity of the temperature fields, circumferrentially around the capsule, occurs only in case of radiant heat put in from one side of the capsule only. In the other set of experiments, in order to eliminate the radiant heat flux from upper flat surfaces of the capsule, three thermocouples were installed (in the middle one temperature fields were measured) into the experimental set up.
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