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A Comparison of R245fa Pool Boiling Measurements to R123, and R245fa/Isopentane on a Passively Enhanced, Horizontal Surface
M. A. Kedzierski

This study presents pool boiling heat transfer measurements for R245fa, R123 and a mixture of R245fa and isopentane on a flattened passively enhanced low-pressure boiling surface. Possibly because the enhanced surface was flattened from a round tube, its performance was sensitive to vapor seeding at start up. Consequently, the surface would operate in fully active and partially active boiling modes. The results indicate that the addition of 0.5% mass isopentane to R245fa increases the heat flux by no more than 11% greater than the heat flux of pure R245fa when the entire surface was actively boiling. By contrast, the heat flux of the R245fa/isopentane mixture was as much as 50% greater than that of pure R245fa for a partially boiling-active surface. In addition, pure R123 pool boiling data was taken on the same passively enhanced surface that was used with R245fa and the mixture. The ratio of the pure R245fa heat flux to the pure R123 heat flux was independent of the fraction of the surface that was actively boiling. This is consistent with the hypothesis that isopentane acts to enhance the site density. That is, the addition of 0.5% isopentane limits the heat transfer degradation that results from the loss of active sites by increasing the number of active sites in the actively boiling regions. From 8 kW/m2 to 100 kW/m2 and for fixed superheat, the R245fa heat flux was 27% greater than that for R123. Considering only the boiling heat transfer characteristics of R245fa, it appears to be a good replacement for R123.

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