Experimental study of the heat transfer in R744/R600a mixtures below the R744 triple point temperature.

• The sublimation heat transfer of R744/R600a mixtures was investigated. • The effects of carbon dioxide crystallisation, the heat flux, and the vapour quality were examined. • The crystallisation and formation of a solid fraction has a huge influence on the heat transfer characteristics. • The heat transfer coefficient values are approximately ten times higher for wet carbon dioxide sublimation than for a single-phase flow. Carbon dioxide, a natural refrigerant, is attracting attention as a direct solution to the legal restrictions on hydrofluorocarbon (HFC) refrigerants. The use of carbon dioxide as a refrigerant is limited by the triple point temperature of −56.5 °C. However, used along with a carrier fluid, carbon dioxide can provide refrigeration to temperatures lower than the triple point. A blend composed of carbon dioxide (R744) and isobutane (R600a) is experimentally studied. An experimental setup was designed, built, and verified in order to obtain the sublimation heat transfer coefficients of R744/R600a mixtures at temperatures lower than −56.5 °C. The test section comprises a horizontal copper tube with an inner diameter of 10 mm and length of 1 m. The tube is electrically heated by a copper wire heater wrapped uniformly around the tube. The tests were conducted with a novel refrigerant blend at heat fluxes from 3630 W m−2 to 8480 W m−2. The heat transfer coefficient decreases with increasing heat flux. Furthermore, a heat transfer coefficient calculation correlation has been developed. [ABSTRACT FROM AUTHOR]