Choi et al. [8] reported the heat transfer characteristics of CO2
through horizontal mini-channels having diameters of 1.5 and
3 mm. They indicated that nucleate boiling was predominant in
the low vapour quality region and a convective boiling heat transfer
contribution appeared in moderate and high vapour quality
regions. The variation of local heat transfer coefficient with heat
flux, mass flux, vapour quality and saturation temperature was discussed.
More vigorous nucleate boiling was observed and studied
when the smaller diameter tube was used. Flow boiling heat transfer
experiments with different refrigerants were continually carried
out by Choi et al. [9]. They indicated that the use of CO2
caused the heat transfer coefficient to be higher than with R-
134a and R-22 fluids