Measurements and Modelling of Convective Vaporization for Refrigerants in a Horizontal Tube

Doctoral thesis, 1996

An evaporator facility has been constructed to measure local values of heat transfer and pressure drop. The evaporator resembles a normal working evaporator tube in a direct-expansion evaporator applicable to large heat pump/refrigeration cycles, typically over 500 kW. The evaporator is brine-heated and uses an expansion valve to control the inlet. Over 54,000 data points have been measured for heat transfer as well as for pressure drop. The pure substances used in the experiments are: CFC12, HCFC22, HCFC142b, HFC134a, HFC143a, HFC125, HFC32, propane and n-butane. The binary mixtures used have been: four mixtures of propane and n-butane; three mixtures of HCFC22 and HCFC142b; R410A and R502. The ternary mixtures used have been R407C and R404A. The experimental ranges were 85-317 kg/m2s, 0.5-45 kW/m2 and -16 to +30 °C for mass flux, heat flux and evaporation temperature, respectively. An experimental comparison of heat transfer and pressure drop were carried out between CFC12, HCFC22 and a suggested replacement fluid. Results indicate the importance of properly setting up the conditions for the comparisons. Such conditions may prove to be more important than the fluid properties. A comparison between local heat transfer data for pure substances and twenty of the most well-known correlations from literature was also made. This revealed several correlations which predicted the data with good agreement, typically having a mean deviation of 20%. An investigation of the surface evaporation and nucleate boiling contributions was carried out using surface evaporation correlations from literature. A clear reduced pressure dependence was seen for the nucleate boiling contribution, which became dominant over a reduced pressure of 0.1 for most experimental conditions. Due to these findings a new correlation for heat transfer is introduced, based partly on previously published correlations. This correlation corresponds to the present data with a mean deviation of 11% for both pure substances and mixtures. An attempt to apply the correlation to data above the mass flux range of this investigation was successful. The experimental values of pressure drop were compared to 15 pressure drop corre-lations. Several of these predicted the data well for both pure substances and mixtures. No mixture effect on pressure drop was observed other than those of the thermo-physical properties.