Energy Efficiency

High Condensing Temperature Heat Transfer Performance of Low Critical Temperature Refrigerants

The condensation tests showed increasing heat transfer coefficients and pressure drops with quality and mass flux. The effect of reduced pressure on heat transfer coefficients is not very significant, while this effect is more pronounced in the pressure gradient, with pressure drops being lower at higher reduced pressures. The data from this study were compared with the available heat transfer and pressure drop models in literature for similar situations, and explanations for agreements/disagreements were provided. In general, no correlation from the literature was successful in predicting heat transfer or pressure under the present conditions with an acceptable degree of accuracy, primarily because the available correlations were developed for different geometries, fluids, or operating conditions. In the absence of other applicable flow regime transition criteria, the criteria developed by Coleman and Garimella (2003) for condensation of R134a were used to designate the prevailing flow regimes for a given combination of mass flux and quality. The condensation data collected in the present study were primarily in the wavy and annular flow regimes. Thus, wavy flow and annular flow heat transfer and pressure drop models were developed. A transition region was then defined to provide a smooth transition between the wavy and annular flow models and thereby remove discontinuities in the overall predictions. The average absolute deviation of the entire data set from the heat transfer model predictions was 6.3%, with 95% of the data predicted within ±15%. The pressure drop model resulted in an average deviation of 7.7%, with
89% of the data predicted within ±15%.

In supercritical cooling, the sharp variations in the thermophysical properties in the vicinity of the critical point were found to have substantial effect on heat transfer coefficients and pressure drops. Due to these abrupt property variations, the heat transfer coefficients show sharp peaks while pressure drops show sudden jumps near the critical temperature. Based on the variations of the specific work of thermal expansion (contraction) for each fluid, the data from the supercritical tests were grouped into three regimes: liquid-like, pseudo-critical transition and gas-like regimes. Supercritical heat transfer coefficient and pressure drop correlations for each regime were developed. The average absolute deviation of the entire data set from the heat transfer model predictions was 14.4%, with 85% of the data predicted within ±25%. The pressure drop model resulted in an average deviation of 14.3%, with 89% of the data predicted within +/- 25%.

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