Heat Transfer Flow Regimes of Refrigerants in a Horizontal-Tube Evaporator

Abstract

An experimental study of flow boiling heat transfer of refrigerants in a horizontal-tube evaporator was conducted. A single-tube evaporation test facility was designed and developed to measure the evaporation characteristics of alternative refrigerants. Measurements were made in several instrumented, horizontal copper tubes with inside tube diameters ranging from 0.277 to 0.430 in and lengths ranging from 4 to 8 ft using R-12, R-22, R-134a, and a 60%/40% azeotropic mixture ofR-32/R-125. The two main flow regimes found during objective and visual evaluation of the flow patterns during adiabatic and diabatic flow in smooth, horizontal tubes were wavy-stratified flow and annular flow. High speed pressure and differential pressure measurements were taken for a variety of mass flux and quality combinations, and were analyzed both spectrally and statistically. The normalized power spectral density of these measurements had sharp peaks near zero frequency, indicative of separated flows. Analysis of the standard deviation of pressure drop divided by the mean pressure drop showed that wavy-stratified flow occurred for values above 0.20, while annular flow occurred for values below 0.10. For annular flow at low heat fluxes, convective boiling was the dominant mode of heat transfer. As heat flux increased, nucleate boiling enhanced the heat transfer coefficient, especially for low qualities and high reduced pressures. For wavy-stratified flows, convective boiling was diminished due to loss of available convective surface area, while nucleate boiling did not appear to be suppressed at higher qualities or lower heat fluxes. The heat transfer coefficients were well correlated using an asymptotic model, which combined the benefits of the "greater of the two" and superposition models for flow boiling heat transfer. A Froude number dependent term accounted for stratification effects on the heat transfer coefficient. The heat transfer correlation and previously developed pressure drop correlations were combined in a computer program which simulated the two-phase portion of evaporators. This program was used to examine whether an optimum diameter existed for evaporators with fixed airside resistances and refrigerant mass flow rates. The results revealed that over a wide range of diameters, the required length of the evaporator was relatively insensitive to the tube diameter, but the required surface area had a definite minimum. As the diameter became sufficiently small, pressure drop decreased much of the driving temperature difference, increasing the length of the evaporator dramatically.