Characteristics of Refrigerant Film Thickness, Pressure Drop, and Heat Transfer in Annular Flow

Common refrigeration and air conditioning cycles are dependent on two-phase flow for efficient heat transfer with minimal pressure drop. Design of heat exchangers for these systems is aided by an understanding of the refrigerant pressure drop and local heat transfer coefficient. An estimate of the liquid fraction is also important for predicting the charge required in a system. The present work develops a semi-analytical model for predicting liquid fraction, pressure drop, and heat transfer for pure refrigerants in the annular flow regime. The model uses the approach of coupling a uniformly thick, turbulent liquid film layer with a turbulent vapor core. Model predictions are compared to experimental evaporation and condensation data for Rll, R12, R134a, and R22. These refrigerants represent the low, medium, and high pressure ranges found in common refrigeration systems. The uniform film model, when compared to experimental data, provides a reference for understanding some of the mechanisms that are important to refrigerant two-phase flow.Air Conditioning and Refrigeration Project 7