Conference Year
2018
Keywords
Pressure drop, model, condensation, vapor-compression system, superheated vapor
Abstract
A new pressure drop model based on flow regime map is proposed for condensation inside horizontal smooth round tubes accounting for the non-equilibrium in a vapor compression system. Conventionally, a pressure drop model for two-phase flow only accounts for the prediction between bulk quality 1 and 0. The temperature gradient during condensation, however, creates the non-equilibrium that guarantees two-phase flow beyond bulk quality 1 and 0. The new model determines the onset and end of condensation by tracing the development of the liquid film when the superheated vapor is condensed on the tube wall. The flow regime map designed specifically for condensation from superheated vapor is used to predict the flow regime when the flow is two-phase. Two flow regime transitions are recognized. One is from annular flow to the stratified flow under low mass fluxes; the other is from annular flow to the intermittent flow under high mass fluxes. The annular flow is treated as a uniform annular ring; the stratified flow is treated as a combination of annular flow on the upper part of the tube and liquid pool at the bottom part of the tube; the intermittent flow is treated as a combination of annular flow and single-phase liquid flow that occur intermittently. The weights designated to each flow regime is calculated from the void fraction model used in the flow regime map. The construction of the new model is guided by the flow visualizations of several different refrigerants under various working conditions in tubes with diameters of 4 and 6 mm. The prediction of the new model is compared with experimental data of R32, R134a and R1233zd(E) mass fluxes from 100 to 400 kg m-2 s-1, heat fluxes from 5 to 15 kW m-2 and tube diameters of 4 and 6 mm at saturation temperatures of 30 oC. The comparison shows that the new model provides good agreements with experimental data. Additionally, by accounting for the non-equilibrium in the condensation process, the new model seamlessly connects the single-phase and two-phase regions with the corresponding mechanisms that occurs in a real vapor compression system.