Flow regime map, condensation, vapor-compression system, superheated vapor
An update on the flow regime map for condensation inside horizontal smooth round tubes accounting for the non-equilibrium existed in a vapor compression system is presented. Current flow regime maps are made in the range of bulk quality between 1 and 0. However, the temperature gradient required by condensation means that the thermal equilibrium assumed in a thermodynamic point of view does not exist in a real condenser, especially at the entrance of a condenser in vapor compression systems. By focusing on the development of the liquid film when the superheated vapor is condensed on the tube wall whose temperature is below saturation temperature at the corresponding pressure, the real onset and end of condensation can be calculated. The flow regime map, therefore, should be constructed in between the real onset and end of condensation that are manually set as superficial quality 1 and 0. Two-phase flows of R32, R134a, R1234ze(E), R245fa and R1233zd(E) under mass fluxes from 100 to 400 kg m-2 s-1, heat fluxes from 5 to 10 kW m-2, and tube diameters of 1, 4 and 6 mm at saturation temperatures of 30 and 50 oC are observed in transparent tube-in-tube heat exchangers where refrigerants are cooled by glycol. The screenshots of the high-speed videos are presented in the paper. The visualizations shows three deficiencies in a conventional flow map. First, a conventional flow map does not provide any information beyond quality 1 and 0 where there is two-phase flow as mentioned above. Second, at the entrance stages of the condensation, the film-forming mechanism guarantees an annular flow. This mechanism, however, is usually not employed in a conventional flow map. In addition, the mechanism that forms misty flow at the entrance stages is also doubtful regardless of the mass flux. Last but not least, some transition lines need to be readjusted because when the non-equilibrium is taken into account, the structure of the two-phase flow does not necessarily follow what is predicted in a thermally and hydraulically fully developed two-phase flow. The new flow regime map addresses these issues. A comparison between visualization and the prediction of the new flow map shows good agreement.