Modeling heat rejection in horizontal smooth round tubes and experimental validation for R1234ze(E), R134a and R32

Heat transfer in condensers is typically divided into 3 zones: superheated, two-phase and sub cooled region. These regions, in general, are considered to be independent of each other and various correlations are available in literature to predict the heat transfer coefficient (HTC) in these regions separately. These correlations, if plotted as a function of quality for the three regions, will show discontinuity at qualities of 0 and 1. The aim of the thesis is to bridge the discontinuity by establishing the interdependency of these regions and propose a unified model to predict HTC throughout the condensers. Experimental data suggests the HTC near x=1 in de-superheating region to be significantly higher than predictions due to presence of liquid when the wall temperature drops below saturation temperature. Similarly, HTC below x=0 has been seen to decrease linearly before following Gnielinski correlation due to presence of vapor as seen in sight glass at the end of test section. The newly proposed model takes into account the presence of liquid in de-superheating and sub-cooled liquid in two-phase zone. The model has been developed independently and compared to experimental data for R134a, R1234ze(E) and R32 for mass fluxes of 100-300 kgm-2s-1, saturation temperatures of 30 0C - 50 0C and from sub-cooling of 20 0C to superheat of 50 0C in a horizontal smooth tube with 6.1 mm inner diameter. Cavallini et al. (2006) and Gnielinski correlations have been used as a baseline correlation to calculate HTC in two-phase and single phase zone respectively. The model predicts the HTC satisfactorily within an accuracy of 16 %. Another objective of the work is to form a baseline for the heat transfer characteristics in condensation for R1234ze(E) which can be a potential replacement in automotive systems for R134a on account of low GWP. The properties of R1234ze(E) is fairly well know to be close to R134a, however, the performance data under similar operating conditions as R134a is not widely published. To enhance the performance, the use of refrigerant mixture is also a possibility. Hence, R32 which is known to have higher heat transfer coefficient for its favorable thermo physical properties is a viable option to be considered as a mixture with R1234ze with a trade-off in GWP. Experiments conducted for R134a, R1234ze(E) and R32 at various mass fluxes, saturation temperature and heat fluxes helps in analyzing the effect of various parameters on heat transfer coefficient. The work in this thesis can be used as reference to study the effect of mixtures on heat transfer coefficient