Condensation inside Horizontal and Inclined Smooth Tubes at Low Mass Fluxes

Abstract

Condensation has been extensively investigated from as early as 1914. However, there are several gaps in the literature, especially for in tube condensation at low mass fluxes and inclined tubes. Until now, no study has systematically investigated the influence of temperature difference, vapour quality, and inclination on the heat transfer coefficients and pressure drops during condensation inside horizontal and inclined smooth tubes at low mass fluxes. Thus, the purpose of this study was to increase the fundamental understanding of two – phase flow behaviour at low mass fluxes by experimentally investigating the heat transfer, flow pattern, and pressure drop characteristics during condensation inside horizontal and inclined smooth tubes at low mass fluxes. An existing experimental set‐up was modified to accommodate the “low mass flux” needs of this study and the initial results were successfully validated against literature. A smooth circular copper tube in tube test condenser with an inner tube 1.49 m long, an inner diameter of 8.38 mm and an outer diameter of 9.54 mm was designed and built. The annulus had an inner diameter of 14.5 mm and an outer diameter of 15.88 mm. Heat transfer and pressure drop experiments were conducted for mass fluxes of 50, 75, 100, 150, and 200 kg/m2s, at 15 different inclination angles from −90° (vertically downwards) to +90° (vertically upwards). The temperature differences (differences between the average refrigerant saturation temperature and tube wall temperature) were varied from 1 C to 10 C, while the average saturation (condensation) temperature was maintained at 40 C. The mean vapour qualities were varied between 0.1 to 0.9. R134a was used as the test fluid while water was used in the annulus to cool the test section. A total of 2 178 videos, 2 920 mass flow rate measurements, 56 301 temperature measurements and 1 536 pressure drop measurements were taken. The flow patterns were recorded in grey levels with two high-speed video cameras installed at the inlet and outlet of the test section through sight glasses made from borosilicate. To improve the image quality and ensure uniformity in the distribution of the light, a uniform (LED) backlight was used. This LED backlight was a 99% uniform, 50 by 50 mm red light. An uncertainty analysis ii showed that the maximum uncertainties of the pressure drops, heat transfer coefficients and vapour qualities presented in this study were 9%, 12%, and 5% respectively. For horizontal flow, it was found that the flow patterns were predominantly stratified and stratified wavy. It was also found that the heat transfer coefficients were dependent on the temperature difference between the temperature of the wall on which condensation occurs and the temperature of the condensing refrigerant. Furthermore, it was found that the heat transfer coefficient decreased with an increase in the temperature difference. When comparing the heat transfer results at low mass fluxes to the literature, it was found that the absolute mean deviation varied by up to 42%. An amendment was suggested in a stratified heat transfer coefficient term from literature. It was found that with this amendment, the heat transfer coefficients of low mass fluxes could be estimated with errors of an average of ± 5%. For inclined flow, six flow patterns namely ─ stratified, stratified wavy, annular, annular wavy, intermittent, and churns flows were observed. Bubbly flow was not observed on its own but was observed during intermittent flows. These flow patterns were adopted using the descriptions of flow regimes as prescribed by Thome. It was found that the inclination angles significantly influenced the flow patterns and the heat transfer coefficients. Downwards flows accounted for an increase in heat transfer coefficient with the maximum heat transfer coefficient found at inclinations of −15 and −30 at the corresponding minimum temperature difference tested for in each case. The maximum inclination effect was approximately 60% and was obtained at the lowest mass flux of 50 kg/m2s. In general, it was concluded that the heat transfer coefficients were more sensitive to the temperature difference for downwards flows than for upwards flows. Furthermore, there was no significant effect of the temperature difference on the heat transfer coefficients for upwards flows. It was also found that the vertical downwards (-90o) and upwards (+90o) orientations were almost independent of the temperature difference. With respect to the inclination effect, it was found that in general, they decreased with increase in temperature difference, but increased with a decrease in mass flux and vapour quality. With respect to pressure drops in smooth and inclined tubes, it was found that they increased with an increase in mass flux, temperature difference and vapour quality. Furthermore, the lowest and highest measured pressure drops were obtained during the downward and upward flows respectively. On the other hand, the opposite was found for the frictional pressure drops. Keywords: Inclination angles, condensation, temperature difference, mass flux, smooth tube, vapour quality, heat transfer coefficient, flow patterns, pressure drop, inclination effect.