Experimental Investigations on Condensation in the Framework of ENhanced COndensers in Microgravity (ENCOM-2) Project

Abstract

We report preliminary results on experimental investigations on condensation in the framework of the European Space Agency funded programme Enhanced Condensers in Microgravity (ENCOM-2) which aims at better understanding underlying phenomena during condensation. The first experiment is a study on condensation of HFE on external curvilinear surface of 15 mm height during reduced gravity experiments. It is found that the local minimum of the film thickness exists at the conjugation area of condensed film and the meniscus at the bottom of the fin; this leads to the local maximum of the heat transfer coefficient, which we also found moves towards the fin tip. The second experiment is a study of falling films hydrodynamics inside a vertical long pipe. In particular, characteristics of wavy falling films produced employing intermittent liquid feed are examined in order to assess wave effects on film condensation. Preliminary results suggest that intermittent feed simply divides the film in two autonomous regions with the wave feature of each one depending only on its flow rate. The processing of registered film thickness data can lead to the estimation of the transverse velocity profile in the film, which is mainly responsible for heat transfer during condensation. The third experiment looks at in-tube convective condensation at low mass fluxes (typical of Loop Heat Pipes and Capillary Pumped Loops) of n-pentane inside a 0.56 mm diameter channel. The results show that the mean heat transfer in the annular zone when it is elongated may be less than the mean heat transfer when it is shorter, due to the interface deformation involved by surface tension effect. When the length of this annular zone reaches a critical value, the interface becomes unstable, and a liquid bridge forms, involving the release of a bubble. The heat transfer due to the phase-change in this isolated bubble zone appears to be very small compared to the sensible heat transfer: the bubbles evolve and collapse in a highly subcooled liquid. The last experiment concerns in-tube condensation of R134a inside a square channel of 1.23 mm hydraulic diameter at mass fluxes of 135 kg m−2 s−1 and 390 kg m−2 s−1 for three different configurations: horizontal, vertical downflow and vertical upflow. For the calculated heat transfer coefficient it is found that gravity has no effect on condensation in downflow configurations at 390 kg m−2 s−1 and in upflow conditions at both values of mass velocity. The effect of gravity on the condensation heat transfer coefficient becomes noteworthy in downflow at mass velocity G = 135 kg m−2 s−1 and vapour quality lower than 0.6

Condensation heat transfer in microgravity conditions

“…The derived heat of condensation was found to increase in hypergravity and decrease noticeably during microgravity. The thickness of the condensate film on a curvilinear surface of 15 mm height was measured with the same optical setup employed by Glushchuk et al 68 during a parabolic flight campaign 69 . Under microgravity, a local minimum in the liquid film thickness was detected at the conjugation area of condensed film and the meniscus at the bottom of the fin.…”

“…Mean absolute deviation between predicted HTCs/film thickness data and experimental values lower than 20%, but the dependence on vapor quality is not accurately foreseen. Bortolin et al 69 Collection of experimental results from ESA ENCOM project (parabolic flight) Axially symmetrical curvilinear fin with 15 mm height HFE-7100 Natural convection T sat = 51 °C Liquid film thickness measurements Flow visualizations Maximum local HTC detected at the tip of the fin and at the conjugation area of condensed film and the meniscus at the bottom of the fin. Brendel et al 23 , 24 Experimental (parabolic flights) Fin-and-tube condenser (6.2 mm ID, 16.8 m total coil length) in a vapor compression cycle R134a T sat = 25.6–31.6 °C G = 15.2–35.1 kg m −2 s −1 Flow visualizations Check of the stability of the system Condensation pressure rise by 3% under microgravity compared to normal gravity.…”

Study on the instability of FC-72 vapor-liquid interface in a rectangular channel under different gravity conditions

“…Experimental and numerical studies have been conducted to investigate the effects of various parameters, such as channel geometry [7] , fluid properties [8] , and flow conditions [9,10] , on the stability of the interface.…”

The effect of gravity on R410A condensing flow in horizontal circular tubes

“…The results showed that gravity effects were pronounced at lower mass flux. Bortolin et al [8] reported four experiments 4 conducted under reduced gravity, including condensation of HFE on external surfaces, falling films condensation, in-tube condensation of n-pentane, and convective condensation of R134a in a square channel.…”