Heat transfer in finned evaporator channels of two-phase thermal control system for microgravity conditions

The evaporators of two-phase thermal control system have to provide high heat transfer intensity. It allows increasing system efficiency due to achieving lower temperature difference between working fluid and heat dissipating equipment. Evaporator with finned internal surface is used to increase the heat transfer surface and to promote the capillary catching of fluid in a gap between fins.
Experimental investigations of heat transfer in a horizontal evaporator profile with upper heat load and R134a as working fluid were done. Extruded aluminum profile with two side shelves was studied. Heaters were mounted on top shelf. Heat flux per shelf area is in the range 0.7…7.6 W/cm2, mass velocity is in the range 150…290 kg/(m2s) and saturation temperature is in the range 30.2…32.7 oC. Temperature distribution along the profile as well as along of perimeter was defined. Processed experimental data allowed to obtain heat transfer coefficient (HTC) as a function of vapour quality. The dependency of total profile thermal resistance on the vapour quality was defined. The minimum of total thermal resistance at x=0.4 was found. Prediction of HTC for microgravity conditions was made. The stratification of two-phase flow in ground conditions occurred and part of interfinned gaps were dried. As a result, the local HTC in the upper part is lower than in the down part of profile. The investigation of temperature distribution along the perimeter was made. The numerical distribution was compared with experimental one. HTC increasing by 33% in comparison with ground conditions and the total thermal resistance is lower by 12…13% with the assumption that interfinned gaps are filled by liquid in the microgravity conditions.
Well-known correlations (Kandlicar, Gunger&Winterton , Scrock&Grossman) developed for the smooth tubes describe the experimental data with unsatisfactory result. It could be explained by dissimilarity of flow in smooth and finned tubes.