Experimental study of steady and transient subcooled flow boiling. (January 2021) - Scheiff, V.
Highlights: Heat transfer in subcooled nucleate flow boiling regime is investigated in steady state with set heat flux or set temperature. Characteristic bubble sizes and velocities are determined from image processing and correlations for the bubble diameter et velocity are derived. In transient nucleate boiling regime with a constant heating rate, a correlation for the prediction of the wall heat flux in transient regime versus the wall temperature is provided. Abstract: This study aims to better characterize the heat transfer and flow structure in the fully developed nucleate flow boiling regime in a semi-annular duct. Experiments with a refrigerant HFE7000 were performed in the range of Reynolds numbers from 13 000 to 40 500, subcoolings close to 15 ∘ C, for constant heating power, constant wall temperature and constant heating rates (linear increase of the wall temperature). With constant heating power, the wall heat flux is well predicted by a Chen-type correlation based on a contribution due to the forced convection and a contribution due to nucleate boiling, including the effect of the liquid subcooling. A thin layer of bubbles sliding along the wall is observed. The characteristic diameter of the bubbles increases with the heat flux and decreases with the liquid velocity and its subcooling. The bubble diameters can be well predicted versus 3 dimensionless numbers: the Reynolds number of the flow, the Jakob number based on the liquid subcooling and the Boiling number. A drag coefficient of the bubbles sliding on the wall is estimated from the measurements of the bubble relative velocities and is in good agreement with the recent numerical simulation of Shi et al. [1] for a spherical bubble moving close to a wall in a shear flow. In the experiments with a constant set temperature, a non-homogeneity of the surface temperatures is observed as well as high fluctuations of temperatures and heat fluxes. The heat transfer is strongly degraded ( ≃ 60%) by comparison with heating with a set power. Finally a transient nucleate boiling regime with a constant temperature increase dT / dt is investigated. For dT / dt < 50 ∘ C.s − 1, the results are similar to those of Auracher and Marquardt and a correlation for the prediction of the wall heat flux versus the wall temperature in the transient nucleate boiling regime is provided.