Vapor bubble nucleation, dynamics, and heat flux in turbulent Rayleigh-Bénard convection

Abstract

Experimental work on 2-phase turbulent Rayleigh-Benard convection was conducted. Vapor bubbles uniquely nucleated at cylindrical micro-cavities etched on the superheated bottom plate (bp). Five different cavity separations (cs) were studied. Turbulent convection (1-phase flow) under the same thermal forcing as for the 2-phase flow was the reference. The circular heated/etched area located in the center of the bp was smaller than the cooling area. Vapor bubbles led to heat-flux enhancement (hfe), which increased with larger superheat and depended weakly on the cavity number. At a given large superheat, the hfe per active site increased with decreasing active site density and it saturated for the case of a very small density. The bubbles affected the bulk temperature; in 2-phase we found a more stable thermal gradient than in 1-phase flow, a reduction of the temperature drop across the thermal boundary layer (bl) of the bp, and a reduction of the temperature standard deviation. The hfe correlated with these effects in the bulk. The skewness of the temperature pdf was positive and constant in 1-phase flow, and was increasingly reduced in 2-phase flow for increasing bp superheat. Blocking the large-scale circulation (LSC) from the nucleating area on one hand, and isolating the liquid column above it from the rest of the flow on the other, led to an even larger hfe. Lateral shadowgraph visualization of 1-phase flow showed plumes not forming a well-defined LSC. In 2-phase flow the LSC was better defined, and the reduced skewness for large superheat associated to the flow dynamics. Bubbles condensed rapidly after departure due to large gradients across the bl. Close to the bp the LSC dragged the bubbles horizontally, decelerating them. In the bulk, their condensation rate and vertical deceleration were constant. There was a large difference between bubble and LSC velocities. The travelled distance until bubbles fully condensed correlated to the maximal vertical velocity along single bubble trajectories. At the bp bubble volume grew linearly with time and at departure neither strongly depended on superheat nor on cs. For all cs the departure frequency exponentially increased as a function of superheat. For a given superheat, the larger the cs was, the more irregular in time they departed. The latent heat required for bubble growth contributed up 25% to the enhanced heat transferred by the surface. The contribution due to effective buoyancy increased with superheat.