Evaluation of pool boiling heat transfer over micro-structured surfaces by combining high-speed visualization and PIV measurements

The present work introduces an alternative approach which combines heat transfer measurements, high speed visualization and PIV to infer on the effect of surface micro-structuring in the various heat transfer parcels. The PIV provides particularly interesting information on the bubbles vertical velocity, allowing to infer how the microstructures affect the bulk induced flow. The micro-patterns are composed by arrays of cavities with fixed shape and depth, only varying the distance between cavities, S. The results confirm the importance of enhancing the pool boiling performance, by promoting the increase of the bubble frequency and active nucleation sites density with micro-patterned surfaces. The role of the patterns is different, depending on the properties of the fluids. Hence, for liquids with very high latent heat of evaporation and surface tension, the parcel of heat transfer related to the phase change is very important. Based on the force balance on the departing bubbles, it is evident that in this case the micro-patterns will mainly affect the bubble coalescence, which may lead to steep deterioration of the heat transfer coefficient. On the other hand, for liquids with lower thermal properties (and lower surface tension) the micro-patterns play an important role in increasing and stabilizing the vertical velocity of the bubbles, thus favoring a very efficient boiling performance, with high boiling frequencies and large active nucleation sites density. This role of the micro-patterned surfaces is well captured by the representation proposed here, which relates the heat transfer coefficient with the distance between micro-patterns S, made dimensionless by the characteristic dimension (Lc=(?lv/(?l-?v))1/2). This representation actually shows the best performance of the HFE7000 in comparison to that of ethanol, when the micro-patterned surfaces are used, despite the lower thermal properties of HFE7000. This trend was confirmed experimentally in our measurements