Capillary column enhanced CHF microgravity flow boiling

Flow boiling heat transfer under microgravity conditions can be extended and enhanced by means of using porous stacks, or capillary columns, arranged on top of a flat heated surface. Under these conditions, body forces are negligible to remove the generated vapor away from the hot surface, which eventually hinders liquid from reaching it. It is possible to increase the critical heat flux (CHF) by having porous stacks symmetrically arranged on this surface; which draws the liquid phase towards it by means of capillary forces. Various flow regimes in the capillary enhanced surface flow boiling can be identified. These include: the regime where the liquid is supplied between the columns, the regime where the liquid flow is controlled by liquid capturing and the viscous drag-capillarity in the columns, and the critical heat flux. For the theoretical model, the expression for the interfacial lift-off model critical heat flux was interpreted based on customizable parameters instead of those imposed by the physics of the flow. This study indicates a potential improvement in CHF by having an inter-column spacing smaller than the critical wavelength for a plain surface. There is also a potential benefit of having the wetting contact to wavelength ratio to be larger than the constant of 0.2 found in experimental studies. The CHF regime can occur by a limitation of the stacks to have access to the liquid phase, as it happens when they are completely submerged in a vapor phase, or by reaching the maximum capillary pressure drop in the stack (as per the Darcy–Ergun momentum equation), or by reaching an entrainment limit of the vapor flow passed the capillary columns. Therefore the critical heat flux can also be extended as long as the capillary columns protrude over the vapor layer and their viscous capillary and entrainment limits are not reached.