Experimental characterization and modeling of critical heat flux with subcooled foaming solution

Abstract

Boiling relies on the latent heat of vaporization to dissipate large heat fluxes within small temperature budgets, i.e., demonstrates high heat transfer coefficient (HTC). Addition of surfactants in water is a commonly used HTC enhancement technique during boiling. However, the vapor-crowding of the heater surface due to the foaming nature of solution induces a premature boiling crisis. Vapor-crowding also renders typical wettability improvement techniques futile for critical heat flux (CHF) enhancement. Resulting low CHF values limit the use of surfactants to low heat flux applications. Here we perform experiments to show that subcooling induced condensation of rising bubbles can be used to suppress vapor-crowding and enhance the CHF in comparison to saturated boiling with foaming solutions. We include the contribution of effective vapor-removal due to the condensation of rising bubbles in the model for saturated boiling to successfully predict the experimental CHF data over a wide range of subcooling. An enhancement of ≈3.5× in CHF in comparison to saturated condition was observed at 40°C subcooling. We further show that wettability improvement techniques such as nanostructuring are ineffective for CHF enhancement even under subcooled conditions. However, at any subcooling, preferential nucleation within microchannels followed by forced coalescence of otherwise non-coalescing bubbles forms separate vapor-removal and liquid-supply pathways to enhance the CHF (≈15−35%) in comparison to the baseline surface without microchannels. Furthermore, we use these new physical insights to revisit pool boiling with pure and subcooled water to propose an accurate mechanistic model for CHF data in literature. We believe that the physical insights presented in this work can be used to optimize the design of heater surfaces to further improve CHF with foaming solutions.