Lattice Boltzmann study of bubble dynamic behaviors and heat transfer performance during flow boiling in a serpentine microchannel

Abstract

• Mesoscale study of microchannel flow boiling in U bend was conducted. • Detailed bubble dynamics and heat transfer characteristics were presented. • Effect of buoyancy on flow boiling process depends on flow orientation. • The onset of nucleate boiling is delayed as the Re number increases. Understanding the flow boiling process in a serpentine microchannel with U-bends is very important to its design and application in practice. In this study, a hybrid thermal multiphase model consisting of the pseudopotential multiphase lattice Boltzmann model and the finite difference method is employed to investigate the flow boiling heat transfer in a serpentine microchannel. Effects of curvature ratio, flow orientation, heat flux and Reynolds ( Re ) number on the bubble dynamic behaviors and heat transfer performance during flow boiling process are comprehensively evaluated. Bubble behaviors including bubble nucleation, growth, coalescence, departure, recontact, and migration are well captured, and typical flow patterns of bubbly flow and intermittent slug flow are identified. The simulation results show that increasing curvature ratio does not affect the heat transfer performance much, but generates elongated bubbles at the U-bend. Depending on the flow orientation, the buoyancy induced by gravity acceleration has both favorable and unfavorable effects on the bubble dynamic behaviors and local heat transfer characteristics in the serpentine microchannel. In addition, the vapor volume fraction is calculated under different Re numbers and heat fluxes. The lower vapor volume fraction exhibits lower wall superheat and better heat transfer performance.