Effect of wettability on flow boiling heat transfer in a microtube

As one of the main factors, surface wettability significantly affects flow boiling characteristics. In this study, the wettability effect in a microtube was presented by a numerical method. A gas-liquid phase-change model was adopted to research characteristics of two-phase flow and heat transfer. Three types of wettability surfaces (hydrophilic, hydrophobic, and gradient wettability) were explored under different mass fluxes with subcooling inlet and constant heat flux conditions. Numerical results indicate that the slug flow is absent near the outlet under a higher mass flux, mainly due to the backward movement of saturated nucleation boiling. Hence, better heat transfer performance is achieved on both hydrophilic and hydrophobic surfaces. Outstanding performance is exhibited on the hydrophilic surface, mainly in subcooling, bubbly, and confined bubble flow. Since the unstable interface fluctuation and the bubble sliding behavior, the hydrophobic surface's heat transfer performance is improved in the confined bubble and slug flow. As incorporating the contact angle continuous variation effect and the hydrophilic and hydrophobic surfaces' advantages, gradient wettability surface shows sound heat transfer performance in each flow region, especially in the slug flow where shorter bubbles and drastic fluctuation of unstable bubble interface appears. Based on the present investigation, the gradient wettability surface demonstrates considerable potential in equipment cooling applications by regulating the flow boiling characteristics and improving the performance