Scale Effect on Boiling Inception and Bubble Dynamics in Micro/Mini/Macro Tubes

Abstract

The present study experimentally and theoretically investigated the scale effect on boiling inception and following liquid-vapor interfacial evolution in horizontal tubes with inner diameter varying from 0.05 mm to 3.0 mm. Reducing scale, i.e., decreasing tube inner diameter, significantly increases the critical heat flux of bubble nucleation. A novel thermodynamic model and derived q″w,c–Tw curve were used to explore the scale effect on bubble nucleation and corresponding mechanisms. It is concluded that reducing scale remarkably enhances heat transfer performance close to the heated wall and increases temperature gradient of liquid, which consequently suppresses boiling inception and increases the critical heat flux. Diverse interfacial movement behaviors were found, by which tubes were classified into various scales as, micro scale (Di ≤ 200 μm) with explosive emission boiling, mini scale (200 μm < Di < 2.5 mm) with distinct spherical and following oblate bubble growth stages and macro scale (Di ≥ 2.5 mm) with spherical bubble growth stage only. Interaction between liquid-vapor interfacial movement and thin film evaporation, induced by the reduced scale, was considered to be the crucial factor to bring about phenomena of explosive emission boiling in micro tube and spherical-to-oblate bubble growth in mini tube which are quite different from that in macro tube. Different models were developed to describe the diverse liquid-vapor interfacial movement behaviors, which provides insight into the scale effect on interfacial movements.