Hydrodynamics of vertical upward and downward flow boiling in a millimetric tube

Abstract

Upward and downward vapor-liquid vertical flows inside a vertical 6 mm sapphire tube was investigated using HFE-7000 as working fluid. The goal was to investigate the effect of flow direction and wall heating on two-phase parameters such as flow pattern, wall shear stress (τw), interfacial shear stress (τi) and interfacial wave structures. The mass flux, heat flux and vapor quality ranges were 50≤G≤400kg/m2.s,0≤q≤3W/cm2 and 0≤x≤0.7 respectively. Bubbly, slug, churn, annular flow patterns were observed in upward and downward flows. A falling film regime occurred in downward flow at low mass fluxes. Void fractions were higher in downward flow than in upward flow due to gravity effect, and were in good agreement with drift flux models of the literature. The wall shear stress increased with the wall heat flux due to the bubble nucleation at the wall. Following a similar approach to Kim and Mudawar (2013b), a correlation for the wall shear stress taking into account the forced convection and the bubble nucleation was derived and provided a good estimation of the experimental data within ±20%. From image processing of the high-speed visualizations, velocities (Uw) and frequencies (Fw) of the disturbance waves in annular flow were measured. The interfacial shear stress was found to directly depend of the product Uw×Fwand a prediction of the interfacial friction factor was proposed in flow boiling for both upward and downward flows.