Interfacial wave behavior and its effect on the flow and condensation heat transfer in a microtube

Abstract

The transient flow and condensation of R32 in a 0.1 mm microtube were simulated for mass fluxes of 100, 150 and 200 kg m−2 s−1. The model predicted three annular-to-intermittent flow pattern transitions as the mass flux increased that are characterized as injection, injection with wave coalescence, and wave coalescence. The annular flow interfacial waves are transient physical phenomena even though the boundary conditions are steady. The mechanism for their appearance, propagation and growth was investigated. The large interfacial waves squeeze and strongly accelerate the local vapor flow. Oscillations of the radial temperature and velocity gradients cause oscillations of the local heat flux and the wall shear stress in the wavy annular flow which increase the average heat flux and flow friction in the wavy annular flow region. Higher mass fluxes significantly increase the local wall shear stress due to the larger radial velocity gradient at the wall. The radial temperature gradient at the wall is nearly inversely proportional to the liquid film thickness which is independent of mass flux in annular flow, so the local heat fluxes are similar for the three mass flux. The higher mass fluxes weakly enhance the heat transfer in the bubbly and liquid flow region. A new correlation for the local condensation heat transfer in annular flow is proposed, and validated by the simulated data with high agreement.