  This paper discusses the convective heat transfer enhancement of
non-condensing slug flows by using phase separation for local liquid
circulations through a porous-tube insert. Numerical simulations were carried
out to study the air-water slug flow and heat transfer without phase change
(boiling and condensation) in a vertical upward flow. The governing equations
of two-phase flow and heat transfer were numerically solved using the Volume of
Fluid-Continuum Surface Force (VOF-CSF) method in a two-dimensional
axisymmetric computational domain. The rear end (exit) of the porous tube was
open to allow discharge of fluid, but two different designs of the frontal end
were considered; a porous tube with a closed front that prevents the bubbles
from entering the porous tube and keeps the bubbles in the annular gap; a
porous tube with an open front allows the bubbles to flow through the porous
tube. Simulation results show that a counter flow of the bulk liquid inside and
outside the porous tube creates internal liquid circulations in both the axial
and radial directions, resulting in an increased liquid velocity near the tube
wall, thus an enhanced convective heat transfer. It is also found that there
exists an optimum diameter of the porous-tube-insert (or annular gap dimension
between the tube wall and porous tube) for convective heat transfer
enhancement. The maximum Nusselt number of the porous-tube-insert cases used in
this study was about five times higher than that of the liquid-only flow in a
bare tube.
