  Microchannels have increasingly been used to miniaturize heat transfer
equipment, improve energy efficiency, and minimize heat transfer fluid
inventory. A fundamental understanding of condensation in microscale will yield
far-reaching benefits for the different areas of industry. In this study,
microtubes with inner diameters of 250, 500, 600, and 900 um were used to
investigate the effect of microtube diameter, inlet quality, and mass flux on
the liquid/vapor interface near the wall boundaries in condensing flow. After
validation with the experimental results, a transient numerical model (based on
the volume of fluid approach) was developed to investigate the hydrothermal
properties of condensing such as bubble dynamics, flow map transitions,
transient interface shear force, and temperature on flow condensation
performance in terms of heat transfer coefficient and pressure drop. The liquid
film thickness, slug velocity, and location of transition from annular flow to
slug flow inside the microtube were characterized for different microtubes, and
the resultant alteration in condensation flow heat transfer and pressure drop
is discussed in detail. The obtained results indicated that the interfacial
characteristics of condensing flow in microtubes with hydraulic diameters lower
than 500um are majorly different from those with D>500um.
