  The evaporation of vapor bubbles inside a microchannel is important to
realize a device with high cooling performance. The liquid film formed on the
solid surface is essential for evaporative heat transfer from solid to fluid;
its formation process and heat transfer characteristics need to be
investigated. The expansion process of a single vapor bubble via evaporative
heat transfer in microchannels was evaluated via a numerical simulation in this
study. In the calculation model, the working fluid used was saturated FC-72 at
0.1013 MPa and the channel diameter was 200 $\mu$m. The superheat of the
initial temperature field and wall were considered as parameters. To evaluate
the heat transfer characteristics, the time variation of liquid film thickness
was evaluated. The averaged liquid film thickness had a correlation with the
capillary number. Additionally, the dominant heat transfer mode was estimated
by decomposing the heat transfer rate into the heat-transfer rate through the
liquid film, rear edge, and wake. When the superheat was low, the heat transfer
mostly occurred via liquid film evaporation; the heat flux through the liquid
film could be predicted using the liquid film thickness. On the other hand, in
cases of higher superheat, owing to rapid expansion of the vapor bubble, no
evaporative heat transfer occurred through the liquid film around the bubble
head. It could be inferred from this study that the relationship between the
thickness of the thermal boundary layer of the bubble and liquid film thickness
is important for predicting the cooling effect of this phenomena. When the
vapor bubble grows in the high superheat liquid, the rapid growth makes the
liquid film thick, and the thick liquid film prevents the heat transfer between
the liquid-vapor interface and heated wall.
