porosidad

4.3. Microchannel model
Heat transfer and fluid flows in the phase separated microchannel array were modeled using the finite element method. Due to
significant computational requirements involved in solving 3D
flow and conjugate heat transfer problems, we only modeled a single period of the channels (one liquid channel and the two halves
of the adjacent vapor channels). The computational domain
(cross-sectional and top views) for the simulations is shown in
Fig. 3. We note that, by not allowing heat spreading in the transverse direction, this unit-cell model may significantly underpredict
the real-world performance for cooling semiconductor chips of finite widths.
The micro-perforations were not directly modeled in the microchannel model. The evaporation of the liquid in the perforations is
accounted for using an effective evaporation heat transfer coefficient. Corresponding mass sink/source terms were included at
the respective channel wall surfaces to represent mass loss/gain
due to evaporation.
The SiC chip (semiconductor device) is mounted on top of the
cooling device. The chip is 0.5 mm in length and is located
0.5 mm from the edge of the channels in the simulation domain.
An infinitely wide (in the transverse direction) heat source of
length 100lm (in the streamwise direction) is applied on the chip
surface at a constant heat flux of 1000 W/cm
2
.
As mentioned earlier, all channel dimensions are fixed except
the widths of the liquid and vapor channels, denoted by Wl and
Wv, which were varied parametrically. Most simulations were
performed for two different values ofhevap (=10 kW/m2K and 20 kW/m2K). In a few select caseshevap was varied in the range
5–30 kW/m2K to examine its impact. The values obtained from
the numerical simulation of the micro-perforations is corrected
to account for both the accommodation coefficient and the sidewall porosity. Note that for vaporization from a liquid–vapor
interface, we use the accommodation coefficient as the ratio of
the net mass flux at the interface to the mass flux at an interface
with no recondensation. It is always less than unity. The resultant
overall effective heat transfer coefficient is then used in the thermofluid model. This provides a lower bound for the effective heat
transfer coefficient. The change in the saturation temperature due
to the pressure drop along the microchannels is negligible and was
hence not accounted for.
No slip conditions were applied at all walls. Zero temperature
gradients (negligible conductive heat flux) were specified at the inlets and outlets of the channels. All the other surfaces were assumed to be thermally insulated. A uniform pressure boundary
condition was specified at the liquid inlet and the vapor outlet