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whereris the surface tension, Dc is the cavity mouth diameter
(modified to account for contact angle) [19], and qv andhfg are
the vapor density and latent heat of evaporation of the fluid, respectively. Fluids with low surface tension (well wetting fluids like FC-72 used in the present work) make it harder for cavities to trap gas.
Additionally, smaller cavities require larger wall superheats to
nucleate. Furthermore, even if bubbles do nucleate and grow, the
vapor would be vented out to the vapor channel through the pores
rather than growing in the liquid channel against the pressure. In
the present study, for FC-72 at atmospheric pressure and a contact
angle of 10 , the predicted wall superheat required for nucleation is
about 62 C (wall temperature’120 C). For the wall temperatures
expected in this study, nucleation is not expected to occur [20-21].
High-aspect-ratio microchannel geometry will allow us to
achieve significant enhancement in the effective heat transfer coefficient (per fin base area) over that of micro-gap flows between
non-finnednon-porous flat surfaces[21]. The thermal energy is
conducted along the perforated solid walls and then transferred
to the evaporating liquids in the micro-pores in our microchannel
device, which helps minimize the negative impact of the low thermal conductivity of a dielectric fluid.
For realistic thermofluid modeling of the chip/cooling device,
accurate thermodynamic and transport properties of FC-72 are
necessary. The properties of liquid FC-72, such as density (q), thermal conductivity (k), specific heat capacity at constant pressure
(Cp), viscosity (l), and surface tension (r) are readily available
[22]. However, many thermophysical properties of the vapor of
FC-72 are not available in the literature.
We develop models to predict some of the missing vapor properties required for the thermofluid modeling, focusing on near
atmospheric pressures. A four-parameter equation of state (EOS)
[23,24] is used to predict qandcpof saturated and superheated
FC-72 vapor. This EOS evaluates the pure component parameters
using the critical temperature (Tc), critical pressure (Pc) and the
accentric factor (x). The EOS is given as