Phase Change Material Particles and Their Application in Heat Transfer Fluids - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 468

Fig. 16 Surface heat flux as
a function of heater
temperature for pure PAO,
pure ethanol, and PAO
nanoemulsion fluids. These
data were measured in a pool
boiling setup with an
untextured heater surface (Pt
wire, 25.4 lm in diameter),
where the bulk liquid was at
atmospheric pressure and
room temperature (25 C, not
saturated state). The CHF is
determined within an
accuracy of 5 % [ 44 ]

capacity due to the heat of vaporization. The effective heat capacity can be
evaluated using the formula:
C eff ¼ C 0 þ / H droplet DT ð 9 Þ

where / is the volume fraction of the phase-changeable nanodroplets, H droplet is
the heat of vaporization of the nanodroplets per unit volume, and DT is the
temperature difference between the heat transfer surface and the bulk fluid. In this
experiment, if it is assumed that DT = 10 C, the effective volumetric-specific
heat can be increased by up to 162 % for the 4 vol% nanoemulsion fluid when the
ethanol nanodroplets undergo liquid-vapor phase transition. However, this would
provide only 2.7 % enhancement of the heat transfer coefficient according to the
Morgan correlation.
The nanodroplet vaporization can enhance heat transfer mainly through
inducing drastic fluid motion within the thermal boundary layer around the heat
transfer surface. The Ethanol-in-PAO interface constitutes a hypothetically ideal
smooth surface, free of any solid motes or trapped gases, so the heterogeneous
nucleation and ordinary boiling are suppressed. In this case, the ethanol nano-
droplets can be heated to a temperature about 120 C above their normal
atmospheric boiling point (78 C). Such a temperature is very close to the
thermodynamic limit of superheat or the spinodal state of ethanol, and is only
about 10 % below its critical point. The spinodal states, defined by states for
which o P

oV T ; n ¼ 0, represent the deepest possible penetration of a liquid in the
domain of metastable states [ 85 - 88 ]. When those ethanol nanodroplets vaporize
after reaching their limit of superheat, the energy released could create a sound-
shock wave, a so-called vapor explosion. This sound wave would lead to strong
fluid mixing within the thermal boundary layer, therefore enhancing the fluid