Phase Change Material Particles and Their Application in Heat Transfer Fluids - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 467

Fig. 15 Relative thermal
conductivity increase with
ethanol loading in the
ethanol-in-PAO
nanoemulsion fluids. The
estimate from Maxwell's
model is shown for
comparison [ 11 ]

1.1

Experiment
Maxwell's model

1.08

Ethanol in PAO Nanoemulsions

1.06

1.04

1.02

1

0 2 4 6 8 10

Ethanol Volume Fraction,

5.1.3 Phase Change Behavior in Ethanol-in-PAO Nanoemulsion Fluids

The pool boiling of these Ethanol-in-PAO nanoemulsion fluids have been inves-
tigated, in which the dispersed ethanol nanodroplets undergo liquid-vapor phase
transition. In these Ethanol-in-PAO nanoemulsion fluids, the dispersed phase
ethanol has a boiling point of 78 C at 1 atm which is much lower than the boiling
point of the base fluid PAO (277 C) [ 35 ]. The pool boiling heat transfer curves
are plotted in Fig. 16 for the pure PAO and PAO-based nanoemulsion fluids. When
the wire temperature is less than 170 C, the heat transfer coefficient values of the
pure PAO and the PAO-based nanoemulsion fluids appear to be the same. This
indicates that these ethanol nanodroplets have little effect on the fluid heat transfer
efficiency if there is no phase transition in these nanodroplets. This is also con-
sistent with the measured thermal conductivity shown in Fig. 15 . When the heater
temperature is further increased, an abrupt increase in convective heat transfer
coefficient is observed in the PAO nanoemulsion fluids, compared to that of the
pure PAO case. For example, the dissipated heat flux q is found to be 90 and
400 W/cm 2 at T wire = 200 C for the pure PAO and the PAO nanoemulsion fluid,
respectively. What is more interesting is that the CHF of the PAO nanoemulsion
fluids is significantly larger than that of their pure components ethanol and PAO.
The causes of the observed abrupt increase in the heat transfer coefficient can be
first examined by evaluating the Morgan correlation that works for free convection
over a long cylinder [ 84 ],

Nu D ¼ CRa D ð 8 Þ

where Ra is the Rayleigh number, and C and n are constants. The Rayleigh number
is in the range 10 ffi 10 10 ffi 2 for the nanoemulsion experiment, so n = 0.058. A
direct impact of the nanodroplet vaporization would be the enhanced effective heat