Environmental Engineering Reference
In-Depth Information
heat transfer. These fluids with enhanced heat transfer rate are expected to find
applications in various energy conversion systems, heat exchange systems, air-
conditioning, refrigeration and heat pump systems, and chemical thermal pro-
cesses [ 84 , 86 , 89 - 93 ].
5.2 Liquid-Solid Phase Change Nanoemulsion Fluids
Water in FC-72 nanoemulsion fluids are discussed below as an example of liquid-
solid phase change fluids, in which water could undergo liquid-solid transition
with appropriate operation temperature range and thus increase heat transfer rate
of the base fluid FC-72. FC-72 is one of a line of Fluorinert TM Electronic Liquids
developed by 3 M TM , which is used as the cooling fluids in liquid-cooled thermal
management systems [ 94 ]. But it has poor thermal conductivity and heat capacity,
compared to other fluids such as water.
5.2.1 Microstructure of Water in FC-72 Nanoemulsion Fluids
Water in FC-72 nanoemulsion fluids are generated by emulsifying deionized water
into FC-72 with a small amount of perfluorinated amphiphiles. Figure 17 shows
the picture of the prepared water in FC-72 nanoemulsion fluids and the pure FC-72
liquid and schematic diagram of a water nanodroplet dispersed in 3 M's FC-72
thermal fluid.
The Dynamic Light Scattering (DLS) technique is used to measure the size and
Brownian diffusivity of the nanodroplets in the prepared water in FC-72 nano-
emulsion fluid [ 95 ]. The autocorrelation function of the scattered light for the 12
vol% water in FC-72 nanoemulsion fluids is plotted in Fig. 18 . The curve shows a
typical exponential decay of the correlation function versus time [ 15 , 16 ]. The
Brownian Diffusivity and effective hydrodynamic radius of the nanodroplets are
found to be 3.5 9 10 -7
cm 2 /s and 9.8 nm at T = 25 C, respectively.
5.2.2 Thermal Conductivity of Water in FC-72 Nanoemulsion Fluids
Thermal conductivity of the water in FC-72 nanoemulsion is measured for dif-
ferent water loadings, and the results are shown in Fig. 19 . It can be seen in Fig. 19
that a very large increase in thermal conductivity is achieved in the prepared
water-in-FC72 nanoemulsion fluids, with thermal conductivity enhancements of
up to 52 % observed in the nanoemulsion fluid containing 12 vol% (or 7.1 wt%) of
water nanodroplets. The observed enhancement in thermal conductivity is much
larger than that predicted by the effective medium theory (EMT) with assumption
of spherical droplets [ 96 ]. This suggests that the water droplets are column-like
Search WWH ::




Custom Search