Biomedical Engineering Reference
In-Depth Information
transfer during power/electricity generation has assumed an added
importance. There are two ways to increase heat transport efficiency
(Duncan and Peterson, 1994; Eastman et al., 2004):
.
designing improved cooling devices, such as increasing the surface by
fins, microchannels, integrated spot cooling and miniaturized cryode-
vices
.
improving the heat transfer capability or efficiency of working fluids.
The effectiveness of updating the design of cooling devices as a
conventional method to increase the heat transfer rate, however, has
reached a limit (Eastman et al., 2004). With increasing demands for
machines and devices to operate efficiently, the development of new heat
transfer fluids with higher thermal conductivity and greater cooling
efficiency is now an absolute necessity. Most modern large-scale energy
production systems are reliant on the effective working of heat transfer
fluids and any enhancement in their properties would directly benefit current
energy production. Conventional heat transfer fluids have very poor thermal
properties, and improving their thermal properties is thus a key area of
research. Historically, thermal properties of colloidal systems have been of
little interest to the scientific community. However, due to recent advances
in nanoparticle colloid production, such fluids are being explored for new
uses like heat transfer. The use of solid particles as an additive into the base
fluid is one technique for thermal property enhancement.
As solids possess very high thermal conductivity in comparison to
conventional heat transfer fluids (Fig. 10.1), it is expected that thermal
properties should be enhanced by dispersing solids in fluids. Since this idea
was introduced by Maxwell (1904), many scientists and researchers have
made continued efforts to increase the thermal conductivity of fluids by
dispersing millimeter or micrometer sized particles in the fluids. Initially,
experiments started by blending milli- or micrometer sized particles in fluids
to form suspensions. Maxwell (1773) was a pioneer in this area who
presented a theoretical basis for calculating the effective thermal con-
ductivity of a suspension. His efforts were followed by numerous theoretical
and experimental studies, such as those by Hamilton and Crosser (1962) and
Wasp et al. (1977). These models work very well in predicting the thermal
conductivity of slurries.
However, these studies were limited to the suspension of micro- to macro-
sized particles, and such suspensions bear a number of disadvantages,
including:
.
rapid settling of the coarse particles from suspension when not in use/
circulation
.
abrasion of the surface of the channels caused by these particles
