Environmental Engineering Reference
In-Depth Information
The strategy of adding PCM particles to improve thermal performance of heat
transfer fluids has been pursued [ 5 , 7 , 11 - 16 ]. The need for high-performance heat
transfer fluids is driven by the increasing thermal management demand. The
advances in semiconductor materials and more precise fabrication techniques have
the unfortunate side effect of generating higher amounts of waste heat within a
smaller volume. The need for higher capacity and more complex cooling systems
is limiting the full potential of the advances in electronics and power electronics.
The solution is the development of significantly improved heat transfer systems
and their kernel components. One important component of these heat transfer
systems is the cooling fluid used inside. The inherently poor heat transfer prop-
erties of some of the coolants, lubricants, oils, and other heat transfer fluids used in
today's thermal systems limit the capacity and compactness of the heat exchangers
that use these fluids. Therefore, there is an urgent need for innovative heat transfer
fluids with improved thermal properties over those currently available.
This chapter discusses several types of PCM particles that can be potentially
used in the application of heat transfer fluids. These PCM particles can be cate-
gorized into three groups: solid-liquid phase change particles, solid-solid phase
change particles, and liquid-vapor phase change droplets. This chapter starts with
the introduction of two types of PCM microcapsules: paraffin-polymer and neo-
pentyl glycol (NPG)-silica microcapsules. Then the low-melting metallic nano-
particles and NPG nanoparticles, which both are synthesized by physical methods,
are discussed [ 5 , 7 ]. The last part of this chapter is dedicated to phase-changeable
nanoemulsion fluids, in which the dispersed nanodroplets can be liquid-vapor
PCM or liquid-solid PCM, depending on the PCM and the operating temperature
[ 11 - 16 ]. Both material synthesis and property characterization of these PCM
microcapsules and particles will be covered. This chapter is not intended to serve
as a complete description of all phase-changeable particles available for heat
transfer applications. The selection of the coverage was influenced by the research
focus of the authors and reflects their assessment of the field.
2 Use of PCMs to Increase the Fluid Thermal Properties
Most research on thermal fluids to date has focused on how to increase thermal
conductivity [ 17 - 19 ]. This chapter will introduce the concept of using PCM parti-
cles to increase the effective heat capacity as well as the effective thermal con-
ductivity of thermal fluids. Both the heat capacity and thermal conductivity of the
fluids strongly influence the heat transfer coefficient. For example, for a fully
developed turbulent flow of a single-phase fluid, the convective heat transfer
coefficient, h, can be described in terms of thermal conductivity, k f , and specific heat,
C f ,ash / k 0 : 6
f C 0 : f [ 20 ]. If the particles are made of PCMs, the effective specific heat
of the PCM fluid will be increased by a factor of 1 รพ a H PCM
DT C f , where a is the weight
fraction of the PCM nanoparticles in the fluid, H PCM is the latent heat of the PCM per
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