Environmental Engineering Reference
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Coulomb forces. However, because of the viscous forces, the rest of the
fl
uid also
fl
ows with it.
The electroosmotic principle can be used in many different applications in mi-
cro
uidics, e.g. pumping, valving, mixing, splitting, etc. However, this kind of
mechanism may not serve well as the thermal diode mechanism, but the potential to
manipulate the
fl
fl
ow and to enhance the heat-transfer coef
cient might lead to some
future solutions for thermal diode mechanisms.
6.4.2 Ferrohydrodynamics
Ferrohydrodynamics (FHD) relates to the
eld.
This domain should not be confused with magnetohydrodynamics (MHD) (see also
the next subsection on MHD). In FHD there is no need to apply an electrical current
to the
fl
uid motion induced by a magnetic
fl
uid. The driving force that induces the
fl
uid
fl
ow is related to the material
magnetization in an alternating magnetic
eld. More information about the physics
behind ferrohydronamics can be found in Chap.
5
on magnetocaloric
uids. We
also refer to the topic of Rosensweig [
101
], which describes important, pioneering
work carried out since the 1960s in this area.
Ferrohydrodynamics is associated with collodial suspensions of nanofluids that
are formed by magnetic nanosized particles, suspended in the base liquid (e.g.
organic solvents, water). These suspensions are also referred to as ferro
fl
uids. The
particles do not settle (stratify) due to the presence of Brownian motion. They are
also coated with a surfactant to prevent clumping. However, the volume fraction of
nanoparticles in the base liquid is rather small (high volume fractions lead to
clustering), which prevents ferro
fl
uids from being used as successful liquid mag-
netocaloric refrigerants for magnetic refrigeration, as the replacement of the active
magnetic regenerator (of course this relates to the present state of development of
magnetocaloric materials and the associated magnetocaloric effect).
Nano
fl
fl
uids are used as heat-transfer
fl
uids in many applications [
102
104
]. This
-
also holds true for ferro
fl
uids [
105
107
]. For more information on ferro
fl
uids, the
-
reader is referred to the chapter on magnetocaloric
uids, as well as topics by
Rosensweig [
101
] and Odenbach [
108
,
109
]. In this subsection we do not focus on
the behaviour of ferro
fl
uids or their applications to enhance heat transfer, but
instead deal with mechanisms that can be used as ferro
fl
fl
uid thermal switches or
ferro
uid thermal diodes.
A review of the principles of micro magneto
fl
fl
uids can be found in Nguyen
[
110
]. Among the various applications of ferro
uids in magnetic refrigeration, an
interesting solution that represents an enhanced heat transfer as well as the appli-
cation of a thermal switch by the ferro
fl
ow was given by Kitanovski and Egolf
[
4
]. This example is shown in Fig.
6.22
. In the case of Fig.
6.22
, four active
magnetic regenerators (AMRs) are placed inside the cylinder and are
fl
uid
fl
lled with the
ferro
fl
uid. Two cross-
fl
ow micro heat exchangers connected to the heat sink (not
shown in the
gure) comprise the water
fl
uid channels and voids that contain the
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