Environmental Engineering Reference
In-Depth Information
familiar with thermal diode mechanisms, so as to understand this particular appli-
cation of magnetorheological
uid can be suc-
cessfully applied as a thermal switch or thermal valve to provide oscillating heat
fl
fl
uids. The magnetorheological
fl
ux to or from the magnetocaloric material. The activation or deactivation of such a
mechanism will be based on the oscillation (alternation) of the magnetic
eld
intensity. In order to provide an ef
uid
should possess high effective thermal conductivity and, on the other hand, small
thermal mass. The latter is required to enable rapid transfer of heat from or to the
magnetorheological thermal diode. Note that the application of the magnetorhe-
ological
cient heat transfer, the magnetorheologic
fl
uid in this case does not necessarily concern the macroscopic, but rather a
microscopic system.
fl
5.5.1.3 Magnetorheologic Fluids as Actuators for Pump Systems
An advantage of applying a magnetorheologic
uid as pump actuators for pump
systems in magnetocaloric devices is that they may be integrated into the system, as
well as because of the fact that the variation of the magnetic
fl
eld in magnetocaloric
devices, based on the AMR principle, are strongly related to the
fl
uid
fl
ow through
the porous structure of the AMR. The magnetorheologic
uid can therefore serve as
the piston in such systems, or the actuator of the peristaltic pump, membrane pump
or other mechanisms. Note that the movement of
fl
uids (e.g. water) in the AMR
relates to rather small oscillations. In terms of distances that the water has to pass in
a single process, this corresponds to about 10
fl
40 mm, e.g. for a higher frequency of
the operation of a device and depending on the effectiveness of the AMR.
-
5.5.1.4 Magnetorheologic Magnetocaloric Fluids as Refrigerants
In this case one should take care of the thermal mass of the carrier
fl
uid. The
speci
c heat of magnetocaloric materials is small, compared to, e.g. water or oils.
Therefore, the carrier
uid should possess high thermal conductivity and small
thermal mass. The volume fraction of the magnetocaloric material should be high,
but one should also note that this strongly in
fl
fl
uences the rheology of such a
fl
uid.
Also, the magnetic
eld in such devices should be rather high, certainly above
1
1.5 T. If the magnetic
eld is too low, the magnetocaloric effect will not provide
-
suf
uid; therefore, the overall temperature change
due to the magnetocaloric effect will be rather small or not suf
cient heat to affect the carrier
fl
cient.
If some surfactants are applied to provide better dispersion of the magnetocaloric
particles, one should take care of their thermal conductivity, since it may represent
the thermal resistance for heat diffusion from the magnetocaloric particle to the
carrier
uid. This holds true also for magnetocaloric particles, where the smaller
size will have a strong in
fl
uence on the desired rapid diffusion of the magnetoca-
loric effect towards the carrier
fl
fl
uid and vice versa.
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