Environmental Engineering Reference
In-Depth Information
Fig. 6.11 The temperature difference between the magnetocaloric material and the wall of the
microchannel heat exchanger for the thermal contact resistance between the thermal diode and the
wall of the microchannel heat exchanger R contact = 0.01 m 2 KkW 1
on the contact (e.g. 3 MPa for the smooth surface of aluminium material) [ 66 ]. As
can be seen from Fig. 6.11 , in order to achieve a temperature difference smaller than
the considered adiabatic temperature change of 4 K, the gap thickness should be as
small as possible.
A strong in
m, comes
from the thermal conductivity of the contact thermal switch. Since we consider the
thermal diode mechanism to be applied for high-frequency operation of the mag-
netic refrigerator, a frequency of 20 Hz can be taken as a minimum value. For such
a frequency, using the case considered in Fig. 6.11 , the thermal switch will provide
an ef
fl
uence, especially for a gap thickness larger than 100
μ
cient operation over a wide range of thermal conductivities as well as gap
thicknesses. However, the increase in the frequency of the operation to 50 Hz will
drastically reduce the applicability to gap thicknesses, which correspond to
dimensions that are smaller than 100 microns.
The contact thermal resistance of R contact = 0.05 m 2 KkW 1 will, in most cases,
still require a contact pressure; however, smaller than in the case of R contact = 0.01
m 2 KkW 1 . In this particular case, at a frequency of the operation equal to 20 Hz,
the contact thermal switch will also operate efciently for a broad range of
parameters; however, an increase in the frequency to 50 Hz will limit its applica-
tion, mostly to a gap thickness of about 10 microns or smaller, especially for
thermal conductivities of the thermal switch that are lower than 10 Wm 1 K 1
(Fig. 6.12 ).
A further increase in the thermal contact resistance to R contact = 0.1 m 2 KkW 1
will further decrease the required contact pressure. However, such a contact
resistance will enable the ef
cient application of a thermal switch only for fre-
quencies of 20 Hz and gap thicknesses below 100 microns. For a frequency of
operation equal to 50 Hz, the contact thermal switch application is feasible only at
thicknesses up to 100 microns for high thermal conductivities (i.e. between 10 and
100 Wm 1 K 1 ) for any thermal conductivities below 10 Wm 1 K 1 , the thickness
should be up to a few tens of microns (Fig. 6.13 ).
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