Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 225

Fig. 6.12 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 = 0.05 m 2 KkW − 1

Fig. 6.13 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 = 0.1 m 2 KkW − 1

Based on the results of this simple example we can show that the operation of
the contact thermal switch should be limited to rather small distances (e.g. less than
100 microns) between the thermal switch in the off mode and the wall of the
microchannel heat exchanger. In most cases we should try to employ thermal
conductivities of the thermal switch that are higher than 10 Wm − 1 K − 1 . Furthermore,
almost all applications of contact thermal switches will require pressure contacts.
These facts can signi

cantly limit the applicability of contact thermal switches,
especially because we have evaluated a rather simpli ed case by neglecting, e.g. the

thermal mass of the system and the contact resistances between the thermal switch
and the magnetocaloric material. Furthermore, we have neglected the heat fl ux in

the opposite direction, especially because the recti cation factors (the ratio between

the thermal resistance in the ON and OFF operations) were obtained as rather high,
i.e. around 8 for a gap thickness of 1 micron, around 80 for a gap thickness of 10
microns, and between 260 and 800 for a gap thickness of 100 microns.