Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 239

Fig. 6.22 a, two AMRs are magnetized. Therefore, the magnetocaloric material
heats, and the heat is partially transferred to the ferro uid by heat conduction.

The magnet assembly rotates in the counter-clockwise direction in order to pull
the ferro

fl

uid from the magnetized AMR to the HHEX (Fig. 6.22 b). There, the heat
is rejected to the heat sink via the water. Since the ferro

fl

fl uid is incompressible, such

a movement also pushes the rest of the ferro fl uid in a cylinder. Therefore, the

ferro fl uid from the demagnetized AMRs fl ows to the CHEX, where it absorbs heat

from the heat source (via water).
The rotation of the magnet assembly is continued in the counter-clockwise
direction until the second pair of AMRs is being magnetized (Fig. 6.22 c). The
second pair of AMRs heats up, and the ferro uid, which absorbed the heat in the

CHEX, moves towards the demagnetized AMRs. Now, the rotation of the magnet
assembly starts in the clockwise direction (Fig. 6.22 d). The ferro

fl

uid from the void
of the magnetized AMRs is now pulled to the HHEX to reject the heat. Simulta-
neously, the other thermodynamic processes are also performed in the cylinder. At
the end (Fig. 6.22 e), the magnet assembly is at its initial position. It is clear that the
described processes above will lead to the simultaneous production of cold, without
the need for any switching valve mechanism. Despite the fact that the ferro

fl

fl uid in

this particular case represents the heat-transfer fl uid, it may also be considered as a

thermal diode mechanism that exploits the ferro ow induced by the magnetic

eld (especially when the consider system in the Fig. 6.22 is small).
Based on the possibility of using ferro

fl uid fl

uids as thermal diode mechanisms, we
can distinguish between the following solutions:

fl

Ferro fl uid thermal diode that applies the anisotropy of thermal conductivity,

￿

ferro fl uid thermal diode that applies the magnetically induced fl uid fl ow,

￿

ferro fl uid thermal diode that acts as the thermal contact switch and

￿

ferro fl uid and magnetowetting principle.

￿

6.4.2.1 Ferro uid Thermal Diode that Applies the Anisotropy

of Thermal Conductivity

fl

This principle is similar to solid-state thermal recti cators. However, in this par-

ticular case the magnetic eld induces thermal recti cation due to the magnetic-

eld-dependent thermal conductivity of the ferro fl uid. Ferro fl uids can therefore be

used as thermal diode recti cators, where the anisotropy of the thermal conductivity

is manipulated with the magnetic eld strength as well as with the direction of the

magnetic eld [ 111 - 114 ]. The recti cation factor is rather low (e.g. up to 300 %) to

be ef ciently applied in magnetic refrigeration. In magnetic refrigeration, the

minimum recti cation factor (the ratio between the thermal resistance in the OFF

and ON operation of the thermal diode) should be at least 10 (1,000 %), or pref-
erably even higher.