Magnetocaloric Fluids - Magnetocaloric Energy Conversion: From Theory to Applications - page 198

uids for the magnetorheologic magnetocaloric refrigerant are
actually liquid metals. Since mercury is toxic, one could think of other

The best carrier fl

fl uids, based

on NaK or gallium-indium alloys. For the rst, one should take care about the

hazard of applying NaK and its potential contact with water. For the second, one
should keep in mind the high costs of such a fl uid.

The pumping system for these kinds of fl uids should be carefully selected, and

one should not neglect the possibility of applying the magnetohydrodynamic

principle.

5.5.2 Applications of Ferro fl uids (Including Magnetocaloric

Ferro fl uids)

In this particular case, the applications will be restricted to a small volume fraction
of the solid particles. Therefore, the effect that can be provided by magnetorheo-
logic

fl uids for seals, valve or pump systems, would not actually be very ef cient

with the application of ferro fl uids. Despite what we may think about the application

of ferro uids, our investigations unfortunately show that the

effect of the higher heat-transfer coef

fl uids for heat-transfer fl

cient will be neglected by the higher friction

losses. Therefore, the overall ef ciency of the magnetocaloric device will not be

better than that of applying a cheaper and natural uid, for example, water.

Because of the above facts, we may think about a few basic applications where
magnetocaloric

fl

fl uids can be applied.

5.5.2.1 Magnetocaloric Ferro fl uids as Refrigerants

In this case the facts that we have pointed out for magnetorheologic refrigerants
have to be considered. However, the rheological properties of the ferro

uid offer
better possibilities for application of different pumping systems. The friction during
the

fl

uids.
Because of the small volume fraction of the solid magnetocaloric material in the
ferro

fl ow of such a fl uid will be smaller compared to magnetorheologic fl

uid
should be liquid metal (when speaking about near-room-temperature applications).
It is therefore not a coincidence that Rosensweig [ 15 ] considered the NaK

fl uid, in most cases when the ferro fl uid is applied as refrigerant, the base fl

fl uid to

be a base fl uid for the magnetocaloric ferro fl uid. In this case, a permanent-magnetic

eld sources can also be applied, but they should provide very high magnetic fl ux

density of about 2.5 - 3 T. So high magnetic fl ux density will certainly increase the

ef ciency. However, the cost and the weight restriction of permanent magnets are

too large. Oils or other water-based fl uids represent too large thermal mass, which

will be re fl ected in the small overall magnetocaloric effect of the magnetocaloric