Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 102

also requires a simultaneous fl uid fl ow during the (de)magnetization process. This

means that the fl uid fl ows during all four processes, i.e. isothermal magnetization

(a b), iso eld fl uid fl ow at a higher magnetic eld (b c), isothermal demagneti-

- -

zation (c d) and iso eld fl uid fl ow at a low magnetic eld (d a), see Fig. 4.7 .It

- -

should be noted that during the demagnetization and iso eld fl uid fl ow, the fl uid

fl ow is in the counter- fl ow direction (v f < 0) compared to the magnetization and

iso ow (v f > 0), as is also shown in Fig. 4.7 b. A similar idea of com-

bining the (de)magnetization and the

eld fl uid fl

ow process, despite not being exactly
related to the study of thermodynamic cycles, has been presented and analysed by
Bj

fl uid fl

rk and Engelbrecht [ 18 ].
In order to ensure isothermal (de)magnetization (or at least to be close to it), the
mass

ø

ow during that time must be appropriate for the
magnetocaloric effect. However, since the magnetocaloric effect along the AMR
and its properties (mostly the speci

fl ow rate of the fl uid fl

c heat) are not constant, but rather strongly
temperature dependent, it is practically impossible to ensure a truly isothermal (de)
magnetization process with a spatially constant mass ow rate (which is

unavoidable in real devices) along the entire length of the AMR. Therefore, we can
only talk about quasi-isothermal (de)magnetization. In order to ensure the best
possible conditions for the isothermal (de)magnetization, the required

fl

fl uid fl ow

during the (de)magnetization process can differ from the optimum fl uid fl ow during

the isoeld process.

4.1.2 Characteristics of a Hybrid Brayton - Ericsson-like
AMR Cycle

By combining the best features of the Brayton-like and Ericsson-like AMR cycles,
we are led to the development of a Hybrid Brayton Ericsson-like AMR cycle

(Fig. 4.8 )[ 9 , 10 ]. In this particular case, the thermodynamic cycle consists of six
processes, as follows: adiabatic magnetization (a

-

b), isothermal magnetization

-

(b c), iso eld fl uid fl ow at a high magnetic eld (c d), adiabatic demagnetization

- -

(d e), isothermal demagnetization (e f), and iso eld fl uid fl ow at a low magnetic

- -

Ericsson-like AMR cycle is, in the periodic steady
state, schematically represented in the T

eld (f a). The Hybrid Brayton

- -

s diagram in Fig. 4.8 a, together with the

-

required magnetic ow regime (Fig. 4.8 b).

In the case of such a cycle, the (de)magnetization process is divided into the
adiabatic and the isothermal process (like in the case of the Carnot-like AMR cycle,
as shown later). The AMR is

eld pro le and fl uid fl

rst magnetized to certain but not the maximum,

magnetic ow). A subsequent

process of magnetization is then performed quasi-isothermally with a simultaneous
fl

eld in a quasi-adiabatic process (without fl uid fl

uid fl ow. The process of demagnetization is analogous ( rst adiabatic and then

isothermal with the counter- ow). The ratio of the

adiabatic and isothermal (de)magnetization should be optimized for each particular

fl ow direction of the fl uid fl