Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 125

4.3.4.2 The Flow Maldistribution

The fl ow maldistribution, also referred to as fl ow channelling, results from a non-

uniform porosity distribution along the fl uid fl ow direction in the AMR. It mostly

depends on the geometry and the ow rate. For a packed-bed AMR, the

porosity near the wall is typically greater than in the middle of the AMR. Due to the
smaller pressure drop near the wall the velocity is increased and the resulting cold
or hot bypasses decrease the regeneration ef

fl uid fl

ow maldis-
tribution in a packed-bed AMR strongly depends on the ratio between the outer
AMR dimensions and the particle diameter [ 103 ]. We do not know whether the
impact of the

ciently [ 33 , 102 ]. The fl

ow maldistribution has yet been directly included in the numerical
model of a packed-bed AMR. However, there are several models which enable the
calculation of the radial porosity distribution [ 104 ] as well as the radial velocity
distribution in the packed bed [ 105 ], but since the great majority of the packed-bed
AMRs are 1-D, this cannot be directly applied. As explained previously in this
chapter, in the 1-D models the thermohydraulic properties are included by using
appropriate correlations (mostly Nusselt number and friction factor correlations).
Since these correlations are also usually obtained through the experimental testing
of a packed bed [ 81 , 106 ] they, to the certain extent, already include the impact of
the

fl

ow
maldistribution, depending on the ratio between the outer AMR dimensions and the
particle diameter and the manufacturing technique applied.
A

fl ow maldistribution. However, each AMR can have a different degree of fl

ow maldistribution is also observed for the parallel-plate AMR and results
from non-uniformly distributed plates causing a non-equal distribution of the
velocity and the

fl

ow. Since the parallel-plate AMRs are usually well into the
micro-channel region, the manufacturing tolerance might be signi

fl uid fl

cant. It was

shown that it can have a very signi cant effect, especially for AMRs with a channel

thickness below 0.3 mm [ 107 ]. Jensen et al. [ 108 ] and Nielsen et al. [ 109 ] devel-
oped a technique and a model that enables an estimation of the reduction of the
Nusselt number due to the ow maldistribution for a particular AMR based on its

plate distribution standard deviation. They suggested the application of the Nusselt
scaling factor, which represents the ratio of the effective Nusselt number of the
particular regenerator and the ideal Nusselt number of the uniformly distributed
regenerator. This can later be directly included in the AMR model of a particular
AMR by multiplying it with the Nusselt number based on the applied correlation
(for which it can be assumed that it is obtained for the uniformly distributed plates).

fl

4.3.4.3 Heat Losses to the Surroundings

As described in Nielsen et al. [ 33 ], most AMR models assume perfect insulation
with respect to the ambient. Thermal interactions with the regenerator housing and
parasitic losses to the surroundings are therefore ignored. However, it was shown