Environmental Engineering Reference
In-Depth Information
indium and tin), different alcohols (ethanol, ethylene glycol, propylene glycol, and
glycerol) and different mixtures of water and ethanol. They concluded that the
AMR shows by far the best cooling characteristics if liquid metals are used as a
working
fl
uid (especially at higher operating frequencies (>2 Hz)), while among
other
uids pure water showed the best performance. Similar conclusions were
obtained also by Silva et al. [ 127 ], who theoretically compared the performance of
different
fl
uids (water, mercury, gallium and different alcohols) used in a high-
frequency micro-size magnetocaloric refrigerator. They concluded that gallium
shows even better performance than mercury, which is highly toxic and in any case
cannot be used in a real magnetic refrigeration or heat pump device.
This section presents a numerical comparison of the AMR performance with six
different heat-transfer
fl
fl
uids. The analysed
fl
uids and their relevant thermohydraulic
properties are shown in Table 4.7 .
The analysis was performed using the AMR numerical model [ 16 , 17 ] with
gadolinium as the magnetocaloric material in two different geometries (packed-bed
and parallel-plate) and 1 T of magnetic
eld change. The packed-bed AMR is
constructed with spheres of 0.25 mm diameter, while the parallel-plate AMR with
plates having a thickness of 0.25 mm and a spacing of 0.1 mm. The outer
dimensions of both AMRs are: 80 mm (length)
×
×
40 mm (width)
10 mm (height).
uids were selected with respect to the current state of the art.
Since the majority of the magnetic refrigerator prototypes apply water or a mixture
of water with different alcohols, we chose water, two mixtures of water and ethanol
and pure ethylene glycol. Silicone oil was selected as it is often used in various
hydraulic and thermal applications. Since some earlier works [ 15 , 119 , 126 , 127 ]
showed a clear advantage when applying liquid metals as the working
The evaluated
fl
uid in the
magnetic refrigerator (mercury and gallium) we evaluated Galinstan (mainly con-
sisting of gallium, indium, and tin), which has low toxicity. Lately, it has been used
as a replacement for mercury and has the potential to be used in a future magnetic
refrigerator or a heat pump.
fl
Table 4.7 Thermohydraulic properties of the working fluids evaluated in the analysis
Speci c
heat
(Jkg 1 K 1 )
Density
(kgm 3 )
Thermal con-
ductivity
(Wm 1 K 1 )
Thermal dif-
fusivity
(m 2 s 1 )
Dynamic
viscosity
(Pas)
1.44 × 10 7
1.00 × 10 3
Water
4,180
998
0.599
10 7
10 3
Water + wt.
20 % ethanol
4,350
968
0.465
1.10
×
2.23
×
10 7
10 3
Water + wt.
50 % ethanol
3,470
922
0.342
1.07
×
2.31
×
10 7
10 2
Ethylene
glycol
2,303
1,120
0.303
1.17
×
3.00
×
1.18 × 10 7
1.29 × 10 3
Silicone oil
1,620
855
0.163
10 4
10 3
Galinstan
370
6,440
16.05
6.74
×
2.40
×
Search WWH ::




Custom Search