Experimental and Numerical Analysis of a PCM-Supported ...

evaporation rate on the hot water load with reference to air as a very poor
conductive medium.
As it can be seen in these figures in comparison with air, it is clear that the
evaporation rate increases at higher solid thermal conductivity. Nevertheless, the
proportionality of evaporation rate is neither uniform with k s nor linear with the water
mass flow rate. It is also important to point out that maximum evaporation rate
occurs when k s is around 1.14 W.m -1 .K -1 as for the case of Pyrex glass and fired clay
brick as listed in table (7.2) . It seems that around this value of k s , the best balance
between sensible and latent heat components at different interfaces can be
obtained. This means that the evaporation rate is enhanced at the expense of the
sensible heat component at the liquid-gas interface due to increasing the sensible
heat transfer at the solid-gas interface with better stratification in the bed. As the
thermal conductivity is further increased, this optimum balance is broken down and
evaporation rate is decreased.
On comparing the productivities of the Pyrex glass and fired clay bricks, it is
recommended that, using smaller size (40-50 mm diameter) of spherical fired clay
bricks and would be the ideal candidate for the present application due to its lower
cost, rather than the Pyrex glass which is quite expensive, heavy, and has high
vulnerability for breaking.
In figure (7.11), the highly conductive media such as aluminium and iron show a
declining trend for the percentage improvement in the evaporation rate over the air,
as an approximately non-conductive medium, when increasing the water load. The
lower conductive media show a peak at 500 l/h, and then follow the same declining
trend as the water load is further increased. Although increasing the water load
increase the productivity due to increasing the wetted area and heat and mass
transfer coefficients at water-air interface, the area of dry patches decrease with the
same magnitude of increasing the wetted area which decreases the heat transfer at
the solid-air interface and hence the percentage improvement over the air spheres
decreases as well. The phenomenological process known as the MEHH, which is an
important topic addressed in the current study, stands behind this interpretation.
Table 7.2: Effect of different packing media on the evaporation rate (Mhw=1000 [l/h])
Glass, Brick
PCM (HS
Medium Properties
Iron Aluminum Water
Air
Pyrex (fired clay)
58)
k (W/m.K) 1.14 1.13 70 229 0.64 0.6 0.027
ρ (kg/m 3 ) 2240 2310 7880 2701.1 1000 1280 1.084
c (J/kg) 840 922 511 938.3 4180 2.51E+05 1015
M d [l/h] 62.52 62.38 60.87 59.92 60.73 59.82 56.89
M d Increase (%)
relative to air
9.9% 9.65% 6.99% 5.32% 6.74% 5.14% 0.00%
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