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

temperature is at its maximum level, and moves downwards due to stratification of
both liquid and gas phases as well as the low effective thermal conductivity of PCM
along the packing height. Thereafter, a sensible heat component increases the PCM
temperature until reaching the steady state conditions. Since the melting front moves
in time downward in the bed, this looks like propagating waves or a “sea snake”
rising up from the bottom. Little peaks of temperatures above the melting range can
be observed near the top of the bed, these may represent some locations just
around the breakeven points during the transient period with the gas phase
temperature profile (reference is made to figure 5.4), and perhaps also due to
melting of PCM over a mushy region rather than a sharp melting front.
Table 6.4: Inlet and geometrical parameters within a sample simulation run
Parameter Value Units
Inlet water temperature 358 K
Water flow rate 400 l/hr
Packed height 0.39 m
Column diameter 0.4 m
Inlet and initial air temperature 298 K
Initial PCM temperature 298 K
Air velocity 0.13 m/s
Figure (6.7) shows the temperature history of liquid water along the packing height.
The water temperature decreases through its path from top to bottom, where it
reaches the minimum. The outlet water temperature increases over time due to
continuous decrease in the temperature difference and heat transfer rates between
water and both gas and solid phases. Figures (6.8) and (6.9) represent the
temperature and concentration fields respectively of the gas phase during
simultaneous heating and humidification processes. It can be seen from the figure
that the concentration also increases with time and packed bed height, to reach its
maximum value of 21mol/m 3 . The time evolution and final profiles of the fluid
temperatures and concentration fields is really governed by the existence of a
conductive packing medium and its thermophysical properties, otherwise the system
would behave stationary and reach steady state in a very short time like other
distillation columns and cooling towers packed with non-conductive media.
6.5 Conclusions
In this chapter, the numerical solution procedures and strategy were introduced and
applied for solving the governing energy and mass balances of the evaporator
mathematical model as an example. Geometric representation of the complex
packed bed geometry was replaced by representation of uni-dimensional domain.
This enabled to solve the mathematical model both in COMSOL Multiphysics and
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