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

It could be interpreted that this particular behavior of PCM based system represents
some first signs on the potentially important role of establishing multiple effects of
heating and humidification due to existence of thermally conductive elements in the
system. It is worth to mention that this behavior can also be realized using other
kinds of sensible heat storage or conductive media since it is not associated with the
solid-liquid phase change phenomenon inside the packing medium but rather related
to its thermal conductivity.
The phenomena of multiple effects of heating/humidification and
cooling/dehumidification have been elaborated in chapters 3 and 4. For the PCM
packing there is complex interaction between various boundary conditions and the
geometrical parameters. For instance, increasing the water mass flow rate increases
the wetted area, enhances axial thermal stratification in all phases in the bed, and
increases the solid-gas temperature difference while decreasing the dry interfacial
area between solid and gas phases.
On the other hand, for a specific inlet hot water temperature and mass flow rate, the
heat transfer between air and both water and solid phases depend on the air mass
flow rate, which in turn has effects on the heat flow between water and solid. As
mentioned earlier, there is a trade-off between direct contact heat and mass transfer
at the liquid-gas interface and indirect heat and mass transfer resulting from the
MEHH phenomena. When the water vapor diffusion at the liquid-gas interface
dominates, for instance due to large interfacial area or high mass transfer coefficient
or both, this will come at the expense of the MEHH and vice versa.
The energy flow between all and each pair of phases plays the major role in plotting
the system behavior under each specific boundary and geometrical conditions. For
instance, the energy flows in the evaporator are mainly dependent on:
The heat capacity flow of inlet hot water,
Air to water mass flow ratio,
Interfacial areas,
Packing height, and
The thermal conductivity of the packing (i.e. Biot numbers).
The maximum sensible heat transfer rates between solid and gas occur if the PCM
is infinitely conductive and fluid-solid heat transfer coefficients are infinite. In this
case the temperature of the PCM will be always at the heat source temperature (i.e.
water in the evaporator or gas in the condenser), which may give rise to the MEHH
to dominate. These particular characteristics have to be seen as the basic
background for describing and reading the comparative results between PCM (as a
conductive medium) and Non-PCM packing types.
115