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

packing where it gets in direct contact with the concurrent flow of hot humid air at
liquid-gas interface. The air continuously circulates in closed loop cycles, as a carrier
medium, to transfer the generated water vapor from the evaporator to the condenser
where it is condensed back into highly purified fresh water.
3.3 Concept of Multi-Effects of Heating and Humidification (MEHH)
The main physical difference between the two phase flow in the PCM evaporator
and condenser investigated in the present study and a conventional packed bed is
that the latter is equipped with a nearly non-conductive packing elements. The nonconductive
packing usually has a negligible heat capacity. This implies that the
energy balance and energy flow in the conventional packed beds are only governed
by the interaction between both fluid phases (i.e. hot and cold streams).
On the other side, for the PCM (or any other conductive material) as packing media,
the solid phase contributes to the energy balance and the interstitial energy flow
between fluid phases. In the latter, since there are wetted areas and dry patches,
there is simultaneous heat and mass transfer entirely between all phases in both
evaporator and condenser. Therefore, the physical problem under investigation
involves multiscales; on the pore scale level or the micro scale as well as on the
macro scale level or the overall balance which mutually affects the sensible and
latent heat components at different interfaces on the micro-scale level.
3.3.1 Micro scale level
Existence of PCM packing (or any conductive medium) in the evaporator and
condenser creates a parallel path for heat and mass transfer between gas and liquid
through the packing elements due to partial wetting of the packing surfaces as
shown in figure 3.2. Let the subscript “s” identify the solid particles and subscripts “l”
and “g” designate the liquid water and humid air (air-water vapor gas mixture)
respectively.
Consider a rectangular finite slab which has thermal conductivity k covered on one of
its sides by hot liquid water and subjected to ambient air on the other side as shown
in figure (3.2b).
Due to the higher liquid temperature, there will be two sensible heat transfer
components between water and gas (Q lg ) and water and solid (Q ls ).
The interaction between gas and solid phases (Q gs ) arises due to the
temperature difference between both of them.
At the liquid gas interface, the latent heat of vaporization becomes part of the
energy balance, and the mass diffusion (m v ) caused by the vapor pressure
gradient affects both heat and mass balances.
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