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

Table 7.1: Reference boundary conditions for the evaporator and condenser
Parameter Units Value
Mass flow of hot water “M hw ” kg/h 1000
Mass flow of cold water “M cw ” kg/h 1000
Velocity of air “v a ” m/s 0.55
Temperature of hot seawater water “T w2 ” ºC 83
Temperature of cooling water “T w3 ” ºC 25
Temperature of inlet air to the evaporator “T a1 ” ºC 25
Temperature of inlet air to the condenser “T a2 ” ºC 66
7.2 Evaporator Performance
Fundamental variables and critical parameters mentioned above and the effect of
using different types of packing media will be discussed and analysed. For each
case, only the value of each targeted parameter was varied, while other parameters
were kept constant at the specified value.
7.2.1 Effect of packing size
It is known that there is an inverse relationship between packing diameter and
specific surface area (i.e. packing area per unit volume of the bed; m 2 /m 3 ), as
depicted in figure (7.1). For a specific column diameter, all heat and mass transfer
coefficients depend on the specific area of the packing (a), as well as the packing
diameter itself, according to equations (4.43) and (4.48) to (4.50). As the flow rate
and diameter of spherical particles are increased under a given diameter of the
packed column, the mean heat and mass transfer coefficients increase, while the
interfacial areas of contact decrease, such that it becomes difficult to optimize the
bed characteristics.
In conventional packed beds, a is inversely related to both the porosity (ε) and
particle size through equation (4.48), while the porosity in turn is directly related to
the particle diameter as suggested by equation (4.43). Combination of all of these
considerations with energy and mass balance equations (4.14) to (4.17) shows the
opposing effects that the particle size and specific surface area have on the thermal
performance of the system. The effect of packing diameter on the evaporation rate is
demonstrated in figure (7.2), which clearly explains why heat and mass transfer rate
densities do not increase substantially in consistence with figure (7.1) as the
dimension of the sphere diameter (d) becomes smaller especially under lower mass
flow rates of hot water. The productivity decreases on increasing the packing
diameter, but this effect becomes more significant at higher mass flow rates.
However, it is excepted that smaller packing size leads to a higher heat capacity flow
between hot water and air, but it seems that its effect is not the predominating one.
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