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

In the second case, water heat capacity flow is decreased due to lower inlet hot
water mass flow rate, while the air flow rate is maintained high. When Air flow rate
has two impacts on the system performance, where the energy transfer rate through
dry batches between solid and air (i.e. MEHH) increases with increasing air flow
rate, both sensible and latent heat transfer at the air-liquid interface increases until
reaching an optimum value of air/water mass flow ratio. Beyond this point the high
sensible heat transfer rate negatively influences (i.e. comes at the expense) of the
mass transfer rate at the interface. Moreover, most of the heat and mass transfer
take place at the upper region of the evaporator, which causes the solid packing in
the lower part of the column to be colder than the gas phase.
Table 5.1: Chosen variable parameters for experiments and range of values
Parameter Units Low level High level
Mass flow of hot water “m hw ” l/h 250 500
Corresponding water mass flux “L” kg /m 2 .s 0.55 1.1
Temperature of inflow hot water “T w2 ” ºC 65 83
Mass flow of cold water “m cw ” l/h 370 670
Air velocity “v a ” m/s 0.23 0.55
Corresponding air mass flux “G” kg /m 2 .s 0.25 0.59
Packing height “Z” m 0.38 0.78
Table 5.2: Basic sets of designed experiments according to boundary conditions
Case No.
Parameter 1 2 3 4 5 6
Air velocity “v a ” (m/s) 0,55 0,55 0,23 0,23 0,55 0,23
Mass flow of hot water “m hw ” 500 250 500 250 500 500
Temperature of inlet hot water “T w2 ” (ºC) 83 83 83 83 65 65
Mass flow of cold water “m cw ” (l/h) 670 670 670 670 670 670
When lowering the water mass flow rate the gas-solid interfacial area increases,
while the wetted area (between water and solid) decreases. At constant inlet hot
water temperature and constant air flow rate, this has two opposing effects; as it
leads to higher rate of energy retrieval from solid to gas, but decreasing the water
heat capacity flow rate decreases all sensible and latent heat transfer components in
the system as well as thermal stratification along the packing height.
In the third case of boundary conditions, water heat capacity flow rate is maintained
at the highest level, while air velocity is low which lowers all sensible and latent heat
components between the gas phase and both solid and liquid phases.
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