Experimental and Numerical Analysis of a PCM-Supported ...
Experimental and Numerical Analysis of a PCM-Supported ...
Experimental and Numerical Analysis of a PCM-Supported ...
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A<br />
m <br />
w<br />
= the plan or cross sectional area <strong>of</strong> the column<br />
= the mass flow rate <strong>of</strong> water<br />
After Singham (56), equation (D.4) will be considered as the basic formulation <strong>of</strong><br />
Merkel’s equation. The main designer task is to ensure equality <strong>of</strong> the two sides <strong>of</strong><br />
this equation (i.e. I M <strong>and</strong> I P ).<br />
D.1.2 Number <strong>of</strong> transfer units (NTU)<br />
Tow closely related expressions for the right h<strong>and</strong> side integr<strong>and</strong> I P are commonly<br />
used for practical purposes. The first practice expresses the performance <strong>of</strong> the<br />
packing as the number <strong>of</strong> transfer units (NTU), while the second measures the heat<br />
transfer performance as a packing dimensionless characteristic quantity (KaV sp /L),<br />
where K is the overall mass transfer coefficient between water <strong>and</strong> air based on the<br />
enthalpy difference (kg.m -2 .s -1 ), a is the packing specific surface area, V sp is the<br />
packed bed volume per unit cross sectional area, <strong>and</strong> L is the water superficial mass<br />
flux per unit cross sectional area (kg.m -2 .s -1 ).<br />
According to the first practice, definitions <strong>of</strong> the effectiveness (NTU) is in close<br />
similarity with those used for the heat exchanger design, hence is defined by [56]:<br />
NTU<br />
a e<br />
(D.5)<br />
G<br />
where G is the gas superficial mass flux per unit cross sectional area (kg.m -2 .s -1 ),<br />
<strong>and</strong> the dimensionless quantity , which is a function <strong>of</strong> Reynolds number, is a<br />
G<br />
form <strong>of</strong> nondimensional heat transfer coefficient Stanton number defined as:<br />
hlg<br />
St (D.6)<br />
Gc p , g<br />
Thus NTU can be regarded as the product <strong>of</strong> Stanton number <strong>and</strong> a geometric<br />
feature <strong>of</strong> the packing a (56). A modified Stanton number based on the effective<br />
wetted area a w instead <strong>of</strong> the total specific packing area a is also used [57]:<br />
St<br />
<br />
<br />
Gc<br />
hlga<br />
aw<br />
<br />
V<br />
p,g<br />
<br />
<br />
<br />
(D.7)<br />
where<br />
h lg<br />
a is the volumetric heat transfer coefficient, <strong>and</strong> V is the packed bed<br />
volume. The relationship between the two performance measures was shown to be<br />
[56]:<br />
203