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TECHNICAL BRIEFS

Entropy Generation in Counter Flow Gas to Gas Heat Exchangers

[+] Author and Article Information
Hany Ahmed Mohamed

Mechanical Engineering Department, Faculty of Engineering, Assiut University, Assiut, Egypthah@aun.eun.eg

J. Heat Transfer 128(1), 87-92 (Apr 13, 2005) (6 pages) doi:10.1115/1.2130407 History: Received October 02, 2004; Revised April 13, 2005

Analysis of heat transfer and fluid flow thermodynamic irreversibilities is realized on an example of a counter flow double pipe heat exchanger utilizing turbulent air flow as a working fluid. During the process of mathematical model creation and for different working and constructing limitations, total thermodynamic irreversibility is studied. The present work proves that the irreversibility occurred due to unequal capacity flow rates (flow imbalance irreversibility). It is concluded that the heat exchanger should be operated at effectiveness, ε, greater than 0.5 and the well operating conditions will be achieved when ε approaches one where low irreversibility is expected. A new equation is adopted to express the entropy generation numbers for imbalanced heat exchangers of similar design with smallest deviation from the exact value. The results obtained from the new equation are compared with the exact values and with those obtained by Bejan (Bejan, A., 1997, Advanced Engineering Thermodynamics, Wiley, New York).

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 7

Comparisons between Ns values obtained from the new equation with the corresponding exact one at SL=20, Re2=5×104, rP=1, rd=0.5, Ω=1 and different Tr

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Figure 1

Schematic drawing for the double pipe heat exchanger model

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Figure 2

Effect of the heat exchanger effectiveness on the entropy generation number at various temperature ratios for balanced heat exchangers

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Figure 3

Effect of the heat exchanger effectiveness on the entropy generation number at various values of hot stream Reynolds number for balanced heat exchangers

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Figure 4

Effect of the heat exchanger effectiveness on the entropy generation number at various rd, rP and SL for balanced heat exchangers at constant Re2=5×104

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Figure 5

Entropy generation number in a balanced counter flow heat exchanger at zero pressure drops

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Figure 6

The entropy generation number for imbalanced counter flow heat exchangers as dependent on Ω and Tr at zero pressure drop

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Figure 8

Comparisons between the new and Bejan (4) equations with the corresponding exact values at SL=20, Re2=5×104, rP=1, rd=0.5, Ω=1, Tr=0.7–0.9 and ε=0.6–0.9

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Figure 9

Entropy generation number due to temperature difference, NsT, and pressure drops, NsP, against ε at Ω=1 for different Tr

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Figure 10

Entropy generation number, Ns, against 1∕Ω at ε=0.5 for different Tr

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Figure 11

Entropy generation number, Ns, against ε at 1∕Ω=0.5 for different Tr

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Figure 12

Entropy generation number, Ns, against ε at Tr=0.2,0.6 for different 1∕Ω

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Figure 13

Entropy generation number, Ns, against ε at Tr=0.4,0.8 for different 1∕Ω

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Figure 14

Entropy generation number, Ns, against ε at Tr=0.5,0.9 for different 1∕Ω

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