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RESEARCH PAPERS: Heat Exchangers

Porous Medium Interconnector Effects on the Thermohydraulics of Near-Compact Heat Exchangers Treated as Porous Media

[+] Author and Article Information
K. Sumithra Raju

Heat Transfer and Thermal Power Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras Chennai, Tamil Nadu 600036, India

Arunn Narasimhan1

Heat Transfer and Thermal Power Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras Chennai, Tamil Nadu 600036, Indiaarunn@iitm.ac.in

1

Corresponding author.

J. Heat Transfer 129(3), 273-281 (Jun 05, 2006) (9 pages) doi:10.1115/1.2427074 History: Received November 07, 2005; Revised June 05, 2006

Abstract

A novel approach of treating near-compact heat exchangers (NCHX) (surface to volume ratio, $α=100–300m2∕m3$ with hydraulic diameter $DM∼6mm$) as a “global” porous media, whose thermohydraulic performance is being influenced by the presence of “local” tube-to-tube porous medium interconnectors, connecting the in-line arrangement of tubes $(D=2mm)$ having square pitch of $XT=XL=2.25$, is investigated in this study using numerical methods. The thermohydraulics of the global porous media (NCHX) are characterized by studying the effect of transverse thickness $(δ)$ and permeability (represented by $Dai$) of the local metal foam type porous medium interconnectors on the global heat transfer coefficient (Nu) and nondimensional pressure drop $(ξ)$. The fluid transport in the porous medium interconnectors is governed by the Brinkman–Darcy flow model while the volume averaged energy equation is used to model energy transport, with the tube walls kept at constant temperature and exchanging heat with the cooling fluid having $Pr=0.7$ under laminar flow $(10. For the chosen NCHX configuration, $ξ$ and Nu increases for an increase in Re and also with an increase in the thickness $(δ)$ of the interconnecting porous medium. However, as the local Darcy number $(Dai)$ of the interconnecting porous medium increases, the $ξ$ decreases but the Nu increases. Treating the heat exchanger as a global porous media this result translates to an increase in the $ξ$ and Nu as the global permeability (represented by $Dag$) decreases, where the decrease in $Dag$ is because of either an increase in $δ$ or a decrease in $Dai$. Separate correlations predicting $ξ$ and Nu as a function of Re and $Dag$ (which in turn is correlated to $δ$ and $Dai$) have been developed for the chosen NCHX configuration, both of which predict the numerical data with $±20%$ accuracy.

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Figures

Figure 8

Isotherms for the tube bank arrangement with interconnecting porous medium thickness δ=D and Ki=4.4×10−10. (a) Re=10 and (b) Re=100.

Figure 9

Effect of δ on Nu versus Re for Ki=4.4×10−10

Figure 1

Problem configuration: (a) schematic of the problem, (b) boundary conditions and computational domain, and (c) grid used

Figure 2

Validation of numerical method: (a) Nulocal versus ω for flow around cylinders, (b) dimensionless longitudinal pressure distribution of the tube for flow over bank of five tubes, and (c) validation of porous medium model used: variation of Nu with Pe for cylinder embedded in porous medium

Figure 3

Local dimensionless velocity U variation with Y at x=0.01125 for Re=10

Figure 4

Streamlines for the tube bank arrangement with interconnecting PM thickness δ=D=0.45 and Ki=4.4×10−10. (a) Re=10 and (b) Re=100.

Figure 5

Variation of pressure drop with average velocity (a) 0⩽δ⩽0.6 and (b) δ=0.8 and 1.0

Figure 6

(a) Effect of δ on ξ versus Re for Ki=4.4×10−10 and (b) effect of δ and Ki on ξ versus Re

Figure 7

Parity plot for data versus proposed ξ correlation, Eq. 15

Figure 11

Nu versus Re for several interconnecting porous medium permeabilities

Figure 12

Parity plot for the Nu data and correlation, Eq. 17

Figure 10

For several local permeabilities Ki, at Re=100 and δ=0.45. (a) Velocity and (b) temperature profiles at the exit.

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