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Applied Thermal Engineering 29 (2009) 1–16 www.elsevier.com/locate/apthermeng

Parametric study and multiple correlations on air-side heat transfer and friction characteristics of fin-and-tube heat exchangers with large number of large-diameter tube rows Gongnan Xie a, Qiuwang Wang a,*, Bengt Sunden b,1 a

State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China b Division of Heat Transfer, Department of Energy Sciences, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden Received 9 August 2007; accepted 8 January 2008 Available online 26 January 2008

Abstract In the present study, for industrial applications of large inter-coolers employed in multi-stage compressor systems the air-side laminar heat transfer and fluid flow characteristics of plain fin-and-tube heat exchangers with large number of tube rows and large diameter of the tubes are investigated numerically through three-dimensional simulations based on the SIMPLE algorithm in Cartesian coordinates. The effects of parameters such as Reynolds number, the number of tube rows, tube diameter, tube pitches and fin pitch are examined, and the variations of heat transfer due to variations of fin materials are also observed. It is found that the heat transfer and fluid flow approach fully developed conditions when the number of tube rows is greater than six, and the tube diameter as well as the fin pitch have much more significant effects than the tube pitches, and the heat transfer of high-conductivity material is larger than that of low-conductivity material especially in the high Reynolds number regime. Due to the fact that the existing correlations are not valid for large tube diameters and number of tube rows, the heat transfer and flow friction of the presented heat exchangers are correlated in the multiple forms. The correlation is so obtained that it can be used for further studies such as performance prediction or geometrical optimization. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Plain fin-and-tube heat exchanger; Heat transfer; Friction factor; Parametric study; Multiple correlation

1. Introduction To increase thermal performance of heat exchangers, it is necessary and effective to employ extended surfaces (or referred to as finned surfaces) on the gas-side to compensate for the low heat transfer coefficient, which maybe 10to-100 times smaller than that of the liquid-side. Compact heat exchangers (CHEs), including two types of heat exchangers such as plate–fin types and fin-and-tube (tube–fin) types, are widely used for gas–gas or gas–liquid applications. CHEs own merits of compactness, small vol*

Corresponding author. Tel./fax: +86 29 82663502. E-mail addresses: [email protected] (G. Xie), wangqw@mail. xjtu.edu.cn (Q. Wang), [email protected] (B. Sunden). 1 Tel.: +46 46 2228605; fax: +46 46 2224717. 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.01.014

ume, low weight, high effectiveness and low cost. Fin-andtube heat exchangers (FTHEs) are employed in many power engineering and chemical engineering applications such as compressor inter-coolers, air-coolers, fan coils as well as HVAC&R (heating, ventilating, air-conditioning and refrigeration). FTHE (as shown in Fig. 1) is one of the successful improvements of the tubular heat exchangers. Because the relatively large thermal resistance, which often accounts to 90% of the overall thermal resistance, is encountered on the gas-side (generally air-side), fins are employed on the gas-side to enlarge the heat exchanger surface area and increase the disturbance of the flow. According to the traditional understanding of the heat transfer enhancement mechanism, an accepted conventional idea adopted in fin surfaces is to disturb the pattern of flow and destroy the boundary layer. Therefore, to satisfy the

G. Xie et al. / Applied Thermal Engineering 29 (2009) 1–16

Returning back to the analysis of applying the correlations by Wang et al. [8–11], a comparison of the results are now shown in Fig. 17. Clearly, the predicted results by such correlations are far from the present computations. This is maybe because the established correlations have some certain application ranges, and the present geometrical parameters, e.g., the tube diameter and tube longitudinal pitch are 18 mm and 34 mm, respectively, are beyond the bounding values of 12.7 mm and 32 mm, respectively. Although the correlations developed by Wang et al. are so-far of much accuracy, the predicted results by those correlations are not valid for design the present heat exchangers. At this point, it is essential and necessary to develop corresponding correlations based on the numerical computations, and in turn more works such as optimization or prediction will be moved extensively forward. 7. Conclusions In this paper, 3D-computations on air-side laminar flow and heat transfer of plain fin-and-tube heat exchangers are conducted to reveal the effects of Reynolds number, the number of tube rows, diameter of tube, fin pitch and tube pitches on the overall Nusselt number and friction factor. Also the effect of the fin surface material is observed. Based on the numerical results, the heat transfer and flow friction correlations are established. The major findings are summarized as follows: (1) The Nusselt number and friction factor decrease with the increase of the number of tube rows, and the laminar flow and heat transfer are fully developed when the number of tube rows is larger than six. (2) An increase of tube diameter or fin pitch decreases the heat transfer and pressure drop. The effects of tube pitches are relatively smaller than that of tube diameter and fin pitch. (3) The heat transfer for a fin surface material with a large thermal conductivity can be enhanced due to the decrease of the thermal resistance. The operational air velocity has also a significant effect. (4) Due to the lack of suitable correlations applicable for the present heat exchanger, multiple correlations of the Nusselt number and friction factor have been established. Acknowledgement This work is supported by National Nature Science Foundation of China (No.50521604) and Program for New Century Excellent Talents in University (No. NCET-04-0938). References [1] F.C. McQuiston, D.R. Tree, Heat transfer and flow friction data for two fin–tube surfaces, ASME J. Heat Transfer 93 (1971) 249–250.

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