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Heat Transfer Research
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ISSN Imprimir: 1064-2285
ISSN On-line: 2162-6561

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Heat Transfer Research

DOI: 10.1615/HeatTransRes.2016009813
pages 49-67

RELIABLE ONE-DIMENSIONAL MODEL APPLIED TO AN INSULATED RECTANGULAR DUCT CONSIDERING THERMAL RADIATION

Ho-Chiao Chuang
College of Mechanical and Electrical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan
Shih-Shih Ku
College of Mechanical and Electrical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan
King-Leung Wong
Kun-Shan University

RESUMO

The heat-transfer characteristics of an insulated long rectangular duct, considering thermal radiation, are analyzed by using the one-dimensional Plane Wedge Thermal Resistance (PWTR) model and Plate Thermal Resistance (PTR) model in this study. It is found that the errors generated by the PWTR model are all positive and the errors generated by the PTR model are all negative. Thus, the Combined Plate Wedge Thermal Resistance (CPWTR) model generated by paralleling PWTR and PTR models with the proportion factors 0.6 vs. 0.4 (64-CPWTR model) or 0.7 vs. 0.3 (73-CPWTR model) can neutralize the positive and negative errors and yield very accurate results in comparison with the two-dimensional numerical solutions analyzed by a CFD software. The 64-CPWTR model returns better results for practical sizes and practical insulated thickness, and the errors are mostly within 2%; on the contrary, the 73-CPWTR model returns better results for practical sizes and a very cold duct with very large insulated thickness. For the results obtained by the same method as the present study except neglecting thermal radiation, it is found that neglecting the thermal radiation effect is likely to produce very large errors in noninsulated and quite large errors in thinly insulated ducts under conditions of low external convection effect introduced by ambient air and higher surface emissivity. The 73-CPWTR model, without considering thermal radiation, can also generate acceptable results in situations of a very cold rectangular duct with thicker insulation, even when an insulated surface is actually with ε = 0.8.


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