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强化传热期刊
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ISSN 打印: 1065-5131
ISSN 在线: 1026-5511

强化传热期刊

DOI: 10.1615/JEnhHeatTransf.v15.i3.40
pages 227-242

Enhancement Mechanisms for Single-Phase Turbulent Heat Transfer in Micro-Fin Tubes

Xiao-Wei Li
School of Aerospace, Tsinghua University, Beijing 10084, P. R. China
Ji-An Meng
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education School of Aerospace, Tsinghua University
Zhi-Xin Li
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education School of Aerospace, Tsinghua University

ABSTRACT

The enhancement mechanisms for single-phase turbulent heat transfer in micro-fin tubes were numerically investigated by simulating the flow in a helical micro-fin tube and a straight micro-fin tube (with the fins aligned with the flow). Flow in parallel plate passages with various height roughness elements was also simulated to further investigate the enhancement mechanisms. The numerical results for the helical micro-fin tube agree well with experimental data. The helical micro-fins begin to enhance heat transfer at a Reynolds number of 10,000 with a 110% increase in the heat transfer rate for Reynolds numbers above 30,000. The heat transfer in the straight micro-fin tube was almost the same as in a smooth tube for Reynolds numbers from 10,000 to 80,000. The additional turbulence generated by the roughness elements or the helical fins is the key mechanism for the heat transfer enhancement. If the roughness elements or the helical fins are inside the viscous sublayer, no additional turbulence is generated and the heat transfer is not enhanced, but when they extend above the viscous sublayer, additional turbulence is generated and the heat transfer is enhanced, hence a critical Reynolds number exists. Straight fins do not generate additional turbulence, hence, the straight micro-fin tube has little effect on the heat transfer enhancement. Additional turbulence is usually accompanied by additional pressure resistance (form drag). Therefore, the key to increasing the efficiency of enhanced heat transfer tubes is to generate additional turbulence near the tube wall with minimum form drag. The high efficiency of the micro-fin tube is due to the effective turbulence generated near the tube wall.


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