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Non-Boiling Heat Transfer of Air/Water Mist Flow in a Square Duct With Orthogonal Ribs

[+] Author Affiliations
Yi-Hsuan Huang, Chiao-Hsin Chen, Yao-Hsien Liu

National Chiao-Tung University, Hsinchu, Taiwan

Paper No. HT2016-7158, pp. V002T08A010; 9 pages
doi:10.1115/HT2016-7158
From:
  • ASME 2016 Heat Transfer Summer Conference collocated with the ASME 2016 Fluids Engineering Division Summer Meeting and the ASME 2016 14th International Conference on Nanochannels, Microchannels, and Minichannels
  • Volume 2: Heat Transfer in Multiphase Systems; Gas Turbine Heat Transfer; Manufacturing and Materials Processing; Heat Transfer in Electronic Equipment; Heat and Mass Transfer in Biotechnology; Heat Transfer Under Extreme Conditions; Computational Heat Transfer; Heat Transfer Visualization Gallery; General Papers on Heat Transfer; Multiphase Flow and Heat Transfer; Transport Phenomena in Manufacturing and Materials Processing
  • Washington, DC, USA, July 10–14, 2016
  • Conference Sponsors: Heat Transfer Division
  • ISBN: 978-0-7918-5033-6
  • Copyright © 2016 by ASME

abstract

Heat transfer of mist flow in a rib-roughened square duct was experimentally determined using infrared thermography. The mist flow was generated by introducing fine dispersed water droplets into the air stream. A constant heat flux was applied to the surface during the test and the surface temperature was kept below the boiling point. The heat transfer measurement was performed on a heated surface located inside a vertical square duct with a hydraulic diameter of 4cm. The air/water mist flow was carried upward by air flow from a blower placed at the bottom of the duct. The flow passed through a flow straightener and entered the heated region of the square duct. The Reynolds numbers of the carrier fluid were 7900, 16000 and 24000.

The results showed that mist flow cooling achieved higher heat transfer rates compared to the air cooling. Thin liquid films formed on the heated surface by the mist flow and the evaporation from the droplets and liquid film contributed to a higher heat removal rate. The heat transfer enhancement on the smooth surface by the mist flow was 4 to 6 times higher compared to the air flow.

Rib turbulators were typically applied in channel walls for heat transfer enhancement in gas turbine blades or heat exchangers. Ribs caused flow reattachment and promoted flow mixing. The higher Nusselt number induced by flow reattachment can be visualized using infrared thermography. For the ribbed case, the heat transfer contours were reported based the regions between ribs. Square brass ribs were used and the rib height-to-hydraulic diameter ratio was 0.05. The rib pitch-to-height ratios were 10 and 20 in the current study.

For the mist flow in the ribbed duct, the intense flow mixing and secondary flow caused by the ribs blew away liquid films on the surface. The heat transfer enhancement near the reattachment region was mainly influenced by the droplet impingement on the surface. In the ribbed duct, the heat transfer enhancement from using the mist flow was 2.5 to 3.5 times higher compared to the air flow.

Copyright © 2016 by ASME

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