Yongsiri K.   Chuwattanakul V.   Eiamsa-ard P.   Nanan K.   Eiamsa-ard S.  

Visualization of thermal characteristics for impinging swirl jet using thermochromic liquid crystal sheet

Reporter: Eiamsa-ard S.

Jet impingement is one of the active techniques for heat transfer enhancement. As the re-sults of its promising characteristics of high heat transfer rate and low cost, the technique has been widely applied in many engineering areas for example cooling of hot steel plate, temper-ing of glass, drying of food products, papers, textiles and films, cooling of turbine blades, elec-tronic equipment, cooling of outer wall of the combustion chamber. In common, an impinging jet brings extremely high heat transfer rate around the stagnation region. Then, the heat trans-fer rate rapidly decays in a wall jet region due to the development of thermal boundary layer on the impinged surface.

In this paper. The effects of jet-to-plate spacing on heat transfer characteristics of swirling impinging jet by using thermochromic liquid crystal sheet have been investigated and reported. The aim of using swirling impinging jets (SIJ) is to create swirl flow structure on the impinged surface leading to enhance heat transfer. In this study, the SIJ was equipped into the pipe noz-zle for producing the swirling flows to the impinged surface.  The effects of locating nozzle at different jet-to-plate spacings of L/D = 1, 2, 4, 6 and 8 were also tested. The experiment was carried out at constant jet Reynolds number of 25,000. The heat transfer patterns via thermo-chromic liquid crystal sheet were visualized. The imaged was then analysed with image pro-cessing technique to obtain the distribution of local Nusselt number.  Twisted tape was twisted at tape pitch ratio of y/W = 3 for producing swirl flow.   It is found that the local Nusselt num-ber and average Nusselt number (2.0<r/D<2.0) of SIJ performed the higher than those the typi-cal impinging jet (IJ) of all jet-to-plate spacings (L/D).


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