Experimental and numerical investigation of heat transfer from rib-roughened surfaces with different impinging jets

Impinging jets are jets of fluid impinging on a surface that needs to be cooled or heated. Impinging jets are used in many applications where high heat and mass transfer rates are required.In the first part of this study, the effect of jet-to-plate distance and Reynolds number on local and average heat transfer characteristics of multiple circular impinging jets was experimentally investigated. In the parametric study, Reynolds number and dimensionless jet-to-plate distance was varied from 2000 to 10000, and dimensionless jet-to-plate distance from 2 to 12, respectively. Maximum averaged Nusselt number was obtained at H/d=2 and decreases with increasing H/d.In the second part, detailed heat transfer measurements over a smooth surface under impingement of in-line multiple elliptic and rectangular jet arrays were investigated using thermal infrared camera. The aspect ratios (AR) of elliptical and rectangular jets with three different values, 1, 2 and 0.5, jet Reynolds number ranging from 2,000 to 10,000, and jet-to-target spacing ranging from 2 to 10 were considered to investigate the impingement heat transfer performance. As a result, it was shown that elliptic and rectangular jets could be used as a passive enhancement mechanism, especially in the stagnation region.In the third part, heat transfer measurements over a surface with V-shaped ribs (V-SR) and convergent-divergent shaped ribs (CD-SR) by a circular impinging jet-array was investigated using thermal infrared camera. Both V-SR and CD-SR configurations with an angle of 45o were considered. The effects of different rib heights on the impinging heat transfer along the wall were studied. During the experiments, the Reynolds number was varied from 2000 to 10000, the jet diameter-to-rib height ratio from 0.6 to 1.2, and jet-to-plate spacing from 2 to 12. The heat transfer results of the rib-roughened plate were compared with those of a smooth plate. Best heat transfer performance was obtained with the V-SR arrangements. The average Nusselt number values for the V-SR plate showed an increase ranging from 4% to 26.6% over those for the smooth plate. Correlations were developed for the average Nusselt number for the rib-roughened surfaces.In the fourth part, the flow field of smooth surfaces and surfaces with V-shaped ribs (V-SR) was studied experimentally with a Laser-Doppler Anemometry (LDA) system. For each H/d position, profiles were collected from x/d=0 to 6 axial locations. Along the target plate, different boundary layer profiles were obtained for smooth and V-SR plates at H/d=3 and 12. Positions of maximum radial and axial velocities and turbulence intensities were determined for smooth and V-SR plates. For low jet-to-plate spacings, the production of turbulence kinetic energy is higher for the V-SR surfaces as compared to smooth surfaces. For H/d=3, the radial velocities are higher for the V-SR surfaces as compared to smooth surfaces, but for H/d=12, the radial velocities were only changing slightly at all x/d locations.In the last part, numerical investigations were carried out in order to study the influence on the heat transfer and velocity distribution due to multiple impinging arrays. X direction jet-to-jet distances of 3d, 4d, 5d and a constant y direction pitch of 5d were considered. The equations were presented starting with a general form. After that, assumptions and simplifications were made on these equations according to the problems studied. The LB low Reynolds number model was used for the solutions. The model was not appropriate in determining the heat transfer rate at every point on the target plate, but was able to predict the general changes in profiles. In addition, numerical and experimental studies were compared.Key Words : Impinging jet, LDA, rib-roughened surface, heat transfer