Heat Transfer and Friction in Rectangular Channels With Ribbed or Ribbed-Grooved Walls

Abstract

The effect of compound turbulators on friction factors and heat transfer coefficients in rectangular channels with two opposite ribbed-grooved walls was determined for a Reynolds number range of 10,000 to 50,000. The channel width-to-height ratio was 10. The fully developed heat transfer coefficients and friction factors on the ribbed-grooved and smooth side walls of each test channel were measured for six rib-groove spacings (p/e = 8, 10, 15, 20, 25, and 30). The fully developed friction and heat transfer in similar aspect ratio rectangular channels with two opposite ribbed walls with two rib spacings (p/e = 8.5 and 11.5) was also measured for comparison. The results show that the heat transfer performance of the rib-groove roughened duct is much better than the rib roughened duct. The rib-groove roughened wall enhances the heat transfer 3.4 times and pays 6 times the pressure drop penalty, whereas the rib roughened wall, with similar rib height and rib spacing, enhances the heat transfer 2.4 times and pays about the same pressure drop penalty. Semi-empirical friction and heat transfer correlations were obtained. Flow measurements show that the roughened ducts have flatter velocity profiles than the smooth duct and rib-groove roughened duct produces higher turbulence intensity than the rib roughened duct. The flatter velocity profile and higher turbulence intensity are responsible for producing higher heat transfer.