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In this study we proposed a method of selecting preferred rib height of roughness-enhanced tubes by combining the boundary layer district theory with Blasius friction correlation. This method calculates the thickness of every layer for the smooth tube, which is helpful in determining the range of optimum roughness size for roughen-surface enhanced tubes. We computed the flow and heat transfer in these optimum tubes by numerical simulation, investigated the three ways of performance evaluation criteria, and selected the best rib height by directly choosing the tube which had the highest heat transfer efficiency. For the air flow at Re number 10 000 in a converging-diverging tube with a 44 mm inner diameter, the optimal rib height is in the range from 1.6 mm to 2.4 mm; and in a transversely-ridged tube with an inner diameter of 34 mm, from 1.0 mm to 1.5 mm. The preferred rib height changes with flow Re number, fluid properties, and inner diameter of tube, also varies with the tube shape. However the tube shape does not exert so much impact as other factors.
In this study we proposed a method of selecting preferred rib height of roughness-enhanced tubes by combining the boundary layer district theory with Blasius friction correlation. This method calculates the thickness of every layer for the smooth tube, which is helpful in determining the range of We calculate the flow and heat transfer in these optimum tubes by numerical simulation, investigated the three ways of performance evaluation criteria, and selected the best rib height by directly choosing the tube which had the highest heat transfer efficiency. For the air flow at Re number 10 000 in a converging-diverging tube with a 44 mm inner diameter, the optimal rib height is in the range from 1.6 mm to 2.4 mm; and in a transverse cross-ridge tube with an inner diameter of 34 mm, from 1.0 mm to 1.5 mm. The preferred rib height changes with flow Re number, fluid properties, and inner diameter of tube, also varies with the tube shape. However the tube shape does not exert so much impact as other factors.