A numerical study on heat transfer enhancement and the flow structure in a heat exchanger tube with discrete double inclined ribs

This paper presents the results of a numerical study on heat transfer enhancement and the flow structure in a heat exchanger tube fitted with discrete double inclined ribs characterized by a discrete and inclined distribution. Water was selected as the working fluid, and Reynolds numbers between 3390 and 20340 were considered. The numerical results show that heat transfer in the ribbed tube was approximately 1.8 -3.6 times greater than that in a smooth tube, while the friction factor was approximately 2.1-5.6 times greater. The performance evaluation criterion (PEC) values based on the same pump power were found to vary from 13 to 23. Visualization of the flow in the ribbed tube showed that three pairs of counter-rotating vortices or longitudinal swirl flows were generated inside the tube and that the mainstream flow was divided into six helical streams, resulting in a long flow path and relatively intense turbulent mixing between the wall and the core flow regions. In addition, a parameter study with respect to the rib length ratio, rib pitch ratio, and rib inclination angle was conducted to optimize the geometric configuration of the ribs. Based on the optimization results, a rib length ratio of 4, a rib pitch ratio of 5, and a rib inclination angle of 37.5 degrees are recommended for practical applications. (C) 2015 Elsevier Ltd. All rights reserved.