Turbulent Flow in Circular Tubes with Twisted-Tape Inserts and Axial Interrupted Ribs

Local heat transfer distributions and pressure drop in smooth circular tubes with twisted-tape inserts and axial interrupted ribs were investigated. Experimental data for isothermal friction factors and regionally averaged Nusselt numbers are presented for turbulent air flows for Reynolds numbers ranging from 17,000 to 82,000. The circular tube is composed of ten isolated copper sections with a tube length-to-diameter ratio (L/D) of 15. Three different 360 degrees twisted-tape ratios are studied: H/D = 6(2.5 turns), 7.5 (2 turns), and 10 (1.5 turns). Three axial interrupted rib configurations attached to the inner wall of three test tubes with twisted-tape inserts are investigated: e/D = 0.0625 in-line rib, e/D = 0.0625 staggered rib, and e/D = 0.125 staggered rib. The results show that the heat transfer and pressure drop in the tube with twisted-tape inserts increase by increasing the number of twisted-tape turns. The tube with twisted-tape inserts provide 1.5-2.0 times the heat transfer augmentation with 3-4 times the pressure drop penalty. The tube with twisted-tape inserts and staggered ribs produce higher heat transfer and pressure drop than that with twisted-tape inserts and in-line ribs, and significantly higher than without ribs. The tube with twisted-tape inserts (H/D = 7.5) and e/D = 0.125 staggered ribs provides 1.8-2,8 times the heat transfer enhancement with about 9-10 times the pressure drop penalty. The tube with twisted-tape inserts (H/D = 6) and with staggered ribs (e/D = 0.125) gains 2.2-3.2 times the heat transfer while paying 13-14 times the pressure drop. The best heat transfer performance in the tube with twisted-tape insert and staggered ribs (e/D = 0.125) is about 25-40% higher than that with the twisted-tape only for different H/D for a constant pumping power.