Experimental investigation and numerical simulation of natural convection heat transfer in converging-diverging tube with twisted tapes

The natural convection heat transfer of water in a vertical converging-diverging tube (CD) with regularly-spaced twisted tapes was experimentally investigated and numerically simulated, and the natural convection heat transfer coefficient varying with the twisted tape number was explored. Then, the results were compared with those of smooth tubes, and the distributions of velocity and temperature, as well as the variations of heat transfer coefficient and heat flux with axial distance, were simulated with Fluent. It is found that the heat transfer coefficient remarkably increases with the number of twisted tapes and keeps unchanged when 7 twisted tapes are inserted, and that the heat transfer enhancement is independent of the type of twisted tapes. Simulated results indicate that the flow resistance increases with the number of twisted tapes and that the optimal number of regularly-spaced twisted tapes is 7. The authors also primarily analyze the mechanism of the natural convection heat transfer enhancement interrelated by the CD and the regularly-spaced twisted tapes, discuss the change of average tangential velocity with the axial distance and point out that the enhancement of heat transfer is due to the increase of tangential wall velocity.