Investigation for Natural Convection Heat Transfer of Sucrose Solution Using a Converging and Diverging Tube with Regularly-spaced Twisted Tapes

The natural convection heat transfer of a 60% sucrose solution in a vertical converging-diverging tube （CD） withregularly-spaced twisted tapes （RSTT） has been investigated numerically and experimentally. The effects of wall temperatureand number of RSTT on the Nusselt number were studied in detail. The distributions of velocity and temperature inthe 60% sucrose solution were studied and the simulated results of CD with RSTT were compared with those of the smoothtube. The influence of Rayleigh number and RSTT on the Nusselt number was conducted experimentally. The results indicatethat the Nusselt number of the 60% sucrose solution obviously increased with the number of RSTT but increased inconspicuouslywith 2 and more twisted tapes. The simulation shows that the distance for achieving an optimal heat transferperformance is 46 times the diameter of the tube. The mechanism of the natural convection heat transfer enhancement of the60% sucrose solution in relationship with the CD and the RSTT was analyzed, and the change of average tangential velocitywith the axial distance was presented to demonstrate that the enhancement of heat transfer was realized mainly because of the increase in tangential velocity.