Experimental and numerical investigation of convective heat transfer and fluid flow in twisted spiral tube

Turbulent flow characteristics and heat transfer performances of twisted tri-lobed tube (TTT) and twisted oval tube (TOT) with Reynolds number of 8000-21,000 are experimentally compared in this research work, indicating that TTT provides better heat transfer performance and higher friction factor than TOT. Furthermore, numerical investigation of the influence of different geometrical parameters on heat transfer performance and flow characteristics for different twisted spiral tubes using computation fluid dynamics with a validated SST k-ω turbulence model is conducted. These geometrical parameters include different cross section shapes, twisted pitches, twisted direction and lobed numbers. The numerical result reveals that the tubes with larger diameters of out-scribed circle (D2) offer higher Nusselt number and friction factor. It is noted that the heat transfer performance and friction factor increase with the reduction of twisted pitch length P. It is also found that twisted tube with right-left hand rotation shows better heat transfer performance than the tube with right hand rotation. In order to understand the heat transfer mechanism of the TTTs, streamlines, velocity and temperature distributions are also given. From the analysis it can be concluded that the heat transfer enhancement of TTTs is attributed to the helical flow and secondary flow induced by twisted curved tube wall. Radial velocity and tangential velocity are generated by centrifugal forces, leading to higher temperature gradients near the wall of lobed region inside twisted tube. Then the field synergy degree between velocity vector and temperature gradient is improved and the heat transfer is enhanced.