Swirling flow and heat transfer in a pipe: Decay and transition to axial flow.

• Decay of swirling flow and heat transfer parameters in a long straight pipe (l/d = 150) is experimentally studied. • Eight blade-type swirl generators with central body are tested. • Flow and heat transfer parameters are summarised and correlated using the swirl flow number. • Conclusions are made regarding transition from swirling to axial flow in terms of the energy loss reduction and heat transfer decay. • Heat transfer demonstrates the earliest transition to axial flow conditions. This paper describes an experimental study of heat transfer, local and total swirl flow parameters in the pipe 80 mm in a diameter and 150 diameters long. The pipe consists of 12 separate pipes of different length, connected to each other using 20 mm ring spacers. These spacers were also used to provide the flow and heat transfer measurements. Overall, eight blade-type swirl generators with central body and profiled according to the power low, are installed at the pipe inlet. The actual swirl intensity S of swirl generators is ranged from 0.38 to 1.29. The profiles of mean axial and tangential velocity, surface heat flux, swirl flow angle, static and total pressure, flow and surface temperature are measured. A few novel scientific results and equations are obtained, including the axial and tangential speed maximum, axial and tangential momentum flux, swirl number decay and energy loss transformation. The common link between the local and total swirl parameters, one-valued correlation between tangential and axial velocity were obtained. The results, presented in the paper led to the conclusion that transition from swirling to axial flow occurs at S ≈ 0.19 in terms of the heat transfer decrease, but at S ≈ 0.10 – in terms of the energy loss reduction. [ABSTRACT FROM AUTHOR]