Flow and heat transfer measurements in swirl tubes with one and multiple tangential inlet jets for internal gas turbine blade cooling

In this paper, we present a detailed experimental and numerical study of the flow phenomena and the heat transfer in swirl tubes with one and multiple tangential inlet jets. Such tangential jets induce a highly 3D swirling flow and an enhanced turbulence in the tube, increasing the convective heat transfer. Thus, a swirl tube is considered as an effective cooling method for technical applications with high thermal loaded components like gas turbine blades. The flow field, the heat transfer and the pressure loss are examined in a swirl tube with three different inlet jet configurations with one (MI1), three (MI3) or five (MI5) tangential inlet jets in axial direction. For this purpose, we measured the flow field via stereo-PIV (Particle Image Velocimetry) and the heat transfer by applying a transient technique using thermochromic liquid crystals for several Reynolds numbers. The numerical simulations are performed via Detached Eddy Simulation. The PIV results reveal a complex axial velocity changing after each inlet due to the additional mass flow. Two main structures occur in the swirl tube with five inlet jets: a vortex in the tube center in a wave-like form and large spiral vortices around the tube axis especially near the inlet jets. In the inlet region(s) the highest heat transfer occurs and decreases continuously until the next inlet or towards the tube outlet for the swirl tube with one inlet. The swirl tubes with multiple inlets show lower maximum heat transfer rates compared to the swirl tube with only one inlet due to lower inlet jet velocities. However, the heat transfer distribution is more homogeneous over the entire tube length at a much lower pressure loss. The homogeneous heat transfer can be explained by two mechanisms. At the inlets, the tangential jets impinge onto the concave wall and cause an enhanced convective heat transfer correlating with large spiral vortices. Secondly, the axial velocity becomes stronger further downstream after each inlet jet and causes an enhanced heat transfer between the inlet jets. The thermal performance parameter for all investigated swirl tube configurations is in the same order of magnitude. Thus, all swirl tube configurations are suitable for cooling. If one is interested in a maximum heat transfer paid by a high pressure loss, the swirl tube with one inlet is the best choice. If a lower but more homogeneous heat transfer with a low pressure loss is desired, one should choose the swirl tube with multiple inlets.