Flow and heat transfer investigations in swirl tubes for gas turbine blade cooling

Abstract

A swirl tube is a very effective cooling technique for high thermal loaded components like gas turbine blades. Such a tube consists of one or more tangential inlet jets, which induce a highly 3D swirling flow. This swirling flow is characterized by large velocities near the wall and an enhanced turbulence in the tube which both increase the convective heat transfer. In the present work, the flow phenomena and the heat transfer in swirl tubes are studied experimentally and numerically. Therefore, a generic swirl tube with tangential inlets at the upstream end of the tube and a novel application-oriented swirl tube geometry with multiple tangential inlet jets in axial direction are investigated in detail. In strong swirling flows, the flow field is dominated by the circumferential velocity which is characterized by a Rankine vortex with a solid body vortex in the tube center and a potential vortex in the outer region. A stability analysis reveals that the solid body vortex is unstable and hence explains the transformation of the solid body vortex into a stable potential vortex towards the tube outlet. In addition, the axial velocity shows a backflow region (vortex breakdown) in the tube center over the entire tube length. It is shown that a vortex breakdown occurs in swirl dominated flows. The measurements indicate that the heat transfer in swirl tubes increases with increasing Reynolds number and swirl number, respectively. Near the inlet, the maximum heat transfer occurs due to the large circumferential velocity component. With decreasing swirl and velocity towards the tube outlet, also the heat transfer decreases continuously. The investigation of the swirl tube with multiple tangential inlet jets reveals a very complex axial velocity which changes after each inlet due to the additional mass flow. However, the circumferential velocity stays almost constant since the swirl strength is re-enhanced with each inlet jet, respectively. For each inlet jet, a high heat transfer can be observed. However, the maximum heat transfer is lower than for the swirl tube with only one inlet because of the lower inlet jet velocities. On the other hand, the heat transfer distribution is more homogeneous over the entire tube length at a much lower pressure loss. For the investigated swirl tubes with one, three or five inlets, the thermal performance is in the same order of magnitude and hence all swirl tube configurations are suitable for cooling.