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Nomenclature A Area [m 2 ] c Heat capacity [J/(kg K)] D Tube diameter [m] f Friction factor = ∆pD/(qL) h Height [m] h Heat transfer coefficient [W/(m 2 K)] I φ Angular momentum [kg m 2 /s 2 ] I z Axial momentum [kg m/s 2 ] k Turbulent kinetic energy [m 2 /s 2 ] k Thermal conductivity [W/(m K)] L Length [m] m Mass flow [kg/s] n Number of inlet ducts Nu Nusselt number = hD/k p Pressure [kg/(m s 2 )] q Dynamic pressure [kg/(m s 2 )] r Recovery factor R Tube radius [m] Re Reynolds numberGreek symbolsAbstract In technical applications, an efficient cooling is necessary for high thermal load components such as turbine blades. One potential and promising technique is a swirling tube flow in comparison with an axial flow. The additional circumferential velocity and enhanced turbulent mixing increase the heat transfer. But the complex flow field and heat transfer mechanisms are still under research. Furthermore, the reliability of a swirl chamber regarding different outlet conditions is of great interest for a robust cooling design. Therefore, we investigated the influence of a straight, a tangential and a 180 • bend outlet. To gain understanding of the flow phenomena, we measured the velocity field by means of stereo-PIV (particle image velocimetry). We experimentally studied the cooling capability measuring the heat transfer coefficients using thermochromic liquid crystals. For an accurate cooling design, we used the local adiabatic wall temperature as the correct reference temperature for calculating the heat transfer coefficients. We will show the velocity field, the pressure loss and the heat transfer results for realistic Reynolds numbers from 10,000 to 40,000 and for swirl numbers between 2.36 and 5.3. The obtained heat transfer is more than four times higher compared to an axial tube flow. Our measurements indicate that the here investigated outlet redirection has no significant influence on the flow field and the heat transfer coefficients.

Swirl cooling is a very efficient method for turbine blade cooling. However, the flow in such a system is quite complicated. In order to gain understanding of the flow structure, the velocity field in a leading edge swirl cooling chamber with two tangential inlet ducts is experimentally studied via Particle Image Velocimetry (PIV). The examined swirl tube is 1 m long and has a diameter of 50 mm. It represents an upscaled generic model of a leading edge swirl chamber. The Reynolds number, defined by the bulk velocity and the swirl tube diameter, ranges from 10,000 to 40,000, and the swirl number is 5.3. Velocity fields are measured in the center plane of the tube axis with stereo- and tomographic-PIV using two and four CCD cameras respectively. Tomographic-PIV is a three-dimensional PIV technique relying on the illumination, recording, reconstruction and cross correlation of a tracer particle distribution in a measurement volume opposed to a plane in stereo-PIV. For statistical analysis 2,000 vector maps are calculated and evaluations show a sample size of 1,000 ensembles is sufficient. Our experiment showed, that the flow field is characterized by a vortex system around the tube axis. Near the tube wall we observed an axial flow towards the outlet with a circumferential velocity component in the same order of magnitude. In contrast the vortex core consists of an axial backflow (vortex breakdown). The gained understanding of the flow field allows to predict regions of enhanced heat transfer in swirl chambers.