On coherent structures and heat transfer in strongly swirling impinging jets

This work is devoted to the experimental study of coherent structures in swirling jets flowing onto a heated surface. To measure the velocity fields with a high acquisition rate of 3.5 kHz Stereo Particle Image Velocimetry (PIV) measurement system is used. Simultaneously with PIV measurements, the temperature of the heated surface is measured by high-speed thermometry. The experiments are carried out for three different cases of swirling and two distances from the nozzle to the impingement surface. It is shown that only in the case of strongly swirling jet breakdown of vortex core and bubble-type recirculation zone appear. In this flow regime, large-scale vortex structures of spiral shape are found. The contribution of coherent structures to the fluctuations of velocity and temperature is analyzed for a strongly swirling jet by using Spectral Proper Orthogonal Decomposition (SPOD) method and spatial Fourier transform. It is found that the strongly swirling jet for H/d = 1 provides most effective overall cooling of the surface for the considered flow configurations.