Simultaneous visualization of the velocity and wall temperature fields in impinging swirling jets

Laser-based three-dimensional velocimetry techniques have been recently undergone a fast and significant development enabled by the improvements in both hardware and processing methods. The benefits of this progress are huge especially in the field of the investigation of complex turbulent flows. In this paper, the application of time-resolved tomographic particle image velocimetry (TPIV) to the analysis of submerged impinging swirling jets is presented. Swirling flows recur both in natural phenomena and industrial processes and impinging swirling jets have been extensively investigated for heat transfer applications; however, three-dimensional and time-resolved investigations on the impinging flow fields of swirling jets are rare in the literature. The current study focuses mainly on the effects of the swirl number on the formation and development of the coherent vortex structures arising in this kind of flow. Five values of the swirl number, including the non-swirling case (S=0, 0.23, 0.43, 0.61 and 0.74) are investigated for fixed values of the Reynolds number (Re=5,100) and the impingement distance (equal to two nozzle exit diameters). TPIV measurements are carried out both in the near field (in proximity of the nozzle exit) and in the impingement zone (in proximity of the wall). In the latter case, the water jet is set to a temperature higher than the ambient one and the wall temperature distribution is measured via time-resolved infrared (IR) thermography with the aid of a transparent IR window. The simultaneous velocity and temperature measurements elucidate the role of the vortex structures in both the flow dynamics and transport of internal energy, which is a passive scalar in the present operating conditions. Modal decomposition techniques, in particular the proper orthogonal decomposition, are also applied to identify the most characteristic structures of the turbulent flows.