How does impinging jet pulsation affects vortex generation which governs heat transfer?

The impinging jet is defined as a high-velocity jet of cooling fluid forced through a hole or slot which impinges on the surface to be cooled, and results in high heat transfer rate between the wall and the fluid. Impinging jets can be classified as a submerged jet or a free jet depending on the density and nature of the fluid issuing the jet. They are widely used in industrial processes, since high heat and mass transfer coefficients can be attained. Unfortunately, their complex flow physics are difficult to numerically simulate and depend on a large number of parameters. In essence, literature has it that the flow field in an impinging jet issued from a converging nozzle is characterized by a free jet region, an impingement or stagnation region and a radial wall jet region. These parameters have been widely studied. In fact, recent publications have reported of a secondary heat transfer coefficient peak, which is the result of local thinning of the boundary layer thickness due to the generation of secondary vortices by impinging toroidal (primary) vortices that are created in the free jet. So far, little is known about the interaction between the primary and secondary vortices. In addition, very little can be found regarding the effect of impinging jet pulsation on the flow field characteristics in the wall jet and impingement region and the resulting heat transfer characteristics.

Although the primary secondary vortex interaction is a well-accepted heat and mass transfer mechanism in impinging jets, there are no published results on the possible effect of jet pulsation on the generation of secondary vortices and their interaction with primary vortices. On this account, researchers from Technion – Israel Institute of Technology: Dr. Moti Raizner and Professor René van Hout conducted a study to elucidate the possible effect of jet pulsation on the generation of the secondary vortices and the interaction between primary and secondary vortices in the radially expanding wall jet generated by a partially confined, pulsating impinging round jet issued from a nozzle (H/D = 2.0). Their work is currently published in the International Journal of Heat and Mass Transfer.

In other words, their work focused on assessing the effect of impinging air jet pulsation on the generation of toroidal (“primary”) vortices near to the nozzle exit, their subsequent downstream evolution, the creation of “secondary” vortices and their interaction along the radial wall jet. To realize this, the researchers performed experiments using particle image velocimetry. Data analysis was based on the determined instantaneous vorticity and swirling strength fields.

The authors reported that jet pulsation significantly affected both the primary and the secondary vortex numbers, areas, and strengths. However, the actual change in vortex strength was seen to depend on the phase and the jet Reynolds number. Secondary vortex strengths were seen to increase up to twice the value obtained for the steady jet case. Nonetheless, no enhancement was measured when the Strouhal number exceeded 8.5×10⁻³.

In summary, the study investigated the effect of jet pulsation on primary and secondary vortices generated by an impinging round jet (H/D = 2.0) at three different Reynolds numbers ranging from 4606 to 13,513. Measurements were performed by planar PIV covering the wall jet up to r/D = 6.5. Based on conditional sampling of the presented data set, one could conclude that secondary vortex generation for the pulsating jet is similar as for the steady jet case. In a statement to Advances in Engineering, Professor René van Hout highlighted that based on their results, jet pulsation had a significant effect on primary and secondary vortex strengths, numbers, and areas both in the free jet (just before impingement) as well as in the radially expanding wall jet. The effects can be linked to changes in the heat transfer characteristics measured in a prior study.