Passive control of wall shear stress and mass transfer generated by submerged lobed impinging jet

Abstract

Particle Image Velocimetry was used to investigate the flow field in an impinging lobed daisy hemispherical nozzle jet in comparison to its counterpart round jet, at a Reynolds number of 5620 based on the exit velocity and the equivalent diameter De of the nozzle.The limitations of the PIV technique in the vicinity of the target wall due to the laser scattering were addressed by using the electrodiffusion (ED) technique to determine the wall shear rate distribution. The distribution of the mass transfer coefficient is also obtained using the ED technique. The target wall is placed at a distance H = 2De from the plane tangent to the nozzle, at the center of the orifice. The entrainment of ambient fluid in the free jet region, which is larger in the lobed jet compared to the round jet, feeds in turn the wall jet region.The maximum wall shear rate was found significantly higher in the daisy jet, with an excess of 93% compared to the reference round jet. The maximum mass transfer is 35% higher in the former compared to the latter. Therefore, the hemispherical daisy nozzle is an excellent candidate in passive strategies to enhance local skin-friction and the subsequent local mass transfer at a constant exit Reynolds number.

Heat and mass transfer enhancement strategies by impinging jets: A literature review

“…These distributions also point out a secondary peak at the wall region where secondary vortices appear, the magnitude of this secondary peak being dependent on nozzle shape. Increasing inertia results in an increase of the magnitude of the secondary peak [63,64]. Jets from lobed nozzles display particular phenomena due to the complexity of their three-dimensional flow, as reported by Trinh et al [65].…”

Vortical structures and behaviour of an elliptic jet impinging upon a convex cylinder

“…Despite significant progresses made in terms of understanding impinging jet vortex dynamics through the use of DPIV technique, the majority of these investigations focused on circular jets that produce axisymmetric vortical structures along their shear layers. On the other hand, noncircular jets such as cross-shaped jet, lobed jet, slot jet and elliptic jets produce comparatively far more complex vortical structures and behaviour that potentially extend their applicability towards heat transfer applications (Rau et al, 2014;Sodjavi et al, 2015;Achari and Das, 2015;Long and New, 2015;Sodjavi et al, 2016 andTrinh et al, 2016, take for instance). Among them, elliptic jet leads to non-uniform initial momentum thicknesses along its perimeter, which further results in non-uniform self-inductions and complex threedimensional vortical motions.…”