Abstr:Impinging jet

Semi-Confined Flows

Underlying Flow Regime 3-09

Abstract

The flow under consideration consists of a turbulent jet issuing from a circular pipe and impinging on a flat plate. A schematic view of the flow is shown below.

Figure 1. Schematic view of the flow.

The turbulent jet impinging orthogonally onto a plane surface produces, in the vicinity of the stagnation point, among the highest levels of Nusselt number encountered in single-phase convection. This is thus a flow configuration which is extensively used in the process industries to achieve intense heating, cooling (present case) or drying rates. From the point of view of turbulence modellers, the turbulent impinging jet is an excellent test case for validation, since it differs in several important respects from flows parallel to walls which are primarily used to calibrate the model. Indeed, in the vicinity of impingement point:

• turbulence is generated by normal straining (shear in parallel flows);
• the velocity fluctuations normal to the wall are larger than those parallel to the wall (in a parallel flow, fluctuations normal to the wall are much smaller than other components);
• the local turbulent length scales are strongly affected by the length scales of the jet turbulence (in a parallel flow, length scales are usually determined by the distance from the wall alone);
• convective transport of turbulence energy towards the stagnation point is important (in a parallel flow, convective effects are usually negligible);
• just beyond the impingement region, the flow structure will be significantly affected by the strong curvature;
• at greater radii, the flow turns into a radial jet;
• the Nusselt number exhibits a hard-to-predict distribution along the plate, with a global maximum at the stagnation point, and a secondary local maximum or at least a plateau around r/D=2 which appears when the nozzle-to-plate distance is small enough.

---

Contributors: Remi Manceau - Université de Poitiers