Characterization of the Behavior of Confined Laminar Round Jets

Abstract

Confined laminar fluid jets have many practical applications in industry. Several examples include expansions in pipes and flow of gas into a large plenum. While much consideration has been given experimentally to heat transfer and pressure gradients within the confinement, little attention has been paid to quantify the velocity profiles and transitions between various flow behaviours. Using a finite volume CFD code, OpenFOAM ®, the Navier-Stokes equations were solved for varying expansion ratio, 1/ε = renclosure/rj, and varying Reynolds numbers. In the present analysis, Reynolds number based on the inlet jet diameter is varied from 30 to 70, well within the accepted range for laminar jet behavior. The expansion ratio, 1/ε is varied from 20–200. Of primary focus in the current study are compact correlations for the jet centreline velocity as a function of jet Reynolds number, Rej and expansion ratio. Similar functional dependences for the “linear” decay region of the jet, and the location of the stagnation point on the enclosure wall, are also investigated. These are all important features of the global flow field for the confined jet. Results suggest that initially, the flow characteristics are identical to a free jet. At some downstream location, the presence of the enclosure is felt by the jet and deviations begin to be seen from free jet behavior. This transition region continues until at a sufficiently large downstream location, the flow becomes fully developed, internal Poiseuille flow. In this paper, we analyse these transition regions and offer explanations and practical correlations to successfully predict the important flow physics that occur between free jet behavior and Poiseuille flow. Key dimensionless parameters are identified, the magnitude of which can be used to classify the flow conditions.

Numerical Investigation of Sheath and Aerosol Flows in the Flow Combination Section of a Baron Fiber Classifier

“…The effects of confinement may also be understood within a steady-state description (Landfried et al, 2012). When a steady-state jet discharges into an unconfined quiescent fluid, the external fluid is entrained, and the jet spreads (Pai, 1954).…”

“…If, however, there is also a back wall, obstructing the axial flow exterior to the jet nozzle, recirculation of the ‘dead fluid’ is induced as the jet spreads. If the back wall is sufficiently far upstream from the nozzle, the recirculation occurs exterior to and upstream of the nozzle, and the jet spreads, essentially as it would were there no back wall present (Landfried et al, 2012). If, however, the back wall is flush with the nozzle, then the recirculation is forced downstream and interferes with the spreading of the jet—this is precisely the geometry of the Baron FCS.…”