Confined Vortices in Flow Machinery

Local recirculation or extensive regions of reversed flow, periodic fluc­ tuations in pressure and velocity, and high levels of kinetic-energy dissi­ pation are the principal characteristics of (sufficiently intense) confined vortex flows. The present article represents a necessarily descriptive rather than analytical discussion of swirling-flow behavior in various practical devices where these effects may be either the desired result of design or unavoidable, possibly unforeseen, side effects. One of the most widespread technical applications of swirl is for flame stabilization in furnaces and combustion chambers, whereby fuel and air are mixed in a zone of recir­ culating flow leading to a stable, compact flame. In this instance, discrete­ frequency noise and vibration may be undesirable side effects. Vibration due to excess swirl also represents a problem in the draft tubes of water turbines, and the features of vortex flows can also adversely affect the performance of axial-radial outlet casings of axial turbines and com­ pressors. The basic characteristics of fluidic vortex valves, which range in size from miniature fluidic devices to flood-dam control valves, are a consequence of the dissipative nature of vortex flows. Examples of other devices where the vortex character of the flow plays a central role include dust cyclones, hydrocyclones, and the Ranque-Hilsch refrigeration tube. As a preliminary, we review some basic aspects of confined vortex flows-in particular, the concepts of supercritical and subcritical flow and vortex breakdown.