Swirling flow states in finite-length pipes with various geometries and inlet flow profiles

Abstract

These solutions include the base columnar flow state, a decelerated flow along the centerline, an accelerated flow along the centerline, a vortex-breakdown state and a wall-separation state. The problem is then reduced to the columnar (axially independent) SLE, with centerline and wall conditions for the solution of the outlet flow streamfunction. The numerical simulations realize the various flow states and show correlation between time-asymptotic states and steady states predicted according to the SLE and the columnar SLE problems. The simulations also shed light on the stability of the various steady states. The computed results provide the bifurcation diagrams of steady states in terms of the incoming swirl ratio and size of pipe divergence or contraction. Critical swirls for first appearance of the various types of states are identified. Results show that pipe divergence promotes the appearance of breakdown states at lower inlet swirl levels while pipe contraction delays the appearance of vortex breakdown to higher swirl levels and promotes formation of wall-separation states. The influence of various in- let swirling flow profiles on the manifold of steady states in a straight, finite-length pipe and on flow dynamics is also investigated. Depending on the inlet profiles, flows may first exhibit vortex breakdown while others wall-separation states.