Simulation of orifice internal flows including cavitation and turbulence

Changhai Xu, Purdue University

Abstract

Using laminar axisymmetric calculation with a recently developed homogeneous two-phase pseudo density model, various parameters' effect on an orifice internal cavitating flow have been investigated. The present study focuses on the unsteadiness caused by the hydrodynamic instability of the vena-contracta or the presence of Cavitation in this region. This instability leads to the oscillation in flow mass and the presence of cavitation enhances both the magnitude and frequency of this oscillation. Both the mean and oscillatory components of cavitation length and discharge coefficient are affected little by Reynolds number when it is greater than 20,000. Shorter orifice tends to have increased oscillation in mass flow rate. The maximum frequency of discharge coefficient oscillation occurs at modest cavitation length. Small amount of inlet rounding reduces the variations of the mass flow rate greatly, and the average value of the mass flow rate is increased. The two-dimensional and axisymmetric laminar two-phase flow solvers were extended by incorporating the k − ω model to simulate turbulent cavitating flow. The turbulence model is a variable density formulation, but it does not explicitly account for turbulence created by cavitation bubbles. The two-dimensional code has been tested by an incompressible, turbulent flow over a backward-facing step. Then, the solvers were utilized to simulate the cavitating flows in a slot and an orifice. The turbulence model improves the prediction of the discharge coefficient of the orifice flow compared with the results for a laminar calculation and that of the measurement. The computed cavitation length of the flow in the slot shows different agreement with the measured result for various length/diameter ratio slots. The internal flow field in the slot were also analyzed and the computed velocity profiles and turbulence intensities shows reasonably well agreement with experimental results. The two-dimensional code was also utilized to simulate the flow through a two-dimensional nozzle to study the flow field in the closure region of an attached cavitation. However the turbulence model fails to match the measured turbulence intensities and the measured boundary layer thickness distributions. A three-dimensional, unsteady, two-phase Navier-Stokes solver was used to simulate the cavitating flow in an orifice driven by a manifold cross flow. The effect of the cross flow velocity on cavitation length and discharge coefficient has been investigated. The increase in cross velocity tends to have an effect of increasing the extent of cavitation and decreasing the mass flow rate. This is in agreement with experimental observation. Also the calculation shows strong vortex interaction exists in both cavitating and non-cavitating conditions. However, the results show that the vortex structure of an cavitating flow is different from that of a non-cavitating flow.

Degree

Ph.D.

Advisors

Blaisdell, Purdue University.

Subject Area

Aerospace materials

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