Gas Particle Movement in a Nozzle

Gas Particle Movement Through the Nozzle, CFD Simulation Tutorial by ANSYS Fluent

Description

This simulation models gas-particle movement through a convergence-divergence nozzle using a two-way DPM model in ANSYS Fluent, with the nozzle operating under grossly overexpanded conditions.

Convergence-divergence (also known as convergent-divergent or de Laval) nozzles are designed to accelerate flow from subsonic to supersonic speeds, with the narrowing throat section followed by a diverging outlet. When the exit pressure of such a nozzle is significantly lower than the surrounding ambient pressure, the flow is described as overexpanded — a condition that produces complex shock structures, flow separation, and pressure oscillations downstream of the throat. Understanding particle behavior under these conditions is particularly important in the gas and petrochemical industry, where nozzles of this type are widely used in gas transport, flow metering, and pressure-letdown applications, and where entrained solid or liquid particles can significantly affect equipment performance and erosion behavior.

The 3D geometry was built using Design Modeler, and the domain was meshed in ANSYS Meshing with an unstructured grid totaling 16,245,216 cells.

Methodology

Several assumptions were applied to simulate this model: a pressure-based solver was used, only fluid behavior was examined (heat transfer was not simulated), and gravitational effects were ignored.

Key simulation settings included:

• Viscous model: Realizable k-epsilon with scalable wall functions

• Phases: air as the primary phase, gas particles as the discrete phase, using an explicit formulation

• Boundary conditions: velocity inlet at 5 m/s with an initial gauge pressure of 448,000 Pa and discrete phase escape condition; pressure outlet with 0 Pa supersonic gauge pressure and discrete phase escape condition; stationary wall with standard wall motion

• Solution methods: phase-coupled pressure-velocity coupling, PRESTO! for pressure discretization, and first-order upwind schemes for momentum, specific dissipation rate, and volume fraction

• Initialization: hybrid method, with a water velocity of 52 m/s in the y-direction and particle velocity initialized to 0 m/s in all directions

Analysis

The results yield two-dimensional and three-dimensional contours of velocity, static enthalpy, and turbulence kinetic energy. The simulation illustrates how gas particles enter the nozzle from the inlet and travel through its convergent-divergent geometry, revealing how the nozzle's overexpanded shock structure and pressure distribution influence particle velocity under the given simulation conditions.