Steam Ejector: Wet Steam, Condensation

Wet Steam for Condensation inside a Steam Ejector, ANSYS Fluent

Description

This project simulates the steam condensation process occurring within an ejector using ANSYS Fluent. An ejector is a mechanical device that uses an actuator (driving) fluid to draw in a secondary material — the two actuator fluids and the suction substance ultimately mix and exit together as a single stream. Ejectors serve two primary functions: vacuuming/suctioning gases and mixing fluids.

Structurally, an ejector takes the form of a convergent-divergent tube. As the driving fluid enters and passes through the nozzle's converging section, the reducing cross-sectional area increases flow velocity according to the continuity equation — effectively converting the fluid's potential energy into kinetic energy. Per Bernoulli's principle, this increasing velocity corresponds to a drop in fluid pressure, which in turn drives the suction effect central to the ejector's operation.

The 2D geometry was designed in Design Modeler, representing an axisymmetric plane of the ejector: 0.411 m in length, with a 0.02 m wide exit, a 0.0038 m wide first entrance, and a 0.0165 m wide second entrance. The model's lower edge was defined as an axis of rotation, allowing the 2D geometry to represent the full 3D structure — a simplification made possible by the ejector's perfectly symmetrical geometry, reducing computational cost significantly.

The domain was meshed in ANSYS Meshing using a structured grid totaling 25,984 cells.

Methodology

The Wet Steam multiphase model solves two coupled sets of transport equations: the mass fraction of the condensed liquid phase, and the number/concentration of droplets per unit volume. This phase-change model captures the formation of liquid droplets during a homogeneous, non-equilibrium condensation process, based on classical non-isothermal nucleation theory.

As superheated dry steam rapidly expands through the ejector, it cools and forms a nucleating core — ultimately producing a two-phase mixture of saturated steam and liquid droplets known as wet steam. A density-based solver was used throughout to capture this compressible, phase-changing flow.

Conclusion

The resulting pressure drop generates a compressive vacuum within the ejector, drawing the secondary material into the flow. The primary driving fluid and the entrained secondary material mix and compress together within the diffuser section downstream.

Results include 2D contours of pressure, velocity, temperature, turbulent kinetic energy, and the rate of liquid mass generation (equivalent to the condensation rate) — with 3D contours obtainable by rotating these 2D results around the central axis. The liquid mass generation rate stands out as one of the most important results, offering direct insight into how effectively and where condensation occurs throughout the ejector's convergent-divergent geometry.