Venturi Flow in a Tube for Air Suction, VOF Multi-Phase, ANSYS Fluent Training

Description

Description

In the present problem, a two-phase CFD simulation of airflow inside a Venturi and accurate modeling of air bubbles as a separate phase in water by ANSYS Fluent software is carried out. The venturi effect reduces the pressure in the fluid when the fluid passes through the narrow part of the pipe.

As the pipe diameter decreases according to the continuity equation, the velocity increases, and the pressure decreases due to energy conservation. Kinetic energy is balanced by pressure drop or pressure gradient. The present model is designed in three dimensions using SOLIDWORKS and then imported into the Design Modeler.

The meshing of the present project has been done using ICEM software. The elements are first used as hexahedral with fewer cells and better quality. The element number equals 193932, and the mesh type is structured. Moreover, due to the nature of the present problem, the transient solver has been enabled.

Venturi Methodology

In the present problem, a two-phase CFD simulation of airflow inside a Venturi using the VOF multiphase model and accurate modeling of air bubbles as a separate phase in water is carried out. The circulated mixed air stream enters the venturi with a volume fraction of 0.7.

After passing this stream through the bottleneck, the flow rate increases, and its pressure decreases. This pressure drop causes air to be sucked out of the hole located in the venturi throat. As air is added to the stream, the air volume fraction increases.

All inputs and outputs of this issue were at a pressure of 30 bar. This study evaluates the amount of air sucked by the venturi and its injection into water.

The VOF model is the best and simplest model suitable for determining the interface boundary between phases of multiphase flow. It is a Tracking Volume model based on the older MAC: cell and Marker model, a tracking-surface model.

In the VOF model, a set of momentum equations is solved jointly for all phases, and for each phase, a volume fraction equation of the continuity equation is solved. Using the Set Level model with VOF in the (VOF + Set Level Couple) section, it is possible to simulate the boundary between phases as accurately as possible.

VOF is designed and developed to track and determine the boundary between phases. We can say that this model is specifically used to simulate immiscible multiphase flow with collisions with definite boundaries between phases.

The Venturi tube has two different inlets. The mixture of air and water, containing 70% (volume fraction 0.7) water, enters through the main inlet boundary. Due to the pressure changes at the bottleneck, air can enter through the top boundary due to the pressure inlet boundary condition type.

Venturi Conclusion

At the end of the solution process, two-dimensional contours related to the velocity, pressure, air and water volume fraction, streamlines, etc., inside the domain are obtained.

As seen in the water volume fraction contour, the velocity of the incoming water flow causes air to be sucked into the venturi, and the sucked air is combined with water, creating a two-phase (air-water) fluid. Air bubbles can be seen in the water fluid.

Also, based on the calculated data from the Fluent software, the graph that represents the sucked air magnitude through the air inlet in terms of time is as follows,

When water passes through a bottleneck in a venturi tube, a vacuum is created at the end of the bottleneck. The hole in the pipe at the point where the vacuum occurs causes air to be sucked into the mainstream, leading to a turbulent flow.