Venturi Flow in a Tube for Air Suction, VOF Multi-Phase, ANSYS Fluent Training
In the present problem, two-phase CFD simulation of airflow inside a Venturi using the VOF model, and accurate modeling of air bubbles as a separate phase in water is carried out.
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The venturi effect reduces the pressure in the fluid when the fluid passes through the narrow part of the pipe. Venturi’s work is named after the Italian scientist Giovanni Battista Venturi. 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. To calculate the pressure drop in a venturi tube, an equation can be obtained by combining the equation and the Bernoulli equation.
The limitation of the venturi effect is when the venturi tube becomes so narrow that the fluid reaches a state of choking. In this case, the velocity of the fluid reaches the sound speed. When the fluid chokes, the mass flow rate of the fluid does not increase with further pressure reduction. However, the fluid is compressed in a compressible fluid, and its density increases (speed increases).
According to the Bernoulli equation in non-compressible fluid conditions, the local pressure drop equation p2 – p1 in the narrowed part of the pipe is as follows:
When passing through the venturi, the velocity of the fluid increases due to the reduction of the passage section, and the pressure of the venturi chamber decreases, creating a much higher suction than other sections of the pipe. By creating a hole in the passage of this section, the passing fluid is removed to prevent the speed of sound. Fluid pressure and velocity analysis in this system is done by Fluent software, and its point-to-point change diagram is given in the results.
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 is of the continuity equation is solved. With the addition of 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 main applications of this model are in shipbuilding industry, offshore structures, water, oil, and gas. In this project, this method is used in Fluent to simulate the Venturi flow.
In the present problem, two-phase CFD simulation of airflow inside a Venturi using the VOF 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 ratio 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 amount of air volume fraction increases. All inputs and outputs of this issue were at a pressure of 30 bar. This study aims to evaluate the amount of air sucked by Venturi and its injection into water.
First, Venturi geometry is designed in SolidWorks software. Then, it is prepared for meshing with Design Modeler software, and geometry file is implemented in ICEM CFD software to mesh generation and name selection of boundary conditions.
The mesh generation is done in ICEM CFD software. The elements are first used as a hexahedral with fewer cells and better quality. The element number is equal to 193932, and the mesh type is structured.
The problem solver is pressure-based, and the solver is transient with current number 1. Gravitational acceleration is also active in this issue. The Venturi tube has two different inlets, including the air input, and the main input of the water and air flow mixture contains a 0.7 volume fraction of water. The total pressure of this device is equal to 30 bar.
The following table represents a summary of the defining steps of the problem and its solution:
|velocity magnitude||0.43 m.s-1|
|Volume friction||0.7 water|
|Volume friction||1 air|
|gradient||Less squares cell based|
|momentum||first order upwind|
|turbulent kinetic energy||first order upwind|
|specific dissipation rate||first order upwind|
|gauge pressure||30 atm|
Here is the graph that represents the sucked air magnitude through the air inlet in terms of time:
When water passes through a bottleneck in a venturi tube, a vacuum is created at the end of the bottleneck. The hole made in the pipe at the point where the vacuum occurs causes air to be sucked into the mainstream, which leads to a turbulent flow. An example of this mechanism can be seen in the venturi tube. When there is a minimum pressure difference between the inlet and outlet of the venturi tube, a vacuum occurs in the suction hole. Venturi flow has become a common method in recent years. In the present project, the amount of air injection in a venturi tube is analyzed using computational fluid dynamics (CFD) modeling. These analyzes are performed by VOF method. This VOF is mostly used for free surface and wave modeling. The results obtained from Fluent were fully shown in album section.
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.
The simulation of this problem is done in transient mode because it is possible to use geo-reconstruct discretization to discretize the volume fraction equations. With increasing time, the amount of sucked air increases and then becomes constant. This simulation showed that this mechanism is well effective for injecting and combining air bubbles in water or producing carbonated water even without the use of a compressor.
There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.