Orifice for Methane Gas Flow, CFD Simulation by ANSYS Fluent
The present problem simulates methane gas flow through an orifice within the canal using ANSYS Fluent software.
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The present problem simulates methane gas flow through an orifice within the canal using ANSYS Fluent software. The orifice is one of the most common devices for measuring mass flow using pressure difference transmitters. The simplest orifice model consists of a perforated plate perpendicular to the desired channel’s fluid flow path. The orifices’ operating mechanism is such that when the fluid passes through an orifice, its velocity increases due to the reduction of the cross-sectional area of the flow and according to the law of mass conservation and flow constant, and according to Bernoulli principle. Also, the flow pressure decreases consequently.
Similarly, when the fluid exits the orifice, the flow velocity decreases again due to the increase in the cross-sectional area of the flow. As a result, the flow pressure increases. Therefore, a certain amount of pressure difference appears on both sides of the orifice, measured using pressure transmitters and built-in pressure sensors. As a result, the orifice flow meter uses the same amount of pressure difference to calculate the flow rate of the fluid passing through the desired channel (taking into account the amount of pressure drop and the ratio of the diameter of the orifice hole to the channel diameter).
The fluid studied in this simulation is methane gas, a density of 0.6679 kg.m-3 and a viscosity of 0.00001087 kg.m-1.s-1. This flow enters the channel at speed equal to 0.033 m.s-1 and exits at a pressure equal to atmospheric pressure.
Geometry & Mesh
The present model is designed in three dimensions using Design Modeler software. The model is a channel 12 inches long and 0.5 inches in diameter, in the middle of which a cross-sectional reduction means that there is an orifice with a diameter of 0.25 inches.
We carry out the meshing of the model using ANSYS Meshing software, and the mesh type is structured. The element number is 54648. The following figure shows the mesh.
We consider several assumptions to simulate the present model:
- We perform a pressure-based solver.
- The simulation is steady.
- The gravity effect on the fluid is ignored.
The following table represents a summary of the defining steps of the problem and its solution:
|near wall treatment||standard wall functions|
|velocity magnitude||0.033 m.s-1|
|gauge pressure||0 pascal|
|wall motion||stationary wall|
|momentum||second order upwind|
|turbulent kinetic energy||first order upwind|
|turbulent dissipation rate||first order upwind|
At the end of the solution process, two-dimensional and three-dimensional contours related to velocity and pressure are obtained. Also, the diagram of pressure and velocity changes in the direction of the channel’s central axis and crossing the section related to orifice has been obtained. As can be seen from the diagrams and contours, when the fluid flows through the orifice, due to a sudden change in cross-sectional area, the flow velocity suddenly increases and then decreases. The pressure drop in the fluid flow in the path passing through the orifice section can also be seen.
There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.