Q. How to I implicitly model an axial fan in CFX, using a fan curve?
A. You can implicitly model a fan in CFX without actually having to include any of its geometry by using a momentum source instead. To do this for an axial fan, you must mesh your geometry such that you have a cylindrical 3D mesh region (perhaps a frozen solid in CFX-Mesh) that represents the fan within the flow domain as a whole. This is so that within CFX-Pre you can create a subdomain using that 3D region, and create the equation sources there. At this point, you should create and name the subdomain but dont set up any sources. You may also want to create a thin wall on the curved surface of the cylindrical region to represent any ducting on the fan.
Next, you must create a Function, using the Interpolation and One Dimensional options. This will represent your fan curve and you must enter the data points in pairs from your curve taking care that the argument and result units are correct. Usually the argument is a flow rate and the result a pressure. Try to enter a point at each extreme of the curve, but make sure you set the extend max/min to off.
Next you should set up two simple expressions equal to the radius and axial length of the cylindrical 'fan' region. Next, create anotherexpression representing the flow rate through the fan region. This will look something like:
FanLength = 0.2 [m]
Fanrad = 0.3 [m]
FanQ = max(1E-6 [m^3 s^-1], volumeAve(Velocity)@fansub*3.14159*fanrad^2)
Here, 'fansub' is assumed to be the name given to the subdomain, and 'fanrad' is the name of the expression defining the radius of the cylindrical region.
Next, edit the fan subdomain and check the Sources box on the Source tab. Select Momentum Sources and a General Momentum Source. Hopefully, the axis of your fan is aligned with one of the global coordinate axes and so you need enter a non-zero value in one component only. The component expression should read (eg. Z direction):
Momentum So
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Q. How to I implicitly model an axial fan in CFX, using a fan curve? A. You can implicitly model a fan in CFX without actually having to include any of its geometry by using a momentum source instead. To do this for an axial fan, you must mesh your geometry such that you have a cylindrical 3D mesh region (perhaps a frozen solid in CFX-Mesh) that represents the fan within the flow domain as a whole. This is so that within CFX-Pre you can create a subdomain using that 3D region, and create the equation sources there. At this point, you should create and name the subdomain but dont set up any sources. You may also want to create a thin wall on the curved surface of the cylindrical region to represent any ducting on the fan. Next, you must create a Function, using the Interpolation and One Dimensional options. This will represent your fan curve and you must enter the data points in pairs from your curve taking care that the argument and result units are correct. Usually the argument is a flow rate and the result a pressure. Try to enter a point at each extreme of the curve, but make sure you set the extend max/min to off. Next you should set up two simple expressions equal to the radius and axial length of the cylindrical 'fan' region. Next, create anotherexpression representing the flow rate through the fan region. This will look something like: FanLength = 0.2 [m] Fanrad = 0.3 [m] FanQ = max(1E-6 [m^3 s^-1], volumeAve(Velocity)@fansub*3.14159*fanrad^2) Here, 'fansub' is assumed to be the name given to the subdomain, and 'fanrad' is the name of the expression defining the radius of the cylindrical region. Next, edit the fan subdomain and check the Sources box on the Source tab. Select Momentum Sources and a General Momentum Source. Hopefully, the axis of your fan is aligned with one of the global coordinate axes and so you need enter a non-zero value in one component only. The component expression should read (eg. Z direction): Momentum Source X Component = 0. [kg m^-2 s^-2] Momentum Source Y Component = 0. [kg m^-2 s^-2] Momentum Source Z Component = fancurve(FanQ)/FanLength which assumes 'fancurve' is the name given to the 1D function representing the curve. You may need to put a minus sign in front of this expression to ensure the fan is driving the flow in the right direction. A positive expression should drive the flow in the positive component direction. Lastly, you may need to use a physical timescale that is smaller that you might usually use, because this fan representation canbe unstable and may oscillate. You can check this by monitoring the Source value in the Solver Manager, or monitor an expression equal to FanQ or fancurve(FanQ). Reducing the timescale should eliminate any oscillations. |
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