Coefficient of Discharge

[MAHLY361]

I am attempting to replicate the test using the paper https://files.thunderheadeng.com/femtc/2014_d2-11-takacs-paper.pdf, which validates the test in accordance with EN 12101-2, Annex B, using FDS/PyroSim. However, the pressure values I am outputting are slightly lower than those in the paper, resulting in a higher discharge coefficient (Cd).

Should the pressure be obtained using the pressure gas-phase device as shown below, or do I need to incorporate an additional value?

[drjfloyd]

"slightly different" could have a number of meanings. A couple of comments:

-There is a range of c_d values for an orifce that depends on the geometric details of the contraction and expansion area. Any obstructions inside the vent will also impact the c_d. Your geometry does not appear to be indentical to the paper. Some of this could be your geometry has an expected c_d that differs from the paper

-The paper states:

"The chamber pressure was calculated from integrating the time and space average of the values of the manometer placed near to the four walls. To be noted, these manometers did not indicate any considerable difference in space. The pressure values in time showed some fluctuations. For time integration, the average value was taken into account in a simulation time of 20-25 s. "

You appear to be doing something different. The paper doesn't have the input file or a very detailed description of the pressure measurements. You could try contacting the authors for details on exactly what they did.

[Er9y714]

I had reproduce a few cases from this paper and I remember mesh size, compartment size, measurement points, inlet/outlet VENT locations and flow direction were very important and can change the results. I agree with the comments above. I recommend to model the setup by closely following the information.

[MAHLY361]

Thanks, @drjfloyd and @Er9y714, for your responses. However, this example relates to the coefficient of discharge for rectangular openings, which is typically tested to be around 0.7.
In this paper, although I didn’t fully replicate the position of the pressure points, I did replicate essential data such as the opening dimensions and flow speed and conducted a mesh sensitivity study. I have consistently observed a Cd of 0.78 in this case and others, which is significantly higher than the expected average of 0.7.
I was wondering if I might be missing something, such as adding an adjustment value. My approach involves subtracting the pressure after the orifice from the ambient pressure (0 Pa).

[rmcdermo]

I cannot tell what your grid resolution is here relative to your geometry, but this has a big impact on whether you capture discharge coefficients correctly. You have to resolve (10-20 cells in the recirc zone) the recirculation region that causes the vena contracta.

If you cannot afford to resolve the vena, then FDS provides an HVAC model where you can specify the loss coefficient exactly.

[drjfloyd]

You also need to measure pressure outside the region where the flow contracts and expands.