HVAC / Coefficient of Discharge

[ShehabGamrah]

I am trying to include the coeficcient of discharge and compare to hand calculations. As seen in the picture below of pressure values. I modelled an HVAC Duct with an area of 1m2 and a forward and reverse loss (K) of 1.451 in the duct. This should be equivalent to a cd of 0.83 based on K=1/cd^2. Then using the formula for pressure drop across rectangular orifice. delta P = (Q/Cd* A)^2 ^0.5 * density = 21.77 Pa based on a flow rate of 5m3/s passing through the opening. However, monitoring the pressure values at upstream and downstream of the orifice the pressure drops to -28 Pa. So is something wrong in my calculations or is the CFD values offset from the hand calcs, and have to change the loss value in the HVAC duct.

[drjfloyd]

There are DEVC outputs for HVAC that will show you the exact values at the nodes.

The greenish area above the connection looks to be about -8. If the bottom is -28 then that is -20 total considering this is just using slice data and trying to eyeball the color is pretty close to the -22 you are expecting.

Without a simple input file to look at can't say much more.

[ShehabGamrah]

Thanks. I understand that when hand calculations are performed for pressure drop due to flow through orifice, the pressure upstream and downstream is considered. It is considered that the pressure beyond the orifice should be the pressure drop subtracted from the atmospheric pressure, so it should be -22 Pa downstream not -28 Pa. The pressure in the vicinity of the orifice is expected to drop to allow the air to flow.

[Er9y714]

An orifice loss includes the loss of contraction, the element at the opening and the expansion. If you want to achieve K=1.451, this represents all of above. If you add a loss only the HVAC element as in your example, you only change the loss of element.