Issue: Nozzle Discharge Coefficient

LiamH · October 16, 2015, 15:18

Hey CFD experts,

I am having trouble predicting the discharge coefficient (Cd) for a standard 2.5mm Nozzle (or just a simple orifice).

The nozzle Cd, defined as the ratio between the actual mass flow rate and the theoretical mass flow rate, has to have a value less than 1. In my latest simulations, I am getting values as high as 1.5 which makes no sense.

The model conditions are:

Fluid: Water
Energy turned off
Turbulence Model: k-w SST
Throat Size: 2.5mm
Pressure Based – Coupled solver
2nd order
Psuedo-transient
Pressure In: 2400psi
Pressure Out: 0psi
Operating Pressure: 1atm

I know that the true Cd=~0.99 and I was able to get this in the past. Now I am getting values of 1.5 and it makes no sense. Does anyone have thoughts on this?

Thanks,

Liam

skewness abyss · October 17, 2015, 10:53

Can you post a picture of your mesh? It might be a meshing issue. Also have you tried to refine your grid to see if your Cd value changes?

Thumper · February 25, 2017, 16:45

How did you manage to plot the discharge coefficient? On fluent there isn't an option for it.

LuckyTran · February 25, 2017, 18:04

Quote:

Originally Posted by Thumper

How did you manage to plot the discharge coefficient? On fluent there isn't an option for it.

You are correct, there is no such option. You have to calculate it independently.

October 16, 2015, 15:18	Issue: Nozzle Discharge Coefficient	#1
LiamH New Member Liam Join Date: Sep 2013 Location: Calgary, Alberta Posts: 19 Rep Power: 13	Hey CFD experts, I am having trouble predicting the discharge coefficient (Cd) for a standard 2.5mm Nozzle (or just a simple orifice). The nozzle Cd, defined as the ratio between the actual mass flow rate and the theoretical mass flow rate, has to have a value less than 1. In my latest simulations, I am getting values as high as 1.5 which makes no sense. The model conditions are: Fluid: Water Energy turned off Turbulence Model: k-w SST Throat Size: 2.5mm Pressure Based – Coupled solver 2nd order Psuedo-transient Pressure In: 2400psi Pressure Out: 0psi Operating Pressure: 1atm I know that the true Cd=~0.99 and I was able to get this in the past. Now I am getting values of 1.5 and it makes no sense. Does anyone have thoughts on this? Thanks, Liam

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October 17, 2015, 10:53		#2
skewness abyss New Member Danny Join Date: Feb 2015 Posts: 13 Rep Power: 11	Can you post a picture of your mesh? It might be a meshing issue. Also have you tried to refine your grid to see if your Cd value changes?

February 25, 2017, 16:45		#3
Thumper New Member DavidB Join Date: Nov 2016 Location: Lon Posts: 1 Rep Power: 0	How did you manage to plot the discharge coefficient? On fluent there isn't an option for it.