3 replies to this topic

[Estiqaatsi]

Hello everyone,

I am a bit struggling with the plots showing the Coefficient of Discharge for restriction orifices.

This coefficient C is usually plotted, for subcritical flow and Re < about 30,000, as a function of Re itself and Beta (the ratio between the orifice and the pipe diameter).

In Perry 10-17 I read that "C is a function of the Reynolds Number based on orifice diameter and velocity, and diameter ratio Beta". 

In other sources though, e.g. in the Standard ISO 5167-2, I see that C is expressed as a function of Re_D and Beta, where D clearly referes to the pipe diameter.

Now, given the same (inlet) density and viscosity, Reynolds Number calculated with reference to the orifice diameter is larger to the one calculated with reference to the pipe diameter (it is scaled by a factor 1/Beta), providing different Coefficient of Discharge and ultimately different sizing.

Am I missing something? Any advice?

Thanks

[breizh]

Hi,

Take a look at these documents.

Good luck

Breizh

[latexman]

Now, given the same (inlet) density and viscosity, Reynolds Number calculated with reference to the orifice diameter is larger to the one calculated with reference to the pipe diameter (it is scaled by a factor 1/Beta), providing different Coefficient of Discharge and ultimately different sizing.

Am I missing something? Any advice?

Thanks

 

You have to pick one correlation, and use it's basis (orifice diameter OR pipe diameter) for your calculations to determine the C.

 

Do not use orifice diameter for N_Re in a correlation that uses pipe diameter for N_Re.  You will get wrong results.

[Estiqaatsi]

 

Now, given the same (inlet) density and viscosity, Reynolds Number calculated with reference to the orifice diameter is larger to the one calculated with reference to the pipe diameter (it is scaled by a factor 1/Beta), providing different Coefficient of Discharge and ultimately different sizing.

Am I missing something? Any advice?

Thanks

 

You have to pick one correlation, and use it's basis (orifice diameter OR pipe diameter) for your calculations to determine the C.

 

Do not use orifice diameter for N_Re in a correlation that uses pipe diameter for N_Re.  You will get wrong results.

 

 

who would have thought!