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Discharge coefficent (?)

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Old   July 24, 2015, 03:52
Default Discharge coefficent (?)
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matteo teone
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Good morning, i am new to this forum.
I work for a company we do spare parts.
I am talking about injectors for diesel engines.

I shoud admit i don't have knoledge about CFD.



American customer sent us a table of common rail nozzles (bosch type DLLA ecc...)

One column of the table have a value "Cd" ... serching the web it shoud be "discharge coefficent" I tried to understand what it is but I am not able.

I send 3 lines from the table:

INJ. NOZZLE HOLES SPRAY HOLE DIAMETER FLOW Cd

*** ****** 7 0.195 1814 0.94
*** ****** 7 0.198 1879 0.94
*** ****** 7 0.163 1211 0.9


Flow is 100bar pressure value is in ml/min

HANY HELP WELLCOME
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Old   July 24, 2015, 04:11
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shahrooz
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I guess it is not related to CFD, is it?
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Old   July 24, 2015, 05:45
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Lucas
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shahrooz is right - At the moment the question is not related to CFD.

Anyways - The discharge coefficient is a ratio of the actual mass flow of the injector and the ideal mass flow.

Cd = actual mass flow / ideal mass flow

The actual mass flow is determined in experiments (e.g.: with a "Bosch Tube"). The ideal mass flow is the maximum amount of mass that can pass through the nozzle per time and related to the nozzle diameter, number of nozzles, nozzle type, fuel, and many more. A very simple way to calculate the ideal mass flow is using Bernoulli's law. By restating the equation you get the following:

Ideal mass flow = n * A * \sqrt{2*\rho*(p_{inj}-p_{amb})}

n ... Number of nozzle holes
A ... Hole surface area
\rho ... Fuel density
p_{inj} ... Injection pressure
p_{amb} ... Back pressure

Your values of Cd ~ 0.9 point out that it is a very new nozzle, because Cd is close to 1. The more use of the nozzle, the smaller Cd (due to depositions in the holes decreasing the actual mass flow).

Last edited by Edeluc; July 24, 2015 at 09:21.
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Old   July 24, 2015, 10:06
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matteo teone
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Thankyou very mutch.

But why you are telling me this question is not related to CFD?

CFD softwares are not made for this type of works, calculations?
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Old   July 24, 2015, 14:45
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Quote:
Originally Posted by teone View Post
Thankyou very mutch.

But why you are telling me this question is not related to CFD?

CFD softwares are not made for this type of works, calculations?
They are capable of such things, reading your post at first I thought it is a question about simulation of injector but it turned out to not be about CFD at all.
I didn't know the answer of your question but wanted to inform you this is not CFD related so you could ask it somewhere else that has a higher chance **of getting an answer.

Last edited by shahrooz.omd; July 25, 2015 at 08:50.
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