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
cfd help
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Re: cfd help
Welcome Matteo,
First of all I would say I'm not specialist at this field, so take my findings with reserve.
I think I have found useful information at http://www.engineeringtoolbox.com/orifi ... d_590.html . Perhaps equation (3b) will be most interesting for you:
I guess that if someone needs to replace some nozzle he needs the nozzle with same volume (or mass) flow, which you can find by equation (3b).
A1 = inner section nozzle area
A2 = outer section nozzle area
p1 - p2 = inner - outer pressure
rho = density of liquid
cd = discharge coefficient changes results from ideal form of equation to real depending on the specific conditions.
At the webpage, mentioned above, there is table of discharge coefficients for nozzles depending on Reynolds number and diameter ratio. But if I have table with cd given by producer of nozzle I would use given value, which I expect to be measured by producer at some typical working case.
Edit:
(3b) is only a bit more complicated form of the continuity equation:
volume flow = discharge coefficient * cross-section area * velocity
First of all I would say I'm not specialist at this field, so take my findings with reserve.
I think I have found useful information at http://www.engineeringtoolbox.com/orifi ... d_590.html . Perhaps equation (3b) will be most interesting for you:
- mass_flow_eqv3b.png (2.08 KiB) Viewed 1124 times
A1 = inner section nozzle area
A2 = outer section nozzle area
p1 - p2 = inner - outer pressure
rho = density of liquid
cd = discharge coefficient changes results from ideal form of equation to real depending on the specific conditions.
At the webpage, mentioned above, there is table of discharge coefficients for nozzles depending on Reynolds number and diameter ratio. But if I have table with cd given by producer of nozzle I would use given value, which I expect to be measured by producer at some typical working case.
Edit:
(3b) is only a bit more complicated form of the continuity equation:
volume flow = discharge coefficient * cross-section area * velocity
Re: cfd help
Maybe this is a little late but hopefully it helps someone.
The only good way to get a Cd value is to measure it.
My recommendation is to use the equation fandaL provided and back out the Cd value from the measured inputs.
Also, the Cd values will change depending on if the flow is 'choked' or not. Liquids don't really choke but the Cd will still change and you will get an effect where the flow rate won't increase unless you increase the upstream pressure.
If your customer wants the Cd values over a range of delta pressures, he will want to know the Cd curve and not just the Cd value.
The only good way to get a Cd value is to measure it.
My recommendation is to use the equation fandaL provided and back out the Cd value from the measured inputs.
Also, the Cd values will change depending on if the flow is 'choked' or not. Liquids don't really choke but the Cd will still change and you will get an effect where the flow rate won't increase unless you increase the upstream pressure.
If your customer wants the Cd values over a range of delta pressures, he will want to know the Cd curve and not just the Cd value.