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-   -   injector model in dieselFoam (http://www.cfd-online.com/Forums/openfoam/67959-injector-model-dieselfoam.html)

leejc September 1, 2009 21:56

injector model in dieselFoam
 
I am trying to run dieselFoam to simulate an experiment with a single injector running at constant flowrate into a large domain. So, what I like to input is the cone angle, droplet distribution, and mass flow rate (I also know the average droplet velocity at the nozzle exit) and use dieselFoam to predict the spray pattern's development downstream where measurements were made. Further, there is little evaporate, so I set evaporationmodel off. atomizationModel, collisionModel, and dispersionModel are all off.

The dieselFoam tutorial uses injectorType = unitInjector, and injectormodel=hollowConeInjector. I kept them.

I suppose unitInjectorProp is the right place to specify what I need to specify.

My question: How to adjust the parameters under unitInjectorProp to match what I have (i.e. total liquid mass flow rate through the opening). I know the followings: (1) the diameter of the opening, (2), the total mass flowrate (kg/s), (3) cone angle (solid cone), (4) droplet size distribution.

I think for item (3) and (4), I can modify the entries under hollowConeInjectorCoef in sprayProperties

I am having a hard time with dealing with (1) and (2). I don't quite understand the variables under unitInjectorProps: mass, diameter, nParcels, and the array under massFlowRateProfile. In the tutorial, diameter=0.00019, mass=6e-6, and nParcel=5000.

For example, in massFlowRateProfile, there is a list of <t,y>. It is obvious that the first entry is the start time and the last is the end time. What is y? My first guess was kg/s of liquid through the opening of size specified by the value of diameter. BUT I see values like 9.4. But 9.4 kg/s for a hole of diameter 0.00019m makes no sense (9.4kg/s is about ten thousand gallon an hours, it's like the total fuel consumption rate of a 747 at takeoff!).

Now the massFlowRateProfile that came with the tutorial runs, so, whatever y means, I tried to make it a constant.

I changed the array to
(0 0.1)
(1e-5 1)
(0.000125 10)
(0.0002 10)
(0.0003 10)
(0.1 10)
(1 10)

Thinking it will just ramp up the flow rate quickly to 10 and remain there for I just integrate in time from 0 to 0.1 sec. The code bombs out at 0.0003. The original array reaches the maximum at 0.000125 and the comes back down. So obviously, y is not what I think it is.

Say, my injector of size 1mm diameter handles 0.03kg/s (which remains constant). What should I do?

Any help will be greatly appreciated.

chbenz September 2, 2009 01:24

Hi Jerry,

first have a look at this page:

http://openfoamwiki.net/index.php/Contrib_dieselFoam

Here you can find explanations for most of spray entries. I think this would help.

(3) and (4) you can modify in $FOAM_CASE/constant/sprayProperties

If you know the flowrate you can estimate the total mass, which is injected.
The flowrateProfile only discribe the progress of injection. This is not the real flowrate.

I hope this help.

Christof

leejc September 2, 2009 12:25

even more puzzled
 
Thank you Christof.

My guess is as follows.

In the first part of unitInjectorProps,

position and direction are the coordinate of the injector hole and the direction of the spray axis.

diameter is the diameter of the injector hole in meter
Cd is nozzle discharge coefficient
mass is the mass for each parcel in kg
temperature is the temperature of the liquid being injected
nParcels is the total number of parcel to be injected in the time interval specified in the array under massFlowRateProfile

so, the total mass of liquid injected in the time interval is nParcels*mass

So far am I far off?

I know the array under massFlowRateProfile (t_i,y_i),i=0,...last denotes the start and end time of injector by t_0 and t_last. But what is the meaning of the "progress variable" y_i? It is obviously not kg/sec, nor is it Parcel/sec.
I integrated the profile and came up with 0.0087 which is neither the total mass injected nor is it the total number of parcels.

If anyone knows the secret or the/or one of the authors of dieselFoam sees this, I would appreciate it if you would respond.

Thank you.

chbenz September 2, 2009 15:13

Hi,

mass is the mass you inject per hole. when you want to inject 60 mg of fuel through a 6 hole injector, every hole injects 10 mg. thats the meaning of mass in injectorProperties.
Please have a look at http://openfoamwiki.net/index.php/Co...lowRateProfile

"The absolute values are not important since they will be recalculated so that the integrated value match the injected mass. The start of injection, tsoi and end of injection, teoi, are defined by the first and last entries in the massFlowRateProfile, i.e"

here you can find some infos. please look also on the other pages on http://openfoamwiki.net/index.php/Contrib_dieselFoam

There is all you need for dieselSprays.

Christof

leejc September 4, 2009 11:40

Thank you very much Christof.... much appreciated.

Rickard.Solsjo June 24, 2010 08:35

This was a very informative thread! I have one question about the nparcels.
I have made some approximations which makes the injected fuel act just as a gaseous phase. Can I just keep the nparcels to 5000 or should i increase it ?

Regards
R

ENKIME May 30, 2014 14:05

Hi Cristof
Sorry for bother you my friend but I have the same issue than Jerry, the difference is that I have sprayFoam instead of dieselFoam so this is the threat.
I have coneNozzelInjector as injector type, so the mass in the tutorial is 6e-06 I need 40 mg to be injected. When I change the mass I have janafthermo error so I can't change the mass and I can reproduce the penetration of any experiment.
My question is, hopping you can answer it, How can I change the mass? by changing the number of parcels or making constant the flow rate?
I read what openFoam wiki said about the flow rate so the problem is not the flowrate.

Some advice will be very appreciate.
Kind regards

Rickard.Solsjo June 1, 2014 05:13

ENIKME,
if you only change the fuel mass and not the injector duration you will inject a lot of mass in a short time.
look at your injection velocity


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