spray simulation
 

hi, anybody has experience of doing spray simulation in CFX5? My CFD model is like a 0.5mm orifice injecting 10um droplets(100m/s injection velocity, 10 degree spray angle and 0.005kg/s mass rate)into a cylinder full of static air. The orifice is at the center of the bottom of the cylinder. Due to the high particle mass rate, the particle tracking computation has to be coupled with fluid.

I tried a big time, but couldn't get reasonable result. For example, simulations often give me extremly high air velocity (over 500m/s) near the droplet tracking paths which doesn't make any sense. Convergence is also a problem.

My questions are: 1. which fluid model is suitable? (K-omega, k-e, etc...) 2. since I am dealing with static air in a closed system, how should i set boundary conditions?

Appreciate any other suggestions or reference. Thanks a lot!

Re: spray simulation
 

Hi Adam,

The same problem faced me as well. After many trials I arrived to the following results (if it is correct).

1. To spray any solution you must use particles tracking technique. 2. In particle tracking, you can define the inlet velocity of the particle (you can not defined it as a pressurized fluid, like orifice). Therefore, there is no meaning to make a simulation for the orifice flow. You only need to use the outlet orifice diameter as inlet (create a hole by orifice diameter). 3. In my simulation, I ignored the orifice and created a spray point. By using spray point you can define spray angle, speed, and mass flow rate

Please inform me if you get a better way to simulate this problem

Regards

Re: spray simulation
 

Hi,

I too doing spray modelling.. My CFD model is like a orifice injecting 100um droplets(20m/s injection velocity, 25 degree spray angle, 0.175kg/s mass rate per injection and totally 150 injection points..so total mass flow throu' all the injection is 26.25kg/s )into a cylinder full of Hot steam entering one end and leaving thro other end.

i defined the point injection in the domain as Mr. Neser said.

mass flow rate of hot steam (mainflow) =11.5kg/s mass flow rate of all injection =26.25kg/s

My problem is..

1) Reg. Mass imbalance..?

Mass inlet= 11.55kg/s

Exit is pressure outlet.

In the post processing, it shows after 700iteration,

Mass inlet = 11.55kg/s

Mass out at pressure outlet = -11.50kg/s (the problem

is not converged)

Mass imbalance =(min-mout)/min i.e. Mass out is goes on reducing, which is physically incorrect.. Am i right..? Definately, the Mass out should increase and in turn Mass imbalance will go in -ve values....!!!But this is not the case now. why does it happen...?

and My doubt is, Will the Injected water mass be added to the mass out?

Injected mass is (0.175kg/s per injection)

=0.175*150injector=26.25kg/s

so, Final mass out of the domain after convergence is expected to be 11.55 + 26.25 = 37.8kg/s.

In that case mass imbalance of 26.25kg/s will be there....

Should i hav to change/activate some panels..?

Do clarify me in this regard..? Neser..?

Thanks in Advance, M K Prabakaran

Re: spray simulation
 

Hi Prabakaran,

I am a new user as well but as far as I Know

1. In mass and energy balance -Ve sign means output and +ve sign means input. Therefore, your -ve sign just means it is outlet quantity.

2. The mass outlet is for the fluid only (no particles)

3. Your mass balance with about 0.43%, which is not bade. The recommended value is about 0.10 % to 0.15 % or less (as fare as I know).

4.Try to run your model without the particles (just fluid without any spraying) and save this CFX solver file (when the flow will converge). Then try to start CFX solver with your full model, with spraying, starting from your result of flow without spray.

Re: spray simulation
 

Dear all,

Thanx neser for ur response.

Initially, I converged the problem with out spray.. Then, i gave the injection.

At present, the mass out is -500kg/s, which is definately impractical and nonphysical...

Any valid suggestions from u all is always welcome..!

Thanx n advance,

M K Prabakaran

Convergence problem
 

hello dear members
I am running a spray simulation using CFX in which a cold water injects through a hot air stream.the geometry is a straight pipe and there is an injection region in the bigenning of the pipe. i have did the setup according cfx help, but I have big problems in convergence. momentom residuals stand still like a horizontal line...???
If anybody have any useful advice I will appreciate.
Thank you in advance.
Reza

hi adam if the geometry is 3d the best model is RMS but it needs a strong CPU and along time but it is better to use RNG k-e becouse it can solve swril flow.

Hi! In my case I'm simulating the injection of a size distribution of water droplets in calm air through an orifice with a radius of 5 mm. From my experience I have obtained good results for the convergence using one way coupling instead of fully coupled.

Best luck.

daniel

Convergence is far easier in 1 way coupling compared to 2 way coupling. So only use 2 way if it is required.

And as a general comment: do not be distracted by Ali's comment. It is wrong. If people have questions about what turbulence model they should use then feel free to start a new thread and discuss their application.

Dear Daniel
Can you explain more about your simulation,
I did a similar simulation for a water spraying in the channel.
but it seems that the spray water mass flow rate is not participate in the main flow mass flow rate. Below is a brief description of the modeling boundary conditions:
the modeling geometry is the rectangular channel with a water spraying in the middle of the channel.
Air Inlet mass flow rate is 1.165 kg/s
Water spray mass flow rate is 0.5 kg/s
after solving the equations, a very well convergence in the equations were achieved, but the problem is that the inlet and outlet mass flow rate after convergence are reported as follows:
Inlet mass flow rate: 1.165 kg/s
outlet mass flow rate: 1.165 kg/s
it seems that the spray water mass flow rate is not participate in the main flow mass flow rate.
do you have any idea?
thanks a lot.