Smoke Extraction Simulation

[Anonymous]

Hi!

I am trying to simulate fire and smoke extraction in garage. At the moment I am checking is everything OK with the simulation setup by doing the steady state analysis. I encountered on few issues...

First of all pressure, temperature and scalar convergence plot was really bumpy (picture in attachment Scalar 1.jpg)...I reduced scalar to 0,9 in intelligent solution control and now scalar works perfectly (picture in attachment Scalar 0,9.jpg)...But still I have problems with temperature and pressure...Any ideas what is wrong?!

Here is the support file of simulation - https://www.wetransfer.com/downloads/f989a9e8800b0389c6eb37864683c70520150614185009/4a81ede823d8d111...

Any comments on simulation setup by the Autodesk experts?!

Furthermore, I have few questions about transient analysis which I am going to simulate after resolving the issue with temperature and pressure (hopefully 🙂 😞

1. As I sad I lowered Scalar to 0,9 in steady state simulation. Should I return it to 1,0 for transient?!

2. Initial Conditions?! On what should I put Initial conditions!? I am doing this simulation based on tutorial from wildej -

Car Park Smoke Extraction and Visibility With Best Practices Autodesk® Simulation CFD™ and from that tutorial isn't clear on what should I put initial conditions?!

3. Should I turn on the radiation?!

4. My idea is to define energy of the fire with the volumetric heat, transient boundary condition so that the energy of the fire rises with the rise of time. But when the temperature above fire reaches 68 °C the energy of fire will start to reduce because the sprinkler system will engage. Is there any way I can tell that to CFD that boundary condition starts with one equation (start of fire) and when the temperature in simulation reaches 68 °C that it switches to second equation (fire extinguishing equation)?!

5. Jet fans should start working after certain amount of time - let's say 4 minutes after the start of simulation. Is there any way I can tell that to CFD?! Or I must STOP/START simulation?! Is that way the good way because in wildeys tutorial I saw that it is not advised to stop and start the solver during a transient analysis?!

6. Time step size (0,5s) and inner iterations (3) from wildejs tutorial are good start for my simulation?! My idea is that whole simulation lasts for 8 minutes.

Hopefully someone will have patience for reading my whole post and then answering it 😄

Best regards, Branimir.

[Anonymous]

Hi!

I am trying to simulate fire and smoke extraction in garage. At the moment I am checking is everything OK with the simulation setup by doing the steady state analysis. I encountered on few issues...

First of all pressure, temperature and scalar convergence plot was really bumpy (picture in attachment Scalar 1.jpg)...I reduced scalar to 0,9 in intelligent solution control and now scalar works perfectly (picture in attachment Scalar 0,9.jpg)...But still I have problems with temperature and pressure...Any ideas what is wrong?!

Here is the support file of simulation - https://www.wetransfer.com/downloads/f989a9e8800b0389c6eb37864683c70520150614185009/4a81ede823d8d111...

Any comments on simulation setup by the Autodesk experts?!

Furthermore, I have few questions about transient analysis which I am going to simulate after resolving the issue with temperature and pressure (hopefully 🙂 😞

1. As I sad I lowered Scalar to 0,9 in steady state simulation. Should I return it to 1,0 for transient?!

2. Initial Conditions?! On what should I put Initial conditions!? I am doing this simulation based on tutorial from wildej -

Car Park Smoke Extraction and Visibility With Best Practices Autodesk® Simulation CFD™ and from that tutorial isn't clear on what should I put initial conditions?!

3. Should I turn on the radiation?!

4. My idea is to define energy of the fire with the volumetric heat, transient boundary condition so that the energy of the fire rises with the rise of time. But when the temperature above fire reaches 68 °C the energy of fire will start to reduce because the sprinkler system will engage. Is there any way I can tell that to CFD that boundary condition starts with one equation (start of fire) and when the temperature in simulation reaches 68 °C that it switches to second equation (fire extinguishing equation)?!

5. Jet fans should start working after certain amount of time - let's say 4 minutes after the start of simulation. Is there any way I can tell that to CFD?! Or I must STOP/START simulation?! Is that way the good way because in wildeys tutorial I saw that it is not advised to stop and start the solver during a transient analysis?!

6. Time step size (0,5s) and inner iterations (3) from wildejs tutorial are good start for my simulation?! My idea is that whole simulation lasts for 8 minutes.

Hopefully someone will have patience for reading my whole post and then answering it 😄

Best regards, Branimir.

[Jon.Wilde]

You could try film coefficients on those inlets - so if air recirculates it should reset to the right temperature. Not ideal but it should work!

A value of 5 W/m2/K at ambient.

[Jon.Wilde]

You could try film coefficients on those inlets - so if air recirculates it should reset to the right temperature. Not ideal but it should work!

A value of 5 W/m2/K at ambient.

[Anonymous]

Hi wildej!

I tried what you suggested and unfortunately it is same as before :S

Just to be sure I attached scenario that I am trying to do. It is typical scenario from your presentation just without any jets, fans in first minute or so.

If you have time to look to my transient simulation I uploaded it here:

https://jumbo.iskon.hr/download/b764b205-6035-4311-bef9-7fc4ebddda10

Best regards, Branimir

[Anonymous]

Hi wildej!

I tried what you suggested and unfortunately it is same as before :S

Just to be sure I attached scenario that I am trying to do. It is typical scenario from your presentation just without any jets, fans in first minute or so.

If you have time to look to my transient simulation I uploaded it here:

https://jumbo.iskon.hr/download/b764b205-6035-4311-bef9-7fc4ebddda10

Best regards, Branimir

[Anonymous]

And convergence plot seems like a bumpy ride.

[Anonymous]

And convergence plot seems like a bumpy ride.

[Jon.Wilde]

Hi Branimir,

That convergence looks pretty good to me - you have flat lines between each major iteration as you have inner iterations set. This is a good thing so long as the lines are not too long (wasted iterations).

I did mean - set a film coefficient to the outlet, not inlet (sorry). So that if air does get re-entrained, CFD knows what temperature to assign it. This is not ideal but should prove to be a sufficient solution I hope.

Thanks!

Jon

[Jon.Wilde]

Hi Branimir,

That convergence looks pretty good to me - you have flat lines between each major iteration as you have inner iterations set. This is a good thing so long as the lines are not too long (wasted iterations).

I did mean - set a film coefficient to the outlet, not inlet (sorry). So that if air does get re-entrained, CFD knows what temperature to assign it. This is not ideal but should prove to be a sufficient solution I hope.

Thanks!

Jon

[Anonymous]

Hi wildej!

I let simulation ran a bit more so I am attaching what happened later? Do you think it is still OK?

I have one more question...When I engage jets after certain period of time should I then lower the timestep (now is 0,5)? Should I start with 0,25 when jets are engaged and then later change it to 0,5?

Best regards, Branimir

[Anonymous]

Hi wildej!

I let simulation ran a bit more so I am attaching what happened later? Do you think it is still OK?

I have one more question...When I engage jets after certain period of time should I then lower the timestep (now is 0,5)? Should I start with 0,25 when jets are engaged and then later change it to 0,5?

Best regards, Branimir

[Jon.Wilde]

OK, that looks a little worse... What is the time step? It could be outlet recirculation (CFD would not know the scalar value to use).

It is not recommended to stop and re-start a transient solution. How about ramping up the jets over a few steps?

[Jon.Wilde]

OK, that looks a little worse... What is the time step? It could be outlet recirculation (CFD would not know the scalar value to use).

It is not recommended to stop and re-start a transient solution. How about ramping up the jets over a few steps?

[Anonymous]

Time step is 0,5.

What does it mean to ramp up the jets over a few steps and how do I do that?

[Anonymous]

Time step is 0,5.

What does it mean to ramp up the jets over a few steps and how do I do that?

[Jon.Wilde]

Good point, you would need to stop and start it as you have internal fans (my apologise), as we can't have them time dependent. Not recommended but unavoidable 🙂

So yes, you could lower the time step at this point. 

[Jon.Wilde]

Good point, you would need to stop and start it as you have internal fans (my apologise), as we can't have them time dependent. Not recommended but unavoidable 🙂

So yes, you could lower the time step at this point. 

[Anonymous]

And how much should I lower time step? Is 0,25 OK? Can I start first few steps with 0,25 then stop the simulation and restart with 0,5?

Best regards, Branimir

[Anonymous]

And how much should I lower time step? Is 0,25 OK? Can I start first few steps with 0,25 then stop the simulation and restart with 0,5?

Best regards, Branimir

[Jon.Wilde]

Try it. If 0.25 diverges, it was most likely too large a step.

Ideally we don't want to stop/start but see how it looks when you do. Hopefully the convergence stays smooth.

[Jon.Wilde]

Try it. If 0.25 diverges, it was most likely too large a step.

Ideally we don't want to stop/start but see how it looks when you do. Hopefully the convergence stays smooth.

[Anonymous]

Hi wildej!

Just a short update about simulations and a small problem...

Till now I have done few smoke simulation transient analysis (with your suggested setup) and all looked fine. Now I added fake cars (represented as boxes) into the simulation, and guess what? It diverges when I turn on radiation. I uploaded simulation (with your setup which worked fine before adding fake cars) and added fake cars so if you have time to quickly check up what is wrong:

https://www.wetransfer.com/downloads/2ddc8d2a7cb14dbd0c470ea8d1cc438120150923175458/33dfc6

Best regards, Branimir.

[Anonymous]

Hi wildej!

Just a short update about simulations and a small problem...

Till now I have done few smoke simulation transient analysis (with your suggested setup) and all looked fine. Now I added fake cars (represented as boxes) into the simulation, and guess what? It diverges when I turn on radiation. I uploaded simulation (with your setup which worked fine before adding fake cars) and added fake cars so if you have time to quickly check up what is wrong:

https://www.wetransfer.com/downloads/2ddc8d2a7cb14dbd0c470ea8d1cc438120150923175458/33dfc6

Best regards, Branimir.

[Jon.Wilde]

Hi Branimir,

I see two openings here. Is there a clearly defined inlet and outlet?

[Jon.Wilde]

Hi Branimir,

I see two openings here. Is there a clearly defined inlet and outlet?

[Anonymous]

Actually not...because my intention is that system in the begining is left to itself...so there is no clear indication which opening is inlet and which outlet...

it worked when I didnt have false cars in simulation :s

[Anonymous]

Actually not...because my intention is that system in the begining is left to itself...so there is no clear indication which opening is inlet and which outlet...

it worked when I didnt have false cars in simulation :s

[Jon.Wilde]

Hmm, not sure if that can work. I am surprised it did before to be honest!

Let me take a look and see if I can figure anything out.

[Jon.Wilde]

Hmm, not sure if that can work. I am surprised it did before to be honest!

Let me take a look and see if I can figure anything out.

[Anonymous]

Hi wildej!

I have done some testing to help you out. I tried this new simulation (with fake cars) with and without fans. With fans I have p=0 boundary condition and V = 20700 m3/h boundary condition so theoretically CFD should know the way where should smoke go. Without fans I have two p=0 boundary conditions and the system is let to itself.

Result is that both simulations diverged, simulation with fans pretty early and simulation without fans a bit later (both convergence plots are attached to this post) and this new test simulations can be downloaded from here:

https://www.wetransfer.com/downloads/ad5a1c41c8ee9903ad0135589c7035ac20150927120627/30da6e

Waiting for your opinion and hopefully way how to resolve this issue. 🙂 Tnx!

Best regards, Branimir

[Anonymous]

Hi wildej!

I have done some testing to help you out. I tried this new simulation (with fake cars) with and without fans. With fans I have p=0 boundary condition and V = 20700 m3/h boundary condition so theoretically CFD should know the way where should smoke go. Without fans I have two p=0 boundary conditions and the system is let to itself.

Result is that both simulations diverged, simulation with fans pretty early and simulation without fans a bit later (both convergence plots are attached to this post) and this new test simulations can be downloaded from here:

https://www.wetransfer.com/downloads/ad5a1c41c8ee9903ad0135589c7035ac20150927120627/30da6e

Waiting for your opinion and hopefully way how to resolve this issue. 🙂 Tnx!

Best regards, Branimir

[Jon.Wilde]

Hi,

It might be wise to have a flow rate at the inlet and not the outlet. Or your pressure has to go from 0 at the inlet to a negative value.

Then we would probably need a longer outlet, I was testing this here and have quite a lot of reciculation here:

Even so, this is at iteration 61 for the model with fans 🙂

Hope that helps,

Jon

[Jon.Wilde]

Hi,

It might be wise to have a flow rate at the inlet and not the outlet. Or your pressure has to go from 0 at the inlet to a negative value.

Then we would probably need a longer outlet, I was testing this here and have quite a lot of reciculation here:

Even so, this is at iteration 61 for the model with fans 🙂

Hope that helps,

Jon

[Anonymous]

Hi wildej.

Strange thing is that everything is same like in simulation that you sent me and that is working. In this new simulation I just added fake cars and it starts to diverge...Without radiation this simulation works normal...

Best regards, Branimir

[Anonymous]

Hi wildej.

Strange thing is that everything is same like in simulation that you sent me and that is working. In this new simulation I just added fake cars and it starts to diverge...Without radiation this simulation works normal...

Best regards, Branimir

[Jon.Wilde]

Hi Branimir,

Is this a new file compared to the one I had running?

Thanks,

Jon

[Jon.Wilde]

Hi Branimir,

Is this a new file compared to the one I had running?

Thanks,

Jon

[Anonymous]

Which one?

09-23-2015 10:58 AM in reply to: wildej

09-27-2015 05:21 AM in reply to: wildej

Both files are practically same. Difference is that in second one I have done testing of few scenarios.

But both of this files are different from the simulation that you solved few months ago (inside this topic). This scenarios are different because I added fake cars (=boxes) inside simulation.

Hope everything is clear.

Best regards, Branimir

[Anonymous]

Which one?

09-23-2015 10:58 AM in reply to: wildej

09-27-2015 05:21 AM in reply to: wildej

Both files are practically same. Difference is that in second one I have done testing of few scenarios.

But both of this files are different from the simulation that you solved few months ago (inside this topic). This scenarios are different because I added fake cars (=boxes) inside simulation.

Hope everything is clear.

Best regards, Branimir

[Jon.Wilde]

Hi Branimir,

I ran the file that you posted on the 27th with a flow rate at the inlet rather than the outlet and it was fine. Was this the one that was a problem before?

Thanks,

Jon

[Jon.Wilde]

Hi Branimir,

I ran the file that you posted on the 27th with a flow rate at the inlet rather than the outlet and it was fine. Was this the one that was a problem before?

Thanks,

Jon