Water droplet evaporation and doplet size distribution

[MIA_CZ]

Hello,

I solve a huge domain model where a water mist spray is inserted in space with high temperature (approx. 300 °C). The space is a room 4 x 4 x 2 m, divided into 4 approximately same meshes. There are 8 wooden bonfires symmetrically deployed in the front half of the room (mesh2 and mesh4). Their burning is specified with HRRPUA = 413 and RAMP function. Air can flow in the room through the window, 1.2 m x 2 m, in the front wall. Two nozzles are situated on the side wall of the room in the mesh3 and are oriented to the space of mesh3 and mesh1. The scheme of meshes and the picture of the domain are in the supplement. 

The basic spray settings are:

&SPEC ID='WATER VAPOR' /

&PART ID='Water',

SPEC_ID='WATER VAPOR',

DIAMETER=60.0,

QUANTITIES='PARTICLE DIAMETER','PARTICLE TEMPERATURE',

DISTRIBUTION='LOGNORMAL',

SIGMA_D=1.5,

AGE=10.0,

SAMPLING_FACTOR=1

CHECK_DISTRIBUTION=.TRUE.,/

 

&PROP ID='Water Spray',

PART_ID='Water',

OFFSET=1.0E-3,

K_FACTOR=2.15,

OPERATING_PRESSURE=10.0,

FLOW_RAMP='Generic Commercial Spray_FLOW_RAMP',

PARTICLE_VELOCITY=37.53,

SPRAY_ANGLE=0.0,55.0/

SMOKEVIEW_ID='NOZZLE'/

I tried several versions variated in these &PART and &PROP parameters:

MINIMUM_DIAMETER=20.0,                                                                                                           MINIMUM_DIAMETER=20.0,

MAXIMUM_DIAMETER=200.0,                                                   or                                                 MAXIMUM_DIAMETER=500.0,

PARTICLES_PER_SECOND=1000 or 5000 or 10 000                                  PARTICLES_PER_SECOND=1000 or 5000 or 10 000

No matter which size interval or how many particles were inserted per second, warnings were appeared during the computing and three of six models were stopped. The warning was exactly written as WARNING: DELTA TMP_G Mesh3 Particle 1191.  What does it mean? And could it cause the break of computing?

I tried also small domain cases for the same spray settings with just one burning bonfire and the warning did not appeared. Models were stable and were finished without any problems. Could the instability be caused by a huge amount of heat present in the domain and very quick evaporation of droplets?

The minimum diameter of droplets in the huge domain cases depicted by SMV is between 6 to 8 μm. Can I say this is the minimum FDS can model?

In FDS User´s Guide is written that droplets less than a specified MINIMUM_DIAMETER are assumed to evaporate in a single time step. Does it mean the vapor generated in the single time step is present in the domain or the droplets just disappear?

My next questions are about the particle size distribution. When I define the minimum and maximum range of particles (It is not important if I talk about 20-200 μm or 20-400 μm, because the generated CDF and CNF were exactly the same.) why is the range of diameters in CHID_Water_cdf.csv file 1 – 140 μm? I also do not understand the generated CNF because it looks like the spray consists just from droplets smaller than the minimum diameter. One example of CHID_Water_cdf.csv file is in the supplement. I must admit I did not try to check the CHID_Water_cdf.csv file for spray model in domain without any source of heat, but I understand the line CHECK_DISTRIBUTION=.TRUE. as the control mechanism, which give me feedback about size distribution near the nozzle. Could you please help me to understand it better?

Thank you very much

[Kevin McGrattan]

What version of FDS are you using?

Can you create a simple example and post it to the Issue Tracker. 

[Michaela Pechová]

I use FDS 6.7.1. 

I will simplify the huge domain model and the description of the problem and post it on the Issue Tracker.

čt 23. 5. 2019 v 14:36 odesílatel Kevin McGrattan <mcgr...@gmail.com> napsal:

What version of FDS are you using?

Can you create a simple example and post it to the Issue Tracker. 

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[dr_jfloyd]

FDS sets a kill radius that is 0.5 % of the mass of the MINIMUM_DIAMETER particle. In your case that is about 3.4 um. This is why you are seeing 6 um in Smokeview.

Within the evaporation routine FDS first tries to evaporate the drop using the current time step. It then does some checks to see if the equilibrium condition is overshot or if the local gas temperature changes too much. If this is the case it tries again at half the timestep. This process is repeated until the droplet passes the checks or the timestep drops too much. If the timestep drops too much a warning is written. In your case the warning is saying that even at a very small sub timestep the gas temperature is still seeing large changes. 

You image shows a narrow cone angle compared to your input. Your OFFSET maybe too small for your grid resolution.

In terms of your CNF, what have you set for DIAMETER? This controls the median for the distribution.

[Michaela Pechová]

Thank you for explanation of the process.

The DIAMETER was set to 60 um. 

pá 24. 5. 2019 v 15:14 odesílatel dr_jfloyd <drjf...@gmail.com> napsal:

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