[zahrae]

Hello!
I have been trying to write a UDF to compute the deposition rate on a wall using the DEFINE_DPM_EROSION, I have used the trap BC at the wall and when I use the following UDF I didn't get anything.
Any help, please? can anyone help me to see the
I am confused I don't know what I have done wrong!! I am a beginner in UDF's.
Regards.

[/#include "udf.h"


DEFINE_DPM_EROSION(dpm_accr, p, t, f, normal, alpha, Vmag, Mdot)
{
real A[ND_ND], area;
Thread *t0;
cell_t c0;
c0 = F_C0(f,t);
t0 = THREAD_T0(t);
F_AREA(A,f,t);
area = NV_MAG(A);
F_STORAGE_R(f,t,SV_DPMS_ACCRETION) += Mdot / area;
}
]

[vinerm]

Instead of trap, try reflect bc. Check for the values of Mdot; it is quite possible that it is 0 for trap, as it should be.

[zahrae]

Thanks for your reply, sir!
I have to use trap BC, the aim of my study is to investigate the deposition of particles (they stick to the surface and then compute the deposition rate).
I have tried to compute it manually by the following eq:
md= (Nd.mp)/A.t
where:
Nd= the number of deposited particles
mp= mass of particles
A=area of the domain
t=the computation time
and I use also the accretion presents already in fluent. the value obtained manually was 7,81e-5 [Kg/m².s] while for accretion I get a very low value of deposition (6E-21 Kg/m²). I am not sure which is the correct one so I decided to use the erosion macro.
I used unsteady tracking is that mean that the accretion calculated concern the last time step? because as you can see the value was found in kg/m² .

[zahrae]

Any guidelines please!!

[vinerm]

Accretion calculated by Fluent is a rate of deposition and not the accumulated one. If you want accumulated value, then use sampling with accumulation enabled.

[zahrae]

Thanks, sir for your reply!
I found that the accretion is related to the particle flow rate indirectly ( via the parcel mass) I increase it and I get an increased value of accretion. But I don't know if it will influence the volume fraction of particles ( I should stay in a volume fraction less than 10% of the volume of the fluid to use the DPM) and also I have to consider the one-way coupling approach, as I understand from the definition of volume fraction, it will not influence it !!
so sir, how can I choose the value of particle mass flow rate, based on what exactly?
Thanks in advance.

[vinerm]

Accretion certainly is dependent on particle flow rate because more particles going in the domain imply higher availability of the particles and higher probability of sticking; in other words, more the sugar in the water, sweeter it gets.

As far as the particle flow rate is concerned, no one can tell you that. It depends on the actual system you are modeling. You have to take that number from the field or the experiment that you are trying to validate against or working on. Do NOT use one-way coupling if volume fraction is higher than 1%. More specifically, check Stokes number value and if it is closer to or larger than 1, use two-way coupling even if the volume fraction is less than 1%.

It is not one-way coupling, but DPM itself is invalid if the volume fraction is more than 10-12%.

[zahrae]

Thank you, sir!!
Can you please tell me how can I compute particle Reynolds number in Fluent?
and also for particle volume fraction, I consider that all the particles injected at the domain are trapped and compute it as follow:
volume fraction= Ntrapped*volume of each particle
is it correct?

[vinerm]

Particle Re is available under Particle Tracks Display. If you want it in the UDF, then also is directly available.

And volume fraction is not be calculated by what is happening in the Fluent, rather based on the inputs. You are specifying a certain mass flow rate of the particles and you also know the density and particle size. So, you can calculate the volume flow rate. On the other hand, you can also calculate the volume flow rate of the continuous fluid. Take the ratio of particle volume flow rate and the sum of particle and continuous phase volume flow rate. That will indicate volume fraction.

[zahrae]

Sir, I can't understand this part
"Take the ratio of particle volume flow rate and the sum of particle and continuous phase volume flow rate".

should I divide the particle flow rate by the total number of injected particles in specific time duration(because I use unsteady tracking)and then divide the obtained value by the volume flow rate of the continuous phase??

[vinerm]

While determining volume fraction, we don't need to include any aspect of CFD or modeling, such as, number of particles tracked or captured, etc.. Just calculate volume flow rate of particles, which is ratio of mass flow rate of particles and its density. Lets call it . Then calculate volume flow rate of continuous phase, which is mass flow rate of continuous phase divided by its density; call it . Then volume fraction is given by

[zahrae]

Thank you so much sir for your valuable help!