I am using ANSYS Fluent to model a 2D microfluidic channel. The channel is 50um tall and 1.5cm long. This model is of cells on the bottom surface consuming oxygen and producing dissolved waste gas, which steadily builds up in the channel.
To perform this model, I am using the Energy Model, the Laminar Flow model, and the Species Transport model for multicomponent diffusion. I have also enabled wall surface reactions.
The components modeled are liquid water with oxygen and nitrogen dissolved, therefore water is the bulk medium and last added to the species list. Silicon (Si) is also added to the site-specific species in the species window.
The inlet O2 mass fraction is 6.88e-08, and the pressure is 3.6Pa, giving the microfluidic channel a 7.5e-5m/s maximum flow rate.
The temperature of the fluid in the channel is 37C/310.15K, giving the oxygen and nitrogen a diffusion coefficient of 3e-9m2/s. Because of this high diffusion coefficient, the flow should be diffusion-dominant.
The reaction modeled is 1O2+Si(site) -> 1N2 + Si(site) with an Arrhenius pre-exponential factor of 1, a temperature coefficient of 0, and 0 activation energy.
During this simulation, I find that both the O2 and N2 rapidly diffuse through the reaction wall, with N2 that is produced by this reaction rapidly diffusing out of the chamber through the reaction wall instead of steadily building up. I would like to find out how to specify zero species diffusive flux through the reaction wall.
Alternatively, I have written the following species wall flux UDF:
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#include "udf.h"
DEFINE_PROFILE(pressure_profile,t,i)
{ real x[ND_ND]; /* this will hold the position vector */
real y;
cell_t c;
cell_t c0; /* this will hold the face-adjacent cell */
face_t f;
Thread *t0; /* this will hold the face-adjacent cell thread*/
begin_f_loop(f,t)
{ c0 = F_C0(f,t); /* this will hold the face-adjacent cell */
t0 = THREAD_T0(t); /* this will hold the face-adjacent cell thread*/
F_CENTROID(x,f,t); /* this will find the centroid of the face-adjacent cell*/
y = x[0]; /* this extracts the x-position of the face-adjacent cell thread centroid*/
F_PROFILE(f,t,i) =.0001*y*y - .000005*y + .0000000688; /* this fits the wall profile species concentration to a pre-defined quadratic curve*/
} end_f_loop(f,t) }
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This UDF achieves the desired curve but is not considered a solution because it depends on foreknowledge of the solution.
Because of this, I have written another UDF to achieve the desired curve:
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#include "udf.h"
DEFINE_PROFILE(pressure_profile,t,i)
{
real x[ND_ND]; /* this will hold the position vector */
real y;
cell_t c;
cell_t c0; /* this will hold the face-adjacent cell */
face_t f;
Thread *t0; /* this will hold the face-adjacent cell thread*/
begin_f_loop(f,t)
{
c0 = F_C0(f,t); /* this will hold the face-adjacent cell */
t0 = THREAD_T0(t); /* this will hold the face-adjacent cell thread*/
F_CENTROID(x,f,t); /* this will find the centroid of the face-adjacent cell*/
y = x[0]; /* this extracts the x-position of the face-adjacent cell thread centroid*/
F_PROFILE(f,t,i) =C_YI(c0, t0, 1)*.9999999999999999999999999999999999999999; /* this extracts the species concentration of oxygen of the face-adjacent cell thread centroid and multiplies it by a number less than 1*/
}
end_f_loop(f,t)
}
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Can you please help me figure out how to prevent species diffusion through the surface reaction wall?
Or alternatively can you help me write a UDF that sets the boundary condition for species concentration such that the mass fraction at a given position is concentration-dependent for that position?
Or alternatively, can you help me write a UDF that specifies the species wall flux as a boundary condition?
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