simulation of cvd-reactor

Christian

Hello Fluent-users,

I'm working on the simulation of an atmospheric cvd-reactor. So far I'm using only a simplified 2d-geometry. The carrier-fluid is hydrogen, the other reactant is trichlorosilane Si-H-Cl3. I've defined one surface reaction on the two side walls, which are at 1473 K:

H2 + SiHCl3 -> Si + 3 HCl

In my first version the surface reaction rate is defined with the Arrhenius model, in a second version with an UDF. All results for the surface deposition rates are quite reasonable, but the concentration of hydrogen in the reactor gives me a headache. The concentrations of hydrogen and trichlorosilane CLOSE to the walls in direction of the flow become smaller - as expected. Trichlorosilane and hyrogen react on the walls, silicon is deposited on the walls and HCl is created. BUT the contour plot of the concentration of hydrogen shows, that after a couple of centimeters behind the reactor entrance (infront of the reactor is an inlet channel with the same diameter and 1000K wall-temperature) the LOWEST concentration of hydrogen is in the MIDDLE between the two parallel walls and NOT on the walls, where the surface reaction takes place!!?? (concentration of SiHCl3 is decreasing from the sidewalls to the center, concentration of HCl is increasing from the sidewalls - as expected) I'm using the dilute-approx. model for mass diffusivity with polynomial coefficients - we've checked the values.

Trying to summarize the effects: the gas mixture arrives at the reactor entrance with a temperature of 1000 K and a parabolic velocity profile. The walls in the reactor are at 1473 K. The gas is heated from 1000 K to 1473K, density decreases, velocity increases. Surface reactions on the sidewalls: H2 + SiHCl3 -> Si + 3 HCl i.e. 2 molecules "disappear" from the volume, 3 HCl molecules go into the volume, 1 Si stays on the surface. Concentration of HCl increases in the volume beginnig from the sidewalls. Concentration of SiHCl3 decreases in the volume beginning from the sidewalls. HYDROGEN: on the first few centimeters in the reactor the concentration of H2 decreases beginning from the sidewalls. Then comes a little area with homogenous concentration of H2 between the sidewalls. In the following 80% of the reactor the concentration of H2 is LOWER in the center of the reactor than close to the sidewalls, where the surface reaction happens.

Does anybody understand this effect? Can anybody give me an idea what's wrong?

Thanks, Christian