Lecture 8: Modeling Surface Reactions 15.0 Release
Advanced Combustion Modeling
Outline •
Applications
•
Surface reaction models in Fluent –
Resolved and Unresolved surface reaction models
•
Key features
•
Types of surface reactions
•
Material set-up
•
Surface reaction solvers
•
Appendix A: Examples –
Kleijn CVD reactor
–
Catalytic combustion in a single channel
–
Carbon canister
Applications •
•
•
Modeling deposition –
Chemical Vapor Deposition (CVD)
–
Alternate Layer Deposition (ALD)
Modeling catalytic reactions –
Catalytic combustion
–
Selective catalytic reduction
Modeling adsorption/desorption adsorption/desorption –
CO2 adsorption
–
Hydrocarbon capture (Carbon canisters)
Surface Reaction Models Resolved Surfaces Model •
Surface reaction on resolved wall surfaces
Growth Rate of Gallium Arsenide in a vertical rotating disc reactor
Un-resolved Surface Model •
Surface reactions in porous media
Hydrocarbon capture in carbon canister
Surface Reactions: Key Features •
•
•
Multi-step reactions with multiple sites and site species –
Surface reaction products as reactants in other surface reactions
–
Local site balancing and desorption of gas-species from surface
–
Gas phase and surface species in a reaction
–
Ability to handle pure surface reactions
Different reaction mechanisms on different surfaces –
Different surface reactions on each surface and boundary
–
Deposition of multiple bulk species on the same surface
User defined outputs –
Separate tracking of deposition/etch rates for each bulk species
–
Contour plots of site species concentration and open sites
–
UDFs to customize the surface reaction rate (DEFINE_SR_RATE) UDFs to access adsorbed species concentrations on the surface
Tungsten film growth rate and uniformity in a 300-mm ALTUS system
Unresolved Surface Reactions •
Surface reactions in porous media –
Catalyst represented represented as porous media
•
Specify anisotropic thermal conductivity
•
Specify anisotropic species diffusion
•
Actual velocity calculation –
•
Accounts for fluid acceleration effects
Non-uniform porosity and resistance distributions in porous media
•
UDF functionality
•
Non-equilibrium thermal thermal model (NETM)
NETM for Porous Medium •
Porous medium (solid) and fluid are not in thermal equilibrium
•
Separate governing equations for fluid and solid zones –
Dual cell approach •
•
Fluid and solid zone interacts with each other through convective heat transfer at the interface –
•
Automatically creates creates cell zone for porous solid spatially coinciding with the porous fluid
Interfacial area per unit volume and heat transfer coefficient need to be specified
Useful for start up/cyclic processes (Unsteady)
Types of Species Involved •
•
•
Gas Species
Gas phase species –
Species in the gas mixture
–
Denoted by (g)
Flow Surface
Surface/Site species –
Species on the top-most layer of the solid
–
Denoted by (site)
Bulk/Solid species –
–
Species in the solid below the surface layer Denoted by (s) or (b)
Site
Solid
Bulk
Types of Surface Reactions •
Adsorption reaction –
–
AsH3(site) (site) + + Ga(b) AsH3(site)
Desorption reaction –
AsH3(site)
AsH3(g) (g) + + Open(site)
–
Can be using atomic site or open site
Adsorption using atomic site AsH3(g) Open(site)
AsH3(g) + Ga(site)
–
Si
Si(b)
AsH3(site) + Ga(b)
AsH3(site)
Adsorption using open site
Deposition reaction –
AsH3(site) Ga(b)
Using open site AsH3(g) (g) + + Open(site) Open(site)
•
Ga(site)
Using atomic site AsH3(g) (g) + + Ga(site)
•
AsH3(g)
AsH3(g) AsH3(site)
Open(site)
Material Set-up •
•
•
•
Gas, site and bulk species to be defined as type fluid Switch on wall surface reactions in the species transport panel Include required species appropriately Gas species
Define reactions – –
Volumetric Wall surface
Site species
Bulk species
Surface Reaction Set-up
Surface Reaction Set-up (Cont…)
Surface Reaction Import
Surface Reaction Import (Cont…) Allowed in Fluent •
• • •
Arrhenius reactions with third-body efficiencies Sticking coefficients (STICK) Duplicate reactions (DUP) Surface coverage modification (COV)
Not Allowed in Fluent •
• • •
Ion-Energy Dependent reaction (ENRGDEP) Bohm rate expressions (BOHM) Ion-Enhanced reaction Motz-Wise correction (MWON and MWOFF)
Surface Chemistry Solver • • •
Robust surface chemistry solver ODE solver when Newton solver fails User controls to select the type of solver manually using rpvariables –
Default approach, calls Newton solver first, if it fails, then calls ODE solver with same initial guess •
–
Use only Newton solver •
–
(rpsetvar 'species/surf-stiff-chem-method 2)
Option to disable reaction-diffusion balance – –
Essentially specify infinite diffusivity Can be controlled using TUI •
•
(rpsetvar 'species/surf-stiff-chem-method 0)
Use only ODE solver •
•
(rpsetvar 'species/surf-stiff-chem-method 1)
define/models/species/disable-dif define/models/ species/disable-diffusion-reaction-b fusion-reaction-balance alance
Options to include heat and mass source due to mass deposition
, − , , , ,
, , =
Appendix A: Examples 15.0 Release
Advanced Combustion Modeling
Example-1: Kleijn CVD reactor Inlet
Susceptor
Inlet Temperature Temperature = 300 K
Inlet
Outlet
Inlet Silane (SiH4) mole fraction = 0.001
Rotational Periodic
Inlet Helium mole fraction = 0.999
(Angle= 45O)
Inlet velocity = 0.0964 m/s
Susceptor rotational speed = 1000 RPM Susceptor Temperature Temperature = 1000 K
Domain in Fluent
Models •
•
Flow is laminar 26 volumetric and 13 surface reactions –
•
•
Kleijn
Stiff chemistry solver with ISAT error tolerance of 1e-8 is used Discretization schemes –
–
Pressure
PRESTO!
Momentum, Species and Energy Second Order Upwind
Overall cell count is 100 K
Volumetric Reactions
Surface Reactions
i
sticking coefficien t
0 for
Si3 H 8
M
Results: Axial Velocity 125.0
112.5
100.0
87.5
) 75.0 s / m m ( y t i 62.5 c o l e V l a 50.0 i x A
SPIN Fluent
37.5
25.0
12.5
0.0 0
1
2
3
4
5
6
7
8
9
Height above S usceptor (mm)
10
11
12
13
14
15
Results: Temperature 1000
950
900
SPIN
850
Fluent
800
750
) K 700 ( e r u t a 650 r e p m 600 e T 550
500
450
400
350
300 0
1
2
3
4
5
6
7
8
9
Height from Susceptor (mm)
10
11
12
13
14
15
Results: Silicon Deposition Rate 2.5
2.4
2.3
) s / 2.2 m n ( e t a 2.1 R n o 2 i t i s o p e 1.9 D
SPIN - 1D[Ref = Kleijn] 2D - Sim[Ref = Kleijn] 3D-Fluent
1.8
1.7
1.6 0
10
20
30
40
50
60
70
80
90
100
Radial Coordinate (mm)
11 110
12 120
13 130
14 140
15 150
Example-2: Single Channel Case Pt Catalyst, Catalyst, T=1290K CH4 air
D=2mm L = 10cm
•
Boundary Conditions – Volume Fraction: 3% CH4 – Inlet velocity: 5 m/s – Inlet temperature: 600 K
•
Gas species –
•
CH4, O2, H2, H2O, CO, CO2, N2, OH
Surface species –
Pt(s), H(s), O(s), OH(s), H2O(s), H3(s), CH2(s), CH(s), C(s), CO(s), CO2(s)
Surface Reaction Mechanism Reaction
A
B
E(J/kmol)
H2+2PT(s) => 2H(s) 2H(s) => H2+2PT(s) O2+2PT(s) => 2O(s) O2+2PT(s) => 2O(s)
4.36E7 3.7E20 1.8E17 2.01E14
0.5 0.0 -0.5 0.5
0.0 6.74E7 0.0 0.0
2O(s) => O2+2PT(s) H2O+PT(s) => H2O(s) H2O(s) => H2O+PT(s) OH+PT(s) => OH(s) OH(s) => OH+PT(s) H(s)+O(s) => OH(s)+PT(s) H(s)+OH(s) => H2O(s)+PT(s) OH(s)+OH(s) => H2O(s)+O(s) CO+PT(s) => CO(s) CO(s) => CO+PT(s) CO2(s) => CO2+ PT(s) CO(s)+O(s) => CO2(s)+PT(s) CH4+2PT(s) => CH3(s)+H(s) CH3(s)+PT(s) => CH2(s)+H(s) CH2(s)+PT(s) => CH(s)+H(s) CH(s)+PT(s) => C(s)+H(s) C(s)+O(s) =>CO(s)+PT(s) CO(s)+PT(s) => C(s)+O(s) OH(s)+PT(s) => H(s)+O(s) H2O(s)+PT(s) => H(s)+OH(s)
3.7E20 2.37E8 1.0E13 3.25E8 1.0E13 3.7E20 3.7E20 3.7E20 7.85E15 1.0E13 1.0E13 3.7E20 2.3E16 3.7E20 3.7E20 3.7E20 3.7E20 1.0E17 1.56E18 1.88E18
0.0 0.5 0.0 0.5 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2.13E8 0.0 4.03E7 0.0 1.93E8 1.15E7 1.74E7 4.82E7 0.0 1.25E8 2.05E7 1.05E8 0.0 2.00E7 2.00E7 2.00E7 6.28E7 1.84E8 1.15E7 1.74E7
H2O(s)+O(s) =>OH(s)+OH(s)
4.45E20
0.0
4.82E7
Mass Fraction of Major Gas Species
Surface Coverage Profiles Major species
Minor species
Example-3: Carbon canister •
•
•
Devices used in automotive to capture hydrocarbon vapor losses Activated carbon elements in the canister adsorb fuel vapor and feed it back into the engine Saves the fuel and reduces hydrocarbon emission Carbon canister
Test case •
n-butane vapor + air
•
Porous zones
n-butane vapor and air enter at the inlet –
Mass flow flow rate = 1e-5 kg/s
–
n-butane mass fraction = 0.675
n-butane is captured at three porous zones using surface reactions –
• •
Made up reaction rates
Outer walls: adiabatic Fluid and solid temperatures in the porous zones are monitored using non equilibrium thermal model in Fluent 14
Setup: Species properties •
n-butane and air
•
Properties from Fluent database
•
n-butane-site
•
Standard state enthalpy –
Assumed such that the adsorption reaction is exothermic
•
Standard state entropy = 0
•
Open-site –
Molecular weight = 0
–
Standard state enthalpy = 0
–
Standard state entropy = 0
Set-up •
Reactions: Reaction C4H10 + Open_s C4H10 _s
•
Pre- exponentia exponentiall constant 10000
C4H10 _s
10
C4H10 + Open_s
Solid material: Carbon-BAX-1500 448 kg/m3
–
Density
–
Specific Heat
–
Thermal Conductivity
900 J/kg-K 1.5 W/m-K
Setup: Non equilibrium thermal model •
Inputs
•
Interfacial area density –
Surface area to volume ratio
•
Heat transfer coefficient
•
Note:
Set surface area to volume ratio in reaction tab as well
in the Fluid dialog dialog box, in order to save settings in the Porous NOTE: Before enabling the non-equilibrium thermal model, click OK in Zone tab.
If it is not saved, they will be reset to the default values while enabling the non-equilibrium thermal model
Solution parameters •
Pressure based segregated solver
•
P-V coupling: SIMPLE
•
Discretization: –
Momentum, species and energy •
•
Second order
Simulation: Transient –
Time step size: 10s
–
Total flow time: ~1500s
Hydrocarbon (n-butane) capture
Temperature variation Fluid side
0
Solid side
Time
t
Temperature at center of porous zone-1
) K ( e r u t a r e p m e T
(s)
Higher temperature on fluid side when reaction front reaches this location
Summary Surface reactions •
Resolved & unresolved surfaces
•
Options to import mechanism in CHEMKIN format
•
Robust surface stiff chemistry solvers
Examples & applications •
Kleijn CVD reactor
•
Single channel case
•
Carbon canister
Tutorials •
Catalytic combustion tutorial
•
CVD tutorial
