Non-Premixed Combustion in a Rocket Application
 

Overview

I have a bipropellant rocket engine combustion simulation utilizing gaseous propellants that I'd like to model within ANSYS Fluent but had several questions and clarifications. I have listed the model setup below.
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Model Setup

SST K-omega: All default settings.

Energy Option (On)

Species: Non-Premixed Combustion
Compressibility Effects: (On)
State Relation: Chemical Equilibrium
Energy Treatment: Non-Adiabatic
Equilibrium Operating Pressure: 14.7 PSI
Fuel Stream Rich Flammability Limit: 0.6875
Boundary:
CH4, Fuel: 1
O2, Oxid: 1
All else, 0
Temperature for both, 300K
Table: Default Table Parameter Settings except for,
Minimum Temperature: 273 K

Inlets
2 mass-flow inlets; 1 for the methane inlet, 1 for the oxygen inlet.
Turbulence
Intensity: 5%
Hydraulic Diameter: (Fluid diameter of inlet).

Outlet (Pressure-outlet)
Turbulence:
Intensity: 5%
Hydraulic Diameter: (Fluid diameter of exit nozzle)
Backflow Total Temperature: 1700K (an arbitrary guess)


Solution

Methods: All methods default except for,
Pressure: Selected PRESTO!
Controls: All controls default except for,
Body Forces: Input "0.8"
Hybrid Initialization
Time Scale Factor: 1
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Questions
1. A rocket engine gradually increases in speed from subsonic (~0.3) in the combustion chamber to Mach 1 at the throat then proceeds to go through the supersonic regime out of the nozzle. Is utilizing the "Compressibility Effects" option as part of the Non-Premixed Combustion model applicable? Official tutorial documents state to "use caution" with the compressible model.

2. Is the "Equilibrium Operating Temperature" within the Non-Premixed Combustion menu supposed to be the design Chamber Pressure of the rocket engine? Or is this the ambient pressure?

3. The flammability ternary diagram for methane and oxygen is below (Source; Mashuga, Crowl 1999). For a mixture without nitrogen, the highlighted scale is used to get the mixture fraction for oxygen/methane. The fuel-rich flammability limit in this diagram is around 0.6875 (corresponding to an oxygen fraction of 0.3125). On pg. 2077 of the ANSYS Fluent User Guide, it says that "A fuel stream rich flammability limit of approximately twice the stoichiometric mixture fraction is appropriate". For ox/methane mixtures the stoichiometric mixture fraction is 0.2 (as per the Fluent PDF calculation) therefore the calculated fuel stream rich flammability limit would be (2 * 0.2) = 0.4. However that is still below the 0.6875 that is shown in the graph. Which value is correct to use in this application?

4. What is "Backflow Total Temperature"? I was instructed to input an arbitrary guess but do not understand the calculation behind it. Is it the backflow of the fluid or the ambient environment recirculating?

https://imgur.com/a/bqeAFxv


Thank you!

I don't know about the correct value for flammability limit but when I applied 0.68, the simulation crashed. While I applied 0.061 which is true for CH4 and Air combustion scenario, the simulations crashed. So I believe the warnings that show up after applying any number other than what FLUENT need to be set shows good recognition in the FLUENT software.


The Boundary condition for outlet can be setup as 'prevent backflow at outlet' condition, then you don't need to setup the outlet backflow temperature

Comments on several points of yours
 

- turbulence model: better use k-epsilon with enhanced wall treatment. The blending funcito of sst does not perform well in combustion environment.

- species: this mode, is old and does not perform well. Either use flamelet, or finite rate.
falemet is very buggy in fleunt. it takes time to get used to it. finite rate with Eddy-dissipation-concept model is what i used to use. Unfortunatelly, it is very expensive.


- Solution: i end up using a coupled solver. But the initialisation you can do e.g. with simple.

Answers to your questions:
1. don't know
2. don't know
3. don't know
4. the temperature which gases would have if they would be entering the domain from the outlet. "total" means accodring to bernoulli static + dynamic. This means that if your gases are faster, theri actual (static) temperature would be lower. Better use "static temperature " for htis boundary condition.