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Non-Premixed Combustion in a Rocket Application

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Old   July 25, 2022, 11:43
Question Non-Premixed Combustion in a Rocket Application
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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.
________________________________________

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
________________________________________
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?




Thank you!
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