Combustible Objects Model
When a solid material burns, it does not simply disappear. It converts into gas, releasing heat, generating pressure, and continuously reshaping the flow domain as the solid recedes.
The Combustible Objects Model in FLOW-3D simulates the conversion of a solid material into gas, accounting for the evolving geometry of the burning solid, the mass and energy released by combustion, and the resulting compressible gas dynamics. The model applies to any engineering problem where solid material is progressively consumed by a combustion reaction. In the example shown below the model is applied to the solid propellant in a rocket motor.
A 10-second solid propellant combustion simulation in FLOW-3D. The combustible component is shown in orange, the steel casing in gray. Streamlines are colored by gas velocity magnitude. The solid propellant volume plot shows its diminishing size over the burn.
How the Model Works
The solid propellant is represented as a special type of geometry component that is stationary but changes shape over time as it burns. The FAVORTM method tracks the changing geometry at every time step, recomputing the area and volume fractions that define the solid boundary as it recedes.
The combustion rate is governed by an empirical pressure-dependent burning law. The burned solid mass is converted to gas at the solid-gas interface, and the energy released by combustion is applied as a heat source at the interface. The combustion gas is then transported away by the flow towards the nozzle.
The mass source at the burning interface is treated as a stagnation source, meaning the initial velocity of the combustion gas is assumed to be zero. This is consistent with the assumption that gas is generated at rest at the propellant surface and is subsequently accelerated due to the buildup of pressure at the interface.
Coupling with Other Models
The Combustible Objects Model works in combination with other FLOW-3D physics models:
- The Compressible Flow Model, since combustion generates high-pressure gas that cannot be treated as incompressible
- The Fluid Structure Interaction Model can be optionally activated when elastic stresses within the solid propellant need to be computed in response to high gas pressures and temperatures, for example, to predict structural failure of the propellant grain
- Turbulence Models can be applied to the gas flow in the chamber and nozzle, though direct turbulence effects on the burn rate are not included.
Model Assumptions
The Combustible Objects Model uses the following simplifying assumptions:
- The combustion rate is controlled by empirical coefficients; the detailed chemistry of the combustion reaction is not modeled
- The spent solid material is converted into gas with equivalent mass, which is then transported by the computed flow field
- Combustion gas density is computed from the ideal gas equation of state
- Direct turbulence effects on the burn rate are ignored
Applications
The Combustible Objects Model is used in FLOW-3D for:
- Solid propellant rocket motor performance analysis, including thrust generation, chamber pressure evolution, and burn time prediction
- Propellant grain geometry optimization to achieve target thrust profiles
- Combustion chamber and nozzle flow analysis, including pressure and temperature distribution, velocity field, and combustion gas transport
- Structural integrity assessment of propellant grains under combustion pressure, in combination with the Fluid Structure Interaction model