ceaToFoam

From a NASA CEA chamber solution to a ready-to-run OpenFOAM thrust-chamber case.

Mach number (upper half) and temperature (lower half) in an RP-1/LOX thrust chamber at 500 psia and O/F 2.4, sized for 2250 lbf at sea level with the frozen mixture and computed with the k-ω SST model.

ceaToFoam takes the propellants, chamber pressure, mixture ratio and design thrust of a liquid-rocket engine and writes an axisymmetric rhoCentralFoam case for the flow through its conical thrust chamber. The chamber composition computed by CEA is frozen and evaluated with one set of thermodynamic and transport data, and this one description of the gas both sizes the nozzle and defines the thermophysical model of the simulation. The design point and the CFD therefore describe the same gas, and the difference between them is the multidimensional and viscous loss of the thrust chamber rather than an artefact of the handoff.

What it does

flowchart LR
    A["Operating point<br/>propellants, O/F, p<sub>c</sub>,<br/>thrust, ambient"] --> B["NASA CEA<br/>(CEA_Wrap)"]
    B --> C["Frozen chamber mixture<br/>(Cantera)"]
    C --> D["Design point<br/>throat, exit, c*"]
    C --> F["janaf and Sutherland<br/>fits"]
    D --> E["Conical contour<br/>wedge mesh"]
    E --> G["rhoCentralFoam case"]
    F --> G
    G --> H["ceatofoam post<br/>thrust, mass flow,<br/>exit state"]

1. CEA is run for the propellants, chamber pressure and mixture ratio, with shifting equilibrium and frozen at the chamber.
2. The chamber composition is frozen and evaluated with Cantera: molecular weight, \(c_p(T)\), \(h\), \(s\) and viscosity.
3. The throat and exit are sized for the design thrust at matched expansion, by default along the isentrope of the frozen mixture.
4. A conical contour is built (cylinder, fillet, convergent cone, throat arcs, divergent cone) with the chamber volume given by \(L^*\).
5. A 5° wedge blockMeshDict and a rhoCentralFoam case are written, with janaf and sutherland coefficients fitted to the same frozen mixture.
6. After the run, ceatofoam post evaluates the thrust, the mass flow rate and the exit state against the design point.

Results in brief

The procedure has been verified by a grid convergence study and validated against wall-pressure and discharge-coefficient measurements in conical nozzles (see Verification and validation):

Check	Result
Frozen sizing reproduces CEA's frozen solution	exit temperature, Mach number and \(I_{sp}\) within 0.1 %
Simulated exit pressure vs. design, frozen sizing, inviscid	within 2.03 % (RP-1, CH₄, H₂ with LOX; 500 and 1500 psia)
Simulated mass flow rate vs. design, frozen sizing, inviscid	within 0.542 %
Wall pressure, Back et al. (1965)	RMS deviation of \(p/p_t\) at most 0.018
Discharge coefficient, Back and Cuffel (1971)	within the measurement uncertainty for 5 of 7 nozzles
Base mesh vs. finest mesh	thrust, mass flow, \(p_e\), \(I_{sp}\) within 0.27 %

Sizing the same nozzles with the equilibrium isentropic exponent \(\gamma_s\) instead gives exit pressures 16 to 21 % below the design value in the frozen-flow simulation, while the mass flow rate stays within 0.7 % of it. The Method page explains why.

Quick start

git clone https://github.com/MCF2-Lab/ceaToFoam.git && cd ceaToFoam
pip install .

ceatofoam design examples/rp1-lox-500psi.yaml             # design point
ceatofoam case   examples/rp1-lox-500psi.yaml cases/rp1   # write the case
cd cases/rp1 && ./Allrun                                  # run OpenFOAM
ceatofoam post   .                                        # -> results.json

The CEA executable has to be compiled once on Linux and macOS; see Installation.

Scope

ceaToFoam is meant for preliminary thrust-chamber design studies, for separating the losses of a conical nozzle from its thermochemistry, and for teaching. The composition is frozen at the chamber (no finite-rate or shifting chemistry in the nozzle), the gas is ideal, the walls are adiabatic and the contour is conical. Injectors, combustion and wall cooling are not modelled.