Combustor Design

Combustor Design (propulsion/gas-turbine-cycle/combustor-design)

Use when the task is the gas turbine combustor design point: the stoichiometric and operating fuel-air ratio from fuel properties and burner flows, the equivalence ratio, the combustion efficiency, the heat release and temperature rise across the combustor, and the adiabatic flame temperature estimate from a simple constant-specific- heat energy balance.

Domain quick reference

All values below are computed by scripts/combustor_design_logic.py (stdlib only, deterministic) and verified by running it.

Kerosene-class fuel: carbon mass fraction c = 0.86, hydrogen mass fraction h = 0.14, lower heating value LHV = 43.2 MJ/kg. Air is 23.2% oxygen by mass.

• Oxygen demand per kg of fuel: m_O2 = (32/12) * c + 8 * h. For kerosene: 2.6667 * 0.86 + 8.0 * 0.14 = 3.4133 kg O2 per kg fuel.
• Stoichiometric fuel-air ratio: far_st = 0.232 / m_O2 = 0.0680. (Methane, c = 0.75, h = 0.25, gives far_st = 0.0580; a carbon-rich fuel has a higher far_st because carbon needs less oxygen per kg.)
• Operating fuel-air ratio: far_op = m_fuel / m_air. At m_fuel = 2.0 kg/s and m_air = 100 kg/s: far_op = 0.0200.
• Equivalence ratio: phi = far_op / far_st = 0.0200 / 0.0680 = 0.2943 (fuel-lean, phi < 1).
• Combustion efficiency: eta_b = actual rise / ideal rise. With actual = 706.563 K and ideal = 713.7 K: eta_b = 0.9900.
• Heat release: Q = eta_b * m_fuel * LHV = 0.99 * 2.0 * 43.2e6 = 85.54e6 W, about 85.5 MW.
• Temperature rise across the combustor: delta_T = Q / (m_air * cp) = 85.54e6 / (100.0 * 1150.0) = 743.8 K. At compressor exit T2 = 700 K the combustor exit is T3 = 1443.8 K, near 1444 K.
• Adiabatic flame temperature estimate (constant specific heat): T_ad = T_in + eta_b * LHV * far / (cp_products * (1 + far)). Lean point (far = 0.0200, cp = 1300 J/(kg K)): T_ad = 1345.1 K. Stoichiometric point (far = 0.0680): T_ad = 2793.8 K, which overestimates the real flame temperature (roughly 2300 to 2400 K for kerosene in air) because dissociation is not modeled.

Units: flows in kg/s, LHV in J/kg, cp in J/(kg K), temperatures in kelvin, heat release in watts.

Workflow

1. Fix the fuel composition (c, h mass fractions) and the lower heating value; compute far_st with stoichiometric_far.
2. Fix the burner fuel and air flows; compute far_op with operating_far and the equivalence ratio with equivalence_ratio.
3. Determine the combustion efficiency from the measured and ideal temperature rise with combustion_efficiency.
4. Compute the heat release with heat_release and the temperature rise with temperature_rise; add the rise to the compressor exit temperature for the combustor exit temperature.
5. Estimate the adiabatic flame temperature with adiabatic_flame_temperature at the operating and stoichiometric fuel-air ratios.
6. Report fuel flow, heat release, combustor exit temperature, and flame temperature estimate.

Pitfalls

• Confusing this leaf with gas-turbine-cycle: that leaf computes the ideal Brayton cycle efficiency and station temperatures from the pressure ratio; the combustor block (fuel-air ratio, heat release, temperature rise, flame temperature) belongs here.
• Confusing this leaf with turbofan-cycle: bypass ratio, propulsive efficiency, and specific thrust are turbofan-cycle; the combustor design point of the core sits in combustor-design even for a turbofan engine.
• Using the higher heating value instead of the lower heating value: LHV must be used because water leaves the combustor as vapor.
• Treating the constant-cp flame temperature as exact: dissociation lowers the real stoichiometric flame temperature to about 2300 to 2400 K for kerosene in air; report the simple value as an estimate.
• Calling the operating fuel-air ratio stoichiometric: far_st for kerosene is near 0.068; far_op is typically 0.015 to 0.03, and phi = far_op / far_st is the lean/rich measure.
• Using the primary-zone air flow instead of the total combustor air flow: the mean exit temperature rise uses the full air flow including dilution air.
• Unit slips: LHV in J/kg with flows in kg/s gives watts; cp in J/(kg K); temperatures in kelvin, never Celsius.
• Fuel composition checks: c + h must sum to about 1 (trace elements are not supported); the functions raise ValueError on non-physical inputs instead of returning nonsense.

Behavior contract (gate 3)

The combustor relations above are exercised by the gate 3 contract test: scripts/test_combustor_design.py against scripts/combustor_design_logic.py (stdlib unittest, offline, 26 test methods). Run:

python3 skills/propulsion/gas-turbine-cycle/combustor-design/scripts/test_combustor_design.py

Compliance

• Standards referenced, not reproduced: FAR-33 is US government work (public domain) and covers engine type certification, not combustor analysis methods; the fuel-air ratio, heat release, and flame temperature relations are common-knowledge combustion thermodynamics, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.