afterburner-design

name	afterburner-design
description	Afterburner (reheat) design — V-gutter flameholder, fuel injection, combustion efficiency, turbine exit conditions, augmentation ratio, screech/buzz instability, military jet engine thrust augmentation.
metadata	{"priority":7,"promptSignals":{"phrases":["afterburner design","reheat combustion","flameholder","thrust augmentation","afterburner efficiency","V-gutter","military jet engine"],"minScore":3}}

Afterburner Design — Complete Skill

Afterburner (Reheat) Purpose and Performance

Purpose: augment thrust by burning additional fuel downstream of turbine Primary benefit: large thrust increase with relatively simple hardware Cost: very high specific fuel consumption during augmentation

Thrust augmentation ratio: AR_T = F_AB / F_dry [typically 1.5–1.8 for military fighters]

Fuel flow during augmentation: ṁ_f,AB = ṁ_f,dry × (AR_T - 1) × TSFC_dry / TSFC_AB

SFC in afterburner: TSFC_AB = 3–5× TSFC_dry (very high; only used for short durations: < 5–30 min total)

Thermodynamics of Afterburning

Afterburner Entry Conditions (Station 7)

Conditions from turbine exit (station 5): T_7 = T_5 ≈ T₀₄ / (T₀₄/T₅) — depends on turbine expansion ratio Typical T_7 = 700–1100 K O₂ content: bypass duct provides excess O₂ (turbofan); or excess from lean primary zone (turbojet)

Stoichiometric temperature (maximum T for AB): T_7_stoich ≈ 2500–2700 K (depends on inlet T₇ and excess O₂)

Afterburner exit temperature T₉: T₉ = T₇ + η_AB × f_AB × LHV / C_p [f_AB = fuel-air ratio added; η_AB = AB efficiency] Limit: T₉ ≤ T₉_limit (turbine liner material; typically 1800–2100 K for T₉ with thrust modulation)

Thrust Gain from Temperature Rise

Velocity ratio from temperature: V₉/V₇ ≈ √(T₉/T₇) [for unchoked nozzle]

Thrust increase: ΔF = ṁ × (V₉ - V₇) = ṁ V₇ (√(T₉/T₇) - 1)

Maximum T₉ before limit: typically set by liner material or nozzle constraints

Flameholder Design

V-Gutter Flameholder (Most Common)

V-shaped cross-section; creates recirculation zone for flame stabilization Function: low-velocity wake downstream of V-gutter; recirculated hot gases ignite incoming mixture

V-gutter geometry: Opening angle: 90° typical (inner angle of V); bluntness maximizes recirculation Width w: 30–80 mm; larger → more stable but more pressure drop Drag: C_D ≈ 2.0 (based on frontal area)

Blockage ratio B: B = Σ (gutter frontal area) / duct area = 0.20–0.35 (30–40% typical) Higher B → more stable flame; more pressure drop

Stability parameter: Damköhler number Da = τ_recirculation / τ_combustion > 1 for stable flame τ_recirculation ≈ w / V_approach [s] τ_combustion ≈ δ_L / S_L [s; flame thickness / laminar burning velocity]

Ignition: spark plug or hot streak igniter at V-gutter apex; ignite during light-off

Radial Flameholder Arrangement

Multiple V-gutters on concentric rings + radial spokes Inner zone: rich combustion; outer zone: lean; mixed to meet T₉ requirement Typically: 1 inner ring + 1 outer ring + radials connecting

Fuel Injection

Manifolds: spray bars or ring manifolds parallel to V-gutters Atomization: pressure atomizing nozzles; V₂ > 30 m/s airstream → aerodynamic shatter

Fuel placement: inject upstream of V-gutter for pre-vaporization Pre-vaporization length: 100–300 mm; SMD (Sauter Mean Diameter) of droplets: 50–100 μm

Fuel-air ratio control: Low augmentation (partial AB): inject only in one zone Full augmentation: all zones active; f_AB = 0.04–0.06 (stoichiometric)

Staging: Stage 1 (pilot): inner zone; most stable; always lit during augmentation Stage 2: intermediate zone Stage 3: full augmentation; outer zone

Combustion Efficiency

Afterburner combustion efficiency: η_AB = (T₉_actual - T₇) / (T₉_max_stoich - T₇) × (f_stoich / f_AB)

Typical η_AB = 0.85–0.97 for well-designed AB Lower efficiency: poor atomization, short residence time, instability

Residence time: τ_res = L_AB × A / Q_fuel_air [L_AB = afterburner length; Q = total volumetric flow]

Combustion loading parameter (stability chart axis): Ω = P₇^n × V_ref × exp(-E_a/R T_7) [Longwell parameter; n ≈ 1.8]

Screech and Buzz Instabilities

Screech (Transverse Acoustic)

High-frequency (500–4000 Hz) combustion oscillations; destructive Mechanism: combustion heat release couples with transverse acoustic mode of duct Screening check: f_transverse = c/(2D) × m [m = mode number; D = duct diameter] If combustion oscillation coincides → screech possible

Prevention: Anti-screech liners: perforated tiles with acoustic damping (Helmholtz resonator array) Asymmetric gutter arrangement: breaks circumferential symmetry → reduces coupling Placement of fuel injection: avoid coupling with specific acoustic modes

Buzz (Longitudinal Acoustic)

Lower frequency (100–500 Hz); oscillating flame location Prevention: stable combustion at wide range of f_AB; maintain Damköhler > 1; avoid lean blowout

Afterburner Pressure Drop

Cold pressure loss (without combustion): ΔP/P = ξ_total × (V_ref/c₇)² = K × B² [B = blockage; K = configuration constant] Typical: ΔP/P_7 = 2–5% cold; 5–10% hot (combustion effect adds rayleigh loss)

Rayleigh flow (heat addition at constant area): P₉/P₇ = (1 + γM₇²)/(1 + γM₉²) T₀₉/T₀₇ = [(1 + γM₇²)/(1 + γM₉²)]² × (M₉/M₇)²

Mach number at afterburner exit: usually 0.3–0.5 to avoid excessive Rayleigh loss

Nozzle Integration

Convergent-divergent nozzle (CD nozzle): required for supersonic AB exit Throat area A₈ controlled to set P₈ for matched expansion Nozzle area ratio: A₉/A₈ = function of P₉/P_ambient (set by flight condition)

Variable nozzle area requirement: as fuel flow changes → T₉ changes → ρ changes → area must adjust Iris nozzle or C-D with variable petals: Ae/A₈ ratio = 1.5–4.0 depending on Mach

Materials and Cooling

Liner: Hastelloy X, Haynes 230, or ceramic matrix composite (CMC) T_liner_metal: must be < 1150°C for life; cooling required if T_gas > this

Cooling methods: Film cooling: cool air through slots or holes at upstream end; film on liner Convection cooling: air through liner double wall Augmentation ratio penalty from cooling: ṁ_cooling reduces net augmentation

Output

Provide: augmentation ratio AR_T, afterburner entry temperature T₇ [K], exit temperature T₉ [K], flameholder blockage ratio B, V-gutter width w [mm] and arrangement (rings/spokes), fuel injection zone count and staging, combustion efficiency η_AB, residence time τ [ms], pressure loss ΔP/P_7 [%], screech frequency check [Hz], nozzle throat area A₈ [cm²] and area ratio A₉/A₈, liner material and T_liner_max [°C], TSFC augmentation [kg/N·hr].