Combustors

The combustor is where the cycle's heat is added, and it has the most difficult brief in the engine. It must burn fuel completely in a few milliseconds, in air moving at tens of metres per second, at pressures from a fraction of an atmosphere at high altitude to forty atmospheres at take-off. It must relight in flight after a flameout, deliver gas to the turbine with a carefully controlled temperature profile, lose as little pressure as possible, and emit as little nitrogen oxides, carbon monoxide, unburned hydrocarbons and soot as possible. Emissions are now as important as efficiency in combustor design, and Dr. Nakamura asks Arjun to prepare the Swift 900's emissions summary for certification planning, checked against the aircraft engine emission standards Transport Canada applies, which follow ICAO Annex 16 Volume II.

Combustor layout

Modern engines use annular combustors: a ring-shaped liner between the compressor and turbine, with fuel nozzles spaced around the front. Air leaving the compressor first passes through a diffuser that slows it, because no flame can survive at compressor exit speeds of 100 to 150 m/s. The air then divides. Part enters the front of the liner through swirlers around each fuel nozzle; the rest flows along the outside of the liner and enters through rows of holes along its length, and through small cooling holes or slots that keep the liner wall itself from melting.

The liner is divided into zones. In the primary zone, near the fuel nozzle, the swirling flow creates a recirculation region where hot products mix back into the incoming mixture and keep the flame stable; the local mixture is near stoichiometric so it burns fast and hot. The secondary zone adds air to complete the burning of carbon monoxide and soot. The dilution zone adds the remaining air to cool the gas to the temperature the turbine can accept and to shape the radial temperature profile, hotter at mid-span where the blades are strongest and cooler at the root and tip.

Fuel-air ratio and equivalence ratio

The overall fuel-air ratio follows from an energy balance across the combustor. With combustion efficiency \(\eta_b\), the fuel needed to raise the gas from the compressor exit temperature \(T_{t3}\) to the turbine inlet temperature \(T_{t4}\) is

Comparing it with the stoichiometric ratio, about 1/14.7 for kerosene, gives the equivalence ratio \(\phi = f/f_{st}\). Overall equivalence ratios in aircraft combustors are low, around 0.3 to 0.4 at take-off, because most of the air is used for dilution and cooling. A flame cannot burn at such lean mixtures, which is exactly why the air is staged: the primary zone sees a much richer mixture.

# Combustor sizing checks for the Swift 900 core at take-off: fuel-air ratio for the required
# temperature rise, overall and primary-zone equivalence ratios, residence time, and CO2 and H2O
# produced per kg of kerosene (taken as C12H23).
LHV, cp_g = 43.2e6, 1180.0
f_st = 1 / 14.7                                  # stoichiometric fuel-air ratio for kerosene, approx.
T3, T4, eta_b = 790.0, 1600.0, 0.995             # compressor exit and turbine inlet at take-off, K
f = cp_g * (T4 - T3) / (eta_b * LHV - cp_g * T4)
phi = f / f_st
print(f"fuel-air ratio {f:.4f}, overall equivalence ratio {phi:.2f}")
share_primary = 0.30                             # fraction of combustor air entering the primary zone
print(f"primary zone with {100*share_primary:.0f} percent of the air: equivalence ratio {phi/share_primary:.2f} (rich-burn)")
print(f"primary zone with 60 percent of the air: equivalence ratio {phi/0.60:.2f} (lean-burn)")
# residence time from volume, mass flow and density
mdot, p3, V = 8.0, 2.4e6, 0.008                 # kg/s core air at take-off, Pa, m^3 combustor volume
rho = p3 / (287 * (T3 + T4) / 2)
t_res = rho * V / mdot
print(f"mean gas density {rho:.1f} kg/m3, residence time about {1000*t_res:.1f} ms")
# products: C12H23 + 17.75 O2 -> 12 CO2 + 11.5 H2O
M_fuel = 12 * 12.011 + 23 * 1.008
print(f"per kg of fuel: {12*44.01/M_fuel:.2f} kg CO2 and {11.5*18.015/M_fuel:.2f} kg H2O")
fuel_kgph = f * mdot * 3600
print(f"take-off fuel flow per engine {fuel_kgph:.0f} kg/h -> {fuel_kgph*3.16/1000:.2f} t CO2 per hour")
for EI in (8, 15, 25):
    print(f"NOx emission index {EI:2d} g/kg -> {EI*fuel_kgph/1000:.1f} kg NOx per engine-hour at take-off")

fuel-air ratio 0.0233, overall equivalence ratio 0.34
primary zone with 30 percent of the air: equivalence ratio 1.14 (rich-burn)
primary zone with 60 percent of the air: equivalence ratio 0.57 (lean-burn)
mean gas density 7.0 kg/m3, residence time about 7.0 ms
per kg of fuel: 3.16 kg CO2 and 1.24 kg H2O
take-off fuel flow per engine 670 kg/h -> 2.12 t CO2 per hour
NOx emission index  8 g/kg -> 5.4 kg NOx per engine-hour at take-off
NOx emission index 15 g/kg -> 10.0 kg NOx per engine-hour at take-off
NOx emission index 25 g/kg -> 16.7 kg NOx per engine-hour at take-off

The program finds a fuel-air ratio of 0.0233 at take-off, an overall equivalence ratio of 0.34. If 30 percent of the air enters the primary zone, it burns slightly rich at 1.14; if 60 percent does, it burns lean at 0.57. These two numbers are the two families of low-emissions combustor described below. The mean residence time in a combustor of 8 litres is about 7 milliseconds, long enough for kerosene to burn completely at take-off pressure, but not by much; at high altitude, where the pressure is a tenth as high, reactions slow sharply, which is why relight at altitude is a certification test. Each kilogram of fuel gives 3.16 kg of CO₂ and 1.24 kg of water, figures used in every aviation emissions inventory, and one engine at take-off burns about 670 kg/h, producing 2.1 tonnes of CO₂ per hour.

Emissions and how combustors control them

Combustor emissions fall into two groups. CO₂ and water are set by the fuel burned and cannot be reduced by the combustor; only less fuel, or fuel from non-fossil carbon (Lesson 12), reduces them. Nitrogen oxides, carbon monoxide, unburned hydrocarbons and non-volatile particulate matter (soot) depend on how the fuel is burned, and here combustor design has a large effect.

Nitrogen oxides form mostly in the hottest parts of the flame, above about 1800 K, and their formation rate rises exponentially with temperature and with the time spent there. Higher engine pressure ratios, which raise compressor exit temperature and flame temperature, make NOx harder to control, so the efficiency gains of Lesson 3 come with an emissions challenge. Two main approaches are used. Rich-burn, quick-quench, lean-burn combustors burn rich in the primary zone, where there is too little oxygen for much NOx, then mix in air very quickly to jump past the stoichiometric peak temperature, and finish burning lean. Lean-burn combustors premix fuel and air and burn the whole primary zone lean, keeping the flame temperature low throughout; they give the lowest NOx but are harder to keep stable and need staging, with a separate pilot flame at low power.

Emissions are regulated at the engine level by ICAO standards, adopted nationally. Certification measures emission indices, grams of pollutant per kilogram of fuel, over a standard landing and take-off cycle of idle, take-off, climb and approach at set thrusts and times, and the limits depend on the engine's pressure ratio and thrust. Standards for non-volatile particulate mass and number, introduced in the last decade, now apply to new engine types; Canada and the United States have adopted them through the aircraft engine emission standards Transport Canada applies, which follow ICAO Annex 16 Volume II. Cruise emissions are not directly regulated, but they matter for climate: NOx at altitude affects ozone and methane, and soot and water vapour can form persistent contrails.

Other combustor requirements

Pressure loss across the combustor, 3 to 6 percent of total pressure, comes partly from the diffuser and partly from the jets through the liner that create the mixing. Lower loss helps the cycle; more loss improves mixing. The temperature profile at the exit, described by the pattern factor (the peak exit temperature above the average, divided by the temperature rise), must be small and stable, because the turbine is designed for the average and a hot streak shortens blade life. Ignition at the ground and relight at altitude, stability at idle and during rapid throttle changes, and freedom from combustion instabilities, acoustic pressure oscillations that can damage the liner, are all certification items under the engine standards in Chapter 533 of the Transport Canada Airworthiness Manual.

Fuel injection and the fuel system

Kerosene must be broken into fine droplets that evaporate quickly. Pressure atomisers force fuel through a small swirling orifice at high pressure; airblast atomisers spread the fuel into a thin sheet and shatter it with fast-moving air, giving good atomisation at high power and a more uniform spray. Many engines use dual-orifice or staged injectors so that atomisation stays good from idle to take-off. Droplet sizes of 20 to 50 micrometres are typical; larger drops burn slowly and raise smoke and carbon monoxide.

The engine fuel system pumps fuel from the aircraft tanks, heats it with oil coolers so that ice crystals cannot block filters, meters it with the engine control, and supplies the nozzles. Modern engines use full-authority digital engine controls, which schedule fuel flow against spool speed, temperatures and pressures to give the thrust the pilot selects while keeping every limit: surge margin, turbine temperature, overspeed. Fuel also serves as the coolant for the engine oil and, in some designs, for electronics, which limits how hot the fuel may become.

Arjun's emissions summary for the Swift 900 lists the engine maker's certified emission indices at the four landing and take-off points, the characteristic values compared with the limits for the engine's pressure ratio, and the margin for each pollutant. It also notes the engine's particulate results, which matter for local air quality around airports. The margins are comfortable for carbon monoxide and hydrocarbons and narrower for NOx, as expected for a high pressure ratio core.