Combustion Fundamentals

If Chapter 1 was about the engineering constraints, Chapter 2 is about the physical rules that all designs must obey. Lefebvre establishes here that temperature is the master variable — it controls how fast chemistry happens, how much pollution forms, and ultimately whether the flame survives or dies.

Two fundamentally different flames

The first distinction Lefebvre draws is between premixed and diffusion flames. The choice between them is not aesthetic — it has deep consequences for stability, emissions, and safety.

Diffusion (non-premixed) flames

In a diffusion flame, fuel and oxidizer are introduced separately. They mix by molecular and turbulent diffusion, and combustion happens at the interface where the local fuel-air ratio is within the flammability limits. The candle flame is the classic example.

Why conventional gas turbines use diffusion flames: You can inject fuel directly into the primary zone airflow without any risk of the flame propagating back into the fuel supply. The flame is inherently stable because it is always burning at the stoichiometric surface — the locus where fuel and air arrive in equal proportions.

The cost: The stoichiometric surface is the hottest part of the flame (~2500 K for kerosene in air), and it is exactly here that thermal NOₓ forms most rapidly. Diffusion flames also produce soot because locally fuel-rich pockets form carbon before enough oxygen arrives to oxidize it.

Premixed flames

Fuel and air are mixed to a uniform composition before reaching the flame front. Lean premixed (LPM) combustion — operating well below stoichiometric — keeps flame temperatures below ~1900 K, which dramatically reduces NOₓ.

The cost: Premixed flames are prone to flashback (the flame propagates upstream into the mixer at high-pressure conditions) and autoignition (the mixture ignites before it reaches the intended flame zone). They are also inherently unstable and susceptible to combustion oscillations (covered in Lesson 7). Modern low-emissions combustors use premixed zones carefully, always with a pilot diffusion flame to anchor stability.

What limits a flame: chemistry vs. mixing

A key conceptual move Lefebvre makes in this chapter is separating two reasons why combustion might be slow or incomplete.

Chemically controlled combustion: Fuel and air are mixed, but the temperature is low enough that molecular collisions lack the energy to initiate reaction. This governs cold starts, idle conditions, and high-altitude operation. The rate-limiting step is the chemical reaction itself.

Mixing-controlled combustion: The chemistry would proceed instantly if the reactants were in contact, but turbulent mixing is the bottleneck. Most gas turbine combustion at operating conditions falls here — the diffusion flame is waiting for the flow to bring fuel molecules to the oxygen-rich region.

This distinction matters for design: if you are mixing-limited, adding more fuel does not help — you need better aerodynamics. If you are chemistry-limited, you need more temperature or more time.

Flammability limits

Not every fuel-air mixture will sustain a flame. The lean flammability limit (LFL) is the minimum fuel concentration (by equivalence ratio $\phi$) at which a flame can propagate. The rich flammability limit (RFL) is the maximum.

The equivalence ratio $\phi$ is defined as:

$\phi = \frac{(F/A)_\text{actual}}{(F/A)_\text{stoichiometric}}$

where $F/A$ is the fuel-to-air mass ratio. $\phi < 1$ is lean (excess air), $\phi > 1$ is rich (excess fuel), and $\phi = 1$ is the stoichiometric ideal.

For Jet-A in air at atmospheric conditions:

• Lean limit: $\phi \approx 0.5$
• Rich limit: $\phi \approx 3.0$

Critical observation: As pressure and inlet temperature increase — as they do from idle to full power, and across engine generations — the flammability limits widen. A modern high-pressure engine is actually easier to keep lit at the primary zone level. The difficulty shifts to keeping emissions low while maintaining that stability.

The Arrhenius equation

The rate at which a chemical reaction proceeds depends on temperature in a way that is not linear — it is exponential. Arrhenius formalized this:

$\dot{\omega} \propto \exp\!\left(-\frac{E_a}{RT}\right)$

where $\dot{\omega}$ is the reaction rate, $E_a$ is the activation energy (the energy barrier molecules must overcome to react), $R$ is the universal gas constant, and $T$ is the absolute temperature.

The consequence is dramatic. A temperature increase of a few hundred Kelvin can increase the reaction rate by orders of magnitude. This is why:

• Cold starts are difficult: low temperature means near-zero reaction rate even with fuel and air well mixed
• High-altitude relight is hard: thin, cold air suppresses both temperature and pressure
• The primary zone must run hot: you need high temperature to sustain the reaction within the available residence time

Adiabatic flame temperature

The adiabatic flame temperature $T_{ad}$ is the maximum temperature reached if all the fuel's chemical energy is converted to sensible heat with no heat loss.

For a hydrocarbon-air mixture, $T_{ad}$ peaks at stoichiometric conditions (~2500 K for Jet-A) and falls on both the lean and rich sides. This bell-shaped profile is fundamental: it tells you simultaneously where the heat release is highest, where NOₓ forms most aggressively, and where flame stability is strongest.

Lean combustion drives the equivalence ratio down the left side of this curve. You gain lower temperatures (less NOₓ), but you also move closer to the lean flammability limit and reduce the margin against blowout.

The master trade-off

Temperature controls everything simultaneously — in opposing directions:

The entirety of combustor design — swirler geometry, air staging, fuel injection strategy, cooling allocation — is an argument about how to navigate this single fact.

In Lesson 3, we leave the primary zone and work backwards to the combustor entrance: the diffuser, which sets up the flow conditions that everything else depends on.