Aerodynamics

The diffuser delivers slow, relatively uniform air to the combustor entrance. What happens next — inside the liner — is the subject of this lesson. The aerodynamics of the combustor are not accidental flow patterns. They are deliberately engineered structures whose purpose is to hold a flame in a high-velocity environment.

The central tool is the swirler. It is, in a meaningful sense, the most important single component of a gas turbine combustor.

The swirler and the Central Recirculation Zone

A swirler is a ring of inclined vanes mounted at the dome of the combustor, surrounding the fuel nozzle. As compressor air passes through these vanes, it acquires angular momentum — it begins to rotate about the combustor axis.

Rotating flow at high angular velocity is centrifugally unstable: the rotating fluid wants to move outward, away from the axis. This creates a low-pressure region on the centerline. The pressure difference drives flow from downstream back upstream along the axis.

This is the Central Recirculation Zone (CRZ): a torically shaped region of recirculating hot combustion products that continuously flows toward the fuel nozzle. Every parcel of fresh fuel-air mixture entering the primary zone is immediately enveloped by hot recirculating gas. Ignition happens continuously, everywhere, without relying on a spark.

The CRZ is the flame-holder. Without it, the flame would be swept downstream and extinguish. With it, even if the engine is throttled back sharply, the recirculating products maintain enough temperature and radical species to reignite the incoming fresh mixture.

Swirl number

The intensity of the swirl is characterized by the Swirl Number $S_n$, the non-dimensional ratio of angular momentum flux to axial momentum flux times a reference radius:

$S_n = \frac{G_\phi}{G_x \cdot R} = \frac{\int_0^R \rho U W r^2 \, dr}{R \int_0^R \rho U^2 r \, dr}$

where $U$ is the axial velocity, $W$ is the tangential velocity, and $R$ is the swirler outer radius.

The practical thresholds:

• $S_n < 0.4$: Weak swirl. The flow rotates, but the centrifugal effect is insufficient to create a recirculating core. The flame will likely stabilize on a bluff body or not at all.
• $S_n > 0.6$: Strong swirl. A well-defined CRZ forms. The flame is robustly anchored.
• $S_n \gg 1$: Very strong swirl. The CRZ extends too far upstream, the recirculating hot products can reach the fuel nozzle body and cause overheating or coking of the nozzle tip.

Most production combustors operate between $S_n = 0.6$ and $S_n = 1.0$. The specific value is tuned to balance flame stability, primary zone temperature distribution, and pressure loss.

Axial vs. radial swirlers

Swirlers can be implemented in two geometric families:

Axial swirlers have vanes angled relative to the engine axis. Air enters axially and exits with both axial and tangential components. They are simple, compact, and the most common type in aircraft engines.

Radial swirlers admit air from the outer radius and discharge it axially at the inner radius, imparting swirl through the curved passage geometry. They offer some aerodynamic advantages in high-pressure applications but are more complex to manufacture.

Many modern designs use counter-rotating dual swirlers — an inner swirler and an outer swirler rotating in opposite directions — which improve mixing rates and can reduce NOₓ by spreading the heat release over a larger volume.

Liner hole aerodynamics

The swirler is not the only path for air into the combustor. The liner — the cylindrical or annular "can" — contains rows of discrete holes that admit compressor bypass air at specific axial locations.

Primary zone jets are positioned just downstream of the dome. Their purpose is to provide additional oxygen to the primary zone and to shape the CRZ. In many designs, opposing jets from the inner and outer liner walls impinge on each other at the combustor axis, reinforcing the recirculation pattern and ensuring the primary zone stays compact.

Intermediate zone jets provide the secondary air that burns out CO and partially oxidized products escaping the primary zone. The equivalence ratio here drops from ~1.0 to ~0.7–0.8, maintaining high enough temperature for CO oxidation while avoiding the peak-temperature NOₓ production zone.

Dilution zone jets are the largest holes in the liner. Their goal is rapid, deep penetration to quench the combustion products from ~2000 K to the turbine's required inlet temperature. The jets must penetrate all the way to the combustor axis, not just cool the outer region.

The penetration depth of a jet into crossflow scales approximately as:

$\frac{y}{d} \sim C \left(\frac{\rho_j V_j^2}{\rho_m V_m^2}\right)^{0.5}$

where $d$ is the hole diameter, $\rho_j V_j^2$ is the jet momentum flux, and $\rho_m V_m^2$ is the mainstream momentum flux. A deep-penetrating dilution jet requires high momentum ratio — either a high-velocity jet (large hole pressure drop) or a dense jet fluid, neither of which is free.

Pressure loss and its meaning

Every vane in the swirler and every hole in the liner creates a pressure drop. The overall combustor pressure loss $\Delta P / P_t$ is typically defined as:

$\frac{\Delta P}{P_t} = \frac{P_{t,\text{in}} - P_{t,\text{out}}}{P_{t,\text{in}}}$

This is the "cost" of the aerodynamics. Higher pressure drop means higher velocities through holes and swirler vanes, which means more vigorous mixing and shorter, more stable flames. It also means more energy extracted from the gas stream, reducing engine thermal efficiency.

The industry target of 3–6% is not arbitrary — it is the range where the mixing benefits outweigh the efficiency penalty for most engine classes.

Pattern Factor: the aerodynamic report card

The Pattern Factor (also called Exit Temperature Traverse Quality or OTDF — Overall Temperature Distribution Factor) measures the non-uniformity of the combustor exit temperature profile:

$\text{PF} = \frac{T_{\max} - \bar{T}_\text{exit}}{\bar{T}_\text{exit} - T_\text{inlet}}$

where $T_{\max}$ is the peak local exit temperature, $\bar{T}_\text{exit}$ is the mean exit temperature, and $T_\text{inlet}$ is the compressor exit temperature.

A perfect combustor would have $\text{PF} = 0$ — uniform temperature everywhere. In practice, values of 0.15–0.25 are typical. Turbine designers account for the worst-case hot streak when specifying blade cooling requirements.

The aerodynamics-to-emissions connection: A poor Pattern Factor does not just threaten turbine blades. Hot streaks in the dilution zone indicate inadequate mixing, which means portions of the gas stream spent too long at high temperature — producing more NOₓ than the average exit temperature would suggest.

The intuition: a controlled tornado

The mental model that makes all of this cohere: the swirler creates a controlled tornado inside the combustor. The eye of the tornado is relatively calm and hot — this is the CRZ, where the flame lives. The outer annular flow of high-momentum air contains the flame zone. The liner jets are carefully aimed surgical tools that shape the tornado's axial structure.

The entire aerodynamic design is, in the end, about giving the chemistry enough time, temperature, and local mixing rate to occur in a controlled way — and then stopping it at precisely the right moment before the gases reach the turbine.

Lesson 5 looks at how we quantify whether all of this is actually working: efficiency, stability, ignition, and the loading parameters that tell an engineer whether the combustor is sized correctly.