Gas turbine combustion chamber: requirements and types

Requirements

• Complete combustion with high combustion efficiency over the whole operating range.
• Low total pressure loss (typically 2 to 8% of the compressor delivery pressure).
• A stable flame over a wide air-fuel ratio range, with no blow-out at altitude or during rapid throttle changes.
• Uniform outlet temperature (low "pattern factor") to protect the turbine blades.
• Reliable ignition on the ground and at altitude, small size and weight, low smoke and emissions, and long life.

Zones of a combustor

 air in -> | primary | secondary | dilution | -> to turbine
           |  zone   |   zone    |   zone   |
 fuel -->  (burn at  (complete   (mix cool air,
            near      burning,    set outlet
            stoich)   cooling)    temperature)

1. Primary zone: about 15 to 20% of the air; burns the fuel near the stoichiometric ratio at high temperature; a swirler produces recirculation that anchors the flame.
2. Secondary (intermediate) zone: more air, admitted through holes, completes combustion and reduces dissociation losses.
3. Dilution zone: the remaining air (about 50 to 60%) reduces the gas temperature to the turbine inlet limit and evens out the temperature profile. A portion also cools the liner wall.

Types

Definitions

• Combustion efficiency: $\eta_b = \dfrac{\text{actual heat release}}{\text{ideal heat release}} = \dfrac{(\dot m_a + \dot m_f) h_3 - \dot m_a h_2}{\dot m_f \cdot LHV}$; 98 to 99% at design.
• Pressure loss factor (PLF): the stagnation pressure loss across the chamber divided by a reference dynamic head based on the maximum cross-sectional area $A_m$ of the chamber:

$$PLF = \frac{\Delta p_0}{\dot m^2 / (2\rho_2 A_m^2)} = K_1 + K_2\left(\frac{T_{03}}{T_{02}} - 1\right)$$

$K_1$ represents the "cold" (friction and mixing) loss in the liner and $K_2$ the "fundamental" loss due to the momentum rise caused by heating. The overall fractional loss $\Delta p_0/p_{02}$ (2 to 8%) follows from the PLF and the flow conditions at inlet.