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COMBUSTION CHAMBER

DOI: 10.1615/AtoZ.c.combustion_chamber

Combustion chambers are one of the main units of air jet and rocket engines or gas-turbine plants that heat up the original components (working medium) from an initial temperature T0 to a preset Tg temperature through the calorific power of the burnt fuel Hu. In an air jet engine, the heat delivered to 1 kg of air in a typical combustion chamber at a constant pressure—and with an allowance for combustion efficiency and heat losses ζ through the walls—is determined by the equation

where Cpo and Cps are the specific heat capacities of the original working medium and the combustion products respectively; the product αL0 is the ratio of working medium to fuel flow rate and depends on the oxidizing medium, e.g., air. The theoretical quantity of oxidizing medium needed for complete burning of 1 kg of fuel is L0. α is the excess coefficient (is the factor by which the stoichiometric air requirement is multiplied to take account of excess air). Thus, burning hydrocarbon (petroleum) fuel in air requires L0 = 0.115C + 0.345H − 0.043O, where C, H, and O are, respectively, the mass fractions of carbon, hydrogen, and oxygen in the fuel. For instance, L0 = 14.9 for aviation kerosene (84-86% C, 14-16% H). For CH4 and H2, L0 = 17.2 and 34.5, respectively.

Calorific power, or the lowest heat of fuel combustion, is defined as the quantity of heat in Joules that is released as a result of the complete combustion of 1 kg of fuel in air at tO = 15°C and p = 0.1 MPa during cooling of combustion products to 15°C. This does not consider the heat of condensation and water vapor content. It is roughly estimated by:

For instance, Hu = 42,900 to 43,100 kJ/kg for aviation kerosene and 49,500 and 116,700 for CH4 and H2, respectively.

The combustion products of hydrocarbon fuels are CO2 and CO, NO and NO2, water, hydrocarbons CxHy, etc. Their composition affects the combustion chamber from the environmental standpoint. A deterioration of combustion efficiency, ζ < 1, raises the quantity of CO, CxHy and gives rise to soot and smoke. Ejection of nitrogen oxides NOx increases as combustion temperature rises and the length of time combustion products are in the combustion zone grows. The allowable levels of NOx, CO, CxHy and smoke for most types of engines are thus subject to state control.

The oxidizer (air) excess coefficient α = Ga/Gf represents the combustion regime. The mixture produced as a result of combustion is stoichiometric at α = 1; rich at α < 1; and lean at α > 1. With an excess or a deficient oxidizer, the temperature of combustion products Tg is lower than the maximum closest to stoichiometric due to the heat consumption of the surplus fuel and oxidizer. With a significant change in α, the steady-state combustion in the chamber stops. These are called "rich" and "poor" flame-out, respectively.

Combustion chambers of power units should be able to provide a high combustion efficiency (in up-to-date gas-turbine engines, ζ = 0.995 and higher), low pressure losses of the working medium flow across the chamber (σ = pout/pin in gas-turbine engines, is 0.94 — 0.96), high reliability and longer service life (in gas-turbine engines, up to 10,000 hours). These can be assured by the absence of overheat, carbon deposit, etc. Variation of ζ and σ coefficients with the air flow rate (or Mcomb.ch) and the value of preheating Tg/T0 are said to be the characteristics of the combustion chamber. As Mcomb.ch and Tg/T0 grow, σ drops. Combustion efficiency ζ is enhanced with increasing Tg/T0 and attains a flat optimum if plotted versus Mcomb.ch.

Of particular significance in gas-turbine engines is a high uniformity of the fields of circumferential gas temperatures at the combustion chamber outlet (for a reliable operation of a nozzle device) and of the temperature versus radius profile (for reliability of blades), with temperature diminishing toward the upper and lower ends of the blade. The fields are produced by the development of oxidizer (air) and fuel flows in the combustion and mixing zones.

Flammability of homogeneous hydrocarbon fuel-air mixtures ranges between 0.5 < α < 1.7. The velocities of flame front propagation are not high: 0.5 to 2.0 m/s for kerosene and 210 m/s for hydrogen. Therefore, to ensure stable combustion at mean flow velocities much higher than the velocity of the flame front propagation, a combustion stabilizer with a reverse current zone can be devised that assures a reliable mixture inflammation in the combustion zone for all operating regimes of the combustion chamber. Figure 1 shows the structure of such a flow in the combustion zone behind the combustion stabilizer. Addition of air to combustion products in the mixing zone reduces average temperature values and raises α values. For example, the characteristic values of α for a poor flame-out in the Combustion chamber of an air jet engine normally vary from 20 to 50. In rocket engines, the flame front stabilization is generally affected by the system of vortices near the oxidizer and fuel jets.

Combustion in the presence of a flame stabilizer.

Figure 1. Combustion in the presence of a flame stabilizer.

Combustion chambers are classified according to engine type (air jet, rocket, and other engines), the purpose it is designed for (the main combustion chamber or afterburner in an air jet engine), combustion character (subsonic or supersonic), fuel pressure (high and low-pressure), the type of atomizers and fuel atomization (centrifugal, high-turbulence, evaporation), the number of combustion zones, and design (axial, radial, reverse-flow, tubular-type, annular, etc.).

REFERENCES

Lefevre, A. H. (1983) Gas Turbine Combustion, McGraw Hill, 1983.

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