Efficient air/fuel mixing

The function of a burner is to mix oxygen, in the form of air, with the fuel so that the fuel will burn more efficiently. Burners are available in a variety of different designs, all engineered with the intent to maximize air/fuel mixing efficiency, and in more recent times we have the added concern of also minimizing the formation of atmospheric pollutants. Please be aware that fuel will burn at the end of a pipe with no burner at all but the combustion will be far from efficient. 

Combustion. The primary reaction carried out in the gas turbine combustion chamber is oxidation of a fuel to release its heat content at constant pressure. Atomized fuel mixed with enough air to form a close-to-stoichiometric mixture is continuously fed into a primary zone. There its heat of formation is released at flame temperatures deterruined by the pressure. The heat content of the fuel is therefore a primary measure of the attainable efficiency of the overall system in terms of fuel consumed per unit of work output. Table 6 fists the net heat content of a number of typical gas turbine fuels. Net rather than gross heat content is a more significant measure because heat of vaporization of the water formed in combustion cannot be recovered in aircraft exhaust. The most desirable gas turbine fuels for use in aircraft, after hydrogen, are hydrocarbons. Fuels that are liquid at normal atmospheric pressure and temperature are the most practical and widely used aircraft fuels kerosene, with a distillation range from 150 to 300 °C, is the best compromise to combine maximum mass —heat content with other desirable properties. For ground turbines, a wide variety of gaseous and heavy fuels are acceptable. 

Off-Gas Treatment. Before the advent of the shear, the gases released from the spent fuel were mixed with the entire dissolver off-gas flow. Newer shear designs contain the fission gases and provide the opportunity for more efficient treatment. The gaseous fission products krypton and xenon are chemically inert and are released into the off-gas system as soon as the fuel cladding is breached. Efficient recovery of these isotopes requires capture at the point of release, before dilution with large quantities of air. Two processes have been developed, a cryogenic distillation and a Freon absorption. 

New units can be ordered having dry, low NO burners that can reduce NO emissions below 25 ppm on gaseous fuels in many cases, without back-end flue-gas cleanup or front-end controls, such as steam or water injection which can reduce efficiency. Similar in concept to low NO burners used in boilers, dry low NO gas turbine burners aim to reduce peak combustion temperatures through staged combustion and/or improved fuel—air mixing. 

Combustors All gas turbine combustors perform the same function They increase the temperature of the high-pressure gas at constant pressure. The gas turbine combustor uses veiy little of its air (10 percent) in the combustion process. The rest of the air is used for cooling and mixing. The air from the compressor must be diffused before it enters the combustor. The velocity leaving the compressor is about 400-500 ft/sec (130-164 m/sec), and the velocity in the combustor must be maintained at about 10-30 ft/sec (3-10 iTi/sec). Even at these low velocities, care must be taken to avoid the flame to be carried downstream. To ensure this, a baffle creates an eddy region that stabi-hzes the flame and produces continuous ignition. The loss of pressure in a combustor is a major problem, since it affecls both the fuel consumption and power output. Total pressure loss is in the range of 2-8 percent this loss is the same as the decrease in compressor efficiency. 

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