Power plants – Combustion products used as motive fluid
Reexamination Certificate
1999-10-20
2002-03-26
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
C060S039300, C060S747000, C060S748000
Reexamination Certificate
active
06360525
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to combustors for gas turbine engines, including the associated fuel burners, otherwise known as fuel nozzles or injectors. It particularly, but not exclusively, relates to combustors suitable for operating in a so-called “low emissions combustion mode” because they can sustain a premixed lean-burn combustion processes.
2. Description of the Related Art
Some known types of combustor utilize a premixed lean-burn combustion mode to reduce emissions of pollutants—such as nitrous oxides (“NOx”), carbon monoxide (“CO”) and unburned hydrocarbons (“UHC”)—by enabling combustion temperatures to be kept within certain limits known to minimize pollutant production. To achieve such a low emissions combustion mode, air is mixed with the fuel before initial injection of the fuel/air mixture into a combustion zone in the combustion chamber. The term “lean-burn” implies that the amount of air mixed with the fuel is more than that required for complete combustion of the fuel.
Gas turbine engines operate over a wide range of engine speed and load conditions, varying from initial start-up conditions, through various engine speed/load combinations up to a maximum. When an engine's combustor is operating in a premix lean-burn low emissions combustion mode, it is well known that flame stability is difficult to achieve in the lower power ranges, because the combustion process is being operated close the weak extinction limit of the air/fuel mixture. An associated problem is non-uniformity of the combustion process, leading to undesirable variation in the temperature and composition of the combustion products leaving the primary combustion zone and increased temperature gradients in the combustor walls nearest the combustion process. Such non-uniform combustion is caused by insufficiently rapid homogenization of the fuel/air mixture before it ignites in the flame front established in the combustion chamber.
A well known way of stimulating good mixing of air and fuel is to impart a swirling motion to the fuel and air, either before, during or after the injection of fuel into an airstream. Published European patent specification EP 0 378 505 B1 discloses how, in an annular combustion chamber, fuel injectors having associated air swirlers can be used to project fuel and air along or parallel to the axial centerline of the combustion chamber. The fuel injectors are arranged in radially inner and outer circumferentially extending tiers or rows in the combustor head and the fuel is mixed with the swirling air after the fuel leaves the ends of the injectors within the combustion chamber. The air swirlers associated with the injectors in one row swirl the air in one rotational direction, whereas the swirl imparted to the air by the air swirlers in the other row is in the opposing rotational direction. Additionally, the fuel injectors in each row are in angular registration with the gaps between injectors in the other row so that the fuel and air discharged from injectors in one row interacts with the discharge from two adjacent fuel injectors in the other row. This is said to establish a reinforced fuel/air swirl pattern in the front or head of the combustor for stabilizing the burning. However, the object of this prior invention is to increase the intensity of combustion in non-premixed diffusion controlled combustion processes, whereas for premixed lean-burn combustion processes aimed at reduced emissions, the combustion intensity has to be controlled within certain limits.
Stable premixed lean burn combustion can be extended into somewhat lower power ranges by the expedient of performing the combustion process in a number of stages, for example as shown in International patent publication number WO92/07221. This discloses a sequentially staged combustion process comprising primary, secondary and if necessary tertiary stages of combustion occurring in sequentially arranged combustion chambers, with the chamber containing the primary combustion zone feeding its combustion products into the secondary stage and so on. A disadvantage of such sequentially staged combustion processes is the extra length required of the combustor in order to ensure an adequately complete combustion process in each stage before further premixed fuel and air is added to the combustion products to initiate the next stage of combustion.
European patent publication EP 0728 989 A2, to the present assignee, discloses a lean burn combustor in which the main premixing fuel burner or injector comprises an annulus surrounding a central pilot fuel burner or injector. The main premixed fuel-lean fuel/air mixture is injected into the combustion chamber as an initial radially inwardly moving swirling flow which meets an axially directed air-blast from the pilot burner. This enhances homogenization of the main fuel-lean fuel/air mixture when it enters the combustion chamber, and aids in avoiding its premature ignition. The pilot air-blast is configured to provide a region of sheltered combustion which supports the combustion process just downstream of the pilot burner and increases stability of the premixed lean burn combustion process at part power engine conditions. This arrangement facilitates a so-called “parallel staged” or “fuel staged” combustion process, in which both primary and secondary combustion processes take place at the same streamwise axial position, fuel being appropriately apportioned between the pilot and main burners according to a schedule of fuel flows against power level. Both a low emissions combustion mode over a wide power range and a relatively short combustion chamber are achieved. Nevertheless, the necessary proximity of the premixed lean-burn main flame and the fuel-rich pilot flame to the pilot burner tends to cause high temperatures in the exposed end of the pilot burner, and the fuel-rich pilot flame also causes the formation of some NOx at the low engine power levels for which it is used.
Of course, practical designs are inevitably a result of compromises between conflicting requirements, but it will be understood from the above that designs of relatively short combustors are required which facilitate not only stable low emissions combustion over a wide power range but also a uniform combustion process in a primary combustion zone which has adequate separation from the combustor head.
SUMMARY OF THE INVENTION
According to the present invention, a combustor for a gas turbine engine comprises
an annular combustion chamber having radially inner and outer concentric walls and a combustor head wall at its upstream end, and
a plurality of fuel injectors disposed in the combustor head wall,
the fuel injectors being circumferentially spaced around the combustor head wall and disposed as a first radially outer row of injectors and a second radially inner row of injectors, the rows being concentric with the combustor head wall, the injectors in the first and second rows being disposed such that injectors in each one of the rows are in angular registration with spaces between injectors in the other row;
wherein each fuel injector comprises
means for producing a fuel/air mixture having a swirling motion, and
a mixing duct located downstream of the means for producing the fuel/air mixture, the mixing duct opening into the combustion chamber through the combustor head wall and having a length sufficient to allow at least partial homogenization of the fuel/air mixture before entry to the combustion chamber as a divergent swirling stream,
the mixing ducts in the first and second rows of injectors having longitudinal axes oriented to coincide with generating rays of respective first and second imaginary conical surfaces, which conical surfaces intersect within a primary combustion zone in the combustion chamber, whereby mixing ducts located in different rows are angled towards each other in the downstream direction and the divergent swirling streams of fuel/air mixture coming from different rows cross in the combustion chamber in an interdigitati
Maidhof Sebastian
Senior Peter
Alstom Gas Turbines Ltd.
Kirschstein et al.
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