FIELD OF THE INVENTION
The present invention relates to the field of combustion chambers of gas turbines working on liquid fuel.
Such gas turbines can be illustrated by the system shown in FIG. 3. This assembly comprises a compressor (20) whose outlet is connected to the inlet of combustion chamber (1) where a liquid fuel (fuel oil or kerosine) is injected. The gases burnt in this chamber are then expanded in a trubine (30) which thus supplies the desired power to the main shaft driving compressor (20).
BACKGROUND OF THE INVENTION
It is well-known that combustion in this type of gas turbines leads to the formation of nitrogen oxides of various origins:
<<prompt>> NO results from complex fast reactions between the fuel and the nitrogen of the air. It forms in a very short space of time generally mush less than one millisecond.
<<fuel>> NO is produced by reactions between the nitrogen contained in the fuel in N form and the oxygen of the air. This type of nitrogen oxide is mainly formed in a lean medium when the air is in excess in relation to the fuel.
thermal nitrogen oxide is produced at high temperature from the nitrogen of the air N2. Nitrogen oxide is commonly produced at temperatures above 1500° C., in view of the residence times in the combustion chamber, which is then of the order of a few ten milliseconds. The rate of the reactions leading to thermal nitrogen increases exponentially as a function of the temperature.
It is the last-mentioned type of nitrogen that poses problems, as explained hereafter.
In the combustion chambers of gas turbines, combustion at the level of the flame is generally achieved around stoichiometry as this provides good flame stability. However, the global fuel/air ratio imposed by the conditions of the thermodynamic cycle of the machine is very low, of the order of 0.15 to 0.3, according to the operating conditions. Local operation under rich conditions or around stoichiometry, with air preheated by the compressor, locally leads to very high temperatures in the chamber (of the order of 2000 to 2500 K). Measurements show that, under such conditions, most of the nitrogen oxide formed is <<thermal NO>>.
Several solutions are known to decrease nitrogen oxides emissions. They can globally be divided into two main types:
wet processes based on injection of steam or water in the combustion chamber,
dry processes based on an improvement of the combustion conditions.
Wet processes give rather satisfactory results from the technical point of view, but they are often more complex and more difficult to implement than dry processes.
Furthermore, they are more expensive than dry processes because of the steam that is necessarily injected either in the liquid or in the gas phase.
Dry processes are generally aimed at achieving combustion of a previously obtained lean premix of air and fuel. Patent application EP-A2-0,769,657 illustrates a system of this type. Combustion stability and ignition of the main premix are provided by a low-power pilot flame whose purpose is also to ensure operation of the machine at idle speed. The mixture strength in the chamber being determined by the respective proportions of premixed air and fuel, it is possible to limit the flame temperatures and therefore the thermal nitrogen oxide.
This technology can be implemented quite easily with a gaseous fuel. In case of a liquid fuel, the problem is more complex since it requires vaporization prior to mixing it with air. Evaporation can be achieved by evaporating a liquid film on a hot wall or by injecting the fuel in spray form in a pipe where it mixes with the air: this is the case of the aforementioned European document.
Current combustion technologies using premixing give no satisfactory results with liquid fuel. Furthermore, this technique requires a pilot burner allowing to ensure flame stability, notably under lean conditions. This burner ensuring operation of the machine during idling phases, a flow of fuel that can reach almost a third of the total flow passes therethrough. For certain applications, it works under operating conditions close to stoichiometry, therefore under unfavourable operating conditions from the viewpoint of nitrogen oxides production.
The present invention allows to solve notably all the above-mentioned problems. It is an alternative solution to combustion chambers using premixing or to wet processes as mentioned above.
The present invention is aimed at achieving a diffusion flame by combining certain air and liquid fuel injection conditions.
There are already diffusion flames in other technical fields than that of the combustion chambers of gas turbines. Boiler burners such as those described for example in patent FR-2,656,676 allow to create diffusion flames. Similarly, patent U.S. Pat. No. 5,562,437 discloses this type of structure fitted to a boiler burner however.
Nevertheless, in this type of combustion, the operating conditions are fundamentally different.
The mixtures are much richer in burners than in turbines. Burners operate around stoichiometry or with a slight excess of air, whereas the global mixture strength in turbine chambers usually ranges between 0.15 and 0.35,
Combustion is performed under pressure (that of the compressor outlet), whereas burners work at atmospheric pressure,
Besides, the heat densities are considerably higher in the combustion chambers of turbines, commonly several ten times as high.
There are also well-known elementary flame techniques in the field of oilwell testing burners. Here again, the operating conditions are very different, notably the pressure which is here the atmospheric pressure. French patent application FR-2,741,424 describes a burner of this type.
These different operating conditions impose constraints and therefore specific structures suited to these particular functions.
SUMMARY OF THE INVENTION
The object of the present invention is a combustion chamber of a gas turbine working on liquid fuel, comprising a tubular enclosure having at least one air inlet, a liquid fuel injection means positioned on or in proximity to the longitudinal axis of the tubular enclosure, an outlet to the turbine, at least two types of pressurized air inlets placed close to each other : the first one taking in the air helically around the longitudinal axis of the combustion chamber, the second inlet taking in the air tangentially to the enclosure in order to create, around the fuel jets, counterrotating flows intended to improve mixing of said fuel and air.
According to the invention:
Said fuel injection means comprises a series of orifices arranged so as to create separate fuel jets, said jets being arranged in the direction of the generatrices of a cone with an angle ranging between 30° and 60° at the vertex thereof,
the assembly working at a pressure ranging between 2 and 30 bars and with a fuel/air ratio ranging between about 0.4 and about 0.8, and the residence time of the fluids in the enclosure is less than 50 milliseconds.
In particular, the first air inlet allows to introduce 30% to 70% of the total amount of pressurized air entering the combustion chamber, the rest being injected through the second pressurized air inlets.
According to the invention, said injection means has 5 to 12 orifices intended for injection of the liquid fuel, and preferably 6 to 10 orifices.
Besides, the air inlets and the injection means are so positioned that the swirl ratio N ranges between 0.2 and 0.4, N being defined by
R1 and R2 are respectively the inner radius and the outer radius of air inlet (7), expressed in meters,
ρ is the density of the air in kg/m3,
Vax is the axial velocity of the fluid at the outlet of inlet (7),
Vtg is the tangential velocity of the fluid at the outlet of inlet (7), the velocities being expressed in m/s.
According to a particular feature of the invention, the injection means comprises a central disk positioned on the longitudinal axis of the tubular enclosure, around which a ring pierced with said orifices is arranged, the surface of the ring being a truncated cone.
Specifically, the tangential inlet comprises a series of inserts distributed on the periphery of the enclosure, which leads the air tangentially to the wall of the enclosure in the opposite direction to the direction of rotation of the main flow.
The air inlets can be so dimensioned that the velocity of the air in the combustion chamber ranges between 20 and 120 m/s.
Besides, the angle at the vertex of the injection cone preferably ranges between 35° and 45°.