Fuel/air mixing device

Abstract: This invention relates to a fuel/air mixing device for two stage lean flow pre-mix combustion chambers in gas turbines. The primary and secondary combustors are formed as venturi nozzles and the primary venturi is mounted tangientally to a flametube (3), which surrounds the secondary venturi (2). The mixing device of the invention consists of a perforated cone providing for the penetration of the fuel air mixture into the primary flame.

Description: This invention relates to a fuel/air mixing device for combustion chambers in gas turbines with a primary combustor and a secondary combustor, where in the first stage fuel is introduced to the primary combustor, and in the second stage, at increasing load, more fuel is fed to the secondary combustor, until at full load the fuel/air ratio in both combustors is the same, where primary and secondary combustors are formed as venturi nozzles and the primary venturi is mounted tangientally to a flametube, which surrounds the secondary venturi. The combustion chambers are lean two stage design concepts with a lean fuel/air mixture in both stages, so that the lowest possible level of pollution is achieved, for all engine conditions from idle to full load. In order to minimize pollution (NOx) it is important to ensure a fully vaporized and uniform mixture of air and fuel. This is achieved by the use of venturi nozzles, which ensure a velocity difference between the fuel drops and the air, due to the inertia of the fuel. Previously, a swirler was used to introduce the secondary mixture into the primary flame. This gave the mixture an angular momentum, which counteracted the rotation from the primary flame and forced the heavier mixture outwards. The swirler was costly to produce, had mechanical problems with attachment, showed a tendency to produce unacceptable combustion pulsations, and was vulnerable to burn-out. EP application 445 652 describes a device for combustion chambers of gas turbines with transverse mixing tubes to a central mixing tube, where the mixing tubes resemble venturi nozzles and where the secondary venturi has a swirler. Other types of combustors are disclosed in JP 59-183202 and US-A-4898001. The object of this invention is to produce a fuel/air mixing device which avoids the above problems and which reduces the level of the pollutants CO and NO x and which has a longer life. This is achieved by a device of the aforementioned type, and which is defined in the claims herewith. This invention is a simple air/fuel mixing device for gas turbine combustion chambers, which ensures an improved penetration of the air/fuel mix into a hot air stream, while presenting a reduced danger for flame-holding and burn-out, and remarkably improves the fuel economy. The invention is formed such that design requirements relating to ruggedness, cost effectiveness and mechanical integrity for fuel/air mixing devices are satisfied. It also provides a sufficiently powerful, stable injection of a fuel/air mixture into a hot air stream thus avoiding unacceptable pressure pulsation levels. The design of the fuel/air mixing device also ensures sufficient cooling by the injection of a relatively cold fuel/air mixture with high velocity into the combustion chamber. By way of example, the accompanying drawings illustrate the invention in more detail and its application, and show the following: Fig. 1 is a plan view of a primary venturi, which in accordance with this invention is for the ejection of the primary fuel/air mixture into the combustion zone, and a secondary venturi, which in accordance with this invention is placed inside the cylindrical combustion chamber. Fig. 2 is a front view of a fuel/air mixing device, which in in accordance with this invention is attached to the free end of the secondary venturi. Fig. 3 is a cross section of the device in fig. 2 The drawing shows a primary combustion chamber 1 combustion chamber 2 connected tangentially to a flametube 3, which surrounds the secondary venturi 2. Fig. 2 illustrates in particular a closed end piece with perforated walls 5, attached to the free end of the secondary venturi 2. The closed end piece 5 is preferably in the form of a perforated conical nozzle 4, which extends in the extension of the venturi nozzle and where the perforations or holes 5 are distributed arbitrarily over the whole surface, and where the apex of the cone is placed centrally in relation to the secondary venturi. There is, in addition, at the apex of the cone, a hole 6 for the ejection of the fuel/air mixture in an axial direction. The size of this hole is determined by the required cooling effect. The number and size of the holes 5, i.e. the total flow area, is determined by the required mass flow of fuel/air mixture in the secondary venturi, the pressure drop available and a coefficient of discharge for the holes 5. This coefficient has been verified experimentally and agrees with well established and publically available theory. Ref. "Gas Turbine Combustion" by A. H. LeFebre and "Handbook of Hydraulic Resistance" by I. E. Idelchik. The number and positioning of the holes as shown in figure 4 is meant only as an example and not limitation, the exact values depending upon application. The cone 4 is cooled internally by the passage of cold fuel/air mixture and the ejection of high velocity mixture through the holes 5. The positioning of the holes 5 is determined by the cooling requirements of the secondary venturi 2. The length of the cone is a compromise between radial penetration and the total combustion chamber length. Lengthening of the cone 4 will lead to less space for secondary combustion and therefore more CO. As mentioned earlier the number of holes 5 is determined by the required penetration depth into the hot gas stream. The required penetration in the example is to the flametube 3. The hole diameter for the required penetration distance has been calculated by well established and publically available material and has been verified experimentally. As mentioned in the introduction combustor pulsations are a problem inherent in many lean pre-mix combustor designs. In accordance with the present invention this problem is dramatically reduced compared to conventional designs by the provision of strong high velocity jets of fuel/air mixture into the flametube. In accordance with the present invention, and combustor pulsations being no problem, the fuel distribution between combustor stages can be optimized to minimize pollution and not combustor pulsations.

Claims: A fuel/air mixing device for combustion chambers in gas turbines comprising a primary combustor (1) and a secondary combustor (2), where in a first stage fuel is introduced to the primary combustor (1), and in a second stage, at increasing load, more fuel is fed to the secondary combustor (2), until at full load the fuel/air ratio in both combustors (1,2) is the same, where primary and secondary combustors are formed as venturi nozzles and the primary venturi is mounted tangientally to a flametube (3), which surrounds the secondary venturi (2), characterized in that a closed end piece (4) has perforated walls (5) extending in the extension of the venturi nozzle, said end piece is attached to the end of the secondary venturi (2), and said end piece (4) is formed as a perforated conical nozzle with perforations or holes (5) distributed over the whole surface and with the apex of the nozzle placed centrally in relation to the secondary venturi and a single hole (6) is placed centrally in the conical nozzle. A device according to claim 1, characterized in that the holes (4) are arranged arbitrarily. A device according to claim 1 or 2, characterized in that the number and size of the holes (4, 6), i.e. the total opening area, is calculated empirically on the basis of the mass flow and pressure drop in the combustor, together with the achievement of the maximum cooling effect. A device according to claims 1 to 3, characterized in that the length of the end piece (4) is determined by the radial penetration and the total length of the combustor.

Inventors: WILSON ANDREW JAMES WILLIAM, WILSON, ANDREW JAMES WILLIAM
Applicants: ULSTEIN TURBINE AS
Application year: 1995
Main classification: F23R3
IPC codes: F23R3