Abstract:
A combustion device for the combustion of fuel, includes a supply duct for supplying a fuel to a combustion zone. The fuel is capable of being guided through the supply duct in a fluid stream with a direction of flow and at a nominal velocity lying within a nominal operating interval. Further, the supply duct is narrowed in an uncoupling region, such that at the nominal velocity, sound waves running opposite to the direction of flow in the fluid stream from the combustion zone are at least partially reflected in the uncoupling region.

Description:
COMBUSTION DEVICE AND METHOD FOR THE COMBUSTION OF A FUEL 
     This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/EP99/09401 which has an International filing date of Dec. 1, 1999, which designated the United States of America. 
     1. Field of the Invention 
     The invention relates to a combustion device for the combustion of a fuel, the fuel being capable of being supplied to the combustion as a fluid stream via a supply duct. The invention also relates to a corresponding method. 
     2. Background of the Invention 
     In the book “Technische Strömungslehre” [“Technical Fluid Mechanics”] by Willi Bohl; 10 th  edition, Vogel-Verlag, Wurzburg 1994, outflow processes are described in Chapter 5.6. Illustrated in more detail are processes for the outflow of a fluid from a vessel, in which the fluid is stored at the pressure p i  and the density ρ i . The fluid emerges from the vessel as a jet, the jet pressure P a  prevailing in the jet. The pressure ratio in the case of which, in the event of a given vessel state, that is to say a given vessel pressure p i  and a given fluid density ρ i  and also in the case of a given vessel orifice from which the fluid emerges, the mass flow of fluid no longer changes, is designated as the critical pressure ratio (P a /P i ) (k) . Depending on the size of the pressure ratio P a /P i , a distinction is made between two types of outflow processes: 1. Subcritical outflow; 2. Supercritical outflow. 
     A Laval nozzle is described in section 5.6.2 of the same book. The Laval nozzle serves for expanding the outflowing fluid beyond the critical pressure ratio and for consequently increasing the flow velocity beyond sound velocity. For this purpose, the fluid is first compressed by a narrowing duct, the flow velocity increasing up to sound velocity. This is followed by a widening duct section, in which the fluid expands and the flow velocity reaches the supersonic range. Such a Laval nozzle serves, for example, for achieving maximum outflow velocities for thrust gases of rocket propulsion units. FIG. 5.25 illustrates various operating states of a Laval nozzle. In the operating state illustrated first, the outlet pressure of the fluid is above the critical pressure. The Laval nozzle behaves, here, in the same way as a Venturi tube. More detailed particulars as to the definition of a Venturi tube follow further below. 
     Compression flows are described in section 5.7 of the same book. Section 5.7.1 explains the functioning of a subsonic diffuser. Subsonic diffusers are ducts which are widened in the direction of flow and in which a flow running in the subsonic range is decelerated. The deceleration results in a pressure rise. Subsonic diffusers are found, for example, in jet appliances, Venturi tubes and in the guide wheels and outlet casings of turbocompressors. Section 5.7.2 describes a supersonic diffuser, in which the duct cross section narrows in the direction of flow. 
     European Standard EN ESO 5167-1 relates to throughflow measurements of fluids by means of throttle devices. Diaphragms, nozzles and Venturi tubes in full-throughflow lines of circular cross section are described in Part 1. FIG. 10 shows a classic Venturi tube. A fluid flows through the Venturi tube in a direction of flow. The Venturi tube consists of an entry cone narrowing in the direction of flow and of a widening exit cone following the entry cone in the direction of flow. A pronounced pressure loss occurs in the entry cone. This pressure loss is for the most part compensated again by the exit cone, so that the overall pressure loss occurring through the Venturi tube, as compared with a tube of invariable cross section and the same length, remains low. 
     In the book “Berechnung der Schallausbreitung in durchströmten Kanälen von Turbomaschinen unter besonderer Berücksichtigung der Auslegung von Drehtonschalldämpfern” [“Calculation of the sound propagation in throughflow ducts of turbomachines, taking particular account of the design of rotational sound dampers ”] by Christian Faber, Verlag Shaker, Aachen 1993, section 3.4 illustrates how discontinuities in flow ducts influence the propagation of sound in a fluid flowing in these flow ducts. Scatter, reflection and transmission factors are derived, from which it is possible to calculate which part of incident sound energy passes the discontinuity and which part is reflected. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to specify a combustion device which has beneficial properties in terms of controlling and influencing the propagation and formation of sound waves induced by combustion. A further object of the invention is to specify a corresponding method. 
     These and other objects are achieved, according to the invention, by specifying a combustion device for the combustion of fuel, with a supply duct for supplying the fuel to a combustion zone, the fuel being capable of being guided through the supply duct as a fluid stream with a direction of flow and at a nominal velocity lying within a nominal operating interval, and the supply duct being narrowed in a decoupling region, in such a way that, at the nominal velocity, sound waves running opposite to the direction of flow in the fluid stream from the combustion zone are at least partially reflected in the decoupling region. 
     During combustion, combustion oscillations may arise, in that, in the event of a fluctuation in a release of power during combustion, a pressure pulse occurs in the fluid stream. Such a pressure pulse in the fluid stream results, in turn, in unevenness in the mass flow of the fluid stream entering the combustion zone. This again leads to a release of power fluctuating in time during combustion. Depending, for example, on the geometric designs of the supply duct, positive feedback may be produced between pressure pulses in the fluid stream and the fluctuating release of power during combustion. A combustion oscillation is formed. Such a combustion oscillation may have a disturbing effect, for example, in the form of considerable noise pollution. In the case of large releases of power, however, vibrations may also occur in the combustion device, which may ultimately result in damage. The invention proceeds from the knowledge that the propagation of sound waves in the fuel via the supply duct into further acoustically coupled regions is conducive to the tendency to the formation of such combustion oscillations. This mechanism is prevented by an acoustic decoupling of the supply duct or else of a plurality of supply ducts for the fuel. Such acoustic decoupling is achieved by a narrowing of the supply duct or supply ducts. 
     By virtue of such narrowing in the direction of flow, known hitherto only with regard to airborne sound dampers, the flow velocity of the fluid is increased. In this case, the flow velocity may be increased to an extent such that sound waves running toward the narrowing opposite to the direction of flow are reflected. The narrowing is designed in such a way that, at a nominal velocity of the fluid stream in the supply duct, such a high acceleration of the fluid is obtained at the narrowing that a high proportion of the sound waves running toward the narrowing is reflected. The nominal velocity is, for example, within a velocity interval which corresponds to those operating states of the combustion device in which there is a high tendency to the formation of combustion oscillations. 
     The uncoupling region is designed preferably as a continuous narrowing of the supply duct in the direction of flow. At such a continuous narrowing, lower flow and pressure losses due to turbulence are obtained, as compared with a discontinuous narrowing. Such a continuous narrowing could, for example, be designed in a similar way to the supersonic diffuser described by Willi Bohl in the above-mentioned book. 
     Preferably, the decoupling region is followed in the direction of flow by a pressure increase region which corresponds to a widening of the supply duct. The pressure in the fluid stream is increased by means of such a pressure increase region. This is carried out as a result of the widening of the supply duct. The passage consisting of the decoupling region and of the pressure increase region thus corresponds, for example, to the Venturi tube illustrated in the European standard specified above or to a Laval nozzle. Such an embodiment is advantageous, in particular, when a high fluid mass flow has to be provided. Thus, what is achieved by the combination of the uncoupling region and pressure increase region is that in the combustion device, a large release of power can be obtained with the aid of a high fluid mass flow, while at the same time an effective acoustic decoupling of the combustion zone and supply duct is provided. 
     The combustion zone is preferably located in a combustion chamber. The combustion chamber may have any desired shape, but it is particularly important to have a tubular or annular combustion chamber. In a combustion chamber, combustion oscillations may be formed as a result of interaction between a power fluctuation during combustion and characteristic acoustic modes of the combustion chamber. Such combustion chamber oscillations may be propagated into fluidically coupled spaces, for example advancing into the fuel or air supply lines and, in some circumstances as far as a supply pump, which may thereby be subjected to high mechanical load. Acoustic decoupling by means of the narrowing of the supply duct prevents such a propagation of the combustion chamber oscillations. Moreover, the tendency to the formation of combustion chamber oscillations at all is reduced, since the acoustic resonance space available for the combustion chamber oscillations is reduced as a result of the decoupling of the supply duct. 
     The combustion device is preferably a gas turbine, in particular with an annular combustion chamber. In a gas turbine, there is a particularly high release of power during combustion. Combustion oscillations may therefore lead, here, to particularly serious noise pollution and damaging vibrations. In an annular combustion chamber, it is virtually impossible to predict characteristic acoustic modes due to the complicated geometry, so that the formation of combustion chamber oscillations is particularly difficult to prevent here. Acoustic decoupling between the annular combustion chamber and the supply ducts for the combustion media is therefore of particular importance here. 
     Objects are further achieved, according to the invention, by specifying a method for the combustion of fuel, the fuel being supplied to a combustion zone as a fluid stream with a direction of flow and at a nominal velocity lying within a nominal operating interval, and the fluid stream being narrowed in a decoupling region, in such a way that, at the nominal velocity, sound waves running opposite to the direction of flow in the fluid stream from the combustion zone are at least partially reflected in the decoupling region. 
     The advantages of such a method emerge correspondingly from the above statements regarding the advantages of the combustion device. 
     The fluid stream is preferably narrowed continuously in the direction of flow. Preferably, the pressure in the fluid stream is increased by a fluid stream widening which follows the narrowing. Also preferably, the fuel used is natural gas or oil. The fuel is preferably burnt in a combustion chamber of a gas turbine, in particular an annular combustion chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are explained in more detail with reference to the drawing in which: 
     FIG. 1 shows a combustion device, and 
     FIG. 2 shows a gas turbine, diagrammatically and not true to scale. 
     Identical reference symbols have the same significance in the various figures. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a combustion device  1 . Arranged concentrically in an air duct  3  of circular cross section is a fuel duct  5 , likewise of circular cross section, which constitutes a supply duct  5 . Air  6  in the form of an air stream  7  is guided in a direction of flow  8  in the air duct  3 . Fuel  14 , for example oil, from a fuel tank  12  is guided as a fluid stream  9  in a direction of flow  10  in the fuel duct  5 . The air  6  and the fuel  14  are burnt in a flame  13  in a combustion zone  11 . A fluctuation in the release of power during combustion gives rise to a sound wave  15  in the fluid stream  9  of the fuel  14 . This sound wave  15  travels upstream, opposite to the direction of flow  10 , in the fluid stream  9 . In the case of a supply duct  5  of invariable cross section, the sound wave  15  could pass through the entire supply duct  5  and, for example, travel as far as a fuel pump, not illustrated, and possibly damage this. In such versions used hitherto, therefore, the combustion zone  11  had acoustically coupled to it by means of the supply duct  5  considerably extended spaces, through which the combustion oscillations could be propagated in the combustion device  1  and which, moreover, constitute resonance spaces which may be conducive to the formation of combustion oscillations. 
     By contrast, in the combustion device  1  shown here, acoustic decoupling of the supply duct  5  from the combustion zone  11  is achieved by means of a decoupling region  17 . The decoupling region  17  is formed by a narrowing of the supply duct  5  in the direction of flow  10 . The flow velocity of the fluid stream  9  is therefore increased in the decoupling region  17 . The decoupling region  17  is designed in such a way that, at a nominal velocity of the fluid stream  9  in the supply duct  5 , this flow velocity is increased sharply in the decoupling region  17 , preferably to a value near sound velocity in the fluid stream. The sound wave  15  is thereby to a large part reflected in the decoupling region  17  as a reflection wave  19 . The remaining part continues to run upstream through the supply duct  5  as a residual sound wave  21 . The nominal velocity is within a nominal operating interval which corresponds to an interval of operating states near a full load and a full-load state. The full-load state of the combustion device I is the maximum value for a release of power during combustion. In operating states of the combustion device  1  which correspond to a lower release of power than full load, a lower reflection of the sound wave  15  takes place. Combustion oscillations may be particularly disturbing and harmful in proximity to the full-load state, since a high release of power occurs here. Where lower load states are concerned, therefore, a lower reflection of the sound wave  15  and consequently a greater propagation of the sound wave  15  are acceptable. The decoupling region  17  is followed by a pressure increase region  23 . The decoupling region  17  forms, together with the pressure increase region  23 , a reflection section  24  with a length  24  of the supply duct  5 . The pressure increase region  23  corresponds to a widening of the supply duct  5 , in this case to the cross section of the supply duct  5  which is also present in front of the decoupling region  17  in the direction of flow  10 . 
     The reflection section  24  is a Venturi tube. The pressure increase region  23  is preferably designed in such a way that, at the nominal velocity, a maximum pressure increase in the fluid stream  9  is obtained. The decoupling region  17  has an inlet region  25  and an end region  27 . The end region  27  is at the same time an inlet region  29  of the pressure increase region  23 . The pressure increase region  23  terminates at an outlet region  31 . A diagrammatic illustration of the pressure profile in the decoupling region  17  and in the pressure increase region  23  is also depicted in FIG. 1. A marked pressure loss occurs in the fluid stream  9  between the inlet region  25  of the decoupling region  17  and the end region  27  of the decoupling region  17 . This pressure loss is for the most part compensated again in the pressure increase region  23 ; so that, overall, an only slight pressure loss Ap occurs, as compared with a pressure loss arising over this segment of the supply duct  5  in the case of an invariable cross section of the supply duct  5  (illustrated by broken lines). 
     FIG. 2 shows diagrammatically a combustion device  1  designed as a gas turbine. A compressor  45  and a turbine  47  are arranged along an axis  43 . Between the compressor  45  and turbine  47  is connected a combustion chamber  49  which is designed as an annular combustion chamber. A plurality of burners  51  open into the combustion chamber  49 , only one burner  51  being illustrated here for the sake of clarity. The burner  51  has an air duct  3  which is fluidically connected to the compressor  45 . The burner  51  has, furthermore, a supply duct  5  for the supply of natural gas  14 . Here, therefore, combustion media are air  6  from the compressor  45  and natural gas  14 . These burn in the combustion chamber  49 . The hot fuel gases  53  occurring as a result drive the turbine  47 . By virtue of the high release of power in such a gas turbine  1 , combustion oscillations having particularly high amplitudes may arise. Such combustion oscillations may be formed as combustion chamber oscillations in the combustion chamber  49 . 
     In order to prevent such combustion chamber oscillations from being propagatedvia the supply duct  5  to the entire natural-gas delivery system, not illustrated in any more detail, a decoupling region  17  is provided in the supply duct  5 . This decoupling region is followed in the direction of flow by a pressure increase region  23 . The effects and advantages of the decoupling region  17  and of the pressure increase region  23  correspond to those explained with regard to FIG.  1 . The natural-gas delivery system, not illustrated in any more detail, is thus acoustically decoupled effectively from the combustion chamber  49 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.