Abstract:
A method for active suppression of hydrodynamic instabilities in a combustion system in which liquid or gaseous fuel is premixed with combustion air and the fuel/air mixture is then burnt. The mass flow of the supplied fuel is modulated on the basis of a selected time function. Simplification and increased functional reliability are achieved by the modulation which is carried out using fluidics.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of combustion technology. It relates to a method for active suppression of hydrodynamic instabilities in a combustion system. It also relates to a combustion system for carrying out the method. 
     BACKGROUND OF THE INVENTION 
     Thermoacoustic oscillations represent a danger to all types of combustion applications. They lead to high-amplitude pressure fluctuations, to constriction of the operational range, and can increase undesirable emissions. This affects, in particular, combustion systems with little acoustic damping, such as those used in gas turbines. Active control of the combustion oscillations may be required to guarantee high performance with regard to pulsations and emissions over a wide operating range. 
     Various techniques for controlling and suppressing combustion instabilities by means of an active control system have already been proposed, in which, using either an open or a closed controller, the supply of fuel and/or combustion air to the burner or to the burners is controlled or modulated in a defined manner. A prior, not previously published application from the applicant relates, for example, to active control of the instabilities in a premixing burner and is illustrated, for example, in FIG. 1 of EP-B1-0 321 809. In such a premixing burner, the fuel flows in the two outer fuel lines ( 8 ,  9  in FIG. 1 of EP-B1-0 321 809) are modulated asymmetrically in an open loop at frequencies between 0.3 Hz and 5 kHz, preferably between 5 Hz and 200 Hz. The modulation process is carried out with the aid of two fuel valves which are inserted in the fuel line. 
     A disadvantage with the use of mechanically moving, electrically driven fuel valves is that they have mechanically moving parts which are subject to increased wear at the modulation frequencies that are used, and whose functional reliability is subject to restrictions. Another disadvantage is the power required by the valves themselves, which makes additional circuit measures necessary. 
     SUMMARY OF THE INVENTION 
     The object of the invention is thus to specify a method for active control of combustion instabilities, which is simple and functionally reliable and presents only minor requirements in terms of hardware preconditions. 
     The essence of the invention is to use fluidics methods rather than unreliable mechanically operated valves for modulation of the fuel supply, that is to say to vary the fuel flows by hydrodynamic means without any moving parts, by using fluidic switches and control elements. 
     One preferred embodiment of the method according to the invention is distinguished in that, within the combustion system, the fuel is passed to two separate fuel lines for premixing, and in that, in order to modulate the supplied fuel, the fuel mass flow is alternately split in a different manner between the two fuel lines by fluidics means. Such alternate splitting is particularly suitable for premixing burners of the type mentioned above since this advantageously results in the axial symmetry of the combustion flame being disturbed and the axial symmetrical vortex structures and pressure fluctuations associated with axial symmetry being suppressed, or their creation being prevented. The alternate splitting can, for example, be achieved by supplying a first unmodulated partial mass flow of fuel equally via the two fuel lines, while a second partial mass flow is additionally supplied alternately via one of the two fuel lines. This process does not utilize the full modulation depth in the fuel supply. 
     However, it is also conceivable, according to a preferred development of the embodiment, for the (entire) fuel mass flow to be passed alternately via one of the two fuel lines (full modulation depth). 
     The modulation process is preferably carried out using a periodic time function, at a predetermined frequency and with a predetermined amplitude. The frequencies are in this case governed by the geometry and method of operation of the combustion system, and are normally in a range which has already been mentioned further above in conjunction with the prior art. 
     The destruction of the symmetries in the flame or combustion chamber which promote oscillations can in this case be achieved on the one hand by the fuel being passed via the two fuel lines to a single premixing device and being injected at different points there. 
     However, it is also conceivable for the fuel to be passed via the two fuel lines to different premixing devices (for example premixing burners) within the same combustion system and to be injected there, which leads to symmetry suppression within the entire system comprising a plurality of premixing devices. 
     In the combustion system according to the invention, which comprises a premixing device for mixing the fuel with the combustion air, at least one fuel line for supplying the fuel to the premixing device, and means for modulation of the mass flow of the supplied fuel, is distinguished in that the modulation means comprise a fluidics element. 
     Another preferred embodiment of the combustion system according to the invention is distinguished in that the fuel is supplied via two fuel lines and in that the fluidics element is designed and is connected to the two fuel lines such that, when modulation occurs, at least a portion of the supplied fuel mass flow is switched alternately to one of the two fuel lines. In particular, the two fuel lines lead to the same premixing device, and the premixing device is designed such that the fuel from each of the fuel lines is injected at a different point in the premixing device. 
     The fluidics element which is used preferably comprises a fuel inlet and two fuel outlets which branch in a Y-shape from the fuel inlet and are connected to the fuel lines, and two mutually opposite control channels, which run transversely with respect to the fuel inlet, that open into the fuel inlet in the region of the branch of the fuel outlets. By applying increased pressure or reduced pressure, the element allows the fuel mass flow entering the fuel inlet to be diverted from one fuel outlet to the other. 
     The desired modulation is achieved in a particularly simple manner with the aid of this fluidics element if the two control channels are connected to one another in a closed circuit by means of a connecting tube of predetermined length running outside the fluidics element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are disclosed in the following description and illustrated in the accompanying drawings, in which: 
     FIG. 1 shows a first exemplary embodiment of a combustion system according to the invention having a premixing burner which is supplied with fuel via two different fuel lines, modulated by means of a fluidics element; 
     FIG. 2 shows a second exemplary embodiment of a combustion system according to the invention having two premixing burners which operate in parallel and each of which is supplied with fuel via a fuel line, modulated by means of a fluidics element; 
     FIG. 3 shows a third exemplary embodiment of a combustion system according to the invention having a mixing tube into which fuel is injected from two opposite sides in the region of a swirl element, which fuel is supplied via two fuel lines, modulated by means of a fluidics element; 
     FIG. 4 shows the internal design of a fluidics element as is preferably used in the exemplary embodiments shown in FIGS. 1 to  3 ; and 
     FIG. 5 shows the preferred configuration of the fluidics element from FIG. 4 as an automatically oscillating changeover element. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a first exemplary embodiment of a combustion system according to the invention. The combustion system  10  comprises a (schematically illustrated) premixing burner  17  which, by way of example, is in the form of a double-cone burner, as is shown in FIG. 1 of EP-B1-0 321 809. A (gaseous) fuel is injected on two opposite sides into the premixing burner  17 , and is mixed with the required combustion air. For this purpose, the fuel for the premixing burner  17  is passed via two separate fuel lines  15  and  16 , which are fed from a common fuel inlet  12  via a fluidics element  11 . 
     The fluidics element  11  is preferably internally designed as shown in FIG. 4 (schematically). The fuel inlet  12  branches in a Y-shape, after a constriction in the interior of the element, into two obliquely emerging fuel outlets  31  and  32 , to which the fuel lines  15 ,  16  are connected. Two mutually opposite control channels  27  and  28  are also provided in the interior of the fluidics element, which run transversely with respect to the fuel inlet  12  and open into the fuel inlet  12  in the region of the branch of the fuel outlets  31 ,  32 . The operation of the fluidics element  11  is based on the principle of the Prandtl diffuser and the Coanda effect. The mass flow flowing in through the fuel inlet  12  has the natural tendency to flow out through one of the fuel outlets  31 ,  32  owing to the Coanda effect (in FIG. 4, the arrows indicate that, in this example, the flow emerges through the upper fuel outlet  31 ). The fuel mass flow entering through the fuel inlet  12  can be diverted from one fuel outlet  31  to the other fuel outlet  32 , and vice versa, by applying increased pressure in one control channel ( 27  in FIG. 4) or reduced pressure in the other control channel ( 28  in FIG.  4 ). 
     Thus, if the fluidics element  11  in FIG. 1 is driven from a controller  14  via a control line  13  with appropriate periodic pressure surges to the control channels  27 ,  28  of the fluidics element, it distributes the fuel mass flow at the fuel inlet  12  on a periodically switching basis to one of the two fuel outlets  31 ,  32 , and thus to one of the two fuel lines  15 ,  16 . The switching frequency and thus the modulation frequency of the fuel supply is in this case governed by the controller  14 . 
     The modulation arrangement is particularly simple if the controller  14  (shown by dashed lines) and the control line  13  are entirely dispensed with. In this case—as shown in FIG.  5 —the two control channels  27  and  28  are connected to one another externally by means of a connecting tube  29 , and thus form a closed circuit. Such a configuration of the fluidics element results in automatic changeover oscillations, resulting in the flow being switched periodically between the fuel outlets  31  and  32 . The geometry of the circuit, in particular the effective length of the connecting tube  29 , in this case governs the changeover frequency and can be selected so as to produce an optimum modulation frequency for suppressing the combustion oscillations. The particular advantage of this arrangement is that no supply or control devices whatsoever are required for modulation. 
     In the example in FIG. 1, the entire fuel supply to the premixing burner  17  is modulated (100% modulation). However—as already mentioned above—within the context of the invention it is also feasible and worthwhile switching only part of the flow between the two fuel lines  15  and  16  periodically, while the rest of the fuel flow flows equally through both lines. In FIG. 1, bypass lines have been provided in a manner corresponding to this method from the fuel inlet  12  to the fuel lines  15 ,  16 , and these bridge the fluidics element  11 . 
     While in the exemplary embodiment in FIG. 1, the modulation of the fuel supply itself has a disturbing influence on the symmetry in the connected premixing burner  17  as a result of the periodic process of switching backward and forward between the two fuel lines  15 ,  16 , the desired symmetry disturbance in a combustion system  20  in which a plurality of premixing burners  18 ,  19  operate in parallel in one combustion chamber, it is also possible, according to FIG. 2, for the two (modulated) fuel lines  15 ,  16  coming from the fluidics element  11  to be connected separately to the various premixing burners  18 ,  19 . In this case, the interaction between the two premixing burners  18 ,  19  prevents the formation of thermoacoustic instabilities. 
     Finally, it is also feasible within the context of the invention to modulate a mixing tube  21 , instead of a premixing burner, as shown in FIG.  3 . In this mixing tube  21 , the fuel lines  15 ,  16  coming from the fluidics element  11  are connected to two opposite injection apparatuses  23 ,  24 , through which the fuel is injected in the region of a swirl element  25  arranged in the interior of the mixing tube  21 , and by means of which combustion air flowing in through the air inlet  22  is mixed by vortex action. Appropriate modulation in the fluidics element  11  then results in the suppression of instabilities in the air/fuel mixture emerging through the outlet  26 . The mixing tube  21  together with the swirl element  25  can in this case be designed in a similar way to that described in U.S. Pat. No. 4,226,083. 
     Although this invention has been illustrated and described in accordance with certain preferred embodiments, it is recognized that the scope of this invention is to be determined by the following claims.