Abstract:
Apparatus and a method of detecting combustor rumble in a gas turbine engine having combustion equipment that is susceptible to combustor rumble. The method has steps of:
       locating a dynamic pressure transducer remotely from   the combustion equipment;   connecting a conduit between the transducer and a location in the engine remote from the combustion equipment and subject to pressure fluctuations due to the combustor rumble; and   measuring the pressure fluctuations using the transducer to detect combustor rumble.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is entitled to the benefit of British Patent Application No. GB 0823085.6, filed on Dec. 19, 2008. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to apparatus and a method for detecting combustor rumble in a gas turbine engine. It is applicable to a gas turbine engine having lean burn combustion equipment. It is particularly, though not exclusively, applicable to an aero gas turbine engine. 
       BACKGROUND OF THE INVENTION 
       [0003]    The environmental drive to reduce emissions from gas turbine engines, particularly for use to propel aircraft, leads to a need to use lean burn combustion equipment to eliminate fuel-rich pockets within the combustor. This means the reduction of the air to fuel ratio, in some cases to the extent that the temperature in the combustor falls below the stoichiometric level, which generates problems with flame stability. At lean air to fuel ratios a slight change in the ratio results in a large change in heat release. Hence the heat exchange reaction in the combustor is unsteady and that drives pressure and velocity fluctuations. These fluctuations manifest as an audible rumbling sound known as combustor rumble, being thermo-acoustic oscillations at discrete frequencies. Combustor rumble may cause fatigue failure of components in the engine and/or may cause passenger discomfort, depending on the frequency of the rumbling. There is a need to detect combustor rumble so that control can be applied to cancel it out to reduce or negate its negative effects. 
         [0004]    Industrial gas turbine engines have formerly had more restrictive emissions targets than aero gas turbine engines and have, therefore, used lean burn technology for longer. A typical lean burn industrial gas turbine engine has a rumble probe mounted onto the outer combustor casing in order to detect combustor rumble. Electrical signals generated by the probe are then passed to engine control means, which attempts to control the rumble. The rumble probe typically comprises a dedicated dynamic pressure sensor, such as a piezoelectric transducer. Such a rumble probe mounted to the combustor casing of a conventional industrial gas turbine engine experiences temperatures of around 700K (450° C.) and must be able to withstand prolonged periods subjected to this environment. 
         [0005]    One problem with applying the prior art rumble probe to an aero gas turbine engine is that the combustor casing of such an engine is a considerably harsher environment, experiencing temperatures of around 950K (700° C.). Currently available dynamic pressure transducers are not reliable at such elevated temperatures, which would instil unacceptable variability and uncertainty into combustor rumble detection if used. The transducers are also prohibitively expensive. 
         [0006]    A further problem is that the weight of the engine is increased by the addition of a rumble probe. It is probable that two rumble probes would be required to provide redundancy, which further adds to the weight and cost of the engine. Additional complexity is also introduced, both to the engine hardware and to the control system. 
         [0007]    The present invention seeks to provide apparatus and a method for combustor rumble detection that seeks to address the aforementioned problems. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, the present invention provides a gas turbine engine that has combustion equipment susceptible to combustor rumble; a dynamic pressure transducer located remotely from the combustion equipment; and a conduit connecting the transducer with a location in the engine remote from the combustion equipment, the location subject to pressure fluctuations due to the combustor rumble; whereby the transducer is arranged to detect combustor rumble dependent on the pressure fluctuations received. 
         [0009]    This is beneficial in terms of weight reduction and accuracy of measurement. 
         [0010]    The combustion equipment may be a lean burn combustor. 
         [0011]    The gas turbine engine may further comprise a signal processor. When the transducer generates an analogue signal, the signal processor may include a filter, preferably a band pass filter. When the transducer generates a digital signal, the signal processor may include a fast Fourier transform. The signal processor may include a rumble threshold comparator. It may have a fault integrator, particularly an incremental counter. 
         [0012]    The transducer may be an air pressure sensor. 
         [0013]    The location in the engine remote from the combustion equipment may be at the exit of a compressor stage or at the exit of a propulsive fan. 
         [0014]    The transducer may be arranged to detect core engine damage, particularly high pressure compressor damage, in addition to detecting combustor rumble. 
         [0015]    A second aspect of the present invention provides a method of detecting combustor rumble in a gas turbine engine comprising the steps of: 
         [0016]    locating a dynamic pressure transducer remotely from combustion equipment susceptible to combustor rumble; 
         [0017]    connecting a conduit between the transducer and a location in the engine remote from the combustion equipment and subject to pressure fluctuations due to the combustor rumble; and 
         [0018]    measuring the pressure fluctuations using the transducer to detect combustor rumble. 
         [0019]    This is advantageous because the dynamic pressure transducer is subjected to a less harsh environment by being located remotely from the combustion equipment. 
         [0020]    There may be a further step of processing the signal measured by the transducer, preferably by filtering the signal, by applying a root mean square calculation to the signal or by applying a fast Fourier transform to the signal. The processing step may also include the step of comparing the signal to a rumble threshold. 
         [0021]    There may be a further step of applying fault integration to the signal, such as by increasing a counter by an increment for each signal above a rumble threshold and decreasing the counter by an increment for each signal below the threshold. 
         [0022]    There may be a further step of setting a rumble detection flag depending on the measured pressure fluctuations. 
         [0023]    The measured pressure fluctuations or the rumble detection flag setting may be passed to an engine controller. 
         [0024]    The signal may be processed to detect core engine damage, particularly high pressure compressor damage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a sectional side view of a gas turbine engine according to the present invention. 
           [0026]      FIG. 2  is a block diagram of a first embodiment of the present invention. 
           [0027]      FIG. 3  is a block diagram of a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    An exemplary embodiment of the present invention comprises a gas turbine engine  10  as shown in  FIG. 1 . The engine  10  comprises an air intake  12  and a propulsive fan  14  that generates two airflows A and B. The gas turbine engine  10  further comprises, in axial flow A, an intermediate pressure compressor  16 , a high pressure compressor  18 , a combustor or combustion equipment  20 , a high pressure turbine  22 , an intermediate pressure turbine  24 , a low pressure turbine  26  and an exhaust nozzle  28 . A fan case  30  surrounds the gas turbine engine  10  and defines, in axial flow B, a bypass duct  32 . 
         [0029]    Mounted to the outside of the fan case  30  is engine control means  34 , for example an engine electronic controller (EEC). Alternatively, the engine control means  34  comprises an engine monitoring unit (EMU). The engine control means  34  comprises a dynamic pressure transducer  36 , which is integral with the engine control means  34  but alternatively may be connected thereto. A pipe  38  connects the transducer  36  to a location in the engine  10  that is upstream of the combustor  20  and which is subject to pressure fluctuations caused by combustor rumble, for example at the exit of the high pressure compressor  18 . The transducer  36  is preferably a dynamic pressure transducer that is already provided on the engine  10  for other purposes of control and/or monitoring. This reduces the weight, cost and complexity of the engine  10  by using one component for more than one purpose. For example, a P30 (high pressure compressor exit) pressure sensor can perform the functions of transducer  36 . The pipe  38  is designed to have minimal length and as few bends as possible to reduce weight, avoid signal attenuation and avoid trapping water at bends, which block the pressure fluctuations from being transferred through the pipe  38 , although drainage holes may be provided at the bends. 
         [0030]    A first embodiment of the apparatus and method according to the present invention is shown in  FIG. 2 . The pipe  38  is shown that passes the pressure fluctuations from the engine location, such as at the exit to the high pressure compressor  18 , to the dynamic pressure transducer  36  which is located remotely from the combustor  20 . The signal generated by the transducer  36  is then passed to signal processing means  40  to process the signal to indicate when combustor rumble is occurring. The signal received at the transducer  36  may be analogue, in which case the signal is passed to a filter  42 , preferably a band pass filter, to extract the frequencies that relate to combustor rumble. From the filter  42  the signal is passed through a calculation block  44  that calculates the root mean square value of the signal. This value is then sampled by an analogue to digital converter  46  to digitise the signal. The digitised signal is passed to a comparator  48 , which also receives a rumble threshold  50 . The rumble threshold  50  may be an absolute value or may be variable depending upon the point in the flight cycle, aircraft altitude, engine speed or any other factors that affect the pressure and absolute fuel flow in the engine  10 . The transducer  36  measures the pressure fluctuations at predetermined time intervals. In the case of a piezoelectric transducer, the frequency of measurements is high but for other types of transducer  36  the frequency may be lower. This can be chosen to suit the application. 
         [0031]    Hence a set of digitised signals are provided, separated by small time intervals. Where one of the digitised signals is greater than the rumble threshold  50 , the comparator  48  passes a signal that rumble is detected to a fault integrator  52 . Conversely, where one of the digitised signals is less than the rumble threshold  50 , the comparator  48  passes a signal to that effect to the fault integrator  52 . 
         [0032]    The fault integrator  52  is in the form of a counter, which is scaled to count between 0 and 1 in increments of N ‘up counts’ and M ‘down counts’. Thus, when a rumble detected signal is passed to the fault integrator  52  the counter increases by an increment 1/N. When the counter reaches 1 this sets the rumble detection flag  54  to true which is then passed to control and/or monitoring functions, for example in the engine control means  34 , for mitigating action to be triggered and engine health monitoring data to be logged. When a signal indicating no rumble is received by the fault integrator  52 , the counter decreases by an increment 1/M. When the counter reaches 0 it sets the rumble detection flag  54  to false which is passed to the control and/or monitoring functions. Thus, any mitigating action is cancelled and further engine health monitoring data can be logged. 
         [0033]    By providing the fault integrator  52  in the form of a counter with multiple increments, false triggering of rumble mitigation actions is prevented or reduced and combustor rumble is confirmed prior to action being commanded. The multiple increments introduce a small delay between the first detection of combustor rumble, or of cessation of rumble, and commanding action in response. 
         [0034]    A variation on the first embodiment is shown in  FIG. 3 , which shows a second embodiment of the apparatus and method of the present invention, in which the transducer  36  measures pressure fluctuations and provides a digital output. In this case the filter  40 , root mean square calculation block  44  and analogue to digital converter  46  are not required. In their place the signal processing means  40  comprises a fast Fourier transform function  56 . This function  56  not only processes the signal but samples it so that the digitised, sampled signal can be passed to the comparator  48 . The comparator  48  and fault integrator  52  function in the same manner as in the first embodiment. 
         [0035]    Although specific embodiments of the apparatus and method according to the present invention have been described, variations and modifications are possible within the scope of the invention as defined in the accompanying claims. For example, although the signal processing means  40  for the digital transducer signal has been proposed as a fast Fourier transform function, alternative digital signal processing is contemplated to fall within the scope of the present invention. For analogue signals produced by the transducer  36 , a different type of filter  42  may be used, for example a low pass filter. 
         [0036]    The fault integrator  52  may be omitted so that the rumble detection flag  54  is set immediately the digitised signal received by the comparator  48  exceeds the rumble threshold  50 . The fault integrator  52  may take a different form to the described counter. 
         [0037]    The pipe  38  may connect the transducer  36  to a different location in the engine  10  that is upstream of the combustor or combustion equipment  20 , for example the exit of the intermediate pressure compressor  16  or the rear of the propulsive fan  14 . Instead of the pipe  38  that has been described, any conduit or other enclosed volume that is capable of passing pressure fluctuations from the location in the engine  10  to the transducer  36  may be substituted with equal felicity. 
         [0038]    Although the present invention has been described with respect to an aero gas turbine engine, the apparatus and method may equally be applied to an industrial or marine gas turbine engine. Similarly, although a ducted fan gas turbine engine has been described by way of illustration, a turbojet or unducted fan gas turbine engine may also benefit from the application of the present invention. Although lean burn combustion equipment is more susceptible to combustor rumble than conventional combustion equipment, the present invention is also applicable to gas turbine engines having such combustion equipment. 
         [0039]    Although the present invention has been discussed in relation to the detection of combustor rumble, the signals generated by the transducer  36  may be processed to provide detection of core damage, particularly high pressure compressor  18  damage.