Abstract:
A low-pressure turbine of a gas turbine is disclosed. The turbine comprises a number of stages arranged one behind the other in an axial manner in the flow-through direction of the turbine. Each stage is formed from a fixed vane ring having a number of vanes and from a rotating blade ring having a number of blades. Each stage is characterized by a characteristic value vane-to-blade ratio that indicates the ratio of the number of vanes to the number of blades within a stage. One of the stages of the turbine is designed in such a manner that, in the event of noise-critical conditions of the turbine, the characteristic value vane-to-blade ratio of this stage is between a lower cut-off limit for mode k=−1 of the blade-passing frequency (BPF) of said stage and an upper cut-off limit for the mode k=−2 of the blade-passing frequency (BPF) of this stage.

Description:
The present invention relates to a turbine, in particular a low-pressure turbine of a gas turbine, in particular of an aircraft engine. 
     BACKGROUND 
     Gas turbines, in particular aircraft engines, are made up of multiple subassemblies, namely among other things a compressor, preferably a low-pressure compressor and a high-pressure compressor, a combustion chamber, and at least one turbine, in particular a high-pressure turbine and a low-pressure turbine. The compressors and the turbines of the aircraft engine preferably include multiple stages which are positioned axially one behind the other in the flow direction. Each stage is formed by a stationary vane ring and a rotating blade ring, the stationary vane ring having multiple stationary guide vanes and the rotating blade ring having multiple rotating blades. Each stage is characterized by a characteristic quantity which indicates the number of guide vanes to the number of rotating blades ratio within the stage. This characteristic quantity is also referred to as the vane-to-blade ratio (V/B). 
     The low-pressure turbine of an aircraft engine in particular is a noise source not to be disregarded. The low-pressure turbine emits noises in particular at frequencies which are an integral multiple of the so-called blade-passing frequency (BPF). The blade-passing frequency of a stage is the frequency at which the rotating blades of the stage rotate past a stationary guide vane of the respective stage. 
     For minimizing the noise emission of the low-pressure turbine of an aircraft engine, it is known from the related art to establish the vane-to-blade ratio of downstream stages of the low-pressure turbine at a value of approximately 1.5 in order to muffle the noise of the blade-passing frequency. Despite these measures known from the related art, the low-pressure turbines of aircraft engines known from the related art still emit a high noise level under noise-critical operating conditions, in particular during the landing approach or during taxiing on the tarmac of an airport. 
     An object of the present invention is to create a novel turbine, in particular a low-pressure turbine of a gas turbine, in particular of an aircraft engine. 
     The present invention provides a turbine, in particular a low-pressure turbine of a gas turbine, in particular of an aircraft engine, having multiple stages positioned axially one behind the other in the flow direction of the turbine, each stage being formed by a stationary guide vane ring having multiple guide vanes and a rotating blade ring having multiple rotating blades, and each stage being characterized by a vane-to-blade ratio characteristic quantity which indicates the number of guide vanes to the number of rotating blades ratio within a stage. According to the present invention, at least one stage of the turbine is designed in such a way that its vane-to-blade ratio characteristic quantity under noise-critical operating conditions of the turbine is between a lower cut-off limit for mode k=−1 of the blade-passing frequency (BPF) of this stage and an upper cut-off limit for mode k=−2 of the blade-passing frequency (BPF) of this stage. 
     The design principle according to the present invention for a turbine of an aircraft engine makes it possible to noticeably minimize the noise level emitted by the turbine. The noise emission in the range of the blade-passing frequency (BPF) may be clearly reduced with the aid of the present invention. 
     According to a preferred refinement of the present invention, at least one of the stages of the turbine is designed in such a way that its vane-to-blade ratio characteristic quantity in noise-critical operating conditions of the turbine is between a lower cut-off limit for mode k=−1 of the double blade-passing frequency (2BPF) of this stage and an upper cut-off limit for mode k=−2 of the double blade-passing frequency (2BPF) of this stage. 
     With the aid of this preferred refinement of the present invention, it is also possible to minimize the noise emission with frequencies which correspond to the double blade-passing frequency. 
     According to another preferred refinement of the present invention, at least one of the stages of the turbine situated upstream in the flow direction is designed in such a way that its vane-to-blade ratio characteristic quantity under noise-critical operating conditions of the turbine is between a lower cut-off limit for mode k=−1 of the blade-passing frequency (BPF) of this stage and an upper cut-off limit for mode k=−2 of the blade-passing frequency (BPF) of this stage, and, furthermore, at least one of the stages of the turbine situated downstream in the flow direction is designed in such a way that its vane-to-blade ratio characteristic quantity under noise-critical operating conditions of the turbine is between a lower cut-off limit for mode k=−1 of the double blade-passing frequency (2BPF) of this stage and an upper cut-off limit for mode k=−2 of the double blade-passing frequency (2BPF) of this stage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Exemplary embodiments of the present invention are explained in greater detail on the basis of the drawing without being limited thereto. 
         FIG. 1  shows a diagram for illustrating the design according to the present invention of the vane-to-blade ratio of the stages of a turbine with regard to modes k=−1 and k=−2 of the blade-passing frequency (BPF), and 
         FIG. 2  shows a diagram for illustrating the design according to the present invention of the vane-to-blade ratio of the stages of a turbine with regard to modes k=−1, k=−2, and k=−3 of the double blade-passing frequency (2BPF). 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is described in greater detail in the following with reference to  FIGS. 1 and 2 . 
     The present invention relates to a design principle for the stages of a turbine, namely a low-pressure turbine of an aircraft engine. Such a low-pressure turbine includes multiple stages which are situated axially behind each other in the flow direction of the low-pressure turbine. Each stage is formed by a stationary guide vane ring and a rotating blade ring. The guide vane ring has multiple stationary guide vanes. The rotating blade ring of each stage has multiple rotating blades. The present invention relates to a design principle with which the vane-to-blade ratio of the stages of a low-pressure turbine may be adapted in such a way that the low-pressure turbine emits a noise level as low as possible, i.e., under noise-critical operating conditions of the turbine or the aircraft engine. Such noise-critical operating conditions are, for example, a landing approach of an aircraft or movement of the aircraft on the tarmac of an airport. The noise emitted is characterized by frequencies which are integral multiples of the blade-passing frequency (BPF). 
     According to the present invention, at least one stage of the low-pressure turbine is designed in such a way that under noise-critical operating conditions of the turbine the vane-to-blade ratio (V/B) is between a lower cut-off limit for mode k=−1 of the blade-passing frequency (BPF) of this stage and an upper cut-off limit for mode k=−2 of the blade-passing frequency (BPF) of this stage. 
       FIG. 1  shows a diagram  10  for a low-pressure turbine having a total of seven stages, six of the seven guide vane rings V 2  through V 7  and the seven moving blade rings B 1  through B 7  being plotted on the horizontal axis of diagram  10 . The vane-to-blade ratio V/B is plotted on the vertical axis of diagram  10 . Reference numeral  11  in  FIG. 1  indicates a lower cut-off limit for mode k=−1 of the blade-passing frequency, while reference numeral  12  indicates the upper cut-off limit for mode k=−1 of this blade-passing frequency. Mode k=−1 of the blade-passing frequency (BPF) is dampened above upper cut-off limit  12  and below lower cut-off limit  11 . However, in area  15 , which is situated between lower cut-off limit  11  and upper cut-off limit  12  for mode k=−1 of the blade-passing frequency, mode k=−1 of the blade-passing frequency propagates almost undampened. Reference numeral  13  in  FIG. 1  indicates a lower cut-off limit for mode k=−2 of the blade-passing frequency. Reference numeral  14  indicates the upper cut-off limit for mode k=−2 of the blade-passing frequency. Mode k=−2 thus propagates almost undampened in area  16  between lower cut-off limit  13  and upper cut-off limit  14  for mode k=−2 of the blade-passing frequency (BPF), proper dampening being achieved for mode k=−2 below lower cut-off limit  13  and above upper cut-off limit  14 . 
     In the prior art, the vane-to-blade ratio of the downstream stages (V 5  through B 7 ) is selected in such a way that, for the downstream stages, it is above upper cut-off limit  12  for mode k=−1 of the blade-passing frequency. This is achieved according to the related art in that the vane-to-blade ratio V/B is established at a value of approximately 1.50 for these stages. In contrast, a vane-to-blade ratio V/B of approximately 0.90 is selected for the upstream stages (V 2  through B 4 ) according to the the present invention as shown by reference numeral  17 . However, such a vane-to-blade ratio is within area  15  so that, according to the related art, sound waves at frequencies in the range of the blade-passing frequency (BPF) are not dampened in the upstream stages. 
     Another problem of design principle  17  known from the related art arises from  FIG. 2  in which the propagation characteristics and the dampening characteristics of modes k=−1, k=−2, and k=−3 of the double blade-passing frequency (2BPF) are discussed. Reference numeral  20  in diagram  19  of  FIG. 2  indicates the lower cut-off limit for mode k=−1 of the double blade-passing frequency (2BPF). Reference numeral  21  in  FIG. 2  indicates the upper cut-off limit for mode k=−2 of the double blade-passing frequency (2BPF) and reference numeral  22  in  FIG. 2  indicates the lower cut-off limit for mode k=−2 of the double blade-passing frequency (2BPF). In the area  23  of  FIG. 2 , which is situated between upper cut-off limit  21  and lower cut-off limit  22  for mode k=−2 of the double blade-passing frequency (2BPF), mode k=−2 of the double blade-passing frequency (2BPF) propagates almost undampened. Moreover, a corresponding area  24  for mode k=−3 of the double blade-passing frequency (2BPF) is shown in  FIG. 3  which is situated between an upper cut-off limit  25  and a lower cut-off limit  26  for mode k=−3 of the double blade-passing frequency. 
     Reference numeral  17  in  FIG. 2  again indicates the design principle of the vane-to-blade ratio for the stages of the low-pressure turbine known from the related art. As is apparent from  FIG. 2 , for the design principle known from the related art, the vane-to-blade ratio V/B in the area of the downstream stages (V 5  through B 7 ) is situated above lower cut-off limit  20  for mode k=−1 of the double blade-passing frequency. According to the related art, mode k=−1 of the double blade-passing frequency is not dampened in the area of the downstream stages. Moreover, in the area of the upstream stages (V 1  through B 4 ), the vane-to-blade ratio V/B of these stages is in area  23 , from which it follows that for these stages mode k=−2 of the double blade-passing frequency (2BPF) is not dampened. 
     A particularly preferred design principle for the vane-to-blade ratio for the stages of a low-pressure turbine is indicated with reference numeral  18  in  FIGS. 1 and 2 . 
     As is apparent in particular in  FIG. 1 , the upstream stages (V 2  through B 4 ) situated in the flow direction of the turbine are designed in such a way that their vane-to-blade ratio V/B under noise-critical operating conditions of the turbine is between the lower cut-off limit  11  for mode k=−1 of the blade-passing frequency (BPF) and upper cut-off limit  14  for mode k=−2 of the blade-passing frequency (BPF). In the area of these stages, the vane-to-blade ratio is preferably in a range between 0.6 and 0.8, in particular in a range of approximately 0.7. In the area of the upstream stages, the vane-to-blade ratio V/B is thus established in a window between lower cut-off limit  11  of mode k=−1 of the blade-passing frequency and upper cut-off limit  14  of mode k=−2 of the blade-passing frequency. Modes k=−1 and k=−2 of the blade-passing frequency (BPF) are thus properly dampened in the area of these stages. 
     In the area of the downstream stages (V 5  through B 7 ) of the low-pressure turbine, their vane-to-blade ratio is established in a range above upper cut-off limit  12  of mode k=−1 of the blade-passing frequency, according to  FIG. 1 . Moreover, the vane-to-blade ratio for these stages is selected in such a way that, in the area of these stages, it is between lower cut-off limit  20  of mode k=−1 and upper cut-off limit  21  of mode k=−2 of the double blade-passing frequency (2BPF), according to  FIG. 2 . This is achieved in that the vane-to-blade ratio V/B in the area of the downstream stages of the turbine assumes a value which is in a range between 1.3 and 1.5, preferably approximately 1.4. 
     Furthermore, it is apparent from  FIG. 2  that due to the vane-to-blade ratio V/B for the upstream stages (V 2  through B 4 ), already discussed in connection with  FIG. 1 , which is preferably in a range between 0.6 and 0.8, it may be achieved that it is outside of area  23  in which mode k=−2 of the double blade-passing frequency (2BPF) may propagate almost undampened. Moreover, the less critical mode k=−3 of the double blade-passing frequency (2BPF) is positioned in area  23  for these stages. 
     The above-described design principle for the vane-to-blade ratio of the stages of a low-pressure turbine directly results in that, using the present invention, modes k=−1 and k=−2 of the blade-passing frequency (BPF) and modes k=−1 and k=−2 of the double blade-passing frequency (2BPF) may be dampened. A turbine configured in this way is thus characterized by low sound emission of frequencies in the range of the blade-passing frequency and the double blade-passing frequency. Using the present invention makes it possible to design all stages of a low-pressure turbine in such a way that the low-pressure turbine exhibits an optimal noise performance. 
     As mentioned above,  FIGS. 1 and 2  only show a preferred exemplary embodiment of the present invention. It should be pointed out that, based on the present invention, it is of course possible to select the vane-to-blade ratio for all stages of the low-pressure turbine in such a way that it is between a lower cut-off limit for mode k=−1 of the blade-passing frequency (BPF) of the respective stage and an upper cut-off limit for mode k=−2 of the blade-passing frequency (BPF) of the respective stage. 
     It is also possible to determine the vane-to-blade ratio for the upstream stages in such a way that, for the upstream stages, it is between a lower cut-off limit for mode k=−1 of the double blade-passing frequency (2BPF) and an upper cut-off limit for mode k=−2 of the double blade-passing frequency (2BPF), while the vane-to-blade ratio for the downstream stages is between a lower cut-off limit for mode k=−1 of the blade-passing frequency (BPF) and an upper cut-off limit for mode k=−2 of the blade-passing frequency (BPF). Also proper dampening of the sound propagation and thus a noise minimization of the low-pressure turbine is possible in low-pressure turbines designed in this way.