Abstract:
A method of starting a gas turbine engine having a rotor comprising at least a shaft mounted compressor and turbine, with a casing surrounding the rotor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Turbine engines, and particularly gas or combustion turbine engines, are rotary engines that extract energy from a flow of combusted gases passing through the engine onto a multitude of turbine blades. Gas turbine engines comprise a core formed of a compressor section, combustion section, and turbine section. The compressor section and turbine section comprise compressor blades and turbine blades that are mounted to a common drive shaft, and are collectively referred to as a rotor, which is surrounded by a casing. In some gas turbine engines there are multiple rotors, such as a low pressure rotor and a high pressure rotor, with the drive shafts being coaxial. 
         [0002]    Gas turbine engines have been used for land and nautical locomotion and power generation, but are most commonly used for aeronautical applications such as for aircraft, including helicopters. In aircraft, gas turbine engines are primarily used for propulsion of the aircraft, along with power generation. In terrestrial applications, turbine engines are primarily used for power generation. 
         [0003]    Gas turbine engines for aircraft are designed to operate at high temperatures, approximately 2000° C., to maximize engine efficiency, therefore cooling of certain engine components may be necessary. Typically, cooling is accomplished by ducting cooler air, approximately 900° C., from the high and/or low pressure compressors to the engine components which require cooling. 
         [0004]    When the turbine engine is turned off after operating at such high temperatures, the heat stratifies in the engine core and the top of the rotor will become hotter than the bottom due to rising heat. The stratification can often lead to a 500° C. difference between the top and bottom of the rotor, which leads to asymmetrical thermal expansion between the top and bottom of the rotor. Under such a temperature difference, the top of the rotor thermally expands a greater radial amount causing what is referred to as a bowed-rotor condition. The bowed-rotor condition may occur within 10 minutes of the engine being turned off and may last up to 8 hours. 
         [0005]    The asymmetrical thermal expansion moves the center of mass upward and out of alignment with the rotational axis of the shaft, resulting in an out of balance condition when the rotor is turned. This upward movement also reduces the clearance between the tips of the blades and the casing. When the rotor is again rotated, the out of balance caused will cause vibration during rotation. The vibrations will increase rotor deflections, especially when passing through a vibratory mode, such as a rotational natural frequency for the rotor. The vibrations can accelerate normal cracking and fatigue, leading to earlier and more frequent maintenance. They also can accelerate wear and tear on seals and similar structures. 
         [0006]    The thermal expansion may be great enough that when the upper part of the rotor is rotated to the lower part of the surrounding engine casing, it may contact the lower portion of the casing, which has not radially expanded as much as the upper portion of the engine casing. The repeated contact or rubbing with the engine casing during rotation of the rotor with a bowed-rotor condition may cause parts to break off and thus may cause foreign object damage to the engine. 
         [0007]    Prior solutions to the bowed rotor phenomena are directed to prevention and mitigation once the bow has occurred. The most common current solution is to rotate the engine with the starter for an extended period when it is desired to start the engine until the bow dissipates. This can take several minutes and has several undesirable effects. First, there may be uncomfortably loud cabin noise due to the rotor vibration being carried through the aircraft structure. Second, the aircraft auxiliary power unit life is consumed due to the longer time at high power required to rotate the engine starter. Third, the added starting time causes airport congestion as the aircraft must remain in the taxiway during engine start, blocking other aircraft movements. Fourth, if the engine rotor has a natural frequency in the speed range of the starting sequence, the damage described above can occur. Previous solutions to these problems have proposed to externally rotate the rotor at a low speed, approximately 1 rpm until the bowed rotor phenomena disappears, which is often more than a half an hour, which is an unacceptable downtime in the operation of aircraft, especially commercial airliner. 
         [0008]    Typically, upon shutting off the turbine engine, the rotor would be rotated at low speed, approximately 1 rpm, by an external power source, such as a pneumatic drive or electric motor, to prevent the bowed rotor. Such preventative action requires an additional step to the shutdown of the engine, which may be forgotten by a ground crew. Alternatively, the turbine engine would not be shut down, which consumed relatively substantial amounts of fuel. The bowed rotor phenomena can occur as quickly as within 10 minutes of shut down. Thus, even if a ground grew takes action to slowly rotate the engine, they may not act fast enough. 
         [0009]    Once the bowed-rotor condition is present, it may naturally last for up to 8 hours, which is an undesirably long time for the aircraft to be out of operation. Thus, given the relatively short time needed for the bowed-rotor condition to arise and the relatively long time for it to naturally subside, it is important to prevent or address the bowed rotor phenomena for normal operation of the aircraft. Otherwise, once the bowed rotor phenomena occurs, the most common current solution is to rotate the engine with the starter for an extended period until the bow dissipates. This can take several minutes and has several undesirable effects. First, there may be uncomfortably loud cabin noise due to the rotor vibration being carried through the aircraft structure. Second, the aircraft auxiliary power unit life is consumed due to the longer time at high power required to rotate the engine starter. Third, the added starting time causes airport congestion as the aircraft must remain in the taxiway during engine start, blocking other aircraft movements. Fourth, if the engine rotor has a natural frequency in the speed range of the starting sequence, the damage described above can occur. Previous solutions to these problems have proposed to externally rotate the rotor at a low speed, approximately 1 rpm, until the bowed rotor phenomena disappears, which is often more than a half an hour, which is an unacceptable downtime in the operation of aircraft, especially commercial airliner. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0010]    A method of starting a gas turbine engine having a rotor comprising at least a shaft mounted compressor and turbine, with a casing surrounding the rotor. The method comprises an acceleration phase, a bowed-rotor cooling phase, during the acceleration, and a combustion phase. The bowed-rotor cooling phase comprises a time where the rotational speed of the rotor is maintained below a bowed-rotor threshold speed until a non-bowed condition is satisfied, wherein the air forced through the gas turbine engine cools the rotor. The combustion phase occurs after the bowed-rotor cooling phase and upon reaching the combustion speed, wherein fuel is supplied to the gas turbine engine is turned on. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0011]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine for an aircraft. 
           [0012]      FIG. 2  is a graph of a normal start sequence without the invention. 
           [0013]      FIG. 3  is a flow chart of a normal start sequence. 
           [0014]      FIG. 4  is a graph of a start sequence with a bowed rotor according to a first embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  illustrates a gas turbine engine  10  for an aircraft. The turbine engine  10  comprises, in axial flow order, fan section  12 , compressor section  14 , combustion section  16 , turbine section  18 , and exhaust section  20 . The compressor section  14 , combustion section  16  and turbine section  18  collectively define an engine core  22  that is surrounded by a casing  24 . The compressor section comprises a low pressure compressor  26  and a high pressure compressor  28 . The turbine section comprises a high pressure turbine  30  and a low pressure turbine  32 . A first drive shaft  34  connects the rotating elements, generally blades, of the high pressure compressor  28  and high pressure turbine  30 . A second drive shaft extends coaxially through the first drive shaft and connects the rotating element of the low pressure compressor  26  and the low pressure turbine  32 . The fan section  12  comprises a fan  38  which further comprises fan blades  40  coupled to the second drive shaft  36 . A nacelle  46  may surround a portion of the fan blades  40 . 
         [0016]    Collectively, the rotating elements of the compressor section  14  and turbine section  18  along with the connecting shafts are referred to as a rotor R. In some turbine engines, the compressor section  14  and turbine section  18  only have a single compressor and turbine connected by one rotating shaft, which would define the engine rotor. In the illustrated example, the turbine engine  10  has two compressors and turbines, low and high pressure, which, along with the first and second drive shafts, define the rotor R. There can be any number of compressor and turbine combinations, which are not limiting on the invention. 
         [0017]    A turning motor system  50  is provided on the casing  24  and is operably coupled to the rotor R. A rotational source, external to the turbine engine, may be coupled to the turning motor system  50  to initiate rotation of the rotor. A well-known example of such a rotational source is a turbine air starter (not shown), which is not germane to the invention. 
         [0018]    As described in greater detail above, when the turbine engine  10  is shut down after running in normal operation, heat stratifies in the engine core  22  as the heat naturally rises from a lower section  44  of the casing  24  toward a upper section  42  of the casing  24 , resulting in the upper section  42  becoming hotter than the lower section  44 . Correspondingly, an upper section R u  of the rotor will thermally expand in the radial distance an amount greater than a lower section R l , leading to an asymmetrical radial expansion of the upper and lower sections R u , R l  of the rotor R relative to the rotational axis  37  of the shafts  34 ,  36 . This asymmetrical radial expansion caused by the temperature differential created by the stratification is referred to in the industry as a bowed-rotor. The issue is that the asymmetrical expansion causes the center of mass of the rotor to move radially upward, away from the rotational axis  37  of the shafts,  34 ,  36 , which leads to a rotational imbalance. 
         [0019]    In a specific engine example, an out of balance condition caused by the bowed-rotor will cause vibration below 500 RPM and/or when going through critical speeds when the turbine engine  10  is accelerating. Certain turbine engines  10  are more sensitive to a bowed-rotor condition if there is a vibratory mode, such as a natural frequency, near the maximum rotor speed the starting system is capable of producing. For the above example, a peak vibratory mode for the turbine engine  10  may be at 3500 RPM and the maximum starting rotor speed may be 4250 RPM. Since the peak mode is below the maximum rotor speed, high vibrations and rubbing will occur when the turbine engine  10  accelerates through the peak mode at 3500 RPM if a bowed-rotor condition exists. 
         [0020]    Under a normal start sequence for the turbine engine  10 , a turbine engine  10  with a bowed-rotor condition can experience vibrations associated with the bowed-rotor condition. This is best seen with reference to  FIG. 2 , where the normal start sequence  100  for a turbine engine  10 , which represents the rotor speed during start as a function of time. A typical start sequence  100  begins with a first acceleration phase  102  where the rotational speed of the rotor R is increased by external power until it reaches a combustion speed  104 , which is where the engine is producing sufficient compression for combustion. Upon reaching the combustion speed  104 , fuel is provided to the turbine engine and the ignition system is turned on to ignite the fuel. After commencing combustion at the end of the first acceleration phase  102 , the rotational speed of the rotor R is increased during a second acceleration phase  106  using engine-generated power, instead of external power, to an idle speed  108 . The second acceleration phase  106  typically has a greater rate of acceleration because the power generated by the turbine engine is greater than the external power during the first acceleration phase. 
         [0021]    Referring to  FIG. 3 , to practically implement the start sequence  100 , at the time of engine start up, the flight crew initiates the start sequence using the Flight Management System of the aircraft at  110 , which turns authority over to a Full Authority Digital Engine Control (herein after referred to as “FADEC”) that carries out an auto start sequence at  112 . The FADEC evaluates the turning motor  50  status at  114 , and opens a starter air valve (“SAV”) (when using a turbine air starter) at  116  to supply the starting air and begin the first acceleration phase  102 . During the first acceleration phase  102 , the FADEC monitors the vibration of rotor R for a period of time as the rotor increases speed to the combustion speed  104  at  118 . Once the combustion speed  104  occurs and the vibration is sensed to be low enough for acceleration to the idle speed  108 , ignition and fuel supply are initiated at  120 . At that point, the FADEC will initiate the second acceleration phase  106  and the rotor will accelerate to at idle speed  108 . After reaching the idle speed  108 , the turbine engine then continues to run at the idle speed  108  in an idle phase. If the monitored vibration is too great, which can be indicative of a bowed-rotor condition, the SAV will be shut and the rotation will be stopped, which is highly undesirable. 
         [0022]    The invention eliminates the need to shut down the turbine engine in this instance and takes corrective action when a bowed-rotor condition is present. In general, the invention takes advantage of the air being drawn through the turbine engine during the start sequence to rapidly cool the rotor R and relieve the bowed-rotor condition during start. This is accomplished during the start sequence by adding a sub-mode  122  for a bowed-rotor cooling phase to the start sequence where the rotor is rotated below a predetermined bowed-rotor speed threshold, preventing damage to the rotor, until a non-bowed-rotor condition is satisfied. The non-bowed-rotor condition may be a predetermined time of rotation that is sufficient to ensure air being drawn through the turbine engine provides sufficient cooling for relief of enough of the bowed-rotor condition for safe operation, or it may be engine parameter, such as a temperature of the rotor or an imbalance of the rotor. 
         [0023]      FIG. 4  illustrates one embodiment of the bowed-rotor cooling phase sub mode where air drawn through the turbine engine is used to cool the rotor and relieve the bowed-rotor condition. The start sequence  200  of  FIG. 4  carries out the invention by adding a bowed-rotor cooling phase in the form of a dwell phase  202  to the first acceleration phase  102  of the start sequence of  FIG. 3 . In this sense, the start sequence  200  is similar to the start sequence  100  except for the addition of the dwell phase  202 . Thus, the prior description of the start sequence  100  applies to the start sequence  200 . For brevity, only the aspects related to the additional dwell phase  202  will be described. 
         [0024]    During the first acceleration phase  102 , if the FADEC determines an imbalance that is indicative of a bowed-rotor condition, instead of shutting off the turbine engine, the FADEC initiates the bowed rotor cooling phase in the form of the dwell phase  202 , which comprises temporarily ceasing the acceleration of the rotor R and rotating the rotor R at a substantially constant speed until the bowed-rotor condition is relieved, such as by satisfying a non-bowed-rotor condition, such as a non-bowed rotor threshold. Upon relief of the bowed-rotor condition, the first acceleration phase  102  is continued, and the start sequence  200  completes in the same manner as the start sequence  100 . 
         [0025]    Several non-bowed-rotor thresholds may be used to determine the relief of the bowed-rotor condition. One non-bowed-rotor threshold is to operate the dwell phase  202  for a predetermined time, which can be a time that is determined by testing, that is sufficient for the particular rotor R rotating at the dwell speed to sufficiently cool to eliminate the bowed-rotor condition. Another non-bowed-rotor threshold is the FADEC sensing the rotor imbalance during the dwell phase. As the rotor cools, the expansion disparity within the rotor disappears, which reduces the amount of the imbalance sensed by the FADEC. Once the amount of imbalance drops below an acceptable threshold for the given turbine engine, the FADEC will terminate the dwell phase  202  and resume the first acceleration phase  102 . As the amount of imbalance is generally related to the magnitude of vibrations at a given speed, the FADEC may use motion sensor inputs that are indicative of the vibrations to determine the degree of imbalance. Another non-bowed rotor threshold is monitoring the temperature of one or more portions of the rotor. When the temperatures are within a predetermined range of each other or below an absolute threshold, the FADEC may determine that the bowed-rotor condition is relieved and resume the first acceleration phase  102 . The temperature can be determined by the FADEC receiving temperature sensor inputs for one or more portions of the rotor, casing, and/or casing interior. The previously described non-bowed-rotor thresholds may be used separately or in any combination. Other non-bowed-rotor thresholds may also be used alone or in combination with those described above. 
         [0026]    It should be noted that while only one dwell phase  202  is illustrated, it is contemplated that multiple dwell phases  202  may be used. If after the completion of the first dwell phase  202  and the first acceleration phase is being completed, the FADEC determines an imbalance indicative of a bowed-rotor condition, another dwell phase  202  could be entered. As many dwell phases as needed could be added. This would lead to a stair-step-like profile to the first acceleration phase, with the rise of the stair step being part of the first acceleration phase  102 , and the run being a dwell phase  202 , where the dwell phases  202  would likely, but not necessarily, have sequentially increasing rotational speeds. 
         [0027]    While the dwell phase  202  is illustrated as being during the first acceleration phase  102 , it should be noted that the dwell phase  202  can be applied to any phase of the start sequence  200 , including the second acceleration phase  106 . The dwell phase  202  is illustrated as being in the first acceleration phase  102  as that is the phase where the rotor speed is likely to be below the bowed-rotor threshold speed, where it is not detrimental to the turbine engine to rotate the rotor with a bowed-rotor condition. However, depending on the turbine engine, the bowed-rotor threshold speed may occur during the second acceleration phase  106 . The bowed-rotor threshold speed is dependent on the vibration modes of the rotor and/or the clearance between the rotor R and the surrounding casing  24 . In most cases, the bowed-rotor threshold speed is a speed below that speed at which the rotor R would contact a portion of the casing  24  because of vibrations associated with the current imbalance attributable to the bowed-rotor condition. 
         [0028]    It should further be noted that while the bowed rotor cooling phase is illustrated as a dwell phase  202  having a constant speed, this need not be the case. The bowed rotor cooling phase could just use a much slower acceleration rate than the first acceleration phase  102 , for example. The goal during the dwell phase  202  is to provide sufficient time for the rotor R to be sufficiently cooled to eliminate the bowed-rotor condition before the rotational speed of the rotor R reaches the bowed-rotor threshold speed for the corresponding imbalance. As the bowed rotor-condition is relieved, the corresponding bowed-rotor threshold speed will necessarily increase because the rotor R continuously shrinks as it is being cooled, resulting in a continuous reduction in the imbalance of the rotor R attributable to the bowed-rotor condition. 
         [0029]    The bowed rotor cooling phase could also be a combination of acceleration and deceleration steps, where the acceleration continues until the FADEC determines an unacceptable imbalance and then the rotor is slightly decelerated. After the passage of a predetermined time and/or the satisfying of another non-bowed-condition threshold, rotor R would once again be accelerated until an unacceptable imbalance is encountered. The acceleration/deceleration would continue until the imbalance is relieved. 
         [0030]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.