Patent Application
20220170419
The application relates generally to gas turbine engines and, more particularly, to combustors thereof.
A plurality of factors are considered in the design of a gas turbine engine, and these include weight, reliability, durability and cost. Moreover, the design of the individual components must often take into account the effect of growth due to temperature and/or pressure which can occur between different operating conditions, or between a given operating condition and a cooled down, inoperative condition. Differences in growth can lead to potential stress at the mechanical interface between components, and such stress can be undesirable, such as when it can cause low cycle fatigue to components or the like. In fabricated assemblies, one can sometimes replace a component which has failed due to such stresses by disassembling and replacing the component, which is typically undesirable. In the context of non-fabricated assemblies, such as where components are soldered or brazed to other components, it can occur that an entire assembly will need to be replaced due to the failure of a single one of its components, which can be even less desirable.
One of the areas of the gas turbine engine which is the most subjected to growth is within and around the combustor, where much of the combustion occurs, and which is typically also subjected to high pressures during operation (another source of growth). The high temperatures which are sustained in the combustor during operation often imposes significant constraints to the choice of materials which can be used in the components of the combustor, and can thus greatly reduce design freedom.
Such issues have been taken into consideration by engineers over the years, and have been addressed to a certain degree. But there always remains room for improvement.
In one aspect, there is provided a gas turbine engine combustor comprising a gas generator case having a first coefficient of thermal expansion, a liner inside the gas generator case, the liner delimiting a combustion chamber, a service tube extending inside the gas generator case, outside the liner, the service tube having a second coefficient of thermal expansion, the second coefficient of thermal expansion being materially higher than the first coefficient of thermal expansion.
In another aspect, there is provided a gas turbine engine comprising, in serial flow communication, a compressor for pressurizing air, a combustor for mixing the compressed air with fuel and igniting for generating an annular stream of hot combustion gases, and a turbine driving the compressor via a shaft using energy extracted from the hot combustion gases, the shaft being supported by bearings, the combustor having a gas generator case having a first coefficient of thermal expansion, and a service tube extending radially across the gas generator case for supplying the bearings with oil, the service tube having a second coefficient of thermal expansion, the second coefficient of thermal expansion being materially higher than the first coefficient of thermal expansion.
In a further aspect, there is provided a method of operating a gas turbine engine, the method comprising, simultaneously: pressurizing air using a compressor, mixing the compressed air with fuel and igniting for generating an annular stream of hot combustion gases in a combustor, extracting energy from the combustion gasses using a turbine, the turbine connected to the compressor via a rotary shaft supported by bearings; supplying said bearings with oil via a service tube extending across a gas generator case of the combustor, said service tube being maintained at a lower temperature than the gas generator case by the oil; maintaining the colder service tube in a state of thermal growth compatible with the state of growth of the hotter gas generator case, due to a greater coefficient of thermal expansion of the service tube.
Reference is now made to the accompanying figures in which:
The combustor 16 can be comprised of a gas generator case 40 which acts as a vessel to the pressurized air exiting the compressor section 14, and the generator case 40 can house one or more liners 42. The gas generator case 40 can thus be said to have an inlet fluidly connected to the compressor flow path. The liners 42 are typically apertured components delimiting a combustion chamber 44. The compressed air can thus enter the combustion chamber 44 through the apertures in the liner 42, a fuel nozzle can be secured to the liner 42 for introducing a jet of fuel in the combustion chamber 44, and the combustion is typically self-sustained after initial ignition. The liner 42 can be said to have an outlet 46 fluidly connected to the turbine section 18.
The compressor 14, fan 12 and turbine 18 have rotating components which can be mounted on one or more shafts 48. Bearings 20 are used to provide smooth relative rotation between a shaft 48 and casing (non-rotating component), and/or between two shafts which rotate at different speeds. An oil lubrication system 22 including an oil pump 24, sometimes referred to as a main pump, and a network of conduits and nozzles 26, is provided to feed the bearings 20 with oil. Seals 28 are used to contain the oil. A scavenge system 30 having cavities 32, conduits 34, and one or more scavenge pumps 36, is used to recover the oil, which can be in the form of an oil foam at that stage, from the bearings 20. The oil pump 24 typically draws the oil from an oil reservoir 38, and it is relatively common to use some form of air/oil separating device in the return line.
One of the contexts where differences in growth can perhaps be the most significant, is situations where components which are mechanically interfaced with one another have materially different coefficients of thermal expansion while being subjected to similar temperatures, and/or are subjected to materially different temperatures and/or pressures during operation. In this context, materially involves more than within a measurement error, and typically a level of significance in the context of the intended use in the gas turbine engine.
One of the areas which is perhaps the most sensitive to differences in growth may be the case of a service tube 50 which must extend across the combustor 16 to convey relatively cool oil to bearings 20. Indeed, in such a case, the service tube 50 may remain materially cooler than the surrounding portions of the combustor 16, such as its gas generator case 40, during normal operation due to the circulation of relatively cool oil in the service 50 tube. If the service tube 50 is cast in the gas generator case 40, it can generate stress in its vicinity during operation. If the service tube 50 is a distinct tube extending inside the cavity of the gas generator case 40, and mechanically interfaced with the gas generator case 40, and has the same coefficient of thermal expansion than the gas generator case 40, the service tube 50 can experience materially less thermal growth than the gas generator case 40. Moreover, this difference in thermal growth can be exacerbated by an additional difference in growth due to pressure. Indeed, the gas generator case 40 is pressurized during operation and the pressure can thus additionally stress its structure in an orientation of growth, at least on its radially outer wall, while the oil pressure inside the service tube 50 may not be a source of dimensional increase. It was found that in some cases, the difference in growth could reach 0.2-0.3% of the components dimensions for instance, and that this can generate a significant source of stress. Similar issues may arise in other gas turbine engine components subjected to similar circumstances.
Different approaches can be considered to address such issues. The component's mechanical interfaces can be designed with sliding joints, for instance, but this can be less than desirable in some embodiments because it can impart additional weight or costs, or affect durability, for instance, particularly when compared with a soldered or brazed mechanical interface, for instance.
It was found that in at least some embodiments, a useful approach can be to design the colder component with a material having a coefficient of thermal expansion materially higher than the coefficient of thermal expansion of the hotter. Indeed, in such cases, the greater coefficient of thermal expansion of the colder component can be harnessed to generate a greater thermal growth, and thereby partially or fully compensate for the colder temperature.
An example embodiment is presented in
The service tube 50 can be made of a first material having a first coefficient of thermal expansion, whereas the gas generator case's 40 radially outer mechanical interface 52 can be made of a second material having a second coefficient of thermal expansion. The first coefficient of thermal expansion can be greater than the second coefficient of thermal expansion in a manner to impart comparable/compatible growth notwithstanding the differences in temperature.
Indeed, the difference in coefficients of thermal expansion can be significant, such as perhaps being different by more than 5%, more than 10%, more than 15%, and perhaps around 20%.
In the context of a gas generator case 40, there can be a limited set of commercially available materials which are adapted to withstand the harsh operating conditions of the context, but there can nonetheless remain sufficient degree of freedom to achieve the goal. Indeed, the gas generator case 40 can be made of stainless steel, particularly 400 series stainless steel and notably Greek Ascoloy, which can have coefficients of thermal expansion in the order of 11-12×10−6° C., but perhaps also 300 series stainless steel, which can have coefficients of thermal expansion in the order of 10*10−6° C. The service tube can be made of Inconel, such as perhaps Inconel 718 or Inconel 625, which can have coefficients of thermal expansion in the order of 13*10−6° C./16*10−6° C., for instance. A typical difference in the coefficient of thermal expansion of stainless steel and Inconel can be around 20%, for instance.
In situations where the difference of thermal expansion coefficients is deemed too great given the expected temperature differences, i.e. where the difference of thermal expansion coefficients between Inconel and stainless steel would tend for the Inconel component to overcompensate for its lower temperature, it can be suitable to pre-stress the lower temperature component in the orientation opposite to the expected growth during assembly, for instance.
Accordingly, during operation of the gas turbine engine 10, the following processes can occur simultaneously: A) the air is pressurized by the compressor; B) the compressed air is mixed with fuel and ignited in the combustor 16 to generate a an annular stream of hot combustion gasses; C) energy from the hot combustion gasses is extracted using a turbine 18, and used to drive the compressor 14 via a rotary shaft 48 supported by bearings 20; D) the bearings 20 are supplied with oil via a service tube 50 which extends inside the gas generator case 40 of the combustor 16, the oil maintaining the service tube 50 at a temperature lower than the surrounding temperature in the gas generator case 40; E) the colder service tube 50 is maintained in a state of thermal growth compatible with the state of growth of the hotter gas generator case 40, due to a greater coefficient of thermal expansion of the service tube 50.
The embodiments described in this document provide non-limiting examples of possible implementations of the present technology. Upon review of the present disclosure, a person of ordinary skill in the art will recognize that changes may be made to the embodiments described herein without departing from the scope of the present technology.
For example, while an example embodiment presented above was applied to a service tube extending in a gas generator case, outside a liner, it will be understood that other embodiments can be applied to other components facing similar or otherwise comparable issues. In one embodiment, the gas generator case can include both a radially outer wall and a radially inner wall, but in alternate embodiments, the gas generator case can include solely a radially outer wall, or a portion of a radially outer wall, while the radially inner wall can be formed by a different component, possibly made of a different material.
In one embodiment, the service tube can be made integrally of a single material. In other embodiments, the service tube can have a body made of a first material, and another component, such as a coupler, made of another material. Typically, a key aspect will be that a portion of the service tube which has a significant effect in the process of thermal growth be made of a material having a higher coefficient of thermal expansion, whereas other portions of the service tube can be made of a material having the same coefficient of thermal expansion than the gas generator case component the service tube mechanically interfaces with, for instance.
Moreover, it will be noted that while the example presented above and illustrated used the example context of a turbofan engine, other embodiments can be applied to other contexts such as a turboprop or turboshaft gas turbine engine for instance, or any other engine subjected to comparable issues and which could benefit from the proposed solution.
Yet further modifications could be implemented by a person of ordinary skill in the art in view of the present disclosure, which modifications would be within the scope of the present technology.
