Patent Description:
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.

<CIT> discloses a prior art gas turbine engine combustor as set forth in the preamble of claim <NUM>.

In one aspect, there is provided a gas turbine engine combustor as recited in claim <NUM>.

In a further aspect, there is provided a method of operating a gas turbine engine as recited in claim <NUM>.

<FIG> illustrates a gas turbine engine <NUM> of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan <NUM> through which ambient air is propelled, a compressor section <NUM> for pressurizing the air, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases around the engine axis <NUM>, and a turbine section <NUM> for extracting energy from the combustion gases.

The combustor <NUM> is comprised of a gas generator case <NUM> which acts as a vessel to the pressurized air exiting the compressor section <NUM>, and the generator case <NUM> houses one or more liners <NUM>. The gas generator case <NUM> can thus be said to have an inlet fluidly connected to the compressor flow path. The liners <NUM> are typically apertured components delimiting a combustion chamber <NUM>. The compressed air can thus enter the combustion chamber <NUM> through the apertures in the liner <NUM>, a fuel nozzle can be secured to the liner <NUM> for introducing a jet of fuel in the combustion chamber <NUM>, and the combustion is typically self-sustained after initial ignition. The liner <NUM> can be said to have an outlet <NUM> fluidly connected to the turbine section <NUM>.

The compressor <NUM>, fan <NUM> and turbine <NUM> have rotating components which can be mounted on one or more shafts <NUM>. Bearings <NUM> are used to provide smooth relative rotation between a shaft <NUM> and casing (non-rotating component), and/or between two shafts which rotate at different speeds. An oil lubrication system <NUM> including an oil pump <NUM>, sometimes referred to as a main pump, and a network of conduits and nozzles <NUM>, is provided to feed the bearings <NUM> with oil. Seals <NUM> are used to contain the oil. A scavenge system <NUM> having cavities <NUM>, conduits <NUM>, and one or more scavenge pumps <NUM>, is used to recover the oil, which can be in the form of an oil foam at that stage, from the bearings <NUM>. The oil pump <NUM> typically draws the oil from an oil reservoir <NUM>, 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 <NUM> which must extend across the combustor <NUM> to convey relatively cool oil to bearings <NUM>. Indeed, in such a case, the service tube <NUM> may remain materially cooler than the surrounding portions of the combustor <NUM>, such as its gas generator case <NUM>, during normal operation due to the circulation of relatively cool oil in the service <NUM> tube. If the service tube <NUM> is cast in the gas generator case <NUM>, it can generate stress in its vicinity during operation. If the service tube <NUM> is a distinct tube extending inside the cavity of the gas generator case <NUM>, and mechanically interfaced with the gas generator case <NUM>, and has the same coefficient of thermal expansion than the gas generator case <NUM>, the service tube <NUM> can experience materially less thermal growth than the gas generator case <NUM>. Moreover, this difference in thermal growth can be exacerbated by an additional difference in growth due to pressure. Indeed, the gas generator case <NUM> 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 <NUM> may not be a source of dimensional increase. It was found that in some cases, the difference in growth could reach <NUM>-<NUM>% 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 arrangements 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 <FIG> and <FIG>. As shown in <FIG>, a service tube <NUM> distinct from the structure of the gas generator case <NUM> and of the structure of the compressor, extending from a radially outer mechanical interface <NUM> with the gas generator case <NUM> to a radially inner mechanical interface <NUM> leading ultimately to one or more bearings <NUM>. In this case, the service tube <NUM> and the gas generator case <NUM> are a non-fabricated assembly <NUM>, as best seen in <FIG>, with the service tube <NUM> inlet section <NUM> being provided in the form of a male component received in a female aperture <NUM> defined in the radially outer mechanical interface <NUM> of the gas generator case <NUM>, and where the outer face <NUM> of the service tube <NUM> inlet section <NUM> is brazed to the inner face <NUM> of the gas generator case's <NUM> receiving aperture <NUM>. In such a non-fabricated assembly, one can strategically select the service tube's <NUM> material to be a non-hardenable material, whereas the gas generator case <NUM> can be made of a hardenable material, in which case, the brazing can occur during the hardening of the gas generator case <NUM>. As known in the art, hardening is a metallurgical metalworking process used to increase the hardness of a metal. A hardenable material is one which can be hardened by this metallurgical process, whereas a non-hardenable material is one for which the hardness is unaffected by this metallurgical process. If the gas generator case is intended to be hardened, which can simultaneously involve brazing the service tube, for instance, it can be preferred that the service tube be made of a material which will be unaffected by this hardening process.

The service tube <NUM> can be made of a first material having a first coefficient of thermal expansion, whereas the gas generator case's <NUM> radially outer mechanical interface <NUM> 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 <NUM>%, more than <NUM>%, more than <NUM>%, and perhaps around <NUM>%.

In the context of a gas generator case <NUM>, 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 <NUM> can be made of stainless steel, particularly <NUM> series stainless steel and notably Greek Ascoloy, which can have coefficients of thermal expansion in the order of <NUM>-<NUM> X <NUM>-<NUM> °C-<NUM>, but perhaps also <NUM> series stainless steel, which can have coefficients of thermal expansion in the order of <NUM> X <NUM>-<NUM> °C-<NUM>. The service tube can be made of Inconel, such as perhaps Inconel <NUM> or Inconel <NUM>, which can have coefficients of thermal expansion in the order of <NUM> X <NUM>-<NUM> °C-<NUM> to <NUM> X <NUM>-<NUM> °C-<NUM>, for instance. A typical difference in the coefficient of thermal expansion of stainless steel and Inconel can be around <NUM>%, 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 <NUM>, 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 <NUM> to generate a an annular stream of hot combustion gasses; C) energy from the hot combustion gasses is extracted using a turbine <NUM>, and used to drive the compressor <NUM> via a rotary shaft <NUM> supported by bearings <NUM>; D) the bearings <NUM> are supplied with oil via a service tube <NUM> which extends inside the gas generator case <NUM> of the combustor <NUM>, the oil maintaining the service tube <NUM> at a temperature lower than the surrounding temperature in the gas generator case <NUM>; E) the colder service tube <NUM> is maintained in a state of thermal growth compatible with the state of growth of the hotter gas generator case <NUM>, due to a greater coefficient of thermal expansion of the service tube <NUM>.

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.

Claim 1:
A gas turbine engine combustor (<NUM>) comprising:
a gas generator case (<NUM>) having a first coefficient of thermal expansion, the gas generator case (<NUM>) acting as a vessel to pressurized air exiting a compressor section;
a liner (<NUM>) inside the gas generator case (<NUM>), the gas generator case (<NUM>) housing the liner (<NUM>), the liner (<NUM>) delimiting a combustion chamber (<NUM>);
a service tube (<NUM>) having a second coefficient of thermal expansion, the second coefficient of thermal expansion being higher than the first coefficient of thermal expansion, characterised in that
the service tube (<NUM>) extends inside the gas generator case (<NUM>), outside the liner (<NUM>); and
the service tube (<NUM>) is brazed or soldered to an aperture (<NUM>) formed in the gas generator case (<NUM>).