Patent Application
20120144841
The present invention relates to deoiler or breather assemblies, and more particularly for deoiler or breather assemblies for use with gas turbine engine gearboxes.
Gas turbine engines and other mechanical devices can include gearboxes and/or bearing assemblies that utilize an oil flow for cooling and lubricating purposes. It is often desired to avoid pressuring bearing compartments and gearboxes, but instead to vent such compartments and allow them to “breathe”. In such an arrangement, oil can become mixed with vented air, causing oil saturation in that air. It is further desired to reclaim oil present in the vented air. The presence of oil in vented air that leaves an engine is unsightly and aesthetically undesirable. In particular, for gas turbine engines used in commercial airline applications, the visible clouds of oil in exhaust streams may be unpleasant to customers or passengers who prefer such exhaust streams to appear transparent—even if such exhaust streams are harmless and within accepted operating parameters.
In a typical prior art deoiler/breather assembly (the terms “deoiler” and “breather” are used synonymously herein), a fluidic mixture of oil and air in a bearing or gearbox compartment is passed through a rotating separator that draws oil out of the mixture. The oil removed from the mixture can then be returned to a primary lubrication circuit for further use. Remaining air from the mixture can leave the rotating separator through a tube or shaft located along a central axis of rotation and can be exhausted from the engine (and its nacelle) to ambient air. Such prior art deoiler/breather assemblies are able to efficiently retain oil to avoid losing too much oil through the vented air, though some small amount of oil typically remains in the exhaust stream of the remaining air. In a typical gas turbine engine, air in the deoiler/breather assembly is at elevated temperatures generally in the range of approximately 121-177° C. (250-350° F.). At elevated temperatures, oil can exist as vapor (i.e., in a gaseous state). However, condensation of small, dispersed oil droplets can exist in vented exhaust streams under certain circumstances. In particular, when vented air containing oil vapor is cooled by adiabatic expansion (i.e., a decrease in pressure) or by mixing with colder air, the oil vapor can condense into tiny droplets (i.e., liquid state droplets) that can reflect light in the visible spectrum and appear as “white smoke”, that is, as a visible cloud of material that can appear to be smoke from a combustion process to an unfamiliar observer.
Prior art solutions to the problem of visible oil in exhaust streams from deoilers/breathers include dispersing such exhaust streams in a fan bypass stream from the engine, which combines the oil-containing exhaust stream with such a large volume of oil-free air that the oil is greatly dispersed and not readily visible. However, this solution requires that an exhaust port for the deoiler/breather to have a particular location in relation to the fan bypass air stream (typically an exhaust port near an aft end of the engine), which is not always feasible for certain engine and nacelle configurations. In the past, efforts have also been made to improve air/oil separation so that less oil is present in exhaust streams from a deoiler/breather. However, even with such efficiency improvements, the separation process is not 100% efficient and some small amount of oil will remain in exhaust streams that may become visible. In addition, some deoiler/breather assemblies have included a cruciform structure on an interior of a rotating exhaust shaft or tube to eliminate a “free” vortex that can lead to oil condensation in the exhaust stream by regulating vortex rotation with the cruciform structure. However, because such cruciform structures rotate with the shaft of the separator, they must be rotationally balanced, which is difficult to accomplish.
Thus, an improved deoiler/breather assembly is desired.
A breather assembly for use with a gas turbine engine according to the present invention includes a static housing for accepting a fluidic mixture of substances, a rotatable separator having one or more fluid inlets and arranged about an axis of rotation, an exhaust outlet defined in the housing and positioned coaxially with the rotatable separator to accept fluidic exhaust from the rotatable separator, and a static diffuser supported by the housing at or near the exhaust outlet downstream from the rotatable separator. A portion of the static diffuser extends within the rotatable separator. The static diffuser includes a flow-straightening structure configured to reduce vortex flows in fluid flows passing through the exhaust outlet.
Deoiler or breather assemblies (the terms “deoiler” and “breather” are used synonymously herein) are used in gas turbine engines to separate oil from air within vented lubrication compartments before venting that air in an exhaust stream. However, prior art breather assemblies can produce a visible cloud (“white smoke”) in an exhaust stream if oil remaining in the exhaust stream condenses forming tiny dispersed droplets (i.e., liquid state oil droplets) that reflect light in the visible spectrum. Visible materials of any sort in an exhaust stream can be aesthetically undesirable, with a general preference being for exhaust streams to appear transparent. It has been found that fluid entering a shaft or tube to be exhausted from a rotating air/oil separator of a breather assembly tends to have a strong rotational component, and conservation of angular momentum in that fluid can form a vortex at an inner diameter or center of that shaft/tube (e.g., the vortex can be formed generally along an axis of rotation of the separator). Such vortices can be intense, like tornados, with a relatively low pressure inside the vortex relative to pressure elsewhere in the exhaust stream. Rapid cooling of fluid in the vortex due to adiabatic expansion causes flash condensation of oil vapor present in the exhaust stream, which produces tiny dispersed droplets of oil. Exhaust fluid then typically mixes with relatively cold ambient air, which can exacerbate droplet formation. Because of these factors, chilled oil droplets in exhaust streams are slow to evaporate and disperse, making it difficult to avoid the presence of visible clouds of oil droplets.
In general, the present invention provides a static (i.e., non-rotating) core diffuser structure that can extend in a cantilevered manner into a rotating portion of an air/oil separator of a breather assembly. The core diffuser can help redirect and straighten fluidic exhaust flows in order to convert rotational kinetic energy into axially oriented kinetic energy to help reduce vortex formation and adiabatic expansion in exhaust flows. This, in turn, helps reduce condensation of oil vapor that may be present in the exhaust flows, which helps such exhaust flows maintain a transparent appearance without visible clouds of material. In some embodiments, the core diffuser can be configured with a generally cylindrical support tube attached to a stationary housing of the breather assembly, a plurality of plates attached to the support tube that form a plurality of stages for redirecting fluid flow, and an optional flow straightener secured at a downstream end of the support tube. In other embodiments, the core diffuser can include outer diameter flow guides and a central cruciform flow guide of varying sizes rather than a plurality of plates. The present invention thus provides for a reduction of visible material in exhaust streams, while providing a breather assembly that is relatively simple to manufacture and install in a variety of settings compared to prior art designs. Those of ordinary skill in the art will recognize additional features and benefits of the present invention in view of the accompanying figures and the description that follows.
The breather assembly 12 includes a housing 26, a shaft 28, an input gear 30, an air/oil separator 32, a core diffuser 34, and an outlet 36. The housing 26 can be stationary, that is, rotationally fixed relative to mounting location in the engine 10. The term “stationary” is used herein to describe rotationally fixed components that may be present in an engine of a movable vehicle. The shaft 28 is rotatable, and defines an axis of rotation A. In the illustrated embodiment, the shaft 28 includes two sections of different diameter, with at least one of those sections being hollow. The input gear 30 is fixed to the shaft 28 for co-rotation, and can accept rotational input power from suitable mating gearing (not shown), such as an accessory gearbox drive shaft powered by the gas turbine engine 10. The air/oil separator 32 is secured to the shaft 28, and rotates with the shaft 28 when rotational power is supplied by the input gear 30. In one embodiment, the separator 32 can include a conventional metallic foam material or other structure that accepts a fluidic mixture 38-1 of air and oil delivered from the passages 24. The incoming fluidic mixture 38-1 is generally at an elevated temperature (e.g., approximately 121-177° C. (250-350° F.)), and typically contains air saturated with oil vapor as well as finely dispersed oil droplets. The separator 32 helps remove oil droplets from air, returning the removed liquid oil 38-2 to the housing 26 through generally radial outward outlets and passing remaining fluid 38-3 radially inward to the shaft 28. The removed oil 38-2 can be collected in the housing 26 for recirculation in the engine 10 in a conventional manner. The remaining fluid 38-3 is mostly air with trace amounts of oil predominantly in a vapor state. The shaft 28 is configured with a hollow section that defines a fluid passage connecting the separator 32 and the outlet 36. From the shaft 28, remaining fluid 38-3 from which the oil 38-2 has been removed is exhausted (i.e., vented) through the outlet 36 and out of the engine 10 in an exhaust stream 40. As shown in
The core diffuser 34 extends at least partially into the shaft 28, and is secured in a rotationally fixed manner to the housing 26 at or near the outlet 36. In this way, the core diffuser 34 extends in a cantilevered configuration along the axis A into the shaft 28. The core diffuser 34 influences flow of the fluid 38-3 through the shaft 28 and the outlet 36 to reduce a risk of oil vapor condensation in the exhaust stream 40 by helping to straighten fluid flow and reduce vortex generation. In particular, the pressure of fluid 38-3 in downstream portions of the core diffuser 34 and in the exhaust stream 40 can be substantially equal at radially inward and radially outward locations relative to the axis A, thereby avoiding a low pressure core associated with vortices. The configuration and operation of embodiments of the core diffuser 34 are explained further below.
The core diffuser 34 of the illustrated embodiment includes a substantially cylindrical support tube 46, a flow straightening structure 48, and an optional flow guide 50. The core diffuser 34 can be stationary, that is, rotationally fixed relative to the housing 26. The support tube 46 is fixedly secured to the housing 26 at or near the outlet 36, and extends in a cantilevered configuration along the axis A inside of the shaft 28. A labyrinth-type seal can be created between the housing 26 and the shaft 28 (with a gap between the housing 26 and the shaft 28), which can also create an air curtain seal between the shaft 28 and the support tube 46 to help ensure that the oil wetted fluid 38-1 does not bypass the separator 32 entirely and escape via the exhaust stream 40. In further embodiments, optional circumferential openings (not shown) can be provided in the support tube 46 to allow radially inward fluid flow into the support tube 46.
The flow guide 50 is fixedly secured to the support tube 46 at or near a downstream end of the support tube 46, which is located at the outlet 36. In the illustrated embodiment, the flow guide 50 has a cruciform shape, though other configurations are possible in alternative embodiments. A central opening 50-1 can be formed through the flow guide 50 along the axis A. The flow guide 50 helps maintain a relative straight flow of the remaining fluid 38-3 and discourage circumferential rotation of that fluid 38-3 when leaving the breather assembly 12 in the exhaust stream 40.
The flow straightening structure 48 can be secured to the support tube 46 at or near an upstream end of the support tube 46. The flow straightening structure 48 is static, that is, rotationally fixed relative to the support tube 46 and the optional flow guide 50, and in turn relative to the housing 26. In the illustrated embodiment, the flow straightening structure 48 is axially aligned with the openings 44 in the shaft 28, though other arrangements are possible in alternative embodiments. Furthermore, in the illustrated embodiment the flow straightening structure 48 includes four diffuser stage plates 48-1, 48-2, 48-3 and 48-4. A larger or smaller number of discrete stages can be provided in further embodiments, as desired for particular applications. In the illustrated embodiment, a diameter of each sequential diffuser stage plate 48-1, 48-2, 48-3 and 48-4 is sequentially larger in the downstream direction, such that the diffuser stage plate 48-1 furthest upstream has the smallest diameter and the diffuser stage plate 48-4 furthest downstream has the largest diameter. The diffuser stage plates 48-1, 48-2, 48-3 and 48-4 can be separate components secured together and to the support tube 46 by brazing or other suitable attachment methods. Alternatively, the flow straightening structure 48 can be formed as a monolithic structure that integrally defines different stages. The flow straightening structure 48 and the support tube can be made of a metallic material, such as aluminum, and preferably are made of a material having a coefficient of thermal expansion that is similar or identical to that of a material of the housing 26.
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. For instance, the particular shape and size of passages and other features of a core diffuser according to the present invention can vary as desired for particular applications.
