Aerodynamic design of a cooled cooling air system for an aero gas turbine

This paper examines the aerodynamic design requirements of a cooled cooling air system for a large, high by-pass ratio, high overall pressure ratio aero-engine. This can be broken into aerodynamic sub-systems each with their own set of requirements and challenges. A low pressure system is required to deliver air bled from the bypass duct to a heat exchanger for use as a heat sink. Similarly, a high pressure system removes a portion of the hot core engine air from a location downstream of the compressor and ducts this to-and-from the HX for cooling. This cooled air must then be returned, across the main gas path, for use in component cooling. The challenge is to design these sub-systems such that satisfactorily perform their own function whilst integrating into the existing engine architecture. This paper presents an overview of a number of studies which use CFD to explore the design space and develop appropriate designs which were subsequently experimentally validated on several isothermal test facilities. Ultimately the feasibility of designing the aerodynamic sub-systems was demonstrated and a future design strategy established

A multi-objective factorial design methodology for aerodynamic off-takes and ducts

Fluid off-takes and complex delivery ducts are common in many engineering systems but designing them can be a challenging task. At the conceptual design phase many system parameters are open to consideration and preliminary design studies are necessary to instruct the conceptualisation process in an iterative development of design ideas. This paper presents a simple methodology to parametrically design, explore and optimise such systems at low cost. The method is then applied to an aerodynamic system including an off-take followed by a complex delivery duct. A selection of nine input variables is explored via a fractional factorial design approach that consists of three individual seven-level cubic factorial designs. Numerical predictions are characterised based on multiple aerodynamic objectives. A scaled representation of these objectives allows for a scalarisation technique to be employed in the form of a global criterion which indicates a set of trade-off geometries. This leads to the selection of a set of nominal designs and the determination of their robustness which will eventually instruct the next conceptual design iteration. The results are presented and discussed based on criterion space, design variable space and contours of several flow quantities on a selection of optimal geometries

A multi-objective factorial design methodology for aerodynamic off-takes and ducts

Fluid off-takes and complex delivery ducts are common in many engineering systems but designing them can be a challenging task. At the conceptual design phase many system parameters are open to consideration and preliminary design studies are necessary to instruct the conceptualisation process in an iterative development of design ideas. This paper presents a simple methodology to parametrically design, explore and optimise such systems at low cost. The method is then applied to an aerodynamic system including an off-take followed by a complex delivery duct. A selection of nine input variables is explored via a fractional factorial design approach that consists of three individual seven-level cubic factorial designs. Numerical predictions are characterised based on multiple aerodynamic objectives. A scaled representation of these objectives allows for a scalarisation technique to be employed in the form of a global criterion which indicates a set of trade-off geometries. This leads to the selection of a set of nominal designs and the determination of their robustness which will eventually instruct the next conceptual design iteration. The results are presented and discussed based on criterion space, design variable space and contours of several flow quantities on a selection of optimal geometries

Aerodynamic influence of a bleed on the last stage of a low-pressure compressor and S-duct

Abstract This paper uses Computational Fluid Dynamics to investigate the effect of an engine handling bleed situated on the outer casing downstream of the last rotor stage of a low-pressure compressor and upstream of the outlet guide vane and S-shaped duct. The model, validated against existing experimental data, utilized an unsteady RANS solver incorporating a Reynolds stress closure to examine the unsteady component interactions. The results showed that at bleed rates less than 25% of the mainstream flow the bleed effects were negligible. However, at higher bleed rates performance was significantly degraded. A uniform flow extraction hypothesis was employed to separate the positional bias effects from the bulk flow diffusion. This revealed that the bleed-induced radial flow distortion can significantly affect the OGV loading distribution, which thereby dictates the position and type of stall within the OGV passage. Extraction of the rotor tip leakage via the shroud bleed, combined with the radial flow distortion, contributed to a 28% reduction in duct loss at 10% bleed and up to 50% reduced loss at 25% bleed. The actual amount of flow required to be extracted for an OGV stall to develop, was 30%. That was independent of the bleed location and the type of stall. For bleeds up to 20%, the S-duct displayed a remarkable resilience and consistency of flow variables at duct exit. However, a stalled OGV deteriorated the radial flow uniformity that was presented to the high-pressure compressor

The aerodynamic design of the low pressure air delivery ducts for a cooled cooling air system

As aero gas turbines strive for higher efficiencies and reduced fuel burn, the trend is for engine overall pressure ratio to increase. This means that engine cycle temperatures will increase and that cooling of various engine components, for example the high pressure turbine, is becoming more difficult. One solution is to employ a cooled cooling air system where some of the compressor efflux is diverted for additional cooling in a heat exchanger fed by air sourced from the by-pass duct. Design of the ducting to feed the heat exchangers with coolant air is challenging as it must route the air through the scenery present in the existing engine architecture which leads to a convoluted and highly curved system. Numerical predictions using ANSYS Fluent demonstrated that a baseline design was unsuitable due to large amounts of flow separation in the proximity of the heat exchangers. This paper is mainly concerned with the aerodynamic design of this component of the duct. In order to produce a viable aerodynamic solution a numerical design methodology was developed which significantly enhances and accelerates the design cycle. This used a Design of Experiments approach linked to an interactive design tool which parametrically controlled the duct geometry. Following an iterative process, individually optimized 2D designs were numerically assessed using ANSYS Fluent. These designs were then fed into an interactive 3D model in order to generate a final aerodynamic definition of the ducting. Further CFD predictions were then carried out to confirm the suitability of the design. RANS CFD solutions, generated, using a Reynolds stress turbulence model, suggested that the new design presented significant improvement in terms of diffusion and flow uniformity

The aerodynamic design of the low pressure air delivery ducts for a cooled cooling air system

As aero gas turbines strive for higher efficiencies and reduced fuel burn, the trend is for engine overall pressure ratio to increase. This means that engine cycle temperatures will increase and that cooling of various engine components, for example the high pressure turbine, is becoming more difficult. One solution is to employ a cooled cooling air system where some of the compressor efflux is diverted for additional cooling in a heat exchanger fed by air sourced from the by-pass duct. Design of the ducting to feed the heat exchangers with coolant air is challenging as it must route the air through the scenery present in the existing engine architecture which leads to a convoluted and highly curved system. Numerical predictions using ANSYS Fluent demonstrated that a baseline design was unsuitable due to large amounts of flow separation in the proximity of the heat exchangers. This paper is mainly concerned with the aerodynamic design of this component of the duct. In order to produce a viable aerodynamic solution a numerical design methodology was developed which significantly enhances and accelerates the design cycle. This used a Design of Experiments approach linked to an interactive design tool which parametrically controlled the duct geometry. Following an iterative process, individually optimized 2D designs were numerically assessed using ANSYS Fluent. These designs were then fed into an interactive 3D model in order to generate a final aerodynamic definition of the ducting. Further CFD predictions were then carried out to confirm the suitability of the design. RANS CFD solutions, generated, using a Reynolds stress turbulence model, suggested that the new design presented significant improvement in terms of diffusion and flow uniformity

Experimental investigation of the effect of bleed on the aerodynamics of a low-pressure compressor stage in a turbofan engine [GT2023-102260]

The compression system in modern turbofan engines is split into several stages linked by s-shaped transition ducts. Downstream of the low-pressure system, a handling bleed is often required for off-design performance and/or to extract ice/water and foreign debris prior to the air entering the highpressure compression stages. The inclusion of this bleed and various structural vanes can introduce unwanted component interactions and compromise the aerodynamic performance of the upstream low-pressure compressor stage and downstream transition duct. This paper presents an experimental investigation of the aerodynamic performance of a compressor transition duct and bleed for a very high bypass ratio turbofan. A fully annular, low-speed test facility incorporating a 1½ stage axial compressor was used to examine the mean and unsteady flow in the last stage of a low-pressure compressor and the downstream transition duct. The transition duct incorporated load bearing struts, including a so-called King strut with twice the thickness of the regular struts. The bleed utilized a 360° annular slot located on the casing immediately downstream of the low-pressure rotor and upstream of the outlet guide vane. The results showed that the King strut, caused a similar flow distortion and redistribution in the OGV like the Regular struts, and had otherwise imposed a negligible effect on overall performance over a range of rotor flow coefficients. The addition of bleed had a more notable effect, generating an increasing outboard bias in the rotor efflux, as the flow migrated towards the offtake. At the design flow operating point, the OGV were relatively insensitive to this until the highest bleed rate (18%) where evidence of stall was observed. At a lower operating point, the change of rotor swirl and additional OGV incidence caused earlier onset of stall and a full OGV stall was observed above 10% bleed. Increasing bleed was observed to cause a gradual increase in duct loss up to the point of OGV stall when losses increased more rapidly.</p

Experimental investigation of the effect of bleed on the aerodynamics of a low-pressure compressor stage in a turbofan engine [GT2023-102260]

The compression system in modern turbofan engines is split into several stages linked by s-shaped transition ducts. Downstream of the low-pressure system, a handling bleed is often required for off-design performance and/or to extract ice/water and foreign debris prior to the air entering the highpressure compression stages. The inclusion of this bleed and various structural vanes can introduce unwanted component interactions and compromise the aerodynamic performance of the upstream low-pressure compressor stage and downstream transition duct. This paper presents an experimental investigation of the aerodynamic performance of a compressor transition duct and bleed for a very high bypass ratio turbofan. A fully annular, low-speed test facility incorporating a 1½ stage axial compressor was used to examine the mean and unsteady flow in the last stage of a low-pressure compressor and the downstream transition duct. The transition duct incorporated load bearing struts, including a so-called King strut with twice the thickness of the regular struts. The bleed utilized a 360° annular slot located on the casing immediately downstream of the low-pressure rotor and upstream of the outlet guide vane. The results showed that the King strut, caused a similar flow distortion and redistribution in the OGV like the Regular struts, and had otherwise imposed a negligible effect on overall performance over a range of rotor flow coefficients. The addition of bleed had a more notable effect, generating an increasing outboard bias in the rotor efflux, as the flow migrated towards the offtake. At the design flow operating point, the OGV were relatively insensitive to this until the highest bleed rate (18%) where evidence of stall was observed. At a lower operating point, the change of rotor swirl and additional OGV incidence caused earlier onset of stall and a full OGV stall was observed above 10% bleed. Increasing bleed was observed to cause a gradual increase in duct loss up to the point of OGV stall when losses increased more rapidly.</p

Aerodynamic design of a cooled cooling air system for an aero gas turbine

This paper examines the aerodynamic design requirements of a cooled cooling air system for a large, high by-pass ratio, high overall pressure ratio aero-engine. This can be broken into aerodynamic sub-systems each with their own set of requirements and challenges. A low pressure system is required to deliver air bled from the bypass duct to a heat exchanger for use as a heat sink. Similarly, a high pressure system removes a portion of the hot core engine air from a location downstream of the compressor and ducts this to-and-from the HX for cooling. This cooled air must then be returned, across the main gas path, for use in component cooling. The challenge is to design these sub-systems such that satisfactorily perform their own function whilst integrating into the existing engine architecture. This paper presents an overview of a number of studies which use CFD to explore the design space and develop appropriate designs which were subsequently experimentally validated on several isothermal test facilities. Ultimately the feasibility of designing the aerodynamic sub-systems was demonstrated and a future design strategy established