Alastair Duncan Walker

Integrated Outlet Guide Vane Design for an Aggressive S-Shaped Compressor Transition Duct

Within gas turbines the ability to design shorter aggressive S-shaped ducts is advantageous from a performance and weight saving perspective. However, current design philosophies tend to treat the S-shaped duct as an isolated component, neglecting the potential advantages of integrating the design with the upstream or downstream components. In this paper, such a design concept is numerically developed in which the upstream compressor outlet guide vanes are incorporated into the first bend of the S-shaped duct. Positioning the vane row within the first bend imparts a strong radial gradient to the pressure field within the vane passage. Tangential lean and axial sweep are employed such that the vane geometry is modified to exactly match the resulting inclined static pressure field. The integrated design is experimentally assessed and compared to a conventional nonintegrated design on a fully annular low speed test facility incorporating a single stage axial compressor. Several traverse planes are used to gather five-hole probe data which allow the flow structure to be examined through the rotor, outlet guide vane and within the transition ducts. The two designs employ almost identical duct geometry, but integration of the vane row reduces the system length by 21%. Due to successful matching of the static pressure field, the upstream influence of the integrated vane row is minimal and the rotor performance is unchanged. Similarly, the flow development within both S-shaped ducts is similar such that the circumferentially averaged profiles at duct exit are almost identical, and the operation of a downstream component would be unaffected. Overall system loss remains nominally unchanged despite the inclusion of lean and sweep and a reduction in system length. Finally, the numerical design predictions show good agreement with the experimental data thereby successfully validating the design process.

Aerodynamic Performance of a Coolant Flow Off-Take Downstream of an OGV

Within the compression system of a gas turbine engine a significant amount of air is removed to fulfil various requirements associated with cooling, ventilation and sealing. Flow is usually removed through off-takes located in regions where space is restricted, whilst the flow is highly complex containing blade wakes, secondary flows and other flow features. This paper investigates the performance of a pitot style off-take aimed at providing a high pressure recovery in a relatively short length. For this to be achieved some pre-diffusion of the flow is required upstream of the off-take (i.e. by making the off-take larger than the captured streamtube). Although applicable to a variety of applications, the system is targeted at an intercooled aero-engine concept where the off-take would be located aft of the fan Outlet Guide Vane (OGV) root and provide coolant flow to the heat exchangers. Measurements and numerical predictions are initially presented for a baseline configuration with no off-take present. This enabled the OGV near field region to be characterised and provided a datum, relative to which the effects of introducing an off-take could be assessed. With the off-take present a variety of configurations were investigated including different levels of pre-diffusion, prior to the off-take, and different off-take positions. For very compact systems of short length, such that the gap between the OGV and off-take is relatively small, the amount of pre-diffusion achievable is limited by the off-take pressure field and its impact on the upstream OGV row. This pressure field is also influenced by parameters such as the non-dimensional off-take height and splitter thickness. The paper analyses the relative importance of these various effects in order to provide some preliminary design rules. For systems of increased length a significant amount of flow pre-diffusion can be achieved with little performance penalty. However, the pre-diffusion level is eventually limited by the increased distortion and pressure losses associated with the captured streamtube.

Integrated OGV Design for an Aggressive S-Shaped Compressor Transition Duct

Within gas turbines the ability to design shorter aggressive S-shaped ducts is advantageous from a performance and weight saving perspective. However, current design philosophies tend to treat the S-shaped duct as an isolated component, neglecting the potential advantages of integrating the design with the upstream or downstream components. In this paper such a design concept is numerically developed in which the upstream compressor outlet guide vanes are incorporated into the first bend of the S-shaped duct. Positioning the vane row within the first bend imparts a strong radial gradient to the pressure field within the vane passage. Tangential lean and axial sweep are employed such that the vane geometry is modified to exactly match the resulting inclined static pressure field. The integrated design is experimentally assessed and compared to a conventional non-integrated design on a fully annular low speed test facility incorporating a single stage axial compressor. Several traverse planes are used to gather five-hole probe data which allow the flow structure to be examined through the rotor, outlet guide vane and within the transition ducts. The two designs employ almost identical duct geometry, but integration of the vane row reduces the system length by 21%. Due to successful matching of the static pressure field, the upstream influence of the integrated vane row is minimal and the rotor performance is unchanged. Similarly the flow development within both S-shaped ducts is similar such that the circumferentially averaged profiles at duct exit are almost identical, and the operation of a downstream component would be unaffected. Overall system loss remains nominally unchanged despite the inclusion of lean and sweep and a reduction in system length. Finally, the numerical design predictions show good agreement with the experimental data thereby successfully validating the design process.Copyright

An Aggressive S-Shaped Compressor Transition Duct With Swirling Flow and Aerodynamic Lifting Struts

In a multistage intermediate pressure compressor an efficiency benefit may be gained by reducing reaction in the rear stages, and allowing swirl to persist at the exit. This swirl must now be removed within the transition duct that is situated between the intermediate and high pressure compressor spools, in order to present the downstream compressor with suitable inlet conditions. This paper presents the numerical design and experimental validation of an initial concept which uses a lifting strut to remove tangential momentum from the flow within an S-shaped compressor transition duct. The design methodology uses an existing strut profile with the camber line modified to remove a specified amount of the inlet tangential momentum. A linear strut loading was employed in the meridional direction with a nominally constant loading in the radial direction. This approach was applied to an existing aggressive S-duct configuration in which approximately 12.5° of swirl remains at OGV exit. 3D CFD predictions were used for preliminary assessment of duct loading and to determine how much swirl could be removed. Consequently, a fully annular test facility incorporating a 1½ stage axial compressor was used to experimentally evaluate four configurations; an unstrutted duct, a non-lifting strut and lifting struts designed to remove 50% and 75% of the inlet tangential momentum. Despite the expected large increase in loss caused by the introduction of struts there was not a significant additional loss measured with the inclusion of turning. No evidence of flow separation was observed and the data suggested that it may be possible to remove more swirl than was attempted. Although the turning struts did not remove the entire targeted swirl due to viscous deviation the data still confirm the feasibility of using a lifting strut/duct concept which has the potential to off-load the rear stages of the upstream compressor.Copyright

Aerodynamic Performance of a Coolant Flow Off-Take Downstream of an Outlet Guide Vane

Within the compression system of a gas turbine engine a significant amount of air is removed to fulfill various requirements associated with cooling, ventilation, and sealing. Flow is usually removed through off-takes located in regions where space is restricted, while the flow is highly complex containing blade wakes, secondary flows, and other flow features. This paper investigates the performance of a pitot style off-take aimed at providing a high pressure recovery in a relatively short length. For this to be achieved some prediffusion of the flow is required upstream of the off-take (i.e., by making the off-take larger than the captured streamtube). Although applicable to a variety of applications, the system is targeted at an intercooled aero-engine concept where the off-take would be located aft of the fan outlet guide vane (OGV) root and provide coolant flow to the heat exchangers. Measurements and numerical predictions are initially presented for a baseline configuration with no off-take present. This enabled the OGV near field region to be characterized and provided a datum, relative to which the effects of introducing an off-take could be assessed. With the off-take present a variety of configurations were investigated including different levels of prediffusion, prior to the off-take, and different off-take positions. For very compact systems of short length, such that the gap between the OGV and off-take is relatively small, the amount of prediffusion achievable is limited by the off-take pressure field and its impact on the upstream OGV row. This pressure field is also influenced by parameters such as the nondimensional off-take height and splitter thickness. The paper analyses the relative importance of these various effects in order to provide some preliminary design rules. For systems of increased length a significant amount of flow prediffusion can be achieved with little performance penalty. However, the prediffusion level is eventually limited by the increased distortion and pressure losses associated with the captured streamtube.

The Impact of Compressor Exit Conditions on Fuel Injector Flows

This paper examines the effect of compressor generated inlet conditions on the air flow uniformity through lean burn fuel injectors. Any resulting nonuniformity in the injector flow field can impact on local fuel air ratios and hence emissions performance. The geometry considered is typical of the lean burn systems currently being proposed for future, low emission aero engines. Initially, Reynolds-averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) predictions were used to examine the flow field development between compressor exit and the inlet to the fuel injector. This enabled the main flow field features in this region to be characterized along with identification of the various stream-tubes captured by the fuel injector passages. The predictions indicate the resulting flow fields entering the injector passages are not uniform. This is particularly evident in the annular passages furthest away from the injector centerline which pass the majority of the flow which subsequently forms the main reaction zone within the flame tube. Detailed experimental measurements were also undertaken on a fully annular facility incorporating an axial compressor and lean burn combustion system. The measurements were obtained at near atmospheric pressure/temperatures and under nonreacting conditions. Time-resolved and time-averaged data were obtained at various locations and included measurements of the flow field issuing from the various fuel injector passages. In this way any nonuniformity in these flow fields could be quantified. In conjunction with the numerical data, the sources of nonuniformities in the injector exit plane were identified. For example, a large scale bulk variation (+/−10%) of the injector flow field was attributed to the development of the flow field upstream of the injector, compared with localized variations (+/−5%) that were generated by the injector swirl vane wakes. Using this data the potential effects on fuel injector emissions performance can be assessed.

Numerical Design and Experimental Evaluation of an Aggressive S-Shaped Compressor Transition Duct With Bleed

The ability to design S-shaped ducts with high aerodynamic loading is advantageous from a performance and/or weight saving perspective. However, the radial pressure gradients required to turn the flow produce strong pressure gradients in the axial direction. This promotes the likelihood of flow separation from the inner casing as the loading is increased. The current paper presents a novel approach to accommodating the increased loading by bleeding an amount of air from the critical inner casing. The process through which the air is bled re-energizes the boundary layer sufficiently to enable it to remain attached despite the high duct loading. A bled duct is numerically developed and experimentally evaluated using a fully annular isothermal facility, with representative inlet conditions provided by a single stage axial compressor. The measurements indicate successful operation of this new design concept with a reduction in the overall system length, compared to a conventional design, of approximately 30% and a reduction in loss of approximately 20%. The data also demonstrate, to a limited degree, the ability to control the flow distribution at duct exit ultimately improving flow uniformity. Furthermore, the pressure of the bled flow is higher than at rotor exit where, in current engine architectures, flow is typically removed from the main gas path. In other words current engine bleed locations could be replaced by a bleed flow within the transition duct, and this flow is of sufficient pressure to meet the existing requirements associated with cooling, sealing and/or zone ventilation.© 2011 ASME

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.

Analysis of single hole simulated battle damage on a wing using particle image velocimetry

Particle Image Velocimetry (PIV) has been used to map the complex flow field generated by simulated battle damage to a two-dimensional wing. Previous studies have relied on surface flow visualisation techniques to study the flow but here PIV data has enabled the flow field away from the surface to be analysed for the first time. Damage was simulated by a single hole with a diameter equal to 20% of the chord, located at mid-chord. Wind tunnel tests were conducted at a Reynolds number of 500,000 over a range of incidences from 0-10 with two-component PIV measurements made on three span-wise planes; on the damage centre line and o set by 0.5 and 1.0 hole radii. The PIV data was seen to be in good agreement with existing surface flow visualisation showing strong evidence of the formation of a horse shoe vortex, a counter-rotating vortex pair and reverse flow regions. Large variations in the flow structure were observed over the range of incidences tested as the jet transitioned from weak at lower angles to strong at higher angles. The data also revealed some significant differences in the flow compared to classic Jets In Cross-Flow (JICF) behaviour. Notably in the case of battle damage the jet never fully occupies the hole and jet velocity pro le is highly skewed towards the rear of the hole. Additionally, the measured velocity ratios are much less than would be expected for typical JICF. For example, strong jet behaviour is observed at a velocity ratio as low as 0.22 whereas JICF studies would suggest a much higher ratio (> 2) is required. Increasing velocity ratio has been related to a reduction in lift and an increase in drag. At the highest incidence tested (10 ) the velocity ratio of 0.32 resulted in a reduction of the lift coe fficient by 0.18 and an increase in the drag coeffi cient of 0.035.

Experimental study of the unsteady aerodynamics the compressor-combustor interface of a lean burn combustion system

To meet the technological challenges of lean burn, low emission combustion knowledge based design methodologies must be developed. This paper presents back-to-back, time-averaged and time-resolved aerodynamic measurements made using a unique state-of-the-art fully annular isothermal test facility incorporating a 1/2 stage axial compressor and a typical lean burn, low emission combustor geometry and either a clean or a strutted OGV/pre-diffuser system. The various measurement techniques employed (miniature five-hole probe, hot-wire anemometry, PIV) show a good level of agreement highlighting both the effect of including pre-diffuser struts and the notable unsteadiness present in the OGV/pre-diffuser system. The data presented provide the first evidence of the highly unsteady nature of the flow issuing from a pre-diffuser and potentially influencing the downstream external combustor aerodynamics.