Nick Atkins - Academia.edu (original) (raw)

Papers by Nick Atkins

Research paper thumbnail of Infrared thermography techniques for boundary layer state visualisation

Experiments in fluids, Jun 1, 2024

Research paper thumbnail of The Influence of Turbine Stator Well Coolant Flow Rate and Passage Configuration on Cooling Effectiveness

Market competitiveness for aero engine power plant dictates that improvements in engine performan... more Market competitiveness for aero engine power plant dictates that improvements in engine performance and reliability are guaranteed a priori by manufacturers. The requirement to accurately predict the life of engine components makes exacting demands of the internal air system, which must provide effective cooling over the engine duty cycle with the minimum consumption of compressor section air. Tests have been conducted at the University of Sussex using a turbine test facility which comprises a two stage turbine with an individual stage pressure ratio of 1.7:1. Main annulus air is supplied by an adapted Rolls-Royce Dart compressor at up to 440 K and 4.8 kg s−1. Cooling flow rates ranging from 0.71 to 1.46 Cw, ent, a disc entrainment parameter, have been used to allow ingress or egress dominated stator well flow conditions. The mechanical design of the test section allows internal cooling geometry to be rapidly re-configured, allowing the effect of jet momentum and coolant trajectory to be investigated. An important facet to this investigation is the use of CFD to model and analyse the flow structures associated with the cavity conditions tested, as well as to inform the design of cooling path geometry. This paper reports on the effectiveness of stator well coolant flow rate and delivery configurations using experimental data and also CFD analysis to better quantify the effect of stator well flow distribution on component temperatures.

Research paper thumbnail of Winglets for Improved Aerothermal Performance of High Pressure Turbines

Journal of turbomachinery, May 2, 2014

This paper investigates the design of winglet tips for unshrouded high pressure turbine rotors co... more This paper investigates the design of winglet tips for unshrouded high pressure turbine rotors considering aerodynamic and thermal performance simultaneously. A novel parameterization method has been developed to alter the tip geometry of a rotor blade. A design survey of uncooled, flat-tipped winglets is performed using Reynolds-averaged Navier–Stokes (RANS) calculations for a single rotor at engine representative operating conditions. Compared to a plain tip, large efficiency gains can be realized by employing an overhang around the full perimeter of the blade, but the overall heat load rises significantly. By employing an overhang on only the early suction surface, significant efficiency improvements can be obtained without increasing the overall heat transfer to the blade. The flow physics are explored in detail to explain the results. For a plain tip, the leakage and passage vortices interact to create a three-dimensional impingement onto the blade suction surface, causing high heat transfer. The addition of an overhang on the early suction surface displaces the tip leakage vortex away from the blade, weakening the impingement effect and reducing the heat transfer on the blade. The winglets reduce the aerodynamic losses by unloading the tip section, reducing the leakage flow rate, turning the leakage flow in a more streamwise direction, and reducing the interaction between the leakage fluid and end wall flows. Generally, these effects are most effective close to the leading edge of the tip where the leakage flow is subsonic.

Research paper thumbnail of The Effect of Circumferential Lengthscale on Hot Gas Ingestion

This paper demonstrates that long wavelength circumferential pressure variations, an order of mag... more This paper demonstrates that long wavelength circumferential pressure variations, an order of magnitude smaller than those created by vanes and blades, are a mechanism of hot gas ingestion. Using experimental, analytical, and numerical methods, it is shown that ingestion depends on both the amplitude and wavelength of a pressure variation, and is driven by the superposition of variations with different wavelengths. A model is presented which demonstrates the sensitivity of hot gas ingestion due to these variations. The model uses the linearised Euler equations to calculate the seal velocity field, and track the movement of mainstream gas into the rotor-stator cavity. Following this understanding, full annulus numerical simulations show that the introduction of long wavelength boundary conditions allow accurate prediction of hot gas ingestion for the double-lip seal tested, using unsteady RANS simulations.

Research paper thumbnail of Heat Transfer in Turbine Hub Cavities Adjacent to the Main Gas Path

Journal of turbomachinery, Nov 8, 2012

Please read the page proofs carefully and: 1) indicate changes or corrections using e-annotation;... more Please read the page proofs carefully and: 1) indicate changes or corrections using e-annotation; 2) answer all queries; 3) proofread any tables and equations carefully; 4) check that any special characters have translated correctly. 5) upload annotated pdf file to the URL address above. Special Notes: Your Login and Password are valid for a limited time. Your prompt attention to and return of page proofs will help expedite publication of your work. Thank you for your cooperation.

Research paper thumbnail of Unsteady Effects on Transonic Turbine Blade-Tip Heat Transfer

Journal of turbomachinery, Aug 27, 2012

In a gas turbine engine the blade tips of the high-pressure turbine are exposed to high levels of... more In a gas turbine engine the blade tips of the high-pressure turbine are exposed to high levels of convective heat transfer, because of the so-called tip-leakage phenomenon. The blade-lift distribution is known to control the flow distribution in the blade–tip gap. However, the interaction between upstream nozzle guide vanes and the rotor blades produces a time-varying flow field that induces varying flow conditions around the blade and within the tip gap. Extensive measurements of the unsteady blade-tip heat transfer have been made in an engine representative transonic turbine. These include measurements along the mean camber line of the blade tip, which have revealed significant variation in both time-mean and time-varying heat flux. The influences of potential interaction and the vane trailing edge have been observed. Numerical calculations of the turbine stage using a Reynolds-averaged-Navier-Stokes-based computational fluid dynamics code have also been conducted. In combination with the experimental results, these have enabled the time-varying flow field to be probed in the blade-relative frame of reference. This has allowed a deeper analysis of the unsteady heat-transfer data, and the quantification of the impact of vane potential field and vane trailing edge interaction on the tip-region flow and heat transfer. In particular, the separate effects of time-varying flow temperature and heat-transfer coefficient have been established.

Research paper thumbnail of Flow in a Rotating Cavity With Axial Throughflow at Engine Representative Conditions

The prediction of compressor drum cavity heat transfer is an important factor in the overall desi... more The prediction of compressor drum cavity heat transfer is an important factor in the overall design of an aero engine. The rotationally dominated flow field within the cavity governs the heat transfer conditions by suppressing the motion of the fluid. Without heating, the fluid in the outer region of the cavity can approach solid body rotation. The outer cavity fluid is disturbed by the bore flow at the inner radius. The resultant bore flow vortex has been shown to exhibit many different modes of behaviour, dependent on the Rossby number. At higher Rossby number the bore flow vortex has been shown to break down into a precessing radial arm. It has also been shown that the hot drive arm (shroud) between the compressor stages destabilises the flow field through natural convection. This paper presents data from the Sussex Multiple Cavity Rig, which matches the fluid dynamic conditions of a compressor bore in terms of axial throughflow, rotational Reynolds number and Grashof number. It features titanium alloy discs, which are instrumented with surface thermocouples. This paper presents data which helps to separate the effects of throughflow Reynolds number, rotational Reynolds number and Grashof number on the dimensionless disc temperature profiles. In order to illustrate the flow structures this paper presents a hybrid RANS/LES model for the two highest Reynolds number cases. For these cases, the numerical simulations show a change from stable to unstable stratification with an increase in the bore to shroud temperature ratio in good qualitative agreement with the measured data.

Research paper thumbnail of Performance Measurements in a Transient Turbine Test Facility- Preliminary Instrumentation Development

This paper describes some of the preliminary work that has been carried out in preparation for ae... more This paper describes some of the preliminary work that has been carried out in preparation for aerodynamic performance or efficiency measurements in the Oxford Rotor Facility (ORF) (Ainsworth (1988)). The current working section features a 62% scale, 1½ stage, high-pressure shroudless transonic turbine. The turbine is driven by an isentropic light piston tunnel (ILPT). The facility operates in a transient manner with a quasisteady state run time of approximately 100ms. Engine representative specific speed, pressure ratio, gas-to-wall temperature ratio, Mach and Reynolds numbers are all simulated. This paper outlines solutions to the unique technological issues presented by performance testing in a transient facility. The three main challenges discussed are the measurement of shaft power, mass-flow rate, and total inlet enthalpy.

Research paper thumbnail of A Rim Seal Ingress Model Based on Turbulent Transport

Recent experimental work from the present authors demonstrated that interactions between the main... more Recent experimental work from the present authors demonstrated that interactions between the mainstream and cavity/rim seal flows lead to ingestion mechanisms with a range of length scales. In addition to the (well known) effect of the vane and blade pressure fields, it was demonstrated that the shear layer instabilities between the mainstream and rim seal flows can affect ingress. Building upon these observations and the understanding in the literature, this paper presents a model which relates rotor-stator cavity seal effectiveness to purge flow rate based on turbulent transport. The main assumption is that all length scales of ingress lead to an effective eddy diffusivity. This eddy diffusivity drives ingress across the seal concentration gradient. Following Prandtl’s mixing length hypothesis for eddy viscosity, the model uses an empirical constant representing an equivalent mixing length. This assumption is shown to be sufficient across a limited range of dimensionless flow rates. An extension of the model is presented to account for the reduction in turbulent mixing in the rim seal recirculation region as it becomes washed out with increasing purge flow. The rate at which the effect of the rim seal recirculation region gets washed out is modelled with a purge-to-mainstream blowing ratio term and the volume fraction of the seal occupied by the rim seal recirculation. The differences in volume fraction and blowing ratio between the different experiments in literature are defined by the geometry and flow condition only. By fitting, it is shown that the model is sufficient to capture a wide variety of experimental data in the literature and that of the present authors. The results and the model derivation provide an encouraging first step and a framework towards a model that is sensitized to both geometry and flow conditions.

Research paper thumbnail of Experimental Investigation of Turbine Stator Well Rim Seal, Re-Ingestion and Interstage Seal Flows Using Gas Concentration Techniques and Displacement Measurements

Journal of engineering for gas turbines and power, Jun 21, 2012

Gas turbine engine performance requires effective and reliable internal cooling over the duty cyc... more Gas turbine engine performance requires effective and reliable internal cooling over the duty cycle of the engine. Life predictions for rotating components subject to the main gas path temperatures are vital. This demands increased precision in the specification of the internal air system flows which provide turbine stator well cooling and sealing. This in turn requires detailed knowledge of the flow rates through rim seals and interstage labyrinth seals. Knowledge of seal movement and clearances at operating temperatures is of great importance when prescribing these flows. A test facility has been developed at the University of Sussex, incorporating a two stage turbine rated at 400 kW with an individual stage pressure ratio of 1.7:1. The mechanical design of the test facility allows internal cooling geometry to be rapidly reconfigured, while cooling flow rates of between 0.71 C W, ENT and 1.46 C W, ENT , may be set to allow ingress or egress dominated cavity flows. The main annulus and cavity conditions correspond to in cavity rotational Reynolds numbers of 1.71 Â 10 6 < Re u <1.93 Â 10 6. Displacement sensors have been used to establish hot running seal clearances over a range of stator well flow conditions, allowing realistic flow rates to be calculated. Additionally, gas seeding techniques have been developed, where stator well and main annulus flow interactions are evaluated by measuring changes in gas concentration. Experiments have been performed which allow rim seal and re-ingestion flows to be quantified. It will be shown that this work develops the measurement of stator well cooling flows and provides data suitable for the validation of improved thermo-mechanical and CFD codes, beneficial to the engine design process.

Research paper thumbnail of Boundary layer control for low Reynolds number fan rig testing

Journal of the Global Power and Propulsion Society, Feb 8, 2023

Ultra-high bypass ratio turbofans offer significant reductions in fuel and pollution due to their... more Ultra-high bypass ratio turbofans offer significant reductions in fuel and pollution due to their higher propulsive efficiency. Short intakes might lead to a stronger fan-intake interaction, which creates uncertainty in stability at offdesign conditions. Due to the prohibitive cost of full-scale experimental testing, subscale testing in wind tunnels is used to understand this behaviour. The low Reynolds number of subscale models results in unrepresentative laminar shock-boundary layer interactions. The boundary layer state thus needs to be conditioned to better represent full-scale transonic fans. This paper proposes the use of an inexpensive and robust flow control method for the suction side of a fan blade. Design guidelines are given for the location and height of the discrete roughness elements used to control the boundary layer state. This paper also presents a rapid experimental validation methodology to ensure and de-risk the application of the boundary layer trip to 3D rig blades. The experimental methodology is applied to a generic aerofoil representative of a fan tip section. The experimental method proves that it is possible to reproduce boundary layers and pressure distributions of a full-scale fan blade on a 1/10 subscale model. The results obtained confirm that the boundary layer trip method successfully promotes transition at the location representative of full-scale blades, avoiding unrepresentative laminar shock wave boundary layer interactions. This highlights the importance of conditioning boundary layers in low Reynolds number fan rig testing.

Research paper thumbnail of The Influence of Boundary Conditions on Tip Leakage Flow

Journal of turbomachinery, Jun 1, 2015

Much of the current understanding of tip leakage flow has been derived from detailed cascade stud... more Much of the current understanding of tip leakage flow has been derived from detailed cascade studies. Such experiments are inherently approximate since it is difficult to simulate the boundary conditions that are present in a real machine, particularly the secondary flows convecting from the upstream stator row and the relative motion of the casing and blade. The problem is further complicated when considering the high pressure turbine rotors of aero engines, where the high Mach numbers must also be matched in order to correctly model the aerodynamics and heat transfer of the leakage flow. More engine-representative tests can be performed on high-speed rotating turbines, but the experimental resolution achievable in such setups is limited. In order to examine the differences between cascade and engine boundary conditions, this paper presents a numerical investigation into the impact of inlet conditions and relative casing motion (RCM) on the leakage flow of a high-pressure turbine rotor. The baseline calculation uses a simplified inlet condition and no relative endwall motion, in typical cascade fashion. Only minor changes to the leakage flow are induced by introducing either a more realistic inlet condition or RCM. However, when both of these conditions are applied simultaneously, the pattern of leakage flow is significantly altered, with ingestion of flow over much of the early suction surface. The paper explores the physical processes driving the changes, the impact on performance and the implications for future experimental investigations.

Research paper thumbnail of Modeling of Compressor Drum Cavities With Radial Inflow

Reynolds-Averaged Navier-Stokes (RANS) computations have been conducted to investigate the flow a... more Reynolds-Averaged Navier-Stokes (RANS) computations have been conducted to investigate the flow and heat transfer between two co-rotating discs with an axial throughflow of cooling air and a radial bleed introduced from the shroud. The computational fluid dynamics (CFD) models have been coupled with a thermal model of the test rig, and the predicted metal temperature compared with the thermocouple data. CFD solutions are shown to vary from a buoyancy driven regime to a forced convection regime, depending on the radial inflow rate prescribed at the shroud. At a high radial inflow rate, the computations show an excellent agreement with the measured temperatures through a transient rig condition. At a low radial inflow rate, the cavity flow is destabilized by the thermal stratification. Good qualitative agreement with the measurements is shown, although a significant over-prediction of disc temperatures is observed. This is associated with under prediction of the penetration of the axial throughflow into the cavity. The mismatch could be the result of strong sensitivity to the prescribed inlet conditions, in addition to possible shortcomings in the turbulence modeling.

Research paper thumbnail of The Measurement of Shaft Power in a Fully Scaled Transient Turbine Test Facility

The Measurement of Shaft Power in a Fully Scaled Transient Turbine Test Facility. [ASME Conferenc... more The Measurement of Shaft Power in a Fully Scaled Transient Turbine Test Facility. [ASME Conference Proceedings 2005, 771 (2005)]. Nicholas R. Atkins, Roger W. Ainsworth. Abstract. Transient test facilities offer the potential ...

Research paper thumbnail of Investigation of a Radial-Inflow Bleed as a Potential for Compressor Clearance Control

The mismatch in thermal response between a High Pressure Compressor (HPC) drum and casing is a li... more The mismatch in thermal response between a High Pressure Compressor (HPC) drum and casing is a limiting factor in the reduction of compressor clearance. An experimental test rig has been used to demonstrate the concept of radial inflow to reduce the thermal time constant of HPC discs. The testing uses a simulated idle - Maximum Take Off (MTO) - idle transient in order to measure the thermal response directly. The testing is fully scaled in the dimensionless sense to engine conditions. A simple closure model based on lumped capacitance is used to illustrate the scope of potential benefits. The proof-of-concept testing shows that HPC disc time constant reductions of the order 2 are feasible with a radial-inflow bleed of only 4% of bore flow at scaled MTO conditions. Using the experimental results, the simple closure modelling suggests that for a stage with a significant mismatch in thermal response, reductions in 2D axis-symmetric clearance of as much as 50% at MTO conditions may be possible along with significant scope for improvements at cruise conditions.

Research paper thumbnail of High Efficiency Cavity Winglets for High Pressure Turbines

The flow leaking over the tip of a high pressure turbine blade generates significant aerodynamic ... more The flow leaking over the tip of a high pressure turbine blade generates significant aerodynamic losses as it mixes with the freestream flow. This paper examines the potential for reducing these losses using winglets with recessed cavities on the tip. These features combine the loss-reduction mechanisms of cavity tips, which reduce the discharge coefficient, and winglet overhangs, which reduce the mixing Mach number, leakage flow angle mismatch, and the driving pressure ratio. RANS calculations are performed for an un-cooled HP rotor blade to explore the cavity-winglet design space and examine the impact on the aerothermal performance of the blade. Relative to a plain-tip design, a cavity tip can reduce the sensitivity to clearance by around 30%. Similar performance can be achieved using a flat-tip winglet with an overhang around the whole blade perimeter. However, by adding a cavity to this winglet it is possible to out-perform the cavity tip at all clearances, and reduce the sensitivity to clearance by 46% relative to the plain tip. This sensitivity is equivalent to a two-fin attached shroud, but the winglet blade will exhibit lower stress and require less coolant flow. Furthermore this cavity-winglet may offer some cooling advantages over the cavity tip.

Research paper thumbnail of A Comparison of Single and Double Lip Rim Seal Geometries

Journal of engineering for gas turbines and power, Jul 19, 2017

The effect of purge flow, engine-like blade pressure field and mainstream flow coefficient are st... more The effect of purge flow, engine-like blade pressure field and mainstream flow coefficient are studied experimentally for a single and double lip rim seal. Compared to the single lip, the double lip seal requires less purge flow for similar levels of cavity seal effectiveness. Unlike the double lip seal, the single lip seal is sensitive to overall Reynolds number, the addition of a simulated blade pressure field and large scale non-uniform ingestion. In the case of both seals, unsteady pressure variations attributed to shear layer interaction between the mainstream and rim seal flows appear to be important for ingestion at off-design flow coefficients. The double lip seal has both a weaker vane pressure field in the rim seal cavity and a smaller difference in seal effectiveness across the lower lip than the single lip seal. As a result, the double lip seal is less sensitive in the rotor-stator cavity to changes in shear layer interaction and the effects of large scale circumferentially non-uniform ingestion. However, the reduced flow rate through the double lip seal means that the outer lip has increased sensitivity to shear layer interactions. Overall, it is shown that seal performance is driven by both the vane/blade pressure field and the gradient in seal effectiveness across the inner lip. This implies that accurate representation of both, the pressure field and the mixing due to shear layer interaction would be necessary for more reliable modelling.

Research paper thumbnail of Aerodynamic Performance Measurement in a Fully Scaled Transient Turbine Test Facility

The practical performance, both the efficiency and durability, of a High-Pressure (HP) turbine de... more The practical performance, both the efficiency and durability, of a High-Pressure (HP) turbine depends on many interrelated factors, including both the steady and unsteady aerodynamics and the heat transfer characteristics. The aerodynamic performance of new turbine designs has traditionally been tested in large scale steady flow rigs, but the testing is adiabatic, and the measurement of heat transfer is very difficult. This paper presents the results of turbine aerodynamic performance measurements at the Oxford Rotor Facility (ORF). Transient test facilities such as the Oxford Rotor allow the simultaneous study of turbine performance and heat transfer. The transient operation gives engine representative Mach number, Reynolds number and gas-to-wall temperature ratios, which are key to the aerothermodynamics of a highly-loaded, transonic, HP turbine stage. Time resolved experimental results are presented together with numerical CFD predictions over a 3% range of non-dimensional speed. The precision uncertainty of the measurements has been resolved to a level comparable with the state of the art in steady flow testing, in the region of ±0.3%.

Research paper thumbnail of The Effect of a Casing Step on the Over-Tip Aerothermodynamics of a Transonic HP Turbine Stage

The casing end-wall of a shroudless high-pressure turbine is subject to severe thermal loading. A... more The casing end-wall of a shroudless high-pressure turbine is subject to severe thermal loading. A backwards facing step positioned upstream of the leading edge of the rotor has been shown to reduce the heat flux in this region. The testing has been conducted at fully engine scaled Mach number, Reynolds number and gas-to-wall temperature ratio. Measurements of local heat-flux and static pressure were made using thin-film and semi-conductor gauges respectively. Normalised heat-flux, static pressure, adiabatic wall temperature and heat-transfer coefficient distributions are compared with those from a previously tested plain annulus baseline geometry. The introduction of the casing step has lowered the casing end-wall heat-load. This reduction is associated primarily with a decrease in adiabatic wall temperature on the casing end-wall. Analysis of the time-resolved static pressure distribution, combined with a Computational Fluid Dynamics simulation, has shown that this reduction in adiabatic wall temperature is the result of work extraction from the fluid entrained behind the casing step. The variation of the blade tip loading with upstream guide vane phase has also been reduced.

Research paper thumbnail of Comparison of Single and Double Lip Rim Seal Geometry

The effect of purge flow, engine-like blade pressure field and mainstream flow coefficient are st... more The effect of purge flow, engine-like blade pressure field and mainstream flow coefficient are studied experimentally for a single and double lip rim seal. Compared to the single lip, the double lip seal requires less purge flow for similar levels of cavity seal effectiveness. The double lip seal has both a weaker vane pressure field in the rim seal cavity and a smaller difference in seal effectiveness across the lower lip. The smaller gradient across the lower lip of the double lip seal suggests that it is less sensitive to mainstream-cavity interactions across all length scales. Unlike the double lip seal, the single lip seal is sensitive to overall Reynolds number, the addition of a simulated blade pressure field and large-scale non-uniform ingestion. In both seals, the addition of blades is seen to suppress unsteady activity attributed to shear between the rim seal and mainstream flows. The data suggests that in the case of the single lip seal, the blade pressure field has a more dominant effect in promoting ingress than the unsteadiness it suppresses at an engine-matched flow coefficient. At higher flow coefficients, increased shear between the rim seal cavity flow and the mainstream drives more mixing, reducing the seal effectiveness for both configurations.

Research paper thumbnail of Infrared thermography techniques for boundary layer state visualisation

Experiments in fluids, Jun 1, 2024

Research paper thumbnail of The Influence of Turbine Stator Well Coolant Flow Rate and Passage Configuration on Cooling Effectiveness

Market competitiveness for aero engine power plant dictates that improvements in engine performan... more Market competitiveness for aero engine power plant dictates that improvements in engine performance and reliability are guaranteed a priori by manufacturers. The requirement to accurately predict the life of engine components makes exacting demands of the internal air system, which must provide effective cooling over the engine duty cycle with the minimum consumption of compressor section air. Tests have been conducted at the University of Sussex using a turbine test facility which comprises a two stage turbine with an individual stage pressure ratio of 1.7:1. Main annulus air is supplied by an adapted Rolls-Royce Dart compressor at up to 440 K and 4.8 kg s−1. Cooling flow rates ranging from 0.71 to 1.46 Cw, ent, a disc entrainment parameter, have been used to allow ingress or egress dominated stator well flow conditions. The mechanical design of the test section allows internal cooling geometry to be rapidly re-configured, allowing the effect of jet momentum and coolant trajectory to be investigated. An important facet to this investigation is the use of CFD to model and analyse the flow structures associated with the cavity conditions tested, as well as to inform the design of cooling path geometry. This paper reports on the effectiveness of stator well coolant flow rate and delivery configurations using experimental data and also CFD analysis to better quantify the effect of stator well flow distribution on component temperatures.

Research paper thumbnail of Winglets for Improved Aerothermal Performance of High Pressure Turbines

Journal of turbomachinery, May 2, 2014

This paper investigates the design of winglet tips for unshrouded high pressure turbine rotors co... more This paper investigates the design of winglet tips for unshrouded high pressure turbine rotors considering aerodynamic and thermal performance simultaneously. A novel parameterization method has been developed to alter the tip geometry of a rotor blade. A design survey of uncooled, flat-tipped winglets is performed using Reynolds-averaged Navier–Stokes (RANS) calculations for a single rotor at engine representative operating conditions. Compared to a plain tip, large efficiency gains can be realized by employing an overhang around the full perimeter of the blade, but the overall heat load rises significantly. By employing an overhang on only the early suction surface, significant efficiency improvements can be obtained without increasing the overall heat transfer to the blade. The flow physics are explored in detail to explain the results. For a plain tip, the leakage and passage vortices interact to create a three-dimensional impingement onto the blade suction surface, causing high heat transfer. The addition of an overhang on the early suction surface displaces the tip leakage vortex away from the blade, weakening the impingement effect and reducing the heat transfer on the blade. The winglets reduce the aerodynamic losses by unloading the tip section, reducing the leakage flow rate, turning the leakage flow in a more streamwise direction, and reducing the interaction between the leakage fluid and end wall flows. Generally, these effects are most effective close to the leading edge of the tip where the leakage flow is subsonic.

Research paper thumbnail of The Effect of Circumferential Lengthscale on Hot Gas Ingestion

This paper demonstrates that long wavelength circumferential pressure variations, an order of mag... more This paper demonstrates that long wavelength circumferential pressure variations, an order of magnitude smaller than those created by vanes and blades, are a mechanism of hot gas ingestion. Using experimental, analytical, and numerical methods, it is shown that ingestion depends on both the amplitude and wavelength of a pressure variation, and is driven by the superposition of variations with different wavelengths. A model is presented which demonstrates the sensitivity of hot gas ingestion due to these variations. The model uses the linearised Euler equations to calculate the seal velocity field, and track the movement of mainstream gas into the rotor-stator cavity. Following this understanding, full annulus numerical simulations show that the introduction of long wavelength boundary conditions allow accurate prediction of hot gas ingestion for the double-lip seal tested, using unsteady RANS simulations.

Research paper thumbnail of Heat Transfer in Turbine Hub Cavities Adjacent to the Main Gas Path

Journal of turbomachinery, Nov 8, 2012

Please read the page proofs carefully and: 1) indicate changes or corrections using e-annotation;... more Please read the page proofs carefully and: 1) indicate changes or corrections using e-annotation; 2) answer all queries; 3) proofread any tables and equations carefully; 4) check that any special characters have translated correctly. 5) upload annotated pdf file to the URL address above. Special Notes: Your Login and Password are valid for a limited time. Your prompt attention to and return of page proofs will help expedite publication of your work. Thank you for your cooperation.

Research paper thumbnail of Unsteady Effects on Transonic Turbine Blade-Tip Heat Transfer

Journal of turbomachinery, Aug 27, 2012

In a gas turbine engine the blade tips of the high-pressure turbine are exposed to high levels of... more In a gas turbine engine the blade tips of the high-pressure turbine are exposed to high levels of convective heat transfer, because of the so-called tip-leakage phenomenon. The blade-lift distribution is known to control the flow distribution in the blade–tip gap. However, the interaction between upstream nozzle guide vanes and the rotor blades produces a time-varying flow field that induces varying flow conditions around the blade and within the tip gap. Extensive measurements of the unsteady blade-tip heat transfer have been made in an engine representative transonic turbine. These include measurements along the mean camber line of the blade tip, which have revealed significant variation in both time-mean and time-varying heat flux. The influences of potential interaction and the vane trailing edge have been observed. Numerical calculations of the turbine stage using a Reynolds-averaged-Navier-Stokes-based computational fluid dynamics code have also been conducted. In combination with the experimental results, these have enabled the time-varying flow field to be probed in the blade-relative frame of reference. This has allowed a deeper analysis of the unsteady heat-transfer data, and the quantification of the impact of vane potential field and vane trailing edge interaction on the tip-region flow and heat transfer. In particular, the separate effects of time-varying flow temperature and heat-transfer coefficient have been established.

Research paper thumbnail of Flow in a Rotating Cavity With Axial Throughflow at Engine Representative Conditions

The prediction of compressor drum cavity heat transfer is an important factor in the overall desi... more The prediction of compressor drum cavity heat transfer is an important factor in the overall design of an aero engine. The rotationally dominated flow field within the cavity governs the heat transfer conditions by suppressing the motion of the fluid. Without heating, the fluid in the outer region of the cavity can approach solid body rotation. The outer cavity fluid is disturbed by the bore flow at the inner radius. The resultant bore flow vortex has been shown to exhibit many different modes of behaviour, dependent on the Rossby number. At higher Rossby number the bore flow vortex has been shown to break down into a precessing radial arm. It has also been shown that the hot drive arm (shroud) between the compressor stages destabilises the flow field through natural convection. This paper presents data from the Sussex Multiple Cavity Rig, which matches the fluid dynamic conditions of a compressor bore in terms of axial throughflow, rotational Reynolds number and Grashof number. It features titanium alloy discs, which are instrumented with surface thermocouples. This paper presents data which helps to separate the effects of throughflow Reynolds number, rotational Reynolds number and Grashof number on the dimensionless disc temperature profiles. In order to illustrate the flow structures this paper presents a hybrid RANS/LES model for the two highest Reynolds number cases. For these cases, the numerical simulations show a change from stable to unstable stratification with an increase in the bore to shroud temperature ratio in good qualitative agreement with the measured data.

Research paper thumbnail of Performance Measurements in a Transient Turbine Test Facility- Preliminary Instrumentation Development

This paper describes some of the preliminary work that has been carried out in preparation for ae... more This paper describes some of the preliminary work that has been carried out in preparation for aerodynamic performance or efficiency measurements in the Oxford Rotor Facility (ORF) (Ainsworth (1988)). The current working section features a 62% scale, 1½ stage, high-pressure shroudless transonic turbine. The turbine is driven by an isentropic light piston tunnel (ILPT). The facility operates in a transient manner with a quasisteady state run time of approximately 100ms. Engine representative specific speed, pressure ratio, gas-to-wall temperature ratio, Mach and Reynolds numbers are all simulated. This paper outlines solutions to the unique technological issues presented by performance testing in a transient facility. The three main challenges discussed are the measurement of shaft power, mass-flow rate, and total inlet enthalpy.

Research paper thumbnail of A Rim Seal Ingress Model Based on Turbulent Transport

Recent experimental work from the present authors demonstrated that interactions between the main... more Recent experimental work from the present authors demonstrated that interactions between the mainstream and cavity/rim seal flows lead to ingestion mechanisms with a range of length scales. In addition to the (well known) effect of the vane and blade pressure fields, it was demonstrated that the shear layer instabilities between the mainstream and rim seal flows can affect ingress. Building upon these observations and the understanding in the literature, this paper presents a model which relates rotor-stator cavity seal effectiveness to purge flow rate based on turbulent transport. The main assumption is that all length scales of ingress lead to an effective eddy diffusivity. This eddy diffusivity drives ingress across the seal concentration gradient. Following Prandtl’s mixing length hypothesis for eddy viscosity, the model uses an empirical constant representing an equivalent mixing length. This assumption is shown to be sufficient across a limited range of dimensionless flow rates. An extension of the model is presented to account for the reduction in turbulent mixing in the rim seal recirculation region as it becomes washed out with increasing purge flow. The rate at which the effect of the rim seal recirculation region gets washed out is modelled with a purge-to-mainstream blowing ratio term and the volume fraction of the seal occupied by the rim seal recirculation. The differences in volume fraction and blowing ratio between the different experiments in literature are defined by the geometry and flow condition only. By fitting, it is shown that the model is sufficient to capture a wide variety of experimental data in the literature and that of the present authors. The results and the model derivation provide an encouraging first step and a framework towards a model that is sensitized to both geometry and flow conditions.

Research paper thumbnail of Experimental Investigation of Turbine Stator Well Rim Seal, Re-Ingestion and Interstage Seal Flows Using Gas Concentration Techniques and Displacement Measurements

Journal of engineering for gas turbines and power, Jun 21, 2012

Gas turbine engine performance requires effective and reliable internal cooling over the duty cyc... more Gas turbine engine performance requires effective and reliable internal cooling over the duty cycle of the engine. Life predictions for rotating components subject to the main gas path temperatures are vital. This demands increased precision in the specification of the internal air system flows which provide turbine stator well cooling and sealing. This in turn requires detailed knowledge of the flow rates through rim seals and interstage labyrinth seals. Knowledge of seal movement and clearances at operating temperatures is of great importance when prescribing these flows. A test facility has been developed at the University of Sussex, incorporating a two stage turbine rated at 400 kW with an individual stage pressure ratio of 1.7:1. The mechanical design of the test facility allows internal cooling geometry to be rapidly reconfigured, while cooling flow rates of between 0.71 C W, ENT and 1.46 C W, ENT , may be set to allow ingress or egress dominated cavity flows. The main annulus and cavity conditions correspond to in cavity rotational Reynolds numbers of 1.71 Â 10 6 < Re u <1.93 Â 10 6. Displacement sensors have been used to establish hot running seal clearances over a range of stator well flow conditions, allowing realistic flow rates to be calculated. Additionally, gas seeding techniques have been developed, where stator well and main annulus flow interactions are evaluated by measuring changes in gas concentration. Experiments have been performed which allow rim seal and re-ingestion flows to be quantified. It will be shown that this work develops the measurement of stator well cooling flows and provides data suitable for the validation of improved thermo-mechanical and CFD codes, beneficial to the engine design process.

Research paper thumbnail of Boundary layer control for low Reynolds number fan rig testing

Journal of the Global Power and Propulsion Society, Feb 8, 2023

Ultra-high bypass ratio turbofans offer significant reductions in fuel and pollution due to their... more Ultra-high bypass ratio turbofans offer significant reductions in fuel and pollution due to their higher propulsive efficiency. Short intakes might lead to a stronger fan-intake interaction, which creates uncertainty in stability at offdesign conditions. Due to the prohibitive cost of full-scale experimental testing, subscale testing in wind tunnels is used to understand this behaviour. The low Reynolds number of subscale models results in unrepresentative laminar shock-boundary layer interactions. The boundary layer state thus needs to be conditioned to better represent full-scale transonic fans. This paper proposes the use of an inexpensive and robust flow control method for the suction side of a fan blade. Design guidelines are given for the location and height of the discrete roughness elements used to control the boundary layer state. This paper also presents a rapid experimental validation methodology to ensure and de-risk the application of the boundary layer trip to 3D rig blades. The experimental methodology is applied to a generic aerofoil representative of a fan tip section. The experimental method proves that it is possible to reproduce boundary layers and pressure distributions of a full-scale fan blade on a 1/10 subscale model. The results obtained confirm that the boundary layer trip method successfully promotes transition at the location representative of full-scale blades, avoiding unrepresentative laminar shock wave boundary layer interactions. This highlights the importance of conditioning boundary layers in low Reynolds number fan rig testing.

Research paper thumbnail of The Influence of Boundary Conditions on Tip Leakage Flow

Journal of turbomachinery, Jun 1, 2015

Much of the current understanding of tip leakage flow has been derived from detailed cascade stud... more Much of the current understanding of tip leakage flow has been derived from detailed cascade studies. Such experiments are inherently approximate since it is difficult to simulate the boundary conditions that are present in a real machine, particularly the secondary flows convecting from the upstream stator row and the relative motion of the casing and blade. The problem is further complicated when considering the high pressure turbine rotors of aero engines, where the high Mach numbers must also be matched in order to correctly model the aerodynamics and heat transfer of the leakage flow. More engine-representative tests can be performed on high-speed rotating turbines, but the experimental resolution achievable in such setups is limited. In order to examine the differences between cascade and engine boundary conditions, this paper presents a numerical investigation into the impact of inlet conditions and relative casing motion (RCM) on the leakage flow of a high-pressure turbine rotor. The baseline calculation uses a simplified inlet condition and no relative endwall motion, in typical cascade fashion. Only minor changes to the leakage flow are induced by introducing either a more realistic inlet condition or RCM. However, when both of these conditions are applied simultaneously, the pattern of leakage flow is significantly altered, with ingestion of flow over much of the early suction surface. The paper explores the physical processes driving the changes, the impact on performance and the implications for future experimental investigations.

Research paper thumbnail of Modeling of Compressor Drum Cavities With Radial Inflow

Reynolds-Averaged Navier-Stokes (RANS) computations have been conducted to investigate the flow a... more Reynolds-Averaged Navier-Stokes (RANS) computations have been conducted to investigate the flow and heat transfer between two co-rotating discs with an axial throughflow of cooling air and a radial bleed introduced from the shroud. The computational fluid dynamics (CFD) models have been coupled with a thermal model of the test rig, and the predicted metal temperature compared with the thermocouple data. CFD solutions are shown to vary from a buoyancy driven regime to a forced convection regime, depending on the radial inflow rate prescribed at the shroud. At a high radial inflow rate, the computations show an excellent agreement with the measured temperatures through a transient rig condition. At a low radial inflow rate, the cavity flow is destabilized by the thermal stratification. Good qualitative agreement with the measurements is shown, although a significant over-prediction of disc temperatures is observed. This is associated with under prediction of the penetration of the axial throughflow into the cavity. The mismatch could be the result of strong sensitivity to the prescribed inlet conditions, in addition to possible shortcomings in the turbulence modeling.

Research paper thumbnail of The Measurement of Shaft Power in a Fully Scaled Transient Turbine Test Facility

The Measurement of Shaft Power in a Fully Scaled Transient Turbine Test Facility. [ASME Conferenc... more The Measurement of Shaft Power in a Fully Scaled Transient Turbine Test Facility. [ASME Conference Proceedings 2005, 771 (2005)]. Nicholas R. Atkins, Roger W. Ainsworth. Abstract. Transient test facilities offer the potential ...

Research paper thumbnail of Investigation of a Radial-Inflow Bleed as a Potential for Compressor Clearance Control

The mismatch in thermal response between a High Pressure Compressor (HPC) drum and casing is a li... more The mismatch in thermal response between a High Pressure Compressor (HPC) drum and casing is a limiting factor in the reduction of compressor clearance. An experimental test rig has been used to demonstrate the concept of radial inflow to reduce the thermal time constant of HPC discs. The testing uses a simulated idle - Maximum Take Off (MTO) - idle transient in order to measure the thermal response directly. The testing is fully scaled in the dimensionless sense to engine conditions. A simple closure model based on lumped capacitance is used to illustrate the scope of potential benefits. The proof-of-concept testing shows that HPC disc time constant reductions of the order 2 are feasible with a radial-inflow bleed of only 4% of bore flow at scaled MTO conditions. Using the experimental results, the simple closure modelling suggests that for a stage with a significant mismatch in thermal response, reductions in 2D axis-symmetric clearance of as much as 50% at MTO conditions may be possible along with significant scope for improvements at cruise conditions.

Research paper thumbnail of High Efficiency Cavity Winglets for High Pressure Turbines

The flow leaking over the tip of a high pressure turbine blade generates significant aerodynamic ... more The flow leaking over the tip of a high pressure turbine blade generates significant aerodynamic losses as it mixes with the freestream flow. This paper examines the potential for reducing these losses using winglets with recessed cavities on the tip. These features combine the loss-reduction mechanisms of cavity tips, which reduce the discharge coefficient, and winglet overhangs, which reduce the mixing Mach number, leakage flow angle mismatch, and the driving pressure ratio. RANS calculations are performed for an un-cooled HP rotor blade to explore the cavity-winglet design space and examine the impact on the aerothermal performance of the blade. Relative to a plain-tip design, a cavity tip can reduce the sensitivity to clearance by around 30%. Similar performance can be achieved using a flat-tip winglet with an overhang around the whole blade perimeter. However, by adding a cavity to this winglet it is possible to out-perform the cavity tip at all clearances, and reduce the sensitivity to clearance by 46% relative to the plain tip. This sensitivity is equivalent to a two-fin attached shroud, but the winglet blade will exhibit lower stress and require less coolant flow. Furthermore this cavity-winglet may offer some cooling advantages over the cavity tip.

Research paper thumbnail of A Comparison of Single and Double Lip Rim Seal Geometries

Journal of engineering for gas turbines and power, Jul 19, 2017

The effect of purge flow, engine-like blade pressure field and mainstream flow coefficient are st... more The effect of purge flow, engine-like blade pressure field and mainstream flow coefficient are studied experimentally for a single and double lip rim seal. Compared to the single lip, the double lip seal requires less purge flow for similar levels of cavity seal effectiveness. Unlike the double lip seal, the single lip seal is sensitive to overall Reynolds number, the addition of a simulated blade pressure field and large scale non-uniform ingestion. In the case of both seals, unsteady pressure variations attributed to shear layer interaction between the mainstream and rim seal flows appear to be important for ingestion at off-design flow coefficients. The double lip seal has both a weaker vane pressure field in the rim seal cavity and a smaller difference in seal effectiveness across the lower lip than the single lip seal. As a result, the double lip seal is less sensitive in the rotor-stator cavity to changes in shear layer interaction and the effects of large scale circumferentially non-uniform ingestion. However, the reduced flow rate through the double lip seal means that the outer lip has increased sensitivity to shear layer interactions. Overall, it is shown that seal performance is driven by both the vane/blade pressure field and the gradient in seal effectiveness across the inner lip. This implies that accurate representation of both, the pressure field and the mixing due to shear layer interaction would be necessary for more reliable modelling.

Research paper thumbnail of Aerodynamic Performance Measurement in a Fully Scaled Transient Turbine Test Facility

The practical performance, both the efficiency and durability, of a High-Pressure (HP) turbine de... more The practical performance, both the efficiency and durability, of a High-Pressure (HP) turbine depends on many interrelated factors, including both the steady and unsteady aerodynamics and the heat transfer characteristics. The aerodynamic performance of new turbine designs has traditionally been tested in large scale steady flow rigs, but the testing is adiabatic, and the measurement of heat transfer is very difficult. This paper presents the results of turbine aerodynamic performance measurements at the Oxford Rotor Facility (ORF). Transient test facilities such as the Oxford Rotor allow the simultaneous study of turbine performance and heat transfer. The transient operation gives engine representative Mach number, Reynolds number and gas-to-wall temperature ratios, which are key to the aerothermodynamics of a highly-loaded, transonic, HP turbine stage. Time resolved experimental results are presented together with numerical CFD predictions over a 3% range of non-dimensional speed. The precision uncertainty of the measurements has been resolved to a level comparable with the state of the art in steady flow testing, in the region of ±0.3%.

Research paper thumbnail of The Effect of a Casing Step on the Over-Tip Aerothermodynamics of a Transonic HP Turbine Stage

The casing end-wall of a shroudless high-pressure turbine is subject to severe thermal loading. A... more The casing end-wall of a shroudless high-pressure turbine is subject to severe thermal loading. A backwards facing step positioned upstream of the leading edge of the rotor has been shown to reduce the heat flux in this region. The testing has been conducted at fully engine scaled Mach number, Reynolds number and gas-to-wall temperature ratio. Measurements of local heat-flux and static pressure were made using thin-film and semi-conductor gauges respectively. Normalised heat-flux, static pressure, adiabatic wall temperature and heat-transfer coefficient distributions are compared with those from a previously tested plain annulus baseline geometry. The introduction of the casing step has lowered the casing end-wall heat-load. This reduction is associated primarily with a decrease in adiabatic wall temperature on the casing end-wall. Analysis of the time-resolved static pressure distribution, combined with a Computational Fluid Dynamics simulation, has shown that this reduction in adiabatic wall temperature is the result of work extraction from the fluid entrained behind the casing step. The variation of the blade tip loading with upstream guide vane phase has also been reduced.

Research paper thumbnail of Comparison of Single and Double Lip Rim Seal Geometry

The effect of purge flow, engine-like blade pressure field and mainstream flow coefficient are st... more The effect of purge flow, engine-like blade pressure field and mainstream flow coefficient are studied experimentally for a single and double lip rim seal. Compared to the single lip, the double lip seal requires less purge flow for similar levels of cavity seal effectiveness. The double lip seal has both a weaker vane pressure field in the rim seal cavity and a smaller difference in seal effectiveness across the lower lip. The smaller gradient across the lower lip of the double lip seal suggests that it is less sensitive to mainstream-cavity interactions across all length scales. Unlike the double lip seal, the single lip seal is sensitive to overall Reynolds number, the addition of a simulated blade pressure field and large-scale non-uniform ingestion. In both seals, the addition of blades is seen to suppress unsteady activity attributed to shear between the rim seal and mainstream flows. The data suggests that in the case of the single lip seal, the blade pressure field has a more dominant effect in promoting ingress than the unsteadiness it suppresses at an engine-matched flow coefficient. At higher flow coefficients, increased shear between the rim seal cavity flow and the mainstream drives more mixing, reducing the seal effectiveness for both configurations.