Glen Snedden

The Application of Non-Axisymmetric Endwall Contouring in a Single Stage, Rotating Turbine

As turbine manufacturers strive to develop machines that are more efficient, one area of focus has been the control of secondary flows. To a large extent these methods have been developed through the use of computational fluid dynamics and detailed measurements in linear and annular cascades and proven in full scale engine tests. This study utilises 5-hole probe measurements in a low speed, model turbine in conjunction with computational fluid dynamics to gain a more detailed understanding of the influence of a generic endwall design on the structure of secondary flows within the rotor. This work is aimed at understanding the influence of such endwalls on the structure of secondary flows in the presence of inlet skew, unsteadiness and rotational forces. Results indicate a 0.4% improvement in rotor efficiency as a result of the application of the generic non-axisymmetric endwall contouring. CFD results indicate a clear weakening of the cross passage pressure gradient, but there are also indications that custom endwalls could further improve the gains. Evidence of the influence of endwall contouring on tip clearance flows is also presented.Copyright

Optical aberrations in a spinning pipe gas lens

If a heated pipe is rotated about its axis, a density gradient is formed which results in the pipe acting as a graded index lens. In this study we revisit the concept of a spinning pipe gas lens and for the first time analyse both the wave propagation of optical fields through the lens, and determine the optical aberrations introduced by the lens to the laser beam. We show that such lenses are highly aberrated, thus having a deleterious effect on the laser beam quality.

Characterisation of a spinning pipe gas lens using a Shack-Hartmann wavefront sensor

A heated horizontal spinning pipe causes gases inside it to assume dynamics resulting in a graded index lens - a spinning pipe gas lens (SPGL). A CFD model is presented which shows that gas exchanges of the SPGL with the surroundings resulting in a near parabolic density distribution inside the pipe created by the combination of velocity and thermal boundary layers. Fluid dynamic instabilities near the wall of the pipe are thought to have an deleterious effect on the quality of the beam and its wavefront. Measurements of the wavefront of a propagating laser beam shows strong defocus and tilt as well as higher order aberrations, thereby reducing the beam quality factor (M2) of the output beam. Results are presented as a function of pipe wall temperature and pipe rotation speed.

THE PERFORMANCE OF A GENERIC NON-AXISYMMETRIC END WALL IN A SINGLE STAGE, ROTATING TURBINE AT ON AND OFF-DESIGN CONDITIONS

The application of non-axisymmetric end walls in turbine stages has gained wide spread acceptance as a means to improve the performance of turbines in both power generation and aero-derivative applications. Non-axisymmetric end walls are aimed at the control of secondary flows and to a large extent have been developed through the use of computational fluid dynamics and detailed measurements in linear and annular cascades and proven in full scale engine tests. Little or no literature is available describing their performance at conditions other than design. This study utilises 5-hole probe measurements in a low speed, model turbine in conjunction with computational fluid dynamics to gain a more detailed understanding of the influence of a generic end wall design on the structure of secondary flows at both on and off-design flow conditions. Results indicate a 0.4% improvement in rotor efficiency at design but this was reduced at off design and at higher loading the rotor efficiency was reduced by 0.5%. Stage efficiencies were improved for all conditions but with a declining trend as load was increased. Experimental and CFD results are examined to investigate these findings further.

Optimization of Non-Axisymmetric End Wall Contours for the Rotor of a Low Speed, 1 1/2 Stage Research Turbine With Unshrouded Blades

Non-axisymmetric end wall contouring has become an established method for the reduction of the losses associated with secondary flow. To date, the majority of designs have been produced by the aeroengine manufacturing community and as a result access to specific design and methodological details is often limited [1]. In contrast, whilst the details of non-embargoed work are more freely available, much of this work has been carried out in simplified environments, with the most common of these being 2-dimensional, linear cascades, and therefore do not include a number of features which are present in the flow field of a real turbine [2]. Recent work by Snedden et al [3] involved the introduction of “generic”, non-axisymmetric end wall contours, originally designed for a linear cascade (the so-called Durham cascade), into the rotor row of a low speed, 1 1/2 stage research turbine. While an increase in rotor performance was noted, a detailed inspection of the flow results suggested that even greater improvements could be obtained through the design of custom end walls for the turbine. This investigation therefore covers the design of custom non-axisymmetric end wall contours for the rotor row of an annular turbine rig with unshrouded blades (the same rig as that used by Snedden), using a modified version of an end wall design routine originally developed for the production of non-axisymmetric end walls for a linear cascade environment.Copyright

A Computational Fluid Dynamics Model of a Spinning Pipe Gas Lens

When a metal horizontal pipe is heated and spun along its axis, a graded refractive index distribution is generated which is can be used as a lens, thus its name, the spinning pipe gas lens (SPGL). Experimental results showed that though increase in rotation speed and/or temperature resulted in a stronger lens and removed distortions due to gravity, it also increased the size of higher order aberrations resulting in an increase in the beam quality factor (M2). A computational fluid dynamics (CFD) model was prepared to simulate the aerodynamics that show how it operates and, in the process shed some light on the optical results. The results of the model consist of velocity profiles and the resultant density data and profiles. At rest the cross-sectional density profile has a vertical symmetry due to gravity but becomes rotationally symmetric with a higher value of density at the core as rotation speed increases. The longitudinal density distribution is shown to be parabolic towards the ends but is fairly uniform at the centre. The velocity profiles show that this centre is the possible source of higher order aberrations which are responsible for the deterioration of beam quality.

Optical Aberrations in Gas Lenses

Gas lenses work on the basis that aerodynamic media can be used to generate a graded refractive index distribution which can be used to focus a laser beam. An example is a spinning pipe gas lens (SPGL). It is a steel pipe whose walls are heated to a preselected temperature and then rotated along the axis to any desired speed to generate a cooler core of incoming air. A laser beam propagating through these lenses is focussed in space. However, experimental observation has shown that distortions are generated in the beam. We provide a computational fluid dynamics (CFD) model of the lens and experimental results of the Zernike aberrations measured using a Shack-Hartmann wavefront sensor which show that the aerodynamic medium in the lens have a deleterious effect on laser beam quality (M2). The effect on the SPGL is that the beam deterioration increases with rotation speed and temperature though the worst M2 measured at speed 20 Hz and temperature 155 °C was ~3.5 which is fairly good.

Preliminary Design of a Gas Turbine to Drive a South African Commercial Booster Engine

This paper focuses on the preliminary design of a two stage axial flow gas turbine operating in a liquid-propellant rocket engine for the first stage of a commercial launch vehicle (CLV). The CLV is a hypothetical vehicle capable of inserting payloads of between 50 and 500 kg into a 500 km sun synchronous orbit. The paper includes a one dimensional, reduced through-flow design and performance analysis with discussion of future two and quasi-three dimensional analyses. This work is accomplished using Axial and AxCent software by Concepts NREC. The partial admission turbine provides 3243 kW at 18850 rpm to drive kerosene and liquid oxygen pumps through a reduction gearbox.

Unsteady Effects of a Generic Non-Axisymmetric Rotor Endwall Contour on a 1½ Stage Turbine Test Rig at Off Design Conditions

With the current drive to improve fuel efficiency and reduce emissions, in gas turbine engines various methods have been investigated. Previously it has been shown that a generic rotor endwall contour could improve the efficiency of a 1½ stage test turbine at design conditions. The current investigation looked at the increased and decreased loading conditions to determine if the contour introduces detrimental effects at off design conditions.A previous unsteady analysis of the design condition found that the contoured rotor does have an effect on the flow field, reducing the magnitude of the hub endwall secondary flow region as well as reducing fluctuations in the velocity.Experimental results showed that the increased load case presented with an increase in hub endwall secondary flow structure when compared to the design case. This increase was to be expected due to the increased turning of the flow due to the increased loading operating condition. The contoured rotor had a weaker hub endwall secondary flow system, with the high momentum flow distributed more in the span wise direction. The variation in the velocity was also found to be smaller for the contoured rotor.The decreased loading case showed similar improvements, but the extent of the change was less due to the lower turning of the flow (due to a faster rotor).The numerical results show that the hub endwall secondary flow vortex of the contoured rotor was not as tightly wrapped as that of the annular rotor. The rotor outlet flow was thus more uniform due to the more dispersed vortex system. As seen with the experimental results, the extent of the change due to the contoured rotor changes with loading. The differences present in the decreased loading case being relatively insignificant.It was concluded that the generic contour does not introduce any unsteady effects at off design conditions that were not observed in the design case.Copyright

Unsteady Effects of a Generic Non-Axisymmetric Endwall Contour on the Rotor of a 1½ Stage Low Speed Turbine Test Rig

Non-axisymmetric endwall contouring has been used as means to improve the characteristics of the flow field exiting a turbine blade row reducing the secondary flows and thus also the secondary losses. The development of non-axisymmetric endwalls has predominantly been done using CFD and detailed measurements in cascades. It has been shown by several researchers that contouring can improve the performance of a gas turbine engine; however the mechanisms that create the improvement are still not fully understood. The current investigation was aimed at unsteady features, if any, and how the unsteady flow field is altered by a non-axisymmetric endwall contour. A previous steady state investigation found that the contouring improved the rotor efficiency of the current rig by 0.4%. The current investigation is an initial experimental investigation into the unsteady nature of the flow in a turbine that has endwdall contours. The unsteady nature of the rotor exit flow field was investigated using an X-film probe to determine if the contouring affected the flow field in ways that the steady measurement technique could not determine. Contour plots, variation in quantities as well as FFT’s were investigated. The unsteady data shows several differences in the flow field of the annular and contoured rotor exit. The velocity range was reduced specifically in the endwall secondary flow region, but the oscillations in the tip leakage flow region were increased. Pitch wise averaged velocity data showed a decrease in the magnitude of the FFT at the blade passing frequency, with the first and second harmonics also being affected. The velocity contours at the rotor exit reveal that the rotor outlet flow field has been made more homogenous (more aligned with the bulk flow) with the addition of the non-axisymmetric endwall contouring.Copyright