No abstract

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.

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.

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.

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.