This page shows how it is possible to reduce the blade count in LP turbines by approximately 15% relative to the first generation of high lift blading employed in the BMW Rolls-Royce BR715 LPT. This is achieved through a series of experiments that lead to an understanding of the wake affected behaviour of the boundary layers on high lift profiles.Measurements were taken at a Reynolds number of 130,000.
The figure below shows a schematic diagram of the experimental rig. This consists of the cascade of seven blades and a moving bar mechanism that allows bars to be traversed upstream of the leading edge of the cascade. These simulate the effects of an upstream blade row. The bars are fitted to belts and carried on wheels that are driven by a variable speed DC motor. The speed of the bars was set to give the required flow coefficient. The bar to cascade pitch ratio was set to the same as that in an equivalent LP turbine. The traverse mechanism allows measurements to be performed within the blade passage.
Schematic diagram of moving bar cascade
A datum profile produced a pressure distribution with peak suction at about 53% surface length (s) and laminar separation at 74% surface length (s). In further experiments the position of peak suction was moved to 60, 65 and 68%s. For example, a pressure distribution resulting in peak suction at 65%s resulted in laminar separation at 80%s, a rearwards shift of 6%s compared to the datum profile. Steady flow measurements of the boundary layer at 98%s showed that sometimes the separation bubble did not reattach before the trailing edge of the blade. The open separation resulted in a large profile loss. However, when the blade was subjected to incoming wakes, it was found that the boundary layer was always attached, no matter what the loading, or position of peak suction. The next figure shows some of the pressure distributions of the aft loaded profiles as well as the datum profile.
Figure 3 shows the variation in relative suction side boundary layer loss vs. the position of peak suction for each a range of lift coefficients. These measurements are non-dimensionalised by the values for the datum profile with wake affected inflow. All measurements were made with wakes present. The figure shows an obvious trend for the losses produced by blade suction surface to reduce when the position of peak suction is moved aft. The reduction in loss can be attributed to a reduction in surface covered in turbulent boundary layer and also to the reduction in turbulent boundary layer due to the wakes. The scatter of the data for each peak suction position is due to the different loadings that each pressure distribution produced. Those with higher losses at a particular position generally generate higher lift. Some of those profiles produce exceptionally low losses but generate levels of lift below those of the datum profile. These profiles would be rejected as unsuitable in any design exercise. Profile C produced 15% more lift than the datum profile and a similar level of loss. Hot film measurements from this profile showed that wake induced transition occurred just after separation.
In low Reynolds number flows (typically 130,000), the presence of wakes on the suction side boundary layer on a highly loaded LP turbine blade has a profound effect on the losses generated. For a fixed position of peak suction with increased loading, steady flow reattachment of the boundary layer moved further from the trailing edge. This resulted in a large amount of blade surface covered in turbulent boundary layer. Aft loading the pressure distributions decreased the amount of turbulent boundary flow present and so reduced the losses generated. Aft loaded profiles are only viable when they are used with incoming wakes.
A new highly loaded LP turbine profile was designed using the data presented here. With steady inflow the profile, as expected, performed poorly. When subjected to unsteady inflow the profile loss reduced dramatically. The profile generates 15% more lift than the datum. The position of separation was in the same position for the datum and new profile. It may be possible to achieve further loss reductions if aft loading of the profile could be achieved.
Papers by Eric Curtis, Robert Howell, Howard Hodson and others and a thesis by RJ Howell is available in the download section. Papers by Roger Dambach and Howard Hodson are available in the download section. Press here to down load papers and PhD Theses.
Howard Hodson, Robert Howell, Ramesh and Rory Stieger
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