Transition Effects on Secondary Flows in a Turbine Cascade

Abstract

The regions of laminar and turbulent flow have been investigated in a linear cascade of a high tuming HP rotor blades. Measurements of intermittency close to the blade and end wall surfaces have shown substantial areas of laminar and transitional flow. The implications for turbulence modelling are important, and Navier-Stokes computations have been performed to investigate how well transition can be modelled in such a flow. Using the intermittency data to specify transitional areas, the mixing length model of turbulence produces excellent results, although there is some sensitivity to the assumed freestream length scale. High Reynolds k-ε model results show too much turbulence and loss using the measured high inlet length scale, but the results are improved with the Kato-Launder modification. A low Reynolds number model does not seem to predict the transition effects, although more work is required with this model.

The design of three-dimensional turbine blades combined with profiled endwalls

“…Approximately 380 000 cells were used in a H-grid topology. The solver uses a mixing length turbulence model and all boundary layers are defined as fully turbulent, although the cascade exhibits transitional behaviour [15].…”

Improved Turbulence Prediction in Turbomachinery Flows and the Effect on Three-Dimensional Boundary Layer Transition

“…Starting with the midspan region, the typical behavior of varying prescribed turbulence length scales is seen. A value of l T which corresponds to the experimental hotwire measurements [18] results in a higher total pressure loss prediction due to a fully turbulent suction side boundary layer because of the unphysical production of turbulent kinetic energy at the leading edge, cf. [10].…”

“…The main conclusion is that the general fully turbulent inlet boundary layer in a cascade flow (measurements by Vera et al [17] indicate an at least transitional boundary layer at the hub of their low-pressure turbine rig) separates in front of the leading edge due to the pressure gradient in combination with the horseshoe vortex. Downstream of the separation a new boundary layer forms which is laminar based on measurements by Moore and Gregory-Smith [18] and Holley et al [19] or large eddy simulations by Cui et al [20]. This new laminar boundary layer is highly three-dimensional and hence, after Schlichting [21], sensitive against instabilities because of the inflection point within the boundary layer profile.…”

“…The Durham turbine cascade is a widely known and used CFD validation test-case. The cascade has been described in detail earlier, see for instance Walsh [22], Moore [23] and Moore and Gregory-Smith [18]. The cascade consists of six blades which have a profile typical of a high pressure turbine rotor.…”

Non-axisymmetric turbine end wall profiling

“…The ow near the end walls is very complex and is strongly affected by viscous and turbulence effects. It is known that for the planar case there are signi cant regions of laminar ow on the end wall and suction surface (see reference [18]) and the pro ling may well affect these. The CFD was carried out as fully turbulent, so it is not surprising that the CFD and experiments do not agree well near the end wall and that the prediction of loss is not reliable.…”