The aerodynamic optimization design of turbine cascade nonaxisymmetric endwall and the midgap influence assessment

Abstract

The nonaxisymmetric endwall profiling has been proven to be an effective tool to reduce the secondary flow loss in turbomachinery. In the present work, first, without considering the endwall midgap in the real machine, an endwall optimization design procedure for reducing secondary flow losses has been developed, which allowed complete threedimensional parameterization turbine endwall design. The profile of the endwall has been designed using automatic numerical optimization by means of an improved efficient global optimization algorithm based on kriging surrogate model. Next, a large-scale linear cascade with a low-speed wind tunnel has been chosen for the experimental validation of the optimization results. The experimental measurements and numerical simulations both demonstrated that the total pressure loss and secondary flow intensity were reduced with the nonaxisymmetric endwall used in the cascade passage. Then, in order to evaluate the ability of the optimized nonaxisymmetric endwall with the midgap, the midgap was added in for both the baseline flat endwall and the optimized nonaxisymmetric endwall in the numerical simulations analysis. The entropy generation rates analysis were used for the investigation of loss distribution in the passage. For the cascade in the present work, with the midgap added in, the optimized nonaxisymmetric endwall did not perform as well as the situation without the midgap in the loss reduction. In addition, comparing to the baseline flat endwall, the optimized nonaxisymmetric endwall needed more net leakage flow to avoid the ingress of passage flow into the midgap.

The nonaxismmetric endwall aerodynamic optimization design for a large turbine cascade with a midgap

“…The invasion flow from the passage into the midgap is considered in the numerical simulation. In the previous work, 11 when the midgap net leakage fraction reached 1.4%, for a baseline flat endwall case, there is almost no invasion flow into the midgap from the passage. In the present study, this 1.4% net leakage flow is set in the numerical simulation as the plenum inlet condition during the optimization process.…”

“…The optimization design system is almost the same as that in the previous work. 11 The secondary loss reduction effect of the nonaxisymmetric endwall optimization design system has already been validated by the experiment measurement on a large-scale linear turbine cascade in previous work. In the present study, the geometric midgap is added, and the mesh of gap with a plenum is also generated.…”

“…Based on the turbine cascade test rig in the previous study, 11 Table 1 provides the computational fluid dynamics (CFD) predicted C pt and the measured C pt data. The CFD method captures the loss reduction in the nonaxisymmetric endwall (NEW) cascade, although the CFD overpredicts the cascade exit C pt for both based flat endwall (BFW) and NEW in the no midgap case.…”

“…This number is adequate according to the mesh independence validation in the previous work. 11 The grid is refined on the blade and endwall surface to ensure that the nondimensional wall distance of y + < 1 is satisfied close to the walls. The Reynolds number (based on C ax and V 1 ) is approximately 3.4 × 10 5 .…”

“…The present study was an extension of a previous work presented by Liu et al. 11 In that work, the midgap was added after the endwall optimization design. The results indicated that the midgap flow discounted the loss reduction ability of nonaxisymmetric endwall design.…”