Tip Leakage Control Research Articles

Control of squealer-tip leakage flow with perforated-rib coolant injection in an axial turbine cascade

A novel perforated-rib configuration is proposed for controlling the tip leakage flow at the rotor tip of an axial turbine. Three perforated-rib layouts are considered, wherein a perforated rib is installed at (A) the Suction-Side squealer (SS-rib), (B) the Pressure-Side squealer (PS-rib), and (C) the additional squealer along the blade Camber Line (CL-rib). A numerical method is used to show how the novel rib layouts affect the aerodynamic performance of the tip leakage flow. Results show that the coolant jets issuing from the perforated-rib injection holes penetrate deeper into the tip clearance than those in the baseline squealer-tip case, and how the perforated-rib coolant injection affects the tip leakage flow depends strongly on the rib layout. The PS-rib and CL-rib layouts appear promising for controlling the tip leakage flow, playing a significant role in reducing the total pressure loss and improving the turbine blade’s isentropic efficiency. In particular, under an injection mass flow ratio of 1% and a tip clearance of 1% blade span, the PS-rib layout reduces the leakage mass flow rate by 27% and increases the isentropic efficiency by 1.25% compared with those in the baseline squealer-tip case. Meanwhile, the advantages of the PS-rib layout in tip leakage control are confirmed under small and large tip clearances.

Kriging surrogate model applied in the mechanism study of tip leakage flow control in turbine cascade by multiple DBD plasma actuators

The dielectric barrier discharge (DBD) plasma actuator, in which electrodes are asymmetric arranged, has already demonstrated its ability in flow control. In the present work, the configuration of multiple plasma actuators is placed at the suction side of the cascade top to realize the tip leakage control. However, massive configurations appear when the number of plasma actuators increases, resulting in the investigation of actuator configuration for tip leakage flow control becomes a challenge. The surrogate modeling approach provides a cheap and efficient method for investigating the effect of multiple plasma actuators on the tip leakage flow control. By constructing an approximation model, tip leakage mass flow rates of all configurations are obtained in the present work. What's more, the flow structures in the tip clearance controlled by the plasma actuators are explained in the process of topological analysis. The results show that the tip leakage mass flow rate is decreasing with the number of active plasma actuators increasing. However, the decreasing would reach its limits in the process of adding plasma actuators. In the analysis of flow topology, single actuator would generate a small vortex at the suction side to cause an obstacle in the tip leakage flow. While the continuous arrangements of plasma actuator is beneficial to generate a larger obstacle in the tip clearance, which resulting in a significant reduction of m˙CFD and the promotion of Cp.

Control of Tip Leakage in a High-Pressure Turbine Cascade Using Tip Blowing

Blowing from the tip of a turbine blade was studied experimentally to determine if total pressure loss could be reduced. Experiments were done with a linear cascade in a low-speed wind tunnel. Total pressure drop through the blade row and secondary velocity fields in the passage between two blades were measured. Cases were documented with various blowing hole configurations on flat and squealer tipped blades. Blowing normal to the tip was not helpful and in some cases increased losses. Blowing from the bottom of a squealer cavity provided little benefit. With a flat tip, blowing from holes located near and inclined toward the pressure side generally reduced total pressure drop by reducing the effect of the tip leakage vortex. Holes near the axial location of maximum loading were most helpful, while holes closer to the leading and trailing edges were not as effective. Higher jet velocity resulted in larger total pressure drop reduction. With a tip gap of 1.5% of axial chord, jets with a velocity 1.5 times the cascade inlet velocity had a significant effect. A total pressure drop reduction of the order 20% was possible using a jet mass flow of about 0.4% of the main flow. Jets were most effective with smaller tip gaps, as they were more able to counter the leakage flow.

Improvement in the stability of a turbocharger centrifugal compressor by tip leakage control

The occurrence of surge or stall in a centrifugal compressor and the role of the tip clearance flow in the instability in the centrifugal compressor are investigated in this study. A computational method is used to study the flow field in the centrifugal compressor in order to gain a better understanding of the surge or stall mechanism. It is found that, near surge or stall conditions, the tip leakage flow at the leading edge deflects more upstream; as the deflection increases, a more severe spillage occurs which finally leads to instability of the compressor. A ring air jet injection is used to eliminate the instabilities and to extend the stable flow range of the compressor. Using an air jet injection, the stable flow range of the compressor was successfully increased with minimal decrease in the efficiency of the compressor. The effects of different injection parameters such as the mass flow, the yaw angle, the injection angle, the slot width and the slot distance on the compressor flow field are studied, and an optimum design for the air jet injection is developed. Further investigation of the compressor with the optimum injection configuration shows that, near surge or stall conditions, the tip leakage at the leading edge is still under control, manifesting a much smaller spillage than does the Dresser–Rand Datum compressor without an air injection. The dominant factor for the instability of the compressor with an injection is found to be the leading-edge separation rather than the tip leakage.

Casing Convective Heat Transfer Coefficient and Reference Freestream Temperature Determination Near an Axial Flow Turbine Rotor

The present study explains a steady-state method of measuring convective heat transfer coefficient on the casing of an axial flow turbine. The goal is to develop an accurate steady-state heat transfer method for the comparison of various casing surface and tip designs used for turbine performance improvements. The freestream reference temperature, especially in the tip gap region of the casing, varies monotonically from the rotor inlet to rotor exit due to work extraction in the stage. In a heat transfer problem of this nature, the definition of the freestream temperature is not as straightforward as constant freestream temperature type problems. The accurate determination of the convective heat transfer coefficient depends on the magnitude of the local freestream reference temperature varying in axial direction, from the rotor inlet to exit. The current study explains a strategy for the simultaneous determination of the steady-state heat transfer coefficient and freestream reference temperature on the smooth casing of a single stage rotating turbine facility. The heat transfer approach is also applicable to casing surfaces that have surface treatments for tip leakage control. The overall uncertainty of the method developed is between 5% and 8% of the convective heat transfer coefficient.

An innovative passive tip-leakage control method for axial turbines: Basic concept and performance potential

The present paper introduces a new concept for passive turbine tip-leakage control. The basic idea of the method is the connection of the blade leading edge and the blade tip by an internal channel. Due to the difference between the stagnation pressure at the leading edge and the low pressure at the blade tip, a small amount of the working fluid is extracted from the blade passage. At the blade tip, a jet is injected roughly perpendicular to the tip gap flow driven by the blade pressure difference. It is proposed that the jet blocks at least a part of the tip gap flow. Since the tip-leakage losses are proportional to the tip gap mass flow rate, the tip injection results in a reduction of the associated losses. After the introduction of the concept, an analytical model is presented which describes the reduction of the tip gap discharge coefficient due to the tip injection. Furthermore, the analytical model is supported by the results of a preliminary CFD analysis. Finally, the potential of the efficiency improvement by the passive blade tip injection method is reported.

Rotor-Tip Leakage: Part II—Design Optimization Through Viscous Analysis and Experiment

Blade tip losses represent a major efficiency penalty in a turbine rotor. These losses are presently controlled by maintaining close tolerances on tip clearances. This two-part paper outlines a new methodology for predicting and minimizing tip flows, and focuses on the control of tip leakage through minimization of the discharge coefficient to control the normal leakage flow component. Minimization of the discharge coefficient was achieved through viscous analysis and was supported by discharge-rig testing. The analysis for the discharge cross-flow used a stream function-vorticity formulation. Support testing was conducted with a newly developed water table discharge rig in which tip-coolant discharge could also be simulated. Experimental and numerical tip-leakage results are presented on a discharge coefficient parameter for five different tip configurations. In addition, numerical studies were conducted for stationary and rotating blades with and without tip coolant injection.