An Innovative Passive Tip-Leakage Control Method for Axial Turbines: Linear Cascade Wind Tunnel Results

Depending on the blade aspect ratio, tip-leakage losses can contribute up to one third of the total losses in an axial turbine blade row. In unshrouded turbine blade rows, the radial gaps allow working fluid to pass from the pressure to the suction sides. This tip-leakage flow does not contribute to the work output of the turbine stage. Therefore, any technique which tends to reduce tip-leakage losses has the objective to decrease the flow through the tip gaps. A frequently used method of reducing the tip-leakage flow is the modification of the blade tip geometry by so-called squealers or winglets. Since this method decreases the sensitivity of tip-leakage losses on tip gap width, it is called tip desensitization. This paper presents a new method for tip desensitization: the passive blade tip injection. A low speed cascade wind tunnel is used for experimental investigations. Geometry of the turbine cascade is the up-scale of the tip section of a gas turbine rotor row. Three different gap widths in the range from 0.85% to 2.50% chord length are used. Total pressure, static pressure and flow angles are obtained by traversing a pneumatic five-hole probe about 0.3 axial chord lengths downstream of the turbine cascade. For investigations of the tip injection effect, a single blade of the cascade is modified by an injection channel. Based on experimental results, it is shown that the passive tip injection method decreases tip-leakage losses. At small tip gaps, this reduction can be rather significant. Finally, the positive influence of blade tip injection on tip-leakage losses is described by an analytical model based on the discharge coefficient.

Axial Turbine Tip Desensitization by Injection From a Tip Trench: Part 2 — Leakage Flow Sensitivity to Injection Location

In Part 1 of this paper it was shown that discrete jets issuing from a tip platform trench were successful in reducing the total pressure deficit due to tip leakage flow. The specific tip cooling system used in Part 1 had all four injection locations active. This paper examines the effect of the individual location of the injection hole on the tip leakage flow. The investigation was carried out in a large-scale rotating rig. Total pressure downstream of the rotor exit was measured using a Kulite sensor. The measurements were phase-locked and ensemble averaged over 200 rotor revolutions. The injection holes are located at 61%, 71%, 81%, and 91% blade axial chord, in the tip trench of a single blade with a clearance of 1.40% blade height. Individual injection at 61% and 71% chord reduced the leakage vortex size. Coolant injection at 81% chord was the most successful in reducing the total pressure deficit in the leakage vortex. Injection from 91% chord had no effect on the leakage vortex. Injection from combinations of holes had greater effect in reducing the leakage vortex size and the total pressure deficit associated with the vortex. It can be concluded that the individual jets most likely turn the leakage flow towards the trailing edge. Most of the leakage flow that is responsible for the greatest total pressure deficit occurs around 80% chord.

Axial Turbine Tip Desensitization by Injection From a Tip Trench: Part 1 — Effect of Injection Mass Flow Rate

An experimental study of a turbine tip desensitization method based on tip coolant injection was conducted in a large-scale rotating turbine rig. One of twenty-nine rotor blades was modified and instrumented to have a tip trench with discrete injection holes directed towards the pressure side. Time accurate absolute total pressure was measured 0.3 chord lengths downstream of the rotor exit plane using a fast response dynamic pressure sensor in a phase-locked manner. The test cases presented include results for tip gap heights of 1.40% and 0.72% of the blade height, and coolant injection rates of 0.41%, 0.52%, 0.63%, and 0.72% core mass flow rate. At a gap height of 1.40% the leakage vortex is large, occupying about 15% blade span. A reduction in gap height causes the leakage vortex to reduce in size and move towards the blade suction side. The minimum total pressure measured, for the reduced gap, in the leakage vortex is about 4% greater. Coolant injection from the tip trench is successful in filling in the total pressure defect originally resulting from the leakage vortex without injection. At the higher tip injection rates the leakage vortex is also seen to have moved towards the blade tip. The high momentum associated with the tip jets affects the total pressure distributions in the neighboring passages.