Tip clearance

About: Tip clearance is a research topic. Over the lifetime, 2637 publications have been published within this topic receiving 32671 citations.

Gas turbine rotor tip clearance control apparatus

Abstract: A stationary shroud structure 26 surrounding a turbine rotor 22 comprises a ring 30 having a frusto conical inner surface 32 defining a small running clearance 29 with blades 24. The ring 30 is supported from a structure 42, 46 by flexible springs 38, 40 which permit axial movement of the ring from an initial position. In operation a pressure difference is formed axially across the ring 30 and a gas bearing is formed between the ring and the radial extremities of the blades 24. The pressure difference produces a first force on the ring 30 acting in a downstream direction; and the air bearing produces a second force on the ring acting in an upstream direction. Axial movement of the shroud ring from the initial position produces a third spring force on the ring acting towards the equilibrium position. The axial position of the ring 30 is determined by the resultant of the first, second and third forces and controls the running clearance. The gas bearing may be improved by the supply of air from each blade interior 58 to a cavity 52 at the blade tip. Winglets may also be provided at the tip. The ring 30 may include a heat pipe or thermosyphon to reduce thermal gradients.

Transonic Passage Turbine Blade Tip Clearance With Scalloped Shroud: Part II — Losses With and Without Scrubbing Effects in Engine Configuration

Abstract: Loss mechanisms in a scallop shrouded transonic power generation turbine blade passage at realistic engine conditions have been identified through a series of large-scale (typically 12 million finite volumes) simulations. All simulations are run with second-order discretization and viscous sublayer resolution, and they include the effects of viscous dissipation. The mesh (y+ near unity on all surfaces) is highly refined in the tip clearance region, casing recesses, and shroud region in order to fully capture complex interdependent flow physics and the associated losses. Aerodynamic losses, in order of their relative importance, are a result of the following: separation around the tip, recesses, and shroud; tip vortex creation; downstream mixing losses, localized shocks on the airfoil; and the passage vortex emanating from under the shroud. A number of helical lateral flows were established near the upper shroud surfaces as a result of lateral pressure gradients on the scalloped shroud. It was found that the tip leakage and passage losses increased approximately linearly with increasing tip clearance, both with and without the effect of the relative casing motion. For each tip clearance studied, scrubbing slightly reduced the tip leakage, but the overall production of entropy was increased by more than 50%. Also the overall passage mass flow rate, for a given inlet total pressure to exit static pressure ratio, increased almost linearly with increasing tip clearance. In addition, it was also found that there was slight positive and negative lift on the shroud, depending on the tip clearance. At the lowest tip clearance of 20 mils there was a negative lift on the shroud. In the 200-mil tip clearance case there was a positive lift on the shroud. The relative motion of the casing contributed positively to the lift at every tip clearance, affecting more at the lowest tip clearance where the casing is closest to the blade tip. Lastly, it was found that the computed entropy generation for the stationary 80-mils case using the SKE turbulence model was close to that of the 80-mils scrubbing case using the RKE turbulence model. In light of the proposed mechanisms and their relative contributions, suggested design considerations are posed.© 2004 ASME

Effect of Tip Clearance on Fan Noise and Aerodynamic Performance

Abstract: The aerodynamic performance results indicate that the fan adiabatic efficiency was highest with nominal tip clearance (about 91.8% at design speed) and decreased as fan tip clearance increased. Differences in fan efficiency between tip clearance configurations was small below 77.5% fan design speed, about 0.5% maximum, and got larger as fan speed increased, to around 1% maximum at 100% fan design speed. The decreases in efficiency are due to a lower blade loading and higher temperature rise over the outer 20% of the blade span; the mechanism is the tip leakage flow from pressure to suction surface of the fan blade, which increases as the fan tip gap increases. Farfield acoustic results show that changes in the noise level are primarily aft-radiating and are on the order of 1 to 5 dB in fan broadband noise level as measured for the rotor-alone configuration (which had no stators). There were very small changes in noise level with tip clearance for the 54-vane Baseline Outlet Guide Vane configuration. Small changes in noise were also seen for the other two Outlet Guide Vane configurations tested - the 26-vane radial Low Count Outlet Guide Vanes and the 26-vane aft swept Low Noise Outlet Guide Vanes. The rotor alone acoustic results suggest that tip clearance changes induce broadband noise changes at the fan tip, but that the noise differences are masked by the rotor wake/stator interaction noise generated with the Outlet Guide Vanes installed. The magnitude of the rotor/stator interaction tones showed small increases with increasing fan tip gap at fan speeds below transonic, indicating slightly stronger interactions with the larger tip vortex wakes that formed with larger tip gaps. The broadband levels, for the most part, only showed the effect of Outlet Guide Vane geometry on the magnitude of the noise as the tip gap changed. The Laser Doppler Velocimetry flow field results show that the flow downstream of the tip of the blades changes very little with changes in the tip clearance when operating at the approach condition. At both the cut-back and take-off speeds, significant changes in the tip flow occur with changes in the tip clearance. Since these changes in the tip flow are not *

A Microwave Blade Tip Clearance Sensor for Propulsion Health Monitoring

Abstract: Microwave sensor technology is being investigated by the NASA Glenn Research Center as a means of making non-contact structural health measurements in the hot sections of gas turbine engines. This type of sensor technology is beneficial in that it is accurate, it has the ability to operate at extremely high temperatures, and is unaffected by contaminants that are present in turbine engines. It is specifically being targeted for use in the High Pressure Turbine (HPT) and High Pressure Compressor (HPC) sections to monitor the structural health of the rotating components. It is intended to use blade tip clearance to monitor blade growth and wear and blade tip timing to monitor blade vibration and deflection. The use of microwave sensors for this application is an emerging concept. Techniques on their use and calibration needed to be developed. As a means of better understanding the issues associated with the microwave sensors, a series of experiments have been conducted to evaluate their performance for aero engine applications. This paper presents the results of these experiments.