Showing papers on "Tip clearance published in 1987"

Deterioration of Compressor Performance Due to Tip Clearance of Centrifugal Impellers

Abstract: The authors’ theory on the tip-clearance loss of centrifugal impellers is modified to include the variation of slip coefficient of the impeller due to the tip clearance, by deriving a rational relationship between two empirical parameters in the theory. In order to compare experimental data in the literature with prediction, examination was made regarding accuracy of available data and the way to select corresponding flow rates of a compressor with different values of tip clearance. Good agreement between data and prediction was observed. These examples demonstrate the following tendency regarding effects of various parameters on the tip clearance loss. Efficiency drop due to the tip clearance of high-pressure-ratio compressors is less than that of low-pressure-ratio compressors if the tip clearance ratio at the impeller exit is equal. The magnitude of clearance loss becomes smaller as the flow rate is reduced and also at a reduced shaft speed in cases of high-pressure-ratio compressors. The equations in the theory clearly show these tendencies.

Pressure Distributions in the Tip Clearance Region of an Unshrouded Axial Turbine as Affecting the Problem of Tip Burnout

Abstract: The pressure distribution in the tip clearance region of a 2D turbine cascade was examined with reference to unknown factors which cause high heat transfer rates and burnout along the edge of the pressure surface of unshrouded cooled axial turbines.Using a special micro-tapping technique, the pressure along a very narrow strip of the blade edge was found to be 2.8 times lower than the cascade outlet pressure. This low pressure, coupled with a thin boundary layer due to the intense acceleration at gap entry, are believed to cause blade burnout. The flow phenomena causing the low pressure are of very small scale and do not appear to have been previously reported.The ultra low pressure is primarily caused by the sharp flow curvature demanded of the leakage flow at gap entry. The curvature is made more severe by the apparent attachement of the flow around the corner instead of immediately separating to increase the radius demanded of the flow. The low pressures are intensified by a depression in the suction corner and by the formation of a separation bubble in the clearance gap. The bubble creates a venturi action. The suction corner depression is due to the mainstream flow moving round the leakage and secondary vortices.Copyright © 1987 by ASME

Comparisons of the relative effect of tip clearance on centrifugal impellers.

Abstract: Why do some centrifugal impellers exhibit high sensitivity to tip clearance losses while others don’t? This may be understood by considering the geometric and fluid mechanics design parameters of the impellers. On the basis of experiments on five pumps of different specific speed, the relative effect on the stage performances of the pumps due to tip clearance is studied and earlier works in the field are discussed.

Performance of two 10-lb/sec centrifugal compressors with different blade and shroud thicknesses operating over a range of Reynolds numbers

Abstract: Centrifugal compressors often cannot be directly scaled to very small flow sizes because of structural and manufacturing limitations. The inability to directly scale all design parameters leads to a performance loss other than that which can be associated with the lower Reynolds number. A 10-lb/sec centrifugal compressor was scaled down to 2-lb/sec where adjustments to blade and shroud thickness and fillet radii were required. The modified 2-lb/sec compressor was then directly scaled back up to 10 lb/sec so that the effect of the modifications could be determined. The performance of the two 10-lb/sec compressors is compared over a range of speed and mass flow. The effect of variations in Reynolds number, impeller tip clearance, and shroud thickness on compressor performance is also presented.

A Trade-Off Study of Rotor Tip Clearance Flow in a Turbine/Exhaust Diffuser System

Abstract: In a modern gas turbine power plant, the axial exhaust diffuser accounts for up to 10% of the generator power. An unshrouded rotor, due to its highly energetic tip clearance flow, improves the pressure recovery characteristic of the exhaust diffuser, while the power production within the blading suffers a loss as a result of the tip leakage flow. In this paper, these conflicting trends are thermodynamically investigated and nondimensional expressions are derived which facilitate the task of a gas turbine system designer. Conservatively, 1% thermal efficiency gain results from elimination of the last rotor tip clearance flow. The corresponding increase in thermal efficiency of a modern gas turbine power plant due to enhanced diffuser pressure recovery is less than one percent.Copyright © 1987 by ASME

Comparisons of the Relative Effect of Tip Clearance on Centrifugal Impellers

Abstract: Why do some centrifugal impellers exhibit high sensitivity to tip clearance losses while others don’t? This may be understood by considering the geometric and fluid mechanics design parameters of the impellers. On the basis of experiments on five pumps of different specific speed, the relative effect on the stage performances of the pumps due to tip clearance is studied and earlier works in the field are discussed.

Low-cost FM oscillator for capacitance type of blade tip clearance measurement system

Abstract: The frequency-modulated (FM) oscillator described is part of a blade tip clearance measurement system that meets the needs of a wide class of fans, compressors, and turbines. As a result of advancements in the technology of ultra-high-frequency operational amplifiers, the FM oscillator requires only a single low-cost integrated circuit. Its carrier frequency is 42.8 MHz when it is used with an integrated probe and connecting cable assembly consisting of a 0.81 cm diameter engine-mounted capacitance probe and a 61 cm long hermetically sealed coaxial cable. A complete circuit analysis is given, including amplifier negative resistance characteristics. An error analysis of environmentally induced effects is also derived, and an error-correcting technique is proposed. The oscillator can be calibrated in the static mode and has a negative peak frequency deviation of 400 kHz for a rotor blade thickness of 1.2 mm. High-temperature performance tests of the probe and 13 cm of the adjacent cable show good accuracy up to 600 C, the maximum permissible seal temperature. The major source of error is the residual FM oscillator noise, which produces a clearance error of + or - 10 microns at a clearance of 0.5 mm. The oscillator electronics accommodates the high rotor speeds associated with small engines, the signals from which may have frequency components as high as 1 MHz.

A numerical study of the 3D flowfield in a transonic compressor rotor with a modeling of the tip clearance flow

Abstract: Etude numerique de l'ecoulement tridimensionnel dans un rotor de compression transsonique avec une modelisation de l'ecoulement dans le jeu au sommet des aubes

The Influences of Tip-Clearance on the Performance of Nozzle Blades of Radial Turbines : Fluids Engineering

Abstract: The effective turning angle of a nozzle becomes smaller and the pressure loss becomes larger as the tip-clearance of a turbine nozzle becomes larger. The mean flow angle at the nozzle exit was measured in four independent ways, and the results agreed well with each other. In order to predict the influence of the tip clearance on the nozzle characteristics, a simple flow model was proposed based on the measured pressure distribution on the nozzle blades, and the predicted exit mean flow angle and the pressure loss or the flow coefficient were compared with the experimental data with which good agreement was demonstrated.

Moving blade for axial-flow fluid machine

Abstract: PURPOSE:To prevent leakage flow due to the presence of a moving-blade tip clearance from taking place by providing a plate-like part which is placed in a slit-shaped groove extending virtually in the direction of the chord of the blade tip so as to be able to move radially, and then providing the plate-like part in such a manner as to move outward freely by the centrifugal force CONSTITUTION:In a slit-shaped groove 7, which is provided on the blade tip of a moving blade 1 in such a manner as to be curved a little in line with its camber, is fit a plate-like part 6 so that it may be free to travel radially, and then, its movement in the direction of the chord of blade is regulated by means of the leading edge of the groove 7 In addition, on the plate-like part 6 is provided a radial hole 9, and into the hole 9 is inserted a check pin 8, which penetrates the moving blade 1 perpendicularly, so as to guide the radial movement of the plate-like part 6 And further, the centrifugal force resulted from the rotation of the moving blade 1 causes the plate-like part 6 to move outward radially so that it comes onto sliding contact with a casing 2 and thereby, leakage flow from the front side of the moving blade 1 toward the back side thereof, which was caused by the presence heretofore of a clearance between the tip of the moving blade 1 and the casing 2, can now be checked by means of the plate-like part 6 as mentioned above

System to regulate the rotor tip-clearance of a gasturbine compressor

Abstract: In this device, the rotor, in particular wheel discs of a Axialverdichterrotors are intended to be ventilated by means of removed from the compressor compressed air and thermally controlled by the compressed air via the compressor inside the channel side disposed openings (40 ', 43') removed from one or more compressor stages and between adjacent wheel and / or idler pulleys (5, 35, 36) formed annular spaces (45, 46, 45 ', 46') is supplied to the rotor.

The Relationship between Cavitation Development and the Velocity and Pressure Fluctuations of an Axial-Flow Pump

Abstract: The change in spectrums of velocity and casing wall pressure fluctuations due to cavitation was investigated, along with the change in the flow pattern through an axial-flow impeller, analysis being carried out for a range under 10 kHz. The spectrums of pressure fluctuation measured on the impeller casing depend mainly on the tip clearance cavitation development. Levels of high frequency components over 1 kHz increase remarkably as cavitation grows, and there are no discrete components in the high frequency range. The energy of pressure fluctuation is much greater than that of velocity fluctuation under the cavitating condition. Thus, cavitation noise is concluded to be brought about by pressure fluctuations.

An experimental study on the effects of tip clearance on flow field and losses in an axial flow compressor rotor

Abstract: Detailed measurement of the flow field in the tip region of a compressor rotor was carried out using a Laser Doppler Velocimeter (LDV) and a Kiel probe at two different tip clearance heights. At both clearance sizes, the relative stagnation pressure and the axial and tangential components of relative velocities were measured upstream, inside the passage and downstream of the rotor, up to about 20 percent of the blade span from the annulus wall. The velocities, outlet angles, losses, momentum thickness, and force defect thickness are compared for the two clearances. A detailed interpretation of the effect of tip clearance on the flow field is given. There are substantial differences in flow field, on momentum thickness, and performance as the clearance is varied. The losses increase linearly within the passage and their values increase in direct proportion to tip clearance height. No discernable vortex (discrete) is observed downstream of the rotor.

Effect of blade tip configuration on tip clearance loss of a centrifugal impeller.

Abstract: According to the theory presented by the authors, the tip clearance loss of an unshrouded centrifugal impeller is attributed to two parts ; one is the drag due to the leakage flow through the blade tip clearance and the other is the pressure loss to support the fluid in the thin annular clearance space between the shroud and the blade tip against the pressure gradient in the meridional plane without blades. The former is proportional to the leakage flow or the contraction coefficient α of leakage flow. The authors have conducted performance tests of an impeller with sixteen backward leaning blades in three configurations of the blade tip : round edge, sharp square edge and edge with an end plate. The experimental tip clearance effects can be predicted by the theory assuming α=0.91, 0.73 and 0.53, respectively, and if the tip clearance ratio at the impeller exit is 0.1, the impeller efficiency is improved by 1.5 percent by reducing α from 0.91 to 0.53.

Discussion: “Effects of Tip Clearance on Blade Loading in a Planar Cascade of Turbine Blades” (Sjolander, S. A., and Amrud, K. K., 1987, ASME J. Turbomach., 109, pp. 237–244)

Abstract: nent. This implies that little practical benefit would be derived from the higher blade loading induced by the leakage vortices: There would be a small increase in the torque developed by the rotor but a proportionately larger increase in the axial thrust. The latter effect is clearly connected with the tip leakage losses, representing a form of pressure drag or induced drag. In their compressor cascade, Lakshminarayana and Horlock (1967) observed a rise in the normal force coefficient of similar magnitude to the rise in resultant force coefficient observed here. However, they did not present the components of the force.