RU2531203C1 - Method of determining forms and frequencies of natural oscillation of rotor blades of gas turbine engines - Google Patents

Abstract

FIELD: motors and pumps.

SUBSTANCE: application is to determine the forms and frequencies of natural oscillation of rotor blades of gas turbine engines. The essence of invention consists in the fact that every finally produced blade (supplied to the engine) is fixed in the clamp for the shank in a horizontal position, sand is applied to its surface in a thin layer and oscillations of the blade are excited of the disturbing force applied to the free end of the blade, to the occurrence of resonant oscillations when the sand is thrown from all the oscillation places, excluding fixed lines-assemblies, which indicates the frequency coalescence of the excitation with the frequency of the natural oscillation of the blade (f_ex=f_nat) with the appropriate form of the blade oscillation fixed by the form of sand figures, the value of which (f_nat) is recorded in the action of the engine, and for the blades having perforation cooling holes on its surface the forms and frequencies of natural oscillation are determined experimentally of 15-20 blades before and after the production of the perforation holes (representative sample), the mean and standard deviations for frequencies are determined for these samples, and correction Δf is calculated, which is added to the frequency of natural oscillation of each blade (supplied to the engine) obtained prior to the manufacture of perforations on the surface of the blade, and the total value of the frequency is recorded in the action of the engine.

EFFECT: providing ability of reliable determining the form and frequencies of natural oscillation of rotor blades of gas turbine engines.

Description

The invention relates to the field of engineering, mainly to aircraft engine manufacturing, and in particular to a method for determining the forms and frequencies of natural vibrations of the working blades of gas turbine engines.

State of the art

Due to the complex design of modern working blades of aircraft gas turbine engines, the determination of their vibration modes and natural vibration frequencies (CSC) by calculation is not possible. The only accurate way to determine the shape and HSC of the blades are experimental tests in laboratory conditions at special facilities, allowing to excite the blade in a wide frequency range.

These tests are carried out for each blade mounted on the engine, and frequency values are recorded in the engine case.

Tests are widely used in factory and development organizations, in which the final manufactured blade is clamped by the shank, and a disturbing force is applied to the free end, the frequency of which varies over a wide range. Excitation of the scapula is carried out by an electrodynamic vibration exciter, widespread in engine construction, or in another way / C.V. Serensen, M.E. Garf, V.A. Kuzmenko. The dynamics of fatigue testing machines. M .: Mechanical Engineering, 1967 /.

Reporting first blade induced oscillations with a low frequency, then increasing the frequency, at some point there is a resonance phenomenon - a coincidence frequency the exciting force (f _WHO) with a natural frequency of oscillation of the blade (f _GSS) and the corresponding form variations. The recorded lowest frequency of the natural vibrations of the scapula is called the frequency of oscillations in the 1st bending shape (or in the fundamental tone), while the nodal line is located in the root zone. With further increase in the frequency of the excitation force will be observed the phenomenon of resonance (f = f _{WHO GSS)} in the oscillations of the blade to 2 minutes to form the flexural nodal line in the root and top of the blade, etc. / G.S. Zhiritsky, V.A. Strunkin. Design and strength analysis of parts, steam and gas turbines. M .: Mechanical Engineering, 1968, pp. 111-113 /.

A known method of determining the forms and frequencies of the natural vibrations of the blades (determined for each blade supplied to the engine), which includes setting the blade in the clamp in a horizontal position with a thin layer of sand poured on its surface, exciting the vibrations of the blade with a disturbing force applied to the free end of the blade , until resonance oscillations occur, when the sand is discharged from all vibrating places of the scapula and remains on fixed lines - nodes, which indicates the coincidence of the excitation frequency eniya with the natural frequency (f = f _{WHO GSS)} at an appropriate shape of the blade oscillations recorded by mean hourglass shapes whose value (f _GSS) is recorded in the case /G.S engine. Skubachevsky. Aircraft gas turbine engines. M.: Mechanical Engineering, 1969, pp. 274-275 /.

However, the existing method for determining the forms and frequencies of natural vibrations is almost impossible to apply to blades having perforated cooling holes on their surface. These types of blades, mainly turbine blades, are increasingly used in modern aircraft engine designs.

When dry sand is poured onto the surface of the blade and its excitation, small perforations are clogged, which leads to a violation of the cooling system and the inability to use the blades in operation.

Removing sand (after determining the forms and CSC) with compressed air, washing, or otherwise is an extremely time-consuming process and requires a re-determination of the flow characteristics of each blade to ensure that the cooling system is fully restored. These technological procedures do not always lead to a positive result (sand remains in the blade cooling system). In mass production, ~ 20% of blades are rejected.

SUMMARY OF THE INVENTION

The objective of the present invention is to reliably determine the shapes and frequencies of natural vibrations of the blades having perforated cooling holes on the surface of the blades, while reducing material costs by using existing laboratory facilities.

The problem is solved due to the fact that in the known method of experimental determination of the forms and frequencies of natural vibrations, the surface of the blade is sprinkled with fine sand until the perforated cooling holes are made, after which the blade is excited in a horizontal position, the shape of the vibrations is fixed by the shape of sand figures, and the frequency of its natural vibrations is determined , to which a correction for Δf is introduced.

It is known that the frequency of natural oscillations is inversely proportional to the mass of the blade

$f_{\mathrm{from}\mathrm{about}b}\sim \mathrm{one}/m\text{(one)}$

Therefore, after making holes in connection with a decrease in mass, the actual value of the frequency that is written into the engine case should be slightly larger (depending on the diameter of the holes and their number). Therefore, the obtained correction Δf must be added to the frequency of the blade without perforation.

Since the experimental determination of blade frequencies has a statistical spread, the correction value Δf must be determined in a probabilistically statistical aspect.

For this purpose, once for the same design of the blade according to a representative sample (usually 15-20 blades) of perforated blades and blades without perforation, the average values of the natural vibration frequency and standard deviations are determined, from which the maximum possible values for each sample are calculated:

- для лопаток с перфорацией, ${{\text{(f}}_{\text{sob}}\text{)}}_{\text{per}}{}^{\text{max}}{\text{= (f}}_{\text{sob}}{\text{)}}_{\text{per}}{}^{\text{Wednesdays}}{\text{+ 3σ}}_{\text{per}}\text{(2)}$

- for perforated blades,

- для лопаток без перфораций. ${{\text{(f}}_{\text{sob}}\text{)}}_{\text{without lane}}{}^{\text{max}}{\text{= (f}}_{\text{sob}}{\text{)}}_{\text{without lane}}{}^{\text{Wednesdays}}{\text{+ 3σ}}_{\text{without lane}}\text{(3)}$

- for blades without perforations.

Subtracting equation (3) from equation (2), we obtain

$\Delta f=\Delta f_{\mathrm{from}Red}+3\cdot \Delta \sigma \text{(four)}$

where Δf is the maximum possible difference between the frequency of natural vibrations of any blade of the same design with perforation and without perforation,

Δf _media - the difference in the average frequencies of the natural oscillations of the sample of blades with perforation and without perforation,

Δσ is the difference in the standard deviations of the frequencies of the natural oscillations of the sample of blades with perforation and without perforation.

It is known that the probability of a random variable falling into the 3σ interval is 99.7%; therefore, the accuracy of determining the correction is very high and the obtained correction values Δf can be extended to the entire population of manufactured blades of the same design.

Concrete example

The sample of natural vibration frequencies (CSC) for 20 turbine blades with perforated cooling holes had the following values:

- среднее значение ЧСК, ${{\text{(f}}_{\text{sob}}\text{)}}_{\text{per}}{}^{\text{Wednesdays}}=1093Gc$

- the average value of ChSK,

σ _per = 22 Hz - standard deviation.

Before the manufacture of cooling holes, these same blades had the following meanings:

- среднее значение ЧСК, ${{\text{(f}}_{\text{sob}}\text{)}}_{\text{without lane}}{}^{\text{Wednesdays}}=1082\text{Hz}$

- the average value of ChSK,

σ _{without per} = 14 Hz is the standard deviation.

Then, according to (4), the magnitude of the correction will be equal to:

Δf = 11 + 3 · 8 = 35 Hz.

Thus, in order to obtain the actual HSC value of the final manufactured perforated blade and record it in the engine case, 35 Hz must be added to each obtained HSC value of the blade without perforation.

Feasibility or other efficiency

Using the proposed method allows to exclude the rejection of blades with perforation after determining their waveforms and frequencies of natural vibrations, to test on the equipment available at the enterprises, which thereby leads to savings in material costs.

Claims (1)

The method for determining the forms and frequencies of natural vibrations (CSC) of the working blades of gas turbine engines, which consists in the fact that each final manufactured blade (supplied to the engine) is fixed in the clamp by the shank in a horizontal position, sand is applied to its surface with a thin layer and the vibrations of the blades are disturbed by the force applied to the free end of the blade, until resonance oscillations occur, when the sand is discharged from all vibrating places, except for fixed lines-nodes, which indicates ovpadenii excitation frequencies with the natural frequency of the blade vibrations (f _WHO = f _GSS) at an appropriate shape of the blade oscillations recorded by mean hourglass shapes whose value (f _GSS) and recorded in the case of the engine, characterized by the fact that for blades having on its surface perforated cooling holes, determine experimentally the shape and frequency of natural vibrations of 15-20 blades before and after the manufacture of perforation holes (representative sample), determine the average and mean squares for these samples of sul frequency deviation and calculate a correction Δf,
Δf = Δf _media + 3
Where
Δf _medium - the difference between the average values of the sampling frequencies of the blades after and before the manufacture of perforations,
Δσ is the difference between the standard deviations of the sampling frequencies of the blades after and before the manufacture of perforations,
which is added to the frequency of natural vibrations of each blade (supplied to the engine) obtained prior to the manufacture of perforations on the surface of the blade, and the total frequency value is written into the engine case.