A correlation method between erosion and the acoustic properties of bladed turbomachinery by ISERP ISERP

Danuţa-Aurica GRAD et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 333 - 338

A correlation method between erosion and the acoustic properties of bladed turbomachinery

impinging particles and the breaking of material due to repeated deformation caused by particle impact to the material. Reference 2 gives equations to predict erosion trough these mechanisms, based on mass and velocity of the particles and also the angles of impact and physical proprieties of the particles and eroded material.

Experimental results of particles in gas streams are given in Ref.3 by J.H. Neilson. Neilson factors in the shape, size velocity and angles of attack in the effort to determine the erosion-angle of attack characteristic for different specimen materials. For aluminum surfaces, Shelson and Kanhere give out in Ref. 4 the erosion process equations for large particles. The mathematical model derived there is based on the indentation hardness theory. It is shown that said model is valid for relatively low velocities. -leveled equations, graphics, and tables are not prescribed, although the various table text styles are provided. The formatter will need to create these components, incorporating the applicable criteria that follow.

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Abstract— This paper studies the influence of erosion and corrosion on the acoustics of bladed turbomachinery. It is well known that eroded airfoils, such as the ones found in turbomachinery, will tend to have an altered flow pattern, more turbulent than the ideal airfoil and ideal rugosity. By studying the turbulence parameters of the flow we can determine the Sound Power Levels (SPL) of different blades – be it stators or rotors- and thus establish a reference to use in operating conditions. Air breathing turbo machineries operate in various conditions, some of which in erosive and/or corrosive environments. It is important, especially under those conditions to have a constant monitoring of the conditions of the blades during operation. While it is known that only boroscopic examinations are to be used to decide the true condition of a bladed stage, we could use the complementary method of recording the sound emerging from different isolated components of the machinery and compare it to a set of estimated SPLs, that an erosion damaged blade would have. This complementary method would be useful especially in the cogeneration industry where shutting down a power plant for inspections is necessary for boroscopy. Another use would be in the airliner industry where maintenance could have a redundant system to look over the turbomachinery while in use, and gather relevant data for future inspections.

Valeriu DRĂGAN

― POLITEHNICA‖ University Bucharest, Faculty of Aerospace Engineering Str. Gheorghe Polizu, nr. 1, sector 1, 011061 Bucharest, Romania drvaleriu@gmail.com

Danuţa-Aurica GRAD

― Politehnica‖ University of Bucharest, Faculty of Mechanical Engineering and Mechatronics Str. Independentei nr.313 sector 6,Bucharest, Building CD Bucharest, Romania danielapopescu20@gmail.com

Keywords-turbomachinery; erosion; acoustics;

INTRODUCTION

The causes of erosion are many and diverse ranging from solid particles suspended in air such as sand, dust, volcanic ash, ice and hail to liquid particles such as water droplets in air breathing engines or condensation droplets in steam turbine installations. In order to understand erosion, various mathematical models have been devised. Finnie, discusses in Ref.1 the prediction models for both ductile ad brittle materials although stating that he quantitative aspects of erosion are difficult to calculate. J.G.A. Bitter points out that the erosion process is caused by two types of wear: the direct cutting of the material by the

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Another important study is given by Hutchins, Ref 5 in which the author presents a theoretical analysis of spherical particle impinging at normal incidence-quite useful in modeling the erosion patterns of the leading edges. The study is particularly of interest since it deals with aluminum alloy specimens. Also, Ref. 6 offers a theoretical model for metal erosion by particles at normal incidence. Sundararajan and Shewmon use criterion of critical plastic strain to show that the localization model is more useful than the fatigue models described in the early literature. The matter investigated in this paper is weather or not aerodynamic parameters of the eroded airfoil will have a significant impact on the acoustics of the flow. In order to achieve this we used Computational Fluid Dynamics software to simulate flow conditions around the considered shapes: the baseline airfoil and the eroded airfoil. The viscosity model used is a standard k-epsilon.

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Danuţa-Aurica GRAD et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 333 - 338

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f= centered frequency of the 1/3 octave [Hz] (applicable for the BPF harmonics) N=rpm β=blade angle of attack at ¾ of its height The broadband noise is slightly more intuitive and it is generated by the turbulence that arises near the blades due to it’s perturbation of the flow. It too has been mathematically described by various models of which we will mention Proudman’s and Lilley’s B. The Proudman and Lilley Equations Proudman(1953) gives the following equation for the total acoustic power radiated by a volume unit that contains turbulent flow:

(5) where u=turbulent velocity ε=turbulent dissipation c=sound velocity ρ=reference density α= Proudman’s constant

V is the number of stator vanes B is the number of rotor blades n is the order of the harmonic frequency

[m]

In Ref. 7 the most obvious erosion pattern were observed at the trailing edge of the tip of the blade. This can be explained in two ways: 1.The thickness of the blade is lower near the trailing edge than near the leading edge therefore the same material loss will result in more radical airfoil changes. 2. Near the walls washed by any two-phase flow, the suspended aerosol is subject to both impingement and lift. The lift force that a suspended particle is subjected to has a fundamentally different origin than conventional lift, i.e. shear layer lift described by Saffman in 1965. It is hence to be expected that the walls to which the flow is substantially laminar and tangential will have less erosion than those nearer the trailing edge which are substantially turbulent. Bladed rotating machinery noise is generally composed by two components : the broadband and the pure tones or rotor harmonics, which peek above the baseline sound pressure level. The general prescription when designing rotary bladed stages is to adapt the stator vane number to reach an evanescence criterion of the desired harmonic frequency of the rotor, typically the second or third. Tyler and Sofrin demonstrated that the ideal number of vanes is given by Eq.1:

The stator rotor interaction is the main theme in evaluating turbomachinery noise, various formulations have been made: (2)

is the turbulent Strouhal number

BPF is the by-pass frequency, i.e. the first natural harmonic of the rotor N is the number of rotations per minute

(7) (8)

Ф=0,42

Lilley gives the equation :

The acoustic mode radiated by the stator-rotor interaction, m is given by:

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(6)

(3)

II.

SPECTRAL ANALISYS OF THE FAN NOISE

A. General Aspects There are two different aspects of the noise spectrum of any bladed machinery: -Pure tones – in fact very narrow band with-represent the harmonics of the rotor, which are the peeks in SPL. -Broad band noise, generated by the turbulent wakes at the trailing edge of the blades. For pure tones, Soderman and Mort (1983) give the following semi-empirical equation: (4)

(10) III.

THE CFD SETUP

The airfoil used for this particular investigation was a NACA 2406-43 – Fig.1a for the baseline and, by interpolation of photographies given in Ref 7, we have obtained the eroded airfoil seen in Fig.1d. Also Fig. 1b and 1c depict airfoils eroded at the leading edge with and without trailing edge damage. The SPL plots of the four considered airfoils are obtained with the Proudman formalism which was found to be more realistic than the Lilley equation which is more suited rather for nonstationary simulations. The flow parameters are presented below: X velocity=200 m/sec Y velocity=25m/sec Angle of attack=7.125 deg Temperature=293.15 K Static Pressure=101325 Pa Rugosity=0

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RESULTS AND CONCLUSIONS

Figures 3 trough 6 depict the Sound Power Level calculated with Eq.5, the common scale is given in Fig.2. It can be seen that the aerodynamic turbulent noise generated by the eroded airfoil is significantly higher than the noise generated by the baseline airfoil. This will yield an increase in the broadband noise. The most influence on noise production is that of the eroded leading edge. This aerodynamic turbulence is directly linked to the loss of aerodynamic lift to drag ratio and hence, can be correlated with the efficiency of the compressor stage. The velocity deficit, which is the aerodynamic reason for the stator-rotor interaction, is illustrated in figures 5 and 6. The flow field downstream of the eroded blade is highly distorted but with a much thicker velocity deficit area. Therefore the stator rotor interaction will have a longer time hence producing additional tones around the natural harmonics of the rotor.

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A. Conclusions The mathematical aero-acoustic formalism of Proudman has been applied in order to evaluate the Sound Pressure Levels of a baseline airfoil and an eroded airfoil, with good results on both cases. The findings were that an eroded airfoil will develop a more turbulent flow which in turns will get more erosion damage to the trailing edge of the foil. In addition to that there are significant aerodynamic downsides of the eroded profile, which has a very low lift to drag ratio and is, also prone to stalling â&#x20AC;&#x201C; which can produce the entire compressor stage to surge. Aerodynamic noise generated by the eroded airfoil is significantly higher than the baseline, therefore offering a perspective for non-invasive investigations during the turbomachinery operation, cutting maintenance costs trough the elimination of periodical boroscopic examinations. The leading edge erosion is the most influential in noise generation although, geometrically the trailing edge erosion is more spectacular. The study was carried out with the purpose to investigate if acoustic measurements would prove useful in investigating the erosion state of airfoils. However further studies could improve this method by using the FW-H equations and unsteady CFD simulations to determine the frequency spectrum of the SPL for many more damaged airfoils in order to establish a database for practical referencing.

Figure 1. The NACA 2406-43 airfoil (a), with leading edge erosion (b), with trailing edge erosion (c) with both trailing and leading edge erosion (d)

IV.

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Figure 2. The scale of the SPL plots of the airfoils considered

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Figure 3. The NACA 2406-43 airfoil SPL

Danuţa-Aurica GRAD et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 333 - 338

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Figure 4. The NACA 2406-43 with leading edge erosion SPL

Figure 5. The NACA 2406-43 with trailing edge erosion SPL

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Figure 6. The NACA 2406-43 with leading and trailing edge erosion SPL

ACKNOWLEDGMENT

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The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Romanian Ministry of Labour, Family and Social Protection through the Financial Agreements POSDRU/107/1.5/S/76909 and POSDRU/88/1.5/S/60203.

DanuĹŁa-Aurica GRAD et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 333 - 338

ISSN: 2230-7818

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Danuţa-Aurica GRAD et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 333 - 338

REFERENCES [1]

Danuţa Grad is a Ph.D student with the Department of Machine Elements and Tribology of the Mechanical Engineering Faculty of the ― Politehnica‖ University of Bucharest in co-tutelle with the Aerospace Sciences ― Elie Carafoli‖ of the Aerospace Engineering Faculty, also of the ― Politehnica‖ University of Bucharest. Valeriu Drãgan is a Ph.D. with the Aerospace Sciences ― Elie Carafoli‖ of the Aerospace Engineering Faculty, ― Politehnica‖ University of Bucharest.

[2] [3] [4] [5] [6] [7]

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[8] [9]

I. Finnie „Erosion of surfaces by solid particels‖, Wear, Volume 3, Issue 2, March-April 1960, Pages 87-103; J.G.A. Bitter, in „A study of erosion phenomena‖, Wear,Volume 6, Issue 1, January-February 1963, Pages 5-21; J.H. Neilson and A Gilchrist, „Erosion by a stream of solid particles‖, Wear, Volume 11, Issue 2, February 1968, Pages 111-122; G.L. Sheldon si A. Kanhere, „An investigation of impingement using single particles‖, Wear, Volume 21, Issue 1, August 1972, Pages 195209; I.M. Hutchings, „A model for the erosion of metals by sherical particles at normal incidence‖, Wear, Volume 70, Issue 3, 15 August 1981, Pages 269-281; G. Sundararajan si P. G. Shewmon, „A new model for the erosion of metals at normal incidence‖, Wear Volume 84, Issue 2, 15 January 1983, Pages 237-258; Gray SimpsSimpson-FOREIGN COMPARATIVE TEST PROGRAM ON ― RUSSIAN EROSION RESISTANT COATINGS FOR US NAVY GTE COMPRESSORS‖ JTEG MEETING AT LIMA ARMY TANK PLANT 24TANK 24--26 JULY 200126 2001 http://my.fit.edu/itresources/manuals/fluent6.3/help/html/ug/ Thomas Brendel; Dr. Falko Heutling; Wolfgang Eichmann; Markus Ücker; Dr. Thomas Uihlein "MTU solutions against erosive attack and loss of EGT margin in turbo engines – ERCoat nt"

AUTHORS BIOGRAPHY

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