An innovative method to measure stress distribution of a wind turbine model by thermoelasticity

Transcription

1 An innovative method to measure stress distribution of a wind turbine model by thermoelasticity F. Castellani, M. Malerba, R. Marsili, A. Salviuolo Università degli Studi di Perugia - Dipartimento di Ingegneria Industriale Via G. Duranti, Perugia- Italy Tel , Fax: , misure@unipg.it Summary An innovative thermoelastic stress analysis method is proposed in order to study stress distribution on a wind turbine model. Thermoelastic stress analysis (TSA) allows to measure the superficial stress pattern of a mechanical component by observing temperature variations due to the application of a dynamic load on a body. To carry out thermoelastic stress analysis, an high resolution thermocamera has been used. Usually this measurement technique is applied when the observed object is fixed in respect to the thermocamera, i.e. it does not present rigid movement or large deformation under load application. In this work it s shown how it is possible to compensate the movement of the component by using thermal markers during the thermographic acquisition and choosing a fixed Cartesian coordinate system in respect to the surface analysed. Results obtained are successfully postprocessed by a self made software. This new technique has been applied to investigate stress distribution on a 1:100 scale wind turbine model tested in a subsonic closed circuit wind tunnel. 1. Introduction In the last years different numerical methods have been developed for the evaluation of stress fields since their study assumes a key role in the design of new mechanical components. To experimentally validate the theoretical results, it is possible to use different techniques, such as strain gages, the holography, the photoelasticity, thermoelastic stress analysis etc. The TSA is based on the measurement of the superficial temperature changes induced by a dynamic load, is widely used in the case of mechanical particulars not in movement. To the state of the art there are still limits in the case of objects in movement. In this work a new methodology to extend this measurement technique also to objects in rotating or translator motion is developed. The proposed method is based on the use of suitably developed algorithms able to offset the bound effect to the target movement by a post-processing of termographic images. In order to validate the proposed methodology, what typical application example, the stresses induced on the blades of an wind generator model are studied, in his various working conditions. 2. Principle of measurement The measurement principle of TSA is based on the temperature changes, due to application of cyclic loads on the component. It is known that if a gas is compressed his temperature increases; the opposite effect is when it is made expand. This phenomenon happens also in the solid bodies, but the changes of temperature which are produced are very small. In a homogeneous linear elastic and isotropic solid the volume exchange, under the hypotheses of adiabatic conditions and reversibility of the transformation, is bound to the sum of the principal stress, which can be correlated to the temperature change by the following relation (1): T α = ( σ + σ ) 1 2 T C P ρ (1)

2 where: α = Thermal expansion coefficient. T = Absolute temperature of the target. ρ = Density. C P = Thermal capacity at constant pressure. σ1, σ2 = Principal Stresses. Only since a few decades it has been possible to measure small temperature variation (in the order of mk), by using high resolution and sensitivity thermographic systems. The system used in this work is the DeltaTherm 1560, built from the Stress Photonics. The principal element of the device is the infrared thermocamera, which is able to acquire sequences of thermal images with high sampling frequency. In this way it is possible to study the time trend of the superficial temperature, what, as highlighted in the eq. (1), is directly correlated to the first invariant of stress. The thermoelastic effect is a transient phenomenon, since in the moment in which a static load is applied, this produces a field of stresses and then a temperature exchange (eq. (1)). But for the second thermodynamic principium the system temperature, increased because of the stress, aims to balance with that of the ambient. This phenomenon lets less the hypothesis of adiabatic condition under which Kelvin has introduced and shown the eq (1). The adiabatic condition hypothesis can be admitted with good approximation if dynamic loads are used with sufficient frequency, to limit the thermal exchanges between the object and the outside environment and inside the same object. The use of dynamic loads behaves the dual advantage to be able to admit the adiabatic condition hypothesis, allowing to apply the thermoelastic technique correctly. It also make available a carrying frequency to the temperature balance due to the applied dynamic load: the frequency is equal to the load this one [1]. To increase the measure signal/noise relationship, a lock-in amplification technique is used. The process of acquisition, elaboration, amplification is executed in "real time" from a suitable measurement chain, composed by the termocamera and a lock-in amplifier, interfaced with a personal computer. 3. Motion problem An example is opportune to understand the problems lied to the movement of the target in thermoelastic stress analysis: We suppose to make a measure, by the thermoelastic analysis stress (TSA), on an object with a hole, subject to an alternate load that does not produce excessive deformations. The CCD sensor of the thermocamera divides the area of sight in elementary zones whose dimension depends on the type of used optics and on the distance between thermocamera and target. This divisions is shown by the grill on the sample in the following figure (fig.1a). Every elementary area issues radiant energy which is directed by optics on a single pixel of the IR sensor. This does that each pixel of the sensor is dedicated to acquire information from one determinate elementary area. During the cycle of load every single pixel allows to acquire the time trend of the temperature ( fig 1 b ); by its elaboration it is possible measure the middle tension in the elementary area which the pixel is dedicated.

3 Figure 1: the area of sight, divided in elementary zones; temperature time history and typical output thermographic map Image on the right of figure 1 show a typical obtainable result with this technique and provides an immediate visualization of more stressed zones. Let s go to consider the same test made with arbitrary rigid movement and without load and supposing that they there are a difference between the radiant energy issued by the surface of the sample and the background. In figure 2 the relative position of the sample respect to the fixed grill is shown. Figure 2: relative displacement between termocamera and target In this situation, every taken measured temperature change is not due to the application of a load, since it assents, but to the bound effects to the movement. 4. Methods of movement compensation The system for thermoelastic analysis has a tool that allows the movement compensation. This can be used and efficient just when the movement presents good characteristics of repetitiveness and it is due just to the fixed solicitation. Another method proposed for the compensation of the movement is the so-called DeRotator [3], but it allows to make thermoelastic analysis on objects under a pure rotation and not a translation. Unfortunately none of the methods reported above can be applied in case of a generic movement of the target. Against this need we have realised a software able to follow the image, choosing a reference system that is integral with the object in movement. The realized software is based on "pattern recognition" algorithms for the post processing of the data. First of all a thermal film has been recorded, and then analyzed. Of each thermographic image of the moving object we calculate the moving vector and the rotation matrix. In this testing situation we will consider only movements on the optical ais of the thermal camera, all this allows the reduction of the displacement to a vector of dimension 2 ( x, y) and the rotation matrix to a simple graduated θ. To know these parameters gives us the possibility to create a data flux very similar to thsat exported from the thermographic system, where the reference system of the thermal camera has been changed from absolute to relative, solid to the moving object.

4 The pattern recognition algorithms exploit the skill to recognise shapes inside the image, when this last one has enough contrast. In the event of thermographic images it is difficult to achieve this condition. The proposed solution in this activity is to insert inside the measurement volume some thermal markers. They are disposals, created to present a high emission, linked solidly to the moving object. In this way the recognition algorithm has to follow just the markers that, as solid to the moving object, allow to follow the object itself. 5. Software validation From an experimental test we have used a cooling fan as the ones used in cars (object of the test) made up of 5 twisted nylon 66-blades, blocked between two semi-monocoque in light diecasted metal fixed with five anti-unscrewed bolts (Fig 3.). The fan has been blocked on an electro-dynamic shaker (RMS, SW1002 model which can produce sinusoidal forces with a wideness up to 600 N) through two bearings. The blocking system used allows the rotation of the fan around its own axis. light Figure 3: automotive cooling fan linked to the shaker and thermal marker Five thermal markers, made up of bulb lights scoured by electricity and blocked on the hub of the shovel have been positioned. The system has been excited at the first frequency of the resonance, and rotated at a random speed. In these operating conditions we have got a sequence of 2048 thermographic images at a sample frequency of 100 Hz. The movement compensation software has elaborated the output data flow: here following some images showing the object, nevertheless its rotation, it seems to be fixed in the output data. Figure 4: typical sequence of output flow images of the compensation software: the target is fixed inside the realised domain.

5 The acquired data have been analysed through a filtering and lock-in amplifying algorithm, that can be compared with that the thermoelastic system realises real-time. In this case, not having a reference signal, the analysis of the spectral lines inside the loading frequency has been carried out. The tension can be calculated both on the entire and restricted area (Fig 5). Figure 5: typical result obtained working on the thermographic films using the technique of filtering and lock-in amplifying on a wide area on the left and on a restricted one on the right Figure 6 shows the absolute congruence of the result calculated through the software compensation with that obtained from the Delta Therm system with fix object. Figure 6: comparison between the obtained result after software compensation (on the left) and obtained result at fixed object (on the right) 6. Application of the technique to the model of the wind generator As an example of the application of the methodology measurements have been carried out on a 1:100 model of a wind generator put in the wind tunnel.

6 Figure 7: 1:100 model of the wind generator In order to simulate heavy exercise conditions we put a cylinder on the wind generator. In this way in the air flux, once met the obstacle, the well-known Von Karman vortex are created, creating themselves a significant cyclic dynamic load on the blades of the wind generator. Figure 8: positioning of the wind generator in the wind tunnel On the blades of the wind generator three thermal markers and strain gages are applied in order to monitor the deformation on the blade with a consolidate measurement methodology used in this case as reference (fig. 10). Figure 9: The stain gage and the thermal marker applied in the blade The following figure shows a typical trend of the deformations measured by an strain gage put on the base of the blade. You can see a periodic trend of the deformations, at the same frequency of application such as the Von Karman vortex.

7 Figure 10: deformation blade time history We have carried out measurements at the variation of the flux speed in the wind tunnel in a range between 0,5 and 33 m/s The sequences of the thermographic films have been elaborated with the developed software. The typical results are shown in the following images where you can see how the rotation effect of the blade has been totally compensated. The analysis of the thermoelastic map of the moving blade, stressed by the Von Karman vortex at the frequency of 15,3Hz has underlined concentrations of growing tensions at the increasing distance from the hub, as we have underlined in the preliminary analysis by strain gages. Figure 11: thermoelastic map of the rotating blade stressed at 15,3 Hz Even if the highest moment is in the dap, the highest concentration of tension is the area that are the furthest from the rotating centre as in some points, because of the particular shape of the blade, the polar moment of inertia is lower.

8 7. Conclusions In this work we have developed a new methodology based on the thermoelastic principle, for the measurement of the dynamic stress pattern of the wind turbine without any contact. To compensate the effects connected to the movement of the target we have realised a software to process thermographic films able to recognise and compensate the movement, thanks to thermal markers. Preliminary experimental proofs have been carried out in the wind tunnel on the example of a wind generator. The first obtained results show, at a quality level, the validity of the proposed technique. Further analyses are to be done in order to verify the metrological limits of the proposed method. It opens up new interesting possibilities in order to apply this technique for periodic diagnostic controls of the wind turbine in operating conditions. A special thank to General Electric for the indications given about the characteristics of the wind generators. References [1] N. Harwood, W. M. Cummings, Thermoelastic Stress Analysis, IOP Publishing Ltd, Bristol, 1991 [2] User Manual of Stress Photonics, Stress Photonics Inc., Madison Wisconsin, USA [3] G. Brustenga, R. Marsili, M. Moretti, J. Pirisinu, G.L. Rossi, Measurement on Rotating Mechanical Component by Thermoelasticity, Applied Mechanics and Materials Vols. 3-4 (August 2005) pp , 2005 Trans Tech Publications, Switzerland [4] Handbook of Pattern Recognition and ComputerVision, C.H. Chen & P.S.P. Wang edition [5] J. Bigun, Vision with Direction, Ed. Springer