MODAL ANALYSIS OF A SMALL WIND TURBINE WITH WIND CONCENTRATOR

Transcription

1 SISOM & ACOUSTICS 2016, Bucharest May MODAL ANALYSIS OF A SMALL WIND TURBINE WITH WIND CONCENTRATOR The University Politehnica of Bucharest, consni@yahoo.com The small wind turbines have to prove their efficiency in the lower atmospheric boundary layer conditions, generally governed by low wind velocity and quite rapidly changing direction of the incident air flow. The classic response consists in high solidity turbine runners, but recently wind concentrators, able to allow the wind harvesting from extended cross surfaces, have been considered. The paper presents a study dedicated to the modal analysis of a wind turbine runner adopting this design. This study follows a previous one, which considered an early design, involving only a ducted six blade runner. Keywords: small wind turbine, tubed solid runner, foam filled blade. 1. INTRODUCTION Small wind turbines have to run, in most cases, in areas with relatively low wind velocity. The best solution for getting high conversion coefficients for Horizontal Axis Wind Turbines (HAWT) is to use high solidity runners. Additional solutions had to be found to further raise the wind energy conversion factor, most of them moving around the ducted runner configuration, already proposed by the authors and described in [1]. Such solutions span from the Diffuser Augmented Wind Turbine (DAWT), first proposed in [2] and further developed until recently [3], to the wind lens technology proposed in the last years [4]. These types of runners were studied so far from the aerodynamic point of view, with very optimistic forecasted performances [4], but the structural analysis was completely absent in the literature, in spite of potential problems which might occur with the intended design solutions. This paper presents an extension of the study previously made on the ducted design firstly considered by the authors for the HAWT in view. The initial design was modified in order to concentrate the airflow moving through the runner and thus to increase the efficiency of the installation. Two variants have been in view for the modified versions (Figure 1): variant A, with conical inlet shell, and variant B, with a curved inlet shell. The first variant meant a 400 mm increase of the outer diameter of the runner, and the second, a 500 mm increase. Taking always into account the quite large envelope of atmospheric conditions, including occasionally high wind or gusts, a comprehensive evaluation of the static and dynamic behaviour of this structure is required. In the paper, a modal analysis is presented and compared with that found for the first design solution considered for the solid runner, in order to have a good background for choosing the most adequate final solution. 2. ANALYSIS AND RESULTS The numerical models have been created with the ANSYS 15.0 code, based on the initial design obtained with CATIA V5. In this study, the whole structure of the runner was considered, in order to evaluate the combined effect of the overall structure architecture and of the structural masses positioned in the right places. The outer cylinder initially ducting the runner was considered in the thin sandwich type variant [1], as the inlet shell and an additional intermediate layer were contributing with structural elements,

2 8 meant to keep the structural strength and stiffness at appropriate levels. The configuration of the radial beams was also modified, aiming a better stability of the runner in various running conditions and an improved air flow circulation across it. Variant A Variant B Figure 1. Variants of the wind concentrator obtained in the pre-processing phase of FEA. The modal analysis of the whole runner structure, involving mainly thin and thick shell type finite elements, revealed some modes occurring at lower frequencies than those found for the outer cylinder alone in the previous study [1]. The first is the torsion mode (Figure 2), which may occur at a sudden and total brake of the turbine axle. Figure 2. The 1st mode of vibration torsion mode for variant A (left) and variant B (right). The 2nd is an axial mode, implying a relative movement of the outer shell and the blades with respect to the turbine axle (Figure 3). The 3rd and 4th involve rigid body movements of the blades coupled with slight diagonal in-plane bending of the ducting shell (Figure 4). Some figures are containing arrows, making thus easier to follow the vibration move. The next six modes contain mainly the rigid body rotational movements of the blades relative to own spars, with frequencies between 4.2 and 4.7 Hz. The next six ones contain mainly the vibrations of the outer ducting shell, similar to the modes obtained in the previous study for this structural part, when the blades were missing (Figure 5 and Figure 6).

3 9 Modal analysis of a small wind turbine with wind concentrator Figure 3. The 2nd mode of vibration axial mode for variant A (left) and variant B (right). Figure 4.. The 3rd mode of vibration for variant A (left) and variant B (right). One can see some permutation in the last modes when comparing the results obtained for variant A and variant B.

4 10 Figure 5. The first four natural modes involving mainly the outer shell, in variant A. Figure 6. The first four natural modes involving mainly the outer shell, in variant B. Other natural modes of interest are concerning the first vibrations modes of the inclined and radial beams sustaining the outer shell and of the blades themselves, for both design variants (Figure 7, Figure 8 and Figure 9, respectively).

5 11 Modal analysis of a small wind turbine with wind concentrator Figure 7. The 1st natural vibration modes of involving the inclined beams for variant A (left) and variant B (right). Figure 8. The 1st natural vibration modes of involving the radial beams for variant A (left) and variant B (right). Figure 9. The 1st natural vibration modes of involving the blades for variant A (left) and variant B (right). The comparative evaluation of the results obtained during the modal analysis of variants A and B, together with those obtained in the similar analysis of the first design of the runner leads to some interesting conclusions, presented below.

6 12 4. CONCLUSIONS The modal behaviour of the runner in three configurations proves that, for the main structural parts, the natural modes do not change significantly. In spite of the apparent important differences between the two latest design variants studied in this paper and the first one studied in [1], the natural modes of the outer shell do not change much, in both shape and associated frequencies. The modal behaviour was not much sensitive to quite important changes in the geometry and involved materials, or to the effect of added masses. Having in view the estimated maximum rotation speed of the runner of about 200 rot/min, the wide majority of natural frequencies seem to range beyond the frequencies of the in-service induced loads. Nevertheless, the results obtained so far and those obtained during the analysis focused on some critical structures may prove useful for detecting possible threats in having dangerous resonance conditions or noise development, affecting the functionality of the wind turbine or its noise signature. The study will also need further extension towards the modal analysis of damaged structural parts, following most probable scenarios. ACKNOWLEDGMENT. The research work was performed under project 258/2014, financed by the Romanian Ministry of National Education through its dedicated body (MEN-UEFISCDI), in the Partnership in priority domains - PN II programme. REFERENCES 1. Sorohan Şt., Constantin, N., Epuran, C. V., Dynamic behaviour of a solid runner for small wind turbines, Proc. of the Annual Symposium of the Institute of Solid Mechanics - SISOM 2015, 24-29, Lilley, G.M, Rainbird, W.J., A preliminary report on the design and performance of ducted wind mills, report no 102, College of Aeronaurics, Cranfield, UK, Lubitz WD, Shomer A, Wind loads and efficiency of a diffuser augmented wind turbine (DAWT), Proc. The Canadian Soc. Mech. Eng. Int. Congress 2014, CSME Int. Congress 2014, Toronto, Canada, 1-5, Ohya Y, Karasudani T, A shrouded wind turbine generating high output power with wind-lens technology, Energies, 2010, 3,