Air Foil Research Articles

Numerical and Experimental Investigations on Preload Effects in Air Foil Journal Bearings

A detailed elastogasdynamic model of a preloaded three-pad air foil journal bearing is presented. Bump and top foil deflections are herein calculated with a nonlinear beamshell theory according to Reissner. The two-dimensional pressure distribution in each bearing pad is described by the Reynolds equation for compressible fluids. The assembly preload is calculated by simulating the assembly process of top foil, bump foil, and shaft. Most advantageously, there is no need for the definition of an initial radial clearance in the presented model. With this model, the influence of the assembly preload on the static bearing hysteresis as well as on the aerodynamic bearing performance is investigated. For the purpose of model validation, the predicted hysteresis curves are compared with measured curves. The numerically predicted and the measured hysteresis curves show a good agreement. The numerical predictions exhibit that the assembly preload increases the elastic foil structural stiffness (in particular for moderate shaft displacements) and the bearing damping. It is observed that the effect of the fluid film on the overall bearing stiffness depends on the assembly preload: For lightly preloaded bearings, the fluid film affects the overall bearing stiffness considerably, while for heavily preloaded bearings the effect is rather small for a wide range of reaction forces. Furthermore, it is shown that the assembly preload increases the friction torque significantly.

Rotordynamic Performance of Hybrid Air Foil Bearings With Regulated Hydrostatic Injection

Air foil bearing (AFB) technology has made substantial advancement during the past decades and found its applications in various small turbomachinery. However, rotordynamic instability, friction and drag during the start/stop, and thermal management are still challenges for further application of the technology. Hybrid air foil bearing (HAFB), utilizing hydrostatic injection of externally pressurized air into the bearing clearance, is one of the technology advancements to the conventional AFB. Previous studies on HAFBs demonstrate the enhancement in the load capacity at low speeds, reduction or elimination of the friction and wear during starts/stops, and enhanced heat dissipation capability. In this paper, the benefit of the HAFB is further explored to enhance the rotordynamic stability by employing a controlled hydrostatic injection. This paper presents the analytical and experimental evaluation of the rotordynamic performance of a rotor supported by two three-pad HAFBs with the controlled hydrostatic injection, which utilizes the injections at particular locations to control eccentricity and attitude angle. The simulations in both time domain orbit simulations and frequency-domain modal analyses indicate a substantial improvement of the rotor-bearing performance. The simulation results were verified in a high-speed test rig (maximum speed of 70,000 rpm). Experimental results agree with simulations in suppressing the subsynchronous vibrations but with a large discrepancy in the magnitude of the subsynchronous vibrations, which is a result of the limitation of the current modeling approach. However, both simulations and experiments clearly demonstrate the effectiveness of the controlled hydrostatic injection on improving the rotordynamic performance of AFB.

Heat transfer and temperature effects on a dimpled NACA0012 airfoil with various angles of attack

ABSTRACTIn this paper, the effects of heat transfer and temperature on a dimpled NACA0012 airfoil were evaluated. A square-shaped, dimpled air foil was chosen for this study and it was examined for various angles of attack – 10°, 15° and 20°. The Mach nos. 1.2–2.2 were considered for the temperature profile and heat transfer characteristics. The lift and drag coefficients were maximum for squared dimples instead of circular dimples.

A Contribution to the Thermal Modeling of Bump Type Air Foil Bearings: Analysis of the Thermal Resistance of Bump Foils

A detailed analysis of the effective thermal resistance for the bump foil of air foil bearings (AFBs) is performed. The presented model puts emphasis on the thermal contact resistances between the bump foil and the top foil as well as between the bump foil and the base plate. It is demonstrated that most of the dissipated heat in the lubricating air film of an air foil bearing is not conducted by microcontacts in the contact regions. Instead, the air gaps close to the contact area are found to be thin enough in order to effectively conduct the heat from the top foil into the bump foil. On the basis of these findings, an analytical formula is developed for the effective thermal resistance of a half bump arc. The formula accounts for the geometry of the bump foil as well as for the surface roughness of the top foil, the bump foil, and the base plate. The predictions of the presented model are shown to be in good agreement with measurements from the literature. In particular, the model predicts the effective thermal resistance to be almost independent of the applied pressure. This is a major characteristic property that has been found by measurements but could not be reproduced by previously published models. The presented formula contributes to an accurate thermohydrodynamic (THD) modeling of AFBs.

Numerical simulation of VAWT on the effects of rotation cylinder

Based on Finite Element Analysis Method, studying on Vertical Axis Wind Turbine (VAWT) which is added rotating cylinder in front of its air foils, especially focusing on the analysis of NACA6 series air foils about variation of lift to drag ratio. Choosing the most suitable blades with rotary cylinder added on leading edge. Analysis indicates that the front rotating cylinders on the VAWT is benefit to lift rise and drag fall. The most suitable air foil whose design lift coefficient is 0.8, the blades relative thickness is 20%, and the optimistic tip speed ratio is about 7.

A fully coupled air foil bearing model considering friction – Theory & experiment

The dynamics of air foil bearings (AFBs) are not yet fully captured by any model. The recent years have, however, seen promising results from nonlinear time domain models, and simultaneously coupled formulations are now available, avoiding the previous requirements for undesirably small time steps and temporal convergence studies.In the present work, an alternative foil structure model is substituted for the simple elastic foundation model to avoid its inherent limitations. The new foil model is based on a truss representation from the literature, but incorporates the foil mass and a dynamic friction model. As a consequence of the friction model's velocity dependency, the foil mass is included to obtain a set of differential equations that can be coupled to the rotor and fluid domains while allowing a simultaneous solution.Considerations leading to a practically applicable implementation are discussed and numerical results are compared with experimental data. The model predicts natural frequencies and mode shapes well, but it is not yet capturing the unbalance response when friction is considered. Possible causes for this discrepancy are discussed and it is suggested that sticking is a more prevalent state than previously assumed.

Erratum: “Design and Performance Prediction of Hybrid Air Foil Thrust Bearings” [ASME J. Eng. Gas Turbines Power, 2010, 133(4), p. 042501; DOI: 10.1115/1.4002249

Erratum: “Design and Performance Prediction of Hybrid Air Foil Thrust Bearings” [ASME J. Eng. Gas Turbines Power, 2010, 133(4), p. 042501; DOI: 10.1115/1.4002249

Plasma actuators for active flow control based on a glow discharge

In this work a glow discharge based active flow control for high flow velocities and low Reynolds numbers is presented. Unlike common plasma actuators such as dielectric barrier discharge (DBD) or spark jets, this actuator uses small impulse bits at frequencies. The actuator is optimized for frequencies up to 40 kHz to counter Tollmien Schlichting wave effects and so reduce overall air foil drag. Several measurements to prove the non-eroding effect of the actuator and the electrical properties were performed. It was found that the actuator is capable of operating at high frequencies without measurable erosion.

Development and experimental investigation of a 500-W class ultra-micro gas turbine power generator

We experimentally investigate the feasibility of a 500-W class ultra-micro gas turbine power generator. System specifications include a design rotational speed of 400,000 rpm, a compressor ratio of 3:1, and a turbine inlet temperature of 1200 K. In a previous study, we used an integrated test rig to examine the feasibility of start-up and self-sustaining capabilities. Here, we describe our development and testing of a new integrated test rig to measure the performance of electric-power generation during self-sustaining and boosting operations. We controlled a motor-generator using an electric controller in fixed speed mode and guided the rotational speed according to the start-up sequence. We employed a radial-thrust integral metal-mesh bumper air foil bearing with which we obtained the design rotational speed of 400,000 rpm in a standalone motoring test. In the combustor, we used liquefied petroleum gas as the main fuel and kerosene as the ignition fuel and partially as the main fuel during the boost operation. The integrated test rig achieved electric power generation of 30 W at 200,000 rpm, 50% of the design rotational speed.

Transient and steady state behaviour of elasto–aerodynamic air foil bearings, considering bump foil compliance and top foil inertia and flexibility: A numerical investigation

This work gives a theoretical contribution to the problem of modelling air foil bearings considering large sagging effects in the calculation of the non-linear transient and steady state response of a rigid rotor. This paper consists of two parts: the development of a miltiphysics model of the air foil bearing, and a numerical parameter study of a rigid journal supported in an air foil bearing with a partially supported top foil. The mathematical model of the air foil bearing is centred around the finite element models of both the air film and the top foil structure. These finite element models utilise two types of eight-node isoparametric elements. The rotor is modelled as a rigid body without rotational inertia, i.e. as a journal. The bump foil is included via a bilinear version of the simple elastic foundation model. This paper introduces the bilinear simple elastic foundation model, which combined with the top foil structure model, enables a separation of the top foil and the bump foil. A phenomenon associated within areas of the top foil is where the aerodynamic pressure is sub-ambient. The parameter study investigates the performance of three air foil bearings with partially supported top foils and one air foil bearing with a fully supported top foil. The steady state responses of a journal supported by these air foil bearings are investigated for varied rotational speeds and journal unbalances as well as the top foil sagging in the unsupported area. The study reveals that sub-harmonic vibrations associated with a large journal unbalance can be eliminated by a proper design layout of the bump foil, i.e. placement of the unsupported area. The positive effect is attributed to ‘equivalent shallow pockets’ formed by the sagging top foil.

Modeling and Aerodynamic Analysis of Small Scale, Mixed Airfoil Horizontal Axis Wind Turbine Blade

To create a clean energy environment for the future found to be as one of the great challenges of our time. As conventional sources (i.e. oil, coal, Fossil Fuels) of energy depleting very rapidly and will be finished in near future, it is very essential to look for renewable energy sources. To incorporate this challenge in India a systematic new energy plan was developed in 2006 i.e. Ministry of New and Renewable Energy (MNRE). Harvard University study, published in the Proceedings of the National Academy of Sciences of the United States 2009, had calculated that the future world’s wind power potential to be 40 times greater than current power consumption. The most important component of wind energy converter is rotor, which rotate and produces electricity. The objective of this project is to design a rotor blade for Reynolds number 100,000. Design procedure consists of selection of suitable air foil (for small scale Horizontal Axis Wind Turbine), determination of twist and chord along the blade radius, and modelling is done on Creo-Parametric 5.0. Aerodynamic analysis is performed through Blade Element Momentum (BEM) theory which gives optimize torque and power. Performance is analysed for wind velocity 4-7 m/s at different tip speed ratio (TSR). Performance is validated with the help of CFD (ANSYS 14.0). After analysis, it is found that the power developed is 250W with coefficient of power 35.687%.

Optimisation of swept angles for airfoil NACA 6-series

A NACA 6-series is one of the most commonly used commercial passenger aircraft, which is of subsonic and transonic type. This series, a laminar flow type is taken for our study. In this paper the straight wing and the swept back wings are designed using a suitable tool and it is simulated by using CFD software. FLUENT software is used to analyse the different swept angles of 0°, 25°, 30°, 35°, 40° and 45° for different mach numbers ranging from 0.6 to 1.2. Based on these results the drag coefficient (CD ) and lift coefficient (CL ) are plotted. The values for the straight and swept wings indicate that there is a reduction in drag coefficient. Then thus by means of optimisation factor the swept angle is optimised for the NACA 6-series air foil. By the help of the optimising factor we have found that 35° will be more efficient and it is the best choice for a high speed aircraft, because this wing minimises fuel consumption and thereby by remarkably reducing wave drag during flight thus reducing the cost of flight and increase the range of aircraft and at the same time it ensures a safe take-off and a smooth landing.

Optimisation of swept angles for airfoil NACA 6-series

A NACA 6-series is one of the most commonly used commercial passenger aircraft, which is of subsonic and transonic type. This series, a laminar flow type is taken for our study. In this paper the straight wing and the swept back wings are designed using a suitable tool and it is simulated by using CFD software. FLUENT software is used to analyse the different swept angles of 0°, 25°, 30°, 35°, 40° and 45° for different mach numbers ranging from 0.6 to 1.2. Based on these results the drag coefficient (CD ) and lift coefficient (CL ) are plotted. The values for the straight and swept wings indicate that there is a reduction in drag coefficient. Then thus by means of optimisation factor the swept angle is optimised for the NACA 6-series air foil. By the help of the optimising factor we have found that 35° will be more efficient and it is the best choice for a high speed aircraft, because this wing minimises fuel consumption and thereby by remarkably reducing wave drag during flight thus reducing the cost of flight and increase the range of aircraft and at the same time it ensures a safe take-off and a smooth landing.

An analysis of silent flight of owl

The present research studies the various parameters such as its special feather materials, the serrated wings feather, long velvet down materials causing the nearly silent flight of an Owl (Barn Owl). An Owl is a bird that is quite common and is well known for its nearly silent flight. The owl contains the long velvet down feathers found on wings and legs absorb the sound frequencies. The elongated distal barbules of the Owl form a multi-layer grid porous structure which also has a positive effect on the sound absorption quality of the Owl. The leading edge serration and trailing edge fringe improves the pressure fluctuation of turbulence boundary, distribute the air rushing and accumulation of air at the end of the wing which suppress the generation of vortex sound and also solves the problem of fluttering in the wings. The owls can perform this phenomenon even at great heights because of their aerodynamically designed wings and feathers. Silent flight gives Owl ability to capture prey by stealth. The main focus and Interest of this paper is to concentrate on the silent flight of Owl and built a basic of a bionic air foil, which is extracted from the Owl wings... This research not only can give the inspiration for solving the aerodynamic noise of aircraft and engineering machine, but also can provide a new idea for the design of low-noise devices Inspired by the noise absorption structure of Owl wings. The results may be further used to build an Unmanned Air Vehicle (UAV), capable of silent flight like Owls.

INNOVATION OF BUILDING DESIGN BASED ON PREDICTED MEAN VOTE (PMV) INDEX FOR INCREASING HUMAN COMFORT

Concerning towards the quality of air comfort and safety to pedestrians in the University area should be increased as Indonesia economics booming. The objective of this study is to investigate and then to evaluate the distribution of thermal comfort at the University of Brawijaya, Malang. The research is started with study literature and early survey to collect all information of building layout and building shape at the University of Brawijaya. The information is used to create a 3D model in CAD software. The model is simulated to measure the PMV factors of air temperature, relative humidity and air speed in some locations. The two scenarios of maintaining the predicted mean vote (PMV) is presented by designing the new building based on air foil shape dimension to keep the wind speed in higher value for pedestrian and by putting the water pool in front of the building to increase the air temperature. As a result, the air foil based design is capable to reduce PMV index in average 0.4 scale and pool design is only reducing PMV index level in 0.001 scale

Rigid Mode Vibration Control and Dynamic Behavior of Hybrid Foil–Magnetic Bearing Turbo Blower

In this study, experimental and analytical analyses of the vibration stability of a 225 kW class turbo blower with a hybrid foil–magnetic bearing (HFMB) were performed. First, critical speed and unbalance vibration responses were examined as part of the rotordynamic research. Its shaft diameter was 71.5 mm, its total length was 693 mm, and the weight of the rotor was 17.8 kg. The air foil bearing (AFB) utilized was 50 mm long and had a 0.7 aspect ratio. In the experiments conducted, excessive vibration and rotor motion instability occurred in the range 12,000–15,000 rpm, which resulted from insufficient dynamic pressure caused by the length of the foil bearing being too short. Consequently, as the rotor speed increased, excessive rotor motion attributable to aerodynamic and bearing instability became evident. This study therefore focused on improving rotordynamic performance by rectifying rigid mode unstable vibration at low speed, 20,000 rpm, and asynchronous vibration due to aerodynamic instability by using HFMB with vibration control. The experimental results obtained were compared for each bearing type (AFB and HFMB) to improve the performance of the vibration in the low-speed region. The experimental results show that the HFMB technology results in superior vibration stability for unbalance vibration and aerodynamic instability in the range 12,000–15,000 rpm (200–250 Hz). The remarkable vibration reduction achieved from vibration control of the HFMB–rotor system shows that oil-free turbomachinery can achieve excellent performance.

Rotordynamic Performance of a Shaft With Large Overhung Mass Supported by Foil Bearings

This work presents the theoretical and experimental rotordynamic evaluations of a rotor–air foil bearing (AFB) system supporting a large overhung mass for high-speed application. The proposed system highlights the compact design of a single shaft rotor configuration with turbomachine components arranged on one side of the bearing span. In this work, low-speed tests up to 45 krpm are performed to measure lift-off speed and to check bearing manufacturing quality. Rotordynamic performance at high speeds is evaluated both analytically and experimentally. In the analytical approach, simulated imbalance responses are studied using both rigid and flexible shaft models with bearing forces calculated from the transient Reynolds equation along with the rotor motion. The simulation predicts that the system experiences small synchronous rigid mode vibration at 20 krpm and bending mode at 200 krpm. A high-speed test rig is designed to experimentally evaluate the rotor–air foil bearing system. The high-speed tests are operated up to 160 krpm. The vibration spectrum indicates that the rotor–air foil bearing system operates under stable conditions. The experimental waterfall plots also show very small subsynchronous vibrations with frequency locked to the system natural frequency. Overall, this work demonstrates potential capability of the air foil bearings in supporting a shaft with a large overhung mass at high speed.

Effect of Operating Conditions on the Elastohydrodynamic Performance of Foil Thrust Bearings for Supercritical CO2 Cycles

In this paper, a quasi-three-dimensional fluid–structure model using computational fluid dynamics for the fluid phase is presented to study the elastohydrodynamic performance of foil thrust bearings for supercritical CO2 cycles. For the simulation of the gas flows within the thin gap, the computational fluid dynamics solver Eilmer is extended, and a new solver is developed to simulate the bump and top foil within foil thrust bearings. These two solvers are linked using a coupling algorithm that maps pressure and deflection at the fluid structure interface. Results are presented for ambient CO2 conditions varying between 0.1 and 4.0 MPa and 300 and 400 K. It is found that the centrifugal inertia force can play a significant impact on the performance of foil thrust bearings with the highly dense CO2 and that the centrifugal inertia forces create unusual radial velocity profiles. In the ramp region of the foil thrust bearings, they generate an additional inflow close to the rotor inner edge, resulting in a higher peak pressure. Contrary to the flat region, the inertia force creates a rapid mass loss through the bearing outer edge, which reduces pressure in this region. This different flow fields alter bearing performance compared to conventional air foil bearings. In addition, the effect of turbulence in load capacity and torque is investigated. This study provides new insight into the flow physics within foil bearings operating with dense gases and for the selection of optimal operating condition to suit CO2 foil bearings.

Sensitivity of Computational Rotor Dynamics Towards the Empirically Estimated Lubrication Gap Clearance of Foil Air Journal Bearings

AbstractThis contribution is concerned with the computational analysis of a rigid rotor supported by means of two self‐acting foil air journal bearings. Even though the overall equation system is thereby typically written in a nondimensional form, prior knowledge about realistic value ranges of occurring dimensionless numbers is required in order to parameterize and interpret such simulations correctly. Unlike all other quantities, the nominal lubrication gap clearance between the rotating journal and the undeformed foil structure is reported to be only poorly known. Thus, even in the light of an advanced understanding of the bearing rotor system's fundamental behavior, the quantitative reproduction and prediction of experimental results by means of computational analysis need to be viewed critically. In this study, the sensitivity of numerical results towards the assumed nominal lubrication gap clearance will be investigated. To this end, the stability of the system is considered and the characteristics of occasionally observed equilibrium points and limit cycles are addressed. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Multiscale measurement of air foils with data fusion of three optical inspection systems

Measuring the geometry of machine parts with complex geometries creates new challenges towards measurement systems. Due to cost, fast inspection is nevertheless desirable. Additionally the characterization of a parts health and functionality requires geometric information in different scales. We have developed a set of optical measurement systems which together meet those requirements: A borescopic fringe projection system, a macroscopic fringe projection system using newly developed algorithms for fast inspection of turbine blades and a low coherence Michelson interferometer (LCI). The first can detect geometric variances in hard to reach areas, e.g. inside machines or in between parts with highly complex geometries like blade integrated discs (blisks). Using fully adaptable fringe patters, the second system can locate geometric variances with a single fringe pattern. These patterns lead to high sensitivity and high measurement speed. Afterwards the LCI further inspects the micro structure of defects and characterizes the surface structure of the air foil. The presented algorithms provide fast 3D reconstruction with a nanometer resolution and, compared to other systems, a large measurement range. Together, these three inspection systems are capable to detect and quantify defects or geometric variances of industrial parts in different scales. This information improves the prediction of the reliability of a part and helps extending its lifetime to reduce the maintenance costs of machines. As an example a turbine blade was measured with all three systems and the results are visualized in one dataset. Means to merge the measurements are discussed.