Bristle Deflection Research Articles

The Effect of Inlet Swirl on Brush Seal Bristle Deflections and Stability

Abstract This paper considers three-dimensional (3D) computational fluid dynamics (CFD) and structural modeling of brush seals, and investigates the effects of inlet swirl on the bristle pack. The model couples aerodynamic forces generated by CFD to a structural model that includes interaction between bristles. At a critical value of inlet swirl, aerodynamic forces cause circumferential slip of the upstream bristle row. In practice, this may lead to instability of the bristle pack and is consistent with anecdotal reports of seal behavior. The critical swirl velocity was reduced when the downstream pressure level was raised, keeping the same upstream total to downstream static pressure difference. This is caused by the increased dynamic head associated with the inlet swirl. Inclusion of a front plate in the seal design does not offer the intended protection to the bristle pack in highly swirling environments. This is associated with highly swirling flow impinging on the bristle tips. Fitting of roughness elements on the upstream face of the front plate could improve stability by reducing swirl of the flow impacting on the bristles. Increasing the bristle diameter and bristle stiffness does not necessarily prevent slip at higher inlet swirl velocities, but reduces the magnitude of slip of the upstream bristles.

A New Uniformly Ultimate Boundedness Criterion for Temperature Dependent Average Bristle Deflections

Temperature variation intensively affects parameters of the friction force, particularly the average bristle deflections. In our earlier research work, a temperature dependent friction compensation scheme was developed for serial rigid robot manipulators which was comprised of a temperature dependent viscous friction compensation scheme and a temperature dependent disturbance rejection scheme. In this paper, a new criterion for the uniformly ultimate boundedness of the temperature dependent average bristle deflections is proposed in such a way that the temperature dependent average bristle deflections preserve the property of uniformly ultimately boundedness in a larger region. This is an improvement to our earlier research work. As the result of this improvement, it is shown that the proposed method in our earlier work, is applicable to larger temperature dependent disturbance terms. These two improvements are the main contributions of this paper that augment our earlier research work. The idea is supported by a new theorem.

Leakage effects due to bristle deflection and wear in hybrid brush seal of high-pressure steam turbine

This study investigated the effect of clearance between a rotor and hybrid brush seal for leakage flow in terms of the axial deflection and wear. The leakage flow was experimentally measured according to pressure difference, and the Darcian porous medium-based CFD model was used to predict leakage flow according to clearance and rotational speed. The bristle's axial deflection was estimated according to the external force, the brush seal compliance was derived based on the cantilever beam theory, and the bristle wear and oxidation under superheated steam and high-speed operation environments were studied. The accuracy of the CFD model that considered the clearance change was improved by 11% compared to that of the previous CFD model.

Theoretical and numerical investigation on the leakage characteristics of brush seals based on fluid–structure interaction

This paper investigates the leakage characteristics of brush seals including the flow field characteristic of brush seal and the effects of structural parameters on the leakage characteristic of brush seals with the consideration of bristle deflection. The flow field characteristic of brush seal was discussed based on three-dimensional (3-D) computational model of brush seal, two-way fluid–structure interaction and moving grid technique. Then the effect of structural parameters on the leakage characteristic of brush seals was investigated from theoretical and numerical simulation perspectives with the emphasis on an improved prediction formula of leakage flow rate of brush seals. As illustrated in the analysis of the leakage characteristics of brush seals, (1) the leakage with the influence of bristle deflection is closer to the results of experiment relative to that without bristle deflection, and increases with the increasing inlet/outlet pressure ratios, which validate the developed 3-D computational model with bristle deflection to be more reasonable; (2) the flow field characteristics (pressure and velocity) of brush seal are revealed reasonably; (3) with the increasing of the height of backing plate fence, the clearance of brush wire and the axial clearance between brush bristle and back plate, the leakage factor rises and then reaches a stable value when the clearance of brush is larger than 0.3 mm; moreover, (4) with the increase of brush wire diameter, the leakage factor decreases firstly and then tends to stabilization, while rapidly decreases at first, then slowly decreases, and lastly tends to a value when the bristle row number increases; (5) the reliability and accuracy of the proposed prediction equation for brush seals is validated to be high by the CFD computational results. The efforts of this paper provide a useful theoretical and numerical method to clearly understand the leakage characteristics of brush seal, which is beneficial to improve the design of brush seals.

Numerical Investigations on the Heat Transfer Behavior of Brush Seals Using Combined Computational Fluid Dynamics and Finite Element Method

Brush seals have been applied in more and more challenging high-temperature locations. The high speed bristle-rotor friction causes a considerable heat generation which accelerates the bristles wear. The frictional heat generation at bristle-rotor interface becomes another major concern in brush seal applications. This study presented detailed investigations on the heat transfer characteristics and contact mechanics of brush seals using a combined computational fluid dynamics (CFD) and finite element method (FEM) brush seal model. The CFD model of brush seal for mass and heat transfer employed Reynolds-averaged Navier–Stokes (RANS) solutions coupled with non-Darcian porous medium approach. The nonlinear contact model of brush seal was established using FEM with considerations of internal frictions (bristle to rotor, bristle to backing plate, and bristle to bristle) and aerodynamic loads on bristles. The numerical method involved iterations between CFD and FEM models to better evaluate the heat transfer behaviors of the brush seal with consideration of bristle deflections. The frictional heat generation was calculated from the product of bristle-rotor frictional force and sliding velocity. The bristle deflections and temperature distributions of the brush seal were predicted at various operational conditions using the iterative CFD and FEM brush seal model. The effects of pressure differential and rotational speed on the contact behavior, temperature distribution and bristle maximum temperature of brush seals were numerically investigated using the developed approach. The detailed pressure contours and streamline distributions of the brush seal were also illustrated.

Dynamic analysis of planar multi-body systems with LuGre friction at differently located revolute clearance joints

In this paper, the dynamic response of a planar rigid multi-body system with stick–slip friction in revolute clearance joints is studied. LuGre friction law is proposed to model the stick–slip friction at the revolute clearance joints. This is because using this law, one can capture the variation of the friction force with slip velocity, thus making it suitable for studies involving stick–slip motions. The effective coefficient of friction is represented as a function of the relative tangential velocity of the contacting bodies, that is, the journal and the bearing, and an internal state. In LuGre friction model, the internal state is considered to be the average bristle deflection of the contacting bodies. By applying the LuGre friction law on a typical slider–crank mechanism, the friction force in the revolute joint having clearance is seen not to have a discontinuity at zero slip velocity throughout the simulation unlike in static friction models. In addition, LuGre model was observed to capture the Stribeck effect which is a phenomenon associated directly with stick–slip friction. The friction forces are seen to increase with increase in input speed. The effect of stick–slip friction on the overall dynamic behavior of a mechanical system at different speeds was seen to vary from one clearance joint to another.

Numerical Simulation and Analysis of Low Hysteresis Brush Seals

To better understand the bristle behavior and leakage performance in brush seals, a Computational Fluid Dynamics (CFD) model has been developed, in which the brush is treated as an anisotropic porous region with nonlinear resistance coefficients. With the calculated pressure distribution and the cantilever beam theory, aerodynamic and contact forces on bristles are calculated utilizing the Finite Element Method; and bristle deflections in the axial direction and in the orthogonal plane are obtained. Subsequently their influences on friction force and torque are evaluated. Computed different kinds of low hysteresis brush seals, discuss the effect of size parameters of front plate and back plate on low hysteresis brush seals.

Numerical Simulation and Analysis of Low Hysteresis Brush Seals

To better understand the bristle behavior and leakage performance in brush seals, a Computational Fluid Dynamics (CFD) model has been developed, in which the brush is treated as an anisotropic porous region with nonlinear resistance coefficients. With the calculated pressure distribution and the cantilever beam theory, aerodynamic and contact forces on bristles are calculated utilizing the Finite Element Method; and bristle deflections in the axial direction and in the orthogonal plane are obtained. Subsequently their influences on friction force and torque are evaluated. Computed different kinds of low hysteresis brush seals, discuss the effect of size parameters of front plate and back plate on low hysteresis brush seals.

Interconnection and damping assignment passivity‐based control of a class of underactuated mechanical systems with dynamic friction

AbstractThis paper considers the extension of the interconnection and damping assignment passivity‐based control methodology for a class of underactuated mechanical systems with dynamic friction. We present a new damping assignment approach to compensate friction by means of a nonlinear observer. Friction at the actuated joints is assumed to be captured by a bristle deflection model: the Dahl model. Based on the Lyapunov direct method we show that, under some conditions, the overall closed‐loop system is stable and, by invoking the theorem of Barbashin–Krasovskii, we arrive to asymptotic stability conditions. Experiments with an underactuated mechanical system, the Furuta pendulum, show the effectiveness of the proposed scheme when friction is compensated. Copyright © 2010 John Wiley & Sons, Ltd.

Nanoscale Friction Dynamic Modeling

Friction and system models are fundamentally coupled. In fact, the success of models in predicting experimental results depends highly on the modeling of friction. This is true at the atomic scale where the nanoscale friction depends on a large set of parameters. This paper presents a novel nanoscale friction model based on the bristle interpretation of single asperity contact. This interpretation is adopted after a review of dynamic friction models representing stick-slip motion in macrotribology literature. The proposed model uses state variables and introduces a generalized bristle deflection. Jumping mechanisms are implemented in order to take into account the instantaneous jumps observed during 2D stick-slip phenomena. The model is dynamic and Lipchitz, which makes it suitable for future control implementation. Friction force microscope scans of a muscovite mica sample were conducted in order to determine numerical values of the different model parameters. The simulated and experimental results are then compared in order to show the efficacy of the proposed model.

Analytical dynamic tire model

Modeling of tire friction is one of the central problems for vehicle control systems design. LuGre-type dynamic tire model has been proposed and well discussed in previous studies, because it offers a compact form of dynamic model that is convenient in advanced control studies. It has been successfully used in tire slip control design and vehicle state estimation problems. In this article, a concept of time-constrained Stribeck effect is introduced to interpret the mechanism of the LuGre friction model in predicting tire friction characteristics. A modified two-dimensional (2D) dynamic LuGre friction model is introduced to make it compatible with the governing theorem in the steady state. An analytical 2D modified LuGre-type dynamic tire model is developed, in which some fundamental limitations of classical LuGre models are eliminated. The main modifications involve a change in the structure of the 2D LuGre friction model, introduction of load-dependent parameters in 1D and 2D tire models, and a changed structure in the distributed parameter model. The proposed model is compared, in the steady state, to both the Magic Formula and the classical LuGre model. It improves model accuracy in the steady state and gives a physically reasonable distribution of the bristle deflection. A first-order lumped parameter (LP) nonlinear model, which has simpler structure than the distributed parameter model and the classical LP LuGre model, is then derived. Numerical simulations show that the proposed LP model has a good estimation for tire transient dynamics. Thus, the proposed model retains the merits of LuGre-type models and improves the agreement with observation and experimental data on friction force distribution along the patch and on the steady-state friction prediction.

Dynamics of a freely rotating cutting brush subjected to variable speed

Cutting and flicking brushes of street sweeping vehicles sweep the debris that lies in the road gutter. As most of the debris is found in the gutter, it is of interest to understand and improve the behaviour of these brushes. This article investigates the dynamics of a novel oscillatory cutting brush in free rotation, by means of an analytical model derived by the authors. Two oscillatory functions are dealt with: a sinusoidal function and a function developed by the authors. The latter function, called Vanegas Useche–Abdel Wahab–Parker (VAP), depends on a smoothness parameter, b, which controls the smoothness of the angular speed function and the maximum brush accelerations. The parameters that affect brush behaviour are identified. The effects of the type of function, frequency of oscillation, smoothness parameter, ratio between the alternating and mean angular speed, bristle length, mount angle and mount radius are studied. The model indicates that bristle deflections and stresses are proportional to the square of the mean angular speed, provided that the ratio between the alternating and mean angular speed is constant, and depend linearly on the mount radius. The results indicate that, for the cases studied, the first mode dominates bristle vibrations, and that there tends to be resonance when the frequency is close to the first natural frequency of the bristles. Notably, bristle dynamics exhibits a peculiar behaviour, a condition similar to resonance at odd fractions of the natural frequencies for the VAP function. The intensity of this phenomenon could be controlled by adjusting the value of b or changing the frequency of oscillation. It is noted that the results of this work are also applicable to a small-deflection cantilever beam, when this is subjected to a function that is the square of a triangle, sinusoidal, or VAP wave. As the VAP function can achieve a resonant behaviour not only at the natural frequency of the bristle but also at odd fractions of at least the first four natural frequencies, small frequencies may be needed to produce a high-frequency mode resonant characteristic.

Manipulator velocity control using friction compensation

The velocity control of a direct-drive mechanical arm with high friction is considered. The friction at the robot joints is assumed to be captured by a bristle deflection model. Two joint velocity controllers with friction compensation are introduced. Experiments show the superiority of the proposed schemes with respect to the velocity controllers when friction is compensated using the classical viscous plus Coulomb model of friction.

Rotating Brush Seal

This article has no abstract.