Assessment of structural integrity through coupled vibration

A vibration-based technique is employed to evaluate the structural integrity using mode coupling property. The methodology is first demonstrated on a single hollow section beam by analytical simulation and experimental validation. A welded test rig with a fatigue crack is then investigated by applying this technique. The test rig consists of chord members cross-connected by smaller branches. The fatigue crack is generated by a reciprocating mechanism. The proposed approach discriminates the cracked member through the emergence of coupled mode that is observable on FRF plots. The test results of the structure under intact and cracked states are presented. It is suggested that this method has the potential to detect cracks in welded structures

Longwall shearer cutting force estimation

Longwall mining is an underground coal mining method that is widely used. A shearer traverses the coal panel to cut coal that falls to a conveyor. Operation of the longwall can benefit from knowledge of the cutting forces at the coal/shearer interface, particularly in detecting pick failures and to determine when the shearer may be cutting outside of the coal seam. It is not possible to reliably measure the cutting forces directly. This paper develops a method to estimate the cutting forces from indirect measurements that are practical to make. The structure of the estimator is an extended Kalman filter with augmented states whose associated dynamics encode the character of the cutting forces. The methodology is demonstrated using a simulation of a longwall shearer and the results suggest this is a viable approach for estimating the cutting forces. The contributions of the paper are a formulation of the problem that includes: the development of a dynamic model of the longwall shearer that is suitable for forcing input estimation, the identification of practicable measurements that could be made for implementation and, by numerical simulation, verification of the efficacy of the approach. Inter alia, the paper illustrates the importance of considering the internal model principle of control theory when designing an augmented-state Kalman filter for input estimation

Crack identification in hollow section structures through coupled response measurements

This paper present a feasible method for crack identification in hollow section structures based on the coupling vibration behaviour of cracked members. From the last several decades lots of techniques have been developed by many researchers to detect, locate and quantify damage by using changes to modal parameters such as natural frequency, mode shape or damping ratio. However, these approaches suffer from the fact that the structural damage has a low sensitivity to changes in these parameters. A related option is offered through coupled response measurements. A transverse surface crack is well known to produce local flexibility due to the stress-strain singularity in the vicinity of the crack tip. The local flexibility can be represented by the way of a 6x6 matrix for a beam element that includes the crack. This matrix contains off-diagonal terms that cause coupling response along the direction corresponding to these terms. This coupling property due to the crack is evidence of the existence of the cracks. Coupled response of cracked hollow section structures was studied. Hollow section structures demonstrate a more pronounced coupling when cracks occur. In this paper, both an analytical simulation and the early results of experimental implementation are presented. This method is fairly discriminative even for small cracks

Heatline visualization of natural convection in a porous cavity occupied by a fluid with temperature-dependent viscosity

Temperature dependent viscosity effect in buoyancy driven flow, of a gas or a liquid, in an enclosure filled with a porous medium is studied numerically, based on the general model of momentum transfer in a porous medium. The Arrhenius model, which proposes an exponential form of viscosity-temperature relation, is applied to examine three cases of viscosity-temperature relation: constant, decreasing and increasing. Application of arithmetic and harmonic mean values of the viscosity is also investigated for their ability to represent the Nusselt number versus the effective Rayleigh number. Heatlines are illustrated for a more comprehensive investigation of the problem

Effects of temperature dependent viscosity on Bénard convection in a porous medium using a non-Darcy model

Temperature-dependent viscosity variation effect on Benard convection, of a gas or a liquid, in an enclosure filled with a porous medium is studied numerically, based on the general model of momentum transfer in a porous medium. The exponential form of viscosity-temperature relation is applied to examine three cases of viscosity-temperature relation: constant (mu = mu(C)), decreasing (down to 0.13 mu C) and increasing (up to 7.39 mu(C)). Effects of fluid viscosity variation on isotherms, streamlines, and the Nusselt number are studied. Application of the effective and average Rayleigh number is examined. Defining a reference temperature, which does not change with the Rayleigh number but increases with the Darcy number, is found to be a viable option to account for temperature-dependent viscosity variation. (C) 2007 Published by Elsevier Ltd

Effects of temperature-dependent viscosity variation on entropy generation, heat and fluid flow through a porous-saturated duct of rectangular cross-section

Effect of temperature-dependent viscosity on fully developed forced convection in a duct of rectangular cross-section occupied by a fluid-saturated porous medium is investigated analytically. The Darcy flow model is applied and the viscosity-temperature relation is assumed to be an inverse-linear one. The case of uniform heat flux on the walls, i.e. the H boundary condition in the terminology of Kays and Crawford, is treated. For the case of a fluid whose viscosity decreases with temperature, it is found that the effect of the variation is to increase the Nusselt number for heated walls. Having found the velocity and the temperature distribution, the second law of thermodynamics is invoked to find the local and average entropy generation rate. Expressions for the entropy generation rate, the Bejan number, the heat transfer irreversibility, and the fluid flow irreversibility are presented in terms of the Brinkman number, the Péclet number, the viscosity variation number, the dimensionless wall heat flux, and the aspect ratio (width to height ratio). These expressions let a parametric study of the problem based on which it is observed that the entropy generated due to flow in a duct of square cross-section is more than those of rectangular counterparts while increasing the aspect ratio decreases the entropy generation rate similar to what previously reported for the clear flow case

Effects of viscous dissipation and boundary conditions on forced convection in a channel occupied by a saturated porous medium

Forced convection with viscous dissipation in a parallel plate channel filled by a saturated porous medium is investigated numerically. Three different viscous dissipation models are examined. Two different sets of wall conditions are considered: isothermal and isoflux. Analytical expressions are also presented for the asymptotic temperature profile and the asymptotic Nusselt number. With isothermal walls, the Brinkman number significantly influences the developing Nusselt number but not the asymptotic one. At constant wall heat flux, both the developing and the asymptotic Nusselt numbers are affected by the value of the Brinkman number. The Nusselt number is sensitive to the porous medium shape factor under all conditions considered

Effects of viscous dissipation and boundary conditions on forced convection in a channel occupied by a saturated porous medium

Forced convection with viscous dissipation in a parallel plate channel filled by a saturated porous medium is investigated numerically. Three different viscous dissipation models are examined. Two different sets of wall conditions are considered: isothermal and isoflux. Analytical expressions are also presented for the asymptotic temperature profile and the asymptotic Nusselt number. With isothermal walls, the Brinkman number significantly influences the developing Nusselt number but not the asymptotic one. At constant wall heat flux, both the developing and the asymptotic Nusselt numbers are affected by the value of the Brinkman number. The Nusselt number is sensitive to the porous medium shape factor under all conditions considered

Performance of power plants with organic Rankine cycles under part-load and off-design conditions

Low-grade heat can be converted to electricity using power plants based on conventional Rankine cycles but with an organic Rankine fluid. Design and construction of such plants have been known for a long time and they are now a commericial reality. Applications include industrial waste heat recovery systems, solar thermal systems, low-temperature geothermal power plants, stand-alone electricity generators like those used for cathodic protection of pipelines, etc. In the past, simulation studies of such systems have usually suffered from the lack of an efficient, reliable and fast algorithm to predict system performance under part-load and off-design conditions. In this study, an efficient algorithm is introduced to simulate ORC Plant performance and the part-load and off-design efficiencies of ORC Plants