1,721,083 research outputs found
A review analysis of unsteady forces in hydraulic valves
No full textIn the fluid power applications where the typical operating conditions are dynamic the knowledge of the unsteady flow forces that act on the spools of the hydraulic valves represents an important issue to be addressed in order to make a correct design of the valve geometry and its driving system. This paper deals with a rigorous unsteady numerical study of the fluid dynamic behavior of a hydraulic directional control valve. A theoretical approach based upon the application of the momentum equation in the transient fluid dynamic conditions is presented, while a successive numerical analysis is performed by using the commercial FluentTM code that provided, in the past, a correct evaluation of the stationary flow forces.7 Unsteady simulations have been carried out considering three different conditions: constant pressure boundary conditions during the spool movement, inlet pressure ripple at a constant spool position, damped pressure oscillations during the spool opening phase. The main objective is to estimate the critical magnitude orders of the pressure ripple frequency and of the axial spool velocity above which the pseudo-steadiness assumption fails; in order to reach this aim, in some cases, the effects of the fluid dynamic phenomena connected with the unsteady flow conditions have been amplified
Three-Dimensional analysis of flow forces on directional control valves
No full textThis paper deals with a sufficiently complete analysis of flow forces acting on spools of hydraulic directional control valves. The analysis performed, using the very reliable and commercially widespread "Fluent" code, aims to describe global parameters in order to evaluate such forces with a reasonable accuracy and without performing expensive three-dimensional calculations. To this aim, different fluid dynamics analyses have been carried out inside the valve, starting from simple axis-symmefric models, up to full three-dimensional simulations. A comparison of the 2D and 3D results is then presented
Regularity of Minimizers for a Model of Charged Droplets
No full textWe investigate properties of minimizers of a variational model describing the shape of charged liquid droplets. The model, proposed by Muratov and Novaga, takes into account the regularizing effect due to the screening of free counterionions in the droplet. In particular we prove partial regularity of minimizers, a first step toward the understanding of further properties of minimizers
Design and tests of a new damper for a gas turbine thin-shell duct
No full textGas turbine (GT) liners, transition ducts and exhaust diffusers are large thin-shell ducts bounded by two barrels, typically characterized by multiple natural frequencies inside the operating speed range of the engine rotor. In most applications, GT ducts are supported on one side only and they are free to expand inside the gas turbine so to avoid thermal distresses. The GT ducts are typically damped structures able to prevent high cycles fatigue failures. Damping is provided by sliding features as insulation or bolted joints. This paper describes the redesign of a transition duct (TD) after it was discovered that in some operating conditions, duct could crack for high cycle fatigue (HCF). The TD connects the flow path of the gas generator turbine with the flow path of the power turbine. The new TD has been made more robust, but it has also been equipped of dampers capable to operate at high temperature. Starting from the analyses of field data, a predictive FEA model has been developed and validated. After a deep investigation of the TD modes that could be excited by flow path and/or by rotor vibrations, it was decided to add two dampers, one for each barrel of the TD. Due to internal space limitations, a new type of damper has been designed for the external barrel. Both dampers have been sized using FEA. Harmonic analyses rather than forced response transient analyses have been performed so to verify the effectiveness of the new design. In the simulations, dampers have been replaced by harmonic forces able to reproduce the friction force of the dampers. Validation of the method and damper calibration has been done by performing lab tests and full-size TD tests
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