EnginSoft International Conference 2011 |
Modeling Different Contact Conditions in Blade Rows of Turbomachinery |
Bachschmid Nicolò - Franco Tosi Meccanica (Italy) |
Abstract |
Actual trend in steam turbine design is to use blades with integral shrouds, for high pressure and intermediate pressure steam turbine sections, as well as also for the long blades of the low pressure sections. The blades are inserted with their root into the seat on the shaft in such a way that the blades are slightly forced against each other in correspondence of the shrouds. In long blades of low pressure stages the forcing can be obtained by the un-twisting of twisted blades due to the effect of the huge centrifugal forces. The dynamic behavior of these blade rows is difficult to predict due to the non linear effect of the contact forces and due to friction. Different “equivalent” linear models for the contact are proposed and compared. The resulting natural frequencies of the blade rows as function of the different nodal diameter mode shapes are highly depending on the assumed models. For avoiding resonant conditions with engine order excitations, the natural frequencies must be calculated with good accuracy. The aim of this paper is to present some numerical results obtained with ANSYS about natural frequencies and forced response of a couple of blades in different contact conditions and with different engine order excitations. Different dynamical behaviors, with continuous contact and with periodical intermittent contacts, with and without friction, with strong or weak excitation, close to resonance and far from resonance are shown. The setting of calculation parameters (type of friction, type of contact elements, time step, asymmetric contact, contact stiffness updating each equilibrium position, default initial contact stiffness and so on) is discussed. All results show a reduction of vibration amplitude with respect to the free standing blades without contact, as could be expected. From these results a linear equivalent (with restricted validity) model of the complete blade row is obtained by using cyclic symmetry, which requires accurate and suitable choice of symmetry surfaces. The obtained linear model allows natural frequencies evaluation and forced vibration calculations. It will be shown that natural frequencies of blade rows depend strongly on the equivalent contact model (springs and dampers, spring distribution on real contact area, no-separation on full contact area, bonded on real contact area, equivalent “contact pads”) which has been chosen to represent the shrouds in contact. |
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