Effects of heat release on triple flames

Heat release effects on laminar flame propagation in partially premixed flows are studied. Data for analysis are obtained from direct numerical simulations of a laminar mixing layer with a uniformly approaching velocity field. The structure that evolves under such conditions is a triple flame, which consists of two premixed wings and a trailing diffusion flame. Heat release increases the flame speed over that of the corresponding planar premixed flame. In agreement with previous analytical work, reductions in the mixture fraction gradient also increase the flame speed. The effects of heat release and mixture fraction gradients on flame speed are not independent, however; heat release modifies the effective mixture fraction gradient in front of the flame. For very small mixture fraction gradients, scaling laws that determine the flame speed in terms of the density change are presented. © 1995 American Institute of Physics

Frequency response functions and modal parameters of a rotating system exhibiting rotating damping

In the analysis of the stability threshold speed caused by rotating damping in rotating machinery, there is a lack of experimental data. This stability threshold speed can be found theoretically by means of a linear speed dependent model. The accuracy of the model depends highly upon the linearity and especially on the damping type that has been chosen. In this paper, the theoretical model and the importance of the stability analysis is discussed together with an experiment to validate the model. A rotating shaft is used to extract frequency response functions at different speeds. The shaft is excited with an automated impact hammer and the response is measured by eddy current probes. From these frequency response functions, the poles are extracted and compared to the poles derived from the model. It is found that the imaginary part of the poles, or the Campbell diagram, agrees quite well. The decay rate plot shows a similar increase as from the model, but there seems to be an extra stabilizing effect that is not accounted for in the model

Instability due to internal damping of rotating shafts

Rotor internal damping has been indicated as one of the main causes of instability in rotating machinery for more than a century. However, the exact characterisation of this damping is still an unsolved research topic. Therefore, in this thesis the consequences of material damping in rotating shafts are examined more in depth. Two main steps are considered. Firstly, a finite element model of the beam, including viscous and hysteretic damping, is constructed. This model allows to calculate the threshold speed of instability and the resonance frequencies of a shaft. Furthermore it allows to vary the damping parameters and to compare the considered models giving an indication of the general relations between instability and damping properties. Secondly, an experimental approach should elucidate which model fits best for the physical damping. In general, the main purpose is to gain new insights into how the damping should really be modelled to have the most accurate and safe prediction of a designed rotor