Flame stabilization in high pressure LOx/GH2 and GCH4 combustion

Planar laser induced fluorescence (PLIF) of OH is used to examine flame stabilization in high pressure cryogenic flames formed by injecting a central jet of low speed liquid oxygen surrounded by a high speed gaseous stream of hydrogen or methane. In the LO_x/GH₂ experiments injection conditions are transcritical as the chamber pressure is above critical but the temperature is below critical . In the LO_x/GCH₄ experiments the chamber pressure and LO_x injection temperature are below critical , . Hydrogen or methane are injected at room temperature LIF images delineate the flame edge in the injector nearfield. The two flames are stabilized in the vicinity of the liquid oxygen injector lip but the anchor point is found to lie closer to the lip in the LO_x/GH₂ case and its displacement from shot to shot is of a smaller amplitude than that corresponding to the LO_x/GCH₄ flame. Interpretation of these data is based on a previous analysis which indicates that stabilization is essentially controlled by a dimensionless group formed by comparing the lip thickness to the flame edge thickness Ψ = h_s/δ_f. It is found that Ψ slightly exceeds unity in the LO_x/GH₂ case essentially fulfilling the stability condition while Ψ < 1 in the LO_x/GCH₄ case. In this last situation the flame is thicker than the characteristic thickness h_s and it is therefore sensitive to the high speed methane stream. Anchoring is imperfect and the flame edge moves with the turbulent eddies shed from the lip. Global stabilization is achieved dynamically but the reactive layer is not well established and the large amplitude motion of the edge is a symptom of a possible lift-off. Theoretical estimates indicate that LO_x/GCH₄ flame stabilization requires a thicker lip size than the LO_x/GH₂ propellant couple.     

Acoustic disturbance from gas non-uniformities convected through a nozzle

The non-steady flow generated by convection of gas containing non-uniform temperature regions or “entropy spots” through a nozzle is examined analytically as a source of acoustic disturbance. The first portion of the investigation treats the “compact nozzle”, the case where all wave lengths are much longer than the nozzle. Strengths of transmitted and reflected one-dimensional waves are given for supersonic and subsonic nozzles and for one configuration of supersonic nozzle with normal shock at the outlet. In addition to a wave reflected from the nozzle inlet, the supersonic nozzle discharges two waves, one facing upstream and the other facing downstream. For reasonable values of the nozzle inlet Mach number, the pressure amplitude of each wave increases directly as the discharge Mach number.The acoustic perturbations from a supercritical nozzle of finite length, in which the undisturbed gas velocity increases linearly through the nozzle, are analyzed for several inlet and discharge Mach number values and over a wide frequency range. The results which agree with the compact analysis for low frequency, deviate considerably as the frequency rises, achieving pressure fluctuation levels of several times the compact values. It is shown that this result originates in a phase shift between the two waves emitted downstream and that the pressure fluctuations for moderate frequencies may be approximated from the compact analysis with an appropriate phase shift.In all cases, the pressure fluctuations caused by a 2% fluctuation in absolute inlet temperature are large enough to require consideration in acoustic analysis of nozzles or turbine blade channels.     

A method for the transverse modulation of reactive flows with application to combustion instability

A method allowing the transverse excitation of non-reactive or reactive flows is described. The method relies on a characteristic wave modulation applied on the lateral sides of the computational domain. It is shown that this procedure can be used to induce a transverse sloshing motion in the region of interest. Two two-dimensional geometries are studied: in the first, the flow features one or two wakes embedded in a high-speed stream; the second configuration involves a premixed reactive jet flame. The excited flow structure calculated in this last case is found to be similar to that observed in an experiment carried out previously. As the simulations are performed in two dimensions, they cannot describe many of the processes taking place in a turbulent flow. They are, however, valuable when the flow is dominated by a large-scale organized motion induced by a transverse acoustic modulation. The present calculations indicate that the method in combination with a large eddy simulation flow solver could be used to study combustion response to transverse acoustic perturbations. With additional developments this might be used to study liquid propellant rocket motor instabilities coupled by transverse acoustic modes in the high-frequency range.     

An experimental study into the effect of injector pressure loss on self-sustained combustion instabilities in a swirled spray burner

Combustion instabilities depend on a variety of parameters and operating conditions. It is known, especially in the field of liquid rocket propulsion, that the pressure loss of an injector has an effect on its dynamics and on the coupling between the combustion chamber and the fuel manifold. However, its influence is not well documented in the technical literature dealing with gas turbine combustion dynamics. Effects of changes in this key design parameter are investigated in the present article by testing different swirlers at constant thermal power on a broad range of injection velocities in a well controlled laboratory scale single injector swirled combustor using liquid fuel. The objective is to study the impact of injection pressure losses on the occurrence and level of combustion instabilities by making use of a set of injectors having nearly the same outlet velocity profiles, the same swirl number and that establish flames that are essentially identical in shape. It is found that combustion oscillations appear on a wider range of operating conditions for injectors with the highest pressure loss, but that the pressure fluctuations caused by thermoacoustic oscillations are greatest when the injector head loss is low. Four types of instabilities coupled by two modes may be distinguished: the first group features a lower frequency, arises when the injector pressure loss is low and corresponds to a weakly coupled chamber-plenum mode. The second group appears in the form of a constant amplitude limit cycle, or as bursts at a slightly higher frequency and is coupled by a chamber mode. Spontaneous switching between these two types of instabilities is also observed in a narrow domain.     

Detailed measurements of equivalence ratio modulations in premixed flames using laser Rayleigh scattering and absorption spectroscopy

Equivalence ratio non-uniformities may give rise to some of the instabilities observed in modern lean premixed combustion systems. The present work intends to investigate the influence of equivalence ratio perturbations on the dynamics of premixed flames. A burner equipped with a secondary injection system is used to generate equivalence ratio perturbations which are convected by the flow and impinge on a conical flame. Two laser-diagnostics, based on Rayleigh scattering and hydrocarbon infrared absorption, respectively, are employed to give insight into the spatial and temporal evolution of the mixture composition field. Rayleigh scattering images also reveal the flame front dynamics providing an indication on the response of a weakly turbulent flame subject to mixture composition inhomogeneities. Laser light absorption provides a time resolved signal which is used to estimate the equivalence ratio perturbation level. A theoretical model based on the G-equation is used to interpret the experimental data and compare the relative effects of velocity and equivalence ratio perturbations.     

Gromov's Centralizer Theorem

We study the properties of rigid geometric structures and their relation with those of finite type. The main result proves that for a noncompact simple Lie group G acting analytically on a manifold M preserving a finite volume and either a connection or a geometric structure of finite type there is a nontrivial space of globally defined Killing vector fields on the universal cover that centralize the action of G. Several appplications of this result are provided.