Electron beaming instabilities as sources of CME radio emissions

<p>Radio emissions accompanying coronal mass ejections (CMEs) from their flaring sources (type III bursts) to interplanetary shocks (type II bursts) are believed to originate in the electrostatic (ES) wave instabilities, which are excited by the electrons beaming along the intense magnetic fields. Theoretically, radio emissions of fundamental (plasma) frequency $\omega_{p}$ or the second harmonic $2 \omega_{p}$ may result from non-linear three waves interaction of electrostatic Langmuir and ion sound fluctuations. However, it is not clear yet what kind of electron beams and specific CME plasma conditions can determine destabilization of Langmuir waves (ion sound waves may result from non-linear decay). Recent attempts to identify and characterize these unstable regimes suggest very critical and limited conditions for Langmuir instabilities to develop, which may undermine our current understanding of their implication in nonlinear generation of radio waves. Thus, even for a dominance of ES instabilities, conditioned by high beaming velocities, Langmuir waves appear to be in close competition with other ES growing modes (such as electron acoustic instabilities), while for less energetic beams the theory predicts a strong interplay with instabilities of different nature (electromagnetic or hybrid, and propagating obliquely to the magnetic field). </p>

Thermo-Acoustic Analysis of a Realistic Liquid-Fueled GT Combustor

Thermo-acoustic instabilities are an important consideration in the design of modern power generation gas turbine combustors. While the design process must consider many competing requirements, such as temperature profiles, emissions, robustness to auto-ignition and flameholding, thermoacoustics is one of the most challenging to predict, and therefore design for. This is particularly true in the case of liquid-fueled systems, where the phenomenon results from a complex system of coupled multi-physics phenomena: fuel atomization and transport, mixing, reactive kinetics and acoustics. Nevertheless, emissions-compliant liquid fuel capability is becoming increasingly important to GT operators, thus it is critical to be able to predict the thermoacoustic instabilities of these combustors. In this work we present an approach to model the thermoacoustic feedback loop for a realistic liquid fuel nozzle in a single burner configuration. The approach is based on an analytical liquid-fuel diffusion flame model to provide the fluctuating heat release response to inflow perturbations. This is coupled with a 3D FEM description of the acoustic response of the single burner rig through a time-domain Green’s function model to predict the growth and saturation of pressure oscillations. The necessary flame model parameters are calibrated based on a range of test data obtained from the single burner rig with a tunable combustor length. The results are shown to compare well with test data across a range of operating conditions, and for two different nozzle geometries.

Analytic description of flame intrinsic instability in one-dimensional model of open–open combustors with ideal and non-ideal end boundaries

This paper is concerned with the theoretical study of thermo-acoustic instabilities in combustors and focuses upon recently discovered flame intrinsic modes. Here, a complete analytical description of the salient properties of intrinsic modes is provided for a linearized one-dimensional model of open–open combustors with temperature and cross-section jump across the flame taken into account. The standard [Formula: see text] model of heat release is adopted, where n is the interaction index and τ is the time lag. We build upon the recent key finding that for a closed–lopen combustor, on the neutral curve, the intrinsic mode frequencies become completely decoupled from the combustor parameters like cross-section jump, temperature jump and flame location. Here, we show that this remarkable decoupling phenomenon holds not only for closed–open combustors but also for all combustors with the ideal boundary conditions (i.e. closed–open, open–open and closed–closed). Making use of this decoupling phenomenon for the open–open combustors, we derive explicit analytic expressions for the neutral curve of intrinsic mode instability on the [Formula: see text] plane as well as for the linear growth/decay rate near the neutral curve taking into account temperature and cross-section jumps. The instability domain on the [Formula: see text] plane is shown to be qualitatively different from that of the closed–open combustor; in open–open combustors it is not confined for large τ. To find the instability domain and growth rate characteristics for non-ideal open–open boundaries the combustor end boundaries are perturbed and explicit analytical formulae derived and verified by numerics.