Effects of nonuniform reactant stoichiometry on thermoacoustic instability in a lean-premixed gas turbine combustor

Detailed experimental investigations of the amplitude dependence of flame describing functions (FDF) were performed using a stratified swirl-stabilized combustor, in order to understand the combustion-acoustic interactions of CH4/air flames propagating into nonhomogeneous reactant stoichiometry. Phase-synchronized OH planar laser induced fluorescence (OH PLIF) measurements were used to investigate local reaction zone structures of forced flames. To determine the amplitude-and frequency-dependent forced flame response, simultaneous measurements of inlet velocity and heat release rate oscillations were made using a constant temperature anemometer and photomultiplier tubes with narrow-band OH*/CH* interference filters. The measurements were made over a wide range of stratification ratios, including inner stream enrichment ( θ o>θ i) and outer stream enrichment ( θ o>θ i)) conditions, and compared to the baseline condition of spatially and temporally homogeneous cases ( θ o=θ i)). Results show that for the inlet conditions investigated, fuel stratification has a significant influence on local and global flame structures of unforced and forced flames. Under stratified conditions, length scales of local contours were found to be much larger than the homogeneous case due to high kinematic viscosities associated with high temperature. Stratification has a remarkable effect on flame-vortex interactions when the flame is subjected to high-amplitude acoustic forcing, leading to different evolution patterns of FDF (amplitude and disturbance convective time) in response to the amplitude of the imposed inlet velocity oscillation. The present experimental investigation reveals that intentional stratification has the potential to eliminate or suppress the occurrence of detrimental combustion instability problems in lean-premixed gas turbine combustion systems. © 2012 Copyright Taylor and Francis Group, LLC.

UV photoconductivity characteristics of ZnO nanowire field effect transistor treated by proton irradiation

We investigated the UV photoconductivity characteristics of ZnO nanowire field effect transistors (FETs) irradiated by proton beams. After proton beam irradiation (using a beam energy of 10 MeV and a fluence of 10 12 cm -2), the drain current and carrier density in the ZnO nanowire FETs decreased, and the threshold voltage shifted to the positive gate bias direction due to the creation of interface traps at the SiO 2/ZnO nanowire interface by the proton beam. The interface traps produced a higher surface barrier potential and a larger depletion region at the ZnO nanowire surface, affecting the photoconductivity and its decay time. The UV photoconductivity of the proton-irradiated ZnO nanowire FETs was higher and more prolonged than that of the pristine ZnO nanowire FETs. The results extend our understanding of the UV photoconductivity characteristics of ZnO nanowire devices and other materials when irradiated with highly energetic particles. © 2012 Elsevier B.V. All rights reserved.

Effects of nonuniform reactant stoichiometry on combustion instability

This paper analyzes the forced response of swirl-stabilized lean-premixed flames to acoustic forcing in a laboratory-scale stratified burner. The double-swirler, double-channel annular burner was specially designed to generate acoustic velocity oscillations and radial fuel stratification at the inlet of the combustion chamber. Temporal oscillations of equivalence ratio along the axial direction are dissipated over a long distance, and therefore the effects of time-varying fuel/air ratio on the flame response are not considered. Simultaneous measurements of inlet velocity and heat release rate oscillations were made using a hot wire anemometer and photomultiplier tubes with narrowband OH*/CH* interference filters. Time-averaged CH* chemiluminescence intensities were measured using an intensified CCD camera. Results show that flame stabilization mechanisms vary depending on stratification ratio for a constant global equivalence ratio. For a uniformly premixed condition, an enveloped M-shaped flame is observed. For stratified conditions, however, a dihedral V-flame and a detached flame are developed for outer stream and inner stream fuel enrichment cases, respectively. Flame transfer function (FTF) measurement results indicate that a V-shaped flame tends to damp incident flow oscillations, while a detached flame acts as a strong amplifier relative to the uniformly premixed condition. The phase difference of FTF increases in the presence of stratification. More importantly, the dynamic characteristics obtained from the forced stratified flame measurements are well correlated with unsteady flame behavior under limit-cycle pressure oscillations. The results presented in this paper provide insight into the impact of nonuniform reactant stoichiometry on combustion instabilities, which has not been well explored to date. Copyright © 2011 by ASME.

Shock Propagation and MPT Noise From a Transonic Rotor in Nonuniform Flow

One of the major challenges in high-speed fan stages used in compact, embedded propulsion systems is inlet distortion noise. A body-force-based approach for the prediction of multiple-pure-tone (MPT) noise was previously introduced and validated. In this paper, it is employed with the objective of quantifying the effects of nonuniform flow on the generation and propagation of MPT noise. First-of-their-kind back-to-back coupled aero-acoustic computations were carried out using the new approach for conventional and serpentine inlets. Both inlets delivered flow to the same NASA/GE R4 fan rotor at equal corrected mass flow rates. Although the source strength at the fan is increased by 38 dB in sound power level due to the nonuniform inflow, far-field noise for the serpentine inlet duct is increased on average by only 3.1 dBA overall sound pressure level in the forward arc. This is due to the redistribution of acoustic energy to frequencies below 11 times the shaft frequency and the apparent cut-off of tones at higher frequencies including blade-passing tones. The circumferential extent of the inlet swirl distortion at the fan was found to be two blade pitches, or 1/11th of the circumference, suggesting a relationship between the circumferential extent of the inlet distortion and the apparent cut-off frequency perceived in the far field. A first-principles-based model of the generation of shock waves from a transonic rotor in nonuniform flow showed that the effects of nonuniform flow on acoustic wave propagation, which cannot be captured by the simplified model, are more dominant than those of inlet flow distortion on source noise. It demonstrated that nonlinear, coupled aerodynamic and aero-acoustic computations, such as those presented in this paper, are necessary to assess the propagation through nonuniform mean flow. A parametric study of serpentine inlet designs is underway to quantify these propagation effects. © 2013 American Society of Mechanical Engineers.

Asymptotic stability for time-variant systems and observability: Uniform and nonuniform criteria

This paper presents some new criteria for uniform and nonuniform asymptotic stability of equilibria for time-variant differential equations and this within a Lyapunov approach. The stability criteria are formulated in terms of certain observability conditions with the output derived from the Lyapunov function. For some classes of systems, this system theoretic interpretation proves to be fruitful since - after establishing the invariance of observability under output injection - this enables us to check the stability criteria on a simpler system. This procedure is illustrated for some classical examples.