Nicholas A. Worth

Tomographic reconstruction of OH* chemiluminescence in two interacting turbulent flames

The tomographic reconstruction of OH ∗ chemiluminescence was performed on two interacting turbulent premixed bluff-body stabilized flames under steady flow conditions and acoustic excitation. These measurements elucidate the complex three-dimensional (3D) vortex‐flame interactions which have previously not been accessible. The experiment was performed using a single camera and intensifier, with multiple views acquired by repositioning the camera, permitting calculation of the mean and phase-averaged volumetric OH ∗ distributions. The reconstructed flame structure and phase-averaged dynamics are compared with OH planar laser-induced fluorescence and flame surface density measurements for the first time. The volumetric data revealed that the large-scale vortex‐flame structures formed along the shear layers of each flame collide when the two flames meet, resulting in complex 3D flame structures in between the two flames. With a fairly simple experimental setup, it is shown that the tomographic reconstruction of OH ∗ chemiluminescence in forced flames is a powerful tool that can yield important physical insights into large-scale 3D flame dynamics that are important in combustion instability.

Some characteristics of thin shear layers in homogeneous turbulent flow

Tomographic particle image velocimetry measurements of homogeneous isotropic turbulence that have been made in a large mixing tank facility at Cambridge are analysed in order to characterize thin highly sheared regions that have been observed. The results indicate that such regions coincide with regions of high enstrophy, dissipation and stretching. Large velocity jumps are observed across the width of these regions. The thickness of the shear layers seems to scale with the Taylor microscale, as has been suggested previously. The results discussed here concentrate on examining individual realizations rather than statistics of these regions.

Experimental and Numerical Investigation into the Propagation of Entropy Waves

The research leading to these results received funding from the European Community’s Seventh Framework Programme (FP7/2007- 2013)under grant agreement no.265586 and was conducted within the Intelligent Design Methodologies for Low Pollutant Combustors for Aero-Engines project.

FLAME AND FLOW DYNAMICS OF A SELF-EXCITED, STANDING WAVE CIRCUMFERENTIAL INSTABILITY IN A MODEL ANNULAR GAS TURBINE COMBUSTOR

Azimuthal instabilities are prevalent in annular gas turbine combustors; these instabilities have been observed in industrial systems and research combustors, and have been predicted in simulations. Recent experiments in a model annular combustor have resulted in self-excited, circumferential instability modes at a variety of operating conditions. The instability mode “drifts” between standing and spinning waves, both clockwise and counter-clockwise rotating, during the course of operation. In this study, we analyze the flame response to standing wave modes by comparing the flame dynamics in a self-excited annular combustor with the flame dynamics in a single nozzle, transverse forcing rig. In the model annular combustor, differences in flame fluctuation have been observed at the node and anti-node of the standing pressure wave. Flames at the pressure anti-node display symmetric fluctuations, while flames at the pressure node execute asymmetric, flapping motions. This flame motion has been measured using both OH* chemiluminescence and planar laser induced fluorescence of OH radicals. To better understand these flame dynamics, the time-resolved velocity fields from a transverse forcing experiment are presented, and show that such a configuration can capture the symmetric and asymmetric disturbance fields at similar frequency ranges. Using high-speed PIV in multiple planes of the flow, it has been found that symmetric ring vortex shedding is driven by pressure fluctuations at the pressure antinode whereas helical vortex disturbances drive the asymmetric flame disturbances at pressure nodes. By comparing the results of these two experiments, we are able to more fully understand flame dynamics during self-excited combustion instability in annular combustion chambers. NOMENCLATURE

Experimental investigation of the response of premixed and non-premixed turbulent flames to acoustic forcing

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved. This paper describes an experimental investigation of acoustically forced turbulent bluff-body stabilised flames with swirl and with three different degrees of premixing: fully premixed, non-premixed with radial fuel injection and non-premixed with axial fuel injection flames. The flame was imaged using OH* chemiluminescence at 5 kHz. The heat release response of these flames to acoustic forcing at 160 Hz, which was the frequency that gave the maximum oscillation amplitude, was studied quantitatively with the calculation of the Nonlinear Flame Transfer Function (NFTF) for various forcing amplitudes, equivalence ratios, and air velocities. The post-processing analysis also consisted of phase-averaged OH* chemiluminescence images. It was found that non-premixed flames with radial fuel injection exhibited a much greater response to acoustic forcing, followed by premixed and non-premixed flames with axial fuel injection. In the premixed system, the magnitude of the heat release response was greater for higher air velocities, however the effect of equivalence ratio was more complex. Both non-premixed systems showed a reduced sensitivity to input air velocity and global equivalence ratio. Also, it was found that for the conditions studied, all three systems showed a nonlinear response. The qualitative analysis of the flame dynamics showed that in the premixed system the flame roll-up at the bluff body edge plays an important role in the flame response. In the non-premixed system with radial fuel injection, apart from the flame roll-up, the temporal variation in equivalence ratio constitutes an important phenomenon.

Experimental and numerical investigation into the propagation of entropy waves in a small-scale rig

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved. Entropy waves are an important source of indirect combustion noise and potentially contribute to the generation of thermoacoustic instabilities in gas-turbine combustors. Entropy fluctuations generated by unsteady combustion are known to disperse and diffuse as they convect towards the combustor exit. However, very few studies on the decay of entropy waves can be found in the literature, and therefore the main aim of this paper is to better characterise the propagation phenomenon of these waves using both experiments and numerical simulations. Entropy waves were experimentally investigated using a smallscale rig, based on a choked straight duct. Entropy fluctuations were generated by igniting fuel from a pulsed supply, enabling accurate control of the excitation. The fluctuating temperature was measured at several locations along the duct as well as the acoustic pressure, allowing us to characterise the propagation of entropy waves. Numerical simulations, based on a compressible code and LES approach, were also carried out in order to better understand the phenomena occurring during the propagation of such waves. In the LES entropy waves were generated by introducing a fluctuating temperature at the inlet of a straight duct. Several computations were performed varying different parameters including the frequency of excitation and bulk velocity of the flow. Both experimental and numerical results demonstrate that the amplitude of entropy wave fluctuations decays along the duct as a function of wave parameters and propagation distance. Results obtained in the present investigation can be directly exploited to improve the modelling of entropy wave dispersion in low-order acoustic network codes.