James R. Dawson

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.

The formation of turbulent vortex rings by synthetic jets

An investigation is made into the mechanism of pinch-off for turbulent vortex rings formed by a synthetic jet using time resolved particle image velocimetry measurements in air. During formation, measurements of the material acceleration field show a trailing pressure maximum (TPM) forms behind the vortex core. The adverse pressure gradient behind this TPM inhibits vorticity transport into the ring and the TPM is spatially coincident with the termination of vorticity flux into a control volume moving with the ring. A Lagrangian Coherent Structures (LCS) analysis is shown to be in agreement with the role of the TPM in pinch-off and in identifying the vortex ring before separation. The LCS analysis provides physical insights which form the basis of a revised model of pinch-off, based on kinematics, which predicts the time of formation (formation number) well for the present dataset. The delivery of impulse to the vortex ring is also considered. Two equally important mechanisms are shown to play a role: a ma...

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

Dr Timothy Bruce Nickels (1966–2010)

![Figure][1] On Saturday 2nd October, at around 16.00 h, the fluid mechanics community lost one of its most valued members: Tim Nickels. Tim published widely in the fields of vortex dynamics and turbulence, but it is for his beautiful experiments that he will be best remembered. He was,