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|Title: ||Combustion, NOx formation and mixing processes in Helmholtz pulse combustors|
|Authors: ||Williams, Timothy C.|
|Issue Date: ||2000|
|Publisher: ||© T.C. Williams|
|Abstract: ||This thesis presents a laser diagnostic investigation into the combustion, NOx
formation and mixing processes occurring within the optically assessed combustion
chamber of a methane-fired (10kW), fully premixed, self-aspirating, Helmholtz pulse
combustor. The inlet geometry of the combustion chamber consisted of a step
expansion and a bluff body obstacle formed by a stagnation plate. The focus of the
investigation was the effects of the stream-wise position of the stagnation plate on the
pulse combustion processes.
A comprehensive parametric study of the performance of the pulse combustor is
presented with stagnation plate position, air/fuel ratio and tailpipe length as the
variables. The operating frequency and peak pressure amplitude trends were found to
vary in accordance with the Rayleigh criterion. The operation of the combustor was
more stable with the effective heat-release point preceding the resonant acoustic peak.
Operation outside of this regime produce increased levels of CO.
Time-resolved, laser-sheet flow visualisation images are presented of the flow
structures within the combustion chamber. The inlet mixing - between the reactants
and residual gases - was dominated by the formation of two counter-rotating toroidal
vortices. In general, the inlet mixing was found to decrease as the stagnation plate was
moved further into the combustion chamber. However, other mechanisms that tended
to counter this trend were observed. Under certain conditions, significant flow
reversals were imaged with gases penetrating the combustion chamber from the
The combustion event was investigated using cycle-resolved chemiluminescence and
laser induced fluorescence imaging of OH* radicals. Ignition of the fresh reactants by
residual combustion/radical activity was found to occur along the interface between
reactants and residual gases. The increase in reaction zone area generated by the
action of the toroidal vortices provided the necessary mechanism for the rapid
combustion of the reactants.
The reduced mixing associated with moving the stagnation plate further into the
combustion chamber produced a more compact combustion zone with less interaction
between combusting reactants and cooler residual gases. This modification to the
combustion zone was consistent with the measured trends of rising NOx tailpipe
emissions and decreasing N02/NOx ratio.
Under certain conditions, a reversal in the NOx and N02/NO, ratio trends was
observed. This was explained by an augmentation of heat transfer rate out of the
combustion chamber, characterised by increased flow reversal strength, which lead to
cooler residual gases. Additional mechanisms,· which modified the inlet mixing
process, were also identified as contributing to the reversal of the NOx trends.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Mechanical, Electrical and Manufacturing Engineering)|
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