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The free and forced response of a propane/air diffusion flame

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Jumppanen, Paavo (1993) The free and forced response of a propane/air diffusion flame. Unspecified thesis, University of Tasmania.

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Abstract

An experimental investigation into the relationship between acoustic forcing and
coherent structures in propane/air diffusion flames is presented. The relationship
between acoustic forcing and coherent structures is established with the view to
applying controlled acoustic feedback to improve mixing rates, and hence
combustion efficiency. Coherent structures are sensed using the laser Schlieren
technique, and the detector signals are correlated with the acoustic pressure signal
(excitation component) under a variety of excitation conditions.
Impulse response estimates for the acoustic pressure to Schlieren response transfer
function are obtained by a PRBS based cross correlation technique. The flow
response waveforms were processed further using minimum mean square error
identification and homomorphic deconvolution to resolve the propagation delay of
the large scale structures. By combining simple excitation waveforms and the
technique of signal averaging it is possible to identify, at least in a qualitative sense,
some of the non-linear characteristics of the flow process. Impulsive, square wave,
pseudo-noise and sinusoidal signals were used to excite the flow at varying levels of
excitation. The results obtained by each method were consistent with a frequency
selective saturation mechanism with the knee of saturation occurring at an overall
sound pressure level of around 60 dB. These observations appeared to be consistent
with velocity induced vortex shedding caused by the acoustic characteristics of the
combustor. Mathematical modelling of the acoustic characteristics of the combustor
demonstrated the validity of the velocity induced vortex shedding hypothesis. Application of Schlieren feedback significantly altered the flow structures within the
diffusion flame. Schlieren feedback results demonstrated a large reduction in small
scale turbulence and a concentration of flow energy into the coherent resonant
structures. The frequency of resonance of the flow with feedback is dependent upon
the position of the laser Schlieren detector relative to the nozzle exit plane.
Centre-line temperature profiles show that both acoustic forcing and Schlieren
acoustic feedback result in improved mixing rates. In the main it seems that the
outer structure has greatest effect upon mixing rates as excitation at low frequencies
gave the greatest increase in centre line temperature. Inner structure forcing and
feedback also results in improved mixing although the temperature improvement is
not as large.

Item Type: Thesis (Unspecified)
Keywords: Combustion gases
Copyright Holders: The Author
Additional Information:

Thesis (M.Eng. Sc.)--University of Tasmania, 1994. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:32
Last Modified: 11 Mar 2016 05:55
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