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Soot production in a tubular gas turbine combustor

By Qing-ping Zheng

Abstract

Soot production in gas turbine combustors is not desirable since it is the major source of exhaust smoke emission and its thermal radiation to the combustor liner deteriorates the liner durability. Soot formation involves comparatively slow chemistry and equilibrium can not be applied to soot modelling in the combustor flow field. . The exact sooting process in the combustor is poorly understood given both the complexity and the limited experimental data available. The work reported in this thesis seeks to first develop in-situ techniques for retrieving spatially-resolved soot properties, mainly soot particle volume fraction, from within the combustor and also to apply the measured results to comparisons with predicted soot concentrations. Two probing methods have been demonstrated which also incorporate a laser absorption technique. The sight probe proves to be more reliable in the present measurements. The evaluation of the physical probing techniques in sooty laboratory flames reveals that the flame structure will not be substantially distorted by the probe. The disturbance caused by the probe is localised, a feature which is evident in the reported water flow visualization test. The necessary inert gas purge can be minimised to reduce the local aerodynamic perturbation. The measured soot volume fraction distributions are comparable with sooting levels reported in flame studies in the literature. The peak soot volume fractions are located off-axis, characteristic of the fuel atornization. The measurementsin the primary zone are restricted by the multi-phase character of the flow, where soot absorption can not be readily discriminated from fuel droplet scattering. Measurements are reported over a range of air-fuel ratios, inlet pressures and temperatures. Time-averageds calard istributionsa t the nominald ilution sectionh ave beeno btained in addition to the soot measuremenut sing probe sampling and standard gas analysis. Correlationso f carbond ioxide with mixture fraction reveala clear relationshipa t overall lean conditionsc onsistenwt ith widely usedm odelleda ssumptions.T here are less well-correlated relationshipsb etweent emperaturea ndm ixture fraction, possiblyd ue to the influenceo f scalar fluctuationsa nda lsoo f the scalard issipationr ate. Sootl oadingi n the presentf low conditions is characteristicallylo w, basedo n the mixture fraction ands ootv olumef raction data. Thermal radiation in the visible spectrum shows a distinct narrow band spectra in addition to the soot continuum, which is believed to arise fromC2radical emission. The mean radiation intensities, predictedb y usingt he measuredte mperaturea nds ootc oncentrationre sults,a rei n generallo wer than the measured mean intensities. Temperature fluctuation levels may be particularly influential in some of these calculations. Sootm odellingi n the combustohr asb eenu ndertakenb y applyinga n extendedla minar flamelet concept. The two-equations oot formation model has beenp rimarily developedo n laminar flames. The comparisono f the computationa nd measuremenstu ggeststh at this soot model holds promise in the context of prediction in the combustor. In the absenceo f a satisfactoryt heoreticald escriptiono f the fuel-air burning in the combustor,w heret he liquid kerosinee mployedis replacedb y gaseoups ropane,t he computeds calarp rofiles are inconsistent in some importantr espectsw ith the measuredo nes. This exerts a major effect on the soot predictioni n terms of the quantitatived etail in the computationw, hich is howeverc rucial for the soot model development. The original flow field modelling needs to be improved for the purpose of further soot model refinement

Publisher: Cranfield University
Year: 1994
OAI identifier: oai:dspace.lib.cranfield.ac.uk:1826/3910
Provided by: Cranfield CERES

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