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Title: High speed chemical species tomography for advanced fuels and engines
Author: Tsekenis, Stylianos-Alexios
ISNI:       0000 0004 5352 8108
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2014
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Current research in CI combustion aims to reduce PM and NOx emissions by controlling mixture homogeneity. Low CN fuels are suitable due to their auto-ignition resistance, but the in-cylinder mixture stratification level must be carefully visualised and controlled. Numerous diagnostic techniques exist for imaging the in-cylinder hydrocarbon species concentration. Tomographic techniques based on spectroscopic modalities are minimally-intrusive and able to target species of interest even in multi-component fuel blends. The high-speed CST technique applied in this work is based on the NIRAT modality. A number of collimated LASER beams at 1700nm traverse the optically accessible engine combustion chamber and are spectroscopically absorbed by the first overtone of the C-H stretch bond. Non species-specific attenuation mechanisms are suppressed by a DWR scheme utilising a reference wavelength at 1651nm. Ratiometric data is used to tomographically reconstruct the spatially-varying fuel concentration. In this work the first application of NIRAT on a commercial CI engine is presented, using instrumentation capable of imaging 13 frames/CAD at 1200rpm using a 31-beam array. A novel method was developed to experimentally quantify the tomography system’s non-uniform spatial resolution. The method was applied in laboratory experiments involving free-space propane plumes and a map of the spatial resolution was created. The spatial resolution varies between 4mm and 14mm. The mean of 9mm is 72% better than previous estimates in the literature. Regions of poor performance correlated with non-uniformities in the sensitivity matrix, indicating that a regular beam array may contribute towards more accurate and objective reconstructions of unknown concentrations. The characterised tomography system was installed on an optically-accessible Volvo D5 CI engine. The optically-inaccessible CAD region achieved was ±18CAD, a reduction of ±12° from previous works. The vibration-tolerance of the optical access system was verified, concluding that the initial alignment of the beams is the dominant factor that determines beam integrity after prolonged engine operation. The behavior of individual beams was studied, finding strong cycle-to-cycle correlation of the anomalies present. This was exploited to develop a novel, robust analysis algorithm to process the engine data. The algorithm achieved a standard deviation of <10% of the maximum pk-pk magnitude of the transmission signal in the fuel vapour phase. The system was applied to qualitatively visualise the mixing of a 50/50% blend of iso-/n-dodecane in a motored, nitrogen-aspirated engine under a range of operating conditions. A study by simulation of the decomposition of n-dodecane concluded that only 0.492% of the quantity injected is pyrolytically converted during a compression stroke. Spray-phase imaging was not possible due to severe reduction of the optical throughput, lasting for 8-15 CAD for a lean mixture and for 15-30 CAD for a rich mixture. Vapour-phase reconstructions using the enhanced iterative Landweber algorithm were successful in resolving rich fuel pockets consistent with the injection pattern. It was shown that the degree of mixture homogeneity at TDC is dependent upon the initial intake temperature. PLIF was used to cross-validate the NIRAT reconstructions. Localisation of the features reconstructed with NIRAT was excellent, with a maximum angular deviation of ±10°. A swirl motion of the mixture by 1°/CAD was observed using both techniques, confirming the features previously observed in the NIRAT reconstructions. In conclusion, NIRAT has been, for the first time, successfully applied for in-cylinder fuel distribution imaging in a CI engine. The results, created using an original data analysis algorithm, were successfully cross-validated using PLIF. A novel spatial resolution quantification method was formulated and used to characterise the tomography system. The numerous findings and learning points from the individual stages of this work will be used to advance the field of combustion diagnostics as well as contribute towards the development of advanced in-cylinder tomographic imaging systems.
Supervisor: Ozanyan, Krikor Sponsor: Not available
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: NIRAT ; chemical species tomography ; combustion diagnostics ; limited-views tomography ; spatial resolution ; landweber ; PLIF