Although Large Eddy Simulation (LES) has demonstrated its potential for modelling the reaction in simple academic combustors, it is more computationally expensive than Reynolds Averaged Navier-Stokes (RANS) which has been used widely for industrial cases. The aim of this research is to employ LES at minimal grid resolution and computational resource requirements to capture the main characteristics of the reacting flows in a helicopter combustor and exhaust plume with the focus on NOx emissions. Test cases have been carried out to validate the current LES code for non- reacting jet, non-premixed combustion and unstructured grids. Despite the moderate grid refinement and simple chemistry models employed, the findings from these test cases have demonstrated good capabilities of the current LES to capture the mixing, flame and flow characteristics. In a farther test case, a key gas-phase chemical reaction selected for the helicopter exhaust plume modelling has also been tested. The validated LES code is then employed in the numerical study of the reaction in the helicopter combustor. The LES predictions in terms of the temperature and EINOx agree generally well with the combustor design, analytical solutions, previous LES and test measurements. Subsequently, the potential application of LES for the calibration of simpler models has been assessed for the generic and helicopter combustors. The results obtained from LES are compared with those from a one-dimensional combustor performance and emissions code, HEPHAESTUS, developed within the Cranfield University Power and Propulsion Department. The discrepancies between the results are found to be primarily due to specific simplification and assumptions established in the HEPHAESTUS model which can be addressed. Finally, LES has been employed to model the transformation of NO to NO2 in the helicopter exhaust plume. The findings from this research have demonstrated that, even without the implementation of highly dense mesh or advanced reaction model, LES is able to provide results with an acceptable level of fidelity at relatively low computational costs. These advantages make it a powerful predictive tool for future design and emissions optimisation investigations, and calibration of other simpler modelling approaches.

The effect of a nozzle’s internal geometry was studied experimentally to determine the breakup of the emitted water jet when it was injected perpendicularly into a quiescent atmosphere or a subsonic air crossflow. The nozzle’s diameter, nominal surface roughness, length-to-diameter ratio and contraction angle were varied, together with the injection pressure, to find the water column’s breakup length. Photographs of the water jet at the nozzle’s exit, gave a clue as to identify the occurrence of cavitation and a hydraulic flip. On the other hand the water column’s breakup length and trajectory, in a subsonic crossflow, were measured by using a stroboscope in conjunction with a high speed CCD camera. Results agreed with previous literature that the breakup length grew with greater liquid/air momentum flux ratios for non-cavitating flows. This was true regardless of the injector nozzle. The rate of increase decreased at the inception of cavitation. On the other hand even shorter breakup lengths were observed at the inception of a hydraulic flip due to the detachment of the water jet from the internal surface of the nozzle. Increasing the nozzle’s length-to-diameter ratio eliminated the occurrence of hydraulic flip. The jet’s trajectory was correlated with the liquid/air momentum flux ratio and the nozzle’s exit diameter. The results showed that higher water jet trajectories were measured under non-cavitating conditions. Even shorter jet trajectories were measured at the inception of a hydraulic flip.

No description available.

The thermal oxidative stability of middle distillate fuels is a topic of considerable concern. There are several examples of ambient temperature oxidation of fuel, leading to particulate matter and filtration issues. It is shown that particulate matter values vary globally based on region and fuel type, suggesting the problem is more than mere inorganic matter. The variability of filtration times is not dependent on absolute particulate matter present; it is suggested to be dependent upon the nature or morphology of deposit. <br>For a more thorough understanding of the chemistry responsible for deposit formation, flask oxidation was employed to test the Soluble Macromolecular Oxidatively Reactive Species (SMORS) mechanism. Spectral data suggest the presence of alcoholic and carbonylic functionality, which is in agreement with how the SMORS mechanism defines deposit formation. It has also been determined that the introduction of compounds conceivably indigenous to jet fuels has a negative impact on deposit formation. In addition, it has been shown the elemental composition of thermally induced deposit entails significant heteroatom content. <Br>According to the SMORS mechanism, one of the primary reasons for deposit formation is the presence of radical initiators. The paraffinic blending of fuels shows promise in oxidatively stabilizing jet fuels. Research suggests blending reduces oxidation by diluting both the radical initiators and soluble deposit precursors. It is possible the use of this method could improve filter life and decrease operational costs. <br>A better understanding of the chemistry of deposit formation can lead to improved deposit inhibitors. Additives that have shown promise in bomb tubing studies were tested using flask oxidation. Additionally, extracted fuel polars reintroduced into the fuel at 0.3% v/v were tested for antioxidative activity. It was concluded the introduction of ppm levels of polar compounds extracted from fuel back into a fuel is very successful in limiting oxidative product formation. <br>One strategy for inhibiting deposit formation is the use of compounds that can act as oxygen/hydroperoxide scavengers. A linear free energy Hammett plot was developed for the reaction between molecular oxygen and triarylphosphines. Results indicate a very small positive charge buildup, suggesting a nonsynchronous concerted reaction. / Bayer School of Natural and Environmental Sciences / Chemistry and Biochemistry / PhD / Dissertation

Enhancing the performance of mixing and fluid entrainment by excitation of quasi-steady jets has been a subject of research for more than three decades. During the 1980s a special emphasis was placed on mechanically oscillating planar jets and the possibility to augment thrust of V/STOL aircraft. However, during this time, little attention was paid to the classification of flow regimes, the development of coherent structures or the existence of different regions in the flow within the jet near field. For the present study, a large aspect ratio nozzle was oscillated in the direction transverse to the width of the nozzle in simple harmonic motion. For a constant nozzle height, the stroke length, oscillation frequency and jet velocity were systematically varied. Over 240 flow cases were examined using a novel method of phase-locked flow visualisation. Following an initial analysis of the acquired data, a small subset of flow conditions was selected for further quantitative investigation using Particle Image Velocimetry (PIV). The phase-locked flow visualisation led to the identification and classification of three separate flow regimes, the Base Flow, the Resonant Flow and the Bifurcation Flow Regimes. Each regime is linked to the other regimes by the presence of a small number of repetitive coherent structures in the form of starting and stopping vortices. The analysis revealed a relationship between the stroke-to-nozzle height ratio and the ratio of the forcing frequency to the natural vortex shedding frequency in the planar jet. This directly contradicts the relationship between the Strouhal and Reynolds numbers of the jet that was proposed by previous investigators. Comparison of phase-locked PIV and flow visualisation data confirms both, the validity of the new regime classification and the identification of relevant large-scale structures. Time-averaged vorticity data are also used to further illustrate the differences between the three flow regimes. Investigation of the time-averaged qualitative data for the Base and Resonant Flow Regimes show that three distinct flow regions exist within both regimes. Adjacent to the nozzle is the initial formation region, where all large-scale structures form. This is followed by a coherent near-field region in which the jet exhibits very little spread for both the Base and Resonant Flow Regimes. Within this region no pairing of the large-scale vortices from the opposing sides of the flow can be found. This region is followed by a transition region that is marked by the sudden breakup and dissipation of all visible large-scale coherent structures. The vortex formation distance is then investigated using the available PIV data and compared with the results of previous investigations. The data show that the formation distance depends on the jet velocity, oscillation frequency and the stroke length. The agreement with previous data is poor due to differences in the method of measurement. Quantitative data are also used to investigate the centreline velocity decay in relation to changes of the jet Reynolds number and stroke-to-nozzle height ratio. The results show that the velocity decay rate increases with increasing stroke length as is expected from findings of earlier studies. In addition the centreline velocity decay rates in the mean jet transition region appear to be constant for each stroke length in the cases examined. Finally, conclusions are drawn and recommendations for future work are presented. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1349701 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2009

Enhancing the performance of mixing and fluid entrainment by excitation of quasi-steady jets has been a subject of research for more than three decades. During the 1980s a special emphasis was placed on mechanically oscillating planar jets and the possibility to augment thrust of V/STOL aircraft. However, during this time, little attention was paid to the classification of flow regimes, the development of coherent structures or the existence of different regions in the flow within the jet near field. For the present study, a large aspect ratio nozzle was oscillated in the direction transverse to the width of the nozzle in simple harmonic motion. For a constant nozzle height, the stroke length, oscillation frequency and jet velocity were systematically varied. Over 240 flow cases were examined using a novel method of phase-locked flow visualisation. Following an initial analysis of the acquired data, a small subset of flow conditions was selected for further quantitative investigation using Particle Image Velocimetry (PIV). The phase-locked flow visualisation led to the identification and classification of three separate flow regimes, the Base Flow, the Resonant Flow and the Bifurcation Flow Regimes. Each regime is linked to the other regimes by the presence of a small number of repetitive coherent structures in the form of starting and stopping vortices. The analysis revealed a relationship between the stroke-to-nozzle height ratio and the ratio of the forcing frequency to the natural vortex shedding frequency in the planar jet. This directly contradicts the relationship between the Strouhal and Reynolds numbers of the jet that was proposed by previous investigators. Comparison of phase-locked PIV and flow visualisation data confirms both, the validity of the new regime classification and the identification of relevant large-scale structures. Time-averaged vorticity data are also used to further illustrate the differences between the three flow regimes. Investigation of the time-averaged qualitative data for the Base and Resonant Flow Regimes show that three distinct flow regions exist within both regimes. Adjacent to the nozzle is the initial formation region, where all large-scale structures form. This is followed by a coherent near-field region in which the jet exhibits very little spread for both the Base and Resonant Flow Regimes. Within this region no pairing of the large-scale vortices from the opposing sides of the flow can be found. This region is followed by a transition region that is marked by the sudden breakup and dissipation of all visible large-scale coherent structures. The vortex formation distance is then investigated using the available PIV data and compared with the results of previous investigations. The data show that the formation distance depends on the jet velocity, oscillation frequency and the stroke length. The agreement with previous data is poor due to differences in the method of measurement. Quantitative data are also used to investigate the centreline velocity decay in relation to changes of the jet Reynolds number and stroke-to-nozzle height ratio. The results show that the velocity decay rate increases with increasing stroke length as is expected from findings of earlier studies. In addition the centreline velocity decay rates in the mean jet transition region appear to be constant for each stroke length in the cases examined. Finally, conclusions are drawn and recommendations for future work are presented. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1349701 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2009

Thesis (M.S.)--Ohio University, November, 2002. / Title from PDF t.p. Includes bibliographical references (leaves 81-82).

Diss. (sammanfattning) Karlstad : Karlstads universitet, 2007. / Härtill 5 uppsatser.

Thesis--University of Minnesota. / Includes bibliographical references. Also issued in print.

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (leaves [256]-267 ).