Flame—wall interaction in diesel engine conditions

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Copyright: Fattah, Islam Md Rizwanul
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Abstract
This dissertation reports on the improved fundamental understanding of the flame—wall interaction to achieve greater soot emission reduction in diesel engine conditions. Three specific aims addressed in this dissertation were: (1) flame—wall interaction on combustion and soot processes, (2) flame—wall interaction on multiple-injection and (3) applicability of flame diagnostics methods in the presence of flame—wall interaction. For this work experiments were performed in a constant-volume combustion chamber which can mimic diesel engine conditions without associated drawbacks viz. complex flow field, non-stationary nature, high turbulence, and strong interactions etc. and facilitates the visualization of spray flame. Four optical diagnostic techniques were used for this work to obtain liquid and combusting phase information for the test cases viz. Mie-scattering, diffused-background illumination, OH-chemiluminescence, and high-speed natural flame luminosity imaging. Two-colour pyrometry and colour-ratio pyrometry techniques were used to provide further soot temperature and soot KL information. To assess the influence of wall impingement on the combustion and soot processes of the jet, experimental measurements were performed with impinging and free flames at identical ambient and injector conditions of 20.8 kg/m3 ambient density, 6 MPa ambient pressure, 1000K bulk temperature, 15 and 10 vol.% ambient O2 concentration, and 100 MPa injection pressure. To simulate flame—wall impingement, a flat plane steel wall, normal to the injector axis, was placed at 53 mm and 35 mm from the nozzle to study the effect of wall distance variation. It was found that wall impingement resulted in lower soot temperature and soot content compared to that of free flame. The results also revealed that decreasing impingement distance from the nozzle result in reduced soot temperature and soot level for the wall jet. To study the effect of multiple-injection on the flame—wall impingement on the combustion and soot processes, post-injection strategy was implemented, which is a multiple-injection strategy that is commonly used as a particulate matter control measure to reduce soot emissions. Similar ambient and injector conditions of 20.8 kg/m3 ambient density, 6 MPa ambient pressure, 1000 K bulk temperature, 15 vol.% ambient O2 concentration, and 100 MPa injection pressure were used for comparing the characteristics of impinging and free flames. A mass ratio of 80%−20% was maintained between the main and post-injections, with the dwell time between the injections varied from 1.5 to 2.5 ms. The results indicated, presence of wall affects the post-injection combustion process with varying effect depending on dwell time. A shorter-dwell post injection was found to be more conducive to soot reduction than that of a longer-dwell post injection. Analysis also reveals that, an optimal post-injection duration in conjunction with a shorter dwell time would effectively suppress the soot from main injection. A preceding study with post-injection was performed with biodiesel without the presence of impinging wall at ambient density, bulk temperatures, and oxygen concentration of 19.4kg/m3, 900 K and 1000 K, and 15 vol% O2, respectively. It was found that the ignition delays, ignition locations, and flame lift-off lengths of the post-injection flames are consistently shorter than those of the main injections. The two-colour pyrometry data revealed a greater interaction between the main and post injections resulted in more rapid development of the soot zone of the post-injection with higher temperature after ignition. The distribution of the most probable soot concentration factors of the post-injection was also found to be narrower, with lower soot content values. To implement the colour-ratio pyrometry (CRP) method, flame—wall interaction images in ‘semi-confined’ geometry were examined with variable and prescribed soot content (KL) outputs and results were compared with that obtained using the two-colour pyrometry (TCP) technique. Similar ambient and injector conditions of 20.8 kg/m3, 6 MPa, 1000 K, 15 vol.% O2, and 100 MPa injection pressure were also used here. Good correlation in the result trends was observed for the CRP method with fixed KL output and that generated using TCP. Slight discrepancies in the predicted absolute values were observed, which were linked to the difference in the KL value prescribed to the CRP method, and that predicted using TCP. No useful output was obtained with CRP method with variable soot output because of channel noise. A simplified flame transparency modelling was performed to assess the inherent errors associated with the pyrometry methods. The results indicated that the uncertainties arising from the fixing of the KL output appeared acceptable.
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Author(s)
Fattah, Islam Md Rizwanul
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Chan, Qing Nian
Kook, Sanghoon
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Publication Year
2019
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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