Ammonia has shown great potential as a carbon-free fuel, in particular for marine transportation and energy production. Its low laminar flame speed, and the tradeoff between ammonia slip and NOx emission, pose challenges for industrial applications, and a more in-depth understanding of the combustion of ammonia is therefore needed. Raman spectroscopy is a powerful diagnostic often employed to investigate turbulence chemistry interactions and resolve the thermo-chemical structure of hydrogen and hydrocarbon-air flames. However, this technique has been used extensively in hydrocarbons (HCs) and hydrogen flames, but no quantitative Raman spectroscopy is available for ammonia flames, despite the current interest in ammonia combustion. First, this work extends Raman spectroscopy to the instantaneous and spatially resolved measurement of major species concentrations and temperatures in ammonia flames. The lack of detailed ammonia spectra at high temperatures, the strong flame luminosity, and fluorescence interference are the major obstacles to the implementation of Raman spectroscopy to ammonia flames. This thesis introduces a novel approach to estimating the temperature dependence of the Raman signal and fluorescence interference contributions from a series of counterflow diffusion flames. Species concentrations and temperature profiles from measurements are shown, and their accuracy and precision are discussed. Next, this work obtains the first quantitative Raman measurements of temperature, mass fractions, and mixture fractions in two turbulent ammonia/hydrogen/nitrogen diffusion jet flames simulating 14% (CAJF14) and 28% (CAJF28) partial ammonia cracking ratio with Reynolds numbers of 11,200. The scalar structure in turbulent flames is examined using conditional mean and RMS radial profiles, scatterplots in mixture fraction space, as well as statistics conditioned on mixture fraction and physical spaces. Finally, the probability of localized extinction and the differential diffusion (diff-diff) effects are analyzed in two turbulent flames. Lastly, an improved Raman/Rayleigh system is introduced for ammonia combustion at atmospheric pressure, which enables the 1D simultaneous laser-induced fluorescence (LIF) measurements of the amidogen (NH2) radical and interference-free Raman/Rayleigh measurements of major species and temperature in non-premixed and premixed NH3/H2-air flames.

Abstract: Soot from combustion emissions has a negative impact on human health and the environment. Understanding and controlling soot formation is desirable to reduce this negative impact, especially as energy demands continue to increase. In this work, a range of fundamental combustion experiments are performed to better understand the soot formation process, and to develop diagnostics for measuring soot properties. First, studies on the effects of doping the flame with different polycyclic aromatic hydrocarbons (PAHs) was performed to investigate soot nucleation mechanisms. Soot formation was found to be most sensitive to phenylacetylene addition and nucleation through physical dimerization appears to be unlikely. Next, the effects of ammonia addition, a possible future fuel, on soot formation in laminar nonpremixed ethylene counterflow flames was performed. A reduction in soot volume fraction was observed and attributed to chemical effects of ammonia addition. Second, the investigation and development of several types of diagnostics was performed. Soot is typically reported to scale with pressure as Pn where P is pressure and n is a scaling factor. A wide range of scaling factors for ethylene coflow flames have been reported using different types of diagnostics. In this work, a comparison between a light extinction technique and PLII was performed and differences between reported values was explored. Next, the time resolved laser induced incandescence (TiRe-LII) diagnostic was advanced by exploring the effects of SVF on local gas heating. Errors introduced into this model by neglecting local gas heating are explored. Finally, a new diagnostic was developed for 3 dimensional measurements of SVF and velocity in turbulent flames using a technique known as diffuse-backlight illumination extinction imaging. Third, the application of gated 2D TiRe-LII was assessed in pressurized environments on laminar coflow flames. Comparisons between TiRe-LII and thermophoretically captured soot imaged by transmission electron microscopy (TEM) was performed. TiRe-LII was found to have reasonable agreement with TEM measurements if the SNR was high, but due to the large disparity in primary particle size in pressurized environments errors in 2D TiRe-LII can be significant.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22770号 / 工博第4769号 / 新制||工||1746(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 黒瀬 良一, 教授 中部 主敬, 教授 岩井 裕 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The utilization of ammonia as a fuel is a pragmatic approach to pave the way towards a low-carbon economy. Ammonia compromises almost 18 % of hydrogen by mass and accepted as one of the hydrogen combustion enablers with existing infrastructure for transportation and storage. From an environmental and sustainability standpoint, ammonia combustion is an attractive energy source with zero carbon dioxide emissions. However, from a practical point of view, the direct combustion of ammonia is not feasible due to the low reactive nature of ammonia. Due to the low combustion intensity, and the higher nitrogen oxide emission, ammonia was not fully investigated and there is still a lack of fundamental knowledge of ammonia combustion. In this thesis, the computational study of ammonia premixed flame characteristics under various hydrogen addition ratios and moderate or intense low oxygen dilution (MILD) conditions were investigated. Particularly, the heat release characteristics and dominant reaction pathways were analyzed. The analysis revealed that the peak of heat release for ammonia flame occurs near burned gas, which raises a question regarding the physics of this. Further analysis identified the dominant reaction pathways and the intermediate species (NH2 and OH), which are mainly produced in the downstream and back diffused to the leading edge and produce some heat in the low-temperature zone. To overcome low reactivity and poor combustion performance of pure ammonia mixture, the onboard ammonia decomposition to hydrogen and nitrogen followed by blending ammonia with hydrogen is a feasible approach to improve ammonia combustion intensity. With increasing hydrogen amount in the mixture, the enhancement of heat release occurs due to both transport and chemical effect of hydrogen. Another approach to mitigate the low reactive nature of ammonia may be eliminated by applying the promising combustion concept known as MILD combustion. The heat release characteristics and flame marker of ammonia turbulent premixed MILD combustion were investigated. The high fidelity numerical simulation was performed to answer fundamental questions of ammonia turbulent premixed combustion characteristics.