Thesis
Effects of hydrogen addition on soot formation in flames : experimental investigation by laser imaging
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2026
- Thesis identifier
- T18122
- Person Identifier (Local)
- 202286176
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Soot emissions from combustion processes have adverse effects on human health and the environment. As a result, much attention has been paid to the reduction of soot in combustion systems. Hydrogen is of increasing interest as an energy carrier for combustion applications, including its use in blends with carbon-containing fuels. Since hydrogen addition may alter the formation of soot particles and their precursors, depending on the operating conditions, it is important to understand its influence on soot nucleation and growth. In this work, the effects of hydrogen addition were studied in an atmospheric-pressure, ethylene-air, premixed flat flame over a range of carbon-to-oxygen (C/O) ratios, spanning from the nucleation condition to highly sooting conditions (C/O = 0.59 - 0.79). Hydrogen-doped flames were obtained by replacing a portion of the premixed gas with hydrogen, thus maintaining a constant C/O ratio and the total molar flow rate of the feed gas. Laser-induced incandescence (LII) experiments have been carried out to determine how hydrogen affects soot concentration and LII decay times (indicative of primary particle size). In the C/O-ratio measurements, which were carried out at a fixed height above burner (HAB) of 16 mm, hydrogen addition was found to significantly reduce soot concentration and LII decay times in all the flames studied, with the extent of reduction being highly dependent on the C/O ratio. The inhibitory effect of hydrogen on soot concentration and LII decay time was found to increase sharply with increasing C/O ratio from the nucleation condition to C/O = 0.63, before declining more gradually thereafter. Another interesting observation is that the C/O ratio at which the threshold of nucleation was observed in the hydrogen-free flame was found to be only slightly sensitive to the addition of hydrogen, shifting from 0.59 to 0.60. Height-scan measurements were also carried out in the highly sooting flame (C/O = 0.79) and a sparingly sooting flame (C/O = 0.69), in which a reduction in soot concentration and LII decay times was observed at all heights, with the extent of reduction being higher at lower HABs. These observations naturally lead to examining how fluorescence associated with polycyclic aromatic hydrocarbons (PAHs) and related structures responds under corresponding flame conditions. Some of these structures may be linked to soot nucleation. Laser-induced fluorescence (LIF) measurements were therefore carried out using three excitation wavelengths to determine whether the effect of H2 addition on the fluorescence signal reflects the corresponding reduction in soot concentration. The LIF measurements were radiometrically calibrated. This is an important feature of the present work, since the spectra are corrected for the wavelength-dependent response of the detection system and reported on a quantitatively reproducible basis. Contrary to the reduction observed in soot concentration, hydrogen addition increased the LIF signal at all three excitation wavelengths in the fixed-height (16 mm HAB) C/O-scan measurements. In the height-scan measurements, hydrogen addition increased the LIF signal at all heights in the nucleation flame (C/O=0.59), whereas the response in the rich flame (C/O=0.79) was height-dependent, with reductions in LIF signal observed only at lower HABs. Since PAH-related structures are considered to play important roles in soot nucleation and growth, it is therefore surprising that they should be more abundant in the H2-doped flames, despite the corresponding reduction in soot volume fraction observed by LII. Principal component analysis (PCA) was applied to the LIF spectra to identify the dominant modes of spectral variation, which provided a basis for extracting approximate spectral components separately for each excitation wavelength. For 266 and 355 nm excitation, the measured spectra were reproduced well using an approximate UV–visible component and a visible component, whereas the 532 nm spectra were adequately represented by a visible component alone. Fluorescence contributions in the visible emission region have recently been discussed in relation to PAH-related structures potentially involved in soot nucleation, such as resonance-stabilised radicals and aliphatically bridged dimers of PAHs. A notable finding of the present work is that the extracted visible components from all three excitation wavelengths follow very similar trends with C/O ratio, with broadly similar increases in signal following hydrogen addition. These results suggest that the visible components may arise either from the same underlying emitting population or from different contributions that track each other very closely.
- Advisor / supervisor
- Burns, Iain
- Resource Type
- DOI
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