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Title: Development of advanced imaging based diagnostics for flame studies
Author: Wang, Yiran
ISNI:       0000 0004 7431 8476
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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As flame light emission is correlated to flame colour appearance, a unique image postprocessing method introduced in this thesis establishes a relationship between the multiple flame emission species (CH*, C2*, soot and soot precursor) and their corresponding colour signal presences. This method implements simultaneous visualisation and investigation of flame emission from the visible to infrared spectrum using a single image. The captured flame images are converted from the 'Red, Green and Blue (RGB)' to the 'Hue, Saturation and Value (HSV)' colour model space. Multiple emission species can be differentiated from the captured images according to their specific ranges from the hue domain. An infrared-emission-only region is found in between the visible flame chemiluminescence and yellow colour soot emission on the captured two-dimensional (2D) flame images. The water vapour, the carbon dioxide and the direct infrared light emission of polycyclic aromatic hydrocarbons (PAHs) are ruled out as the possible source of the detected infrared emission. However, the comparison with published data indicates that the captured infrared emission associated with soot precursor region, because it is found that the relative distribution of the captured infrared flame emission in both the co-flowing normal diffusion flames (NDFs) and inverse diffusion flames (IDFs) of our experiments coincide with the region of PAH resolved in the published laser experiment. The captured infrared is inferred from the direct infrared light emission of soot precursor which excludes PAHs. It may also attribute to the soot precursor which selectively scattering the soot infrared radiation. It is interesting to note that the ignition process starts with the development of a blue flame in the all of the case studies. Subsequently, the infrared soot precursor and visible yellow sooty flame simultaneously occur. This thesis attempts to investigate the flame ignition and propagation characteristics by applying various experimental diagnostic techniques, which include direct highspeed imaging, schlieren imaging, stereo imaging and two-colour flame temperature measurement techniques. The results have indicated that the ignition location, CO2 concentration and plate surface temperature have significant effects on the initial flame propagation properties, flame temperature and soot formation in the impinging flame ignition process. The flame temperature is evaluated with the help of a modified twocolour method, by measuring glowing thin SiC fibres that are positioned in the flow field. It is evident that soot production is susceptible to ignition location from the stages of ignition to a stable flame condition, and so are the active chemical species such as CH* and C2*. Enormous soot is accumulated during the ignition process when the flame is initiated from the nozzle exit. The increase of CO2 concentration significantly inhibits the soot formation and reduces the flame temperature. The wall quenching phenomenon takes place in each case due to the cool plate effect. The increase of plate surface temperature shortens the time span from flame ignition to a steady state whilst suppressing the soot formation. Furthermore, the flame temperature will increase when the equivalence ratio approaches unity. The absolute soot propagation velocity determination in three-dimensional (3D) space relies on the strength of the flame 3D reconstruction technique via the high-speed stereo imaging technique. The pixel correspondence issue between two sequential frames is resolved by the optical flow Lucas-Kanade algorithm. The correspondence of the 3D coordinate is obtained from its corresponding 2D mapping, thereby determining the instantaneous soot propagation speed. This method can also be utilised in wildfire studies.
Supervisor: Zhang, Yang ; He, Shuisheng Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available