Further Investigation of Hydrogen-air Diffusion Flame Ignition Behaviour and Flame Colour

Hydrogen-air diffusion flame is commonly described as colourless. However, even in controlled laboratory conditions with careful imaging, the flame appears reddish. Although hydrogen flame colour was first described as "faint reddish-brown" by W. F. Barrett in 1872, the misconception about if the hydrogen flame colour is invisible still has not been well addressed until today.
In this research, the ignition process of hydrogen flame is captured by using schlieren imaging and high-speed imaging technique. The flame colour of soapy hydrogen bubble is yellow while of distilled water hydrogen bubble is more reddish. The hydrogen-air diffusion flame direct images are captured by a DSLR camera as well. Various co-flow conditions are applied and the colour of the flames are slightly different but generally red.
The spectra of hydrogen-air diffusion flame in the wavelength range of 590 nm to 700 nm are analysed in this research. Air and oxygen co-flow with different flow rate are applied as comparisons. The intense emission lines in this wavelength range are identified according to the HITRAN and the NIST database. The spectra of hydrogen diffusion flame under different co-flow conditions are similar. H2O and OH contribute significantly to the reddish flame colour, and some other elements from the equipment or atmosphere also have contributions. Furthermore, the results approved that H2O and OH provide more emission lines in the hydrogen flame with co-flows, which might be due to the higher flame temperature. Moreover, the emission line of H-α also presents in the spectra. Therefore, it is highly possible that H-α participates in the reaction and emits light in the combustion process of hydrogen diffusion flame.