Strategy for instantaneous formaldehyde (CH2O) imaging by planar laser-induced fluorescence (PLIF) in a scramjet with intense flame emissions

Abstract

Conducting CH2O PLIF imaging in scramjets is challenging due to strong flame emissions (FEs). The FE interference is comparable to the CH2O fluorescence in the wavelength range of the fluorescence. To deal with this challenge, we proposed a CH2O PLIF imaging strategy considering CH2O excitation at 353.06 nm and FE reduction based on capturing two frames with a reasonable time delay. The fluorescence efficiency with the 353.06 nm excitation was shown to be approximately four times that with the excitation of ∼355 nm in the linear regime, where the two wavelengths corresponded to full-width-at-half-maximum line widths of ∼0.16 and ∼1 cm−1, respectively. During the two frames being captured, the FE background remained nearly constant. Then collecting the CH2O fluorescence in the first frame and subtracting the second frame from the first substantially reduced FE interference. The signal-to-background ratio in the raw image was approximately 1–2, but it could be increased to 10 using the proposed strategy. This strategy enables CH2O PLIF imaging to be conducted in scramjets and scramjet-like combustion facilities with intense FE interference.