Two-dimensional temperature field imaging in laminar sooting flames using a two-line Kr PLIF approach

A two-line Kr PLIF approach is presented for thermometry in moderately sooting flames. This technique leverages the spectral line-broadening phenomenon to choose the two excitation wavelengths whose Kr PLIF signal ratio effectively cancels out the composition dependence while retaining the temperature dependence. Furthermore, the Kr PLIF ratio for the chosen wavelengths also exhibits a monotonic trend with temperature, and span a wide range of values to ensure adequate dynamic range on the measurements. The technique is evaluated in the near field of an ethylene laminar jet flame where the peak soot loading was about $0.15ppm$. Krypton gas was added in small amounts to both fuel mixture and air co-flow. Comparing the Kr PLIF fields with the LII fields showed that the main source of interference to Kr PLIF signal is from the soot interference, which contributed to a maximum of $20-50\%$ of the total signal at different axial locations. Interestingly, the interference from PAH molecules was observed to be less than $1\%$ of the total signal. The soot interference was retained during data processing to obtain an evaluation of the measurement uncertainty caused by the soot interference and the maximum soot loading that could be tolerated. The temperature in the regions away from soot layers exhibit very consistent values with literature, where the value extended from close to $300K$ in the fuel core and air co-flow through about $2200K$ in the reaction zone. The presence of soot, however, caused a noticeable depreciation in the measured temperature by about $200K$ at the peak sooting location. It is further noted that the mean systematic error of $50K$ is expected at $f_v=60ppb$. This limit is observed to be a strong function of the fractional contribution of the soot interference to the overall signal and can be substantially extended by subtracting the soot interference and using higher excitation energies.