Two dimensional rotational temperature measurement by multiline laser induced fluorescence of nitric oxide in combustion flame

Two-dimensional rotational temperature measurement was performed in a stable combustion flame of premixed butane and oxygen using multiline laser induced fluorescence (LIF) of nitric oxide molecules. Multiple rotational absorption lines of A(2) Sigma(+)<--X(2) Pi(0,0) Q(1) and Q(2) lines were excited by laser light around 226 nm, and the LIF signal was observed by an image-intensified digital camera. Temperature was determined through least squares fitting correlation between LIF intensity and excitation rotational quantum number for the Boltzmann distribution function. The measured LIF intensity was approximated by the Boltzmann distribution with good accuracy, and the temperature obtained was between 500 K and 1800 K for the test flame. The measuring error of the temperature was evaluated and found to be 80 K, which corresponded to 8% of the measured fluorescence intensity. The two-line LIF scheme was evaluated by different pairs of excitation lines (Q(1)(31.5)/Q(1)(16.5) and Q(1)(18.5)/Q(1)(16.5)) for comparison with the multiline LIF approach. Temperature which was obtained by two-line LIF scheme corresponded well with multiline LIF results for Q(1)(31.5)/Q(1)(16.5) excitation. However, for Q(1)(18.5)/Q(1)(16.5) excitation, the obtained temperature did not agree with the multiline LIF result because the population of rotational states J=18.5 and J=16.5 is similar at high temperatures. We found that two-line LIF temperature measurement was reliable when excitation lines were suitably selected.