Measurement of NO/sub 2/ concentration in combustion using fluorescence excited by an argon--ion laser

Preliminary results on the measurement of NO/sub 2/ in a premixed hydrogen-air flame are reported. A laser-excited fluorescence detector was constructed for the purpose. Measurements of the effective broadband fluorescence (6700 A to 7500 A) emission cross-sections excited by five Ar/sup +/ laser lines were carried out using NO/sub 2/-air mixtures in the 1 to 1000 ppM range of concentrations, under atmospheric pressure and temperature conditions. These yielded values on the order of 10/sup -25/ cm/sup 2//sr. Fluorescence intensity was linear with NO/sub 2/ concentration in the range 1 to 500 ppM at total pressure of 1 atm and room temperature. Measurement of fluorescence at atmospheric pressure but elevated temperatures up to 700/sup 0/K indicated almost no change in effective emission cross-section when a 1000 ppM NO/sub 2/-air mixture was studied. At higher temperatures, thermal decomposition was observed. The instrument sensitivity under room temperature conditions was about 1 ppM, and under flame conditions was about 7 ppM. When measurement of NO/sub 2/ in a 1.1 equivalence (fuel rich ) ratio flame was attempted, no NO/sub 2/ (greater than 20 ppM) was found. Even when 1000 ppM NO/sub 2/-in-air mixture was used to oxidize hydrogen, NO/sub 2/ concentration greater than 20more » ppM could not be detected in the combustion region. This sensitivity could be obtained only somewhat behind the flame front due to NO/sub 2/ chemi-luminescence radiation generated at the flame front. Improved spatial discrimination should allow more extensive and sensitive measurements which would help towards resolving the questions of existence and mechanisms for NO/sub 2/ formation in combustion.« less