Oxygen atom concentrations and no production rates in a turbulent H2/N2 jet flame

Abstract

An experimental procedure is demonstrated that allows determination of instantaneous oxygen-atom concentrations and thermal NO production rates in turbulent flames. This is accomplished without direct measurement of O atoms by laser-induced fluorescence, a technique that has been shown to be problematic. Simultaneous point measurements of temperature, the major species, OH, and NO are performed in a non-premixed turbulent jet flame of nitrogen-diluted hydrogen fuel. The O-atom concentration for each laser shot is derived from these data, based on the assumption of partial equilibrium of the reaction OH+OH=O+H2O. The derived O-atom concentration is then used with the measured N2 concentration and temperature to calculate the instantaneous thermal (zeldovich) NO production rate. The partial equilibrium assumption is shown to be appropriate in this flame for conditions relevant to thermal NO formation. The maximum conditionally averaged O-atom mole fraction is about 6.5 times the maximum value for full chemical equilibrium. The width in mixture fraction coordinates of the conditionally averaged NO production rate curve is roughly twice that calculated from adiabatic equilibrium values of temperature, [O], and N2. Experimental results are compared with predictions obtained using the Monte Carlo PDF model with detailed chemistry. With some exception, there is good agreement between experiment and prediction when results are plotted against mixture-fraction (scatter plots and conditional averages). Agreement is not as good when radial profiles of averaged scalar quantities are compared. This indicates that the present PDF model predicts the hydrogen and nitrogen chemistry with good accuracy but that improvements are needed in the modeling of the turbulent fluid dynamics and mixing. A comparison of experimental results with steady strained laminar flame calculations shows that the turbulent flame conditions cannot be represented by a combination of the laminar flames.