NOx Emissions of a Premixed Partially Vaporized Kerosene Spray Flame

Abstract

An important question for future aero-engine combustors is how partial vaporization influences the NOx emissions of spray flames. In order to address this question an experimental study of the combustion of partially vaporized kerosene/air mixtures was conducted, which assesses the influence of the degree of fuel vaporization on the NOx emissions in a wide range of equivalence ratios covering the entire lean burning regime. The tests were performed at atmospheric pressure, inlet air temperatures of 313 to 376K, a reference mean air velocity of 1.35m/s, and equivalence ratios of 0.6, 0.7 and 0.9 using Jet A1 fuel. An ultrasonic atomizer was used to generate a fuel spray with a Sauter Mean Diameter of approximately 50μm. The spray and the heated air were mixed in a glass tube of 71mm diameter and a variable length of 0.5 to 1m. The temperature of the mixing air and the length of the preheater tube were used for the control of the degree of vaporization. Downstream of the vaporizing section, the mixture was ignited and the flame was stabilized with a hot wire ring that is electrically heated. For local exhaust measurements a temperature controlled suction probe in combination with a conventional gas analysis system were used. The vaporized ratio of the injected fuel was determined by a Phase Doppler Anemometer (PDA). In order to optimize the accuracy of these measurements, extensive validation tests with a patternator method were performed and a calibration curve was derived. The data collected in this study illustrates the effect of the vaporization rate Ψ upstream of the flame front on the NOx emissions, which changes with varying equivalence ratio and degree of vaporization. In the test case with low pre-vaporization, the equivalence ratio only has a minor influence on the NOx emissions. Experiments made with air preheat and higher degrees of vaporization show two effects: With increasing preheat air temperature, NOx emissions increase due to higher effective flame temperatures. However, with an increasing degree of vaporization, emissions become lower due to the dropping number and size of burning droplets, which act as hot spots. A correction for the effect of the preheat temperature was developed. It reveals the effect of the degree of pre-vaporization and shows that the NOx emissions are almost independent of Ψ for near-stoichiometric operation. At overall lean conditions the NOx emissions drop nonlinearly with Ψ. This leads to the conclusion that a high degree of vaporization is required in order to achieve substantial NOx abatement.