Effects of CO2 dilution on ignition characteristics in lean premixed H2/Air flames

Abstract

The research has examined the effects of CO2 dilution on ignition characteristics in H2/air mixtures. An ignition kernel was initiated by a laser-induced plasma in H2/air/CO2 mixtures with a bulk velocity of 6.5 m/s. An infrared camera was used to measure radiation intensity emitted from H2O and ignition probabilities. A high-speed schlieren image system was utilized to measure ignition kernel areas and ignition time. High-speed chemiluminescence was performed to examine OH* intensities. Numerical simulations were conducted using GRI 3.0 mechanisms to calculate reaction rates and laminar flame speeds. The ignition probability is highest for the undiluted mixtures and decreases with increasing the CO2 dilution fraction at constant adiabatic temperatures. A prolonged ignition time is observed at elevated CO2 dilution concentrations. Increasing the CO2 mole fraction decreases the ignition kernel growth, the integrated OH* intensity, and the integrated radiation intensity for each adiabatic temperature. CO2 reacts with H radicals and decreases the production of OH*, OH, and H2O. The CO2 dilution in the H2/air flames significantly contributes to the reduced reaction rate and flame speed, resulting in the low ignition probability. The high ignition probability is found at the large integrated OH* intensity in all cases. The ignition probability is high at the large integrated radiation intensity under the constant adiabatic flame temperatures. The OH* intensity is a critical parameter to estimate the ignition probability in the diluted lean hydrogen flames.