Abstract
This study aims to develop a lift-off height model suitable for hydrogen autoignited flames in turbulent hot coflow. The differences in flame structure and lift-off characteristics between hydrogen and acetylene flames are compared through experiments. Due to hydrogen's high flame speed, the autoignition spots of hydrogen are undetectable, and the flame zone becomes regular and continuous. Therefore, the flame speed can be used to improve the calculation of the chemical time scale to incorporate the flame propagation factor into the lift-off height model. A novel model is established to consider the combined effects of flame propagation and autoignition on flame stabilization through the Karlovitz number and Damkohler number, respectively. Positive parameters β and 1-β represent the relative contributions of flame propagation and autoignition. The Ka/Da model successfully predicts the variations in the lift-off height of hydrogen flames at various coflow temperatures and fuel mole fractions. In addition, the good correlations based on the Ka/Da model to the lift-off heights of hydrogen, methane, and ethylene flames in the literature prove the model's generality. Unlike hydrocarbon flames (β < 0.5), hydrogen flames' β is bigger than 0.5, indicating that flame propagation plays a more significant role than autoignition in stabilizing autoignited flames.
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