Transient horizontal flame spread under non-steady concurrent airflow

Abstract

The transient horizontal flame spread over thin black cast polymethyl methacrylate (PMMA) sheets under non-steady airflow profiles is examined experimentally. The horizontal flame spread behavior is examined through measurements of the pyrolysis front progression and the heated length (measured as the flame extension length) ahead of the pyrolysis front. The magnitude and frequency of the heated length were found to change directly with the frequency and magnitude of the transient component of the non-steady airflow. The transient response of the flame spread rate was less direct, but peak instantaneous flame spread rate values were three to six times greater than the time-averaged values. The presence of a non-steady inlet flow component was found to result in time-averaged flame spread rates up to 20% higher compared to steady-flow cases without the non-steady airflow component. Dimensional analysis with the Richardson number is performed to characterize the importance of natural convection and forced convection. Nearly all cases are found to be in mixed convection regime with some cases in natural convection regime for a portion of the flow profile. An empirical correlation, involving the Richardson number, is used to compare the flame spread rate by accounting for the heat transfer to the unburned fuel. Similar trends are observed between the measured and predicted flame spread rates. The results indicate that the more pronounced transient flame spread rate observed under oscillatory flow, compared to steady flow, can be understood by analyzing the convection regime.