Effects of stretch and thermal radiation on difluoromethane/air burning velocity measurements in constant volume spherically expanding flames

Abstract

Compared to current refrigerants, next-generation refrigerants are more environmentally benign but more flammable. The laminar burning velocity is being used by industry as a metric to screen refrigerants for fire risk and it is also used for kinetic model development and validation. This study reports measurements of difluoromethane/air flame burning velocities for equivalence ratios from 0.9 to 1.4 in a spherical, constant volume device. Experimental burning velocities produced with the aid of an optically thin radiation model are about 17% greater than those obtained with an adiabatic model. Characterization of flame stretch based on the product of Markstein and Karlovitz numbers indicates that while many experimental data are nearly stretch-free, those for slower burning velocities, smaller flame radii, and leaner conditions may not be. Limiting the data to regions estimated to be stretch-free requires extrapolation away from the experimental conditions to extract burning velocities near ambient conditions, e.g., at (298 K, 101 kPa). Lower uncertainty, desirable for kinetic model validation, is obtained by interpolating between experimental conditions, e.g., at (400 K, 304 kPa). Since thermal radiation and flame stretch were found to affect the inferred burning velocities of difluoromethane/air constant volume spherical flames, these effects should also be considered during data reduction of other mildly flammable refrigerants.