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, they should also be considered during data reduction of other mildly flammable refrigerants.

Laminar flame speed determination at high pressure and temperature conditions for kinetic schemes assessment

“…accuracy of the models for the burned mass fraction [9], stretch effects [10], influence of burnt gases equilibrium state [11] and of radiation [4]) and problems in flame instability detection [12]. This method has received particular attention in recent years [4,10,[13][14][15][16][17].…”

“…Remarkable progress has been achieved with the derivation of clearly identified limits of data to process while guaranteeing negligible effects of stretch ( 0 > 2 ⁄ , with 0 the initial pressure in the chamber), buoyancy ( > 15 / ) and wall heat loss effects ( < 0.75 ⁄ , with the theoretical end-pressure). Neglecting radiation heat loss when interpreting experimental data could lead to uncertainty as large as 15% as demonstrated in [4,13]. This was limited by introducing radiation heat loss while keeping an inexpensive computational cost [4,13,21].…”

“…Neglecting radiation heat loss when interpreting experimental data could lead to uncertainty as large as 15% as demonstrated in [4,13]. This was limited by introducing radiation heat loss while keeping an inexpensive computational cost [4,13,21]. Meanwhile, we have developed an alternative method minimizing uncertainties using the isochoric method without resorting to any physical model [22]: A novel perfectly spherical isochoric combustion chamber with full optical access (OPTIPRIME), which allows the simultaneous recording of the pressure inside the chamber and of the flame radius evolution until the flame vanishes at the wall, making it fully innovative.…”

Measurements and modeling of spherical CH2F2-Air flames

“…al. [12,13], Moghaddas et al [14], Burrell et al [15], and Burgess et al [16] used the constant volume combustion method to deduce the burning velocity from the pressure rise, with experiments in both normal gravity [12] and microgravity [13]. Takizawa et al [13] also used the constant pressure method, employing a 3.9 L cylindrical chamber with Schlieren imaging to deduce the flame speed from the outward propagation speed of the horizontal edges of the flame.…”

An empirical model for refrigerant flammability based on molecular structure and thermodynamics

“…However, it has only recently been calculated for pure HFCs with air (e.g., R-32 (Burgess Jr et al, 2018); C1 and C2 HFCs (Linteris and Babushok, 2018)), and for blends of HFCs with hydrocarbons (Linteris, 1996;Linteris et al, 1998;Linteris and Truett, 1996;Pagliaro et al, 2016a;Pagliaro et al, 2016b). For accurate predictions however, additional research is necessary to understand the important effects of flame stretch (Pagliaro and Linteris, 2016), radiation heat losses (Burrell et al, 2018), and buoyancy (Takizawa et al, 2013). Also, the calculation of the laminar burning velocity of HFC compounds in air requires a detailed kinetic mechanism, and while these are in active stages of development (Babushok and Linteris, 2017;Needham and Westmoreland, 2017;Papas et al, 2017), more work is required (Linteris and Babushok, 2018).…”