Suppression of hydrogen-air explosions by hydrofluorocarbons

This study examines the suppression effectiveness of fluorinated species CHF3 and C2HF5 on the premixed hydrogen-air explosions experimentally and numerically. The results demonstrate that as for stoichiometric and rich hydrogen-air mixtures, both CHF3 and C2HF5 can effectively reduce thermal expansion ratio and increase flame thickness, and then reduce the influence of hydrodynamic instability on the flame acceleration. Laminar burning velocity, the maximum of explosion pressure, the maximum rate of pressure rise and the absolute value of pressure impulse all decrease with increasing suppressant concentration at various equivalent ratios. The maximum of explosion pressures for stoichiometric flames is decreased by 11.54 % with 2 % CHF3 added, and decreased rapidly by 40.39 % with 2 % C2HF5 added. The hydrogen-rich mixtures cannot be ignited when C2HF5 content reaches 10 %. Numerical simulations have confirmed that both CHF3 and C2HF5 can effectively reduce the concentration of active radicals with the decrease order of H > OH > O. C2HF5 is more effective in suppressing explosions than CHF3 by converting more H atoms to HF formation through the elementary reactions CHF2 + H double left right arrow CHF + HF, CF2:O + H double left right arrow CF:O + HF, and CF2 + H = CF + HF. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.