Laminar burning speeds and flame instabilities of isobutane carbon dioxide air mixtures at high pressures and temperatures

Liquefied petroleum gases (LPGs), such as isobutane and propane, find applications as working fluids in refrigeration and as fuels in internal combustion engines. Isobutane is typically used as a precursor molecule in the synthesis of isooctane, a component of gasoline. It can also be used as a natural low global warming potential (GWP) refrigerant, however, there are fire safety concerns. This work investigated the effects of adding carbon dioxide (CO2) to isobutane (iC(4)H(10)) to evaluate its effects on flammability and flame propagation. This paper also compared isobutane results with propane (the major LPG component) results, which were adapted from a previous study. The laminar burning speeds of blends of iC(4)H(10)/CO2/air were measured in a constant volume spherical chamber. The flame stability/instability was evaluated in a cylindrical chamber where the flame propagation was captured with a high-speed camera in a Z-shaped Schlieren system. A power law correlation was developed to determine laminar burning speeds for blends of iC(4)H(10)/CO2/air at CO2 mole fractions of 0-80%, equivalence ratios of 0.8-1.2, temperatures of 298-580 K, and pressures of 0.5-4.2 atm. Results were compared with experimental studies from the literature and numerical data obtained using 1-D premixed flame code from CANTERA. Both experiments and numerical predictions showed that the laminar burning speeds of iC(4)H(10)/CO2 mixtures decreased with increasing CO2 mole fraction, and that the maximum laminar burning speed shifted towards stoichiometric mixture. Carbon dioxide acted as an inhibitor of flame instability, as verified both experimentally by photography and theoretically by calculating the critical Peclet number.