Effects of diluents on laminar burning velocity and cellular instability of 2-methyltetrahydrofuran-air flames

Recent investigations have indicated that 2-methyltetrahydrofuran (2-MTHF) can be a potential alternative fuel, owing to its renewability and environmentally friendly properties. In this study, the effects of carbon dioxide (CO2), nitrogen (N-2), and helium (He) dilution on the laminar burning velocity (LBV) and cellular instability of 2MTHF-air flames were investigated experimentally and kinetically at a temperature of 423 K, a pressure of 5 bar, equivalence ratios (phi) of 0.7-1.5, and diluent ratios of 0-15%. The results illustrated that CO2 exhibited the strongest ability to reduce the LBVs, followed by N-2 and He. The effective Lewis number (Leeff) of the He diluted 2-MTHF-air flames increased as more He was added to the reactant mixture, thereby indicating that the intensity of diffusive-thermal instability could be diminished by He dilution. In contrast, Leeff was insensitive to CO2 and N-2 dilution. The flame thickness increased and the density ratio decreased as the diluent ratio increased, indicating that all three diluents suppressed the hydrodynamic instability of 2-MTHF-air flames. The relative magnitudes of the stabilizing abilities of these diluents were in the order CO2 > N-2 > He. The stability analysis indicated that the monotonically decreasing critical Peclet number and non-monotonically altered flame thickness caused a non-monotonic variation in the theoretical critical flame radius of 2-MTHF-air flames versus phi. The most chaotic state occurred at approximately phi = 1.3 under all test conditions. Notably, CO2 and He exhibited a similar ability to delay the onset of cellular instability, whereas N-2 demonstrated a weaker ability to do so.