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

Abstract

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 (N2), and helium (He) dilution on the laminar burning velocity (LBV) and cellular instability of 2-MTHF-air flames were investigated experimentally and kinetically at a temperature of 423 K, a pressure of 5 bar, equivalence ratios (ϕ) 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 N2 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 N2 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 > N2 > He. The stability analysis indicated that the monotonically decreasing critical Péclet number and non-monotonically altered flame thickness caused a non-monotonic variation in the theoretical critical flame radius of 2-MTHF-air flames versus ϕ. The most chaotic state occurred at approximately ϕ = 1.3 under all test conditions. Notably, CO2 and He exhibited a similar ability to delay the onset of cellular instability, whereas N2 demonstrated a weaker ability to do so.