Numerical studies on intrinsically unstable H2/CO2 flames: Effect of CO2 dilution, equivalence ratio, temperature and pressure

Direct combustion of H${}_2$/CO${}_2$ gas mixtures obtained from the gasification and steam reforming processes can help avoid CO${}_2$ separation costs and safety concerns associated with the fast combustion of pure hydrogen as a fuel. This work studies the effect of varying CO${}_2$ concentrations in hydrogen-air flames through detailed numerical simulations (DNS) as well as canonical flame calculations. The species transport budget calculated using one-dimensional freely propagating flames shows dominance of diffusion against convection with increase in CO${}_2$ percentage. Similarly, an increase in reaction rate and negative Markstein length is observed for flames under strain, suggesting enhanced thermodiffusive response. Quantification of thermodiffusive instability in linear and non-linear flame propagation regimes has been performed for CO${}_2$-rich hydrogen-air mixtures using two-dimensional freely propagating flames in a classical inflow, outflow and periodic domain for a range of equivalence ratios (0.8–1.1), unburned temperatures (300 K–700 K) and pressures (1 atm–8 atm). The growth rate of perturbation amplitude increases with decreasing the equivalence ratio and temperature, and increases with CO${}_2$ dilution and pressure, enhancing intrinsic instability. The flame propagates with a finger-like structure with increasing propensity of subadiabatic and superadiabatic regions for mixtures corresponding to higher growth rates. These findings will further the understanding of burning characteristics of H${}_2$/CO${}_2$/air mixtures when used in practical combustion devices.

Novelty and significance statement

Direct combustion of H${}_2$/CO${}_2$ fuel mixtures can provide different low-cost pathways for grey hydrogen utilization. Such mixtures are known to exhibit intrinsic flame instabilities, especially of a thermodiffusive nature. This work presents a one-dimensional assessment of the convective, diffusive, reactive balance of H${}_2$/CO${}_2$ fuel-air flames and their response to flow-stretch, which confirms the sensitivity of their thermodiffusive nature towards instabilities. The main novelty lies in the demonstration of thermodiffusive instabilities in H${}_2$/CO${}_2$ fuel-air mixtures using two dimensional direct numerical simulations (DNS) and assessing sensitivities of instability growth rates and overall consumption speed to mixture equivalence ratio, unburnt temperature and pressure. Further, the relative impact of flame wrinkling and local differential diffusion on consumption speed enhancement for such mixtures has been quantified for the first time. Such insights make this study significant for understanding the combustion of H${}_2$/CO${}_2$/air mixture when used in practical applications.