A review of laminar flame speeds of hydrogen and syngas measured from propagating spherical flames

Abstract

As promising alternatives to fossil fuels, hydrogen (H2) and syngas are playing important roles in the development and control of high-efficiency, low-emission engines. Achieving accurate prediction of H2-fueled combustion requires a reliable chemical mechanism which, however, still exists considerable uncertainty. The laminar flame speed (LFS) has been widely employed to validate and optimize chemical mechanisms and to model turbulent premixed combustion. While in the literature there are extensive LFS data measured using the outwardly propagating spherical flame (OPF) method for hydrogen/air and syngas/air mixtures at normal temperature and pressure (NTP), the accuracy of the LFS data is not fully explored. This work aims to (i) review the uncertainty in the LFSs measured by different groups for hydrogen/air and syngas/air mixtures at NTP using the OPF method, and (ii) identify underlying sources of the uncertainty. It is found that there are considerable discrepancies in the LFS measurements, leading to these experimental data being unreliable for restraining the uncertainty of chemical models. The underlying sources of uncertainty are discussed in different flame propagation regimes and their contributions to the discrepancies are assessed individually using 1-D simulations. The results show that the contribution of ignition effects to the uncertainty depends strongly on the equivalence ratio and that the ignition effects could be one of the main sources of uncertainty for the LFSs of fuel-rich mixtures. Furthermore, it is found that the accuracy of measured LFSs is strongly affected by the choice of extrapolation model and flame radius range for extrapolation. The nonlinear extrapolation is less sensitive to the flame radius range than linear extrapolation, implying that using nonlinear extrapolation models can reduce the impact of the flame radius range selected on the uncertainty, especially for fuel-rich and/or fuel-lean mixtures. Nevertheless, strong nonlinear behavior between stretched flame speed and stretch rate still makes a major contribution to the very large discrepancies even when the nonlinear extrapolation models are used. To address the nonlinear stretch behavior, a new nonlinear extrapolation model NQH is proposed and it is shown to be more accurate than other models as pressure increases. Moreover, the recommendations on H2and syngas LFS measurements using the OPF method are provided.