Measurements of laminar burning velocity and Markstein length in outwardly-propagating spherical SNG-air premixed flames at elevated pressures

A study has been conducted to measure unstretched laminar burning velocities and burnt Markstein lengths for outwardly-propagating spherical SNG-air premixed flames at elevated pressures. The range of flame radius and extrapolation models (LM, NM I and NM II) were first optimized for CH4-air premixed flames at normal temperature. The presently measured unstretched laminar burning velocities yielded the minimal errors when Aramco 2.0 and NM II were adopted for the optimized range of flame radius. The capability of predicting Markstein length with a composite model was investigated by using different definition of flame thickness and non-dimensional thermal conductivity for four iso-temperature surfaces. The theoretical Markstein lengths with appropriately defined flame thickness and non-dimensional thermal conductivity best trace the presently measured ones for the iso-temperature surface of 600 K. Unstretched laminar burning velocities against equivalence ratio were measured and compared with the predicted ones with Aramco 2.0 for SNG-air premixed flames. Except 0.1 MPa, the predicted ones were deviated appreciably from measured data when chamber pressure increases. The capability of predicting unstretched laminar burning velocities against equivalence ratio was improved via modifying several kinetic data based on sensitivity analysis to Aramco 2.0 Mechanism. The capability of predicting Markstein lengths was also tested for SNG-air premixed flames at elevated pressures up to 0.7 MPa. Choosing the iso-temperature surface of 600 K excellently traced the presently measured data when flame thickness and non-dimensional thermal conductivity are appropriately defined, while several issues are raised up as future works.