Experimental and numerical study on the laminar flame characteristics for PODE3 and PODE3/iso-octane blends under elevated and sub-ambient initial pressures

Polyoxymethylene dimethyl ether (PODEn) is a promising alternative fuel for diesel due to its high oxygen contents (> 40%) and high cetane number, thereby improving the thermal efficiency and reducing the pollutant emissions caused by the internal combustion engine. This study investigated the laminar burning speeds and Markstein length for PODE(3 )and PODE3/iso-octane blends at wide equivalence ratios (0.7-1.6) and initial temperature of 408 K under elevated and sub-ambient initial pressures (0.5-4 bar) in a cylindrical constant-volume combustion vessel with high-speed Schlieren photography method combined with the nonlinear extrapolation model. The numerical simulation was also carried out to validate the accuracy of the mechanisms of PODE3 and primary reference fuel (PRF)/PODEn blends and analyze the reaction sensitivities and the mole fraction profiles of key species. In addition, flame instability was discussed experimentally and theoretically. The results show that the laminar burning speed for PODE3 decreases with the initial pressure, and the laminar burning speed for PODE3/iso-octane basically accords with the linear relationship to the PODE3 volume fractions. The findings indicate that the kinetic effect has a dominant impact on the laminar burning speed for PODE3 when compared with the thermal effect. The results also reveal that the Markstein length is negative only at an equivalence ratio between 1.5 and 1.6 under P = 2 bar, indicating that PODE3 has more enhanced flame stability. Furthermore, the flame instability decreases with the increase of PODE3 volume fractions, increases with the increase of initial pressure, and increases with the increase of the equivalence ratio. The differences between experimental results and theoretical results including the Markstein length, the critical radius and the critical Peclet number might be explained by the stretch rate results.

Automated summary

This study investigated the combustion characteristics and flame instability of Polyoxymethylene dimethyl ether (PODEn) as a diesel substitute fuel through experiments and numerical simulations. The results showed that the laminar burning speed of PODE3 decreased with initial pressure, and the laminar burning speed of PODE3/iso-octane blends was linearly related to the PODE3 volume fractions. In addition, PODE3 exhibited improved flame stability, and flame instability decreased with increased PODE3 volume fractions but increased with increased initial pressure and equivalence ratio.