Experimental and computational investigation of the influence of stoichiometric mixture fraction on structure and extinction of laminar, nonpremixed dimethyl ether flames

Experimental and computational investigation is carried out to elucidate the influence of stoichiometric mixture fraction, ${\xi }_{\mathrm{st}}$, on the structure and critical conditions of extinction of nonpremixed dimethyl ether (DME) flames. The stoichiometric mixture fraction represents the location of a thin reaction zone in terms of a conserved scalar quantity. The counterflow configuration is employed, wherein two reactant streams flow towards a stagnation plane. One stream is made up of DME and nitrogen (N${}_2$) and the other stream is oxygen and N${}_2$. Previous studies have shown that critical conditions of extinction depend on ${\xi }_{\mathrm{st}}$ and the adiabatic temperature $T_{\mathrm{st}}$. Therefore, the present investigation is carried out with the composition of the reactants in the counterflowing streams so chosen that the adiabatic temperature is the same for different values of ${\xi }_{\mathrm{st}}$. The strain rate at extinction, $a_q$, is measured for values of ${\xi }_{\mathrm{st}}$ up to 0.8. Computations are performed using detailed kinetic mechanisms and critical conditions of extinction and flame structures are predicted. The measurements and predictions show that, with increasing ${\xi }_{\mathrm{st}}$, the strain rate at extinction first decreases and then increases. The predictions agree with measurements for ${\xi }_{\mathrm{st}}<0.4$, but significant deviations between measurements and predictions are observed at higher values of ${\xi }_{\mathrm{st}}$. The scalar dissipation rate at extinction, ${\chi }_{\mathrm{st},q}$ is calculated using measured and predicted values of $a_q$. With increasing ${\xi }_{\mathrm{st}}$, the measured and predicted values of ${\chi }_{\mathrm{st},q}$ first increase and then decrease. It is noteworthy that changes in values of ${\chi }_{\mathrm{st},q}$ with ${\xi }_{\mathrm{st}}$ for dimethyl ether flames are similar to those for methane flames, while the changes in values of $a_q$ with ${\xi }_{\mathrm{st}}$ are remarkably different. Flame structures are predicted and they are found to be qualitatively similar to those for hydrocarbon fuels.