Experimental and computational investigation of the influence of ethanol on auto-ignition of n-heptane in non-premixed flows

Experimental and computational investigations are carried out to elucidate the influence of ethanol addition on n-heptane auto-ignition in counterflows. An axisymmetric stream of air, temperature gradually increased, is directed onto the surface of an evaporating pool of a liquid fuel. The air-stream temperature at auto-ignition is measured at various strain rates, defined as the axial gradient of the axial component of the flow velocity at the stagnation plane, for n-heptane, ethanol, and various n-heptane/ethanol mixtures. Critical conditions for auto-ignition are predicted employing the San Diego Mechanism for both fuels and the fuel mixtures, and the results are compared with the measurements. Measurements and predictions show that low-temperature chemistry plays a significant role in promoting auto-ignition of n-heptane at low strain rates, but there is insufficient residence time at high strain rates for low-temperature chemistry to take place, so auto-ignition is promoted by high-temperature chemistry. Experimental and computational results show that addition of ethanol inhibits the low-temperature chemistry of n-heptane. To identify the responsible elementary steps, computations are performed to identify those that dominate oxygen consumption and that contribute to the temperature rise in the reaction zone for n-heptane and n-heptane/ethanol mixtures at low strain rates. For n-heptane oxygen is consumed primarily by the low-temperature steps that result in ketohydroperoxide; the temperature rise is produced by subsequent low-temperature-chemistry steps. For the mixtures, a key step that consumes O2 is O2 + CH3CHOH = HO2 + CH3CHO, and the heat release occurs through the classical high-temperature reaction mechanism. Thus, the inhibition of auto-ignition that is observed to occur when ethanol is added to n-heptane arises from the competition for oxygen between this step and the low-temperature-chemistry addition of O2 to the heptyl radical and to the radical arising from the subsequent isomerization, for n-heptane.