Effects of low-temperature chemistry and turbulent transport on knocking formation for stratified dimethyl ether/air mixtures

Abstract

The effects of low-temperature chemistry (LTC) and turbulent transport with thermal and fuel stratifications on knocking formation are numerically modeled for dimethyl ether/air mixtures with detailed chemistry. The critical conditions of knocking formation with thermal and fuel concentration gradients in both one-dimensional laminar and two-dimensional turbulent flow are examined with and without LTC and with different turbulent timescales, length scales, and Reynolds numbers. The results show that LTC has a significant effect on the ignition-to-detonation transition caused by both thermal and concentration gradients. The results also show that at the same dimensional gradient, LTC can accelerate, enable, or quench the detonation transition, depending on the magnitude of the gradient. These three different LTC effects are demonstrated in the detonation peninsula by using the same ignition delay time and a unified criterion for combined thermal and concentration gradients. Moreover, by using the ignition delay time with and without LTC, the results showed that the critical conditions for the detonation transition are not affected. This result clearly revealed that large heat release rate from hot ignition is required to initiate ignition and shockwave coupling for detonation transition. The results further show that turbulent transport can delay knocking/detonation transition and dramatically reduce detonation strength due to turbulent mixing. It is found that when the turbulent timescale is much shorter than the ignition delay time of a gradient field, the knocking formation will be inhibited. With a given turbulent timescale, a larger turbulent length scale or turbulent Reynolds number will induce a broader mixing in the preheated zone in front of detonation wave and thus reduce the knocking strength. Moreover, knocking formation in a smaller gradient kernel is weakened more by turbulent transport. The present research provides important insights of knocking formation and control of knocking using low-temperature fuel reactivity, thermal and fuel stratifications, and turbulence in advanced engines.