Thermal and chemical effects of low-temperature chemistry on flame initiation and propagation

Abstract

Large hydrocarbon fuels exhibit pronounced low-temperature chemistry (LTC) and negative temperature coefficient (NTC) behaviors. During low-temperature combustion, a small amount of heat is released while massive chemical components are generated, which are of great significance in flame ignition and propagation. However, the roles of thermal and chemical effects of the LTC in combustion evolutions have not been thoroughly investigated before. In this work, the LTC-affected flame initiation and propagation were studied, allowing for the thermal and chemical effects as well as their interactions. Dimethyl ether (DME) as a typical LTC fuel was employed for the study and an ignition energy deposition method was adopted to trigger the transient flame initiation and propagation. Besides, the steady laminar flame speed was also investigated to elucidate the LTC-affected combustion mechanisms. The results show that the LTC facilitates the appearance of cool flames in addition to traditional hot flames. Compared to hot flames, cool flames allow lower minimum ignition energy (MIE), especially under fuel-lean and fuel-rich conditions. Cool flames can coexist with hot flames and promote flame initiation and propagation under fuel-lean conditions. To isolate the specific role of the thermal and chemical effects of the LTC, an artificially modified chemical scheme is adopted for comparative study. The results show that the thermal effects of the LTC promote flame initiation while the chemical effects present an opposite impact. Despite this, the LTC shows a promotion impact on flame initiation in comprehensive due to the leading order significance of the thermal effects. Similar observations can also be found for steady laminar flame speed and adiabatic flame temperature. Sensitivity analysis suggests that low-temperature reactions such as CH2OCH2O2H + O2 = O2CH2OCH2O2H and CH2OCH2O2H = OH + 2CH2O play an important role in hot flame propagation. The current work provides useful insights into the LTC-affected combustion at elevated temperature conditions.