Characterizing combustion and atomization of PODEn and ethanol/PODEn binary droplets

Abstract

Polyoxymethylene dimethyl ethers (PODEn) constitute a promising e-fuel family from CO2 feedstock, while their droplet combustion characteristics are still insufficiently understood. This work reports an experimental investigation on the combustion and atomization characteristics of PODEn (n = 1–4) and ethanol/PODEn binary droplets, with special attentions on the influence of content ratio, highly volatile component, and boiling point difference between components (ΔTbp). History of square of normalized droplet diameter was recorded during the combustion process, from which two stages including stable combustion and atomization can be derived. Three different behaviors, i.e., puffing, ejection, and micro-explosion, were observed in the atomization stage of the investigated binary droplets with ΔTbp generally greater than 50 K, while only puffing and ejection were observed as ΔTbp becomes lower. For PODE1/PODE4 binary droplets, the atomization index which defines the relative reduction of droplet lifetime caused by atomization is found to peak at 70 % PODE1, while for ethanol/PODE4 binary droplets, the atomization index rises quickly at small ethanol content and thus peaks early at 50 % ethanol. This is mainly because that the evaporation propensities of ethanol and PODE4 are both promoted in the ethanol/PODE4 binary droplets as their activity coefficients become greater than one, while those of PODE1 and PODE3 are both suppressed in the PODE1/PODE4 binary droplets. For both PODEn and ethanol/PODEn binary droplets at the fixed content ratio of 50 %/50 %, the atomization delay time and micro-explosion intensity both increase with ΔTbp first and keep almost invariable as the weakly volatile component varies from PODE3 to PODE4, while the atomization index becomes proportional to ΔTbp. On the other hand, the ethanol/PODEn binary droplets generally have higher micro-explosion intensities and consequently higher atomization indices, indicating their stronger atomization propensities than the PODEn binary droplets.