Study on the spray and combustion characteristics of water-emulsified diesel

Emulsified fuel remains a potential solution to meet the increasingly stringent emission regulations for internal combustion engines due to its capability of simultaneously reducing NO_x and particular matter (PM). In this study, emulsified diesels with 10% and 20% water by volume were studied. The stability of the water emulsified diesel was first investigated in terms of the hydrophilic-lipophilic-balance (HLB) value. Based on the stability test, a suitable surfactant composition for the diesel/water interfacial condition was given and the separation tendency of the fuel with different water volumetric ratio was analyzed. The emulsions were later injected and combusted in a pre-burn type constant volume chamber, which is able to provide high ambient temperature and pressure to mimic real engine operation conditions. High speed imaging was used to capture the spray and combustion process under various conditions. Results show longer initial liquid penetration for emulsified diesel under low ambient temperatures. Longer ignition delay of emulsified diesel also provided more air/fuel mixing time, thus significantly lowering the soot luminosity. Although droplet micro-explosion has been intensively studied, its behavior in a burning spray is much less reported. This study in particular focused on micro-explosion in a burning spray. Broadband natural flame images were recorded with intentional overall over-exposure such that the central lift-off region could be illuminated by soot incandescence. Puffing and disruptive droplet combustion was consistently observed at high ambient temperature in the central lift-off region with emulsified diesel indicating the occurrence of micro-explosion in a burning spray flame. It is demonstrated that micro-explosion is not only able to enhance the secondary breakup, but also affect the primary breakup under certain conditions, which to the author's awareness has not been reported in any previous literature. Lower injection pressure and higher ambient temperature favor the occurrence of micro-explosion before primary breakup as a competition between the micro explosion delay time and the primary breakup time.