Abstract
Autoignition of an ethanol-based gel droplet was experimentally investigated by adding 10 wt % of methylcellulose as gellant to liquid ethanol. Experimental studies of the ignition behavior of the gel droplet were found to be quite rare. The initial droplet diameter was 1.17 ± 0.23 mm. The gel droplet was suspended on a K-type thermocouple and its evaporation, ignition and combustion characteristics were evaluated and compared with pure ethanol at an ambient temperature of 600, 700, and 800 °C under atmospheric pressure conditions. The gel droplet exhibited swelling and vapor jetting phenomena. Before ignition, a linear decrease in droplet diameter followed by a sudden increase was repeatedly observed, which was caused by evaporation and swelling processes, respectively. Major droplet swelling was detected just before the onset of ignition at all temperatures. But no further swelling was detected after ignition. For the gel droplet, the ignition delay accounted for 93% of the droplet lifetime at 600 °C, and 88% at 700 °C, but only 31% at 800 °C. Its average burning rate was also evaluated for all temperatures. At 800 °C, the gellant layer no longer exerts any influence on the combustion of the gel droplet.
Highlights
Gel propellants can be prepared by agitating liquid fuels with various gellants, depending on the properties required
Swelling and fuel vapor jetting due to gellant layer were apparently observed during the evaporation process of gel droplet
Major droplet swelling was detected at all temperatures, which was regarded as the onset of ignition in this study
Summary
Gel propellants can be prepared by agitating liquid fuels with various gellants, depending on the properties required. Until the 1970s, gel propellants were studied as missile and rocket fuels, principally in the USA, but their combustion characteristics were less than theoretically expected [2]. Negri and Ciezki [23] showed that aggregation occurred when a gel propellant containing aluminum particles based on jet A-1 was burned. It explained low combustion efficiency than expected. The combustion of a single gel droplet lacking metal particles can be divided into four stages, of which the first is heat-up, similar to the combustion of a typical liquid fuel droplet. Most of researchers used to observe combustion behavior characteristics after an external ignition of the gel droplet. Regarding the autoignition in this study, the gel droplet is to be observed throughout the time interval from evaporation to combustion, which allows the autoignition behavior of the gel droplet to be better understood
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