Single-plume and multi-plume atomisation of ethanol with different levels of water content at hot fuel conditions

Abstract

Producing anhydrous ethanol involves separation of ethanol and water in an energy intensive process. It is possible to use ethanol (without any gasoline) as a fuel in countries where the ambient temperature is high enough that the low vapour pressure of pure ethanol does not give rise to significant cold weather driveability issues. From a cost and carbon footprint perspective, it is highly desirable to use hydrous ethanol, but there is a clear need to understand the effect of water content in ethanol on the spray formation process and hence mixture formation. The present paper presents results from an optical experimental investigation into the effects of 0–15 % water content per volume in ethanol on spray formation, using high-speed imaging and droplet sizing. Experiments were carried out with fuel temperatures in the range 20–110 °C at 1.0 bar and 0.5 bar gas pressure. Iso-octane and water were also tested for comparison. Multi-hole and isolated single-hole injections were studied to decouple effects from plume-to-plume interactions. Single-plume hydrous ethanol fuels showed initial delay out of the nozzle with shorter penetration but gradually achieved or surpassed the liquid penetration of the anhydrous fuel. Increased water content was generally associated with reduced single plume cone angle between 20 and 90 °C, but at 110 °C the order was reversed, with the hydrous ethanol blends having greater cone angle than the anhydrous which has implications for spray collapse upon multi-plume operation. At 20 °C, 1.0 bar, anhydrous ethanol showed larger droplets than iso-octane by ∼5 %, with the addition of 10 % water increasing the SMD by about a further 5 %. Water showed larger SMD than the fuels by 30–40 %. At 110 °C, 0.5 bar, all fuels showed smaller droplets by 20–30 % than at the subcooled condition. When comparing the isolated single-plume spray with the respective plume pair of the multi-hole experiment for anhydrous ethanol and iso-octane at subcooled conditions, it was observed that the plume pair had higher penetration. This could be attributed to near-nozzle effects from interacting adjacent plumes, differences in gas entrainment and drag. At superheated conditions, the penetration of the multi-hole plume pair for iso-octane was similar to that of the isolated single-hole plume spray. However, for ethanol it was clearly shorter due to presence of plume merging and initiation of spray collapse, effects that were absent for the single-plume configuration. The fact that increasing water content was associated with wider near-nozzle plume angle at superheated conditions and stronger propensity for plume merging and spray collapse, despite the slightly lower vapour pressure, suggests that other effects, e.g. related to the higher heat capacity of water and potential temperature saturation upon phase change, could dominate the process.