Droplet Combustion Behavior of Vegetable Oil Surrogate Fuel Enhanced with Alcohols

Internal combustion engines play an essential role in industry and transportation due to their reliability, efficiency, and cost-effectiveness. However, emissions produced from fossil fuels impact environmental degradation, so alternative fuels are needed to increase combustion efficiency and reduce emissions. Vegetable oil is introduced as a renewable energy source, which is potentially used as a substitute for diesel fuel. However, vegetable oils are not fully compatible with diesel fuels, although they have some advantages regarding their properties. Therefore, this work aims to study the mechanism and enhancement of the single isolated droplet combustion behavior of oleic acid as the vegetable oil surrogate compound by adding 20 vol% of alcohol with different molecular structures. The droplet was suspended on the fiber to observe the combustion behavior under static conditions. The droplet dynamics and microexplosive behavior are investigated using spatial and temporal tracking under atmospheric pressure and normal gravity conditions. The post-processing procedure was used to observe the evolution of flame and droplet diameter. The fuel blends exhibit severe volumetric expansion followed by child droplet ejection with various droplet breakup structures due to different nucleation modes. This mechanism is essential to enlarge the fuel reaction zone with ambient air, which shortens the droplet lifetime with a lower ignition point. Methanol significantly reduces the droplet lifetime, while only 2-propanol reduces the ignition delay and burning lifetime among alcohol additives. Methanol addition generates the highest droplet peak temperature due to the faster burning rate, even though it has a lower energy density. Alcohol increases the flame standoff ratio due to the microexplosive behavior. During the unsteady and quasi-steady combustion phases, an elongated flame structure is formed due to natural convection, which is also related to soot formation. Alcohol diminishes the sooting propensity of oleic acid combustion with lower flame luminosity.