Study On Ignition And Combustion Characteristics Of Aluminum/heptane-based Nanofluid Fuel Droplets

How to improve the ignition characteristics is the major problem for hydrocarbon fuels.Nano-aluminum,with more great volume-energy density,can enhance the ignition characteristics of hydrocarbon fuels and lower the ignition temperature,ignition delay time.It also can increase the burn rate at some degree.However,there are few researches about nanofluid fuels.In this paper,nano-aluminum is selected as additives and n-heptane is used as base liquid.The Al/n-heptane based nanofluid fuels are synthesized by ‘two-step method',and the effects of nano-aluminum concentration,particle size and oxygen concentration on the ignition characteristics of Al/n-heptane based nanofluid fuels were investigated by suspend droplet method.The specific conclusions in present work are as follows:Ricinoleic acid is the best surfactant for the stability of Al/n-heptane based nanofluid fuels,and the optimum mass ratio of ricinoleic acid to nano-aluminum particles is 1.5:1.After 50 minutes of ultrasonic dispersion,the stability of Al/n-heptane based nanofluid fuels can be extended for one week.With the temperature increasing,both the ignition delay time and the ignition temperature decrease,which means that the ignition character: stable combustion,micro-explosion and secondary stable combustion.With temperature increasing,the production of soot is increasing when the droplet of heptane is ignited.In addition,the degree of micro-explosion and ejection is enhanced.The burn rate is increasing.Surfactant can increase the ignition delay time and decrease the ignition temperature.The higher of the temperature increases,the reduction of ignition temperature is more obvious.Little amount of surfactant can decrease the production of soot.However,higher concentration of surfactant will increase the production of soot.It is difficult for droplet with higher concentration of surfactant to burn out.After the flame disappears,there is little droplet remaining which would evaporate and run out finally.Nano-aluminum particles can accelerate the evaporation and combustion of Al/n-heptane based nanofluid fuels.With temperature increasing to below 700 ?,the ignition delay time of nanofluid with the same concentration decreases.However,the promotion effects of nanoparticles weaken when the temperature is above 700 ?.With temperature increases,the ignition temperature decreases.When the ambient temperature is 600 ?,lower concentration of nanoparticles will inhibit the ignition of the nanofluid.However,as the ambient temperature increases,the nanoparticles will promote the ignition and the promotion effect is firstly enhanced and then weakened.After the addition of nanoparticles,the production of soot is increasing,and the degree of micro-explosion and deformation of droplet is greater.With the concentration of nanoparticles increasing,the size of ejected droplet increases and the ignition of nano-aluminum is less obvious.The size of nanoparticles has no effects on the ignition delay time,but it can influences the ignition temperature.When the ambient temperature is lower than800 ?,the ignition temperature decreases with the size of added nanoparticles increasing.When the ambient temperature is over 800 ?,the phenomenon is opposite,and the bigger nanoparticles can inhibit the generation of bubbles and micro-explosion.The concentration of oxygen can change the ignition characteristics of Al/n-heptane based nanofluid fuels.With the concentration of oxygen increasing,both the ignition delay time and the ignition temperature decrease,and the droplet exhibits secondary combustion under the flowing atmosphere.At the same time,the nano-aluminum would cluster and burn.The ignition characteristics of nanofluid under the flowing atmosphere with 20 % concentration of oxygen is worse than that of in static atmosphere.