| In the study of hydrocarbon fuels,it is an urgent problem to be solved to reduce the ignition delay period and increase the fuel combustion rate.Nano-metal powder has many excellent characteristics.When the nano-aluminum powder is applied to liquid hydrocarbon-fuels and forms“Al/hydrocarbon based nanofluid fuels”,it can significantly increase the energy density of liquid hydrocarbon-fuels and it is likely to reduce the ignition delay time and improve the combustion performance.However,the study of evaporation,ignition and combustion performance for Al/hydrocarbon based nanofluid fuels is very rare now,so we study the evaporation,ignition and combustion characteristics of Al/hydrocarbon based nanofluid fuels as a function of nano-aluminum powder,nano-aluminum powder content and the mass ratio of surfactant to nano-aluminum powder.In this paper,nano-aluminum particles was selected as additives,and ethanol was used as the base fuel,and a stable nanofluid fuel was prepared using a“two-step method”.The effects of nano-aluminum powder concentration,particle size and surfactant ratio on the evaporation characteristics of nanofluid fuel droplets were studied through an experimental device constructed by ourselves.The synchronization of the high-speed camera image acquisition system and the thermocouple temperature measurement acquisition system was achieved through the setting of the signal generator.Using the self-designed matlab program,the images of the evaporation process were processed in batches to obtain the rule of droplet diameter variation.Combined with the temperature curve,the characteristics of droplet evaporation were analyzed.The ignition and combustion characteristics of nanofluid fuels droplets was also investigatedThe specific conclusions in present work are as follows:(1)The evaporation process of ethanol and oleic acid-added ethanol droplets was studied.It was found that the evaporation process of ethanol droplets with oleic acid was similar to that of ethanol,and both experienced the initial heating stage and the equilibrium evaporation stage following the d~2 law.The evaporation behavior of ethanol droplets and ethanol droplets with oleic acid was used as a reference.It was found that when the concentration of nano-aluminum powder was added above 0.1%,the droplet fragmentation stage occurred during the droplet evaporation process due to bubble generation and fragmentation,indicating the generation and fragmentation of bubbles was due to the addition of nano-aluminum powder.In the droplet fragmentation stage,droplet evaporation didn't follow the d~2 law.(2)To further analyze the evaporation behavior of nanofluid fuel droplets during the unique fragmentation stage,the bubble breaking strength and bubble breaking delay were defined.The bubble breaking mode was classified into needle spray according to the size of the bubble breaking strength.When the bubble breaking strength was low,the liquid droplets broke into the needle-like spray mode,needle-shaped downward jet appeard on the droplet surface and caused it small-scale wrinkling.When the bubble breaking strength was further increased,it turnd into high-intensity injection mode.The increase in the ambient temperature lead to the enhancement of the occurrence of the high-intensity injection mode.(3)At 350°C and 500°C,as the concentration of nano-aluminum powder added increases(0.1-1.0%),droplet break-up generally occurs earlier,with higher strength.This is due to the strong radiation-absorbing nature of the nano-aluminum powder,where the nano-aluminum powder on or near the surface of the droplets are heated above the local boiling point of ethanol,thereby providing the surrounding liquid with multiple vaporization centers,which in turn generate superheated steam.The increase of nano-aluminum powder concentration favors the generation and accumulation of superheated vapor inside the droplet.Under high temperature(500-650°C),the addition of nano-aluminum powder increases the evaporation rate of ethanol droplets,and at an ambient temperature of 500°C.With the increase of the concentration of nano-aluminum powder(0.1%-1%),evaporation rate of ethanol droplets gradually increased.(4)At 350°C,the diameter change in the evaporation process of nanofluid droplets with 80 nm and 150 nm Al powder and were similar to ethanol droplets.The addition of large-sized nano-Al powder had less influence on the formation and movement of bubbles inside the droplets.At ambient temperature of 650°C,the bubble breaking delay time increased with higher particle size of the aluminum powder,with a lower breaking strength.The addition of large particle size nano-aluminum powder reduced the rate of bubble generation within the droplet.The number of vaporization cores inside the droplet was less than that of the droplet with 50 nm aluminum powder.(5)At 650°C,when the mass ratio of surfactant to nano-aluminum powder is 1:2and 1:1.5,the phenomenon of bubble breaking didn't appear,and the droplet breaking strengths of the other three ratios did not differ much.Under the high temperature,the low ratio of oleic acid to nano-aluminum powder makes the agglomeration of nano-aluminum particles in the droplets intensify,which affects the rate and quantity of the nano-aluminum particles as the vaporization center.(6)In the ignition characteristics of nanofluid droplets experiment,it was found that the combustion processes of ethanol droplets and nanofluid droplets were similar,and similar.The combustion process was divided into three distinctive stages:the ignition combustion stage,the flame extinguishment stage and the secondary combustion stage.Compared with the ethanol fuel droplets,the ignition delays of fuel droplets containing ethanol with 0.5 and 2.5%aluminum concentration decreased by14.29%and 23.81%,respectively.The ignition temperature decreased by 6.10%and20.24%,respectively.It was indicated that the addition of nano aluminum powder enhances the ignition characteristics of the droplets.When the concentration of aluminum powder was 2.5%,the ignition delay time of nanofluid droplets with particle sizes of 50 nm and 100 nm decreased by 23.81%and 19.48%,respectively,and the ignition delay time of nanofluid droplet with 50 nm aluminum powder was lower than nanofluid droplet.The ignition temperature of nanofluid fuel droplets with two kinds of particle size is almost similar. |
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