Evaporation And Combustion Characteristics Of Aluminum/Hydrocarbon-Based Nanofluid Fuels

Traditional liquid hydrocarbon fuels have low volume energy density(generally about 35 MJ·L^-1)and long ignition delay time,which make them difficult to meet the requirements of the development of aerospace technology.The dispersion of nano-particles into liquid hydrocarbon fuels can increase their energy density,and improve their ignition and combustion.Aluminum（Al）powder has a high volume energy density(81MJ·L^-1),and it is widely used in solid propellants.Therefore,the above problems can be solved by dispersing nano-sized Al particles（Al NPs）into liquid hydrocarbon fuel to form Al/hydrocarbon-based nanofluid fuels.However,the composition of hydrocarbon fuels varies greatly depending on the type,such as from a single component to a multi-component,from common types to new types of fuel.If Al NPs are added to these different types and components of hydrocarbon fuels,whether the ignition and combustion performance rules are the same,and whether the mechanism of Al NPs promoting the combustion of hydrocarbon fuels is different.In addition,the use of nano-fluid fuel is mostly spray combustion,and whether the multi-droplet combustion has a synergistic effect is unknown.Therefore,further research is urgently needed to provide theoretical guidance for the early application of Al/hydrocarbon-based nanofluid fuels.In view of the above problems,some nanofluid fuels were prepared with good stability such as Al/n-heptane（single-component fuel）Al/kerosene（multi-component fuel）and Al/JP10（new olefin fuel）and take them as representatives to study their evaporation,igntion,combustion performance using the droplet suspension method combined with a variety of experimental methods.Firstly,the evaporation and combustion properties of double droplets of aluminum/n-heptane-based nanofluid fuels were studied and the results were compared to those of the single droplet at 700℃and 0.1 MPa.The effect of the distance of the double droplets on their evaporation behavior,ignition delay time,combustion phenomena,and micro-explosion time were studied.Secondly,the ignition and combustion characteristics of 5 wt.%aluminum/kerosene-based nanofluid fuel droplets at atmospheric pressure（0.1MPa）and high temperature（650,700,750,and 800℃）were studied and analyzed.The combustion mechanism of adding Al NPs to kerosene droplets was discussed in detail.Thirdly,to improve the combustion performance of traditional Al/kerosene based nanofluid fuel,namely,adding nano-copper oxide（n CuO）into Al-based nanofluid fuel.The effect of n CuO on the droplet ignition delay time,ignition temperature,droplet behavior,burning rate,and combustion intensity of Al/kerosene based nanofluid fuels was explored.Finally,in terms of the problem of insufficient solid phase combustion hindering the full release of energy of Al/JP10 nanofluid fuel,the effect of benzoyl peroxide（BPO）on thermal oxidation of aluminum was studied,and the effect mechanism of surfactant Span80 and benzoyl peroxide on combustion of Al/JP10 nanofluid fuel was studied.The specific results obtained are as follows:（1）The double droplets have a synergistic effect on the evaporation behavior,ignition delay time,and micro-explosion of the Al/n-heptane-based nanofluid fuel.The ignition delay time of double droplets of the Al/n-heptane-based nanofluid fuel has debased about 25.3%compared with that of single droplet,while the maximum microexplosion duration has increased 7.2%.Micro-explosion can reduce the duration of the n-heptane combustion stage,and the duration of the n-heptane combustion stage in the double-droplet group is longer than that in the single-droplet group.With the increase of the distance of both droplets,the synergistic effect is first strengthened and then weakened.（2）Al NPs can reduce the ignition delay of kerosene droplets up to24.4%.Aluminum/kerosene nanofluid fuel droplets’time history from ignition to the first micro-explosion is affected by the consumption before burning and the burning rate of the droplet.The time history from ignition to the micro-explosion and the total combustion duration has a good linear positive correlation.The good thermal conductivity of Al NPs can make the evaporation rate of kerosene droplet increase by nearly three times.When the droplet diameter of the Al/kerosene nanofluid fuel decreases to 50%compared with that of the ignition time,Al NPs form a compact core-shell structure on the surface of the droplet,and the surfactant also forms a corresponding coating layer,which prevent the evaporation of kerosene,but the micro-explosion can destroy the core-shell structure of the droplet to a certain extent.（3）Adding 2 wt.%n CuO to a 2 wt.%Al/kerosene-based nanofluid fuel has no effect on the ignition delay time and temperature at varied ambient temperatures such as 700,750,and 800℃.At 750 and 800℃,the droplet diameter changes substantially over time,the burning rate is enhanced by up to21.7%,and the combustion intensity and oxidation efficiency of Al NPs are improved significantly.Therefore,the n CuO has good performance in improving the combustion of Al/kerosene-based nanofluid fuels.It’s an effective approach to boost the performance of Al/kerosene-based nanofluid fuels.（4）Benzoyl peroxide can promote the oxidation of Al NPs and improve its heat release rate per unit mass.The high thermal conductivity and radiative heat absorption capacity of Al NPs,the accumulation of SP80 on the surface of the droplet,and the decomposition of BPO and the reoxidation of the product have an impact on the combustion time of nanofluid fuel and the behavior of the droplet evaporation.They lead to higher thermal expansion rate of the droplet,more intense micro-explosion and shorter combustion cycle.The more complete the combustion of Al NPs is,the higher combustion intensity of Al/JP10 nanofluid droplets is.