| As the ideal propulsion device for future hypersonic vehicles,the ramjet engine has the advantages of wide operating range,high Mach number flight impulse and simple structure.It has become an important development trend to widen the operating range and increase the operating lift of ramjet engines.However,when the ramjet engine flies in the critical space above 20 km,the dynamic pressure of air drops significantly,which causes the pressure drop in the combustion chamber of the ramjet engine and the performance of the combustion chamber deteriorates.In the actual engine,the liquid fuel mainly enters the combustion chamber by spraying.However,the decrease of airflow density in the low-pressure environment leads to the deterioration of liquid fuel spraying performance,the increase of droplet sauter mean diameter(SMD),and the longer period of droplet evaporation combustion in the combustion chamber,which is unfavorable to the stable combustion of scramjet engines with millisecond fuel residence time in the combustion chamber.RP-3 kerosene is a high-carbon hydrogen fuel widely used in domestic aircraft,and it is important to reveal the evaporation,micro-explosion and ignition characteristics of RP-3 and blended fuel under low-pressure and high-temperature environment to improve the fuel atomization level and combustion stability under extreme operating conditions and to expand the working range of ram engines.In this dissertation,the evaporation,micro-explosion and combustion characteristics of RP-3 at low pressure are investigated by using laser diagnosis,high-speed camera and soot thermophoretic diffusion detection techniques.At the same time,the evaporation,micro-explosions and ignition characteristics of RP-3/nanoboron and RP-3/ethanol/water at low pressure and high temperature were investigated for the problems of long ignition delay and poor atomization at low pressure.And the hydrodynamic mechanism of micro-explosions of miscible multicomponent and complex three-component droplets at real spray scale was studied by using levelset coupled volume fraction(VOF)numerical simulation method.The effects of bubble nucleation position,superheat temperature and ethanol/water ratio on the micro-explosion process were revealed.Firstly,a high-temperature pressure-controlled droplet evaporation and combustion experimental device was designed and built independently to study the evaporation,micro-explosion and combustion characteristics of RP-3 droplets under low-pressure and high-temperature environment.And it was found that at 473 K,the change of RP-3 droplet diameter deviated from the classical d~2-law and the evaporation rate gradually decreased.At the remaining temperatures,the droplet evaporation rate increases with increasing temperature and pressure.The decrease in pressure resulted in a circular distribution of combustible vapor before droplet ignition.The decrease in temperature and pressure significantly increased the delay time of spontaneous ignition,and the law was basically in accordance with Arrhenius’law.The analysis revealed that in addition to affecting the chemical delay,the decrease in evaporation rate at low pressure also increased the physical delay,which prolonged the ignition time at low pressure.A sufficiently high ambient temperature(673 K or higher)is a necessary condition for the micro-explosion of RP-3 droplets,and the increase of ambient temperature can effectively promote the occurrence of micro-explosion.Reducing the pressure can inhibit the micro-explosion of RP-3 droplets at high ambient temperatures,but increase the growth rate of bubbles and the degree of droplet fragmentation.Secondly,the combustion characteristics of RP-3 droplets at low pressure were investigated based on the self-designed droplet forced ignition,laser detection and soot collection system.The distribution area of OH*and CH*radicals gradually changes from elliptical to circular as the pressure decreases,and the distribution area and its signal intensity both tend to increase.As the ambient pressure decreases,the droplet combustion rate decreases and the flame temperature increases.The soot emission from droplet combustion is divided into three stages:(1)ignition(2)steady expansion and(3)contraction.The average soot volume fraction in the droplet flame increased with the increase of pressure,with a power of 2.044±0.066.The size of soot particles decreased significantly in the low pressure environment,and it was found that the effect of low pressure on soot concentration was mainly achieved by reducing the particle diameter,while the effect on particle number density was minor.Subsequently,the evaporation,micro-explosion and spontaneous ignition characteristics of RP-3 blended fuels were investigated at high temperature and low pressure.The RP-3/nanoboron reduced the minimum ambient temperature required for puffing to occur and increased its micro-explosion capability.The lower the ethanol content in the droplet,the later the micro-explosion generation time,but the micro-explosion intensity is enhanced.The low pressure environment can improve the micro-explosion intensity of blended droplets.Compared to boron-blended fuels,ethanol/water blended fuels have higher micro-explosion intensity(1000-10000 vs.0.2-15 mm/s),shorter droplet lifetime and spontaneous ignition time at the same ambient temperature.30%water group fuels have the shortest spontaneous ignition/breakup time,23.17%and 26.92%shorter than RP-3 at 1.0 and 0.8 bar pressures,respectively.The principle of fire promotion is to increase the release rate of combustible vapor and its rate and degree of mixing with air by promoting the puffing and micro-explosion of droplets during the evaporation period after entering the high temperature environment,thus reducing the auto-ignition delay time of droplets.Finally,a two-component phase change model was written independently based on levelset coupled VOF numerical simulation technique,and the bubble growth in an infinitely large miscible two-component liquid was numerically calculated and verified in comparison with the theoretical results,and the accuracy of the multi-component phase equilibrium obtained by the table-building interpolation method was also verified.The micro-explosion phenomena of miscible two-component droplets and complex three-component emulsion droplets at the actual spray scale were studied.The miscible two-component micro-explosion process can be divided into three stages:bubble growth,cavity expansion,and recoil.The change in micro-explosion morphology promotes a fuller contact between the boiling surface and the air.Bubbles farther from the droplet surface take longer to burst and produce larger satellite droplets,boiling area and higher boiling mass transfer rates.Unlike miscible two-component droplets,emulsion droplets eject vapor outward from the boiling surface after puffing occurs,continuing to drive water/ethanol droplet intrusion into the droplet interior.The rise in water content in the sub-droplet enhances the destruction of the fuel parent droplet,inhibits the development of Landau-Darrieus and Rayleigh-Taylor instabilities at the sub-droplet/parent droplet interface,and increases the sub-droplet intrusion rate,causing secondary rupture and puffing at the other end of the oil droplet.The increase in ethanol content in the sub-droplet facilitates the increase in the scalar dissipation rate(SDR)of ethanol,which has a 3-shaped distribution at the location of the droplet rupture and reaches a maximum at the oil droplet breakup,and then rapidly decreases with gas phase diffusion.There are high SDR values at the secondary breakage of the oil droplet. |
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