Search

Experimental research was conducted to study the combustion characteristics of clouds composed of micrometer-sized aluminum particles with an oxidizer blend of oxygen and superheated steam. Both ignition and combustion of an aluminum particle are difficult because it is enveloped by an oxide layer that has a high melting point (above 2300 K). To overcome the problem, we induced a particular shape of the combustor that can keep a stable flame of aluminum dust clouds at a lower feeding-rate. The combustion characteristics were examined using a system applied combustor involving a variety of parameters affecting the burning of the dust clouds. Based on these attempts, the aluminum dust clouds had an upper flammability limit above the equivalence ratio (Ф) 1.9 for oxygen diluted by air. Varying the lower limit of flammability, we found, crucially, that the feeding-rate and velocity of the particles are more effective than the equivalence ratio or combustor length. Furthermore, we could confirm the changing of the lower limit from 0.3 to 0.26 because the steam was additionally fed in the combustion of aluminum-oxygen.

Since aluminum responds to various oxidizers and has a high energy density, there are high expectations for its usefulness as a fuel. However, it is covered with an aluminum oxide film, which has a high melting point, and thus, its ignition is difficult. One method suggested to solve this problem is nickel coating; however, in contrast to the extensive amount of research conducted on the overall phenomenon of aluminum combustion, research regarding Ni-coated aluminum is still in nascent stages. This study was carried out to further elucidate the ignition mechanism; thus, millimeter-sized (∼2.38 mm) aluminum particles were used to observe the surface where ignition occurs in air. The spatial and temporal resolutions were heightened by prolonging the heating period. The aluminum particles were nickel coated using electro/electroless methods, and surface analysis by SEM, thermal analysis by TGA/DSC, and species analysis by XRD and EDS were carried out. In addition, two-wavelength pyrometry was used to measure the ignition temperature. The results show that regardless of the nickel content in the coating of the aluminum particles, the ignition temperature was approximately 2400 K, similar to the melting point of aluminum oxide. The thermodynamic and thermophysical characteristics of nickel, aluminum, aluminum oxide, and nickel (II) oxide, and the surface/cross-sectional analysis, thermal and species analysis, and high-speed cinematography of the quenched samples provided a detailed explanation of the ignition process. Through this ignition mechanism, the emitted spectrum of AlO (as an intermediate combustion material) was traced to explain the decrease in ignition delay with increase in nickel content.

In this study, the effects of gaseous methane/oxygen injection velocity ratio on the shear coaxial jet flame structure are analyzed using high-speed imaging along with OH* and CH* chemiluminescence. The images show that, as the velocity ratio is increased, the visual flame length increases and wrinkles of the flame front are developed further downstream. The region near the equivalence ratio 1 condition in the flame could be identified by the maximum OH* position, and this region is located further downstream as the velocity ratio is increased. The dominant CH* chemiluminescence is found in the near-injector region. As the velocity ratio is decreased, the signal intensity is higher at the same downstream distance in each flame. From the results, as the velocity ratio is decreased, there is increased entrainment of the external jet, the mixing of the two jets is enhanced, the region near the stoichiometric mixture condition is located further upstream, and consequently, the flame length decreases.

A slurry fuel is a non-Newtonian shear-thinning fluid with high viscosity. The fuel has a high energy density and has advantages in feed control. When the slurry fuel is injected using a swirl atomizer, the spray characteristics change because the rheological characteristics are different from those of the conventional liquid fuel. In particular, the metallic fuel contained in the slurry is an important parameter that dominates the rheological properties, breakup mechanism, and performance of the slurry swirl spray. In this study, an experimental analysis was performed to investigate the effect of aluminum particles on the spray characteristics of the slurry fuel. The rheological properties (shear-viscosity traces), mass flow rate, and discharge coefficient of the slurry composition were measured, and the influences of the particle content and mean diameter on the spray characteristics of the slurry fuel were explained. The content and mean diameter of the particles were found to be proportional to the viscosity of the slurry, and the flow behavior changes with respect to the particles. As the viscosity increases because of the particles, the mass flow rate and discharge coefficient increase because of the increase in the liquid-film thickness. The kerosene gel and slurry swirl spray undergo a breakup process via an aerodynamic mechanism, forming web-like ligaments, which do not appear in the kerosene because of increase in viscosity. As the injection pressure is increased, the frequency of the pulsation generated in the liquid film increases, and the web-like ligaments tend to be dense. When the particle content decreases, the same phenomenon occurs. As the particle content increases, the breakup length tends to increase with the increase in the thickness of the liquid film. However, the breakup mechanism changes depending on the thickness of the liquid film when the mean diameter of the particles changes. The spray angle is inversely related to the viscosity change due to the particles.

The evolution of multi-dimensional grains of solid rocket motor propellant was carried out using the partial interface tracking method. This method applies the Lagrangian approach to the axisymmetric area of the X–Y plane and the fin area of the Y–Z plane to analyze the multi-dimensional grain burnback. The Lagrangian approach is an effective way to simulate the axisymmetric or 2D shape of a grain, but it is difficult to determine the 3D shape using this method. Therefore, the partial interface tracking method was designed to solve this problem and to get solutions quickly. The grid data resulting from this method is generated in a standard file format, which makes it easy to check the grid shape through commercial modeling software. This method showed good agreement with previous literature and was effective for numerous different grains. Also, it was confirmed that the code can be applicable to the 0D flow field model.

Aluminum particles 15–25 μm in size are widely used in fuel propellants and underwater propulsion systems in national defense research. However, these particles are covered with an aluminum oxide film, which has a high melting point, so ignition is difficult. To improve the ignitability of high-energy aluminum powder and to understand the reaction phenomenon as a function of particle size(15–25 μm, 74–105 μm, and 2.38 mm) and oxidizer(air, CO2, and argon), the natural oxide films are chemically removed. The particles are then coated with nickel using an electro-less method. The degree of nickel deposition is confirmed qualitatively and quantitatively through surface analysis using scanning electron microscopy/energy dispersive spectroscopy. To characterize the nickel coatings, elemental analysis is also conducted by using X-ray diffraction. Thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) enable comparisons between the uncoated and coated aluminum, and the reaction process are investigated through fine structural analysis of the particle surfaces and cross sections. There are little difference in reactivity as a function of oxidant type. However, a strong exothermic reaction in the smaller nickel-coated aluminum particles near the melting point of aluminum accelerates the reaction of the smaller particles. Explanation of the reactivity of the nickel-coated aluminum depending on the particle sizes is attempted.

High-temperature ignition is essential for the ignition and combustion of energetic metal fuels, including aluminum and magnesium particles which are protected by their high-melting-temperature oxides. A plasma torch characterized by an ultrahigh-temperature plasma plume fulfills such high-temperature ignition conditions. A new steam plasma igniter is designed and successfully validated by aluminum power ignition and combustion tests. The steam plasma rapidly stabilizes in both plasma and steam jet modes. Parametric investigation of the steam plasma jet is conducted in terms of arc strength. A high-speed camera and an oscilloscope method visualize the discharge characteristics, and optical emission spectroscopy measures the thermochemical properties of the plasma jet. The diatomic molecule OH fitting method, the Boltzmann plot method, and short exposure capturing with an intensified charge coupled device record the axial distributions of the rotational gas temperature, excitation temperature, and OH radical distribution, respectively. The excitation temperature at the nozzle tip is near 5500 K, and the gas temperature is 5400 K.

Ignition and the burning of air-born single aluminum and magnesium particles are experimentally investigated. Particles of 30 to 106 μm-diameters were electrodynamically levitated, ignited, and burnt in atmospheric air. The particle combustion evolution was recorded by high-speed cinematography. Instant temperature and thermal radiation intensity were measured using two-wavelength pyrometry and photomultiplier tube methods. Ignition of the magnesium particle is prompt and substantially advances the aluminum particle by 10 ms. Burning time of the aluminum particles is extended 3 to 5 times longer than the magnesium particles. Exponents of a power-law fit of the burning rates are 1.55 and 1.24 for aluminum and magnesium particles, respectively. Flame temperature is slightly lower than the oxide melting temperature. For the aluminum, dimensionless flame diameter is inert to the initial particle size, but for the magnesium inversely proportional to the initial diameter.

Flow effects on the collection of submicron particles by a single fiber are investigated by stochastic analysis of the particle deposition evolution. The incident particle-laden stream is simulated by a Lagrangian-Eulerian approach, while the flows around a fiber and particles accumulation are solved using the Lattice Boltzmann method in conjunction with Brownian dynamics to trace the trajectory of randomly moving particles. A boundary surface on the fiber also evolves to include the changing morphology due to particle deposition. The simulation method is validated for collection efficiencies and pressure drop of clean fiber. Brownian effects on particle accumulation were examined in terms of the Peclet number. Predictions of evolving particle-layered filter geometry showed a strong effect of carrier-gas convection on the extent and the morphology of the particle accumulation, which, in turn affected the morphology of the filter. This strong interaction between the carrier-gas convection and the filter membrane causes more active particle accumulation, and thus at all Peclet numbers examined with carrier-gas convection yielded higher collection efficiencies, but with a higher pressure drop.

T-Burner tests of an Al/HTPB propellant in conjunction with a Pulsed DB/AB Method were conducted to find an acoustic amplification factor. Aluminum-fr ee and aluminum-heavy propellants were examined. Instant surface ignition was successfully made by the use of a supplementary propellant of fractionally higher reaction rate. With the prese nce of higher aluminum concentration in the propellants, the pressure perturbations were promptly dampe d down and the pressure fluctuations were no longer dispersive. Addition of aluminum particles into the propellant was advantageous for stabilizing pressure-coupled unstable waves.초 록 연소 시, 입자상 물질에 대한 HTPB/AP 계열 고체추진제의 음향특성을 정량화하기 위해서, Pulsed DB/AB T-burner 실험을 수행하였다. 추진제 전면에서 동시 점화를 위해, 대상 고체추진제보다 연소속도가 빠른 다른 고체추진제를 대상 추진제 앞면에 부착하였다. 다량의 알루미늄이 포함된 고체추진제에서는 T-burner 내부에서 만들어진 압력섭동에 의한 음향학적 불안정성이 매우 빠르게 감쇠되었고, 반대로 알루미늄이 포함되지 않은 고체추진제에서는 상대적으로 매우 느리게 감쇠함을 확인하였다. 본 연구에서는 음향학적 특성값들을 정량화하였고, 이를 통해 연소응답 특성을 계산하였다.Key Words: Solid Propellant(고체추진제), Combustion Response(연소응답), T-burner(T-버너), Fast Ignition Disk(점화보조제), Pulsed DB/AB Method(펄스형 DB/AB 방법)Received 2 December 2013 / Revised 1 March 2014 / Accepted 8 Ma rch 2014

In this study, DC (Direct current) type steam plasma igniter is developed for effective ignition of high-energy density metal aluminum and gas temperature is measu red by emission spectrum of OH radical. Because of the ultra-high gas temperature, the DC plas ma jet is measured by Boltzmann plot method which is the non-contact optical technique and spectrum comparison-analysis. And both methods were applied to experiment after accurate verification. As a result, we could identify that plasma jet temperature is 2900 K ~ 5800 K in the 30 mm range fr om the nozzle tip.초 록 본 연구에서 고에너지 금속 알루미늄의 효과적인 점화를 위해 개발한 직류 방식의 스팀 플라즈마 점화기 가스온도를 OH radical의 방출 스펙트럼을 사용하여 측정하였다. 플라즈마 제트온도는 초고온이므로 비접촉식 광학 계측 방법인 볼츠만 기울기법과 스펙트럼 비교 분석법을 이용하여 측정하였으며 각각의 방법은 정밀하게 검증 후 실험에 적용되었다. 플라즈마 점화기의 노즐 팁으로부터 30 mm 범위에서의 제트온도 측정결과 두 방법 모두 알루미늄의 점화온도(≈2400 K) 이상의 2900 K ~ 5800 K 를 확인할 수 있었다.Key Words: Steam Plasma Igniter(스팀 플라즈마 점화기), High Temperature Measurement(고온측정법), Emission Spectroscopy(방출분광법), Aluminum Ignition(알루미늄 점화)Received 2 June 2013 / Revised 3 January 2014 / Accepted 11 Jan uary 2014

Corresponding author. E-mail: wsyoon@yonsei.ac.krABSTRACT In this study, In order to develop the measurement method of high energy density metal aluminum dust cloud combustion, flame temperature and emission spectrum was measured using spectrometer. Because of the ultra high ㎛-sized aluminum flame temperature more than 2400 K, it was meas ured by non-contact optical technique which is the modified two wavelength pyrometry with 520, 640 nm and spectrum comparison method. These methods were applied to e xperiment after accurate verification. As a result, we could identify that flame tempera ture is more than 2400 K in bottom of combustor in both methods. And on the emission spectrum analysi s, we could measure AlO radical which is occurred dominantly in aluminum combustion.초 록 본 연구에서는 고에너지 금속 알루미늄 군입자 연소 화염 분석을 위한 측정기법 개발 연구로서 스펙트로메터를 사용하여 화염 온도와 자발광 스펙트럼을 측정하였다. 마이크로 크기의 알루미늄 군입자 연소 반응시 발생하는 화염온도는 약 2400 K 이상의 초고온이므로 비접촉식 광학 계측 방법을 사용하였으며, 측정을 위해 개발된 기법은 520 nm, 640 nm를 사용하는 이색법을 응용한 방법과 광대역 파장 비교법으로서 각각의 방법은 정밀하게 검증 후 실험에 적용되었다. 연소실 하단에서 화염온도 측정결과 두 방법 모두 2400 K 이상의 화염온도를 확인할 수 있었으며 자발광 측정 결과 알루미늄 연소 반응시 가장 지배적으로 발생하는 화학종인 AlO를 확인할 수 있었다.Key Words: High Temperature Measurement(고온측정법), Emission Spectroscopy(방출분광법)Received 14 June 2013 / Revised 11 September 2013 / Accepted 18 September 2013

Basically, in order to apply solid propellant as ignition source to high energy metal particle combustion system, we analyzed combustion characteristics of the HTPB/AP/Al, HTPE/AP/Al propellants by using a strand burner. The propellants were tested in a high-pressure windowed strand burner, which was pressurized up to 300 psia with pure argon gas. Strand burner was visualized with two quartz windows and ignition was accomplished by a 10 W laser. The burning rate of propellant was measured with high-speed camera method for frame analysis and photodiode method for combustion time analysis. Emission spectrum was measured with spectrometer at 300 nm ~ 800 nm and 1500 nm ~ 5000 nm and then we analyzed species during propellant combustion.

In this paper, as an altitude simulator, a second-throat exhaust diffuser with no induced secondary flow has been studied. To design a startable diffuser, we first summarize a simple theoretical method based on the normal shock theory. Using the diffusers designed from the present design method, we obtain the diffuser characteristic curves from the small-scale cold-gas tests using cold nitrogen gas as a working fluid. All the experimental test cases are numerically reproduced by using a Reynolds-averaged Navier–Stokes solver, and the numerical method is properly validated with the measured pressure distributions along the diffuser wall andthe pressure in the vacuum chamber. We investigate the effects of the essential geometric factors of a second-throat exhaust diffuser, such as subsonic diffuser, second throat area ratio, nozzle expansion ratio, and nozzle contour, on the starting and evacuation performance. Finally, to get a full picture of a second-throat exhaust diffuser operation, evolving diffuser flows during the starting transient and plume blowback at diffuser breakdown are also studied. Nomenclature A � = cross-sectional area of nozzle throat Ad = cross-sectional area of diffuser inlet Ae = cross-sectional area of nozzle exit As = cross-sectional area of the second throat duct Ax = cross-sectional area of diffuser exit D = diameter of diffuser

The breakup and spray formation of swirl liquid jets with respect to the instant emulsification of heavy fuel oil and water were experimentally investigated. The effects of the degree of emulsion and flashing on the macro- and microscopic spray parameters were measured optically in terms of the rotor speed, mixture ratio, and nozzle exit temperature. Enhanced emulsification and flashing improved the spray characteristics. The effect of flashing on the Sauter mean diameter of the spray decreased as the degree of emulsion increased. However, when flashing occurred, the effect of instant emulsification was small because of the blades in the emulsion chamber and filter prior to the nozzle inlet, which served as a static mixer.

In this study, aluminum flame analysis was researched in order to develop a measurement method for high-energy-density metal aluminum dust cloud combustion, and the flame temperature and UV-VIS-IR emission spectra were precisely measured using a spectrometer. Because the micron-sized aluminum flame temperature was higher than 2400 K, Flame temperature was measured by a non-contact optical technique, namely, a modified two-color method using 520 and 640 nm light, as well as by a polychromatic fitting method. These methods were applied experimentally after accurate calibration. The flame temperature was identified to be higher than 2400 K using both methods. By analyzing the emission spectra, we could identify AlO radicals, which occur dominantly in aluminum combustion. This study paves the way for realization of a measurement technique for aluminum dust cloud combustion flames, and it will be applied in the aluminum combustors that are in development for military purposes.

A simplified analytical modeling of single aluminum particle combustion was conducted. Ignition and quasi-steady combustion (QSC) were separately formulated and integrated. Both the heat transfer from the hot ambient gas and the enthalpy of heterogeneous surface reaction (HSR) served to cause the particle ignition. Conservation equations were solved for QSC parameters in conjunction with conserved scalar formulation and Shvab-Zeldovich function. Limit temperature postulate was formulated by a sink term pertinent to the dissociation of the aluminum oxide near the flame zone. Effective latent heat of vaporization was modified for the thermal radiation. Ignition and QSC of the aluminum particle were predicted and discussed with emphasis on the effect of the aluminum oxide and variable properties. The model was validated with the experiments regarding ignition delay time, burning rate, residue particle size, flame temperature, QSC duration, and stand-off distance of the envelop flame. Agreement was satisfactory and the prediction errors were limited within 10%.

Breakup and spray formation of swirl liquid jets introduced into a low-speed cross-flow are experimentally investigated. Effects of the cross-flows on the macro and microscopic spray parameters are optically measured in terms of jet Weber number and liquid-to-gas momentum ratio. At lower jet Weber numbers, the liquid stream undergoes Rayleigh jet breakup. At higher momentum ratios, bag breakup occurs and tends to distort the liquid column into a loop-like structure. As the jet Weber number rises, stronger aerodynamic interaction and secondary flows cause multi-mode breakup. Regardless of the momentum ratio, the spray profile is hardly altered at higher jet Weber numbers. The cross-flow promotes the jet breakup and renders a finer spray in an entire range of injection velocities.

Breakup and spray formation by impinging liquid jets introduced into a low-speed cross-flow are experimentally investigated. Effects of the cross-flows on the macroscopic and microscopic spray parameters are optically measured in terms of jet Weber number and liquid-to-gas momentum ratio. The liquid stream undergoes Rayleigh jet breakup at lower jet Weber numbers and bag/plume breakup at higher momentum ratio through Kelvin-Helmholtz instability. In particular, the first and the second wind breakup occur at an intermediate jet Weber number. At higher jet Weber numbers, the hydrodynamic impact waves commands and the effect of the convective gas flows is insignificant. The breakup length rises in proportion to the jet Weber number, but starts to decrease when the jet Weber number further rises over 1000. The cross-flow promotes the jet breakup and renders a finer spray in an entire range of injection velocities.

Effects of essential performance parameters on the starting transient of a straight cylindrical supersonic exhaust diffuser (SED) are numerically investigated. Diffuser starting and evacuation transients are examined in terms of SED lengths and pre-evacuation configuration. Preconditioned Favre-averaged Navier–Stokes equations incorporated with a low Reynolds number turbulence model and Sakar’s method to treat turbulence compressibility is solved for strongly turbulent all-Mach diffuser flows. The numerical method is properly validated with the measurements with accuracy. Characteristic locus of diffuser-starting and diffuser-unstarting modes is constructed for the diffusers of three different lengths (L/D = 2, 5, and 20). Flow evolutions visualized in diffuser mode-transition regimes manifest a threshold L/D over which the SED starting transient is unique. An occurrence of plume blowback into the vacuum chamber due to lower initial pressure (PC,INIT/PA = 0.0027) expedites expansion of nozzle exhaust and diffuser choking, and causes faster chamber evacuation than the atmospheric starting.

Nanojet injection in a high pressure environment was simulated using non-equilibrium molecular dynamics. To maintain constant gas pressure, the periodic-shell boundary condition, which can produce a continuous gas flow with constant temperature and velocity, was used. The breakup characteristics of the jet were compared from vacuum to supercritical conditions. Comparing the previous results from vacuum, high pressure effects generally accelerated jet breakup and droplet vaporization processes. Typical supercritical phenomena such as the disappearance of phase interface caused by drastic reduction of surface tension were also observed. At the supercritical state where both pressure and temperature exceed the critical values, a continuous phase transition becomes dominant without droplet formation.

Starting transient and plume blowback at diffuser breakdown of a straight cylindrical supersonic exhaust diffuser with no externally supplied secondary flow are numerically investigated. Fluctuating pressures occurring in the transitional periods are measured by a small-scale cold-gas simulator for high altitudes. The flow-fields evolving in the diffuser-type ejector are solved by a preconditioned Favre-averaged Navier–Stokes equations with a low-Reynolds number k–e turbulence model edited for comprehending compressibility effects. In the steady operation regime, strong momentum transfer by a high-speed jet at diffuser inlet is balanced while it forms a recirculation zone at the diffuser inlet. Flow separation occurs at the diffuser downstream during the starting transient of the diffuser. During the starting transient and diffuser breakdown, abrupt changes in internal shock structures including a transition from Mach to regular shock wave reflection occur. Diffuser choking is advanced by anchoring the sonic line from the nozzle onto the wall of diffuser inlet before the reversed flow from the downstream subsonic region reaches the upstream of the diffuser. Immediately after the diffuser breakdown, higher ambient pressure acts on the upstream flows and the exhaust gases from the nozzle surge into the vacuum chamber. Confronted with the plume blowback, the exhaust flows are blocked and non-monotonic pressure recovery occurs.

With a view to building up a program used in conceptual design of liquid rocket engine system, a preliminary performance-based code for an integrated engine system has been developed by incorporating sub-modular programs for each essential engine component. Modular descriptions for each component were formulated mathematically with essential parameters. In the whole iterative circuits for predicting engine performance, matching conditions of mass flow rate and pressure drop through each engine component have been considered. Mass balance calculations at each inter-component boundary are found smoothly converged. All the pressure drops through engine components as a function of mass flow rate are added up to provide turbo-pump outlet condition. In this paper, the flow chart for each iterative circuit and design methodologies are presented. Resultant predictions are validated with real engine data.

In order to build a conceptual design program for a liquid rocket engine system, performance based sub-programs for each core component of the engine system were made. Parts included were the combustion chamber, supersonic nozzle, centrifugal pump, and impulsive turbine. Simple mathematical models based on classical thermodynamic and inviscid theories were adopted with proper tuning by empirical data. In Part I, aiming to validate each sub-program, we examined the results of each program qualitatively, and parametrically investigated the sensitivity due to the change in design parameters.

To improve the ignitability of high energy aluminum powder, in this study, natural oxide films (alumina, Al2O3) were chemically removed and nickel coatings were deposited using an electroless method for nickel plating. The time-dependent degree of nickel deposition was confirmed qualitatively and quantitatively through surface analysis using scanning electron microscopy/energy dispersive spectroscopy. To characterize the nickel coatings, we also conducted elemental analysis by using X-ray diffraction and weight analysis by using inductively coupled plasma optical emission spectrometry. Thermophysical studies were also conducted using thermogravimetric analysis/differential scanning calorimetry in an airoxidized environment. The results of these analyses explain the ignition enhancement mechanism observed in the nickel-coated aluminum powder. A novel plating method for creating a nickel-coated aluminum powder was established and the mechanism for the enhancement of the ignition and combustion of the powder is elucidated.

Experimental investigation of mixing and mixing-controlled combustion efficiencies for sprays formed by unlike impinging split triplet injector elements was carried out. The quality of mixing was checked by performing cold-flow tests with inert simulant liquids. Measurements of local mass and mixture ratio distributions were made for different injection configurations and different jet momentum ratios. Results show that the quality of macroscopic mixing can be effectively characterized by the jet momentum ratio. The unlike split triplet element with lateral fuel injection is superior in mixing to either unlike doublet or unlike split triplet elements with central fuel injection. Secondary impingement appears to play a significant role in the extent of mixing. Mixing characteristics of the unlike split triplet element and its contribution to the promotion of macroscopic mixing efficiencies are discussed in detail.

We report realistic performance simulation results for a new MWIR camera. It is designed for early detection of long distance missile plumes over few hundreds kilometer in the distance range. The camera design uses a number of refractive optical element and a IR detector. Both imaging and radiometric performance of the camera are investigated by using large scale ray tracing including targets and background scene models. Missile plume radiance was calculated from using CFD type radiative transfer algorithm and used as the light source for ray tracing computation. The atmospheric background was estimated using MODTRAN utilizing path thermal radiance, single/multiple scattered radiance and transmittance. The ray tracing simulation results demonstrate that the camera would satisfy the imaging and radiometric performance requirements in field operation at the target MWIR band.

An experimental study on the secondary spray formation and interaction with primary one for the flash swirl spray was performed. To elucidate the vaporization and atomization characteristics on these spray regions, the local temperatures and droplet sizes were measured and averaged on the axial cross-section using a K-type thermocouple and laserbased optical technique, respectively. In order to obtain the physically meaningful mean droplet size, the time-averaging method was also considered as “total SMD.” Experiments were conducted by ejecting the superheated liquids (water) into an atmospheric pressure environment with variations of essential injection parameters. The elevation of the superheat degree or the injection pressure activates the thermal atomization in the primary spray owing to the increased available energy, which augments the number percent of drops in the secondary spray by the entrained gas effect. On the contrary, the enlargement of the nozzle diameter, in spite of the unchanged available energy, enhances the atomization characteristics of the primary spray by growing the instability of the liquid sheet due to the increment of the vapor production.

Spray formation and characteristics for a superheated hollow-cone swirl spray are experimentally investigated. To prevent any kind of thermal noise pertinent to the phase change, high frequency dielectric heating method is used and laser-based optical technique using Global Sizing Velocimetry (GSV, TSI Inc.) is employed to elucidate the spray characteristics largely altered by complex thermodynamically transition in the void core region. Local spray characteristics are instantly measured and then analyzed on its 2dimensional longitudinal sectional area in terms of dimensionless superheat degree, injection pressure, and nozzle diameter. Flash swirl spray has the relation in the injection pressure and the decline on its mean SMD is larger than that of the sub-cooled liquids. With increasing parameters, its mean SMD linearly decreases and the influence of injection pressure on it increases for the higher dimensionless superheat degree and vice versa. Smaller droplets occur in the void core and they undergo secondary atomization and heterogeneous nucleation with increasing parameters. The region of the secondary droplet jet is axially expanded with increasing dimensionless superheat degree or the injection pressure but it diminishes with that of nozzle diameter.

Ignition and burning characteristics of single aluminum and magnesium particles are experimentally investigated. Burning time, ignition delay, flame temperature, and ignition temperature were measured. The single metal particle (30-100 μm in diameter) is uplifted by an electrodynamic levitator, exposed and ignited by a CO2 laser. The thermal radiation intensity was measured using the photomultiplier tube and combustion history was monitored by high-speed cinematography. Two-wavelength pyrometry measured the temperature of the burning particles. The burning time of the Al particle is approximately 5 to 8 times longer than that of the Mg particle. Exponents of D n -law, for the burning rate of the magnesium and aluminum particles of diameters less than 100 μm, were found to be 0.6 and 1.5, respectively. The instant of the aluminum ignition was clearly distinguished with the ignition delay time little less than 10 ms, however the burning history of the magnesium particle exhibited no clear instant of the ignition. The ignition delay time of the magnesium particle (less than 100 μm) might be in the range from 50 to 200 ns. The flame and ignition temperatures of a single Al particle are slightly lower than the boiling and melting point of Al2O 3

As a preliminary evaluation of system applicability, parametric studies were conducted to figure out primary parameters and their effects with a previously developed simple mechanistic model of single aluminum particle combustion. Initial particle size, ignition temperature, initial oxide film thickness, convection, radiation, heterogeneous surface reaction (HSR), ambient pressure and temperature were selected as the primary parameters. It was intended to deduce a proper ignition temperature from ignition delay time by comparing the predictions with the calculated results of Friedman’s equation. However, the equation of ignition delay was also restrictive in its employment, and thus a melting temperature of aluminum oxide was selected for the ignition temperature as it was before. Ignition delay and burning time were proportional to the particle size increment and flame temperature of small particle was higher than that of bigger one. Influence of initial film thickness on the particle ignition and burning characteristics was insignificant. Ignition delay drastically decreased with the HSR and the radiative heat transfer was as important as the convective heat transfer in quasi-steady combustion (QSC) process. The higher the ambient pressure, the shorter the particle burning time, and burning rate (slope of the D 2 curve) was proportional to P 0.095 . Temperature of both particle and flame increased at higher ambient pressure and flame radius was opposite. Burning time decreased as the ambient gas temperature increased, and burning rate was proportional to T 0.116 .

A simplified modeling of a suspended aluminum particle burning in air is presented. The burning process consists of two stages, ignition and quasi-steady combustion (QSC), and the emphasis is laid on the effects of aluminum oxide (alumina) on the aluminum particle combustion. The film thickness is calculated using the phase diagram of Al-O and lever rule. In ignition stage, the aluminum inside of oxide film is modeled to be melting due to heterogeneous surface reaction (HSR) as well as heat transfer from ambient air until the particle temperature reaches melting point of oxide film. In quasi-steady combustion stage, 1-dimesional spherical symmetric diffusion flame is formulated with a flame sheet assumption. Extended conserved scalar formulations and modified Shvab-Zeldovich functions are used to account for the deposition of metal oxide on the surface of the molten aluminum. Oxide smoke effect is included with Brzustowski’s assumption. Transport properties of materials are expressed as a function of temperature by using ChapmannEnskog kinetic theory except for latent heat. Burning rate, combustion time, flame radius and temperature are compared with the measurements. Evolutions of particle ignition and burning of an aluminum particle is predicted.

The constant area exhaust diffuser (CAED) demands a higher starting pressure than the second throat exhaust diffuser (STED). In the design process, one dimensional (1D) normal shock theory was explored to obtain the optimum starting pressure of STED. Experiments were conducted with a small scale cold gas simulator using nitrogen gas as injectant. The application of 1D normal shock theory for STED design was examined with 20-25% difference in each accuracy in comparison to numerical and experimental evidences. The relation between evacuation quality and essential geometrical parameters such as diffuser– to–nozzle throat area ratio (Ad/At), diffuser–to–the second throat area ratio (Ad/As), and the nozzle expansion ratio (e) is presented for the optimization of STED. The optimum starting pressure increases in proportion to Ad/At and the optimized Ad/As was predicted in the range of 2.2-2.5. After STED starting, the evacuation quality is the same whether the expansion ratio of the nozzle is large or not. However, the range of the transition regime varied according to the nozzle expansion ratio. Nomenclature d A = area of diffuser inlet s A = area of the second throat t A = area of nozzle throat D = diameter e

Numerical analysis is performed on the transition of the Integral Rocket Ramjet using the Reynolds-Averaged Navier-Stokes equations with low Reynolds number k e − turbulence model and finite rate chemistry. Two-dimensional axisymmetric coaxial ramjet is considered from the boost end phase to the steady ramjet operating mode. The mode transition accompanies a certain amount of complex phenomena such as fuel spreading, mixing, ignition and chemical reaction. The detailed transient sequence will be described in the paper. For a fixed total fuel mass flow rate, the unsteady/unstable combusting flow field is driven when the fuel/air mixture ratio is locally changed at the flame holding region. By the investigation of pressure oscillation, heat release rate fluctuation and vortex shedding, it is numerically demonstrated that unsteady vortex shedding is one of the possible reasons of the low frequency combustion instability in ramjet.

Supercritical droplet dynamics in forced-convective environment is numerically studied. A cryogenic oxygen droplet gasifying in axisymmetric gaseous nitrogen stream is analyzed by solving complete sets of time-dependent conservation equations of mass, momentum, energy, and species concentrations. Full account is taken of thermodynamic non-idealities and transport anomaly using a unified property evaluation scheme based on the fundamental equation of state and the extended corresponding-state principle. Results show that extent of thermodynamic and flow properties controls the droplet gasification and deformation. As the velocity of gas stream increases, the droplet aspect ratio gradually increases with pressure. The droplet lifetime is reduced primarily due to augmented convective momentum of nitrogen gas increasing with increase in ambient pressure. With reduction of diffusivity at high pressure, boundary layer at the rear of the droplet is detached. Occurrence of recirculation at the droplet interior is not perceptible.

The combustion responses of the methanol and n-decane sprays reacting with air under the unstable waves are studied with a view to observing the oscillatory spray combustion, and investigating the effects of vaporization on the combustion instabilities. Navier-Stokes solutions for chemically reacting flows are used to calculate the flow variables, mass fractions and droplet histories, which in turn provide necessary data to detect the development of unstable waves. A preconditioned dual-time implicit algorithm is employed to simulate the low Mach flows with strong density variations inside the axisymmetric liquid propellant rocket combustor. Emphasized in the present study is the effect of vaporization-chemical interactions upon unstable waves. The results show that the droplets inside the flame strongly responds to the oscillating pressure, in return, flame is excited by amplifying fuel mass fluctuations. And pure vaporization without the interaction with flame, has much less contribution to amplify the unstable waves.

Homogeneous and heterogeneous nucleations were simulated by molecular dynamics (MD). The behavior of Lennard-Jones molecules was studied inside a liquid-gas system where all dimensions of the wall were periodic and a soft core carrier gas within the system controlled the temperature. In this study, the classical nucleation theory was found to underestimate the homogeneous nucleation rate by five orders of magnitude, which complies with other MD studies. The discrepancy in the nucleation rate between theory and simulation was mainly caused by the fundamental assumption that there are no volumetric interactions in the growth process. In this particular case, however, growth was observed at multiple sites due to Ostwald ripening and coalescence between nuclei by Brownian motion. Furthermore, even though the supersaturation ratio is inadequate for homogeneous nucleation, once a seed is introduced to the system, a cluster can be created. The addition of seeds not only enhances nucleation but also renders coalescence as an important nucleation mechanism in the earlier stages compared to homogeneous nucleation.