Woongsup Yoon

Performance Analysis of Secondary Gas Injection into a Conical Rocket Nozzle

The effects of injection location of secondary jet and nozzle divergent cone angle on the performance of secondary gas injection for thrust vector control (SITVC) were numerically investigated. Three-dimensional Reynolds-averaged Navier-Stokes equations with an algebraic turbulence model solved the complex three-dimensional nozzle flows perturbed by the secondary gas jet. The numerical code was properly validated by experiment. The results showed that downstream secondary jet injection and smaller nozzle divergent cone angles lead to higher efficiencies over a wide range of pressure ratios. The occurrence of reflected shock waves severely lowers SITVC propulsion performance. Upstream jet injection is more effective in a narrow range of deflection.

Effect of Momentum Ratio on the Mixing Performance of Unlike Split Triplet Injectors

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.

Molecular dynamics analysis of multiple site growth and coalescence effects on homogeneous and heterogeneous nucleations

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.

Burning and Ignition Characteristics of Single Aluminum and Magnesium Particle

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

Spray Characteristics of a Pressure Swirl Nozzle under an Atmospheric Condition due to Flash Boiling

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.

Parametric Investigation on the Sensitivity of the Simplified Aluminum Combustion Modeling

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 .

Orifice Diameter Ratio Effect on the Mixing Performances for Split Triplet Injectors

The effect of orifice diameter ratio on the mixing qualities of unlike-doublet and split-triplet impinging elements is experimentally studied. The quality of mixing was checked by performing cold-flow tests with nonreacting immiscible simulant liquids. The local volume fractions are determined by direct measurement of the volume of each liquid. The test matrix comprises combinations of the orifice diameter ratios from 1 to 1.5, with the jet momentum ratios (oxidizer/fuel) in the range of 0.5-6. Results show that impinging elements of unequal orifice diameters exhibit substantially different mixing qualities. Maximum mixing efficiency occurs at higher momentum ratios with increasing diameter ratio. Relative jet velocity ratio for optimum mixing ranges from 0.65 to 0.78 and from 0.87 to 0.92 for unlike-doublet and split-triplet elements, respectively. The split-triplet element is superior to the unlike-doublet element in both the mixing efficiency and the mixing-controlled characteristic velocity in the test range of interest. The diameter ratio effect on the mixing of split-triplet element is less significant than that on the unlike-doublet element mixing. The mixing factor of the split-triplet element for maximum mixing efficiency is 0.75.

Optical Diagnostic Study for Flame Characteristic Analysis in Aluminum Dust Clouds

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

An Experimental Study of Laser-induced Ignition of Solid Propellant with Strand Burner

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.

Design and Analysis of a Second-Throat Exhaust Diffuser for Altitude Simulation

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