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In summer, the high ambient air temperatures and increased solar radiation cause rises in rail temperatures, and rails can experience the buckling. In this study, a thermofluidic analysis was conducted to predict the temperature of a rail. Numerical simulations were performed by changing the orientation of the rails. The south-north rail and the east-west rail was named “0° rail” and “90° rail”, respectively. The numerical results from the 0° and 90° rails were validated by comparing them with measured values. Based on this validation, the temperature distribution of a 45° rail was analyzed. The total amount of energies received by the rail were in the order of 0° > 45° > 90°. The results of this study showed that the temperature of a rail can be sufficiently predicted through numerical simulation, and this approach can be used in a forecast to predict a time when buckling might occur.

Recently, electrically driven pump-fed cycle rocket engines (i.e., ElecPump engines) have received considerable attention. ElecPump engines are superior to pressure-fed engines and will likely replace gas generator cycle engines under a 100 kN thrust level. This study focuses on LOX/methane ElecPump engines with a low-thrust level and estimates the nominal operation point of the engine. The objective variable was set to the maximum velocity increment of the stage. The mixture ratio and combustion pressure were explored as the design variables. Thus, the optimal mixture ratio was proposed as the design variable, considering not only the engine weight but also the structural weight.

Combustion instabilities are often-observed phenomena which take place in acoustically closed spaces such as combustion chambers in propulsion systems. These phenomena should be studied consistently because they can make considerable damage on the combustion chambers and even entire engine systems. Acoustic cavity has been widely used as a passive stabilization device to suppress these combustion instabilities. To elucidate damping capacity of acoustic cavity according to various geometric shapes, a well-known Rijke tube facility has been used to simply include the interaction of heat and acoustic field. The damping capacity has been evaluated quantitatively in terms of bandwidth and amplitude ratio. Present results showed that the acoustic cavities having large orifice area showed better damping capacity. But, in case of orifice length, the length above a certain value makes cavity volume and resultant acoustic stiffness too small, so the damping capacity was decreased above a threshold-like specific length. As for the number of cavities, the damping capacity increases with the number, but, in a certain number or more, the damping capacity has been attenuated and the significant increase of the capacity was not observed. In case of the acoustic cavities which were thought to have sufficient damping capacity, decay time has been measured to quantify the damping capability in time domain. The effect of orifice area on decay time was much higher than those of orifice length. These results from heat and acoustic field interaction through Rijke tube showed that the pure acoustic approach would be insufficient for the fine tuning of acoustic cavity, and further combustion tests would also be necessary to optimize the shapes of the cavity.

Monitoring rail temperature is very important for determining the safe running speed of trains and to prevent buckling. In general, the maximum variation of the internal rail temperature can be >7 ℃ depending on the point of measurement. However, there is as yet no sufficient information about how to predict the measurement point to represent the thermal deformation due to temperature distribution. In this study, the authors report a new point, called the representative measurement point, at which the rail temperature can be measured. This point considers the average deformation of the rail through structural analysis by adopting experimental and actual rail temperature data. The authors designed and installed a measurement system similar to an actual rail environment. Using the system, various data were acquired (internal/surface rail temperature and weather data) for 10 months. On the basis of these data, an analysis was done to calculate the average deformation point through thermal analysis. Finally, the representative measurement point was proposed as the position at which the average deformation point converges regardless of weather or seasons. The authors believe that the method described herein is advantageous in that it could be used in a high-accuracy temperature-monitoring system and for predicting thermal deformation and buckling.

In railway-safety management, the rail temperature, which directly affects the buckling of a rail, is very important. Train companies have been measuring rail temperature directly to prevent buckling and to limit speeds on trains. However, since it is difficult to directly measure the temperature distribution over an entire rail, various models for predicting this temperature using climate information have been developed. In this study, we propose a novel rail-temperature-prediction model based on the energy-equilibrium equation with consideration of the position of the sun. When compared with previous models, the newly proposed model shows higher performance in terms of root-mean-square error and R-squared value. It is expected to be very helpful for train-safety management.

In this study, the auto ignition with low limit temperature of syngas has been numerically investigated using a 2-D numerical analysis. Previous study showed that auto ignition was observed at above 860 K in co-flow jet experiments using syngas and dry air. However, the auto ignition at this low temperature range could not be predicted with existing chemical mechanisms. Inconsistency of the auto ignition temperature between the experimental and numerical results is thought to be due to the inaccuracy of the chemical kinetic mechanism. The prediction of ignition delay time and sensitivity analysis for each chemical kinetic mechanism were performed to verify the reasons of the inconsistency between the experimental and numerical results. The results which were calculated using the various mechanisms showed significantly differences in the ignition delay time. In this study, we intend to analyze the reason of discrepancy to predict the auto ignition with low pressure and low temperature region of syngas and to improve the chemical kinetic mechanism. A sensitive analysis has been done to investigate the reaction steps which affected the ignition delay time significantly, and the reaction rate of the selected reaction step was modified. Through the modified chemical kinetic mechanism, we could identify the auto ignition in the low temperature region from the 2-D numerical results. Then CEMA (Chemical Explosive Mode Analysis) was used to validate the 2-D numerical analysis with modified chemical kinetic mechanism. From the validation, the calculated λexp, EI, and PI showed reasonable results, so we expect that the modified chemical kinetic mechanism can be used in various low temperature region.

The center body diffuser is one of supersonic diffuser that can simulate high-altitude environment. There is center-body structure inside the diffuser, and a complex fluid flow is occurred inside the diffuser because of the interaction of the CB structure with gas exhausted from the nozzle outlet. In this study, starting point and flow characteristics of diffuser were investigated according to changing the CB nose cone angle and the length of distance between nozzle and CB structure. The differences of the supersonic flow were compared through each parameter of CB distance and CB nose angle. First changed parameter was length between nozzle and CB. According to the length of distance between nozzle and CB, axial momentum was developed and oblique shock wave moved front of CB from end of CB nose cone. Also, when CB position was located on a certain length, starting point of CBD decreased. Next change parameter was angle of CB nose cone. According to the angle raised, angle of oblique shock wave was raised and radial momentum of supersonic diffuser developed. But, according to radial momentum of supersonic flow over certain angle, the starting pressure of CBD increased. Because axial momentum which isolated vacuum chamber from atmospheric pressure. Through these CFD analysis results, it was shown that angle and length of distance between nozzle and CB influent performance of CBD.

The objective of present study is to assess the performance of the first stage compressor in a total 3-stage 5000 HP-level turbo compressor. CFD commercial code, CFX has been used to predict three-dimensional flow characteristics inside of the impeller. Shear Stress Transport (SST) model has been used to simulate turbulent flows through Reynolds-averaged Navier-Stokes (RANS) equations. Grid dependency has been also checked to get optimal grid distribution. Numerical results have been compared with the experimental test results to elucidate performance characteristics of the present compressor. In addition, flow characteristics of the impeller only have been studied for various blade configurations. Angular offset in leading edge of the blade has been selected for the optimal blade design. Performance characteristics in region of low mass flow rate and high pressure ratio between the impeller entrance and exit have been investigated for the selection of optimal blade design. Also, flow instability such as stall phenomena has been studied and anti-stall characteristics have been checked for various blade configurations in the operational window.

A numerical study was conducted to investigate the flow and heat transfer characteristics of a supersonic second throat exhaust diffuser for high-altitude simulations. The numerical results were satisfactorily validated by the experimental results. A subscale diffuser using nitrogen was utilized to investigate starting pressure and pressure variation in the diffuser wall. Based on the validated numerical method, the flow and heat transfer characteristics of the diffuser using burnt gas were evaluated by changing operating pressure and geometric shape. During normal diffuser operation without cooling, high-temperature regions of over 3000 K appeared, particularly near the wall and in the diffuser diverging section. After cooling, the flow and pressure distribution characteristics did not differ significantly from those of the adiabatic condition, but the temperature in the subsonic flow section decreased by more than 1000 K. Furthermore, the tendency of the heat flux from the diffuser internal flow to the wall was similar to that of the pressure variations, and it increased with operating pressure. It was confirmed that the heat fluxes of the supersonic and subsonic flows in the diffuser were proportional to the operating pressure to the 0.8 and −1.7 power, respectively. In addition, in the second throat region after separation, the heat flux could be scaled to the Mach number ratio before and after the largest oblique shock wave because the largest shock train affected the heat flux of the diffuser wall.

This paper presents two case studies to evaluate passive stabilization in combustion chambers by quantifying damping capacity characteristics of acoustic cavities in combustion chambers using a three-dimensional linear acoustic analysis that has been applied in a previous study. A feasible interpretation was performed for a FASTRAC thrust chamber, where both successful and failed cases were reported. Regarding the temperature and sonic velocity in an acoustic cavity, the possibility of fine-tuning is evaluated in terms of acoustic absorption and conductance characteristics using the acoustic impedance concept. For a given acoustic cavity, the sonic velocity in the cavity required to achieve an optimal tuning is determined, which agrees with previous experimental results. The results show that the adopted cavities are not tuned appropriately to the frequency of combustion instabilities. Hence, it can be concluded that using an acoustic cavity with a small volume or an orifice having a large cross-sectional area can result in sufficient acoustic damping and the suppression of instabilities. In addition, the feasible estimation of sonic velocity in the acoustic cavity is proposed to ensure sufficient acoustic absorption. The results show similar trends without significant deviations, which can be applied in the design and verification of an acoustic cavity. From the present results, a preliminary fine-tuning of the acoustic cavity can be performed using this pure acoustic analysis.

The chlorides of sodium and potassium and other mineral salts are often present in solid refuse fuel (SRF) in concentrations of at least 1–2%. These salts can generate clinker during combustion and cause serious operational problems, such as blocking of the gas passage, reduction in boiler operational efficiency, and corrosion of the heat exchanger tubes in the boiler. Although many solutions have been proposed to control clinker formation in boilers, herein we investigated the effects of boron compounds (boric acid and borax) on the decomposition of the mineral salts present in SRF to control the formation of fly ash. Using a laboratory-scale reactor, isothermal reactions between the boron compounds and the chlorides were performed at 900 °C for 10 h. HCl gas was released during the decomposition reactions, indicating that a chemical reaction had occurred on the surfaces of the boron compounds. Peaks related to sodium and potassium borates were observed in X-ray diffraction patterns, suggesting that these compounds had been formed during the reactions. Scanning electron microscopy images of the reaction products showed clusters of the mineral salts on the matrix of the borax particles, indicating that changes in the morphology owing to physical adsorption had also occurred during the reactions.

The present study has proposed a numerical methodology based on harmonic analysis of the convective Helmholtz equation to predict acoustic characteristics of combustion chambers with passive stabilization devices such as baffle and acoustic resonators. In order to resolve complex geometries of the acoustic damping devices, a three-dimensional Galerkin finite element method with four-type hybrid elements is adopted. The acoustic energy dissipation in the laminar wall boundary layer is taken into account by a wall damping model based on acoustic admittance. Special effort is devoted to obtain quantified parameters for comparative evaluation of acoustic damping capacities: (1) eigenfrequency shift and damping factor ratio for baffles, and (2) absorption coefficient and conductance for acoustic resonators. The numerical results have been compared with measured data from two different acoustic tests for baffle and Helmholtz resonators, respectively, and demonstrate that the present method is capable of reproducing quantitatively acoustic behaviors of the damping devices in terms of the quantified parameters if the wall damping model is appropriately adjusted.

A numerical study has been conducted to investigate flow and heat transfer characteristics of supersonic second throat exhaust diffusers for high altitude si mulation. By changing pressure and configuration, flow and cooling characteristics of the diffuser have been studied. At the normal operation of the diffuser, there were high temperature regions over 3,000 K without cooling, especially near wall and in subsonic diffuser part. If the cooling system of the diffuser is added, flow velocity is increased due to the cooled wall temperature. 초 록 고고도 모사를 위한 초음속 이차목 디퓨저의 유동 및 열전달 특성에 대한 수치적 연구를 수행하였다. 디퓨저의 유동 특성에 영향을 주는 작동압력과 형상을 변화시켜 유동 특성과 냉각 특성을 파악하였다. 냉각이 없는 경우 디퓨저가 시동 된 후, 디퓨저 벽과 아음속 구간에서 3,000 K 이상의 고온 구간이 나타났다. 디퓨저에 냉각 시스템을 추가하면 벽면 근처가 냉각되면서 유속이 빨라져 유동 길이가 길어지고 유동 박리와 함께 압력 회복이 급격해진다. 디퓨저 내부에 압력 변화를 가져오는 유동 현상과 함께 heat flux의 경향도 유사하게 나타났다. Key Words: High-altitude Simulation(고고도 모사), STED(Second Throat Exhaust Diffuser)(이차목 디퓨저), Plume(플룸), Normal Shock(수직 충격파), Burnt Gas(연소가스)

The flow rate control of a cavitating venturi has been investigated with downstream pressure variation. A set of cavitating venturies for a liquid rocket engine thrust chamber firing test facility have been designed and manufactured. The flow characteristics of the cavitating venturies have been analyzed by experimental and computational methods. Results showed that constant mass flow rate condition was established by the cavitation inside the venturi. However, upstream pressure less than the actual design pressure of the cavitating venturi could not supply a constant flow rate.

The effect of thermal stratification, spark-ignition, and turbulence on the ignition of a lean homogeneous iso -octane/air mixture at constant volume and high pressure is investigated by direct numerical simulations (DNS) with a new 99-species reduced kinetic mechanism developed for very lean mixtures from a detailed mechanism. Two-dimensional DNS are performed in a fixed volume with two-dimensional isotropic velocity spectrums, temperature fluctuations, and an ignition source superimposed on the initial scalar fields. The influence of variations in the initial temperature field imposed by changing the variance of temperature, the ignition-timing by changing the time at which the ignition source is superimposed, and the turbulence intensity and length scale on ignition of a lean iso -octane/air mixture is elucidated. The mean heat release rate increases more slowly and ignition delay decreases with increasing thermal stratification under homogeneous charge compression-ignition (HCCI) conditions since the present mean temperature lies far outside of the negative temperature coefficient (NTC) regime. The spark-ignition induces relatively short ignition delay under spark-assisted compression ignition (SACI) conditions while mildly spreading out the mean heat release rate. For SACI combustion, high turbulence intensity decreases the ignition delay more by increasing the turbulent flame area. Displacement speed and Damkohler number analyses reveal that the high degree of thermal stratification induces deflagration at the reaction fronts, and hence, the mean heat release rate is smoother subsequent to thermal runaway occurring at the highest temperature regions in the domain. For SACI combustion, the heat release rate occurs solely by deflagration prior to the occurrence of the maximum heat release rate and subsequently by mixed modes of deflagration and spontaneous ignition. These results suggest that thermal stratification provides smooth operation of HCCI engines and moreover, spark-ignition can precisely control the ignition timing for SACI combustion.

․ Seong-Ku Kim**ABSTRACT 3-D linear acoustic analysis has been performed to elucidate damping characteristics of large Helmholtz acoustic cavities in FASTRAC thrust chamber. Acoustic impedance concept has been introduced to quantify combustion stabilization capacity. For a given acoustic cavity, sonic velocity in the cavity to achieve an optimal tuning has been determined and satisfactory agreement with the previous results has been obtained. Feasible estimation of soni c velocity in the acoustic cavity has been devised. Results show similar trends without significant deviat ions, which can be used in the procedure of design and verification of acoustic cavity. From t he satisfactory results, investigation of other thrust chambers with acoustic cavities which have shown c ombustion instabilities will be done as future works.초 록 연소시험을 통한 성공/실패 사례가 있는 FASTRAC 연소기에 대해, 효용성이 검증된 3차원 선형 음향해석을 수행하여 음향공의 물성치 변화에 따른 감쇠 특성을 파악하였다. 음향공의 동조주파수와 감쇠능력을 정량화하기 위해 음향 임피던스 개념을 도입하였다. 기하학적 형상이 주어진 상태에서, 음향공 내부의 음속 변화에 따라 최적의 동조가 가능한 음속을 파악하였다. 최적의 동조가 이루어지기 위한 음향공 내부의 음속 예측 방법을 제시하였다. 이를 통하여 여러 기하학적 형상을 가지는 음향공에 대하여 적용하여 감쇠능력을 정량화함으로써 연소안정성의 확보 여부에 대한 검토가 가능할 것이며, 안정성이 확보되지 못한 경우에 대해서는 그 이유를 명확하게 규명할 수 있을 것이다. Key Words: Combustion Instability(연소불안정), Acoustic Cavity(음향공), Resonant Frequency(공진주파수), Acoustic Impedance(음향 임피던스), Absorption Coefficient(흡수계수), Acoustic Conductance(음향 컨덕턴스), Sonic Velocity(음속)

Under the so-called Blue Drive strategy aiming at high fuel efficiency and low CO{sub 2} emission, Hyundai developed the Theta turbocharged GDI 2.0 l engine to replace large-displacement engines. With a new conceptual design using the synergistic combination of various advanced technologies and rigorous optimization of the system, Theta turbo GDI engine has the same performance as 3.5 l naturally aspirated engines with significant improvement in fuel efficiency. (orig.)

Acoustic damping enhanced by gap width in baffled injectors is investigated numerically, which are installed to suppress pressure oscillations in a model acoustic chamber. The previous experimental works reported that baffled injectors show larger acoustic damping with gaps between adjoining injectors than baffled injectors without the gap or conventional baffles. Acoustic-damping behaviors of baffled injectors are simulated numerically and the damping mechanism is examined. Damping factors are calculated as a function of baffle gap and it is found that the maximum acoustic damping is observed at a gap of 0.1–0.2 mm. The enhanced damping by gaps is attributed to viscous dissipation on the surfaces of the injectors or baffle blades. The optimum gap for maximum damping depends on the viscosity of the medium in the chamber and it increases with the viscosity. As a quantitative parameter of viscous dissipation, the dissipation rate of kinetic energy is calculated as a function of baffle gap. The parameter shows its maximum value at a specific gap and especially, the viscous dissipation rate has the same profile as that of damping factor. It verifies that the enhanced damping by gaps is attributed to the viscous dissipation of acoustic energy increased by gaps in baffled injectors.

The effect of C 3 H 6 on the deNO x activity of CuZSM5, FeZSM5 and V 2 O 5 /TiO 2 catalysts by NH 3 has been examined. The catalytic NO removal activity decreased by the continuous inclusion of C 3 H 6 into the reactor feed gas stream. The higher the feed concentration of C 3 H 6 , the more severe the retardation of the catalyst activity. The primary cause for the inhibition is the competitive adsorption of NH 3 and C 3 H 6 onto the catalyst surface and the useless consumption of NH 3 by the side reaction, including the NH 3 oxidation and ammoxidation reactions during the course of NH 3 /SCR reaction with C 3 H 6 .

An acoustic cavity was selected as a stabilization device to control high-frequency combustion instabilities in gas turbines or liquid rocket engine combustors, and the acoustic damping capacity of the acoustic cavity was investigated for various geometric configurations under atmospheric non-reacting conditions. The tuning frequency of the acoustic cavity and the acoustic responses of a model chamber with a single acoustic cavity were studied first. Damping capacity was initially quantified through the frequency width of two split modes and the amplitude-damped ratio. The results showed that the cavity with the largest orifice area or the shortest orifice length was the most effective in acoustic damping of the harmful resonant mode. The effect of the number of cavities on acoustic damping capacity was also studied. Damping capacity was improved by increasing the number of cavities. For a better evaluation of acoustic damping capacity, two quantified parameters; the acoustic absorption, meaning the damping efficiency, and acoustic conductance, meaning the acoustic power loss, were introduced. The case was observed that has had insufficient loss of acoustic power in spite of having the highest absorption efficiency. As a result, fine geometric tuning for the acoustic cavity is required for the sufficient passive control. Also, the choice of the number of cavities is important to optimize the damping efficiency and absolute damping loss in consideration of the restriction of the cavity volume.

Hydrotalcite (HT)-based catalysts have been developed to improve NOx storage-reduction (NSR) activity, particularly at low temperature during the cold start period of automotive engines. The content of noble metals, including Pt and Pd, has been optimized and the effect of the K precursors on NSR activity has been extensively examined by a knowledge-based combinatorial approach using colorimetric assay. The optimum noble metal ratio was determined to be 0.5Pt/0.5Pd or 0.25Pt/0.75Pd. Pd is a more effective metal for the high performance of NSR catalyst than Pt, particularly for reducing NOx from the catalyst at 200 °C. Both CH3COOK and KNO3 are the best K precursor in view of the NSR performance at both low (200 °C) and high temperatures (350 °C). HT-based catalysts reveal excellent deNOx performance compared to Al2O3-based catalysts, mainly due to their superior redox property.

Flow analyses have been performed to investigate the uniformity of propellant flow through the fuel and oxidizer manifolds of a full-scaled gas generator for a pump-fed liquid rocket engines. Injectors were simulated as porous medium layers having equivalent pressure drops. The uniformity of propellants has been analyzed for 3 fuel rings and 3 injector head configurations. The mixture ratio distribution at the exit of injectors has been estimated from the mass flow rates of fuel and oxidizer. The best configuration of fuel ring and injection head was selected through these flow analyses.

The effect of baffle gaps on the damping enhancement of a liquid rocket engine combustor has been elucidated through a series of tests, which include cold acoustic tests under both atmospheric and simulated viscous conditions and simulated combustion tests. The injector-formed baffles, which consist of an array of protruded coaxial injectors, were found to have a much greater acoustic damping effect than conventional planar baffles. For several axial baffle lengths, an optimal acoustic damping capacitance has been achieved at a 0.1 ∼ 0.2 mm baffle gap. The reason there exists an optimal baffle gap is thought to be mainly due to the viscous dissipation at the surface of the injector-formed baffles. Consequently, the axial baffle length can be reduced by taking advantage of the optimal baffle gap, providing a possible solution to the thermal cooling problem persistent with the baffle. Moreover, these optimum characteristics can provide some guidelines for manufacturing and assembling of the baffled injectors in rocket combustors.

Regenerative cooling passage to guarantee the thermal survivability in high performance rocket engine combustors could have complex configurations of the branching/merging of channels and flow turning, etc. By applying the classical hydraulic coefficients which can be found in the literature according to the flow conditions, hydraulic characteristics in regenerative cooling passages can be obtained effectively through dividing the pressure loss into friction loss and local resistance loss. Satisfactory agreement has been obtained by comparing the present results with experimental measurement of water flow test. In addition, the present results were in good agreement with CFD results when the actual coolant, kerosene was used. Therefore, the application of the present method is expected to be useful to design regeneratively cooled combustors.

Stability rating tests using a pulse gun, one type of artificial disturbance device, have been performed to optimize and to limit the axial baffle length of a combustor. To determine whether the combustor is stable when it is exposed to strong external perturbations or pressure oscillations, the decay time and amplitude ratio have been analyzed to quantify the stabilization capacity of the combustor. A baffle not big enough to cover the flame zone considered as the collision plane of the F-O-O-F impinging injector was not able to suppress external perturbations sufficiently. The combustion stability boundary limit of the present combustor can be determined with respect to the axial baffle length.

Simulating combustion tests have been performed to elucidate the effect of baffle gaps on the optimal damping characteristics in liquid rocket combustors where coaxial injectors are installed. Amplitude of pressure oscillation in model combustion chamber and the combustion stability margin are used to quantify the damping capacitance of baffles. Satisfactory agreement has been achieved with the results of cold acoustic tests. Present results have shown that the optimal gap for high acoustic damping capacity has also the large combustion stability margin in simulating combustion tests. Therefore, the present results can be utilized to determine the baffle length and optimal gap in full-scaled rocket combustors.

Elastic-plastic structural analysis for regenerative cooling chamber of gas generator was performed. Uniaxial tension test was conducted for STS316L at room and high temperature conditions to get the material data necessary for the structural analysis of the chamber which was operated under thermal load and high internal pressure. Physical properties including thermal conductivity, specific heat and thermal expansion were also measured. The structural analysis for four different types of regenerative cooling chamber of gas generator revealed that increased cooling performance decreased the thermal load and strain of the cooling channel structure. The results propose that in order for the regenerative cooling gas generator chamber to have high structural stability with endurance to high mechanical and thermal loads, it is important for the chamber to be designed to have high cooling performance.

Cold acoustic tests have been performed to elucidate the effect of baffle gaps on the optimal damping characteristics in a liquid rocket combustor where coaxial injectors are installed. For several axial baffle lengths, an optimal acoustic damping capacitance has been achieved in a certain gap range. Cold acoustic tests for simulating fluid viscosity by changing the pressure in a model chamber have been done to study the main mechanism of optimal damping. Experimental data have shown that the optimal gap for high damping capacity exists mainly due to the viscosity near the gap of baffles. Therefore, axial baffle length can be reduced by using the optimal baffle gap, providing a possible solution of thermal cooling problems. Also, these optimum characteristics can be some guidelines for manufacturing and assembling injectors in full-scaled rocket combustors.

In a combustion chamber of a liquid rocket engine, acoustic fine-tuning of gas–liquid scheme injectors is studied numerically for acoustic stability by adopting a linear acoustic analysis. Injector length and blockage ratio at gas inlet are adjusted for fine-tuning. First, acoustic behavior in the combustor with a single injector is investigated and acoustic-damping effect of the injector is evaluated for cold condition by the quantitative parameter of damping factor as a function of injector length. From the numerical results, it is found that the injector can play a significant role in acoustic damping when it is tuned finely. The optimum tuning-length of the injector to maximize the damping capacity corresponds to half of a full wavelength of the first longitudinal overtone mode traveling in the injector with the acoustic frequency intended for damping in the chamber. In baffled chamber, the optimum lengths of the injector are calculated as a function of baffle length for both cold and hot conditions. Next, in the combustor with numerous resonators, peculiar acoustic coupling between a combustion chamber and injectors is observed. As the injector length approaches a half-wavelength, the new injector-coupled acoustic mode shows up and thereby, the acoustic-damping effect of the tuned injectors is appreciably degraded. And, damping factor maintains a near-constant value with blockage ratio and then, decreases rapidly. Blockage ratio affects also acoustic damping and should be considered for acoustic tuning.

A Helmholtz resonator as a stabilization device to control high-frequency combustion instabilities in liquid rocket engine is adopted and its damping capacity is verified by linear acoustic analysis and atmospheric acoustic tests. To compare the results of acoustic attenuation effect in accordance with uni-resonator`s geometry, quantitative analyses were made in the cases of various orifice diameters and lengths. Next, in the experiments to compare the results of acoustic attenuation effect by a difference in the number of resonators, damping capacity of harmful resonant frequency was improved by the increase of the number of resonators. On the other hand, attenuation efficiency of the frequency tended rather to lower due to over damping from the point of view of absorption coefficient. As the result, tuning the suitable geometry for the resonator to the resonant frequency is required for the control using the resonator. Also, the design of resonator`s geometry and the choice of its number are important to put up the optimal efficiency in consideration of restriction of its volume.

A liquid rocket thrust chamber should have a high confidence in its combustion performance and combustion stability. Expecially, the injector of which function is spraying and mixing propellants plays an important role in getting the confidence. This study was carried out to evaluate combustion stability of a double swirl coaxial injector by using the model similarity method. Besides, in case of a baffle which was used to improve combustion stability, the length and gap effects of the baffle were investigated.

Thermal effect of finite-rate chemistry on linear combustion stability and film-cooling effect are investigated in sample rocket engines. The flow variables required to evaluate stability limits are obtained from CFD data with finite-rate chemistry adopted in three dimensional chamber. Major flow variables are affected appreciably by finite--rate chemistry and thereby, the calculated stability limits are modified. It is found that finite-rate chemistry contributes to stability enhancement in thermal point of view. And film cooling also has the effect of combustion stabilization.

To optimize and limit the axial length of the baffle of the KSR-III engine, stability rating tests using pulse gun as one of artificial disturbance devices have been done. Generally a rocket engine can be considered to be dynamically stable if a certain imposed external perturbation or pressure oscillation in rocket combustion chamber could be suppressed within a short time period. Decay time and other parameters for the evaluation of stabilization ability of an engine to external perturbation have been analyzed to quantify stabilization capacity of engine, in other words, dynamic stability margin. Baffle not covering flame zone enough which can be considered as collision region of injector wasn`t be able to suppress external perturbation sufficiently. The limit of combustion stability margin of engine is assumed to be 50 mm length baffle of the KSR-III engine.