Hwan-Seok Choi

Effects of Injector Recess and Chamber Pressure on Combustion Characteristics of Liquid–Liquid Swirl Coaxial Injectors

Combustion characteristics such as combustion performance and combustion stability have been studied experimentally using a small liquid rocket thrust chamber with 19 liquid–liquid swirl coaxial injectors. Data were obtained from static pressure, temperature, and dynamic pressure sensors installed in propellant manifolds and the combustion chamber. While changing the recess length of the injector, characteristic velocity and pressure fluctuation data were collected and analyzed. In addition, chamber pressure was varied between 42 and 54 bar, which covers the sub- and supercritical pressures of oxygen. The results show that the longer recess length generally promotes combustion performance and the spray interaction between injectors in the multielement combustor increases the characteristic velocity. When the chamber pressure is above the critical pressure of oxygen, the recess length scarcely affects the pressure fluctuation. However, when the chamber pressure is below the critical pressure, the shift from external mixing to internal mixing of oxidizer and fuel sheets by the variation of recess length significantly degrades combustion stability and induces strong low-frequency instability. Accordingly, the effects of both recess length and operating chamber pressure must be taken into consideration when designing liquid–liquid swirl coaxial injectors.

Combustion Dynamics and Stability of a Fuel-Rich Gas Generator

The dynamic characteristics of fuel-rich combustion have been studied using an experimental combustor simulating a gas generator for a liquid rocket engine. The combustor burns liquid oxygen and fuel (Jet A-1) at a mixture ratio of about 0.32 and a chamber pressure ranging from 4.10 to 7.24 MPa, which covers subcritical to supercritical pressures of oxygen. For the investigation of combustion dynamics, pressure fluctuation measurements using piezoelectric sensors have been a major probe throughout the present study. Two different types of injector heads equipped with biliquid swirl coaxial injectors and either a short nozzle or a turbine manifold nozzle have been used in the study. Fuel-rich combustion of both injector heads with the short nozzle revealed pressure pulsation at frequencies of about 128 Hz, which attenuates along with an increase of a chamber pressure. Combustion tests with the turbine manifold nozzle conducted at chamber pressures lower than the oxygen critical pressure showed combustion instabilities at a frequency of 330 Hz, which has been identified as a longitudinal resonant mode by a linear acoustic analysis. The combustion instabilities seem to be induced by inherent pressure fluctuations from the biliquid swirl coaxial injector when the chamber pressure is below the oxygen critical pressure.

Fuel-Rich Combustion Characteristics of Biswirl Coaxial Injectors

An experimental study was performed to investigate the combustion characteristics of liquid–liquid swirl coaxial injectors in fuel-rich conditions. Liquid oxygen and kerosene (Jet A-1) were burned in a range of mixture ratios (0.29–0.41) and chamber pressures (46–65 bar) in a gas generator for a liquid rocket engine. An injector head was connected to a water-cooled chamber and a short nozzle with or without an extension pipe between the chamber and the nozzle. The extension pipe acoustically simulated a turbine inlet manifold. The injector head had 37 identical swirl coaxial injectors. It is found that the characteristic velocity and combustion gas temperature are seldom influencedby the extension pipe, but are only functions of themixture ratio. The dynamic pressure data show that the combustion instability in the fuel-rich gas generator equipped with biswirl coaxial injectors can be significantly affected by themixture ratio and also by the extension pipe, which influences the resonant frequency in the chamber.

Combustion Dynamics of Swirl Coaxial Injectors in Fuel-Rich Combustion

An experimental study was carried out to investigate the combustion characteristics of liquid–liquid swirl coaxial injectors in a fuel-rich gas generator. Propellants of liquid oxygen and kerosene (jet A-1) were burned in a range of mixture ratios (0:29 0:38) and chamber pressures (53:6 57:9 bar) in several combustion chambers with differentdiametersandlengths.Aninjectorheadhad37identicalswirlcoaxialinjectorswithlargerecesslengthsand internal-mixingcharacteristics.Theinjectorheadwasconnectedtocombustionchambersandachokednozzleeither with or without an extension pipe. It was found that the combustion characteristic velocity, combustion gas temperature, and combustion dynamics were seldom influenced by the increase of injector recess length in the present fuel-rich conditions. The results of dynamic pressure data and swirl-injector dynamics suggest that the longitudinal-modecombustioninstabilityinthefuel-richgasgeneratorequippedwithbiswirlcoaxialinjectorscould besignificantlyaffected bytherelationship betweenthe resonant frequencyin thecombustion chamberandthe flow dynamics of the swirl injector.

Computations of nonlinear wave interaction in shock-wave focusing process using finite volume TVD schemes

Abstract An upwind and a symmetric finite difference TVD scheme have been extended to cell-centered finite volume methods. These newly extended schemes have been used to analyze a shock-wave focusing phenomenon that is dominated by complex nonlinear interaction of shock waves. Numerical solutions obtained from these two schemes successfully resolved all the waves evolving through the focusing process with reasonable accuracy and resolution. The phenomenon near the focal region was found to be strongly affected by the gas dynamic nonlinearities of shock waves. The computed solutions showed good agreement with experimental data and, thus, demonstrated the accuracy and reliability of the present numerical schemes as applied to the analysis of complex unsteady shock-shock interaction problems. The present numerical simulation has clearly revealed the mechanism of shock-shock/expansion wave interaction in a shock-wave focusing process, which determines the wave-front geometries at and beyond the focus and confines the pressure amplification at the focal region. In addition, some comparisons have been made for the performance of both schemes considering the effect of the limiter functions and those results are proposed.

Effects of Recess Length on Discharge Coefficients of Swirl Coaxial Injectors

An experimental study has been performed to investigate the effects of the recess length on the discharge coefficients of swirl coaxial injectors. Liquid oxygen and kerosene (Jet A-1) were burned in a range of mixture ratios (1.97–3.31) and chamber pressures (43–80 bar) in small liquid rocket thrust chambers. Each thrust chamber was equipped with 19 liquid–liquid swirl coaxial injectors. While changing the recess length of the injectors, static pressure, temperature, and flow rate data were collected and analyzed to calculate the discharge coefficient. The discharge coefficients obtained from hot-firing tests are compared with those from cold-flow tests. Operating conditions such as oxidizer-to-fuel mixture ratio and chamber pressure have some influence on the discharge coefficients. The recess length of the injectors significantly affects the discharge coefficients in the hot-firing tests, but not in the cold-flow tests. The results show that the longer recess length gradually reduces the discharge coefficients. Especially when the liquid oxygen film collides with the kerosene film inside the fuel nozzle, the discharge coefficients of the outer fuel-side injectors decrease more remarkably than those of the inner oxidizer-side. It is thought that the flame anchored inside the recessed region in the swirl coaxial injector confines propellant flows, and the internal mixing of propellants strongly disturbs the outer fuel flow. Since the discharge coefficient is directly related to the injection pressure drop, the effects of recess length on discharge coefficients must be carefully considered when designing liquid–liquid swirl coaxial injectors.

Combustion Stability Assessment of Double Swirl Coaxial Injectors Using Simulant Propellants

*This study is related to conduct model combustion tests applying various double swirl coaxial injectors to identify their combustion stability characteristics. Gaseous oxygen and mixture gas of methane and propane have been used as simulant propellants. A model combustion chamber was designed for its first tangential frequency to be corresponded to that of a full-scale thrust chamber. They were manufactured four different kinds of injector heads with five elements to examine their stability characteristics. The main idea of the experiment is that a propellant mixing mechanism is considered as a dominant factor significantly affecting combustion stability in the full-scale thrust chamber. Self-excited dynamic pressure measurements in the model combustion chamber show different combustion stability characteristics with respect to a recess length of an oxidizer post. The test result shows that the coupling between combustion phenomena and the 1T frequency in the model combustion chamber becomes strengthened according to the increase of a recess ratio.

A Trade-off Analysis between Combustion and Cooling Performance of a Liquid Rocket Combustor with Fuel Film Cooling Scheme

․ Seong-Ku Kim* ․ Hwan-Seok Choi*ABSTRACT Performance of a liquid rocket thrust chamber with regenerati ve cooling scheme has been numerically analyzed using in-house CFD code which can predict combustion/cooling performance and provide nozzle design parameters. This paper investigates trade -offs between combustion and cooling performance with varying amount of fuel directly injected into the chamber wall to form cooling films and mixture ratios for the peripheral injectors. Further efforts to verify/improve the simulation methodology including comparison with the firing test results a re planned to make it a reliable tool to optimize the film cooling and other major design parameters. 초 록 액체로켓 추력실의 성능 예측 및 초음속 노즐부 형상 설계에 활용 중인 in-house 해석 도구를 이용하여 재생냉각 연소기에 대한 성능/냉각 통합해석을 수행하였으며, 막냉각 유량 및 외곽 분사기열의 혼합비 변화에 따른 연소 성능과 냉각 성능 간 trade-off 경향을 고찰하였다. 향후 막냉각 및 주요 설계인자의 최적화 도구로 활용될 수 있도록 개발 연소기에 대한 시험 결과와의 비교 등을 통하여 수치해석 도구를 검증/개선해나갈 계획이다. Key Words: Liquid Rocket Combustor(액체로켓 연소기), Regenerative Cooling(재생냉각), Fuel Film Cooling(연료 막냉각), Combustion/Cooling Performance(연소/냉각 성능)

Development of Helmholtz Solver for Thermo-Acoustic Instability within Combustion Devices

In order to effectively predict thermo-acoustic instability within real combustors of rocket engines and gas turbines, in the present study, the Helmholtz equation in conjunction with the time lag hypothesis is discretized by the finite element method on three-dimensional hybrid unstructured mesh. Numerical nonlinearity caused by the combustion response term is linearized by an iterative method, and the large-scale eigenvalue problem is solved by the Arnoldi method available in the ARPACK. As a consequence, the final solution of complex valued eigenfrequency and acoustic pressure field can be interpreted as resonant frequency, growth rate, and modal shape for acoustic modes of interest. The predictive capabilities of the present method have been validated against two academic problems with complex impedance boundary and premixed flame, as well as an ambient acoustic test for liquid rocket combustion chamber with/without baffle.

Structural Analysis of Gas Generator Regenerative Cooling Chamber

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.