Woo-Seok Seol

On the method for hot-fire modeling of high-frequency combustion instability in liquid rocket engines

This study presents the methodological aspects of combustion instability modeling and pro-vides the numerical results of the model (sub-scale) combustion chamber, regarding geometrical dimensions and operating conditions, which are for determining the combustion stability boundaries using the model chamber. An approach to determine the stability limits and acoustic characteristics of injectors is described intensively. Procedures for extrapolation of the model operating parameters to the actual conditions are presented, which allow the hot-fire test data to be presented by parameters of the combustion chamber pressure and mixture (oxidizer/fuel) ratio, which are customary for designers. Tests with the model chamber, based on the suggested scaling method, are far more cost-effective than with the actual (full-scale) chamber and useful for injector screening at the initial stage of the combustor development in a viewpoint of combustion instabilities.

Application of Combustion Stabilization Devices to Liquid Rocket Engine

Application of combustion stabilization devices such as baffle and acoustic cavity to liquid propellant rocket engine is investigated to suppress high-frequency combustion instability, i.e., acoustic instability. First, these damping devices are designed based on linear damping theory. As a principal design parameter， damping factor is considered and calculated numerically in the chambers with/without these devices. Next, the unbaffled chambers with/without acoustic cavities are tested experimentally for several operating conditions. The unbaffled chamber shows the peculiar stability characteristics depending on the operating condition and it is found to have small dynamic stability margin. As a result, the acoustic cavity with the present design has little stabilization effect in this specific chamber. Finally， stability rating tests are conducted with the baffled chamber, where evident combustion stabilization is observed, which indicates sufficient damping effect.

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.

Hot-fire injector test for determination of combustion stability boundaries using model chamber

This study realizes the conceptual method to predict combustion instability in actual full-scale combustion chamber of rocket engines by experimental tests with model (sub-scale) chamber. The model chamber was designed based on the methodologies proposed in the previous work regarding geometrical dimensions and operating conditions, and hot-fire test procedures were followed to obtain stability boundaries. From the experimental tests, two instability regions are presented by the parameters of combustion-chamber pressure and mixture (oxidizer/fuel) ratio, which are customary for combustor designers. It is found that instability characteristics in the chamber with the adopted jet injectors can be explained by the correlation between the characteristic burning or mixing time and the characteristic acoustic time. In each instability region, dynamic behaviors of flames are investigated to verify the hydrodynamically-derived characteristic lengths of the jet injectors. Large-amplitude pressure oscillation observed in upper instability region is found to be generated by lifted-off flames.

Combustion stability characteristics of the model chamber with various configurations of triplet impinging-jet injectors

Combustion stability characteristics in actual full-scale combustion chamber of a rocket engine are investigated by experimental tests with the model (sub-scale) chamber. The present hot-fire tests adopt the combustion chamber with three configurations of triplet impinging-jet injectors such as F-O-O-F, F-O-F, and O-F-O configurations. Combustion stability boundaries are obtained and presented by the parameters of combustion-chamber pressure and mixture (oxidizer/fuel) ratio. From the experimental tests, two instability regions are observed and the pressure oscillations have the similar patterns irrespective of injector configuration. But, the O-F-O injector configuration shows broader upper-instability region than the other configurations. To verify the instability mechanism for the lower and upper instability regions, air-purge acoustic test is conducted and the photograph of the flames is taken. As a result, it is found that the pressure oscillations in the two regions can be characterized by the first impinging point of hydraulic jets and pre-blowout combustion, respectively.

Development of Design Program of Regeneratively Cooled Combustion Chamber

A design code validated against the thermal analysis results of CFD and published RTE code for a regeneratively cooled combustion chamber has been developed. The major function of the code is to predict the regenerative cooling performance and stress of the chamber wall. Adopted are the empirical correlation for the evaluation of the heat transfer coefficient of hot gas and coolant, and theoretical formula for the fin effect of the channel rib. The hot-gas-side wall temperature from the present code shows 100 K difference at most compared to RTE results. It shows less than 10 % difference for the heat flux thrall through the chamber wall and hot-gas-side convective heat transfer coefficient. The major cause of the wall temperature difference is due to the underestimation of the fin effect of the channel rib.ﾖ⨀

Characteristics of Dynamic Pressures in Liquid Rocket Thrust Chambers

The objective of this paper is to characterize dynamic pressure traces measured at various experimental conditions of liquid rocket thrust chambers. Stability rating test and self-excited combustion instability results for subscale and full-scale thrust chambers show various aspects of dynamic behavior of the pressure field depending on combustion system hardware and operating conditions. External perturbations facilitated by a shock wave generated from an explosion of solid material are able to alter the stable combusting flow to an unstable one when it has lack of dynamic stability margin. The direct comparison of pressure traces for self-excited and perturbed high frequency instabilities suggests that coupling mechanisms behind these two cases are totally different from each other. Naturally occurring combustion instability seems to be first coupled with a low frequency wave that affects flame more vigorously with the smaller phase difference between heat release rate and acoustics than a high frequency wave. However, for the artificial perturbation, the combusting flow triggered by a shock wave becomes located at physical conditions irrelevant to those before the perturbation.

Study on Turbopump-Gas Generator Open-Loop Coupled Test

초록:30톤급액체산소/케로신액체로켓엔진개발의중간단계로터보펌프-가스발생기개회로연계시험이수행되었다.터보펌프-가스발생기개회로연계시험은엔진시스템작동모사환경시험으로서가스발생기로의추진제는터보펌프출구를통해공급되지만, 가스발생기출구가스는터빈구동에이용되지않고외부로배출된다. 터보펌프-가스발생기개회로연계시험목적, 시험설비구성, 제어시스템의작동조건, 시험수행절차, 연계시험기의구성형태,개회로연계시험결과가제시되었다.터보펌프-가스발생기개회로연계시험결과,연계시험기의예냉절차와시동특성,정격작동성및안정적인종료특성이액체로켓엔진시스템작동환경모사조건에서확인되었다.Abstract: Turbopump-gas generator open-loop coupled tests are performed during the development of a30tonf-LOx/Kerosenerocketengine.Intheturbopump-gasgeneratoropen-looptests,thepropellantstogasgeneratoraresupplied from the outlets of turbopump, while the gas exhausted from the gas generator is vented out to theatmosphere, instead of being used to turbine driving. This paper presents the objectives, procedure, and results of theopen-loopcoupledtest, inadditiontoaschematicrepresentationofthetestapparatusandtheoperatingconditionsforthe test facility system and control system. The results of turbopump-gas generator open-loop coupled test confirmchill-down procedure, startup characteristics, nominal operability and smooth shutdown of the open-loop coupled TestPlantintestconditionssimulatingenginesystemoperationenvironment.

Study of Grid Dependency of Sheet Atomization Model of a Pressure-Swirl Atomizer

An improved spray model of a pressure-swirl atomizer was developed and the grid dependency of the model was investigated. Since the Lagrangian-Eulerian approach was adopted for tracking droplets, very small grids could not be used. However, in order to detect swirl flow accurately, small grids were needed because of the consideration of swirl injection. In order to overcome these limitations, numerical studies were performed by using various grids with cell sizes ranging from 10.0 10 mm to 0.625 0.625 mm. From these calculated results, it was observed that the most efficient grid cell size was 1.25 1.25 mm.

Combustion Characteristics of Fuel-rich Gas Generator with Impinging Injector for a Liquid Rocket Engine

The overall results of hot firing tests of fuel-rich gas generator with impinging injector at design and off-design points are described. The gas generator consists of an injector head with impinging injector, a water cooled combustor wall, a turbulence ring to enhance mixing, an instrument ring measuring temperature and pressure and a nozzle. The combustion tests were successfully performed without damage of gas generator. Test results show that the outlet temperature is not dependent on residence time of hot gas within 4～6msec but dependent on chamber pressure. The relation between outlet temperature and combustion efficiency resulting from measured pressure, mass flow rate and area of nozzle throat is shown. The overall O/F ratio is the critical parameter to determine the outlet temperature and the linear correlation between two parameters is established.⼌?؀〰㐮㔻ἀ䍯浰畴敲⁓捩敮捥…⁉湦潲浡瑩潮