The effects of nitrogen film cooling on ignition transition under different conditions of film cooling injection pressure were observed. Gaseous oxygen and liquid kerosene were used as propellants, injected using a shear coaxial injector. Gaseous nitrogen as the film coolant was injected along the gap between the combustion chamber and outer injector walls. A dynamic pressure transducer and a high-speed camera with a band-pass filter were respectively used. The over-peak pressure and pressure rise time were detected in cases with film cooling. With the increase in the differential pressure, the over-peak pressure decreased, and pressure rise time increased. These phenomena are explained by the interruption of normal propellant injection in the mixing zone of propellant. The combustion instability intensity decreased with the increase in the differential pressure between the combustion chamber and injected film coolant. Flame structures were visualized to confirm the unstable combustion flame during the ignition transition.

An atomization model for plain orifice diesel fuel sprays is developed, which considers both turbulent fluctuation in the jet flow and wave growth by gas inertia force due to the jet velocity. It is assumed that the atomization time scale is the sum of the turbulent and wave growth time scale while the atomization length scale is given by the integral length scale of turbulence. The developed atomization model is implemented in the KIVA code in terms of two numerically different groups, the primary and secondary drop parcels. The computation results are in good agreement with the experimental data for the drop size distribution from Phase Doppler Particle Anemometry (PDPA) as well as the spray tip penetration and spray shape.

Pintle injector is the most suitable injector for thrust control because it can control the area of propellant injection. Accordingly, the combustion test of multiple hole pintle injector and continuous type pintle injector was carried out in this paper using liquid oxygen and gas methane. The combustion performance of the two pintles was verified with the characteristic speed efficiency, and the experimental results were compared according to the O/F and combustion chamber pressure and under similar conditions. The efficiency of the multi hole pintle was generally somewhat higher than continuous pintle when pintle opening distance(the area of dispensing oxidizer) was in a 100% thrust condition.

Supercritical combustion occurs in the high-pressure environment of a rocket engine where supercritical atomization is required. In this experiment, a kerosene surrogate, the primary fuel of a liquid rocket, is selected and injected into a high-temperature, high-pressure chamber above the critical point. The experiment is conducted to analyze the jet penetration sprayed from the subcritical state to the supercritical environment and to confirm the phase change at the jet-ambient gas interface. The atomized jet is generated by impinging a hydrocarbon mixture propellant, and this experiment examines the phase change in a supercritical environment. A mixture of n-decane and methylcyclohexane is used as the surrogate propellant. The propellant is injected into the chamber of the sub/supercritical environment, and the jet core and penetration are visualized using a shadowgraph method. The ambient pressure exhibits two conditions: 2.6 MPa ( $${P}_{r}$$ = 1.00) and 3.6 MPa ( $${P}_{r}$$ = 1.38). These conditions have different pseudo-critical points, and the experiment is carried out while increasing the ambient temperature from 599 K ( $${T}_{r}$$ = 1.00) to 700 K ( $${T}_{r}$$ = 1.17). At 2.6 MPa, the density of the propellant decreases drastically near the critical temperature of the mixture, 602 K. At 3.6 MPa, the density decreases near 630 K, a pseudo-critical temperature. Consequently, it is confirmed through the visualization that the length of the liquidlike core of the jet is shorter at 2.6 MPa than at 3.6 MPa, all else being equal.

The single jet of decane/methylcyclohexane mixed fuel that is surrogate for kerosene was injected into supercritical environment and visualized using shadowgraph technique. The injection pressure drop of the fuel jet of T-r was kept constant at 0.5 MPa and the experiment was conducted above the critical point of the mixed fuel, and the reduced temperatures of the chamber was changed from 1.00 to 1.23, and the reduced pressures was 1.00 and 1.38. As an index for reducing the density of jets sprayed into the supercritical environment, the brightness intensity of the post-processed jet image was observed with the internal temperature and pressure of the chamber. It was confirmed that the decrease in the brightness intensity of the jet when the temperature inside the chamber increased, and when the pressure inside the chamber was higher at the same temperature, the decrease in the brightness intensity of the jet was delayed. When the pressure inside the chamber is high, it is thought that the change in brightness intensity is delayed due to the increase in the pseudo-critical temperature of the fuel and the increase in the temperature required to reduce the density of the fuel jet.

Experimental investigations are conducted to determine the mechanism and characteristics of a jet in an L-shape crossflow simulating the radial swirl injector of a lean premixed-prevaporized (LPP) combustor. To simplify the radial flow of the actual injector while ignoring the centrifugal effect, the L-shaped 2D-channel is used for the crossflow, and water is used as a fuel simulant. The jet breakup is captured using a high-speed camera, and the density gradient magnitude is post-processed to clarify the spray. The Sauter mean diameter (SMD) of the spray is measured via a laser diffraction method with a helium–neon laser optical system (HELOS). The characteristics of the jet in the L-shape crossflow are compared with the characteristics of the jet in a typical crossflow through the flat channel. The results for different outlet heights of the L-shape channel (H/d0) and different injector positions (L/d0) are presented. A dimensionless number (τ) consisting of a time ratio is introduced to describe the jet characteristics. In a previous work, the spraying tendency was demonstrated for different injector positions. In addition, the effect of the recirculation area on H/d0 was empirically shown. H/d0 determines the size of the recirculation area, and the range of τ determines the jet breakup mechanism inside the L-shape channel. The results of this study present the breakup mechanism of the jet in the L-shape channel flow, which simulates a jet in a radial swirler injector for gas turbine engines. It is expected that these results can be used to assist in designing gas turbine engines with more combustion efficiency.

Multi-element injectors have been used in liquid rocket engines to obtain high thrust, and the gas-centered swirl coaxial injector is a representative injection system because it provides high mixing performance. Investigations of the spray characteristics of injection systems have focused on the characteristics of single-element injectors, including the spray angle, breakup, and atomization mechanism of the liquid sheet formation. However, the spray characteristics of multi-element injectors need to be studied because of their usage in real rocket engine systems. In this paper, spray patterns and interacting spray under different injection conditions are analyzed using the backlight imaging technique, and the dominant flow fields are observed using the dynamic mode decomposition method. The spray angle in the case of the gaseous nitrogen with a high Reynolds number is calculated to be lower than the case with a low Reynolds number. From the averaged images, it is found that there are two dominant flow fields: one is located in the vicinity of the injector head and the other is in the interacting spray zone, which is related to a secondary breakup via an adjacent injector. As a result of the dynamic mode decomposition analysis, however, the spray zone near the injector is confirmed to be a more dominant flow field than the interacting spray zone.

The spray combustion of LOX/GCH4 in the pintle injector is simulated to study the combustion characteristics at a subcritical condition by considering the effects of pintle opening distance and pintle tip angle. To perform the simulation, first, the VOF simulation was used to estimate the LOX sheet thickness and breakup length. Second, the breakup constants for the secondary breakup model were calculated using the results of VOF and the Lagrangian approach was used to estimate the Sauter mean diameter around the injector exit. Finally, the spray combustion of LOX/GCH4 simulation was performed using the estimated SMDs data. Subsequently, the spray combustion characteristics were discussed by considering the individual effects of pintle opening distance at fixed pintle tip angle, and pintle tip angle at fixed pintle opening distance using various contours such as temperature, mass fraction of OH, and discrete phase model evaporation of LOX droplets. Following this, the characteristic velocity and combustion efficiency were shown for various cases. The optimised design of the pintle injector based on the simulated results was summarised for future liquid rocket pintle injectors.

The role of the structural baffle injectors and blades in the liquid rocket engine is to block the transverse pressure waves that are caused by combustion instability. Although the protection of the liquid rocket system from high-frequency combustion instability is essential, there are side effects such as increased weight of the rocket and thermal effect. In this study, a fluidic baffle injector was applied to the simulant spray system, expecting that it would operate with the same performance as a structural baffle injector. The development of the additive manufacturing technology allows for the designing of various geometrical injector arrays such as the combination of a gas-centered swirl injector and shear coaxial injector. This study aimed to compare the blockage performance of a structural baffle injector with that of a fluidic baffle injector using damping capacity and various injector arrays. The damping capacity was high in the structural baffle injector because the pressure wave was completely blocked. However, the amplitude was similar in the shear coaxial injector. This means that the blockage performance of the shear coaxial injector as a fluidic baffle injector is noteworthy.

Pintle injector is known to have been adopted as injector of Lunar Module Descent Engine (LMDE) and contributed to success of the Apollo program and recently used in merlin engine. In this study, 500N Lab-Scale pintle injector thruster was manufactured and the combustion experiment with LOx/GCH4 was conducted. However, the proto-type thruster was showed some problems, such as low combustion efficiency and melting of pintle tip. To solve these problems, the flow guide in pintle tip was suggested through the CFD simulation. After addition of flow guide module, the combustion efficiency increased and pintle tip did not melt until the end of combustion.

Two-dimensional axisymmetric large eddy simulation was conducted to model the sheet formation and primary breakup of gelled kerosene and gelled hydrogen peroxide in a pintle injector. This was done by varying the pintle opening distance. To simulate a hollow conical gelled hydrogen peroxide sheet from the center gap and the sheet breakup by the gelled kerosene from the annular gap, volume of fluid (VOF) simulation was employed. Here, the viscosity was considered as a function of the shear rate. The power law expression for hydrogen peroxide and the Herschel-Bulkley extended expression for kerosene were used as the user-defined function codes. The variations of the breakup length, wavelength, and the amplitude of the formed sheet due to disturbance waves using VOF simulation with varied pintle opening distances were studied. The viscosity distributions due to the shear rate of the gel flow and the velocity at the exit of the injector were plotted to investigate the gel flow behavior.

In liquid rocket performance tests, external excitation techniques such as the use of pulse guns have been used to examine combustion stability. However, it is difficult to detect the flame visualization of the gas-centered swirl coaxial injector because of the soot produced by the chemical reaction of oxygen and kerosene during the pressure wave propagation. Due to this soot, the flame might not be exposed during combustion. In this study, a high speed film coolant is injected to remove the soot from the visualization window. A simplified pulse gun with a piston is designed to generate the pressure wave. The film cooling method is confirmed; the soot is removed and the flame structure is detected. In the case with a high momentum flux ratio for the GCSC injector, the combustion exhibits greater stability than in the case with a low momentum flux ratio. In addition, the pulse gun with the piston is confirmed as the external excitation from the hot-firing test.

Injection visualization of heated mixed simulant droplets based on hydrocarbon fuel was performed under supercritical state environment. Mixed simulant consisted of Decane and Methylcyclohexane with different critical pressure and critical temperature. Flows injected into the supercritical state environment created droplet by Rayleigh breakup mechanism, and the Oh number and Re number were determined to confirm the breakup area. The temperature of the mixed simulant varied from Tr=0.49 to Tr=1.34. The flow rate was maintained at 0.7 to 0.8 g/s. Droplet became shorter in breakup length as heated and into a lumped form. Second droplet was formed and when Tr=1.34, the phase was not visible in the supercritical state with local unsteady flow.

Several studies regarding the combustion characteristics of a pintle injector, which is one of techniques for throttling liquid rocket engines, have been conducted; however, the primary focus was on the performance instead of on the flame. To study the fundamental flame characteristics and provide a useful design guide for a pintle injector, gaseous methane and oxygen flames under the laminar condition were visualized by CH* chemiluminescence and Schlieren imaging techniques. The pintle injector used in this study had a fully opened annular injection area, and the combustion chamber was designed as a square cylinder with two windows to visualize the flames. The mass flow rates of methane and oxygen were controlled to change the injection condition, and the pintle opening distance was changed as a geometrical variation. The flame shapes could be classified globally into two types: a shear layer flame, located between two injected propellants and a tip-attached flame located near the pintle tip. The flame angles were measured from deconvoluted flame images, and measured angles were compared with spray angles from previous studies. In conclusion, the flame-anchoring locations and flame shapes were found to be critical for the injection momentum flow rate. Additionally, a new non-dimensional number, the pintle number, K-p, for the normalized angle is proposed, and the pintle number has a strong relation with the flame and spray angles. It is expected that the pintle number can be used to design a pintle injector.

Pintle injectors have several unique features that allow high-performance thrust control of rocket engines; thus, it can be used in reusable space launch vehicles. Because the orifice area is directly controlled in the thrust adjustment process, several geometric parameters of pintle injectors must be considered in the design and analysis. However, the influence of the critical parameters on the injector performance has not been extensively investigated. The research object in this study was the skip distance, as well as the throttling level, which is a key geometric parameter for pintle injectors. It is related to the travel distance of annular flow along the pintle surface. The operating range of the skip distance was 0.25–1.91 and the throttling level based on the mass flow rate was operated in the range of 20–100%. Water and air were used as simulants under atmospheric conditions. In these conditions, the spray angle and droplet size were experimentally obtained, and the gas velocity distribution near the pintle surface was numerically analyzed. In this manner, the gas velocity distribution was determined in terms of the skip distance and throttling level, and the corresponding effects on the spray characteristics were analyzed. In addition, the variation trends of the normalized spray angle and droplet size with the skip distance and throttling level were obtained. Finally, an appropriate skip distance was proposed from a design perspective, considering the standard deviation in each throttling level. The results can help elucidate the influence of the skip distance and the throttling level on the spray characteristics, thereby facilitating the design of pintle injectors.

Gelled propellants for rocket propulsion present various advantages, such as storability with regard to liquid rocket propellants and a high specific impulse with regard to solid rocket propellants. However, one of the main limitations of gelled propellants in rocket propulsion is their low mixing performance resulting from their high viscosity. For these reasons, conventional injectors such as coaxial and pintle injectors are not suitable for spraying gelled propellants. To address this issue, we have herein devised new methods to increase the mixing performance. For the gelled propellant, liquid kerosene was compounded with 5 wt% of Thixatrol ST, a gelling agent, and shear viscosity data according to the shear rate were measured to confirm viscosity. Two types of injectors-flat type and deflector-were used to confirm the spray characteristics, such as spray angle, via the backlight image technique. The spray angle was formulated according to the total momentum ratio for comparison with the measured experimental data, and the result regarding the measured spray angle in the deflector type injector satisfies the expected spray angle. The dominant difference in spray characteristics between the flat type and deflector type injectors is in terms of a liquid jet stream, related to the breakup and atomization processes. In the case of the flat type injector, the liquid jet stream was detected even for a high total momentum ratio. However, in the case of the deflector type injector, the liquid jet stream disappeared with an increase in the total momentum ratio.