Liquid Swirl Coaxial Injector Research Articles

Supercritical combustion instabilities of oxygen/kerosene mixture in a swirl coaxial injector

This study focuses on the fundamental combustion instability issues in staged combustion for liquid oxygen/kerosene rocket engines. A comprehensive and high-fidelity numerical simulation is conducted based on the Peng-Robinson real gas model. This involves combining the large eddy simulation (LES) method with the eddy-dissipation-concept (EDC) model. This study focuses on both the mixing and combustion processes within the gas-centered liquid-swirl coaxial injector and the influence of the swirl number. The results show that the numerical method can accurately predict the change of thermodynamic properties at transcritical conditions. For the injector under investigation, the dominant frequencies of near-field pressure, axial velocity, and radial velocity all coincide at 1,066 Hz. This indicates that these three parameters are coupled, but no periodic fluctuation processes are evident for tangential velocity and temperature. The time-domain pressure characteristics at monitoring points in various directions are compared and analyzed. These comparisons reveal a robust coupling relationship between pressure and axial velocity. Additionally, the numerical simulation results show that the change of the swirl number conditions at the fuel inlet has negligible effect on the evolutionary characteristics of the injector. It is confirmed that tangential velocity is not the cause of pressure fluctuations.

Effect of recess length on flow dynamics in gas-centered liquid-swirl coaxial injectors under supercritical conditions

The flow dynamics in gas-centered, liquid-swirl coaxial injectors at supercritical conditions are comprehensively investigated using the large eddy simulation technique. This class of injectors has been used in many high-performance propulsion and power-generation systems. Gaseous oxygen is axially injected into the center post of the injector at a temperature of 687.7 K, while liquid kerosene is tangentially introduced into the coaxial annulus at a temperature of 492.2 K. The operating pressure of 25.3 MPa substantially exceeds the thermodynamic critical pressures of oxygen and kerosene. Detailed flow structures and mixing layer characteristics are examined, with special attention to the effect of the distance between the end of the center post and the entrance of the taper region, that is, the recess length. Six different cases ranging from no recess to fully recessed are investigated. Various controlling mechanisms of the injector flow dynamics, including recirculating motion near the fuel injection slit, shear-layer instabilities in the recess region, and vortex expansion and merging in the taper region are considered. Dominant frequencies of the axial and azimuthal instability modes are characterized and compared to empirical correlations. Dynamic analyses are conducted for the overall computational domain, as well as individual regions. Among the six cases, only the fully recessed Case 1 exhibits strong pressure oscillations over the entire injector, which are found to be closely related to the longitudinal acoustic oscillations in the center post. In cases with partial or no recess, Case 2 with a long recess achieves the best mixing, and Case 6 with zero recess leads to the worst mixing.

Experimental study on the flow field distribution characteristics of a gas–liquid swirl coaxial injector under ambient pressure

To better understand the atomization characteristics of a gas–liquid swirl coaxial (GLSC) injector under ambient pressure, the spatial distribution of multiple cross-sections was captured using a phase Doppler interferometry system, including the Sauter mean diameter (SMD), velocity, and data rate (particle number collected at the measurement points every second). The effects of the gas–liquid ratio and ambient pressure on the spatial distribution of the GLSC injector were analyzed. In general, the SMD increased with an increase in ambient pressure, while the axial velocity and data rate showed the opposite trend. For the non-recessed GLSC injector under ambient pressure, the atomization quality at low gas–liquid ratios was not better than that for no gas injection. In the area near the injector exit, the ambient pressure positively affected the atomization. Furthermore, the ambient pressure had a negative effect beyond a certain distance from the injector exit. The effect of environmental pressure on the spray cone was more apparent, and the combined effect of pneumatic and pressure atomization was worse, which eventually increased the particle diameter at the axis center. An increase in the gas–liquid ratio had little effect on the axial velocity around the spray cone but caused a significant increase in the axial velocity near the axis.

Surrogate-based modeling for emulation of supercritical injector flow and combustion

High-fidelity simulations play an increasingly important role in understanding fundamental turbulence-chemistry interactions and combustion dynamics in practical propulsion and power-generation systems. These simulations are, however, too computationally expensive and unwieldy for the purposes of design and optimization, given the large group of design parameters and wide design space. In this paper, we present an efficient surrogate-based modeling strategy to emulate spatiotemporal flows and combustion at supercritical pressures with accuracy similar to that of large-eddy simulations (LES). A common kernel-smoothed proper orthogonal decomposition (CKSPOD)-based surrogate model is developed, incorporating computer experiments, projection-based model reduction, kriging, and uncertainty quantification. The surrogate model (emulator) is carefully trained using a database drawn from a set of LES-based simulations that are conducted at designated sampling points in a given design space. A common Gram matrix is built using a Hadamard product to transform reduced spatial basis functions to remedy phase deviations among different design settings. Kriging is then used to obtain temporal spatial functions and associated coefficients for flowfield reconstruction at a new design setting.The framework is examined with two case studies: an emulation of flow dynamics in a simplex swirl injector, and an emulation of mixing and combustion in a gas-centered liquid-swirl coaxial injector. The surrogate model not only faithfully captures the salient features of its LES counterpart, but also shortens the computation time dramatically, by up to five orders of magnitude. The developed surrogate model can be applied to a broad range of engineering systems and will provide support for future engineering innovation and design.

Effect of coaxial injector parameters on LOX/methane engines: A numerical analysis

The liquid oxygen/methane engine is a desirable power for reusable launch vehicle. It is generally believed that the efficiency of the liquid oxygen/methane engine is mainly influenced by the injection conditions of the thrust chamber. However, due to the complexity of combustion experiments, the exact influence of specific injectors on the combustion performance is difficult to acquire. By analyzing a typical sub-scale thrust chamber, this paper presents a three-dimensional numerical model to evaluate the effects of three major injector parameters, i.e. the thickness of liquid oxygen post, the number of injectors and the type of injectors, on liquid oxygen/methane engine combustion performance. The results of the simulation are consistent with existing experimental data in the chamber pressure, combustion stability, as well as wall heat flux. This paper's results have shown that the larger liquid oxygen post thickness and swirl coaxial injector can significantly reduce the heat flux while influence of the injector number is relatively small. Furthermore, the decrement of the liquid oxygen post thickness can improve the combustion efficiency. And the less injectors lead to the more stable combustion. The presented simulation model gives a reliable and cost-effective way to analyze specific injector parameters and provides new principles for future liquid oxygen/methane engine design.

Three-dimensional flow dynamics and mixing in a gas-centered liquid-swirl coaxial injector at supercritical pressure

Three-dimensional flow dynamics and mixing in a gas-centered liquid-swirl coaxial injector at supercritical pressure is numerically studied using the large-eddy-simulation technique. In this class of injectors, typical of liquid-fueled propulsion engines, high-temperature gaseous oxygen (GOX) is axially delivered into the center tube and kerosene is tangentially injected through discrete orifices into the coaxial annulus. The operating conditions and geometry mimic those of the main injector elements used in staged-combustion propulsion engines. The present work details the full three-dimensional flow evolution over the entire injector configuration, including axial and circumferential dynamics that are essential for small-scale mixing between GOX and kerosene. Various key flow structures and instability mechanisms in the injector, including axial and azimuthal shear-layer instabilities, secondary instabilities (baroclinic torque and volume dilatation), centrifugal instability, flow recirculation, and acoustic motion, are identified. The significance of these instability mechanisms is explored in the context of streamwise and azimuthal vorticity transport. The GOX core is found to exhibit a hexagonal shape, mainly due to the interactions of vortex rings detached from the center post and coaxial annulus. For comparison, a cylindrical sector of the configuration is also simulated. The results of the present study will support the design and development of high-performance injectors for future propulsion applications.

Supercritical combustion of gas-centered liquid-swirl coaxial injectors for staged-combustion engines

The combustion characteristics of gas-centered, liquid-swirl coaxial injectors typically used in oxidizer-rich staged combustion cycle engines are numerically investigated at supercritical conditions. Turbulence closure is achieved using large-eddy-simulation techniques, and turbulence/chemistry interaction is modeled by a steady laminar flamelet approach. Gaseous oxygen (GOX) at 687.7 K is injected into the center post while kerosene at 492.2 K is delivered tangentially into the outer coaxial annulus. The operating pressure is 25.3 MPa. Detailed flow structures and flame dynamics are explored. The entire flowfield can be divided into four regimes: propellant injection, flame initialization, flame development, and intensive combustion. The diffusion-dominated flame is anchored in the wake of the GOX post and further enhanced in the downstream taper region. The surface of the coaxial annulus and taper is covered by fuel-rich mixtures and thus protected from thermal flux in the flame zone. Effects of the recess length (from the end of GOX post to the entrance of taper region) on the flow and flame evolution are investigated in depth. The efficiency of propellant mixing and subsequent combustion is found to increase with increasing recess length. The kerosene film is nearly depleted before the exit of the recess region for cases with long recess length, and the flame spreads upwards in the taper region for cases with reduced recess length due to insufficient mixing between GOX and kerosene. In a fully recessed injector without fuel shielding, the injected kerosene behaves like a liquid jet in a crossflow. Two recirculating zones containing fuel-rich mixtures are formed between the injection slit and the headend. A broad flame region is established at the exit of the recess region. In a non-recessed injector, the occurrence of combustion is delayed to the taper region. The flame resides along the taper surface and the injector faceplate, with most of GOX convecting downstream unburned. Results obtained from the present study can also be used to characterize combustion responses to local flow oscillations.

Theoretical supercritical fluid dynamics and mixing revealed in study on liquid rocket engines

Theoretical and numerical results elucidate the flow and mixing behavior of the fuel and oxidizer in gas-centered liquid-swirl coaxial injectors under complex and harsh supercritical conditions.

Supercritical fluid flow dynamics and mixing in gas-centered liquid-swirl coaxial injectors

Gas-centered, liquid-swirl coaxial injectors similar to those used in the main chambers of oxidizer-rich staged-combustion engines are investigated computationally, in terms of supercritical fluid flow dynamics and mixing. Gaseous oxygen (GOX) is axially directed through a center post at a temperature of 687.7 K. Kerosene is tangentially introduced into the outer coaxial swirler at a temperature of 492.2 K. The mean chamber pressure of 253.0 bars substantially exceeds the thermodynamic critical pressures of oxygen and kerosene. The end of the GOX post is recessed from the entrance of the taper region, which is connected downstream to an open domain. A wide range of recess lengths (and correspondingly, fuel shielding collar lengths) is considered to determine the dependence of flow characteristics on this geometric parameter. Special attention is given to the regions downstream of the GOX post end and in the taper section, where primary mixing occurs. Instantaneous and time-averaged flow properties, as well as mixing effectiveness, are examined. Results indicate that the recess length plays a critical role in determining the flow evolution and mixing behaviors. In a fully recessed injector without fuel shielding, the initial kerosene/GOX interaction resembles a swirling transverse jet into a crossflow, and flow recirculation occurs near the kerosene injection slit and the head end. In other injectors with fuel shielding, the kerosene flow is predominantly axial before it enters the mixing zone; the coflow kerosene and GOX streams expand radially and recirculate in the wake of the GOX post. Flow unsteadiness arising from the fluid injection and mixing and vorticity production in the boundary layer of the GOX stream, along the wall of the fuel passage, and at the kerosene/GOX interface cause the development of salient vortical structures in the downstream flowfield. The geometric changes at the entrance of the taper region and at the injector exit further alter the flow dynamics, inducing multiple toroidal recirculation zones and secondary vortex structures.

Effects of self-pulsation on the spray characteristics of gas–liquid swirl coaxial injector

To understand the influence of self-pulsation on the spray characteristics of gas–liquid swirl coaxial injector, a back-lighting photography technique has been employed to capture the instantaneous self-pulsated spray and stable spray images with a high speed camera. The diameter and velocity of the droplets in the spray have been characterized with a Dantec Phase Doppler Anemometry (PDA) system. The effects of self-pulsation on the spray pattern, primary breakup, spray angle, diameter and velocity distribution and mass flow rate distribution are analyzed and discussed. The results show that the spray morphology is greatly influenced by self-pulsation. The stable spray has a cone shape, while the self-pulsated spray looks like a Christmas tree. The main difference of these two sprays is the primary breakup. The liquid film of stable spray keeps stable while that of self-pulsated spray oscillates periodically. The film width of self-pulsated spray varies in a large range with ‘neck’ and 'shoulder’ features existing. The liquid film of self-pulsated spray breaks up at the second neck, and then the second shoulder begins to breakup into ligaments. The self-pulsated spray produces droplet clusters periodically, varies horizontal spray width and mass flux periodically. From the point of spatial distribution, self-pulsation is good for the spray, it uniformizes the mass flux along radius and increases the spray angle. However, when self-pulsation occurs, the SMD distribution varies from an inverted V shape to a hollow cone shape, and SMD increases at all the measuring points. Namely, from the point of atomization performance, self-pulsation has negative effects even when the breakup length is smaller. The effects of self-pulsation on the diameter and velocity distributions of the spray are mainly in the center part of the spray. The periphery of stable and self-pulsated spray has similar diameter and velocity distribution.

Planar Sauter Mean Diameter measurements in liquid centered swirl coaxial injector using Laser Induced Fluorescence, Mie scattering and laser diffraction techniques

An experimental technique is carried out to demonstrate the measurement of planar Sauter Mean Diameter (SMD or D32) distribution in a liquid centered swirl coaxial injector (LCSC) using simultaneous measurements of Mie scattering, Planar Laser-Induced Fluorescence (PLIF) and Laser diffraction technique (LDT). Here water is used as the test fluid with addition of optimized quantities of the organic dye (Rhodamine 6g) for PLIF measurements. Experiments are carried out at three experimental conditions with momentum flux ratios of 6.25, 12.14, and 19.95 respectively. Experiments are carried out to study the effect of dye concentration in LDT. LDT (line of sight) is corrected for multiple scattering effects. The SMD distribution obtained from Liquid Sheet Drop Sizing (LSDS) technique is calibrated using LDT (Malvern particle analyzer) that utilizes the principle of diffraction; the results obtained from both the methods are compared and analyzed using the respective histograms. The variations in the distribution of droplet diameter along the axial and radial locations in the spray field are also studied in detail.

Effects of injector recess on heat flux in a combustion chamber with cooling channels

An experimental study has been carried out to study the effects of recess length in swirl coaxial injectors on heat transfer in a combustion chamber with cooling channels. Liquid oxygen and kerosene (Jet A-1) were burned in a range of mixture ratios (2.12–3.07) and chamber pressures (57.3–82.3 bar) in small liquid rocket thrust chambers. Each thrust chamber had nineteen liquid–liquid swirl coaxial injectors. While changing the recess length of the injectors and operating conditions, temperatures of the cooling water passing through the cooling channels were collected and analyzed to calculate the heat flux in the combustion chamber. As expected, operating conditions such as oxidizer-to-fuel mixture ratio and chamber pressure had a large influence on the heat flux in the combustion chamber. The recess length of the injectors also significantly affected the heat flux in the combustion chamber, which showed that the longer recess length gradually increased the heat flux. It is thought that the variation of flame structure with respect to the recess length in swirl coaxial injectors was the main reason. Since heat flux is a very important factor in designing cooling channels, the effects of recess length on heat flux in a combustion chamber must be carefully considered.

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.

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.

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.

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.