Swirl Injector Research Articles

Stabilization regimes and pollutant emissions from a dual fuel CH4/H2 and dual swirl low NOx burner

Burning hydrogen in gas turbines is a relevant technological solution to decarbonize power production and propulsion systems. However, ensuring low NOx emission and preventing flashback can be challenging with hydrogen. Stabilization regimes and pollutant emissions from partially premixed CH4/H2/air flames above a coaxial Dual Fuel Dual Swirl injector are investigated in a laboratory-scale combustor at atmospheric conditions for increasing hydrogen contents. The injector consists of an external annular swirler providing premixed methane/air and a central channel fed with pure hydrogen. This burner virtually removes the risk of flashback due to the late injection of hydrogen. Flame stabilization regimes, CO and NOx emissions are analyzed for different configurations of the injector and operating points. The effect of swirling the hydrogen stream is investigated together with the influence of the hydrogen injector recess, i.e. its nozzle position with respect to the backplane of the combustion chamber. It is shown that swirling the central hydrogen stream favors aerodynamically stabilized flames resulting in a low thermal stress on the injector and limited NOx emissions. The study also highlights that a small recess of the central hydrogen injector widely extends the operability range of the burner with aerodynamically stabilized flames. With a sufficient inner swirl and a small recess, flames detach from the injector rim when the hydrogen bulk velocity is large enough. In this configuration, it is found that NOx emissions remain low even for operation with pure hydrogen. Moreover, NOx emissions decrease when increasing the thermal power for a fixed equivalence ratio.

Effect of Upstream Valve Opening Process on Dynamic Spray Atomization of Bipropellant Thruster Injector.

In order to develop a new generation of intelligent satellites, fast-response bipropellant thrusters are required to work in minimum impulse mode without limitation. When a valve is opening, the fluctuation affects downstream spray atomization at the injector, which determines the thruster’s impulse performance, involving combustion efficiency and impulse repeatability. Accordingly, the spray atomization under impulse working condition was investigated to optimize the thruster’s dynamic response. The effects of propellant property, switch speed, valve stroke, and throttle orifice layout are respectively compared in simulation cases using OpenFOAM. The fluctuating flowrate caused by valve opening was simulated and then used as boundary conditions for downstream spray. Among these factors, orifice layout plays the most significant roles in transient spray development. Compared with MMH spray, NTO spray from outer swirl injector is more sensitive to upstream fluctuation. When the upstream flowrate stabilizes faster, the atomization stability can also be enhanced, thereby improving the impulse repeatability of thrusters in combustion. This experimental result was in good agreement with the simulation, thereby showing that only when atomization of MMH spray and NTO spray both develop into a steady state within 5 ms after valve opening can the impulse performance be reliably achieved.

Spray characteristics of gel fuel in a gas-centered coaxial swirl injector with variable recess ratios

This paper has studied the spray characteristics of gel in a series of different gas-centered coaxial injectors by experimental observations. Impacts of recess ratio on breakup features, such as the breakup length, spray angle and droplet size distribution, have been also investigated. The gel spray morphology shows that the spray field could be divided into not-fully and fully-atomization zones with the different gaseous flow rates. Then a critical airflow velocity has been proposed to distinguish the different atomization patterns. Experimental results reveal that the larger recess ratio would widen the droplet spatial distribution, increase the spray angle and reduce the Sauter Mean Diameter (SMD) in the spray field. Besides, the contraction impact of inner gaseous flow could also be visualized by these experiments. It could be an effective way to improve its atomization performance of gel solutions based on the present coaxial injectors.

Experimental studies on the unsteady atomization characteristics of a swirl injector

This paper studies the spray and dynamic respond characteristics of a swirl injector under the different unsteady experimental conditions. The flow pulsation and injector geometric parameters, such as the pulsation frequency and swirl chamber length, on the atomization features like average liquid sheet thickness and spray angle have been comprehensively measured. It is indicated that the whole pulsation range could be divided into different parts based on the pulsation frequencies of incoming flow. Experimental results reveal that the liquid sheet thickness and Sauter Mean Diameter (SMD) amplitude would decrease in the square and bimodal regions and then remain unchanged in the triangular region. Both of them would increase with the tangential channel diameter and swirl chamber length while the steady incoming pressure drop shows an opposite impact on them. Besides, the spray performance of swirl injector could be improved by rising the swirl chamber diameter.

Modelling and Validating the Spray Characteristics of a Co-axial Twin-Fluid Atomizer Using OpenFOAM

Today, the applications of sprays cover a wide range of fields. Their role in internal combustion engines is instrumental in maintaining higher engine efficiency. A deeper understanding of the liquid-gas phase interaction in sprays is crucial to the atomization process. The methods and models used in the simulations have their challenges due to the various discretization schemes and solutions used. To develop and validate the computational models, well defined experimental data is required. In the present work, spray characteristics were studied numerically through OpenFOAM. As the spray characteristics are closely linked with the liquid breakup length, this study focuses on the primary breakup phenomena and the breakup length of the liquid jet emanating from the twin-fluid co-axial flow atomizer. Numerical simulations were performed for a wide range of initial conditions and the breakup length of the spray was validated against the experimental observed by Sivadas et al., [26]. These simulations were carried out using a Eulerian based VOF solver that models the fluid as a continuum. K-Epsilon model was used to predict the turbulent nature of the spray. The air and water velocities were varied between 19.0 to 31.3 m/s and 0.7 to 1.8 m/s respectively. The proposed model was able to predict the computed breakup length within 20% of the experimental values. The present model can be further extended to test for a co-axial swirl injector to predict finer spray formation.

Computational analysis of fine droplet in swirl injector for desalination

Desalination refers to a water treatment process that removes salts from water. Low Pressure Flash Evaporation desalination (LPFE) process is conducted experimentally with using of high flow swirl injector. This injector is used to improve the efficiency of the plant. The effects of atomizer’s dimensions and flow parameters were studied and analyzed to get desirable droplet size and good yield. From this parametric study, we can know that droplet diameter effects yield. The theoretical results shows that droplet size should be in the range of 0.9 to 1.0 mm to get maximum yield.The internal flow behavior in high flow swirl injector also studied using a commercial computational fluid dynamics (CFD) software - FLUENT. Through these works, designed a swirl nozzle at high flow rate (1lit/s) with parameter’s study and optimized technically. This paper deals, through CFD analysis that desirable range of droplet size is analyzed and has been proved that to get maximum yield with the effect of various flow parameters. The result gives a valuable output for developing technology in desalination.

DESIGN, EXPERIMENTAL AND NUMERICAL INVESTIGATION OF COAXIAL SWIRL INJECTOR

DESIGN, EXPERIMENTAL AND NUMERICAL INVESTIGATION OF COAXIAL SWIRL INJECTOR

Prediction of combustion instability by combining transfer functions in a model rocket combustor

The flame transfer function (FTF) and acoustic transfer function (ATF) are estimated experimentally and numerically, respectively, and both are combined to predict combustion instability of a model rocket combustor with a single coaxial jet–swirl injector. FTF of the injector is measured experimentally by imposing external velocity excitation on the injector. ATF, i.e., model-chamber response to heat-release fluctuation from flames, is difficult to measure, and alternatively, an idea of numerical evaluation method is suggested by applying an equivalence-ratio perturbation to flames. Then, ATF is calculated by numerical simulations with a three-dimensional model and compared with ATF from the one-dimensional analytic equation. Results show that ATF estimated by the 3-D numerical model improves accuracy in predicting resonant frequencies and growth rates of pressure oscillations in the model chamber. Finally, a closed-loop system transfer function describing dynamic behaviors of the model chamber is formulated by combining FTF with ATF and it is applied to predict combustion instability of the chamber. This prediction is compared with experimental observation, which shows good agreement between them.

Experimental and Numerical Investigations on the Ignition of RBCC Gas-Oxygen/Kerosene Rocket Ejector

The rocket ejector refers to a core component of a rocket-based combined cycle (RBCC) engine. The ignition is of critical significance for rocket ejection. Reliable and stable ignition crucially determines the normal operation of the engine. In this paper, a thrust chamber with coaxial swirl injector for the RBCC rocket ejection was developed and tested. Gas oxygen (GOX) and kerosene acted as propellants. As revealed from the test results, the process of ignition pressurizing comprised four phases. The oxygen prefilling time before ignition slightly impacted the ignition time, whereas it affected the peak pressure of ignition. In a confined range, the peak pressure decreased as the prefilling time was extended. The ignition was simulated by building a numerical model, and the results well complied with the experimentally achieved results. The numerical model is capable of specifically indicating the position of the kernel of fire and the process of flame propagation. The simulation results reveal that the propellant could form a combustible condition within 4 ms. The kernel was 6 mm away from the injector, located at the oxygen and kerosene mixing interface and approaching the upper wall. The above results reflected the vital role of the central recirculation zone formed by the prefilled oxygen. The ignition energy was transported near the injector under the convection effect, which ignited the stoichiometric mixture, and the entire ignition could reach a stable state within 20 ms. The numerical model which was developed in this paper can help clarify the combustion mechanism.

Dynamic Characteristic Analysis of a Gas–Liquid Swirl Injector Using Planar Laser Imaging

The oscillation of the mass flow rate of the input propellant in liquid rocket engines may induce a periodic heat energy release within the combustion chamber, resulting in operational instability. Since propellant components are discharged from the injector head, the injectors can play a role as a source of oscillation in the dynamic system of a liquid rocket engine. Prior studies confirmed that if the liquid spray and gas flow intersect by using gas-centered dual-swirl injector with tangential inlet in the gas flow path, droplets are generated with periodicity (fracture frequency). In this study, the characteristics and mechanisms of this periodicity were analyzed using longitudinal cross-section spray images. Since the multiple scattering signal that inhibits clarity is non-modulated components, it can be removed by using two modulated images with a phase difference of 180 deg (two-phase structured laser illumination planar imaging technique). And the dynamic characteristics of the gas–liquid spray were analyzed by applying the fast Fourier transform method. It was confirmed by this method that the fracture frequency was related to the impinging velocity of gas and hydrodynamic feature of liquid film. That is, it can be said that fracture frequency of the liquid film is dependent on the liquid Weber number and gas Weber number as the entrainment effect and impact force by gas flow are applied to the liquid film.

Reduced-Order Modeling for Complex Flow Emulation by Common Kernel-Smoothed Proper Orthogonal Decomposition

In the present study, we propose a new surrogate model [common kernel-smoothed proper orthogonal decomposition (CKSPOD)] to emulate spatiotemporally evolving flows. The model integrates and extends recent developments in Gaussian process learning, high-fidelity simulations, projection-based model reduction, uncertainty quantification, and experimental design, rendering a systematic, multidisciplinary framework. The novelty lies in the construction of a common Gram matrix: the Hadamard product of Gram matrices of all observed design settings. The common Gram matrix synthesizes the temporal dynamics by transferring proper orthogonal decomposition (POD) modes into spatial functions at each observed design setting, which remedies the phase-difference issue encountered in the kernel-smoothed POD (KSPOD) emulation. The CKSPOD methodology is demonstrated through a case study of flow dynamics of swirl injectors with three design parameters. A total of 30 training design settings and eight validation design settings are included. The CKSPOD emulation outperforms the KSPOD counterpart, and it is capable of capturing small-scale flow structures faithfully. The CKSPOD prediction of turbulent kinetic energy reveals lower uncertainty than KSPOD. The turnaround time of the CKSPOD emulation is about five orders of magnitude faster than the corresponding high-fidelity simulation, which enables an efficient and scalable framework for design exploration and optimization.

Passive Control of Low-Frequency Instability in Hybrid Rocket Combustion

The occurrence of low-frequency instability (LFI) appears to be related to multiple interactions among many complex physical processes, such as vortex shedding, boundary-layer oscillation, and additional combustion in the post-combustion chamber. In this study, two combustion tests were conducted to suppress LFI and to examine which physical processes its occurrence was most sensitive. In the first test, two fuel inserts were used to modify the formation of a boundary layer, vortex shedding at the end of the fuel, and vortex impingement. In the second test, the fuel insert located at the front end was replaced with swirl injection. The first test was aimed at controlling and suppressing the initiation of LFI using fuel inserts, through which a small step appeared gradually due to differences in the regression rates of the two materials, i.e., polymethyl methacrylate and high-density polyethylene. The test results confirmed that (i) there are physical connections among several processes, such as the thermoacoustic coupling between p′and q′ and the oscillations of the upstream boundary flow, and (ii) LFI suppression is possible by disrupting or eliminating the connections among these physical processes. The second test was also aimed to control LFI while minimizing the deviation in combustion performance using proper swirl injection along with a fuel insert. Even when replaced by swirl injection, LFI suppression was still possible and showed reasonable combustion performance without causing too much deviation from the baseline in terms of the oxygen-to-fuel ratio and the fuel regression rate.

Experimental investigation of the effect of gaseous fuel injector geometry on the pollutant formation and thermal characteristics of a micro gas turbine combustor

Experimental investigations of the effects of a new gaseous fuel injector on the performance of a micro gas turbine combustor have been conducted. The new injector which is called swirl injector exerts a swirl-like motion on the fuel flow to increase air-fuel mixing in the combustor. The results, including temperature distribution in the secondary zone of the combustor, NOx, CO, and CO2 emissions, thermal power, and pattern factor are compared with the conical injector configuration. The results show that the temperature distribution is more uniform, both at the secondary zone and outlet section of the combustor in the swirl injector configuration, and the pattern factor of the combustor is reduced by a factor of two. Moreover, the mean outlet temperature of the combustor is higher in the swirl injector configuration which leads to increased thermal power and combustion efficiency up to 12%. Analysis of the emission of the combustor reveals that the CO emission can reduce to half in the swirl configuration at high fuel flow rates. NO emission, however, is two times higher in the new configuration at low fuel flow rates, but at higher flow rates the NO emission is equal to the conical injector configuration.

Theoretical Atomization Model of Liquid Sheet Generated by Coaxial Swirl Injectors

This paper describes the atomization process of a liquid sheet generated by a coaxial swirl injector and it establishes a theoretical breakup model to predict the Sauter mean diameter (SMD) in the entire spray field. The two stage breakup model, which includes Kelvin-Helmholtz (K-H) and Rayleigh-Taylor (R-T) instability, is established and combined with the dispersion relation obtained by linear stability analysis. The theoretical expression to predict the SMD is then determined by full wavelength integration. The validity of the present model is first verified; the results predicted by the theoretical model showed good agreement with previous experimental data. Subsequently, effects of fluid physical properties and flow parameters on the SMD are also investigated. The related conclusions are compared and concur well with the existing literature.

Vortex Breakdown Control by the Plasma Swirl Injector

Vortex breakdown, observed in swirling flows, is an interesting physical phenomenon relevant to a wide range of engineering applications, including aerodynamics and combustion. The concept of using a plasma swirler to control vortex breakdown was proposed and tested in this study. The effect of plasma actuation on controlling the onset and development of the vortex breakdown was captured by particle image velocimetry. Flowfield measurement results suggested that, by varying the strength of the plasma actuation, the location and size of the vortex breakdown region was controlled effectively. The plasma swirl injector offers a method for optimal control and efficient utilization of vortex breakdown. The method being proposed here may represent an attractive way of controlling vortex breakdown using a small amount of energy input, without a moving or intrusive part.

Sheet-breakup characteristics of a closed-type swirl injector considering internal flow instability

Sheet-breakup characteristics of a single liquid swirl injector is important for understanding swirling flow characteristics of the swirl coaxial injector. Also, acquirement of a precise breakup length of a swirl injector could be used as a guideline for designing the injector for small thrusters. Theories for obtaining this breakup characteristic have been developed using the waveform instability of a thin liquid sheet. However, these theories could not explain the effect of internal instability of the swirl injector on the breakup characteristic. In the swirl injector, internal instability could be caused by air core fluctuation and this instability could affect the wave and breakup characteristics of the spray sheet. Therefore, in this study, the effect of internal flow instability of the swirl injector on the spray-sheet breakup characteristics was investigated. At first, it was proposed to substitute the new value of the internal growth rate ωint for the existing growth rate, and an empirical formula for the internal growth rate value was presented based on experimental results. Through this equation, it was confirmed that the breakup length could be more accurately calculated using the swirl injector condition in which the internal flow instability of the injector was the dominant influence. Then, a semi-empirical equation for calculating the breakup length more accurately was proposed.

Local flame and flow properties of propagating premixed turbulent flames during light-round process in a MICCA-type annular combustor

In this paper, local flame and flow properties of propagating premixed turbulent propane/air flames were derived by two-dimensional time-resolved Mie scattering and PIV methodology in a MICCA-type annular combustor. The curvature of flame front, local flame speed and the evolution of leading points during the light-round process were analyzed in detail. All experiments were performed under a constant equivalence ratio of 0.67, with the bulk flow velocity set from 1.95 to 5.85 m/s in five cases to achieve different turbulent intensities. The instantaneous flame fronts and velocity fields of unburned gases were measured to investigate the flame-flow interactions corresponding to four regions of the annular combustor. The results indicate that the slight asymmetry of the propagation in two flame branches is mainly due to the competition between the local swirling injection and the overall circumferential flow along the inner or outer parts of the combustor. Based on the statistical analysis of the local radius of flame curvature, the whole flame front tends to be convex to the unburned mixture. As a characteristic length scale, the inner-cutoff scale εi of the flame front is about 2–3 times the flame thickness δL, which decreases with the increase of turbulent intensity. The value of fractal dimension increases with the increase of turbulence intensity, and its value is about 2.15–2.25 for all cases. Besides, in order to analyze the flame properties during the ignition process, the local flame speed wn was calculated using statistical method as well as the velocity of unburned mixture gas. Moreover, the experimental results (Ub=3.90 m/s) of the leading points’ trajectory were consistent with the large-eddy simulations at the height of h =25 mm, and the leading points of the flame fronts were generally concentrated near (about 5 mm away) the inner tube for both flame branches. The researches of ignition mechanisms in annular combustors have been carried out for almost ten years, but the quantitative measurements of flow and flame fronts are barely reported. Motivated by this, it will provide new insights into the ignition process in annular combustors from this work.

Numerical study of two-phase flow dynamics and atomization in an open-type liquid swirl injector

The three-dimensional, two-phase flow dynamics and underlying physics in an open-type liquid swirl injector are investigated using numerical simulations. The basis for this study is a validated multiphase flow solver, interFoam, in OpenFOAM. Turbulence closure is achieved by means of LES with a one-equation eddy-viscosity turbulence model. A volume-of-fluid method is used for tracking the interface between the gas and liquid phases. The detailed spatio-temporal evolution of the flow field is explored systematically, including the liquid surface wave motion and liquid film characteristics within the injector, the helical air core, and the formation and atomization of the liquid spray cone downstream of the injector. The spectral content of the pressure field is also examined, to reveal the characteristic frequencies and feedback mechanisms of gas-liquid interactions.

A Joint Experimental and Large Eddy Simulation Characterization of the Liquid Fuel Spray in a Swirl Injector

Abstract The quality of liquid fuel spray injection determines to a large extent the performance of aero-engine combustors. This investigation focuses on the detailed characterization of the liquid fuel spray in a test rig targeted at aero-engine applications. The liquid fuel is injected as a hollow cone by a simplex atomizer and the injector comprises a radial swirler. Two features of the droplet distribution are less commonly found. First, the distributions of droplet diameters exhibit nonaxisymmetric patterns, which are investigated for three types of swirlers. Second, it is found that the size-conditioned velocity distributions feature a single wide peak for small droplets and become bimodal for the largest droplets, with a first peak at low velocities, and a second one at higher velocities. Experiments are complemented with large eddy simulations and Lagrangian particle tracking. The spray interacts with the lateral injector surface and requires a droplet–wall interaction model for the liquid film. Simulations do not retrieve the lack of rotational symmetry that is found experimentally indicating that this feature is not linked to the nature of the flow. This is also consistent with further experiments with a different atomizer confirming that this is due to imperfections in the atomizer geometry. Another result is that certain swirler designs are more robust to atomizer imperfections. Simulations accounting for the liquid film yield a bimodal distribution for the droplets' axial velocity distribution which is not obtained without this model, indicating that it is important to represent the droplet–wall interaction.

A comparative study of dynamic mode decomposition methods for mode identification in a cryogenic swirl injector

The dynamic mode decomposition (DMD) is one of the post-processing methods used in analyzing dynamic or transient flow field data from experiments or CFD analyses. This method extracts resonant frequencies, damping coefficients, and spatial structures of mode shapes without a priori physical knowledge of the specific form of structure or power spectrum density analysis. The optimized DMD method is applied to improve the accuracy of DMD results in finding the eigen-components by fitting the nonlinear dynamic system from initial eigenvalues and modes determined by applying the exact DMD. A comparison is made between the optimized DMD and the exact DMD method. Two DMD methods are applied to the numerical data calculated with a cryogenic swirl injector to show validity of their algorithms in identifying acoustic modes of a dynamic system. Compared with the exact DMD, the optimized DMD can extract feasibly the acoustic modes of the dynamic system with higher accuracy. Furthermore, the optimized DMD can find the resonant modes of the system more completely.