Central Jet Research Articles

Velocity field and cavity dynamics in drop impact experiments.

Drop impact experiments allow to model a wide variety of natural processes, from raindrop impacts to planetary impact craters. In particular, interpreting the consequences of the planetary impacts requires an accurate description of the flow associated with the cratering process. In our experiments, we release a liquid drop above a deep liquid pool to investigate simultaneously the dynamics of the cavity and the velocity field produced around the air-liquid interface. Using particle image velocimetry, we analyse quantitatively the velocity field using a shifted Legendre polynomials decomposition. We show that the velocity field is more complex than considered in previous models, in relation to the non-hemispherical shape of the crater. In particular, the velocity field is dominated by the degrees 0 and 1, with contributions from the degree 2, and is independent of the Froude and the Weber number when these numbers are large enough. We then derive a semi-analytical model based on the Legendre polynomials expansion of an unsteady Bernoulli equation coupled with a kinematic boundary condition at the crater boundary. This model explains the experimental observations and can predict the time evolution of both the velocity field and the shape of the crater, including the initiation of the central jet.

Multi-scalar mixing in turbulent coaxial jets

Although natural and industrial flows often transport and mix multiple scalars, relatively few studies of turbulent multi-scalar mixing have been undertaken. In the present work, a novel three-wire thermal-anemometry-based probe – capable of simultaneously measuring velocity, helium concentration and temperature – is used to investigate the evolution of multiple scalars ($\phi _1$,$\phi _2$,$\phi _3$) and velocity in turbulent coaxial jets. The jets consist of (i) a centre jet containing a mixture of helium and air ($\phi _1$), (ii) an annular jet containing pure (unheated) air ($\phi _2$) and (iii) a coflow of (pure) heated air ($\phi _3$). Axial measurements are made at three different momentum flux ratios ($M=0.77, 2.1, 4.2$). Increasing$M$was observed to result in complex, competing effects. Larger momentum flux ratios cause the potential core of the centre jet to decrease in size, greater scalar fluctuations and more rapid correlation of$\phi _1$and$\phi _2$. However, at the same time, certain statistics, including those describing the velocity field, evolve more slowly. Moreover, the flow near the beginning of the fully merged region appears to be less mixed at higher values of$M$. The present work finally demonstrates that differences can be observed in the evolution and mixing of coaxial jets between those in which$M<1$and those in which$M>1$, thereby presenting an opportunity by which the mixing process in coaxial jets may be controlled.

Effect of operational and geometric parameters on the hydrodynamics of a Wurster coater: A CFD-DEM study

A coupled CFD-DEM method was used to study the hydrodynamics of a Wurster coater. Firstly, the CFD part of the model was validated by the accurate prediction of the pressure drops over a pseudo-2D fluidized bed under various gas velocities. The effect of gas velocity, gap height, tube length and batch volume of the particles on the cycle time and the residence time of the particles was thoroughly investigated. The central jet gas velocity u1 was found to speed up the particle cycle but undermine the coating efficiency. The gas velocity at the horizontal transport zone u2 was able to promote the horizontal transport of the particles but should not be too high, otherwise, it would obstruct the normal falling back of the particles in the downward zone. Big gap heights would decrease the coating efficiency but tube length had little impact on that. The increment of batch volume would commonly abase the cycle time and the working efficiency under a given u1. The de-fluidization problem arose when the batch volume increased to 550 mL. However, this problem could be swept out by the optimization of u1 and u2. In a mixture of different sizes, the coarse particles enjoyed higher coating efficiency and could travel closer to the nozzles. This may shield the fine particles from getting enough coating liquids, and thus coarse particles and fine particles were not recommended to get coated in the same batch.

Black hole and galaxy co-evolution in radio-loud active galactic nuclei atz∼ 0.3–4

The relation between the mass of the supermassive black hole (SMBH) in the center of galaxies and their bulge mass or central velocity dispersion is well known. This suggests a coevolution between the SMBHs and their galaxy hosts. Our aim is to study this relation, specifically, for radio loud galaxies, and as a function of redshiftz. We selected a sample of 42 radio galaxies and active galactic nuclei (AGN) with broad emission lines and spectroscopic redshifts betweenz = 0.3 − 4 by cross-matching the low radio frequency sources from Very Large Array (VLA) FIRST with spectroscopically confirmed galaxies from wide-field surveys, including Sloan Digital Sky Survey (SDSS) DR14 ugriz and Dark Energy Survey (DES) DR2 grzY in the optical, Wield Infrared Survey Explorer (WISE), and the Galaxy And Mass Assembly (GAMA) spectroscopic survey. We characterized the stellar mass (M⋆), star formation, and black hole properties (mass of the central SMBH, Eddington ratioη, and jet power,Qjet). The relation between SMBH mass,M⋆,η, andzis placed into context by comparing them with scaling relations (MBH–M⋆,MBH/M⋆–z,MBH–Qjet, andQjet–η) from the literature. On the basis of a multiwavelength spectral energy distribution modeling, our radio sources are broadly consistent with being on the star-forming main sequence. They have sub-Eddington accretion rates,η ≃ 1% on average, as typically found in type I AGN, while higher accretion rates favor more powerful jets to be launched by the central engine. We find overmassive SMBHs in (17 ± 5)% of our radio sources, similarly to previous studies on nearby early-type galaxies. Altogether, an evolutionary scenario in which radio-mode AGN feedback regulates the accretion onto the SMBHs and the stellar mass assembly of the radio sources is discussed, which may explain the observed phenomenology. This pilot study represents a benchmark for future studies using wide-field surveys such as those withEuclidand theVera RubinObservatory.

Assessment of LES Dynamic Smagorinsky-Lilly model resolution for combustion engineering applications

Large Eddy Simulation (LES) has become a powerful tool for dealing with turbulence. Nevertheless, mesh resolution of the simulated domain under the LES approach was shown to be a key matter. Critical regions should maintain an adequate mesh resolution and also the highest possible quality. LES with the Dynamic Smagorisky-Lilly sub-grid model was used for the simulation of SMA1-flame, to assess the influence of the mesh resolution on the instantaneous velocity fields, species profiles, and temperatures profiles. The results of the simulation suggest a comprehensible agreement with experimental data. Nevertheless, some areas in the highly rotational velocity field are not properly solved, mainly due to a poor mesh resolution of such areas. As a result, the central jet’s decay rate was not accurately predicted. On the other hand, the temperature and species profiles were reasonably computed, considering the simple chemistry provided by the Eddy Dissipation Model (EDM).

Use of recirculation theory and reduced dimensionality for efficient calculation of jet flow in cylindrical combustors

This work evaluates use of a dimensionally adaptive (DA) meshing technique to speed CFD modeling of non-reacting pressurized flow in a cylindrical geometry with a center jet and non-axisymmetric secondary jets. The DA-capable approach incorporates an a priori dimensional boundary location and extension of 3-D RANS flow solvers. Recirculation zone length, not flow asymmetry, was the limiting factor in placement of the dimensional boundary between 3-D and axisymmetric meshes. Boundary placement was based on confined turbulent jet theory, which suggests jet attachment is a linear function of cylinder radius, L, expressed as 5.85L. CFD results for a turbulent axial pipe flow showed that as the dimensional boundary was moved upstream, computational time dropped quadratically with the reduction in mesh cells. Further simulations of a 0.2032-m diameter, 1.5-m long reactor with turbulent center primary jet and outer radius secondary jets placed the dimensional boundary at 0.7 m. Results for pressures of 20, 10, and 5 bar and mass flow rates of 75% and 50% of baseline demonstrated two key results. First, flow similarity was preserved across all cases and the predicted jet attachment location was approximately 0.61 m, compared with the theoretical value of 0.594 m. Second, runtime reductions of 79% (pressure) and 82% (flow rate) were seen for the DA meshes compared with fully 3-D meshes. Differences in average exit axial velocities between DA and fully 3-D meshes were 2–4% for the pressure cases and 0.7–1.5% for the flow rate cases. Lastly, comparison with 3-D quarter-geometry calculations showed the DA approach to be ∼10% faster with exit axial velocities within 3–5% of 3-D velocities.

Vibration-induced streaming flow near a sharp edge: Flow structure and instabilities in a large span of forcing amplitude.

The steady streaming generated near solid walls by the periodic forcing of a viscous fluid is known to be strongly enhanced near sharp structures, owing to centrifugal effects that lead to the generation of an intense jet from the sharp tip. This flow has been shown to provide efficient active mixing in microchannels, due to strong transverse velocity. The forcing is often prescribed by acoustic transducers, but it can also be generated from low-frequency time-periodic flow ensured by mechanical vibrations. In this paper, we study the flow structure generated by low-frequency forcing (typically 10Hz) around a sharp tip. Using direct numerical simulations, we extract both the time-periodic and steady responses within a large span of amplitude of vibrations. When the amplitude is smaller than the tip radius of curvature, we recover the flow structure observed at higher frequencies (>1kHz) in previous studies, namely, an intense symmetric central jet and a quadratic dependence for the characteristic streaming velocity with the oscillating velocity v_{s}∼v_{a}^{2}. At higher amplitudes, such a scaling no longer holds and the streaming flow pattern loses its left-right symmetry. We then analyze the mechanisms of the instability from the careful examination of the instationary flow fields, and we propose possible mechanisms for such a flow transition involving the coupling between the streaming jet and instationary vorticity.

Lagrangian mixing of pulsatile flows in constricted tubes

Several Lagrangian methods were used to analyze the mixing processes in an experimental model of a constricted artery under a pulsatile flow. Upstream Reynolds number Re was changed between 1187 and 1999, while the pulsatile period T was fixed at 0.96 s. Velocity fields were acquired using Digital Particle Image Velocimetry for a region of interest (ROI) located downstream of the constriction. The flow is composed of a central jet and a recirculation region near the wall where the vortex forms and sheds. To study the mixing processes, finite-time Lyapunov exponents (FTLE) fields and concentration maps were computed. Two Lagrangian coherent structures (LCS) responsible for mixing fluid were found from FTLE ridges. A first LCS delimits the trailing edge of the vortex, separating the flow that enters the ROI between successive periods. A second LCS delimits the leading edge of the vortex. This LCS concentrates the highest particle agglomeration, as verified by the concentration maps. Moreover, from particle residence time maps, the probability of a fluid particle leaving the ROI before one cycle was measured. As Re increases, the probability of leaving the ROI increases from 0.6 to 0.95. Final position maps rf were introduced to evaluate the flow mixing between different subregions of the ROI. These maps allowed us to compute an exchange index between subregions, EI¯, which shows the main region responsible for the mixing increase with Re. Finally, by integrating the results of the different Lagrangian methods, a comprehensive description of the mixing and transport of the flow was provided.

On the mechanics of droplet surface crater during impact on immiscible viscous liquid pool

We study drop impacts on an immiscible viscous liquid pool and investigate the formation of droplet surface craters using experimental and theoretical analyses. We attribute the formation of air craters to the rapid deceleration of the droplet due to viscous drag force. The droplet response to the external impulsive decelerating force induces oscillatory modes on the surface exposed to the air forming capillary waves that superimpose to form air craters of various shapes and sizes. We introduce a non-dimensional parameter (${\varGamma }$), that is, the ratio of the drag force to the capillary force acting on the droplet. We show that${\varGamma }$is directly proportional to the capillary number. We show that droplets forming air craters of significant depths have${\varGamma }>1$. Further, we demonstrate that Legendre polynomials can locally approximate the central air crater jet profile. We also decipher that the air crater response time scale ($T$) varies as the square root of impact Weber number ($T\sim We^{1/2}$). Further, we generalize the local droplet response with a global response model for low impact energies based on an eigenvalue problem. We represent the penetrating drop as a constrained Rayleigh drop problem with a dynamic contact line. The air–water interface dynamics is analysed using an inviscid droplet deformation model for small deformation amplitudes. The local and global droplet response theories conform with each other and depict that the deformation profiles could be represented as a linear superposition of eigenmodes in Legendre polynomial basis. We unearth that the droplet response in an immiscible impact system differs from the miscible impact systems due to the presence of such a dynamic contact line.

Comparative study on gas-particle transport characteristics subjected to the central and annular coaxial jets

The gas-particle transport characteristics subjected to the central and annular coaxial jets were investigated using particle image velocimetry (PIV) and optical fiber probe technologies. The powder transport behaviors, the distributions of particle velocity, concentration, fluxes, and turbulence properties, as well as the spatial variations of vortex structures and vorticity, were measured with various jet velocities. The results showed that the injection patterns significantly influence the transport characteristics of the gas-particle two-phase coaxial jets. Compared with the annular coaxial jets, the powder jet subjected to the central coaxial jets exhibits better momentum transfer and particle dispersion characteristics, leading to the acceleration and fluctuation in the axial and radial velocities of particles. Correspondingly, the vortex structures formed in the shear layer of the annular coaxial jets do not undergo pairing, coalescing, and breaking up into small-scale eddies as rapidly as the central coaxial jets, which significantly decreases the entrainment and turbulence intensity. The combing interactions of the recirculating entrainment and the wall effect result in the particle accumulation near the wall ( r / R = 0.83– 1). Although the uneven distribution of particles was observed in both coaxial jets, the radial profiles of particle concentration are more uniform in the central coaxial jets than in the annular coaxial jets. • The injection patterns showed a significant effect on powder transport behaviors. • The turbulence properties and the evolution of coherent structures were captured. • The central coaxial jet exhibited better momentum transfer and particle dispersion. • The near-wall accumulation of particles increases with gas jet velocity.

EXPERIMENTAL INVESTIGATION OF UNLIKE TRIPLET SPRAYS USING HIGH-MAGNIFICATION SHADOWGRAPHY: INFLUENCE OF THE JETS VELOCITIES AND CENTRAL JET PROPERTIES

In this study, inert liquid sprays are generated by impinging two symmetric jets of water with a central jet of water, ethanol, or n-dodecane. This configuration, referred to as an unlike triplet injector, can be used in rocket engines to atomize liquid storable propellants, for instance, hydrogen peroxide oxidizer combined to a fuel. Here, the inert sprays are investigated in the so-called impact waves regime, which corresponds to jets a Weber number of higher than 1000. The atomization process is characterized using high-magnification shadowgraphy (HMS) from the impinging point of the jets into a sheet until it breaks up into ligaments and droplets. The HMS technique enables 10 kHz visualizations with an interframe of 4 μs and a spatial resolution up to 6.4 μm/pixel (1024 × 1024 pixels). Characteristic lengths of the primary atomization are measured: breakup length, apparent wavelength, and ligaments size. Similarly, the droplet populations are described based on arithmetic and Sauter mean diameters, shape, and velocity. Statistics of large droplet distributions are analyzed regarding the injection conditions and distance to the impingement point. Compared to like-doublet spray, the like-triplet evidences a slower atomization (longer breakup distance) and generates larger drops that require more distance to stabilize in size, centricity, and velocity. Unlike-triplet sprays exhibit a similar behavior to like-triplet spray while producing larger droplets, probably because of the fuel properties that stabilize the liquid sheet.

The Effect of Mitral Valve Prostheses Design and Orientation on Left Ventricular Flow during Left Ventricular Assist Device Support

The mitral valve affects the shape and timing of incoming flow to the left ventricle of the heart, establishing a strong central jet that generates a vortex ring that conserves momentum during the cardiac cycle. When the vortex is disrupted, regions of stasis are introduced that increase blood residence time and the risk of thrombus formation. This risk is increased when multiple medical devices are combined, as occurs in the fraction of patients with mitral valve prostheses that experience heart failure and receive treatment with a left ventricular assist device (LVAD). LVADs are attached directly to the apex of the left ventricle, continuously pumping blood from the heart into the ascending aorta. While LVADs ameliorate the symptoms of heart failure, they are associated with thromboembolic events that result from disrupted blood flow. In this study, six different mitral prosthesis designs/orientations were studied in combination with LVAD support in a mock circulatory loop. Hemodynamics and the intraventricular velocity field were measured and the vortex characteristics determined. The bioprosthetic valve produced the standard flow pattern with a dominant clockwise vortex that circulated within the left ventricle before exiting through the LVAD. The tilting disk valve exhibited skewed inflow that produced a reverse vortex pattern when oriented towards the septum, which greatly increased stasis and residence time. The bileaflet valves split the incoming flow into two streams directed towards the apex but produced weaker vortices than the bioprosthesis. The results inform the surgical planning for LVAD patients with preexisting mitral prostheses, enabling concomitant repair procedures that would decrease the risk of thromboembolic events.

NUMERICAL SIMULATION OF THREE-DIMENSIONAL CIRCULAR FREE TURBULENT JET FLOW USING DIFFERENT REYNOLDS AVERAGE NAVIER-STOKES TURBULENCE MODELS

The present research work shows the numerical investigation of free turbulent jet behavior as well as thermal characteristics of the jet being issued from the circular nozzle outlet and it is performed by solving the different Reynolds averaged Navier-Stokes (RANS) equation. In this research work, central jet velocity, and turbulent intensity of the jet along with thermal characteristics are calculated along the jet axis to check the coherency, heat-exchanging capability, and widening of the jet. Simulation results of the jet are first validated with the experimental data of previous literature work. The numerical experiment provides a better potential for future improvements in the field of jet flow and exhibits a great deal of resemblance with experimental data with minimum error. Computational fluid dynamics (CFD) is a capable tool for depicting the various flow behaviors and selecting the suitable turbulence model is always required for an accurate prediction of the jet-flow regime. Most of the turbulence models investigated in this research work show good results for turbulence intensity and predicted the near- and far-field region as per the experimental data. From the thermal analysis of the free turbulent jet, it is seen in the present results utilizing the various turbulence RANS models that <i>Nu</i><sub>x</sub> is rather high near the nozzle exit. The standard <i>k-ε</i> model describes the near field and long field of the free jet very well and it is also easy and inexpensive to execute. Furthermore, for the observed free turbulent jet, the most appropriate model is recommended.

Chemically reacting mixing in coaxial miscible liquid jets under variable viscosities and reaction rates

We study the mixing and reactions between two fluids with disparate viscosities in a co-axial flow at high Schmidt number, through numerical simulations. The chemical reactions chosen in this work are competitive-consecutive in nature. Numerical implementation of the reactions proceeds as given by Li and Toor (1986), in a Large-Eddy Simulation (LES) framework. The aim of the present study is to investigate the effect of reaction rate constant ratio on the product and byproduct formations under distinct viscosity ratios between the annular and central jets. Flow features promoting mixing and reactions such as turbulence, engulfment, jet instability, waves and break-up are described. The effect of viscosity difference between the jet and co-flow on the mixing and reactions are also delineated. It is found that selectivities of the undesirable product decrease with reducing viscosity ratio as well as increasing reaction rate constant ratio.

Experimental study of submerged liquid metal jet in a rectangular duct in a transverse magnetic field

A liquid metal flow in the form of a submerged round jet entering a square duct in the presence of a transverse magnetic field is studied experimentally. A range of high Reynolds and Hartmann numbers is considered. Flow velocity is measured using electric potential difference probes. A detailed study of the flow in the duct's cross-section about seven jet's diameters downstream of the inlet reveals the dynamics, which is unsteady and dominated by high-amplitude fluctuations resulting from the instability of the jet. The flow structure and fluctuation properties are largely determined by the value of the Stuart number ${{N}}$ . At moderate ${{N}}$ , the mean velocity profile retains a central jet with three-dimensional perturbations increasingly suppressed by the magnetic field as ${{N}}$ grows. At higher values of ${{N}}$ , the flow becomes quasi-two-dimensional and acquires the form of an asymmetric macrovortex, with high-amplitude velocity fluctuations reemerging.

Mitral repair of myxomatous valves with simple annuloplasty: a follow-up up to 12 years

Diffuse myxomatous mitral valve degeneration (DMD) represents a challenge in the reparative mitral valve surgery. A subgroup of patients with symmetrical DMD can be effectively treated with a simple band-annuloplasty with good early and mid-term results. Here, we evaluate the long-term outcomes in terms of freedom from reoperation, recurrence of moderate or severe mitral regurgitation (MR) and overall survival. Between April 2006 and December 2020, patients with DMD causing severe MR and the echocardiographic features of symmetrical bileaflet prolapse, central regurgitant jet(s), annular dilation and no chordal ruptures were treated using a simple annuloplasty with a semi-rigid band. These patients were prospectively collected and retrospectively analysed. Seventy-five patients were enrolled. The mean clinical follow-up time was 104 [standard deviation (SD): 43] months, and echocardiographic follow-up time was 95 (SD: 43) months. The mean age was 54 (SD: 15) years, and 56% were females. Long-term overall survival was 98.2% [standard error (SE): 1.8], 93.7% (SE: 4.7) and 93.7% (SE: 4.7) at 4, 8 and 12 years, respectively. The freedom from reoperation was 100% at 4 and 8 years and 94.1% (SE: 5.7) at 12 years. The freedom from recurrent moderate or severe MR was 98.3% (SE: 1.7), 98.3% (SE: 1.7) and 92.8% (SE: 5.5) at 4, 8 and 12 years, respectively. Mitral repair with the simple band-annuloplasty for the treatment of MR due to symmetrical DMD seems to be stable and effective in the long term.

Jet cross-flow induced vibrations in rod bundles. Part I: Experimental apparatus and stream-wise vibration results

Jet cross-flow induced vibration of fuel assemblies has recently been observed in some reactors that have fail-safe features for pressure release in the event of a loss of coolant accident (LOCA). The generated pressure in the barrel–baffle region during LOCA is mitigated by discharging the flow through LOCA holes and slots normally to the primary axial coolant flow inside the core. However, the jet flow injected through the LOCA holes and slots can induce vibration in the fuel assembly near the baffle before being mixed with the axial flow, and subsequent it can cause grid-to-rod-fretting (GTRF). In this paper, experimental investigations were performed to characterize the jet cross-flow induced vibration of fuel rods by performing fluidelastic instability (FEI) tests on flexible rod bundles. The effect of the stream-wise which is called ”stand-off distance”, is studied by the changing number of rows in the tested bundle. Furthermore, an offset between the centerlines of the jet and the bundle (jet eccentricity) which means the transverse direction on the jet centerline, is changed to show its effect on rod bundle vibration. An experimental test facility was designed to give the capability to study different jet eccentricities to examine their stability effects. The stream-wise results show that varying jet eccentricity excited the rod bundle with different mechanisms. The rods vibrations occurred with the centered jet with rod bundle case had a lock-in/synchronization phenomenon, where the resonant peak is observed. While, the excitation mechanism is switched to turbulence-induced vibrations by moving the jet flow by 0.25 of pitch. However, the aligned jet-rod case showed a fluidelastic instability phenomenon at relatively high jet velocity. On the other hand, the two resonant peaks are obtained when the rod bundle moved away from the jet flow (i.e larger stand-off distance).

Investigation of Metal Powder Injection into a Combustion Chamber by Large Eddy Simulation for Power to X Studies

AbstractThe numerical modeling and simulation of particle dispersion during the filling process of a tubular combustion chamber are presented. By means of multiphase large eddy simulations (LES) the particle distribution inside a circular diffuser‐reactor chamber is studied to analyze the two‐phase flow dynamics for metal powder combustion in a tubular reactor with flame propagation. Typical particle accumulations are found that form close to the reactor wall and also particles tend to segregate and preferentially concentrate at vortex surfaces in particle bands. Particle segregation and accumulation effects aside vortex structures are analyzed using the LES approach. The causes of particle segregation and inhomogeneities are studied and measures for its reduction by changes in the reactor design are discussed. Alterations of the diffuser geometry are adapted to minimize the central particle jet formation and the particle distribution inhomogeneities during the filling process.

Entrained flow gasification: Impact of fuel spray distribution on reaction zone structure

Entrained flow gasification (EFG) is an important process for generating syngas from biogenic and anthropogenic waste based feedstocks for a future circular economy. The EFG process is characterized by complex interactions between different physical and thermo-chemical sub processes which determine syngas quality and process efficiency. The understanding of these sub processes is essential for the development of validated models, and therefore for design and scale up of EFG reactors. EFG processes using a central jet burner configuration feature flames that can be described as inverse diffusion flames superimposed by a fuel spray. The flames are characterized by (i) the conversion of liquid and slurry droplets and (ii) the oxidation of recirculating synthesis gas with the gasification medium. This work studies the interactions between fuel and oxidizer in the near-flame region of an atmospheric EFG process. The model fuel ethylene glycol was gasified using oxygen-enriched air for two different burner nozzle configurations. Spray imaging, OH-LIF and Fuel Tracer-LIF measurements were carried out in addition to gas temperature measurements to characterize the fuel distribution and the flame structure. The experimental results show that narrower fuel spray distributions result in shorter flames and changes in flame shape from a compact to a hollow cone shape in the downstream flame region. The experiments were accompanied by 2-phase free-jet modeling and RANS based CFD modeling. The models were improved to reflect the experimental findings including the fuel spray distributions. The simulation results predict the observed flame structures well using both models and for both burner nozzle configurations. The changes in flame structure for different spray distributions can be explained by local stoichiometry using the results of the 2-phase free-jet model.

Large eddy simulation of multi-regime combustion with a two-progress variable approach for carbon monoxide

Simulations of two cases in a novel multi-regime burner configuration are undertaken using a presumed joint probability density function (PDF) approach with tabulated chemistry. The flame conditions are varied by changing the central jet equivalence ratio, which produces different multi-regime combustion modes in the non-premixed inner flame. An outer premixed flame and recirculation zone behind a bluff body are present to supply heat and combustion products to stabilise the inner flame. A two-progress variable approach is tested to improve predictions of carbon monoxide (CO) in the post-flame regions, where CO oxidation occurs. The large eddy simulation set-up and sub-grid combustion model are assessed through comparisons with time-averaged measurements for radial profiles at different streamwise locations. The jet break-up length, the shear layers and the mixture fraction distribution are well captured in both cases. The temperature distribution is well captured for the inner flame in each case but the temperature and mixture fraction are over predicted in the downstream regions of the outer premixed flame, which is due to increased dilatation that suppresses air entrainment. Improved predictions of the CO mass fraction are obtained for the outer premixed flames with the two-progress variable approach. Over predictions are seen in the upstream regions of the inner flame when the CO mass fraction is obtained from a look-up table, suggesting that the CO mass fraction should be transported to include the convection/diffusion balance in regions where there is no flame. Furthermore, transporting the CO mass fraction with a one-progress variable approach produces over predictions in the burnt regions, suggesting a two-progress variable model is needed to capture the consumption region of CO. The multi-regime combustion characteristics are observed to be stronger in flame MRB26b, where non-premixed and rich premixed combustion is present. For flame MRB18b, the non-premixed contribution is smaller and weak stratified combustion is observed.