Three-dimensional Direct Numerical Simulations Research Articles

Disturbance energy budget of turbulent swirling premixed flame in a cuboid combustor

In order to clarify combustion oscillation based on the exact disturbance energy budget, three-dimensional direct numerical simulations (DNS) of hydrogen–air turbulent swirling premixed flames are conducted in this study, considering a detailed kinetic mechanism and temperature dependencies of transport and thermal properties. Stoichiometric and lean conditions under two swirl numbers (S=0.6 and 1.2) are investigated. The exact disturbance energy budget is closed to more than 92% for all computational conditions. Investigation of source term contributions reveals that the swirl number affects the order of contribution levels of some sources, while the equivalence ratio influences the fluctuation amplitude of the time derivative value of disturbance energy. It is shown that source terms related to fluctuations of entropy and heat sources are major contributors to disturbance energy growth and decay, respectively. In addition to these major terms, source terms related to non-equilibrium combustion chemistry are also determined to be influential to the time evolution of disturbance energy. The relation between disturbance energy generation and flame characteristics is discussed, based on the time–averaged distributions of some of the abovementioned important sources.

The role of porous metal foam on the unidirectional solidification of saturating fluid for cold storage

We conducted analytical, numerical and experimental investigations on solidification of fluid saturated in highly porous metal foams with open cells. Based on pore-scale thermal equilibrium assumption, an analytical extension of the classical Neumann’s solution was made to predict phase change heat transfer in PCM-foam composites. To explore the heat transfer mechanisms underlying the phase change process and clarify the role of foam insertion, three-dimensional direct numerical simulations on periodically distributed tetrakaidecahedron cells were carried out. Experimental measurements were performed to validate the analytical model and the numerical method, with good agreement achieved. The phase interface was macroscopically flat via experimental visualization but microscopically irregular via pore-scale simulations. Temperature difference between the saturating PCM and metallic ligaments was negligible, so that local thermal equilibrium and one-equation model were applicable especially at low Ste numbers (e.g., 0.22). The present findings provide new insights to cold energy storage design, utilization and economic analysis in HVAC systems.

Modeling and experimental analysis of acoustic cavitation bubble clouds for burst-wave lithotripsy

Understanding the dynamics of cavitation bubble clouds formed inside a human body is critical for the design of burst-wave lithotripsy (BWL), a newly proposed method that uses focused ultrasound pulses with amplitude of O(10) MPa and frequency of O(0.1) MHz to fragment kidney stones. We present modeling and three-dimensional direct numerical simulations of interactions between bubble clouds and ultrasound pulses in water. We study two configurations: isolated clouds in a free field, and clouds near a rigid surface. In the modeling, we solve for the bubble radius evolution and continuous flow field using a WENO-based compressible flow solver. In the solver, Lagrangian bubbles are coupled with the continuous phase, defined on an Eulerian grid, at the sub-grid scale using volume averaging techniques. Correlations between the initial void fraction and the maximum collapse pressure in the cloud are discussed. We demonstrate acoustic imaging of the bubbles by post-processing simulated pressure signals at particular sensor locations indicating waves scattered by the clouds. Finally, we compare the simulation results with experimental results including high-speed imaging and hydrophone measurements. The time evolution of the cloud void fraction and the scattered acoustic field in the simulation agree with the experimental results. [Funding supported by NIH 2P01-DK043881.]

Three-dimensional simulations of viscous folding in diverging microchannels

Three dimensional simulations on the viscous folding in diverging microchannels reported by Cubaud and Mason are performed using the parallel code BLUE for multi-phase flows. The more viscous liquid L_1 is injected into the channel from the center inlet, and the less viscous liquid L_2 from two side inlets. Liquid L_1 takes the form of a thin filament due to hydrodynamic focusing in the long channel that leads to the diverging region. The thread then becomes unstable to a folding instability, due to the longitudinal compressive stress applied to it by the diverging flow of liquid L_2. Given the long computation time, we were limited to a parameter study comprising five simulations in which the flow rate ratio, the viscosity ratio, the Reynolds number, and the shape of the channel were varied relative to a reference model. In our simulations, the cross section of the thread produced by focusing is elliptical rather than circular. The initial folding axis can be either parallel or perpendicular to the narrow dimension of the chamber. In the former case, the folding slowly transforms via twisting to perpendicular folding, or it may remain parallel. The direction of folding onset is determined by the velocity profile and the elliptical shape of the thread cross section in the channel that feeds the diverging part of the cell. Due to the high viscosity contrast and very low Reynolds numbers, direct numerical simulations of this two-phase flow are very challenging and to our knowledge these are the first three-dimensional direct parallel numerical simulations of viscous threads in microchannels. Our simulations provide good qualitative comparison of the early time onset of the folding instability however since the computational time for these simulations is quite long, for such viscous threads, long-time comparisons with experiments for quantities such as folding frequency are limited.

Interaction behavior of triple transitional round fountains in a homogeneous fluid

In this study, the PIV and flow visualization techniques and three-dimensional direct numerical simulation are used to investigate the behavior of triple transitional round fountains, which are aligned along a straight line, interacting with each other in a homogeneous fluid over the ranges of 1 ≤ Fr ≤ 10 and 100 ≤ Re ≤ 1000, at a fixed spacing of D/X0=6, where Re and Fr are the Reynolds and Froude numbers, D is the spacing between the two neighbouring fountain sources, and X0 is the radius of orifices at the fountain source, respectively. The results show that the interaction behavior of triple transitional round fountains, over the ranges of Re and Fr studied, is dominated by the bobbing and flapping motions, and is either steady, or unsteady weakly multi-modal, or unsteady strongly multi-modal. In a steady interaction, the bobbing-flapping motions are only present in its initial development stage and the interaction will attain a steady state in the later stage in which the bobbing-flapping motions are no longer present. In contrast, in an unsteady interaction, the interaction remains unsteady all the time and the bobbing-flapping motions are present at all stages. Among all cases considered, a steady interaction occurs at Re ≤ 300 with Fr=1 and at Re=100 with Fr ≤ 5; an unsteady weakly multi-modal interaction occurs at Re=100 with 6 ≤ Fr ≤ 10, at 200 ≤ Re ≤ 300 with 2 ≤ Fr ≤ 10, and at Re=400 with Fr=1; and an unsteady strongly multi-modal interaction occurs at 600 ≤ Re ≤ 1000 with Fr=1 and at 400 ≤ Re ≤ 1000 with 2 ≤ Fr ≤ 10. Such interaction behavior is found to be similar to that of twin transitional round fountains over comparable ranges of Re and Fr, as discovered by us in an earlier study. It is also found that the two interaction regions formed by the three fountains have essentially symmetrical behavior about the middle fountain for all cases considered. Furthermore, the maximum fountain penetration heights and the maximum thicknesses of the interaction regions of the triple transitional round fountains are quantified with the experimental and numerical results and compared to the available scalings for single fountains at comparable Fr and Re values. Several scaling relations are then developed for these parameters over the ranges of Fr and Re considered.

A Priori Direct Numerical Simulation Modeling of Scalar Dissipation Rate Transport in Head-On Quenching of Turbulent Premixed Flames

ABSTRACTThe statistical behavior and modeling of scalar dissipation rate (SDR) transport for head-on quenching of turbulent premixed flames by an inert isothermal wall have been analyzed in the context of Reynolds averaged Navier–Stokes simulations based on three-dimensional simple chemistry direct numerical simulation (DNS) data. It has been found that the density variation, scalar-turbulence interaction, reaction rate gradient, molecular diffusivity gradient, and molecular dissipation terms, i.e., , and , respectively, act as leading order contributors to the SDR transport away from the wall and the turbulent transport and molecular diffusion terms remain negligible in comparison to the other terms. The leading order contributors to the SDR transport have been found to be in a rough equilibrium away from the wall before the quenching is initiated but this equilibrium is not maintained during flame quenching. The predictions of the existing models for the unclosed terms of the SDR transport equation have been assessed with respect to the corresponding quantities extracted from DNS data. No existing models have been found to predict the near-wall behavior of the unclosed terms of the SDR transport equation. The models, which exhibit the most satisfactory performance away from the wall, have been modified to account for near-wall behavior in such a manner that the modified models asymptotically approach the existing model expressions away from the wall.

Three-dimensional simulations of ignition of composite solid propellants

This paper reports three-dimensional direct numerical simulations of ignition of a composite AP (ammonium perchlorate)–HTPB (hydroxyl-terminated polybutadiene) propellant subjected to a constant flux ϕ. The model includes solid heat transfer, gas-phase combustion with global kinetics and explicit description of propellant microstructure. Simulations show that ignition starts from AP particles because of primary AP/binder flame. Go/no-go computations reveal an unreported intermediate regime between go and no-go with apparent quenching followed by a delayed ignition. This delay is linked to a slow flame spreading from localized scattered hot spots on surface. In accordance with experiments, ignition delays deviate from classical ϕ−2 scaling for high flux and low pressure conditions. For intense flux levels, simulations attest deradiation extinction upon flux termination meaning that there is a critical flux above which ignition is no longer possible. The role of AP particle shape and size distribution on ignition delay is studied and predicted to be limited. The effect of propellant surface conditions is also investigated and can lead to substantial effects on ignition delay. Finally, semi-transparent propellants are also considered and low absorption materials result in longer ignition times, reduced ϕ-dependence, and absence of deradiation extinction. This work eventually highlights the importance of microstructure-based details in the physics of composite propellant ignition and opens the way towards better understanding of the role of AP particles.

Direct numerical simulation of double-diffusive gravity currents

This paper presents three-dimensional direct numerical simulations of laboratory-scale double-diffusive gravity currents. Flow is governed by the incompressible Navier-Stokes equations under the Boussinesq approximation, with salinity and temperature coupled to the equations of motion using a nonlinear approximation to the UNESCO equation of state. The effects of vertical boundary conditions and current volume are examined, with focus on flow pattern development, current propagation speed, three-dimensionalization, dissipation, and stirring and mixing. It was observed that no-slip boundaries cause the gravity current head to take the standard lobe-and-cleft shape and encourage both a greater degree and an earlier onset of three-dimensionalization when compared to what occurs in the case of a free-slip boundary. Additionally, numerical simulations with no-slip boundary conditions experience greater viscous dissipation, stirring, and mixing when compared to similar configurations using free-slip conditions.

Three-dimensional direct numerical simulation of wake transitions of a circular cylinder

This paper presents three-dimensional (3D) direct numerical simulations (DNS) of flow past a circular cylinder over a range of Reynolds number ($Re$) up to 300. The gradual wake transition process from mode A* (i.e. mode A with large-scale vortex dislocations) to mode B is well captured over a range of $Re$ from 230 to 260. The mode swapping process is investigated in detail with the aid of numerical flow visualization. It is found that the mode B structures in the transition process are developed based on the streamwise vortices of mode A or A* which destabilize the braid shear layer region. For each case within the transition range, the transient mode swapping process consists of dislocation and non-dislocation cycles. With the increase of $Re$, it becomes more difficult to trigger dislocations from the pure mode A structure and form a dislocation cycle, and each dislocation stage becomes shorter in duration, resulting in a continuous decrease in the probability of occurrence of mode A* and a continuous increase in the probability of occurrence of mode B. The occurrence of mode A* results in a relatively strong flow three-dimensionality. A critical condition is confirmed at approximately $Re=265{-}270$, where the weakest flow three-dimensionality is observed, marking a transition from the disappearance of mode A* to the emergence of increasingly disordered mode B structures.

Correlations for maximum penetration heights of transitional plane fountains in linearly stratified fluids

In this study, a series of three-dimensional direct numerical simulations (DNS) were carried out using ANSYS Fluent for transitional plane fountains in linearly stratified fluids with the Reynolds number (Re), Froude number (Fr) and dimensionless temperature stratification parameter (s) over 28≤Re≤300, 3≤Fr≤10, and 0.1≤s≤0.5, to study and quantify the effects of these governing parameters on the maximum fountain penetration height, including the initial one during the early developing stage and the time-averaged one at the quasi-steady state, as well as the time to reach the initial maximum penetration height. The results show that both the initial and time-averaged maximum fountain heights as well as the time to attain the initial maximum fountain height increase with Fr but decrease with s, whereas the effect of Re is negligible, and the fluctuations of the maximum fountain penetration height at some specific locations at the quasi-steady state also follow the similar trends. Empirical correlations to quantify the effects of Fr, s and Re on these bulk fountain behavior parameters were obtained from the DNS results over the ranges of Fr, s and Re considered.

Linear instability in the wake of an elliptic wing

Linear global instability analysis has been performed in the wake of a low aspect ratio three-dimensional wing of elliptic cross section, constructed with appropriately scaled Eppler E387 airfoils. The flow field over the airfoil and in its wake has been computed by full three-dimensional direct numerical simulation at a chord Reynolds number of \(Re_{c}=1750\) and two angles of attack, \(\mathrm{{AoA}}=0^\circ \) and \(5^\circ \). Point-vortex methods have been employed to predict the inviscid counterpart of this flow. The spatial BiGlobal eigenvalue problem governing linear small-amplitude perturbations superposed upon the viscous three-dimensional wake has been solved at several axial locations, and results were used to initialize linear PSE-3D analyses without any simplifying assumptions regarding the form of the trailing vortex system, other than weak dependence of all flow quantities on the axial spatial direction. Two classes of linearly unstable perturbations were identified, namely stronger-amplified symmetric modes and weaker-amplified antisymmetric disturbances, both peaking at the vortex sheet which connects the trailing vortices. The amplitude functions of both classes of modes were documented, and their characteristics were compared with those delivered by local linear stability analysis in the wake near the symmetry plane and in the vicinity of the vortex core. While all linear instability analysis approaches employed have delivered qualitatively consistent predictions, only PSE-3D is free from assumptions regarding the underlying base flow and should thus be employed to obtain quantitative information on amplification rates and amplitude functions in this class of configurations.

Numerical simulations of shoaling internal solitary waves of elevation

We present high-resolution, two- and three-dimensional direct numerical simulations of large amplitude internal solitary waves of elevation on the laboratory scale, shoaling onto and over a small-amplitude shelf. The three-dimensional, mapped coordinate, spectral collocation method used for the simulations allows for accurate modelling of both the shoaling waves and the bottom boundary layer. The shoaling of the waves is characterized by the formation of a quasi-trapped core which undergoes a spatially growing stratified shear instability at its edge and a lobe-cleft instability in its nose. Both of these instabilities develop and three-dimensionalize concurrently, leading to strong bottom shear stress. We explore significant regions of Schmidt and Reynolds number space and demonstrate that the formation of shear instabilities during shoaling is robust and should be readily observable in a number of standard laboratory setups. In the experiments with a corrugated bottom boundary, boundary layer separation is found inside each of the corrugations during shoaling. This more complex boundary layer phenomenology precludes the formation of the lobe-cleft instability almost completely and hence provides a different mechanism for fluid and material exchange across the bottom boundary layer. Our analyses suggest that all of these wave-induced instabilities can lead to enhanced turbulence in the water column and increased shear stress on the bottom boundary. Through the generation and evolution of these instabilities, the shoaling of internal solitary waves of elevation is likely to provide systematic mechanisms for material mixing, cross-boundary layer transport, and sediment resuspension.

Natural transition in natural convection boundary layers

The ‘natural transition’ of a natural convection boundary layer adjacent to an isothermally heated vertical surface is investigated by means of three-dimensional direct numerical simulation (DNS). In order to trigger ‘natural transition’ numerically, spatially and temporally random perturbations are introduced into the upstream boundary layer. The propagation of the random perturbations in the streamwise direction is observed. It is found that there exist two competing wavenumbers of spanwise vortical structures, one large and the other small. The large wavenumber dominates in the upstream boundary layer, whereas the small wavenumber dominates in the downstream boundary layer. The streamwise evolution of the mean (time-averaged) streamwise vorticity observed at planes perpendicular to the heated surface in general reveals two- and three-layer longitudinal roll structures. Nonlinear processes in a transitioning natural convection boundary layer are also analysed using Bicoherence method. The transition route and mechanism are discussed based on the power spectra and Bicoherence spectra of the temperature time series obtained in the boundary layer. A spectrum filling process during the ‘natural transition’ to turbulence is also observed, which is qualitatively similar to that observed in Blasius boundary layers.

Effects of fuel Lewis number on localised forced ignition of turbulent homogeneous mixtures: A numerical investigation

The influences of fuel Lewis number LeF (ranging from 0.8 to 1.2) on localised forced ignition and early stages of combustion of stoichiometric and fuel-lean homogeneous mixtures have been analysed using simple chemistry three-dimensional compressible direct numerical simulations for different values of root-mean-square velocity fluctuation and the energy deposition characteristics (i.e. characteristic width and the duration of energy deposition by the ignitor). The localised forced ignition is modelled using a source term in the energy transport equation, which deposits energy in a Gaussian manner from the centre of the ignitor over a stipulated period of time. The fuel Lewis number LeF has been found to have significant influences on the extent of burning of stoichiometric and fuel-lean homogeneous mixtures. It has been shown that the width of ignition energy deposition and the duration over which the ignition energy is deposited have significant influences on the success of ignition and subsequent flame propagation. An increase in the width of ignition energy deposition and the duration of energy deposition for a given amount of ignition energy have been found to have detrimental effects on the ignition event, which may ultimately lead to misfire. For a given value of [Formula: see text] ( LeF), the rate of heat transfer from the hot gas kernel increases with increasing LeF[Formula: see text], which in turn leads to a reduction in the extent of overall burning for both stoichiometric and fuel-lean homogeneous mixtures but the detrimental effects of high values of [Formula: see text] on localised forced ignition are particularly prevalent for fuel-lean mixtures. Detailed physical explanations have been provided for the observed [Formula: see text] and energy deposition characteristics effects.

Three-dimensional direct numerical simulations of turbulent fuel-lean H2/air hetero-/homogeneous combustion over Pt with detailed chemistry

Three-dimensional direct numerical simulations (DNS) with detailed heterogeneous and homogeneous chemistry and transport were carried out to investigate the turbulent combustion of fuel-lean hydrogen/air mixtures (equivalence ratios φ=0.16 and 0.18) in a platinum-coated channel with prescribed wall temperatures of 1250K and 1270K and an incoming bulk Reynolds number of 5700. Over the channel domain where only catalytic reactions were present, temporally and spatially segregated islands with increased concentration of the limiting hydrogen reactant formed on the walls, a manifestation of finite-rate surface chemistry effects. The formation of such islands correlated with high streamwise vorticity, which was in turn responsible for an increased turbulent mass transport towards the catalyst. Fluctuations of either gaseous or surface species were significant (up to 34%) at the upstream channel locations and dropped farther downstream due to the flow laminarization induced by the heat transfer from the hot walls and the growth of the turbulent boundary layer. Such strong fluctuations exemplified the necessity of appropriate catalytic chemistry turbulence closures similar to those used for gas-phase reactions. The homogeneous ignition location exhibited streamwise fluctuations with a standard deviation of three channel half-heights. Downstream of homogeneous ignition, gas-phase combustion was concentrated in elongated streamwise stripes confined close to the walls. Hydrogen was incompletely converted within the gaseous combustion zones, and the leaking fuel reacted on the catalytic walls leading to combined hetero-/homogeneous combustion over the entire post-ignition domain. Local extinction of gaseous combustion was finally observed in spatially isolated regions characterized by high streamwise vorticity. The increased turbulent mass transport towards the walls was responsible for the local flame extinction, which in turn led to an increased catalytic conversion at the extinguished flame locations.

Steady and unsteady mixed convection flow in a cubical open cavity with the bottom wall heated

In this study we analyze experiments and numerical simulations of steady and unsteady mixed convection flow in a cubical cavity located at the bottom of a square channel. The Reynolds numbers based on the mean flow velocity and the channel width are in the range 100⩽Re⩽1500 and the Richardson numbers vary within 0.1⩽Ri⩽10. Particle Image Velocimetry has been used for the measurements in a water channel. Three-dimensional direct numerical simulations have been carried out with a second order finite volume code considering the Boussinesq approximation since, for the experimental conditions considered, the variation of the physical properties with temperature has no significant influence on the overall flow topology. For 100⩽Re⩽1500 and Ri⩽0.1 the flow is steady and it consists in a single roll that exhibits larger velocities as the Richardson number is increased. An unsteady periodic flow is found at Re=100 and Ri=10. Alternate flow ejections from the cavity to the channel occur near the lateral walls while the flow enters the cavity from the channel through the central part of the cavity. A conditional sampling technique has been used to elucidate the evolution of the mean unsteady turbulent flow at Ri=10. It has been found that the alternate flow ejections persist for all the Reynolds analyzed. The computed Nusselt numbers are in general agreement with a previously reported correlation, valid for two dimensional cavities of different aspects ratios.

Three-dimensional instabilities in oscillatory flow past elliptic cylinders

We investigate the early development of instabilities in the oscillatory viscous flow past cylinders with elliptic cross-sections using three-dimensional direct numerical simulations. This is a classical hydrodynamic problem for circular cylinders, but other configurations have received only marginal attention. Computed results for some different aspect ratios ${\it\Lambda}$ from 1 : 1 to 1 : 3, all with the major axis of the ellipse aligned in the main flow direction, show good qualitative agreement with Hall’s stability theory (J. Fluid Mech., vol. 146, 1984, pp. 347–367), which predicts a cusp-shaped curve for the onset of the primary instability. The three-dimensional flow structures for aspect ratios larger than 2 : 3 resemble those of a circular cylinder, whereas the elliptical cross-section with the lowest aspect ratio of 1 : 3 exhibits oblate rather than tubular three-dimensional flow structures as well as a pair of counter-rotating spanwise vortices which emerges near the tips of the ellipse. Contrary to a circular cylinder, instabilities for an elliptic cylinder with sufficiently high eccentricity emerge from four rather than two different locations in accordance with the Hall theory.

Droplet migration in a Hele–Shaw cell: Effect of the lubrication film on the droplet dynamics

Droplet migration in a Hele–Shaw cell is a fundamental multiphase flow problem which is crucial for many microfluidics applications. We focus on the regime at low capillary number and three-dimensional direct numerical simulations are performed to investigate the problem. In order to reduce the computational cost, an adaptive mesh is employed and high mesh resolution is only used near the interface. Parametric studies are performed on the droplet horizontal radius and the capillary number. For droplets with an horizontal radius larger than half the channel height, the droplet overfills the channel and exhibits a pancake shape. A lubrication film is formed between the droplet and the wall and particular attention is paid to the effect of the lubrication film on the droplet velocity. The computed velocity of the pancake droplet is shown to be lower than the average inflow velocity, which is in agreement with experimental measurements. The numerical results show that both the strong shear induced by the lubrication film and the three-dimensional flow structure contribute to the low mobility of the droplet. In this low-migration-velocity scenario, the interfacial flow in the droplet reference frame moves toward the rear on the top and reverses direction moving to the front from the two side edges. The velocity of the pancake droplet and the thickness of the lubrication film are observed to decrease with capillary number. The droplet velocity and its dependence on capillary number cannot be captured by the classic Hele–Shaw equations, since the depth-averaged approximation neglects the effect of the lubrication film.

Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble

Three-dimensional direct numerical simulations (DNS) of a shock-induced laminar separation bubble are carried out to investigate the flow instability and origin of any low-frequency unsteadiness. A laminar boundary layer interacting with an oblique shock wave at $M=1.5$ is forced at the inlet with a pair of monochromatic oblique unstable modes, selected according to local linear stability theory (LST) performed within the separation bubble. Linear stability analysis is applied to cases with marginal and large separation, and compared to DNS. While the parabolized stability equations approach accurately reproduces the growth of unstable modes, LST performs less well for strong interactions. When the modes predicted by LST are used to force the separated boundary layer, transition to deterministic turbulence occurs near the reattachment point via an oblique-mode breakdown. Despite the clean upstream condition, broadband low-frequency unsteadiness is found near the separation point with a peak at a Strouhal number of $0.04$, based on the separation bubble length. The appearance of the low-frequency unsteadiness is found to be due to the breakdown of the deterministic turbulence, filling up the spectrum and leading to broadband disturbances that travel upstream in the subsonic region of the boundary layer, with a strong response near the separation point. The existence of the unsteadiness is supported by sensitivity studies on grid resolution and domain size that also identify the region of deterministic breakdown as the source of white noise disturbances. The present contribution confirms the presence of low-frequency response for laminar flows, similarly to that found in fully turbulent interactions.

Numerical investigation of two-phase flame structures in a simplified coal jet flame

The coal flame structures are investigated based on a three-dimensional compressible point-source direct numerical simulation (DNS) of a turbulent simplified coal jet flame. The coal particles are treated as point sources and tracked in the Lagrangian frame considering the interphase transfers. A comprehensive coal combustion model, including the evaporation, devolatilization and char combustion, is applied. The reaction mechanism of CH4 is adopted for gas combustion. In this paper, the general flame features and particle distributions are investigated and three typical flame structures of coal jet flames, namely interspersed flame, stripe flame and stable continuous flame, are identified. By analyzing the temporal evolution of the flame structures, the stable flame formation process is obtained intuitively. It is also found that these three typical flame zones have similar gaseous reaction and the non-premixed flame dominates. The strong reaction is more likely to appear at the locations with small vorticity and high scalar dissipation rate. Compared with purely gaseous flames, the scalar dissipation rate is not only related to turbulent micro-mixing between the fuel and oxidizer flows but also the devolatilization process. By testing the heat release rate proportion of the three zones, it is found that the contribution of interspersed flame is less. The stripe flame contributes about 26% and is important to the whole stable flame formation.