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Nucleation and grain growth during metal additive manufacturing (AM) still remain debatable since the nature of rapid melting and solidification induced by laser-powder interaction during AM may cause particular liquid metal behaviour especially when in-situ alloying is used. The rationalisation of the role of heterogeneous mixing in the microstructure development is of importance for composition control and hence process optimisation. In this work, coupled thermal-chemical-fluid-microstructure modelling is developed for simulating in-situ AM to understand the chemistry-induced solidification, re-melting and microstructure development. The results indicate that thermal fluid flow and chemical mixing play an important role in the rapidly solidified microstructure. The heterogeneous nucleation resulting from the undercooling due to the large thermal gradient and the large cooling rate initiates grain nuclei forming equiaxed grains, and with the extent of the thermal gradient, more anisotropic columnar grain growth occurs in AM. The keyhole serves as a strong stirrer to enhance chemical species mixing by inducing convective flow motion, which determines the local chemical composition and microstructure in in-situ alloying while melting newly fed powders and re-melting part of the previous layer. This work enhances a fundamental and comprehensive understanding and sets a guideline in tailoring the grain structure in as-fabricated components, which is difficult to observe directly in experiment.

The understanding of flame holding mechanism is very important for combustor design. To elucidate the flame holding mechanism at the injector exit in liquid rocket engines, it is indispensable to know the underlying physics of trans-critical mixing process, because the flame holding location is exactly where trans-critical mixing process, which is still veiled, takes place. In the present paper, a new Large Eddy Simulation (LES) method is developed by gaining deeper physical insights into possible sub-grid-scale (SGS) thermodynamic and fluid dynamic instabilities excitable at thin large-density-gradient-magnitude portions to describe trans-critical processes. The presence of thin large-density-gradient-magnitude portions in trans-critical mixing process is troublesome for numerical stability of supercritical fluid flow LES from various points of view. Our idea is to utilize an SGS model functioning at the crossover between the liquid-like and gas-like supercritical fluids in a hybrid scheme of Lagrangian particle tracking and two-phase flow LES. Reduced dynamic viscosity at high pressure may cause SGS thermodynamic and hydrodynamic instabilities in the trans-critical mixing flow field. They are (1) phase separation due to thermodynamic instability, (2) Landau's hydrodynamic instability occurring at the pseudo-boiling location within a heated injector, and (3) Rayleigh–Taylor instability at the pseudo-boiling location, which is induced by Kelvin–Helmholtz instability vortices. These are analyzed and organized into new SGS models characterizing the trans-critical jet mixing process.

形態: 図版あり, Physical characteristics: Original contains illustrations, Accepted: 2019-10-15, 資料番号: PA2010052000

Physical mechanisms of explosive boiling of an ethanol-in-decane emulsion droplet are investigated by numerical simulation. Vapor bubble growth due to explosive boiling leads to secondary breakup of the droplet. The ethanol mass ratio is chosen in the regime of puffing (vapor ejection and partial breakup), which is generally likely to occur for a fuel spray in a combustor. Using both free and sessile droplet configurations and varying the initial number of boiling bubbles and the depth of the bubble formation in the dispersed sub-droplets, the droplet breakup and vapor mixing processes are discussed. For both free and sessile configurations, the basic dynamics of puffing is the same. When the bubble depth is deeper in the sub-droplet, the bubble growth is eventually larger. When the number of bubbles is more, puffing occurs several times, or the bubbles coalesce leading to larger bubble size, larger breakup and enhanced vapor mixing. In the sessile droplet configuration, the vapor ejection dynamics is essentially similar, but particularly when puffing is strong, the droplet bounces from the wall, first pushed toward the wall by the ejected vapor and then pulled from the wall due to the repelling and recoiling motion. Under such a condition, the initially attached droplet finally detaches from the wall and returns into the ambient gas. This phenomenon may transiently influence the mixture formation in a combustor when a fuel spray collides with the combustor wall. The present study serves as a first step toward a goal of quantitatively evaluating the puffing/microexplosion effect on a real fuel turbulent spray in internal combustion engines as to how large it can contribute to improving combustion performance, and insights have been obtained for future modeling which is expected to fill the gap between the droplet-scale and spray-scale knowledge.

Early fuel spray development and subsequent autoignition are investigated using our turbulent atomization model that has enabled physically-closed primary atomization modeling. Thanks to its modeling methodology, no parameter tuning or adjustment is needed to simulate a turbulent spray. Due to turbulent atomization, a dense droplet layer is formed over the injected liquid core. With evaporation, this region has a triple layer structure, which is composed of the inner layer with many droplets, the vaporization layer and the outer layer exposed to the hot air. With combustion, hot spots are generated in the outer layer, while the inner layer is too cold and hinders the hot gas to penetrate inside. The group combustion number indicates that this is sheath evaporation/combustion mode and its structure is preserved down to the liquid core length. The liquid core thinning is determined by atomization characteristics and under evaporative/reactive conditions where the outer gas temperature is high and thus the kinematic viscosity is also high, the flow motion is larger and the core is unsteadily affected by this motion. Due to this, the Rayleigh–Taylor (RT) atomization mode is further excited and core breakup occurs relatively faster. It is also observed that the unsteady core head flapping motion leads to non-uniform distributions of droplets and vapor. Due to this non-uniformity, the initial autoignition in the outer layer does not occur uniformly. After core breakup, the droplet layer rolls up in the front region and the group combustion number becomes lower, where external group combustion behavior is observed. Multistep chemistry improves the ignition delay estimation compared with the global chemistry, but the spray dynamics is mostly determined by the above fluid dynamic effects. Therefore, the present study indicates that the accurate simulation capability of fluid dynamics, i.e. primary atomization and early spray development, is important in predicting spray combustion behavior.

Metal additive manufacturing is promising for designing advanced metallic parts of complex geometries. The challenge lies in process control on melt flow dynamics, alloy mixing and vapour mass loss, which is significantly vital for the final quality. A high-fidelity thermal-solutal-fluid modelling approach including accurate tracking of surface shape, thermo-capillary dynamics and vaporisation has been developed. Multi-species formulations are also included for multi-metal simulation. Using this method, the physical link between metal vapour mass loss and melt flow process for 21 transition metals and 3 binary alloys is investigated. The mass loss rate is governed by a fluid dynamic parameter of Reynolds number with a simple proportional correlation linked with thermal-fluid behavior of the melt pool, and convective mixing further complicates the behaviour in in-situ binary alloying. The digital materials approach is effective in understanding complex interdependent thermal-fluid flow dynamics and can advance process-based materials design.

A novel turbulent atomization model, which is physically closed itself and free of case-by-case parameter tuning using experimental data, has been formulated and demonstrated in the framework of turbulent spray combustion large-eddy simulation (LES). Based on our accumulated research findings that elementary droplet/ligament generation is a deterministic phenomenon, not something random as considered in the conventional understanding, the model describes two dominant modes of turbulent atomization, i.e. the turbulent resonant mode and the Rayleigh–Taylor (RT) mode, in a physically straightforward manner. Extending the baseline theory proposed in Umemura (2016), to a hybrid turbulent spray LES formulation which includes both an Eulerian liquid jet core and Lagrangian droplets, the subgrid-scale (SGS) atomization characteristics are completely detailed in this study. Using the LES-resolved turbulent Weber and Bond numbers on the liquid core surface, the atomization mode and the SGS atomization characteristics such as droplet size, number, ejection velocity and core regression velocity are all identified locally, and the information is transferred back to the LES code as input information. Test cases of Diesel fuel jets demonstrate that the present formulation well reproduces the turbulent spray behavior. Thanks to the obtained detailed data, the spray formation process can be tracked both temporally and spatially, from the initial head formation with edge atomization to the later core atomization and spray spreading. It is essentially featured that the present turbulent atomization model works well without ambiguous user input, contrary to the conventional way of spray simulation. This is a significant breakthrough to urge paradigm shift in spray simulation, from unclosed/unpredictable to closed/predictable, which enables drastic improvement in the accuracy of spray simulation and may exert a large impact on both research studies and industrial applications.

Digital technology such as metal additive manufacturing (AM) provides flexible process design freedom to fabricate intricate three-dimensional structures layer-by-layer. However, its manufacturability relies on the fundamental understanding of melt pool physics and fluid (metal) dynamics. The effect of metal vapour and porosity induced during the laser-materials interaction can influence the additive manufacturability. In this work, composition-process relationship of laser-based powder-bed fusion ( L -PBF) AM is studied via computational fluid dynamics modelling to rationalise solid-liquid-vapour transformation where empirical-based approach is used to generate thermo-physical property of about 100 nickel-based superalloys at the liquid state. It is found that with larger vapor mass loss, the porosity tends to be higher. However, the higher vapour mass loss means faster cooling rate. This is indicated that the thermal-fluid flow process, which is also governed by the thermo-physical property, strongly affects the additive manufacturability. Additive manufacturability map based upon porosity, cooling rate from liquid to solid, volatile mass loss criteria has been established to link the composition in nickel-based superalloys with their thermo-physical property. This offers a thermal-fluid science based tool in designing compositions of novel superalloys for AM applications.

This paper presents a multi-scale approach coupling a Eulerian interface-tracking method and a Lagrangian particle-tracking method to simulate liquid atomisation processes. This method aims to represent the complete spray atomisation process including the primary break-up process and the secondary break-up process, paving the way for high-fidelity simulations of spray atomisation in the dense spray zone and spray combustion in the dilute spray zone. The Eulerian method is based on the coupled level-set and volume-of-fluid method for interface tracking, which can accurately simulate the primary break-up process. For the coupling approach, the Eulerian method describes only large droplet and ligament structures, while small-scale droplet structures are removed from the resolved Eulerian description and transformed into Lagrangian point-source spherical droplets. The Lagrangian method is thus used to track smaller droplets. In this study, two-dimensional simulations of liquid jet atomisation are performed. We analysed Lagrangian droplet formation and motion using the multi-scale approach. The results indicate that the coupling method successfully achieves multi-scale simulations and accurately models droplet motion after the Eulerian–Lagrangian transition. Finally, the reverse Lagrangian–Eulerian transition is also considered to cope with interactions between Eulerian droplets and Lagrangian droplets. This work was supported by the Engineering and Physical Sciences Research Council of the UK (grant number EP/L000199/1).

[EN] Microexplosion is rapid disintegration of an emulsion droplet caused by explosive boiling of embedded liquid subdroplets with a lower boiling point. Microexplosion and puffing (partial microexplosion) are potentially beneficial to achieving enhanced secondary atomisation. These eruptive secondary atomisation mechanisms will help to meet conflicting requirements for an atomising fuel spray with regard to penetration achieved by large droplets and evaporation/mixing achieved by small droplets. Although with great interest, our understanding of microexplosion is still limited and therefore optimising and controlling microexplosion is not feasible yet. This paper reviews our recent research outcome on microexplosion and puffing of an emulsion fuel droplet from high-fidelity interface-capturing simulation study, which directly resolves the dynamics of boiling and evaporating liquid/gas interfaces, immiscible liquid/liquid interfaces with jump conditions for mass, momentum and heat transfer across a resolved interface. We first unveiled microexplosion and puffing dynamics of an emulsion fuel droplet in a quiescent ambient. Since convective heating has important effects on temperature distribution inside a fuel droplet in realistic engine conditions, which determines the initial nucleation location and thus the microexplosion outcome, a model has been proposed to approximate the temperature distribution inside a droplet, for which momentum and heat transport was found to be only moderately correlated under typical engine conditions. With this model in place that allows for saving considerable computational cost on setting up initial conditions for microexplosion simulation, puffing effects on fuel/air mixing is then investigated, which can be quantified by the scalar dissipation rate (SDR) of the primary fuel decane, the SDR of the secondary fuel ethanol and the cross SDR. We then further extended our simulation studies towards reacting conditions and investigate puffing effects on a droplet wake flame. The series of high-fidelity simulation studies is believed to have considerably improved our understanding of microexplosion dynamics and impact on local fuel/air mixing and combustion. Based on the improved knowledge, microexplosion induced secondary droplet breakup models can be developed and incorporated into hybrid highfidelity simulation of spray atomisation and combustion enhanced by microexplosion., The authors are grateful for the financial support of the Engineering and Physical Sciences Research Council (EPSRC) of the UK under Grant No. EP/J018023/1.

Previous researches on the single-mode Rayleigh-Taylor (RT) instability were mainly focused on the advancing speed of gaseous bubbles, while few of them shed light on the nonlinear dynamics of the liquid spikes (jets) where the droplets are constantly generated in the liquid atomization case. In this paper, a three-dimensional (3D) numerical simulation on a single-mode RT instability (Atwood number is close to unity) with the focus on the development of the liquid spike was conducted by the Coupled Level-Set and Volume-of-Fluid (CLSVOF) method. Nonlinear dynamic characteristics of the RT instability were then analyzed thoroughly based on the detailed information of pressure and velocity fields obtained from the numerical simulation. The numerical results revealed that a local maximum pressure point, where an equilibrium was achieved between the inertial force and pressure gradient, was formed at the root of liquid spike region caused by the horizontal impinging flow. Combined with the theoretical analysis, the relationships among the steady-state characteristic parameters, the initial perturbation wavelength and the inertial acceleration were established. It is found that the velocity and width of the liquid entering the freed liquid spike at the maximum pressure point exhibit the same characteristics as those of a vertical jet emanating downward from an orifice injector under gravity. Thus, theories concerning the low-speed jet can be used to predict the disintegration behavior of a liquid spike caused by RT instability.

A very detailed evaporating/reacting spray simulation has been conducted extending our previous liquid atomization data in order to understand the physical processes leading to group combustion. The head region of a realistic dense Diesel jet spray is considered, and the droplets and diffusive layers are resolved. At this stage, the droplets have the relative velocity to the high-temperature air flow and this generates comparable-size turbulent eddies around the droplets. By these eddies, mixing is enhanced more strongly than molecular diffusion and merged vapor clouds are formed especially in the area of large droplet number density. This suggests that external group combustion is likely to occur and this is also confirmed by the estimation of group combustion number. This finding requires a modification to the point-source model so that the droplet-generated mixing enhancement effect should be more accurately included. A simple time scale analysis shows that final ignition occurs after the liquid jet head disappears and the resident time is sufficiently large enough.

© 2016 by Begell House, Inc. Microexplosion/puffing is rapid disintegration of a water-in-oil emulsion droplet caused by explosive boiling of embedded superheated water sub-droplets. To predict microexplosion/puffing, modeling the temperature distribution inside an emulsion droplet under convective heating is a prerequisite, since the temperature field determines the location of nucleation (vapor bubble initiation from superheated water). In the first part of the present study, convective heating of water-in-oil emulsion droplets under typical combustor conditions is investigated using high-fidelity simulation in order to accurately model inner-droplet temperature distribution. The shear force due to the ambient air flow induces internal circulation inside a droplet. It has been found that for droplets under investigation in the present study, the liquid Peclet number PeL is in a transitional regime of 100 < PeL < 500. The temperature field is therefore somewhat distorted by the velocity field, but the distortion is not strong enough to form Hill's vortex for the temperature field. In the second part of the present study, a novel approach is proposed to model the temperature field distortion by introducing angular dependency of the thermal conductivity and eccentricity of the temperature field. The model can reproduce the main features of the temperature field inside an emulsion droplet, and can be used to predict the nucleation location, which is a key initial condition of microexplosion/puffing.

Using the detailed numerical simulation data of primary atomization, the liquid surface instability development that leads to atomization is characterized. The numerical results are compared with a theoretical analysis of liquid–gas layer for a parameter range close to high-speed Diesel jet fuel injection. For intermittent and short-duration Diesel injection, the aerodynamic surface interaction and transient head formation play an important role. The present numerical setting excludes nozzle disturbances to primarily investigate this interfacial instability mechanism and the role of jet head. The first disturbed area is the jet head region, and the generated disturbances are fed into the upstream region through the gas phase. This leads to the viscous boundary layer instability development on the liquid jet core. By temporal tracking of surface pattern development including the phase velocity and stability regime and by the visualization of vortex structures near the boundary layer region, it is suggested that the instability mode is the Tollmien–Schlichting (TS) mode similar to the turbulent transition of solid-wall boundary layer. It is also demonstrated that the jet head and the liquid core play an interacting role, thus the jet head cannot be neglected in Diesel injection. In this study, this type of boundary layer instability has been demonstrated as a possible mechanism of primary atomization, especially for high-speed straight liquid jets. The effect of nozzle turbulence is a challenging but important issue, and it should be examined in the future.

In this study, the atomization characteristics of Diesel jet front tip have been investigated to elucidate the physical mechanisms by detailed numerical simulation. The computations are carried out with the finest grid resolutions ever that can resolve the final droplet generation by surface tension. The numerical methods are based on level-set interface tracking. The methods were validated by test cases and the grid resolution survey shows that the resolutions for the present study are sufficient. The present flow setup excludes nozzle disturbances to investigate how the disturbances from the liquid jet front would lead to atomization where the liquid jet impacts against the quiescent gas. The liquid jet front becomes an umbrella-like shape. From the front umbrella tip edge, ligament breakup first occurs. Ligament breakup is strongly correlated with the gas motion in the vicinity. The gas region behind the front is highly disturbed by atomization. By the gas recirculation motion here, air and some droplets are entrained and mixed. Also, the disturbances are fed back to the front umbrella by this motion and become synchronized with the breakup. Droplet pinch-off is mainly in the short-wave mode, but some ligaments are elongated by local gas stretch to finally have a long-wave mode shape, namely a mode shift occurs. The above findings of liquid jet front umbrella formation, atomization at the umbrella edge, mixing and atomization loop in the recirculation flow region and droplet generation mode give an insight to the modeling of droplet generation in actual sprays.

Detailed numerical simulations of Diesel liquid jet sprays have been conducted to elucidate the liquid atomization mechanisms. The obtained flow field is very complicated, but the spray behavior can be understood physically. The jet head formation leads to vortex shedding and subsequent atomization from the head edge. The liquid core surface is also unstable due to gas boundary layer instability and atomization also occurs from here. The final process of droplet generation from a ligament is basically the same as the past research results, and can be explained by the capillary wave destabilization. Such findings can be used for droplet modeling for practical-scale simulations.

Primary atomization of liquid injected at high speed into still air is investigated to elucidate physical processes by direct numerical simulation. With sufficient grid resolution, ligament and droplet formation can be captured in a physically sound way. Ligament formation is triggered by the liquid jet tip roll-up, and later ligaments are also produced from the disturbed liquid core surface in the upstream. Ligament production direction is affected by gas vortices. Disturbances are fed from the liquid jet tip toward upstream through vortices and droplet re-collision. When the local gas Weber number is O(1), ligaments are created, thus the ligament or droplet scale becomes smaller as the bulk Weber number increases. Observation of droplet formation from a ligament provides insights into the relevance between the actual droplet formation and pinch-off from a slow liquid jet in laboratory experiments. In the spray, the dominant mode is the short-wave mode driven by propagative capillary wave from the ligament tip. An injection nozzle that is necessary for a slow jet is absent for a ligament, thus the long-wave (Rayleigh) mode is basically not seen without the effect of stretch. By the present simulation, a series of physical processes have been revealed. The present result will be extended to LES modeling in the future.

To validate our proposed atomization theory, a series of microgravity experiments were conducted for a water jet issued into an otherwise quiescent atmosphere. The present paper reports findings from the case of liquid Weber number being nearly equal to unity. The water contained in a syringe is pushed by a piston which moves at a constant speed. The initial overshoot of liquid issue speed produces a long water column, whose length is reduced at every instant of subsequent disintegration interacting with the nozzle exit. The balance between tip contraction speed and liquid issue speed, which is attained by the selection of the Weber number, enables us to observe the disintegration process in detail. It is revealed that the short-wave breakup mechanism is characterized as the local destabilization which takes place at the short neck part bridging the tip bulb and the upstream high pressure crest part and that the neck maintains the nature of propagative capillary wave until a final stage of disintegration.

In this study, flame dynamics of secondary fuel injection control is investigated by large eddy simulation (LES) in a lean premixed combustor to elucidate the experimental two-stage oscillation suppression phenomenon. Without secondary fuel injection, large-amplitude longitudinal oscillations were observed in the experiment. With constant injection of secondary fuel, the oscillation amplitude was reduced and with harmonic feedback injection, it was further reduced. In this process, the flame shape and the dynamic behavior of the flame were changed. To fill the gap of the experimental data and to understand the flame dynamics, large eddy simulations are carried out. The LES results show that the oscillation reduction is attributable to the roles of both the main flame and the secondary flame. The interaction of the main flame and the vortices is reduced when the injection is on; namely, heat fluctuation is reduced. The secondary flame helps the flame base stabilization and directly modulates the heat release in the feedback injection. The phase relations between the pressure, secondary heat release, and injection velocity are also shown. With these effects from the main and secondary flames combined, the oscillations are suppressed by secondary fuel injection. The present study indicates that LES can be a tool to understand flame dynamics with control.

This paper presents a basic study on the characteristics of capillary wave propagation on a water ligament surface. The capillary wave plays an important role in de-stabilizing ligaments created by the turbulent motion of liquid jet. The ligament end contracts due to capillary force and emanates surface waves. The analysis is based on 2D numerical simulation and linear wave analysis, and reveals the basic characteristics such as contraction speed, wave speed and the associated flow field structures. The phenomenon can be parameterized by the Ohnesorge number, and the numerical results are in good agreement with the linear analysis. The higher pressure cases, where the Ohnesorge number is high, will be investigated in the next step.

This paper presents a numerical study on surface melting pattern formation due to aerodynamic heating. When aerodynamic heating is severe enough to melt the surface of a flying body, the interaction between the melted layer and the external flow creates surface patterns. The present study succeeds in reproducing surface melting patterns, and results show the same trend as predicted by theory and experiment. The instability mechanism is mainly governed by surface friction and surface pressure fluctuations. These two factors act as driving forces on the surface. The surface pattern formation is characterized by the amplification factor and the wavelength factor. The results show the same trend as the experiments and the validity of the present analysis has been demonstrated.

This paper presents a numerical study on the formation of diffusion flame islands in a hydrogen jet lifted flame. A real size hydrogen jet lifted flame is numerically simulated by the DNS approach over a period of about 0.5 ms. The diameter of hydrogen injector is 2 mm, and the injection velocity is 680 m/s. The lifted flame is composed of a stable leading edge flame, a vigorously turbulent inner rich premixed flame, and a number of outer diffusion flame islands. The relatively long-term observation makes it possible to understand in detail the time-dependent flame behavior in rather large time scales, which are as large as the time scale of the leading edge flame unsteadiness. From the observation, the following three findings are obtained concerning the formation of diffusion flame islands. (1) A thin oxygen diffusion layer is developed along the outer boundary of the lifted flame, where the diffusion flame islands burn in a rather flat shape. (2) When a diffusion flame island comes into contact with the fluctuating inner rich premixed flame, combustion is intensified due to an increase in the hydrogen supply by molecular diffusion. This process also works for the production of the diffusion flame islands in the oxygen diffusion layer. (3) When a large unburned gas volume penetrates into the leading edge flame, the structure of the leading edge flame changes. In this transformation process, a diffusion flame island comes near the leading edge flame. The local deficiency of oxygen plays an important role in this production process.

This article has been made available through the Brunel Open Access Publishing Fund. In this study, the modulation of turbulence and scalar mixing by finite-size droplets/ligaments in a dense fuel spray is investigated using a DNS (Direct Numerical Simulation) dataset. Ejected from a spray nozzle with a high speed, a liquid-fuel jet deforms and the fuel spray is atomized into many ligaments and droplets. During these processes, the gas flow becomes turbulent due to droplet/ligament dynamics. At the same time, droplet evaporation and mixing with ambient air are affected by the small-scale gas turbulence. An understanding of the mixing characteristics in the dense spray zone is important for modeling spray combustion. In a region where the droplet number density is relatively low, a universal feature of isotropic turbulence was found, although the alignments of strain eigenvectors with vorticity and the mixture fraction gradient are slightly modulated by the presence of droplets, which is a characteristic of particle-laden flows. In gas-phase regions close to droplet surfaces, where the dissipation rate of turbulent kinetic energy is strongly increased, the alignments are more modulated, especially those of the scalar gradient with strain eigenvectors. This can also be seen in the topology similarity among energy dissipation, enstrophy and scalar dissipation in the near field of droplet/ligament surfaces. For the first time, it is found that droplets whose size is comparable to turbulence scales do affect the mixing characteristics in a realistic turbulent spray. This finding has shed new light upon the modeling of flow turbulence and scalar mixing in an evaporating and atomizing fuel spray. © 2014 The Authors.

The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays. This article has been made available through the Brunel Open Access Publishing Fund.

In the present work, results of attempts to capture the high-frequency combustion instability in a rocket combustor equipped with hydrogen (H2) / oxygen (O2) injector are shown. To enhance the coupling between heat release rate fluctuations and pressure oscillations, the injector element is installed at the off-center of the combustion chamber in the simulation. Two-dimensional simulations are conducted for a single-element combustor. The flow forcing by modulating the mass flow rate of O2 injection is attempted to excite the first transverse (1T) mode of chamber. The simulation results indicate that the amplitude of 1T mode is increased by the flow forcing. To clarify a driving mechanism for the growth of pressure oscillations, the simulation results are analyzed using the Rayleigh index. Simulations are then extended to three-dimensional LES for a combustor equipped with five shear-coaxial injector elements. The obtained results of pressure field and the Rayleigh index are compared with those of a single element combustor. The effects of multiple flames on the pressure field are examined.

In the present work, results of numerical simulations to capture the high-frequency combustion instability in a two-dimensional combustor with hydrogen (H2) / oxygen (O2) injector are shown. Three sets of computations, in which the locations to install the injector element are varied with an aim to change the degree of coupling between heat release fluctuation and pressure oscillation, are conducted. In the first computation (named as RUN1), the injector is installed at the center of combustion chamber. In the second and third computations (named as RUN2 and RUN3), the injector is installed at the off-centered location of combustion chamber. In RUN3, the flow forcing is attempted by adding an energy source term in the energy conservation equation to generate the first transverse mode (1T) in the combustion chamber. Although the amplitudes of pressure oscillations in the first two computations are relatively small, the result of RUN2 shows that pressure oscillation at 4kHz, which corresponds to the 1T mode of combustion chamber, is significantly excited. In RUN3, the 1T mode of combustion chamber is successfully generated by the flow forcing. After switching off the flow forcing, however, the amplitude of pressure oscillation decays rapidly and self-sustained oscillation can not be established. The results of three cases are analized by using the balance equation for acoustic energy. The analysis indicates that the Rayleigh term works as a driving mechanism for instabilities, but the other damping term has a large negative value and prevents the glowth of pressure oscillations for all three cases. Finally, the present work is extended to three-dimensional LES and the preliminary result is shown.

The interactions between multiple H2/O2 coaxial jet flames are investigated by a full three-dimensional LES approach. The numerical simulation under atmospheric pressure condition is conducted for a cylindrical combustor equipped with three coaxial injector elements. Although the three flames merge together at the downstream location, the combustion field observed in the current combustor configuration is dominated by diffusion combustion. The analysis of resolved kinetic energy spectrum reveals that turbulent structures are significantly affected by the collisions between turbulent shear layers. The comparison with the result of a single element combustor also reveals that the O2 jet injected from the injector positioned in the center tends to be longer due to the interactions between flames.

This paper presents an attempt to caputure the high-frequency combustion instability of a supercritical LOX/GH2 flame by Large Eddy Simulation (LES) framework. Four sets of computations, in which lengths of the coaxial injector and purge gas inlet are changed with a view to changing the frequency of acoustic mode, are conducted for the combustion experiment by the P8 combustor at the DLR. The high-frequency combustion instability corresponds to the first tangential (1T) mode of combustion chamber is successfully captured by the present LES when the length of purge gas inlet is set to 20mm. By the analysis of acoustic mode of purge gas inlet using the finite element based solver, a possible explanation of observed combustion instability is explored.

Acoustic damping of a cylindrical chamber with open and closed ends is analytically and numerically investigated. In the analysis, based on the analytic solution of resonant acoustic modes inside cylinder, the damping from the open end is calculated by a simple acoustic source modeling for velocity fluctuation. The effect of viscosity is also considered as an attenuation mechanism. Acoustic damping calculated for first longitudinal and tangential modes are in good agreement with that obtained by numerical simulation. When a longitudinal mode exists, the directivity of radiation becomes like monopole and the damping is large. On the other hand, when a tangential mode exists, it is found that the dipole like directivity is obtained and the damping is small compared to that for the longitudinal mode. A configuration of the chamber and an injector installed off-centered is also investigated. Under non-resonant condition between injector and chamber acoustics, it is found that the acoustic radiation from the open end of the injector is negligible but the viscous effect becomes important. Finally we investigate the acoustic damping with hot gas injection numerically and semi-analytically. The obtained mode is found to be a spinning tangential mode due to the asymmetry of the mean flow field. The radiated wave has also a spinning feature and the damping is found to be much larger than that for symmetric dipole like radiation under uniform condition. The simple acoustic radiation modeling applied for the analysis also works well for this general non-uniform mean flow condition.

The aim of the present study is to validate the accuracy of large eddy simulation (LES) framework against liquid oxygen (LOX) and gaseous hydrogen (GH2) coaxial jet flame at supercritical pressure. In the present LES Framework, a laminar flamelet model is employed for chemistry modeling and the real-fluid properties are also taken into account. In order to validate the LES code, numerical simulation is conducted for the supercritical LOX/GH2 combustion experiment by the P8 combustor at the DLR. By analyzing the obtained LES result, the flowfield features of the P8 combustor are explored. The integral length scale of turbulent coherent structures evaluated from the present LES is compared with the experimental data. The comparison shows fair agreement between simulated length scale and experimental data.

This paper briefly reviews recent numerical combustion simulation results at ISTA/JAXA obtained by DNS and LES approaches, and shows some topics towards future combustion research. We have successfully simulated detailed structures of a hydrogen jet lifted flame by DNS and an unsteady combustor flow field in a gas turbine combustor by LES. In these simulations, numerical simulation has been proved very effective, but its applicability is still limited due to long computational time. It is expected that future progress in computer performance will make this kind of simulation more realistic and useful in combustion research and development.

The structure and stabilizing mechanism of a subsonic hydrogen / air jet lifted flame are numerically investigated using DNS. The lifted flame consists of an inner premixed flame and outer diffusion flames. The flame base is premixed, which sustains the vigorously turbulent inner premixed flame. The outer diffusion flame consist of several island flames, which are produced from the inner premixed flame. The stabilization at the flame base is maintained by the balance between the axial velocity and the laminar flame velocity. Deviation of heat release layer from hydrogen consumption is observed in the inner premixed flame.

Acoustic damping of a cylindrical chamber with open and closed ends in resonant modes is analytically and numerically investigated to understand the low damping characteristic of the chamber without chocked nozzle. First, on the basis of the analytic solution of resonant acoustic modes inside a cylinder, the damping by radiation from the open end is calculated analytically using simple acoustic source modeling for velocity fluctuation. The effect of viscosity is also considered as an attenuation mechanism. The values of acoustic damping calculated for the first longitudinal and tangential modes are in good agreement with the corresponding values obtained using numerical simulation. The damping is also investigated for a configuration of the chamber with an injector installed off-center. Finally, we numerically and semi-analytically investigate the acoustic damping for a configuration that includes a hot-gas injection. The obtained mode is found to be a spinning tangential mode and the radiated wave also has a spinning feature. The damping for the spinning tangential mode is found to be larger than that for the symmetric dipole-like radiation under a uniform standing condition, but much smaller than the chamber with a chocked nozzle. Therefore, the chamber with an open end has the low damping characteristic suitable for intentionally generating oscillatory combustion.