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Combustion science and technology
Taylor & Francis, Gordon and Breach Publishing
Combustion science and technology

Taylor & Francis, Gordon and Breach Publishing

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0010-2203

Combustion science and technology/Journal Combustion science and technologySCIISTPEI
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    Large Eddy Simulation of a Turbulent Dilute Ethanol Flame Using the Two-Phase Spray Flamelet Generated Manifold Approach

    Ran YiXu ZhangC. P. Chen
    1-31页
    查看更多>>摘要：ABSTRACT A recently developed spray Flamelet Generated Manifolds (SFGM) approach has been extended to include turbulence–chemistry interactions for studies of turbulent spray flames based on Eulerian–Lagrangian multiphase Large Eddy Simulation (LES). The Eulerian–Eulerian two-fluid formulation of the SFGM was extended to include the mixture fraction variance to construct a 4-D manifold library. The effect of heat loss due to droplet evaporation was considered in the SFGM tabulation, and the SFGM approach was evaluated using the Sydney ethanol spray flame experimental database. The two-phase interaction was investigated from complete analyses of the continuous- and dispersed-phase dynamics. Comparisons between the numerical results and the experimental data indicate that the SFGM formulated from counterflow spray flames can produce a good agreement with the gaseous flow field and the dispersed-phase statistics. By comparing the results in terms of the temperature field with other existing models, it can be inferred that the discrepancies between the flamelet-based results and the experimental data may not depend on the choice of the flamelet approach but on the modeling of the turbulence/chemistry interaction model. A Beta-function of the normalized progress variable can be an alternative to improve the temperature predictions. Moreover, using a well-designed definition of the progress variable by finding the optimal weights of several appropriate species is another possible path to improve the predictions.
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    Analysis of the Evolution of the Surface Density Function During Premixed V-Shaped Flame–Wall Interaction in a Turbulent Channel Flow at Reτ = 395

    Reo KaiAbhishek Lakshman PillaiUmair AhmedNilanjan Chakraborty...
    32-58页
    查看更多>>摘要：ABSTRACT The flame–turbulence interaction and statistical behavior of the surface density function (SDF; i.e. magnitude of the reaction progress variable gradient) in the vicinity of the wall for a stoichiometric methane-air flame are investigated using a three-dimensional direct numerical simulation of a turbulent premixed V-flame interacting with an isothermal inert wall in a fully developed turbulent channel flow at a friction Reynolds number $$R{e_\tau} = 395$$Reτ=395 . The results show that the mean SDF significantly decreases in the viscous sublayer in comparison to the corresponding values for the same reaction progress variable in the unstretched laminar flame. Moreover, the mean values of SDF for a given value of reaction progress variable decrease in the downstream direction with the progress of flame quenching in all zones of turbulent boundary layer. The effective normal strain rate $$a_n^{eff}$$aneff ( $$ = {a_n} + {\bf{\it{n}}}\cdot\nabla {S_d}$$=an+n⋅∇Sd ), which acts to reduce the SDF as it increases, is much higher in the viscous sublayer than in the other layers. In the viscous sublayer, the contribution of the gradient of displacement speed in the flame-normal direction ( $${\bf{\it{n}}}\cdot\nabla {S_d}$$n⋅∇Sd ) to $$a_n^{eff}$$aneff has been shown to dominate the fluid-dynamic normal strain rate ( $${a_n}$$an ). This tendency is qualitatively similar to the previous findings for a V-flame interacting with an isothermal inert wall at $$R{e_\tau} = 110$$Reτ=110 . However, the maximum mean value of $$a_n^{eff}$$aneff at $$R{e_\tau} = 395$$Reτ=395 is approximately twice of that at $$R{e_\tau} = 110$$Reτ=110 , which causes a sharper drop in the SDF in the viscous sublayer at higher $$R{e_\tau}$$Reτ .
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    Cenosphere Formation and Combustion Characteristics of Single Droplets of Vacuum Residual Oils

    Hendrix Yulis SetyawanMingming Zhu
    59-76页
    查看更多>>摘要：ABSTRACT The ignition, combustion characteristics, and cenosphere formation of single droplets combustion of four vacuum residues (VRs) from different refineries with various asphaltene contents were studied experimentally. The single droplets of VRs were suspended at the tip of a silicon carbide fiber and heated in air at temperatures of 973 and 1023 K, respectively, in an electrically heated tube furnace. The ignition and combustion behavior of the VRs were recorded using a CCD camera, which enabled the determination of droplet size, ignition delay time, flame duration, and cenosphere size. The effect of initial droplet size, gas temperature, and asphaltene content on the ignition delay time, flame duration, cenosphere morphology, and particle size were investigated. The whole ignition and combustion process of single droplets of the VRs consisted of five stages in succession: (1) pre-ignition, mainly involving the evaporation of highly volatile components from the droplet surface; (2) steady combustion of fuel vapors evaporated from the droplet surface; (3) splashing combustion of fuel vapors evaporated from droplet interior; (4) disruptive combustion due to thermal decomposition of asphaltene; and (5) solid residue ignition and combustion. A visible and sooty flame was formed upon ignition and lasted during stages 2–4. The droplet size increased sharply in the stage 4 due to the thermal decomposition of asphaltene, which was more profound for VRs with higher asphaltene content and at higher gas temperatures. The ignition delay time increased with increasing initial droplet size and gas temperature but varied little as the asphaltene content in the VRs increased, suggesting that the ignition process of VRs was controlled by the vaporization of high volatile components on the droplet surface. The thermal decomposition of asphaltene produced solid residue, which was in the form of a cenosphere with the shell thickness being ca. 20 μm and a number of blowholes presented in the shell. The VRs with higher asphaltene content had more and bigger blowholes. The ratio of cenosphere particle size to initial droplet size is independent of the initial droplet size but almost increased linearly with the asphaltene content in the VRs.
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    A Numerical Investigation on Effects of Hydrogen Enrichment and Turbulence on NO Formation Pathways in Premixed Ammonia/Air Flames

    Shervin KarimkashiParsa TamadonfarOssi KaarioVille Vuorinen...
    77-106页
    查看更多>>摘要：ABSTRACT NO x emission reduction is one of the major challenges when using ammonia/hydrogen blends as an alternative fuel for carbon-free combustion. In this study, chemical reaction pathways of NO formation in planar premixed flames of stoichiometric ammonia/hydrogen/air at atmospheric pressure and reactants temperature of 298 K are investigated under laminar and decaying turbulent conditions using quasi-DNS with detailed chemistry and the mixture-averaged transport model. The sweep parameter, $$\alpha $$α , is the volumetric ratio of hydrogen to the ammonia/hydrogen mixture. Here, for unstrained laminar conditions, $$\alpha $$α  = 0, 0.4, and 0.6 and for turbulent condition, $$\alpha $$α  = 0.4 within the thin reaction zones region ( $$Ka$$Ka $$ \approx $$≈ 34.2) are studied. Under 1D laminar conditions, increasing $$\alpha $$α enhances NO formation drastically, around 3 times when comparing $$\alpha $$α  = 0 and 0.6. At different $$\alpha $$α , despite the high activity of the reactions which belong to the Zeldovich pathway, the contribution of this pathway is insignificant in NO formation. However, HNO and N2O pathways have the most significant roles. For instance, at $$\alpha $$α  = 0.4, R85 (NH+NO $$ \Leftrightarrow $$⇔ N2O+H) in the N2O pathway and R144 (HNO+H $$ \Leftrightarrow $$⇔ NO+H2) in the HNO pathway have major roles in NO formation. Higher NO formation at larger $$\alpha $$α is found to be due to the increased H and O radicals within the reaction zone as well as the increased reaction rates because of the higher flame temperature. Under the 3D turbulent condition ( $$\alpha $$α  = 0.4), it is observed that turbulence does not switch the pathways trends across the flame brush compared to the laminar condition. In the reaction zone, however, it is observed that NO formation is lower (higher) in the regions convex (concave) toward the reactants. The underlying reason for this effect is the preferential diffusion of H2 to the regions of the flame front convex toward the reactants, which consumes NO and also depletes O and OH radicals, which are necessary for NO formation.
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    Investigation on the Effect of Charge Injection from Non-Thermal Plasma on Soot Formation in Laminar Coflow Diffusion Flame

    Yong Ren TanYichen ZongMaurin SalamancaJacob W. Martin...
    107-128页
    查看更多>>摘要：ABSTRACT A novel, modified coflow burner was developed to study the effect of charge injection from a non-thermal plasma into three helium-diluted laminar coflow diffusion ethylene flames. The frequency of the high voltage (HV) signal was varied to control the ion concentration (charge) injected into the flames. Optical emission spectroscopy was used to characterize the non-thermal plasma while a bias plate methodology was used to gauge the relative amount of charge generated. For different HV signal frequencies, the laser-induced fluorescence of OH, chemiluminescence of CH*, and laser-induced incandescence of soot in flames were measured. The OH and CH* measurements showed that the flames retained the classic flame shape with charge injection. Significant soot reduction was observed at low HV signal frequencies, corresponding to an increase in charge injection. Notably, at low HV signal frequency, soot reduction in highly concentrated (60%) ethylene flame is three times lower than the less concentrated (32%) ethylene flame. This can be attributed to the decrease in the injected charge to soot precursor concentration ratio when the concentration of ethylene in the flame is increased. These results demonstrate that the current system is a promising candidate for studying the charge effect from non-thermal plasma on soot formation in laminar coflow diffusion flames.
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    Spray Flamelet Modeling of Turbulent Two-Phase Reacting Flows with Multi-Component Fuel in a Lean Direct Injection Combustor

    Ran YiC. P. Chen
    129-171页
    查看更多>>摘要：ABSTRACT Towards the numerical evaluation of the performance in aero-engine combustors, implementing detailed reaction kinetics into the large eddy simulation (LES) cost-effectively is highly desirable. In this work, a newly-developed spray flamelet model is combined with a four-component jet fuel surrogate model to allow integration of the detailed kinetics in the open-source CFD code. The spray flamelet approach (SFGM) can take into account the non-adiabatic effect caused by the evaporating spray and can directly reflect the preferential evaporation, as well as multiple combustion regimes present in complex spray combustion systems. The coupling of the SFGM-LES approach is assessed by comparison with the experimental data from a Lean Direct Injection (LDI) combustor, where the real aviation kerosene, Jet-A, was utilized. The SFGM-LES approach well predicts the experimental statistics in both gas and liquid phases. It is capable of capturing the rich physics and complex flame structures, including the preferential evaporation effect and the multi-regime combustion phenomenon. The present work demonstrates that the complex nature of multi-components and the multiple-combustion regimes in the two-phase combustion can be directly included in the spray flamelet approach, and thus this model can provide new insights into the turbulent spray combustion phenomena.
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    Stretch Effects in Large Eddy Simulation of Turbulent Premixed Bunsen Flames

    Yunde SuSeung Hyun Kim
    172-201页
    查看更多>>摘要：ABSTRACT The stretch effects in large eddy simulation (LES) of a turbulent lean propane-air premixed Bunsen flame in the wrinkled flamelet regime are investigated. A simple approach to model the subgrid-scale flame stretch is proposed. It is based on the analysis which shows that, when the surface-filtered strain rate in a subgrid-scale stretch model is approximated using the volume-filtered velocity field solved for in LES, heat release and associated gas expansion in the filtered flame brush tend to artificially alter the wrinkling of resolved flame fronts. To minimize such artifacts, it is suggested that the volume-filtered strain rate on the unburned side of the filtered flame brush be used to approximate the surface-filtered strain rate and projected through the filtered flame brush. The results show the importance of mitigating the artificial heat release effects when considering the strain effects in LES. The relative importance of the curvature stretch is also investigated in terms of the mean and local effects. For a Bunsen flame with a positive Markstein number, the mean flame curvature effect tends to increase the total burning rate by enhancing the laminar flame speed near the flame tip, while the local flame curvature effect tends to decrease the total burning rate by suppressing the wrinkling of the resolved flames. It is found that the competition between the two makes the overall curvature effects not influence the total burning rate much for the flame investigated here, as compared with the strain effects.
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    Numerical Investigations of Pseudo-Boiling and Multi-Component Mixing Under Trans-/supercritical Conditions for Engine Applications

    Jie MaHongsheng LiuLiang LiMaozhao Xie...
    202-239页
    查看更多>>摘要：ABSTRACT The fuel injection and mixing process are often carried out under trans-/supercritical conditions for engine applications; however, the process is still unclear. In this study, based on OpenFOAM, a multi-component trans-/supercritical spray model based on the KT/KNP scheme is developed, which covers the special real fluid equation of state, various mixing rules and modified thermodynamic properties. In addition, the PIMPLE algorithm is extended to deal with the high nonlinearity. First, a one-dimensional advection case is applied to evaluate the performance of the model. Then, the effect of various mixing rules and pseudo-boiling phenomenon on n-heptane/nitrogen jet are thoroughly analyzed under trans-/supercritical conditions. The results show that the jet is extremely sensitive to the changes of the initial jet temperature and chamber pressure, and the “heat shield” effect of pseudo-boiling delays the breakup and mixing in the transcritical jet. With the mixing of different components, unlike the single-component condition, the pseudo-boiling intensity will increase. The increase of pressure and occurrence of mixing will lead to the decrease of pseudo-boiling temperature, and the pseudo-boiling will end earlier, which is obviously different from the single-component fluid case. For the n-heptane/nitrogen mixture, when the mole fraction of n-heptane in the mixture changes from 1.0 to 0.7, and the pseudo-boiling temperature decreases from 571 K to 465 K. With increasing injection temperature or chamber pressure, the pseudo-boiling intensity gradually decreases, and the potential core shortens, the difference among various mixing rules decreases. When the injection temperature is higher than 571 K, the pseudo-boiling phenomenon no longer exists. When the chamber pressure reaches 9 MPa, the pseudo-boiling strength is very low. Therefore, the spray mixing under supercritical conditions will be more sufficient. With the occurrence of mixing, the critical parameter gradually transitions from the critical value of n-heptane to the critical value of nitrogen, which strongly depends on the mixing rule.
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    A Numerical Methodology for the Design of Active Prechambers in Spark-Ignition Engines

    Lorenzo SforzaTommaso LucchiniGianluca D’ErricoGiovanni Gianetti...
    241-275页
    查看更多>>摘要：ABSTRACT Active prechamber spark-ignition engines are a feasible and effective solution in reducing fuel consumption and pollutant emissions for internal combustion engines. However, reliable and low-cost numerical approaches need to be developed to support and speed-up their industrial design, considering their geometric complexity and the involved multiple-flow length scales. In this work, a computational fluid dynamics (CFD) methodology is presented for the design of active prechambers (APCHs) in spark-ignition engines. It consists of two connected steps. First, the fuel injection process inside the APCH is simulated inside a constant-volume domain, including only the APCH geometry and considering the main chamber (MCH) as boundary condition. Then, the power-cycle is simulated on the whole closed-valves domain (APCH and MCH). A flame area evolution model is used to describe the flame propagation process. Experimental data from a research single-cylinder heavy-duty engine are used to assess the proposed methodology. Different operating conditions are considered, to evaluate the effects produced by variations of the nozzles diameter, the MCH air/fuel ratio and the load. Satisfactory results were achieved, demonstrating that the proposed methodology is consistent and reliable.
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    Formation and Evolution of the Numerical Air-Breathing Rotating Detonation Fueled by C12H23

    Chenglong YanJiafeng ZhaoYiheng TongBing Wang...
    276-309页
    查看更多>>摘要：ABSTRACT Conducting an in-depth study of the characteristics of a two-phase rotating detonation combustor is crucial for developing a new type of air-breathing engine that can be used for wide-range flight. This study established the first full-process air-breathing two-phase rotating detonation numerical combustor, based on published experimental data, using the Euler-Lagrangian method to simulate the complete process of fuel injection, breaking, evaporation, blending, and combustion. The simulation model analyzed the cold flow and the combustion flow field after detonation ignition, providing insight into the interaction mechanism between the two-phase detonation wave and fuel. The simulation results showed that the secondary heat release after the detonation wave was caused by the secondary combustion of unburned droplets. The working frequency of the detonation combustor was found to be 2301 Hz, and the propagation speed was 1228.89 m/s. Compared to the experiment, the simulation showed a deviation of approximately 19.5% in terms of frequency and detonation wave propagation velocity. When the Sauter-mean diameter of the droplets D 32 was around 25 μm, the combustor was able to operate stably. The thrust generated by the engine was approximately 1009 N, and the fuel-specific impulse was about 1767.5 s. These findings have significant theoretical value in designing a two-phase air-breathing rotating detonation combustor and engine.
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