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In the present study, we propose a new surrogate model [common kernel-smoothed proper orthogonal decomposition (CKSPOD)] to emulate spatiotemporally evolving flows. The model integrates and extends...

High-fidelity simulations play an increasingly important role in understanding fundamental turbulence-chemistry interactions and combustion dynamics in practical propulsion and power-generation systems. These simulations are, however, too computationally expensive and unwieldy for the purposes of design and optimization, given the large group of design parameters and wide design space. In this paper, we present an efficient surrogate-based modeling strategy to emulate spatiotemporal flows and combustion at supercritical pressures with accuracy similar to that of large-eddy simulations (LES). A common kernel-smoothed proper orthogonal decomposition (CKSPOD)-based surrogate model is developed, incorporating computer experiments, projection-based model reduction, kriging, and uncertainty quantification. The surrogate model (emulator) is carefully trained using a database drawn from a set of LES-based simulations that are conducted at designated sampling points in a given design space. A common Gram matrix is built using a Hadamard product to transform reduced spatial basis functions to remedy phase deviations among different design settings. Kriging is then used to obtain temporal spatial functions and associated coefficients for flowfield reconstruction at a new design setting. The framework is examined with two case studies: an emulation of flow dynamics in a simplex swirl injector, and an emulation of mixing and combustion in a gas-centered liquid-swirl coaxial injector. The surrogate model not only faithfully captures the salient features of its LES counterpart, but also shortens the computation time dramatically, by up to five orders of magnitude. The developed surrogate model can be applied to a broad range of engineering systems and will provide support for future engineering innovation and design.

Liquid vaporization under thermodynamic phase non-equilibrium condition at the gas-liquid interface is investigated over a wide range of fluid state typical of many liquid-fueled energy conversion systems. The validity of the phase-equilibrium assumption commonly used in the existing study of liquid vaporization is examined using molecular dynamics theories. The interfacial mass flow rates on both sides of the liquid surface are compared to the net vaporization rate through an order-of-magnitude analysis Results indicated that the phase-equilibrium assumption holds valid at relatively high pressures and low temperatures, and for droplets with relatively large initial diameters (for example, larger than 10 µm for vaporizing oxygen droplets in gaseous hydrogen in the pressure range from 10 atm to the oxygen critical state). Droplet vaporization under superheated conditions is also explored using classical binary homogeneous nucleation theory, in conjunction with a real-fluid equation of state. It is found that the bubble nucleation rate is very sensitive to changes in saturation ratio and pressure; it increases by several orders of magnitude when either the saturation ratio or the pressure is slightly increased. The kinetic limit of saturation ratio decreases with increasing pressure, leading to reduced difference between saturation and superheat conditions. As a result, the influence of non-equilibrium conditions on droplet vaporization is lower at a higher pressure.

In this study, large eddy simulations (LES) are conducted using both a finite-rate chemistry (FRC) model and a flamelet/progress-variable (FPV) model for a series of piloted partially premixed meth...

This interdisciplinary study, which combines machine learning, statistical methodologies, high-fidelity simulations, projection-based model reduction, and flow physics, demonstrates a new process f...

An efficient finite-rate chemistry (FRC) model is developed for a preconditioned compressible flow solver. The model uses a point-implicit stiff ODE solver and a correlated dynamic adaptive chemistry algorithm. With respect to the conventional FRC model using the double precision variable coefficient stiff ODE solver, the present work achieves an 8.6 times speed-up for chemistry calculation, and 6.4 times for total computation, when using a 20-species kinetics mechanism for methane/air flames. As an example problem, a piloted partially premixed methane/air jet flame (Sandia Flame D), with a relatively low level of local extinction and re-ignition, is considered, and both the new FRC-large eddy simulation (LES) and flamelet/progress-variable (FPV)-LES are conducted. The FRC-LES approach predicts larger time-averaged flame length, and better agrees with the measured value. This is because the instantaneous high-temperature zone for the FPV-LES case is significantly smaller than it's FRC-LES counterpart, especially in the downstream region. For spatial distribution of time-averaged statistics, the FPV-LES result agrees with the experimental data better. For conditional statistics in the mixture fraction space, the FRC-LES approach provides significantly better predictions. Near the stoichiometric region, in comparison with experimental data and the FRC-LES results, the FPV-LES approach predicts higher radical generation, but lower CO generation and heat release.

A novel approach for treating vaporization of liquid droplets with large deformation and high mass transfer rate is developed. The formulation is based on decomposed vaporization mechanisms and accommodates the effects of density and property variations in both the gas and liquid phases. In this model, the mass and energy transfer across the interface are realized by a source layer in the gas phase and a sink layer in the liquid phase around the interface. The flow field in each phase is described by a set of unified conservation equations in the low Mach number limit. The equations are then solved by a modified projection method, in which the nonzero velocity divergence is treated as a source term in the pressure Poisson equation. The vaporization model is implemented in a volume-of-fluid (VOF) based code “Gerris” featuring adaptive mesh refinement. The vaporization of a single n-decane droplet in both quiescent and convective air environments of 1000 K at 1 atm is investigated to verify and validate the present model in terms of interface localization and mass conservation. The results demonstrate the accuracy and robustness of the present work. The vaporization of an impulsively started n-decane droplet with large deformation and breakup is also studied. The droplet surface dynamics are captured well, as are the vaporization behaviors.

In transonic flight within a certain range of Mach number and angle of attack, the flowfield becomes unstable, and it produces an oscillating aerodynamic force: a phenomenon commonly known as trans...

A numerical model based on decomposed vaporization mechanisms has been developed for the study of vaporization of liquid droplets with large deformation and high mass transfer rate. In this model, the vaporization-induced volume, energy, and species source terms in the gas phase are modeled using a body-fitted layer placed around the interface on the gas side. In this layer, continuous distributions of volume, energy, and species sources are specified by the conservation laws. A corresponding body-fitted sink layer is placed on the liquid side of the interface to account for mass, energy and species loss in the liquid phase due to vaporization. The formulation retains the advantages of interface localization methods and mitigates the error and complexity caused by the combined treatment of mass/heat transfer and interface localization. The new model is versatile and does not depend on the specific interface localization method, and can thus be implemented in any of the existing methods. As a specific example, the present approach is implemented in a volume-of-fraction (VOF)-based code “Gerris”. A number of test cases concerning liquid vaporization in both quiescent and convective environments are presented, with the constant-density assumption, to verify and validate this model in terms of interface localization. In particular, the vaporization of a single n-decane droplet in a uniform air flow of 1000 K at 1 atm is investigated. The predicted liquid volume evolution demonstrates the robustness and accuracy of interface localization for cases with large density ratio and high vaporization rate. Vaporization of n-decane droplets with large deformation and high mass transfer is also investigated systematically.

The linear acoustic behavior of an oxidizer-rich staged combustion engine preburner assembly that was similar to that of the RD-170 has been investigated by using real fluid speeds of sound to acco...

This paper presents analysis of results from optical diagnostics in a high-pressure combustor burning gaseous oxygen (GOX) and liquid kerosene RP-2 fuel through a jet-swirl coflow injector. The obj...

Impinging-jet injectors are widely used in liquid propulsion applications, since their simple configuration provides reliable and efficient atomization. The flowfield involves a series of complicated spatio-temporal evolutions. Much effort has been directed toward understanding the underlying physics and developing quantitative predictions of impinging-jet atomization. This paper summarizes the recent advances in this direction, including state-of-the-art theoretical, experimental, and numerical studies, along with representative results. Finally, concluding remarks address remaining challenges and highlight modeling capabilities of high-fidelity simulations. Keywords: Computational fluid dynamics, Flow instability, Flow visualization, Gas/liquid interfacial dynamics, Impinging-jet atomization, Theoretical models, Two-phase flow

Linear stability analysis is a useful tool for the exploration of the initial evolution of flow motions in mixing layers. A real fluid mixing layer exhibits strong property variations and, thus, may present stability behaviors distinct from its ideal gas counterpart. The present study carries out spatial and temporal stability analyses of nitrogen mixing layers at supercritical conditions, with special attention to the density stratification induced by the temperature and velocity gradients across the mixing layer. The differences between the ideal gas and real fluid approaches are discussed. The maximum spatial growth rate and the most unstable frequency evaluated based on the real fluid density profile are found to be substantially lower than their ideal gas counterparts near the critical point, where an inflection of the density distribution occurs in the mixing layer. Across the inflection point, the strong density stratification arising from the real fluid effect tends to stabilize the mixing layer. The maximum growth rate and the most unstable frequency do not show a monotonic trend with the ratios of temperature and density. In the absence of the inflection point, however, the mixing layer is destabilized and features a substantially higher maximum spatial growth rate at lower ratios of density and temperature. The most unstable frequency and the maximum spatial growth rate increase with increasing pressure. The real fluid effect diminishes when the pressure is away from the critical value or when there is no inflection point in the density profile. The temporal stability analysis also indicates that a detailed density distribution plays a key role in dictating the stability characteristics of mixing layers at supercritical pressures.

Simulation of turbulent mixing and combustion at supercritical pressures requires the use of a real-fluid equation of state (EOS) to represent the nonideal, nonlinear thermodynamic behavior of fluids under these conditions. The simplified representation of the filtered EOS in the large eddy simulation methodology introduces inconsistencies in the computed filtered thermodynamic state. This study investigates these inconsistencies and novel subgrid modeling approaches to address these issues, using high-resolution direct numerical simulation of a transcritical mixing layer. Errors incurred by not accounting for subgrid effects in the EOS are quantified, and fundamental insights are drawn regarding the nature of these effects. Then, different modeling approaches are proposed and investigated to obtain a more accurate representation of the filtered EOS. The evaluation of the filtered EOS in terms of the Reynolds-filtered state variables is considered instead of the conventional Favre-filtered variables. A dynamic gradient model is formulated by building upon the ideas of dynamic modeling to render a functional form for the subgrid EOS expressed in terms of the resolved flow gradients. A scale-similarity model formulation for the subgrid EOS is also constructed and examined. Finally, a model for the filtered EOS is derived using a presumed filtered density function that accounts for the effect of subgrid-scale fluctuations. The performance of each model is evaluated using various metrics, and the relative accuracy of each modeling approach is compared and contrasted at different filter sizes.

A comprehensive linear acoustic analysis of the main combustion chamber of an oxidizer-rich staged combustion engine is presented. The theoretical basis is an acoustic wave equation derived from th...

The combustion characteristics of gas-centered, liquid-swirl coaxial injectors typically used in oxidizer-rich staged combustion cycle engines are numerically investigated at supercritical conditions. Turbulence closure is achieved using large-eddy-simulation techniques, and turbulence/chemistry interaction is modeled by a steady laminar flamelet approach. Gaseous oxygen (GOX) at 687.7 K is injected into the center post while kerosene at 492.2 K is delivered tangentially into the outer coaxial annulus. The operating pressure is 25.3 MPa. Detailed flow structures and flame dynamics are explored. The entire flowfield can be divided into four regimes: propellant injection, flame initialization, flame development, and intensive combustion. The diffusion-dominated flame is anchored in the wake of the GOX post and further enhanced in the downstream taper region. The surface of the coaxial annulus and taper is covered by fuel-rich mixtures and thus protected from thermal flux in the flame zone. Effects of the recess length (from the end of GOX post to the entrance of taper region) on the flow and flame evolution are investigated in depth. The efficiency of propellant mixing and subsequent combustion is found to increase with increasing recess length. The kerosene film is nearly depleted before the exit of the recess region for cases with long recess length, and the flame spreads upwards in the taper region for cases with reduced recess length due to insufficient mixing between GOX and kerosene. In a fully recessed injector without fuel shielding, the injected kerosene behaves like a liquid jet in a crossflow. Two recirculating zones containing fuel-rich mixtures are formed between the injection slit and the headend. A broad flame region is established at the exit of the recess region. In a non-recessed injector, the occurrence of combustion is delayed to the taper region. The flame resides along the taper surface and the injector faceplate, with most of GOX convecting downstream unburned. Results obtained from the present study can also be used to characterize combustion responses to local flow oscillations.

Advances in fluid-flow modeling and simulation techniques over the past two decades have improved understanding of the intricate flow physics and combustion dynamics in the supercritical regime. However, there remain many numerical issues to be addressed, including turbulence closure modeling, combustion modeling, and the evaluation of real-fluid thermodynamic and transport properties. The challenges can be broadly categorized into two areas: (1) achieving highly accurate simulation through inclusion of all the necessary physics and (2) developing a computationally efficient framework to achieve simulation results in a reasonable turnaround time. This paper investigates these challenges and presents a systematic approach to achieve high-fidelity and efficient simulation of supercritical fluid mixing and combustion using large-eddy simulation (LES) techniques. The unresolved subgrid-scale (SGS) term in the filtered equation of state (EOS), which is generally neglected for ideal gases, becomes significant for real fluids, especially in regions of strong property gradients at supercritical conditions. The relative error for the filtered density can reach up to 40%, and this uncertainty can propagate and contaminate calculations of the conservation equations. Two closure models for the SGS term in the EOS are proposed: a gradient-based and a mixing-based approach. Both approaches reduce the modeling error considerably. Flamelet-based combustion models are also examined at supercritical conditions. The probability density functions (PDFs) for mixture fraction and scalar dissipation rate are evaluated using a data-driven approach. The presumed beta-function distribution accurately describes the PDF of the mixture fraction at low mixture fraction variance, but deviates at high variance (> 0.01). The lognormal distribution can capture the shape of the extracted PDF of the scalar dissipation rate but underestimates the peak value. An alternative combustion model using finite-rate chemistry integrated with dynamic adaptive chemistry and correlated transport is developed, rendering a computationally efficient and affordable framework. The efficiency of evaluating real-fluid thermodynamic and transport properties, a computationally expensive procedure, is dramatically enhanced using tabulation and correlated dynamic evaluation techniques. Finally, suggestions are provided regarding opportunities for future research.

The present study develops a data-driven framework trained with high-fidelity simulation results to facilitate decision making for combustor designs. Its core is a surrogate model employing a machi...

The flame dynamics of liquid bi-swirl injectors are numerically investigated using the large eddy simulation technique. Liquid oxygen (LOX) and kerosene at subcritical temperatures are injected into a supercritical pressure environment. The theoretical framework is based on the full conservation laws and accommodates real-fluid thermodynamics and transport theories over the entire range of fluid states. Turbulence/chemistry interaction is modeled with a laminar flamelet library approach, the validity of which is demonstrated in the present work. The near-field flow and flame characteristics are carefully studied. The flame is anchored in the wake of the inner injector post by two counter-rotating vortices, and further stabilized by center and corner recirculation zones in the downstream region. Differences in the flow patterns between the cold-flow and combustion cases are recognized. Various geometric parameters, including recess region, post thickness, and kerosene annulus width, are examined in depth to explore their influence on flame characteristics. A recess region is found to be necessary to achieve efficient mixing and combustion. The absence of a recess region increases the penetration depth of the kerosene stream in the downstream region and reduces the thermal protection provided to the injector faceplate. On the other hand, a thicker LOX post or a wider kerosene annulus protects the faceplate more efficiently, and introduces larger recirculation zones near the LOX post surface and thus higher flow residence time to better anchor the flame. However, the flame attachment for thicker post and wider annulus induces a stronger heat flux to the post surface, and thus increases the risk of thermal failure of the injector device. The dynamic characteristics of the flame field are also discussed. The flow oscillations within the injector are found to be dominated by a quarter acoustic wave, while the oscillatory field near the injector exit is characterized by vortex shedding. The characteristic frequency of the vortex shedding is similar for different LOX post thicknesses and annulus widths, and is determined by the exit velocity profiles.

A deep-learning based approach is developed for efficient evaluation of thermophysical properties in numerical simulation of complex real-fluid flows. The work enables a significant improvement of computational efficiency by replacing direct calculation of the equation of state with a deep feedforward neural network with appropriate boundary information (DFNN-BC). The proposed method can be coupled to a flow solver in a robust manner. Depending on the numerical formulation of the flow solver, the neural network takes in either the primitive or conservative variables, including the chemical composition of the system, and calculates all relevant fluid properties for the subsequent routines in the solver. Two test problems are employed to validate the proposed methodology. The first uses a preconditioning scheme with dual-time integration for the simulation of swirl rocket injector flow dynamics under supercritical conditions. The second uses a conservative-variable based formulation for the simulation of laminar counterflow diffusion flames for cryogenic combustion. A parametric analysis is performed to optimize the numbers of hidden layers and neurons per hidden layer. The computational accuracy, efficiency, and memory requirements of the neural network are examined. The DFNN-BC model accelerates the evaluation of real-fluid properties by a factor of 2.43 and 3.7 for the two test problems, respectively, and the overall flowfield simulation by 1.5 and 2.3, respectively. In addition, the memory usage is reduced by up to five orders of magnitude in comparison with the table look-up method.

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

This paper presents a systematic investigation of large eddy simulation (LES) and subgrid scale (SGS) modeling with application to transcritical and supercritical turbulent mixing and combustion. There remains uncertainty about the validity of extending the LES formalism developed for low-pressure, ideal-gas flows to simulations of high-pressure real-fluid flows. To address this concern, we reexamine the LES theoretical framework and the underlying assumptions in the context of real-fluid mixing and combustion. Two-dimensional direct numerical simulations of nonreacting and reacting mixing layers of gaseous methane and liquid oxygen in the thermodynamically transcritical and supercritical fluid regimes are performed. The computed results are used to evaluate the exact terms in the LES governing equations and associated SGS models. Order of magnitude analysis of the exact filtered and subgrid terms in the LES equations and a priori analysis of the simplifications are performed at different filter widths. It is shown that several of these approximations do not hold for supercritical turbulent mixing. Subgrid scale terms, which are neglected in the LES framework for ideal-gas flows, become significant in magnitude compared to the other leading terms in the governing equations. In particular, the subgrid term arising from the filtering of the real-fluid equation of state is shown to be important.

The three-dimensional flow structures and dynamics of shear-coaxial nitrogen jets under supercritical conditions are comprehensively investigated using the large-eddy simulation technique. The theoretical framework is based on the full conservation laws and accommodates real-fluid thermodynamics and transport theories over the entire range of fluid states. Cryogenic liquid nitrogen (132 K) is delivered through the inner tube of a shear-coaxial nozzle, and gaseous nitrogen (191 K) is injected through the outer annulus, into a high-pressure environment at 233 K. Particular attention is paid to the influence of operating parameters on the flow evolution of the coaxial jets. Two supercritical pressure conditions, 4.94 and 10.0 MPa, and three outer-to-inner velocity ratios in the range of 2.03–3.75, are considered. Results reveal that the flowfield downstream of the coaxial jets is characterized by two potential cores and three shear layers. Two counter-rotating recirculation zones are formed next to the central post-tip. The characteristic frequencies of flow recirculation and shear-layer instability are analyzed using power spectral analysis. Increasing the pressure and the velocity ratio enhances turbulent mixing, promotes the entrainment of the outer stream into the inner region, and advances the pinching point of vortical motion to the centerline, thereby leading to a decreased length of the inner dark core. The predicted dark core length and outer spreading angle show good agreement with experimental results. The introduction of transverse acoustic excitation induces large sinusoidal oscillations of the inner dense-fluid stream along the direction of acoustic motion and promotes the mixing of the inner and outer streams.

A systematic study is carried out to investigate the flow dynamics and mixing of liquid oxygen/kerosene bi-swirl injectors at supercritical conditions. The theoretical framework is based on the ful...

Supercritical combustion has attracted significant interest due to its applications in high-pressure combustion devices. Validation of supercritical combustion modeling, however, has not been well ...

A comprehensive study is conducted to enhance the understanding of swirl injector flow dynamics at supercritical conditions. The formulation is based on full-conservation laws and accommodates real...

To investigate the sensitivity of predictions to chemical kinetics models, two different kinetics models, GRI-Mech 3.0 and an 11-species syngas model, are compared by performing 3D finite-rate kinetics-based direct numerical simulations (DNS) of a temporally evolving turbulent non-premixed syngas flame. Dynamic adaptive chemistry and correlated transport techniques are applied to enable computationally efficient simulation with the detailed GRI-Mech 3.0. Both chemical kinetics models, providing comparable qualitative trends, capture local extinction and re-ignition events. However, significant quantitative discrepancies (86–100 K difference in the temperature field) indicate high sensitivity to the chemical kinetics model. The 11-species model predicts a lower radicals-to-products conversion rate, causing statistically more local extinction and less re-ignition. This sensitivity to the chemical kinetics model is magnified relative to a 1D steady laminar simulation by the effects of unsteadiness and turbulence (up to 7 times for temperature, up to 12 times for CO, up to 13 times for H 2 , up to 7 times for O 2 , up to 5 times for CO 2 , and up to 13 times for H 2 O), with the deviations in species concentrations, temperature, and reaction rates forming a nonlinear positive feedback loop under reacting flow conditions. The differences between the results from the two models are primarily due to: (a) the larger number of species and related kinetic pathways in GRI-Mech 3.0; and (b) the differences in reaction rate coefficients for the same reactions in the two models. Both (a) and (b) are sensitive to unsteadiness and other turbulence effects, but (b) is dominant and is more sensitive to unsteadiness and other turbulence effects. At local extinction, the major differences between the results from the two chemical kinetics models are in the peak values and the volume occupied by the peak values, which is dominated by unsteady effects; at re-ignition, the differences are mainly observed in the spatial distribution of the reacting flow field, which is primarily dominated by the complex turbulence–chemistry interaction.

The mixing and combustion characteristics of liquid-oxygen/kerosene bi-swirl injectors are investigated under the supercritical conditions typical of contemporary rocket engines. The basis of the study is a large-eddy simulation technique combined with a unified treatment of real-fluid thermodynamics. The turbulence/chemistry interaction is treated using a laminar flamelet library approach. Emphasis is placed on the near-field flow and flame development downstream of the inner swirler. The flame is found to be stabilized by two counter-rotating vortices in the wake region of the liquid-oxygen post, which is covered by the kerosene-rich mixture. The width of the kerosene annulus is found to significantly affect the injector behavior. A wider annulus induces a larger spreading angle of the liquid-oxygen stream, which intercepts the kerosene stream in a more efficient way. Increasing the annulus width, however, imposes a wake region in a broader zone. The resultant flame becomes relatively unstable if the fl...

Combustion instabilities arise from interactions between the acoustic field and unsteady heat release within a confined chamber. In the present study, a three-dimensional acoustic model is developed to explore the effect of azimuthally nonuniform distribution of heat release on longitudinal combustion instabilities. The governing equations are solved by means of a spectral collocation method with domain decomposition to accommodate the flow discontinuities across the flame. It is shown that the circumferential nonuniformity in heat release has only a marginal effect on instability frequency but may cause a potentially significant decrease in the growth rate. For some combinations of flow parameters, a high degree of heat-release asymmetry may qualitatively change the stability characteristics of the combustor, with a sign change of the growth rate. It is found that vorticity waves produced by the azimuthally nonuniform heat release may contribute damping to the thermoacoustic system.

A new numerical framework for direct numerical simulation (DNS) of turbulent combustion is developed employing on-the-fly adaptive kinetics (OAK), correlated transport (CoTran), and a point-implicit ODE solver (ODEPIM). The new framework is tested on a canonical turbulent premixed flame employing a real conventional jet fuel mechanism. The results show that the new framework provides a significant speed-up of kinetics and transport computation, which allows DNS with large kinetic mechanisms, and at the same time maintains high accuracy and good parallel scalability. Detailed diagnostics show that calculation of the chemical source term with ODEPIM is 17 times faster than with a pure implicit solver in this test. OAK utilizes a path flux analysis (PFA) method to reduce the large kinetic mechanism to a smaller size for each location and time step, and it can further speed up the chemical source calculation by 2.7 times in this test. CoTran uses a similar correlation method to make the calculation of mixture-averaged diffusion (MAD) coefficients 72 times faster in this test. Compared to conventional DNS, the total CPU time of the final framework is 20 times faster, kinetics is 46 times faster, and transport is 72 times faster in this test.

The present work combines numerical and experimental efforts to investigate the effect of nanosecond pulsed plasma discharges on the low-temperature oxidation of C2H4/O2/Ar mixtures under reduced pressure conditions. The nonequilibrium plasma discharge is modeled using a one-dimensional framework, employing separate electron and neutral gas temperatures, and using a detailed plasma and combustion chemical kinetic mechanism. Good agreement is seen between the numerical and experimental results, and both results show that plasma enables low-temperature C2H4 oxidation. Compared to zero-dimensional modeling, the one-dimensional modeling significantly improves predictions, probably because it produces a more complete physical description (including sheath formation and accurate reduced electric field). Furthermore, the one- and zero-dimensional models show very different reaction pathways, using the same chemical kinetic mechanism and thus suggest different interpretations of the experimental results. Two kine...

Conventional aerodynamic theory of lift is based on the Kutta–Joukowski condition, in which the lift and drag coefficients are predicted based on the assumption that flow is attached and steady-state. Recent research on insect flight indicates, however, that the unsteady motion of a wing can generate much higher unsteady aerodynamic force through the flow induced by the shedding vortices. In the present work, therefore, we propose a novel type of stage consisting of rotor and periodically pitched stator to take advantage of unsteady vortex lift. The numerical results suggest that the flowfield around the leading and trailing edges of the blades are substantially modified by the interactions between the rotor and pitching stator. Several transient lift peaks can be generated for each period. If an appropriate pitching phase is chosen for the stator, stage performance can be improved through unsteady lift mechanism effects.

The axial gap between blade rows in turbomachinery should be minimized in order to reduce size requirements and increase efficiency; although, there is a tradeoff between gap width and flow steadiness. The aerodynamic interaction between rotors and stators influences system performance under both steady and transient conditions. To investigate the basic physical mechanisms associated with the rotor/stator interaction, an efficient numerical scheme for solving unsteady, viscous flows on a quasi-three-dimensional coordinate system is established using an immersed boundary method. The data transfer between moving and stationary grids that slip against each other in traditional numerical methods is avoided. The effects of the axial gap between adjacent blade rows are studied by considering the flow evolution through a rotor and stator stage in different Reynolds number regimes. Results indicate that reduced blade gap leads to high lift on the rotor blade and improved stage loading. At the same time, the rotor...

A comprehensive numerical study is conducted to explore the dynamic behaviors of a flame stabilized by a triangular bluff body in a straight chamber. The formulation accommodates the complete set of conservation equations with turbulence closure achieved by a large eddy simulation (LES) technique. A G-equation-based level-set flamelet approach is employed to model the interactions between premixed combustion and turbulence. Both non-reacting and reacting flows are treated, with special attention given to the effect of inlet boundary conditions on the flame evolution. The flow around the bluff body consists of boundary layers, separated shear layers, a recirculation zone and a wake. Their mutual coupling, as well as interactions with acoustic motion and flame oscillation are analyzed in detail. The physical processes responsible for driving combustion instabilities and the mechanism of energy transfer from chemical reactions in the flame zone to acoustic oscillations in the bulk of the chamber are ...