Non-reacting Flow Research Articles

Numerical Study of Velocity and Mixture Fraction Fields in a Turbulent Non-Reacting Propane Jet Flow Issuing into Parallel Co-Flowing Air in Isothermal Condition through OpenFOAM

This research employs computational methods to analyze the velocity and mixture fraction distributions of a non-reacting Propane jet flow that is discharged into parallel co-flowing air under iso-thermal conditions. This study includes a comparison between the numerical results and experimental results obtained from the Sandia Laboratory (USA). The objective is to improve the understanding of flow structure and mixing mechanisms in situations where there is no involvement of chemical reactions or heat transfer. In this experiment, the Realizable k-ε eddy viscosity turbulence model with two equations was utilized to simulate turbulent flow on a nearly 2D plane (specifically, a 5-degree partition of the experimental cylinder domain). This was achieved using OpenFOAM open-source software and swak4Foam utility, with the reactingFoam solver being manipulated carefully. The selection of this turbulence model was based on its superior predictive capability for the spreading rate of both planar and round jets, as compared to other variants of the k-ε models. Numerical axial and radial profiles of different parameters were obtained for a mesh that is independent of the grid (mesh B). These profiles were then compared with experimental data to assess the accuracy of the numerical model. The parameters that are being referred to are mean velocities, turbulence kinetic energy, mean mixture fraction, mixture fraction half radius (Lf), and the mass flux diagram. The validity of the assumption that w߰ = v߰ for the determination of turbulence kinetic energy, k, seems to hold true in situations where experimental data is deficient in w߰. The simulations have successfully obtained the mean mixture fraction and its half radius, Lf, which is a measure of the jet’s width. These values were determined from radial profiles taken at specific locations along the X-axis, including x/D = 0, 4, 15, 30, and 50. The accuracy of the mean vertical velocity fields in the X-direction (Umean) is noticeable, despite being less well-captured. The resolution of mean vertical velocity fields in the Y-direction (Vmean) is comparatively lower. The accuracy of turbulence kinetic energy (k) is moderate when it is within the range of Umean and Vmean. The absence of empirical data for absolute pressure (p) is compensated by the provision of numerical pressure contours.

Application of hydrogen mechanisms in combustion simulation of DLR scramjet combustor and their effect on combustion performance

In view of the promising application of hydrogen in scramjet combustor and the rapid development of hydrogen kinetic mechanisms, present work applies representative mechanisms with good kinetic performance to the combustion simulation of DLR scramjet combustor with a customized solver based on OpenFOAM platform. Different Reynolds-Averaged Navier Stokes (RANS) turbulence models including standard k-ε, SST k-ω and their corresponding modified versions for turbulent round jets, are utilized to simulate the non-reactive and reactive flow fields. Compared with the detailed experimental measurements of schlieren image, streamwise velocity, pressure etc., the modified k-ω SST turbulence model shows good performance and improves the simulation results near hydrogen jet. In combination with partially stirred reactor (PaSR) combustion model, three representative detailed hydrogen mechanisms (marked with Ó Conaire-2004, Varga-2015 and Konnov-2019) are adopted to evaluate their effect on combustion performance under scramjet combustion condition. And their effects on the combustion performance are quantified based on combustion efficiency, net thrust, total pressure loss, etc. All of them can well characterize the thermal combustion characteristics of hydrogen in the combustor. Among them, Ó Conaire-2004 slightly overestimates flame temperature, combustion efficiency and net thrust, and underestimates total pressure loss. Whereas Varga-2015 and Konnov-2019 provide nearly identical results on these performance parameters and give temperature profiles coinciding well with experimental data. Although the newly added excited states in Varga-2015 and Konnov-2019 deepen the understanding of hydrogen combustion processes and can served for optical combustion diagnostics, their effects on thermal combustion characteristics are negligible under present combustion condition, judging from their low content and the analysis on the performance parameters. Present work is conducive to the selection of hydrogen mechanisms for the combustion simulation of hydrogen-fueled scramjet combustor, and facilitates to obtain reliable simulation results for scramjet with turbulent round jets by employing the modified SST k-ω turbulence model.

Evaluation of heat transfer coefficient between a reacting steam-methane mixture and preheated Ni-based catalyst

This work is devoted to the investigation of a heat transfer inside a steam methane reformer filled with pre-heated Ni-based catalyst pellets of various shapes. Various particle shapes were tested: spheres, cylinders and Raschig ring under various inlet steam-to-methane ratios. The chemical kinetics model is based on experimentally derived reaction rate expressions taken from the literature. The distinguishing feature of the developed model is unsteady flow (transient) of reacting mixture over pre-heated Ni-based catalysts for the following initial parameters: inlet velocity – 0.5–1.7 m/s, inlet gas temperature – 400–450 °C, temperature of the preheated catalysts – 927 °C. This model allowed us to investigate the effect of endothermic chemical reactions on heat transfer between the catalysts and gas flow. Moreover, two cases were tested to understand the heat transfer in the reformer: the chemical reaction was switched on and switched off in the model. It was established that the heat transfer coefficient for a reacting flow (≈90W/(m2K)) is about twice higher than the heat transfer coefficient for a non-reacting flow.

Computational fluid dynamics study of the atomic layer deposition process around cylindrical and planar configurations

Computational fluid dynamics (CFD) simulations were used to model atomic layer deposition (ALD) coating processes in the continuum flow regime. CFD model was validated for non-reactive and reactive flows. In ALD CFD simulations, surfaces in various test cases were considered as non-porous aluminum oxide. The effect of trimethylaluminium precursor partial pressure, initial condition, and Reynolds number (Re) on coating timescale were analyzed. The CFD model accurately captured the surface coverage solution for all cases when the domain was initially filled with the precursor. The absence of initial precursor delayed coating time during the surface concentration level development process. As Re increased, coating time significantly decreased due to reduced diffusion limitations and promoted convection swiftly transporting precursors to the surfaces. The provided example showed that 2D polydisperse fixed cylinder bed is fully coated within approximately one flow time through the bed. The benefits of CFD in understanding ALD processes were discussed.

Exploiting Active Subspaces for Geometric Optimization of Cavity-Stabilized Supersonic Flames

With the increased demand for hydrogen for carbon-neutral propulsion technologies, understanding the coupling effect of the fuel injection and cavity on flow characteristics is essential and an open issue for cavity-based hydrogen supersonic flame stabilization and optimizations. In this paper, the active subspace (AS) method is exploited to quantitatively reveal the intrinsic connections between the flow and flame stabilization with various fuel injection and cavity geometric parameters. A flame stabilization metric connecting the flame stabilization mode with nonreacting flows is proposed for the AS analysis and multiobjective optimizations. The results show that there exist one-dimensional active subspaces for key performance metrics, for example, the total pressure loss , residence time , and . The fuel injection pressure, cavity length-to-depth ratio, and cavity depth are three leading factors that govern the total pressure loss while the cavity residence time is dominated by the cavity depth. The proposed reasonably maps the flame stabilization mode with nonreacting flows. Using the response lines obtained by the AS, the tradeoff between and is quantitatively formulated with a clear visualization of candidate sets in the input space, demonstrating the advantages of the AS method for the combustor designs involving multiple objectives.

Mixing and combustion characteristics in a scramjet combustor with different distances between cavity and backward-facing step

The mixing and combustion characteristics in a cavity flameholding combustor under inlet Mach number 2.92 are numerically investigated with ethylene injection. Dimensionless distance is defined as the ratio of the actual distance to the height of the combustor entrance. The cavity shear-layer mode, the lifted cavity shear-layer mode, and jet wake mode with upstream separation are observed respectively with dimensionless distance equals to 1.5, 4.5, and 7.5. In both non-reacting and reacting flow fields, the numerical results are essentially in agreement with the schlieren photography, flame chemiluminescence images, and wall pressure, which verify the reliability of the numerical method. The results of non-reacting flow fields show that the Backward-Facing Step (BFS) can promote the flow separation downstream at a fixed distance. The more forward the separation position is, the larger the separation zone is in the non-reacting flow field. Furthermore, the larger the separation zone is, the higher the intensity of combustion in the reacting flow field is. A reasonable distance can reduce the total pressure loss generated by the shock waves in the combustor. The flame presents remarkable three-dimensional characteristics in the reacting flow fields. When dimensionless distance equals to 4.5, there are flames near the side wall above the cavity and it is difficult for the flame stabilization in the center of the combustor, while the combustion intensity in the center of the combustor is higher than that near the side wall when dimensionless distance equals to 7.5. In the cavity flameholding combustors with a backward-facing step, the higher combustion intensity may bring much total pressure loss to the combustor. Thus, it is a good choice to achieve better thrust performance when dimensionless distance equals to 4.5 compared to the other two combustors.

MCGA-assisted ignition process and flame propagation of a scramjet at Mach 2.0

The ignition process and flame propagation with ethylene fuel in cavity-stabilized scramjet by a Multi-Channel Gliding Arc (MCGA) at Mach 2.0 were investigated. Effects of equivalence ratios on the MCGA-assisted ignition process and flame propagation of the scramjet were recorded by two high-speed cameras from different view angles. The discharge characteristics of MCGA are also collected synchronously with the high-speed cameras. The distributions of temperature, velocity, and equivalence ratios in non-reactive flows of the cavity were simulated by Reynolds Averaged Navier-Stokes (RANS) model. The results show that MCGA can achieve reliable ignition with the Global Equivalence Ratios (GER) between 0.06 and 0.17. The ignition process is composed of flame kernel generation, flame development, and stable combustion. The time from flame kernel generation to the establishment of global flame decreases as GER decreases from 0.17 to 0.08. In the streamwise direction, the flame first develops to the Cavity Leading Edge (CLE) because of the influence of the cavity recirculation zone and then uplifts into the cavity shear layer, and finally develops to the Cavity Trailing Edge (CTE). In the spanwise direction, the flame width is less than 50% of the width of the cavity before developing to CLE and begins to develop towards the two sides of the combustor after reaching CLE, which is affected by the angular recirculation zone on both sides of CLE. The ignition processes by MCGA in the scramjet combustor are significantly affected by local distributions of equivalence ratios and velocity in the cavity.

One-dimensional turbulence modeling of compressible flows: II. Full compressible modification and application to shock–turbulence interaction

One-dimensional turbulence (ODT) is a simulation methodology that represents the essential physics of three-dimensional turbulence through stochastic resolution of the full range of length and time scales on a one-dimensional domain. In the present study, full compressible modifications are incorporated into ODT methodology, based on an Eulerian framework and a conservative form of the governing equations. In the deterministic part of this approach, a shock capturing scheme is introduced for the first time. In the stochastic part, one-dimensional eddy events are modeled and sampled according to standard methods for compressible flow simulation. Time advancement adjustments are made to balance comparable time steps between the deterministic and stochastic parts in compressible flows. Canonical shock–turbulence interaction cases involving Richtmyer–Meshkov instability at Mach numbers 1.24, 1.5, and 1.98 are simulated to validate the extended model. The ODT results are compared with available reference data from large eddy simulations and laboratory experiments. The introduction of a shock capturing scheme significantly improves the performance of the ODT method, and the results for turbulent kinetic energy are qualitatively improved compared with those of a previous compressible Lagrangian ODT method [Jozefik et al., “Simulation of shock–turbulence interaction in non-reactive flow and in turbulent deflagration and detonation regimes using one-dimensional turbulence,” Combust. Flame 164, 53 (2016)]. For the time evolution of profiles of the turbulent mixing zone width, ensemble-averaged density, and specific heat ratio, the new model also yields good to reasonable results. Furthermore, it is found that the viscous penalty parameter Z of the ODT model is insensitive to compressibility effects in turbulent flows without wall effects. A small value of Z is appropriate for turbulent flows with weak wall effects, and the parameter Z serves to suppress extremely small eddy events that would be dissipated instantly by viscosity.

Stability of a layered reactive channel flow

We analyse the linear stability of a reactive plane Poiseuille flow, where a reactant fluid A overlies another reactant B in a layered fashion within a two-dimensional channel. Both reactants are miscible and have the same viscosity, while upon reaction, they produce either a less or more-viscous product fluid C . The reaction kinetics is of simple A + B → C type, and the production of C occurs across the initial contact line of reactants A and B in a mixed zone of small and finite width. All three fluids have the same density. We demonstrate the effects of various controlling parameters such as the log-mobility ratio, Damköhler number, Schmidt number, Reynolds number, position and thicknesses of the reactive zone on the stability characteristics. We show that a tiny viscosity stratification by the reaction destabilizes the flow at a moderate (10–1000) and even at low Reynolds numbers (0.01–1). The maximum growth occurs for shorter waves than for the Tollmien–Schlichting eigenmode, and the ranges of unstable wavenumbers are wider than that known for non-reactive channel flow systems. In most cases, the instability occurs due to the overlap of the critical layer with the viscosity-stratified layer. Surprisingly for some parameters, it is observed that the reaction can make σ M decrease with increasing Reynolds number.

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

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

Direct numerical simulation of supersonic internal flow in a model scramjet combustor under a non-reactive condition

A Mach 1.5 non-reactive flow in a cavity-stabilized combustor of a model scramjet is studied via a direct-numerical simulation approach, and the analysis is focused on the interaction among boundary layer, free shear-layer above the cavity and shock wave. It is found that the impingement of the free shear-layer on the aft wall of the cavity leads to strong turbulence kinetic energy, high local pressure, and a fan of compression waves. The compression waves evolve into an oblique shock, which reflects between the upper and lower walls and interacts with the boundary layers attached to the two walls. The analysis of the turbulence production reveals that the amplification of turbulence in the core of the shear-layer and around the reattachment point is mainly due to the shear production, but the deceleration production mechanism presents a significant impact in the regions above the aft wall of the cavity and around the shock interaction points. The very low frequency commonly observed in shock wave/boundary layer interactions is not observed in the present research, which might be due to the low Reynolds number of the studied case.

Numerical Predictions on the Intermittent Combustion Instability: Implications and Applications

Any application employing combustion places a huge emphasis on safe and effective operations. Several industries make use of relatively small amounts of propellant volumes to generate enormous amounts of energy. For instance, the Aerospace industry requires fuel injection in liquid rocket engines and gas turbine engines for producing thrust. For such cases, combustion comes with its own set of challenges. The predominant challenge among them all is that of combustion instability. Instabilities are physical phenomena occurring in both reacting and non-reacting flows. In the context of engine combustion, they tend to emanate from very small perturbations in the flow field and amplify in magnitude to alter the macroscopic properties of the flow causing problems for the smooth functioning of engines. Since the problem of combustion instability arises from the point of fuel injection itself, this project aimed to look at instability from the viewpoint of intermittent instability in the temporal domain and has redefined it from the perspective of energy loss. For understanding the results in a simplified form, a new zonal system ‘i-SEV’ was established. Owing to the heterogeneous heat and mass transfer, optimization and characterization of combustion performance were carried out parametrically to arrive at a conclusive decision of the key parameters in combustion instability and the governing physics behind it.

The Role of Hydrodynamic Instabilities on Near-Lean Blowout Flame Shapes in a Swirl-Stabilized Spray Combustor

Abstract This study investigates the role of hydrodynamic instabilities on near-lean blowout (LBO) flame shapes in a swirl-stabilized spray combustor. Hydrodynamic instabilities often manifest themselves in swirling flows as a helical vortex that winds around the vortex breakdown bubble. However, the heat released from combustion tends to suppress coherent vortex structures, which can limit the helical vortex to certain combustor geometries and operating conditions. Flame shape changes often accompany changes in hydrodynamic stability because they reposition the heat release and consequently modify the degree of coherent vortex suppression. In this study, laser diagnostics measurements were used to characterize the flow fields and spray patterns corresponding to different flame shapes that were observed in the Air Force Research Laboratory (AFRL) referee combustor. In particular, the flame fluctuated between its original shape, FS1, and a new flame shape, FS2, when the combustor operated on the threshold of LBO. Proper orthogonal decomposition (POD) was used to analyze the measurements. POD showed that the appearance of FS2 coincided with coherent vortex structures that resembled those in the hydrodynamically unstable nonreacting flow field. Furthermore, fuel Mie scattering measurements and phase-averages of the velocity field provided evidence that the FS2 spray was periodically disturbed by a helical vortex. Near the swirler exit, this helical vortex structure involved both outer and inner shear layer vortices that appeared to be synchronized with each other. However, the inner shear layer vortices decayed as the flow progressed downstream and only the outer shear layer vortices remained throughout the measurements' field of view. In contrast, there was no indication of a helical vortex structure in either the flow field or fuel spray measurements corresponding to FS1.

Phosphor thermometry in heat transfer fluids and liquid organic hydrogen carrier systems using (Sr,Ca)SiAlN3:Eu2+

Temperature control in heat exchangers in reacting and non-reacting flows is of great importance for process optimization. In this context, phosphor thermometry is a promising technique for remote planar temperature sensing. The thermometry technique is based on exciting a luminescent material by a laser pulse and analyzing the subsequent phosphorescence signal. A particular interesting application is chemical hydrogen storage using liquid organic hydrogen carrier (LOHC) systems. The knowledge of temperature fields is of special interest for the characterization and understanding of hydrogen release from the carrier liquid. We investigated the luminescence properties of the thermographic phosphor (Sr,Ca)SiAIN3:Eu2+ (‘SCASN:Eu2+’) dispersed in different heat transfer fluids, in particular LOHC systems, using a newly developed calibration cell. As heat transfer fluids may be excited to fluorescence by the laser as well, their absorption and florescence behavior is studied to develop an excitation and detection concept for thermometry. We found strong absorption of the heat transfer fluids from the UV range to a wavelength of about 400 nm. In addition, fluorescence signals were found in the visible wavelength range, which can interfere with the phosphor emissions. These fluorescence signals should therefore be circumvented by utilizing the different luminescence decay times in the chosen detection strategy. For thermometry, the SCASN:Eu2+ particles were excited by a laser sheet of a 532 nm Nd:YAG laser. A spectrometer and photomultiplier tube (PMT) were used to detect the emission spectrum and phosphorescence decay time (PDT). Two temperature evaluation strategies were applied, which are based on either the intensity ratio of two spectral emission regions (two-color laser-induced phosphorescence) or the PDT. The results obtained show an applicable measurement range between 293 K and 598 K for the intensity ratio method with a maximum relative sensitivity of 0.12% K−1 at 293 K. For the PDT method, the phosphor allows measurements between 423 K and 598 K with a maximum relative sensitivity of 0.56% K−1 at 598 K.

Experimental and Numerical Investigation of the Non-Reacting Flow in a High-Fidelity Heavy-Duty Gas Turbine DLN Combustor

A dry, low-NOx (DLN) combustor for a heavy-duty gas turbine using lean premixed technology was studied. A high-fidelity test model was built for the experimental study using particle image velocimetry (PIV). The non-reacting flow in the DLN combustion chamber was investigated experimentally and numerically. The numerical results are in good agreement with the experimental data. The results show that recirculation zones were formed downstream of each swirl nozzle and that the flow pattern in each section was self-similar under different working conditions. For two adjacent swirl nozzles with opposite swirling directions, the entrainment phenomenon was present between their two flows. The two flows gradually mixed with each other and obtained a higher speed. If the two adjacent swirl nozzles had the same swirling direction, then the mixing of the two flows out of the nozzles was not present, resulting in two separate downstream recirculation zones. The interaction of swirling flows out of different nozzles can enhance the turbulent fluctuation inside the combustion chamber. Based on the analysis of the recirculation zones and turbulent kinetic energy (TKE) distribution downstream of each nozzle, it can be found that nozzle coupling results in stronger recirculation and turbulent mixing downstream counterclockwise surrounding nozzles.

Hybrid time-frequency domain dual-probe coherent anti-Stokes Raman scattering for simultaneous temperature and pressure measurements in compressible flows via spectral fitting.

We demonstrate a hybrid time-frequency spectroscopic method for simultaneous temperature/pressure measurements in nonreacting compressible flows with known gas composition. Hybrid femtosecond-picosecond, pure-rotational coherent anti-Stokes Raman scattering (CARS), with two independent, time-delayed probe pulses, is deployed for single-laser-shot measurements of temperature and pressure profiles along an ∼5-mm line. The theory of dual-probe CARS is presented, along with a discussion of the iterative fitting of experimental spectra. Temperature is obtained from spectra acquired with an early, near-collision-free probe time delay (τ 1=0p s) and pressure from spectra obtained at probe delays of τ 2=150-1000p s, where collisions significantly impact the spectral profile. Unique solutions for temperature and pressure are obtained by iteratively fitting the two spectra to account for small collisional effects observed for the near zero probe delay spectrum. A dual-probe pure-rotational CARS system, in a 1D line-imaging configuration, is developed to demonstrate effectively the simultaneous temperature and pressure profiles recorded along the axial centerline of a highly underexpanded jet. The underexpanded air jet permits evaluation of this hybrid time-frequency domain approach for temperature and pressure measurements across a wide range of low-temperature-low-pressure conditions of interest in supersonic ground-test facilities. Single-laser-shot measurement precisions in both quantities and pressure measurement accuracy are systematically evaluated in the quiet zone upstream of the Mach disk. Precise thermometry approaching 1%-2% is observed in regions of high CARS signal-to-noise ratios. Pressure measurements are optimized at probe time delays where the ratio of the late probe delay to the Raman lifetime exceeds four (τ 2/τ R>4). The impact of low-temperature Raman linewidths on CARS pressure measurements is evaluated, and comparisons of CARS pressures obtained with our recent low-temperature pure-rotational Raman linewidth data and extrapolated high-temperature Q-branch linewidths are presented. Considering all measurements with τ 2/τ R≥4.0, measured pressures were on average 7.9% of the computed isentropic values with average shot-to-shot deviations representing a combination of instrument noise and fluid fluctuations of 5.0%.

Status, perspectives, and added value of high fidelity simulations for safety and design

With the increase in computational power, High Fidelity (HiFi) simulations become increasingly important to nuclear thermal hydraulics for fundamental understanding of turbulent flow and heat transfer phenomena and for further development and validation of engineering CFD models. We consider Direct Numerical Simulations (DNS) and Large Eddy Simulation (LES) as HiFi analyses. In this paper, we present an overview of single-phase and two-phase flow DNS simulations relevant for nuclear reactor safety and design purposes. In order to limit the scope, this overview has a focus on DNS simulations of fully turbulent non-reacting incompressible flows. LES approaches are briefly mentioned in certain areas, where LES-to-DNS upgrade is expected to happen in the near future.We first explain the concept of DNS and related concepts like Under-resolved DNS (UDNS) and quasi-DNS (q-DNS). Subsequently, we present the numerical methods which are generally used for HiFi simulations. A wider span of numerical methods can be observed in single-phase flow simulations than in two-phase flow simulations, where the later require additional algorithms for tracking of the interfaces between the two phases. Consequently, two-phase DNS simulations are not immune to modelling errors, and are much more complex than their single-phase counterparts. Next, an overview is given of single-phase flow DNS simulations. We start with basic cases like turbulent channel and pipe flows, which are mainly useful for development and validation of less detailed engineering CFD models. We continue with more recent and more complex cases like, e.g., flows in a rod bundle and a pebble bed, which are directly applicable in nuclear engineering. Next, an overview of two-phase flow HiFi simulations is presented in a similar fashion. The importance of the availability of the presented HiFi simulations is explained by addressing their role for further development and validation of engineering CFD models which are used for the eventual industrial applications. The importance of HiFi simulation data, complementary to experimental data, for uncertainty quantification and machine learning is explained. Finally, future challenges for HiFi simulations, especially for two-phase flow, are elucidated.

Numerical investigation on the performance and combustion characteristics of a cavity based scramjet combustor with novel strut injectors

A three-dimensional numerical investigation has been carried out on the performance and combustion characteristics of a scramjet combustor with and without cavity for various strut injector configurations. The geometrical shape of the injectors considered for the present study are circle, square and rectangle. The numerical model of scramjet combustor has been discretized hexahedrally through finite volume approach using ANSYS ICEMCFD 18.0 and the necessary boundary conditions are specified in ANSYS Fluent 2021 CFD code. The three-dimensional fluid flow variations through a scramjet engine with strut-based injectors and cavity have been analyzed by solving the corresponding governing equations namely continuity, momentum (Navier-Stokes) equation and energy transport equation for the non-reacting flows. In addition to the above equations species transport equation is also solved for the reacting flows. The turbulence induced in the fluid is modelled using standard K-ε turbulence model. The effect of injector shapes, mixing length, combustion efficiency and total pressure loss have been numerically simulated and plotted with and without cavity of a scramjet combustor. The results from the study predicted that circular injector with cavity showed better mixing characteristics and combustion characteristics compared to the rectangle and square model. The total pressure loss of circular injector with cavity is 5.4% higher than the other models investigated, still it is preferred because of high combustion efficiency attained with shorter span of combustor length.

Reacting and non-reacting, three-dimensional shear layers with spanwise stretching

A three-dimensional, steady, laminar shear-layer flow spatially developing under a boundary-layer approximation with mixing, chemical reaction, and imposed normal strain is analyzed. The purpose of this study is to determine conditions by which certain stretched vortex layers appearing in turbulent combustion are the asymptotic result of a spatially developing shear flow with imposed compressive strain. The imposed strain creates a counterflow that stretches the vorticity in the spanwise direction. Equations are reduced to a two-dimensional form for three velocity components. The non-reactive and reactive cases of the two-dimensional form of the governing equations are solved numerically, with consideration of several parameter inputs, such as the Damköhler number, the Prandtl number, chemical composition, and free-stream velocity ratios. The analysis of the non-reactive case focuses on the mixing between hotter gaseous oxygen and cooler gaseous propane. The free-stream strain rate κ* is predicted by ordinary differential equations based on the imposed spanwise pressure variation. One-step chemical kinetics are used to describe diffusion flames and multi-flame structures. The imposed normal strain rate has a significant effect on the width of downstream mixing layers as well as the burning rate. Asymptotically in the downstream direction, a constant width of the shear layer is obtained if the imposed normal strain rate is constant. The one-dimensional asymptotic result is an exact solution to the multicomponent Navier–Stokes equation for both reacting and non-reacting flows, although it was obtained using the boundary-layer approximation. A similar solution with the layer width growing with the square root of downstream distance is found when the imposed strain rate decreases as the reciprocal of downstream distance. The reduced-order asymptotic solutions can provide useful guidance in developing flamelet models for simulations of turbulent combustion.

Identification of flow structures in a closed chamber in the presence of a needle plasma actuator

This study deals with the experimental characterization of the induced flow dynamics by a disk-needle-type plasma actuator driven by a sinusoidal generator and located in a rectangular cross-section burner. Flow characterization was performed using different plasma actuation conditions and standoff distances. Experiments were conducted under non-reactive flow conditions. Electrical characterization was carried out. Airflow behavior was also analyzed using smoke flow visualization. Smoke flow visualization showed the dynamic behavior of the plasma-induced flow. Post-processing of high-quality images was performed by using Proper Orthogonal Decomposition (POD) technique to recognize the dominant flow vortexes and coherent structures. This could support the design of plasma actuation devices in real combustors and be useful for the implementation of numerical models. Moreover, it has been concluded that flow dynamics can be controlled by a variation of the plasma power or the gap distance between two electrodes. Laser Droplet Velocimetry (LDV) was used to investigate the distribution of flow velocities and turbulent kinetic energy (TKE) at different plasma power values of the sinusoidal alternating current generator and standoff distances. From POD and LDV analyses, it has been observed that there is quite a linear relation between the POD energy of the first mode and the maximum TKE. The POD method could be used to identify motions in the flow field carrying the most TKE. TKE peaks are present in the area with the most energetic flow structures, as identified by the POD.