Flow, Turbulence and Combustion最新文献

The interrelation between displacement speed and dilatation rate in premixed turbulent flames has been analysed using three-dimensional Direct Numerical Simulation (DNS) data. The study focuses on statistically planar turbulent premixed flames with single-step chemistry at a unity Lewis number, representing stoichiometric methane-air premixed combustion, and detailed chemistry NH₃-air premixed flames with an equivalence ratio of 1.2. It has been shown analytically that the dilatation rate based on fluid velocity is proportional to the product of the density-weighted displacement speed and the reaction progress variable gradient for unity Lewis number adiabatic conditions. This is demonstrated using unity Lewis number single-step chemistry DNS data and detailed chemistry DNS results of NH₃-air premixed flames with an effective Lewis number close to unity. The study reveals that density-weighted displacement speed positively correlates with dilatation rate based on fluid velocity, though the relationship is non-linear due to the non-linear relationship between density-weighted displacement speed and the reactive scalar gradient magnitude. The density-weighted displacement speed and the dilatation rate based on flame propagation velocity are negatively correlated for the flames belonging to the wrinkled/corrugated flamelets regime, while their joint PDFs in thin reaction zones regime flames show both positive and negative correlation branches. The curvature stretch rate primarily causes these positive and negative correlation branches, a tendency that strengthens with an increase in Karlovitz number. Additionally, the mean dilatation rate in premixed turbulent flames is found to be proportional to the mean reaction rate in Reynolds Averaged Navier-Stokes simulations, suggesting that the mean dilatation rate could be linked to mean reaction rate closure.

The lateral flow around a rectangular plate under unsteady inflow involves complex fluid dynamic phenomena, including asymmetric flow structures and turbulence-induced instabilities. To investigate these effects, a three-dimensional numerical model combining the Volume of Fluid (VOF) method for gas-liquid two-phase flow and Large Eddy Simulation (LES) was employed to simulate the flow fields around rectangular obstacles with varying opening ratios. The analysis focused on time-averaged and instantaneous flow structures, Reynolds stresses, energy distributions, and modal decomposition of flow fields using Proper Orthogonal Decomposition (POD). Key findings reveal that turbulence intensity in the downstream shear layer escalates with increasing opening ratio E. High-intensity turbulence expanded toward the channel crown when E = 0.5, while a weakening trend was observed at E = 0.9. High-velocity zones extended progressively with larger E values. Upstream flow velocity intensified when the dimensionless pressure differential P^* exceeded 0.2, amplifying near-obstacle base velocities. Pressure fluctuations and vortex shedding behind the obstacle varied markedly with E. Instantaneous pressure coefficients at E = 0.5 fluctuated between −6 and 2 at 1 times obstacle thickness above the free end, with phase differences reaching 50°. A dominant vortex shedding frequency of 1 Hz across all cases confirmed periodic pressure pulsations. Reynolds normal and shear stresses concentrated in the rear shear layer, intensifying with higher E values. The high Reynolds stress area spreads to the top of the channel at elevated E. The first 20 POD modes captured the flow field’s dominant energy, with the top 5 modes accounting for 34.22% to 88.14% of total energy. Larger E values concentrated energy in lower-order modes, highlighting the growing dominance of large-scale flow structures. These insights provide a theoretical basis for optimizing underwater structures and offshore platforms under unsteady hydrodynamic loads.

In Constant Volume Combustion (CVC) and wider combustion applications, unwanted transitions to detonation can occur in highly reactive end-gas with a reactivity gradient present. The objective of this present study was to identify variables encountered in practical applications which influence detonation transition behavior. Different temperature gradients were imposed on stoichiometric H₂/C₁₀H₂₂/O₂/Ar mixtures with a 5% H₂ fuel mass percentage in a closed optical vessel through varying the temperature of heating cartridges placed at the top, middle and bottom of the chamber. In Part 1 of this study, ultra-high-speed schlieren records were analyzed to identify the exact mechanism by which detonation transition occurred for a mixture initially at 3 bar with top, middle and bottom heating cartridge temperatures of 453 K, 455 K and 441 K respectively. At this reference temperature gradient, both a vertically and laterally propagating autoignition wave would form. Zones with a reactivity high enough to transition to detonation would form at the intersection of the two waves as well as at the end wall due to the effects of reflecting pressure waves. Part 2 of this study analysed the effects of varying temperature gradient. A stronger temperature gradient condition resulted in end-gas with insufficient reactivity to transition to detonation. A weaker temperature gradient resulted in the development of more zones with sufficient reactivity to transition to detonation when compared to the reference temperature gradient. Although temperature gradient was the ultimate determinant of detonation transition behavior, it was found that changing chamber geometry could work towards suppressing detonations in borderline cases.

Film cooling is widely implemented in highly thermally stressed gas turbine components. Its performance has been extensively investigated for several decades and many results are available in the literature. In conventional gas turbines, regions of supersonic flow are not prevalent and should generally be avoided. For this reason, results relative to film cooling in supersonic flow are limited. Nevertheless, a new interest related to Rotating Detonation Combustors (RDC) and supersonic turbines is growing. The implementation of those engine components in a gas turbine is likely to need film cooling for thermal protection. In this context, it becomes crucial to gain an understanding of how the film interacts with the freestream when operated in a supersonic flow. This paper investigates the effect caused by the injection of film cooling on the morphology of the supersonic flow field. Results obtained by means of schlieren imaging indicated that the coolant injection acts as a wedge inside the flow, determining the local formation of an oblique bow shock around each film cooling hole. The shape, inclination, and strength of the oblique shock showed a dependency on the fundamental dimensionless parameters considered for the characterization of the operating conditions of film cooling. Furthermore, as the amount of mass injected was increased, the inclination of the generated shocks increased and the impingement location of the reflected shock moved upstream along the injection plate. The fluid dynamics of this interaction affected the local pressure distribution on the injection plate, measured by means of Pressure Sensitive Paint (PSP). Different film cooling geometries and main flow conditions were tested at multiple operating conditions. The relative impact of the different parameters is presented, providing useful information for the design of a film cooled engine component exposed to a supersonic flow.

The influence of fuel Lewis number Le_F on the wall-normal variations of mean values of streamwise velocity component, temperature, and Reynolds stresses within turbulent boundary layers have been analysed using a Direct Numerical Simulation (DNS) database of oblique wall quenching of V-shaped premixed flames due to their interaction with inert isothermal walls in a turbulent channel flow configuration. The fuel Lewis numbers of Le_F = 0.6, 1.0 and 1.4 have been considered for the current analysis. It has been found that the flame starts to interact with the wall further upstream for a smaller value of Le_F due to an augmentation of the volume-integrated burning rate with a decrease in fuel Lewis number. The thermal expansion induced by heat release affects the variations of mean values of both streamwise velocity component and temperature in the wall-normal direction and they show significant deviations from the conventional log-law profiles. Recently proposed density-compensated modified wall functions for streamwise velocity and non-dimensional temperature, which were previously validated for head-on quenching configuration for unity Lewis number, are found to capture the corresponding behaviours extracted from DNS data for flame-wall interaction of V-shaped flames with non-unity Le_F. The wall-normal distributions of Reynolds stresses also deviate significantly from the corresponding non-reacting fully developed channel flow profiles during flame-wall interaction and Le_F affects the extent of this deviation. The anisotropy of Reynolds stresses has been found to increase with the progress of flame-wall interaction. The physical explanations for the observed mean velocity, mean temperature and Reynolds stress statistics have been provided and their implications for modelling of flame-wall interaction have been elaborated.

This study investigates preferential diffusion effects on the volume-integrated burning rate in turbulent premixed lean hydrogen/air flames, using 3D direct numerical simulations (DNS) to analyze flames at two global equivalence ratios and varying turbulence intensities. By examining the statistical distributions of local equivalence ratios, the analysis confirms pronounced preferential diffusion effects across all cases. Intenser turbulence tends to amplify these effects. Probability density functions (PDFs) of the local equivalence ratio further confirm stronger preferential diffusion at lower global equivalence ratios and significant sensitivity to the choice of reaction progress variable definitions, particularly between hydrogen-based and water-based definitions. To predict the impact of preferential diffusion, the volume-integrated burning rate is estimated by multiplying the laminar burning velocity for the local equivalence ratio with the probability density function of the local equivalence ratio distribution. The subsequent estimates are compared to corresponding values directly obtained from DNS data. Results show that, in cases where preferential diffusion effects are more pronounced, i.e. at lower global equivalence ratios and relatively higher turbulence intensities, the DNS-derived burning rates based on water and temperature progress variables are best approximated using burning rates computed from the local equivalence ratio field conditioned on positive mean curvature. In contrast, cases less affected by preferential diffusion yield burning rate per unit area values comparable to those in the unstretched laminar flames when evaluated using water- and temperature-based definitions. The findings suggest that the burning rate per unit flame area could be modeled using the laminar burning velocity corresponding to the local equivalence ratio, and presumed PDFs representing the distributions of the local equivalence ratio. The gamma (or beta) distribution has been found to reasonably approximate the PDF of the local equivalence ratio, which can be utilized for the modeling of the volume-integrated burning rate in premixed turbulent lean hydrogen/air flames.

To efficiently harness gas-driven liquid metal for two-phase magnetohydrodynamic (MHD) power generation, three types static mixers was installed normally at the front of the power generation channel. The mixing flow process of the liquid metal gallium (Ga) with the gas R245fa within the power generator was numerically studied using ANSYS software. The results indicate that under identical boundary conditions, the gas-liquid metal two-phase fluid exhibits stratified and annular flows after passing through the HEV and SK type mixers, respectively. In contrast, a churn flow is observed when the fluid passes through the SMV-type mixer. Behind the SMV-type mixer, the velocity of the liquid metal in the power generation channel is 8.65 times greater than at the mixer’s inlet, while the slip ratio of the two-phase fluid is only 1.3. The uniformity of the liquid metal in the power generation channel is 0.611, and the volume flow rates of the liquid metal were increased by 46.7% and 3.1% compared to the that of SK and HEV types, respectively. Additionally, the current density fluctuation in the power generation channel was observed to be more stable over time with the SMV-type mixer.

Implicit Large Eddy Simulations (ILES) are deployed to characterize the effect of swirling boundary conditions on vorticity transport as well as on the Lighthill’s tensor for a cold, supersonic aerospike nozzle jet. Four jets are simulated at a Nozzle Pressure Ratio (NPR) = 3, one jet without swirl and three jets with swirl numbers (mathcal{S} = {0.10,0.20,0.30}). Swirling boundary conditions lead to an increase in vorticity tilting and stretching downstream of the aerospike bluff body, potentially enhancing sound generation. An exact decomposition of the Lighthill’s tensor is undertaken and the magnitude of the obtained source terms is compared. The dominant acoustic source terms are amplified under swirling boundary conditions. Terms describing the interactions between dilatation fields and density gradients balance each other in shock regions. At higher swirl numbers, the convection of density gradients along the flow direction leads to an imbalance that contributes to increased sound generation. Finally, cross-correlations between the near-field pressure and individual source terms reveal that enstrophy correlates more strongly with the near-field acoustics at higher swirl numbers.

A scalar dissipation rate-based mean reaction rate closure modified for premixed flame-wall interaction, which was previously proposed based on a priori Direct Numerical Simulation (DNS) analysis, is implemented for Reynolds Averaged Navier-Stokes (RANS) simulations in two configurations. The first configuration is the oblique wall quenching of a V-shaped premixed flame in a turbulent channel flow, and the second configuration is the head-on quenching of a statistically planar flame in a turbulent boundary layer. To avoid uncertainties associated with wall functions in reacting flows, a low Reynolds number (k - varepsilon ) model that resolves the viscous sub-layer is used. Comparisons between RANS simulations using the modified closure and DNS data reveal satisfactory agreement for Favre mean streamwise velocity and Favre mean temperature. However, quantitative discrepancies are found in the predictions of the Favre-averaged reaction progress variable due to differences in mean reaction rate profiles between DNS and RANS results. This behaviour arises from differences in turbulence quantities (e.g. turbulent kinetic energy and dissipation rate) between RANS and DNS, leading to discrepancies in RANS predictions of the mean reaction rate and Favre-averaged scalar dissipation rate, despite these closures performing well in a priori analysis. Even with these discrepancies, the scalar dissipation-based mean reaction rate closure shows promise in predicting mean values of Favre-averaged streamwise velocity and non-dimensional temperature in premixed flame-wall interaction configurations.

The development of a hydrogen jet injected into quiescent argon was investigated in a temporal jet configuration via direct numerical simulations (DNS). A case of argon mixing in argon was used as the basis for comparison. Both systems were computed at jet Reynolds numbers of 5000 and 10,000. Attention was focused on the mechanism driving the mixing process, as well as the turbulent momentum and scalar transport. The physical properties of argon are very different from those of hydrogen (density ratio (≈20), kinematic viscosity ratio (≈0.1), and Lewis number ratio ((approx 3))), leading to significant differences between the two cases, in jet structure, instantaneous and mean profile characteristics. A common feature in all systems was the emergence of large quasi-two-dimensional rotating structures, responsible for the engulfment of surrounding fluid, which created elongated regions where most molecular mixing takes place, with one difference being faster mixing in the hydrogen cases. An a priori assessment of the classical gradient hypothesis for turbulent fluxes revealed that the turbulent Schmidt number ((mathrm{Sc}_t)) and C_µ are not constant in space nor time, with local values ranging from (0.2-1.4), and (0.6-1.1), respectively, contrasting with the constant values used in Reynolds-Averaged Navier-Stokes (RANS) modeling. Additionally, an evaluation of a two equation RANS model and a dynamic one-equation large eddy simulations (LES) model was made a posteriori by comparison of their predictions with the DNS results. Both approaches exhibited significant deviations from the DNS, primarily at the early stage, but relaxed to similar solutions as time progressed. The properties at the jet edge were less well predicted by the RANS model than by the LES model. This is attributed to both gradient diffusion modeling and the impact of a turbulent/non-turbulent interface. Possible model enhancements are discussed.

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