Flow, Turbulence and Combustion最新文献

To assess the feasibility of implementing a slotted return guide vane with the jet flow in a compact multi-stage centrifugal compressor, this study utilised a cylindrical slotted return guide vane as the simplest shape example within a return channel with a large inner and outer diameter ratio and an inadequate mixing channel. The impact on internal flow, pressure recovery, and residual angular momentum at the device outlet was primarily examined through computational fluid dynamics, in addition to the suppression of the flow instability that may occur in an inward swirling flow. The study also delved into determining the optimal dimensionless momentum for the application of a slotted return guide vane through Bayesian optimisation, with the power ratio serving as the objective function. The findings revealed that the proposed slotted return guide vane with jet flow effectively curbs the occurrence of flow instability, resulting in an improved flow field compared with that of the conventional return guide vane under the constant dimensionless momentum condition. Additionally, the residual angular momentum at the device outlet was reduced, and the outlet static pressure coefficient was higher than that of the conventional return guide vane. Furthermore, under the dimensionless momentum condition identified through Bayesian optimisation as the minimum power ratio from device inlet to outlet, improvements were observed in the residual angular momentum and static pressure coefficient. The study findings are expected to improve the overall system performance and power ratio of multi-stage centrifugal compressors.

The physical mechanisms that drive changes in the velocity field and skin friction caused by a thin plate immersed in a turbulent boundary layer (TBL) are numerically examined via LES. The TBL develops in a water channel at Reynolds number Re_θ = 540–600 based on centreline velocity and momentum thickness. The plate’s chord and width are equal to c = 0.53δ and l ~ 1.3δ, respectively (δ is the thickness of an unperturbed TBL at the plate location). A wake (a velocity deficit region bounded by shear layers) and a pair of edge vortexes are generated by this plate. Their effects are studied at four plate locations in the inner TBL area. The effect of the wake is considered separately for the velocity deficit region and its shear layers. The velocity deficit region isolates the wall flow from the rest of the TBL. Thus, the velocity gradient at the wall is determined by the flow velocity below this region rather than at the center of the channel. The expansion of this region downstream causes a further decrease in the velocity gradient. It becomes minimal, when the lower shear layer of that region disappears. The shear layers of the wake and edge vortexes create normal and spanwise flows that reduce the wall flow velocity and thereby skin friction, but their effect is secondary. Edge vortexes reduce the surface friction beyond the plate width and have almost no effect on the friction in the channel center until the lower shear layer degenerates. The total drag introduced by the plate into the flow exceeds the reduction in skin friction.

The present article is concerned with the modeling of primary atomization in Eulerian-Lagrangian Large Eddy Simulation of liquid jet breakup. Recent numerical studies have demonstrated that spray atomization does not strictly follow a large-to-small breakup cascade. Instead, numerous small droplets can directly be generated by the core jet. As these cannot be resolved in Large Eddy Simulations using interface capturing methods for primary atomization, modeling their formation by releasing small Lagrangian particles from the core jet might improve the predictive capabilities of such simulations. This study discusses a corresponding model proposed in the literature and demonstrates its application for a Large Eddy Simulation of a complex Diesel-like spray. The results indicate a promising performance, but also highlight the need for further investigation.

Waste gas flares frequently encounter turbulent crosswinds, which pose significant challenges to maintaining the EPA-mandated 96.5% combustion efficiency for non-assist flares. Strong crosswinds can distort flame shapes, disrupt mixing, challenge emissions measurements due to variable speeds and directions, and ultimately degrade flare efficiency. This study quantifies the impact of crosswind turbulence intensity on non-assist flare combustion efficiency using large-eddy simulations coupled with a flamelet progress variable approach. Results show that while jet-induced turbulence enhances mixing and improves combustion efficiency, turbulence from crossflows increases local strain rates and consistently reduces efficiency. Combustion efficiency drops by up to 10% at turbulence intensities approaching 20%. A new correlation for combustion efficiency, obtained using symbolic regression, captures both experimental and simulation data well across natural gas flare flow rates of 2–4 m/s and wind speeds of 0–10 m/s. Incorporating a power-law dependence on turbulence intensity significantly reduces data scatter.

The balance of shock propagation and energy release in the detonation diffraction represents one of the most complex and unresolved phenomena relevant to gaseous detonations. Herein, the effects of nitrogen, argon, and helium diluent concentrations on the diffraction behavior of stoichiometric hydrogen-oxygen-diluent detonations at initial pressures of 0.5 and 1.0 bar are investigated. Through experimental analysis utilizing high-speed Schlieren and direct photography techniques, distinct diffraction outcomes were observed for different diluent compositions: increased argon and helium diluent concentrations led to supercritical transmission and affect the diffraction structure. The present study also identifies the detonation Damköhler number, defined as the ratio of the characteristic flow timescale to chemical timescale, as a diffraction outcome predictor. It is shown that detonations with diffraction Damköhler numbers exceeding 160 and 110 for initial pressures of 1.0 and 0.5 bar, respectively, will super-critically diffract.

This study presents three-dimensional large eddy simulations of fuel film flames to clarify the effects of the quiescent ambient pressure and oxygen content on the flame characteristics of isooctane fuel films in a constant-volume chamber. The ambient pressure, P_amb, and oxygen mole fraction, χ_O2, were set to 2–5 bar and 16–30%, respectively. The results indicate that, regarding the effect of P_amb, the flame characteristics of the fuel film transition from a laminar flame with flickering to a turbulent flame where the flame oscillates irregularly in the horizontal direction as P_amb increases. The flame characteristics are principally influenced by the behaviour of the vortex rings that are periodically formed around the flame. As the buoyancy effect increases with increasing P_amb, the axi-symmetry of the vortex rings disappears early in the formation process, leading to rapid breakdown. Consequently, the flame behaviour becomes turbulent. The χ_O2 conditions affect the flame temperature of the fuel film flame. The change in flame temperature with χ_O2 affects not only the buoyancy effect but also the viscous effect acting on the flame. The rate between both effects changes the axisymmetry of the vortex rings formed around the flame, consequently leading to changes in the flame characteristics. This effect is more noticeable when χ_O2 is lower than standard atmospheric conditions. The relationship between the buoyancy effect and the viscosity effect that determine the flame characteristics of the fuel film can be expressed by the Grashof number (Gr), as proposed in the present study, which considers P_amb and flame temperature. The proposed flame Gr is proportional to the square of P_amb but inversely proportional to the cube of the flame temperature.

The flame stabilisation mechanism for single and multiple reacting jets in cross-flow has been investigated using Large Eddy Simulation (LES) with the Conditional Moment Closure (CMC) as the sub-grid combustion model and a detailed chemical mechanism for pure hydrogen fuel. It has been found that a single jet in cross-flow (SJICF) has higher jet penetration depth compared to multiple jets in cross-flow (MJICF). This behaviour is attributed to the proximity of the counter-rotating vortex pairs of the three jets, which induces a downward negative velocity component, thereby influencing the jet stem. The flame stabilisation mechanism has been investigated using the budget of individual terms in the CMC equation. The CMC budget analysis reveals that upstream of the reactive zone, a premixed flame structure is observed with a convection-diffusion balance, whereas further downstream, a non-premixed flame structure prevails with a balance between micromixing and chemical reactions.

Wind tunnel experiments using Particle Image Velocimetry method were conducted to compare the flow characteristics induced by slender square and circular cylinders fully immersed in the boundary layer at gap ratios from 0 to 2. The results indicate that the shedding vortex is suppressed by the presence of the bottom wall in both flows if the wall-cylinder gap distance is lower than a threshold. However, the threshold gap for the square cylinder is lower than that for the circular cylinder. Although the scales of the large-scale turbulent structures are significantly reduced by the presence of cylinder, the two types of disturbed boundary layer have a similar downstream recovery process, without significant scale difference due to the small characteristic size of the disturbing cylinder. Since the near-wake flow behind the square cylinder is larger than that behind the circular cylinder for the same gap, the shedding vortex exhibits noticeably different contributions to the energy fraction of various turbulent modes. For boundary layer disturbed by the square cylinder, the Kármán vortex street becomes the dominant structure in the third and fourth order modes, whereas the effects are visible in the 4-10th order turbulent modes in the circular-cylinder disturbed boundary layer if the gap is high enough.

Turbulent nonpremixed combustion is widely used in the gas turbine combustors and in combustors attached to thermal power plant boilers. Improving the performance of these combustors is essential for reducing their CO₂ emissions. Understanding the effects of flame curvature on the local structure of turbulent nonpremixed flames is necessary for improving the accuracy of the laminar flamelet model and determining how the combustor performance can be improved. This study experimentally clarified the effects of the Lewis number of the fuel flow (Le_F) on curved counterflow nonpremixed flames using methane or propane as the fuel and air as the oxidizer. The effects of Le_F were isolated by comparing observations of methane–nitrogen fuel flows, for which Le_F is unaffected by the dilution rate, with observations of propane–nitrogen fuel flows, for which Le_F changes substantially with the dilution rate. Although it was previously believed that flame curvature effects could be neglected in hydrocarbon–nitrogen flames, this study revealed that even with hydrocarbon fuels, flame curvature can induce Lewis number effects when the flame radius is small. The effects of flame curvature on extinction were observed when the ratio of the flame zone thickness to the flame radius was on the order of 10^− 1, which is expected to occur in actual combustors. These results indicate that the effects of flame curvature should be considered when the laminar flamelet model is used to analyze turbulent nonpremixed flames at Le_F ≠ 1.

This study presents a compact data-driven Reynolds-averaged Navier-Stokes (RANS) model for wind turbine wake prediction, built as an enhancement of the standard (k)-(varepsilon) formulation. Several candidate models were discovered using the symbolic regression framework Sparse Regression of Turbulent Stress Anisotropy (SpaRTA), trained on a single Large Eddy Simulation (LES) dataset of a standalone wind turbine. The leading model was selected by prioritizing simplicity while maintaining reasonable accuracy, resulting in a novel linear eddy viscosity model. This selected leading model reduces eddy viscosity in high-shear regions—particularly in the wake—to limit turbulence mixing and delay wake recovery. This addresses a common shortcoming of the standard (k)-(varepsilon) model, which tends to overpredict mixing, leading to unrealistically fast wake recovery. Moreover, the formulation of the leading model closely resembles that of the established (k)-(varepsilon)-(f_P) model. Consistent with this resemblance, the leading and (k)-(varepsilon)-(f_P) models show nearly identical performance in predicting velocity fields and power output, but they differ in their predictions of turbulent kinetic energy. In addition, the generalization capability of the leading model was assessed using three unseen six-turbine configurations with varying spacing and alignment. Despite being trained solely on a standalone turbine case, the model produced results comparable to LES data. These findings demonstrate that data-driven methods can yield interpretable, physically consistent RANS models that are competitive with traditional modeling approaches while maintaining simplicity and achieving generalizability.