Flow, Turbulence and Combustion最新文献

To enable efficient and clean combustion in internal combustion engines, precise control over fuel injection and atomization is essential. Dynamic oscillation of the injector needle is a primary factor governing internal flow and atomization performance; however, its underlying mechanisms are substantially more complex than the commonly assumed static eccentricity model, and remain insufficiently understood. This study systematically examines this dynamic phenomenon by analyzing the impact of needle oscillation on the internal flow dynamics and vortex evolution in both single-hole and multi-hole injectors, with particular attention to the critical needle opening and closing stages. The results reveal that dynamic needle oscillation induces reverse-rotating large-scale vortices within the sac, which further interact to form small-scale vortex pairs. In the four-hole injector, flutter consistently suppresses the mass flow rate across all orifices, exhibiting pronounced asymmetry; the orifices located outside the flutter plane experience the strongest reduction, especially at low needle lifts. Moreover, under high-pressure (200 MPa) compressible conditions, the suppressive effect of flutter on mass flow rate becomes more significant, accompanied by intensified flow instability and persistent pressure oscillations. The dynamic mechanisms identified in this work overcome the limitations of conventional static simulations. These findings not only offer new insights for improving injection performance in traditional diesel engines but also provide important theoretical and engineering guidance for the design of injection systems for emerging green fuels such as liquid ammonia, methanol, and biodiesel.

This study investigates the effect of flow dynamics on the soot temperature and volume fraction fields in a non-premixed pulsed ethylene flame generated on a Yale burner. Soot properties were characterized through absorption/emission techniques, while Particle Image Velocimetry with zirconium dioxide tracer particles was applied to the entire flame. The resulting velocity fields were used to calculate shear strain rates in different regions of the flame. The results showed that modulation significantly affected the soot distribution, with a variation of (sim)12% in the soot volume fraction between consecutive phases of the cycle. During certain phases a higher injection flow rate is favored by the acoustic pulse, leading to an increased amount of fuel being burned and a higher soot formation. This behavior is related to the flow expansion in the upper regions of the flame, where high velocity zones coincide with an increment in soot temperature due to particle transport and accumulation. Also, areas exhibiting heightened strain rates are linked to elevated levels of mixing and oxidation. These regions are associated with a low soot volume fraction. This is likely due to enhanced mixing and oxidation under elevated strain rates, as suggested by previous numerical work, highlighting the suppression of soot inception and the predominance of oxidation processes during specific phases of the forcing cycle. The interactions between fluid structures and soot-rich zones were identified as a crucial aspect of overall combustion dynamics, with a direct impact on efficiency and emissions.

As an efficient and stable fluid delivery device, the ejector plays a critical role in various gas supply systems. However, existing ejector designs often fail to meet performance requirements under all operating conditions. This study focuses on optimizing the ejector structure for applications involving high flow rates and limited installation space, thereby expanding its applicability. The key geometric dimensions of the ejector were determined using the Sokolov method. A three-dimensional fluid simulation model was developed and validated through experimental testing. Under various operating conditions, the effects of Nozzle Exit Position (NXP), mixing chamber diameter (D_m), mixing chamber length (L_m), and Flow-Distributing Net curvature (K) on ejector performance were analyzed. The optimal structural parameters that meet the design requirements were identified. The results show that when the primary flow pressure is 350 kPa, the optimal design parameters for the ejector are NXP = 15 mm, D_m = 60 mm, L_m = 60 mm, and K = 0. These values yield the best overall performance. This research provides valuable technical support for the application of ejectors in gas fuel supply systems.

Conventional airfoils suffer from aerodynamic limitations, including early stall onset and increased drag at higher angles of attack, which restrict their operational efficiency and versatility. Active flow control methods, such as the Co Flow Jet (CFJ), have shown promise in overcoming these challenges. This study investigated whether a Dual Co Flow Jet (DCFJ) system with independently controlled suction and blowing slots may extend the stable working range of the airfoil. Building on these foundations, airfoils were tested at various free stream velocities and angles of attack, ranging from 0° to 24°, each corresponding to multiple coefficients of jet momentum. Compared to the baseline airfoil, the DCFJ configuration not only greatly improves lift-to-drag ratios but also raises maximum lift coefficients and delays stall. To further reduce the cost and complexity of full-scale testing, numerous machine learning (ML) models, including multi-layer artificial neural networks, were trained on experimental datasets. These models achieved minimal prediction errors for lift and drag coefficients, providing a reliable and computationally efficient alternative to conventional experimental workflows.

Turbulent hydrogen flames with varying operating conditions in the swirl-axial air injection AHEAD combustor were studied computationally with a multi-regime flame closure method, combustion LES / Stochastic fields. The method was validated by comparisons with measurements in isothermal and reacting flows. The velocity fields, flames, mixing fields and thermo-chemical states were analysed in detail. Further comparisons were carried out for different operating conditions to study the effect of global equivalence ratio and axial air injection ratio. On the one hand, it introduces higher axial momentum, which restricts flashback. On the other hand, increasing the global equivalence ratio or axial air injection ratio negatively affects the spatial mixing quality where the axial momentum flux plays an important role. The results also suggest that thermo-chemical states are dominantly controlled by the global equivalence ratio rather than the inlet reactant temperature or flow conditions. The effect of differential diffusion was also studied. Differential diffusion slightly increases the possibility of the upstream occurrence of the flame inner branch, which results in the inner flame branch brush becoming broader. This was found to be related to the changes of the upstream mixing field due to differential diffusion. Nevertheless, the global system is negligibly influenced by differential diffusion due to the high Reynolds number.

Ammonia/methane ((textrm{NH}_3)/(textrm{CH}_4)) co-combustion offers a promising route toward high-efficiency, low-carbon, and low-NO_x combustion, yet experimental insights into local displacement speeds in ammonia-containing flames remain scarce. Here, 2D particle image velocimetry (2D-PIV) was applied to turbulent premixed Bunsen flames at atmospheric pressure, covering ammonia fractions up to 0.6 and equivalence ratios between 0.7 and 1.0. Streamline-based decomposition of the progress-variable transport equation showed that the measured flame propagation speed s_T is broadly consistent with the closure relation s_R + s_F at low turbulence intensities, supporting the applicability of the Bray-Moss-Libby (BML) thin-flame concept to (textrm{NH}_3)/(textrm{CH}_4) flames. The reaction and convection terms dominated the balance, while turbulent fluxes, though weaker, remained non-negligible. Closure errors increased with higher ammonia content or leaner mixtures but were greatly reduced by introducing Markstein corrections and approximating 2D flame surface density as 3D. Analysis of reaction rates further revealed that the near-axis region contributed most, while ammonia addition lowered local reactivity, increased flame height, and promoted local extinction at the root. The findings provide experimental guidance for source-term estimation and turbulence–flame interaction modeling.

Introducing a vegetation barrier in place of one building obstacle within a canonical street canyon of identical geometry yields a coupled vegetation-building (V–B) or building–vegetation (B–V) configuration, depending on whether the vegetation is located on the upwind or downwind side of the canyon, whose flow structure and turbulence characteristics remain less explored compared with the canonical building-building (B–B) case. The extent to which results in V–B and B–V canyons deviate from or converge with those of a generic B–B canyon remains unclear. Using large-eddy simulation across a range of leaf area densities (LAD), this study examines how vegetation placement and porosity modulate canyon-scale vortex geometry, turbulence structure, and air exchange. The zero-w trace delineating the primary canyon vortex, which aligns approximately with the vertical axis in a unit-aspect-ratio B–B canyon, exhibits a monotonic tilt toward the vegetation side as LAD decreases, reflecting progressive deformation and weakening of the mean circulation. Within V–B canyons, turbulence intensity and vertical momentum flux show little dependence on LAD, implying that upwind vegetation primarily acts as a momentum sink that suppresses near-wall shear. In contrast, these turbulence statistics in B–V canyons exhibit stronger sensitivity to LAD ((R^2sim 0.81) and 0.65, respectively), suggesting enhanced turbulence generation through canopy-wake interaction. Both mean and fluctuating air-exchange rates increase smoothly with LAD for vegetated canopies but decline sharply as the system approaches the impermeable B–B limit. This non-monotonic behaviour demonstrates that the (LADtoinfty) limit is dynamically distinct from an impermeable wall, as dense vegetation retains finite permeability and sustains shear-layer instabilities absent in solid obstacles.

This work assesses a POD/SVD-based sparse sensing framework for thermochemical field reconstruction in the LUCY cyclonic combustor operating under MILD methane combustion conditions. A reduced-order representation is built from CFD data and combined with QR decomposition with column pivoting (QRCP) to design sensor layouts and infer reduced coefficients from sparse measurements. Two truncation criteria are compared, the optimal hard threshold of Gavish et al. and a 99.5% cumulative variance, together with Auto-scaling preprocessing, to quantify their impact on reconstruction accuracy and out-of-sample prediction. Beyond unconstrained QRCP, practical deployment constraints are investigated through distance separation and region-limited candidate sets, and the performance of a predefined thermocouple layout is evaluated by augmenting it with additional optimally placed sensors. Results are reported for both training and testing operating conditions, complemented by an uncertainty-propagation analysis based on experimental measurement variability. The study shows that robust reconstruction is achieved with a limited number of sensors when the balance between retained rank and measurements is respected, while spatial constraints can preserve near-optimal performance and improve physical feasibility. The framework provides a computationally efficient soft-sensing strategy to support monitoring-oriented applications in confined MILD combustors.

High-speed diagnostics are essential for understanding the unsteady parameter fluctuations in Rotating Detonation Combustors (RDCs). However, the experimental data from RDCs often exhibit significant stochasticity spatially and temporally due to factors such as lap-to-lap detonation wave fluctuations, measurement uncertainties, and sensor-induced artifacts. Traditional phase-averaging techniques, like the arithmetic mean, can distort the true detonation wave structure by smoothing out key features due to temporal misalignment. This study investigates the application of a soft-Dynamic Time Warping (soft-DTW) based averaging as a unique method for processing high-speed RDC data. Compared to conventional methods, soft-DTW has shown improved resilience to local time axis distortions, which may enable better alignment and preservation of the intrinsic wave structure, particularly by capturing the sharpness of the main peak and secondary features relevant to the detonation process. The study evaluates the capability of soft-DTW to capture essential physical characteristics of rotating detonation waves using dynamic pressure and video data. Additionally, a sensitivity analysis assesses the method’s effectiveness in accurately representing secondary features, such as reflected shock waves, highlighting its potential for more representative RDC data averaging.