Flow, Turbulence and Combustion最新文献

To reveal the gas-water two phase flow characteristic in the anode porous transport layers (PTL) of Proton Exchange Membrane (PEM) electrolyzer, both experimental and numerical (twin numerical model) studies were carried out in this work. An optical visualization test setup was developed, high-speed camera was used for flow visualization, pressure sensor and flowmeter were used to quantify the pressure drop and mass flow rate characteristics of gas-liquid two-phase flows, and the pressure drop and flow regime in the anode free flow channel as well as the PTL were reveal under different gas-to-water ratio (GWR), incline angle, porous size and pores configuration. Besides, a twin numerical model was also developed, and the flow characteristics of gas-liquid two-phase flow were further revealed by simulation. Moreover, the bubble coverage α in the free flow channel and the water proportion in the catalytic interface were quantified, and the threshold GWR under different operation conditions were also determined, which provides insight for water management of PEM electrolyzer.

The use of Computational Fluid Dynamics (CFD) to predict the acoustic signature of marine propellers operating in highly turbulent and non-uniform flow conditions has attracted considerable academic and industrial interest over the past decade. The negative effects of radiated noise from underwater maritime vessels and shipping activities on marine ecosystems and aquatic life are well recognized. This noise originates from various sources, most prominently propellers. This study aims to characterize the noise levels generated by marine vessels through high-fidelity numerical modeling of hydroacoustic phenomena. We employed Large Eddy Simulation (LES), the Schnerr–Sauer cavitation modeling approach, and the compressive volume-of-fluid (VOF) method to simulate cavitating flow over the propeller and predict the far-field radiated noise using the Ffowcs Williams-Hawkings (FW-H) hydroacoustic analogy. This study provides insights into the flow physics of noise generation due to wake structures—such as tip, root, trailing edge, and hub vortices—and cavitation patterns, including sheet, tip, and hub cavitation, over marine propellers operating upstream of a rudder. We characterized the instability of vortical structures in the wake of the marine propeller, including tip/hub vortex oscillation and instability, mutual-inductance instability (leapfrogging effect), elliptic instability, and primary and secondary wake grouping. Additionally, we examined sound levels related to propeller loading, cavitation development, periodic pulsating cavitation, and pressure fluctuations in both the near-field and far-field. Finally, the comparison between modeled and measured noise provided insights into the spectral contributions of propeller- and non-propeller-generated noise, helping to define the range of applicability for the assumption that propeller sources dominate overall noise emissions from the vessel.

The response of a hollow-cone n-heptane spray in a swirling airflow to forced acoustic disturbances is investigated using compressible Large-Eddy Simulations (LES) of the two-phase flow, with the dispersed fuel phase described in a Lagrangian framework. Phase-averaged numerical simulations with respect to the acoustic forcing are compared against phase-averaged phase-Doppler velocimetry (PDV) measurements of gaseous and droplet velocities, as well as phase-averaged measurements of droplet diameters over a forcing cycle. Liquid injection is modeled using the semi-empirical FIM-UR approach, a well-established framework for simulating fuel spray injection in gas turbines. Under steady injection conditions for both the gaseous and dispersed phases, simulations and experiments show excellent agreement in terms of gaseous velocity fields, droplet velocities, and droplet diameter distributions. In the acoustically forced regime, simulations also reproduce the gaseous-phase velocity response with good accuracy. Minor differences in negative velocity fluctuations at the burner outlet, attributed to pressure-drop mismatches across the air swirler, vanish downstream and can be mitigated by refining the mesh resolution in the air injection channels. For the dispersed fuel phase, however, droplet velocities are significantly overpredicted in the acoustically forced simulations. The difference arises because the simulations cannot fully reproduce the acoustically forced swirling flow, particularly in the internal recirculation zone, which is narrower than observed in experiments. Acoustic disturbances induce oscillations of the angle of hollow cone but also spatial and temporal fluctuations of the particle diameter and velocity distributions that are not accurately reproduced by the model. These findings highlight the critical importance of accurately modeling the unsteady response of the fuel injector in order to capture the complex swirling flow-spray-acoustic coupling. This coupling is shown to strongly influence the spray cone angle, as well as the distributions of droplet diameter and velocity.

Porous media are a promising technology to reduce turbulent boundary layer trailing edge noise. However, the fact that the porous material is grazed by turbulent flow on both sides makes its characterization not trivial. This paper describes the modifications resulting from the interaction between the grazing flows through the porous medium, defined as communication. To this end, lattice-Boltzmann simulations of two communicating turbulent channel flows separated by a fully resolved porous medium are carried out. The porous medium is realized as a 75% porous triply periodic minimal surface of type Schwarz’ P. Results are compared against the case with porous medium backed by a solid wall and the smooth wall channel flow. When communication between the two channel flows is allowed, spanwise coherent structures appear that are assimilated to a shear instability at a non-dimensional frequency of (St_t=0.02). Instantaneous flow through the porous medium is observed and is driven by a time-dependent pressure differential between the channels (with a zero mean and 7.8 Pa standard deviation). This leads to a decrease in energy in turbulent scales smaller than 2.5δ and for bulk scaled frequencies greater than (St_b=0.41). These flow modifications are not observed in the non-communicating case, with the wall preventing flow through, where the topology of the fluctuating statistics is similar to the smooth wall case. Finally, the drag is found to increase by over 200% with respect to the non-communicating case and 650% with respect to a smooth turbulent channel flow. The drag increase is found to be driven by the velocity fluctuations impinging on the porous topology. The communication does not follow the asymptotic drag relation for the same equivalent roughness, thus entering a different drag regime.

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.