Flow, Turbulence and Combustion最新文献

Polyhedral Bunsen flames, induced by hydrodynamic and thermo-diffusive instabilities, are characterized by periodic trough and cusp cellular structures along the conical flame front. In this study, the effects of flow velocity, hydrogen content, and equivalence ratio on the internal cellular structure of premixed fuel-lean hydrogen/methane/air polyhedral flames are experimentally investigated. A high-spatial-resolution one-dimensional Raman/Rayleigh scattering system is employed to measure the internal scalar structures of polyhedral flames in troughs and cusps. Planar laser-induced fluorescence of hydroxyl radicals and chemiluminescence imaging measurements are used to quantify the flame front morphology. In the experiments, stationary polyhedral flames with varying flow velocities from 1.65 to 2.50 m/s, hydrogen contents from 50 to 83%, and equivalence ratios from 0.53 to 0.64 are selected and measured. The results indicate that the positively curved troughs exhibit significantly higher hydrogen mole fractions and local equivalence ratios compared to the negatively curved cusps, due to the respective focusing/defocusing effect of trough/cusp structure on highly diffusive hydrogen. The hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are first increased and then decreased with increasing measurement height from 5 to 13 mm, due to the three-dimensional effect of the flame front. With increasing flow velocity from 1.65 to 2.50 m/s, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps decrease, which is attributed to the overall decreasing curvatures in troughs and cusps due to the decreased residence time and increased velocity-induced strain. With increasing hydrogen content from 50 to 83%, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are amplified, due to the enhanced effects of the flame front curvature and the differential diffusion of hydrogen. With increasing equivalence ratio from 0.53 to 0.64, a clear increasing trend in hydrogen mole fraction and equivalence ratio differences between troughs and cusps is observed at constant flow velocity condition, which is a trade-off result between increasing effective Lewis number and increasing curvatures in troughs and cusps.

In the context of fundamental flow studies, experimental databases are expected to provide uncertainty margins on the measured quantities. With the rapid increase in available computational power and the development of high-resolution fluid simulation techniques, Direct Numerical Simulation and Large Eddy Simulation are increasingly used in synergy with experiments to provide a complementary view. Moreover, they can access statistical moments of the flow variables for the development, calibration, and validation of turbulence models. In this context, the quantification of statistical errors is also essential for numerical studies. Reliable estimation of these errors poses two challenges. The first challenge is the very large amount of data: the simulation can provide a large number of quantities of interest (typically about 180 quantities) over the entire domain (typically 100 million to 10 billion of degrees of freedom per equation). Ideally, one would like to quantify the error for each quantity at any point in the flow field. However, storing a long-term sequence of signals from many quantities over the entire domain for a posteriori evaluation is prohibitively expensive. The second challenge is the short time step required to resolve turbulent flows with DNS and LES. As a direct consequence, consecutive samples within the time series are highly correlated. To overcome both challenges, a novel economical co-processing approach to estimate statistical errors is proposed, based on a recursive formula and the rolling storage of short-time signals.

The Air-Core-Liquid-Ring atomizer is a pioneering internal-mixing pneumatic atomization technique designed for energy-efficient spray drying of highly viscous liquid feeds with substantial solid contents. However, it can suffer internal flow instabilities, which may lead to spray droplets with a wide variation in diameter. Experimental investigation of how flow conditions mechanistically determine the resulting droplet sizes is hindered by high velocities near the nozzle outlet. Therefore, this study addressed the issue by implementing a numerical model, employing a coupled Eulerian-Lagrangian approach with adaptive mesh refinement, to simulate the breakup of the liquid into ligaments and droplets. Additionally, Large Eddy Simulation was incorporated to replicate turbulent flow conditions observed in experiments. The numerical model demonstrated significant improvement in predicting liquid film thickness, compared to previous work. Additionally, the simulated droplet size distributions mirrored experimental trends, shifting to smaller sizes as pressure increased. Unfortunately, while reduced, there is a persistent underestimation of the lamella thickness and the droplet sizes at 0.2 MPa. In spite of this, the fact that the error propagates between the two phenomena underscores the effective coupling between Eulerian and Lagrangian approaches.

Critical flow Venturi nozzles (CFVNs) are a state-of-the-art secondary standard widely used for gas flow measurements with high precision. The flow rate correlates with the type and thickness of the boundary layer (BL) inside the nozzle throat. In the cylindrical type—one of the two standard designs of CFVNs—the nozzle throat encompasses a defined axial length in which the BL develops. This numerical study is concerned with the BL effects in a cylindrical CFVN by means of two turbulence models. Compared to experimental data, the k-(omega) SST model predicts the discharge coefficient well for high and low Reynolds numbers, but not in the intermediate regime. The (gamma)-(Re_{theta }) model, on the contrary, agrees well with experimental data in the entire flow range. Relevant quantities and profiles of the BL are separately investigated in the laminar, turbulent, and transitional region. The calculated laminar and turbulent BL thicknesses correspond to predictions based on integral methods for solving the BL equations. Simple representations are proposed for the Zagarola-Smits scaled laminar and turbulent deficit BL profiles removing the effects of axial position, Reynolds number, and pressure gradient. Furthermore, the shape factor is investigated as a characteristic parameter for determining the transitional region.

The present work analyses the effect of water vapour addition on ({text {NO}_text {x}}) emissions of premixed hydrogen flames. In doing so, the adiabatic flame temperature is maintained by increasing the equivalence ratio, or alternatively increasing the unburned gas temperature, for increasing levels of water loading. Thus, it is possible to elucidate the changes in ({text {NO}_text {x}}) production at constant-temperature conditions when the mixture is diluted with water. A consistent reduction of ({text {NO}_text {x}}) emissions for increasing water dilution can be observed from 1-D premixed freely propagating flame simulations. Regarding the chemical kinetics effect of water vapour, the relative importance of different third-body reactions is examined by modifying the corresponding water collision efficiencies individually. For the chemical mechanism adopted, three reactions directly affect the nitrogen chemistry and the remaining relevant reactions are important for the flame structure and radicals concentration. The analysis stresses the importance of indirect effects like the formation and consumption of (text {O}) and (text {H}) radicals in the pre-heat zone, which enhance the subsequent formation of ({text {NO}_text {x}}) within the flame. The presence of steam can lead to a reduction of approximately (50%) in ({text {NO}_text {x}}) emissions under conditions close to stoichiometry and high water loading ((10%) by mass), compared to scenarios without water addition. Furthermore, the efficiency of water in third-body reactions significantly contributes to an emission reduction, and half of (text {NO}) emissions under the same water loading conditions at high equivalence ratio are observed when the third-body reaction efficiency is activated with respect to the case with zero efficiency. This reduction is primarily attributed to effects on radical concentrations. Finally, the chemical effect via the third-body efficiency of water is examined with respect to flame speed. It turns out that the adiabatic flame temperature plays a key role for the relative influence of the chemical kinetics effect of water dilution. A cross-over temperature is found, below which the chemical effect of water reduces the flame speed, whereas the flame speed is increased above it.

This paper presents a comprehensive three-dimensional Direct Numerical Simulation (DNS) investigation of flow characteristics over a roughened wall in a duct, exploring bulk Reynolds number variations from 4400 to 13,200 and considering the influence of side walls. Notably, a significant increase in friction factor highlights heightened resistance to flow due to surface roughness, emphasizing the importance of considering surface conditions in engineering applications. The study reveals three recirculation zones in the streamwise direction, indicating intricate separation phenomena caused by the interaction between the ribbed wall and the flow. Furthermore, the presence of sidewalls creates two major vortices perpendicular to the flow direction, highlighting the complexity of the vortical structures in this configuration, crucial for predicting flow behavior and optimizing system performance. The study demonstrates that the influence of the Reynolds number on these vortices is not well-scalable with respect to outer units, in contrast with respect to smooth ducts. Investigating side-wall effects, increased turbulent production rate, and non-equilibrium turbulence along the side walls highlight the sensitivity of turbulent stresses to Reynolds number and side-wall proximity. In conclusion, the paper provides novel insights into the interplay between Reynolds numbers, roughened surfaces, and boundary conditions in turbulent flows, laying a foundation for a deeper understanding of the flow in duct with high roughness.

The effect of the degree of flow turbulence on detonation processes is analyzed. The relation between the turbulence parameters in front of and behind the shock wave is obtained for the first time. A modified detonation Hugoniot equation is derived, which takes into account the thermal effect and the level of flow turbulence. The equation for determining the velocity of detonation products, which shows how the degree of flow turbulence affects this velocity, was obtained. It is shown that the thermal effect weakens the effect of turbulence. The equation for estimating the effect of heat release and turbulence on the velocity in front of the shock wave is determined.

In this study, the highly nonlinear and multi-scale flame-turbulence interactions prevalent in turbulent premixed flames are examined by using direct numerical simulation (DNS) datasets to understand the effects of increase in pressure and changes in the characteristic scale ratios at high pressure. Such flames are characterized by length-scale ratio (ratio of integral length scale and laminar thermal flame thickness) and velocity-scale ratio (ratio of turbulence intensity and laminar flame speed). A canonical test configuration corresponding to an initially laminar methane/air lean premixed flame interacting with decaying isotropic turbulence is considered. We consider five cases with the initial Karlovitz number of 18, 37, 126, and 260 to examine the effects of an increase in pressure from 1 to 10 atm with fixed turbulence characteristics and at a fixed Karlovitz number, and the changes to characteristic scale ratios at the pressure of 10 atm. The increase in pressure for fixed turbulence characteristics leads to enhanced flame broadening and wrinkling due to an increase in the range of energetic scales of motion. This further manifests into affecting the spatial and state-space variation of thermo-chemical quantities, single point statistics, and the relationship of heat-release rate to the flame curvature and tangential strain rate. Although these results can be inferred in terms of an increase in Karlovitz number, the effect of an increase in pressure at a fixed Karlovitz number shows differences in the spatial and state-space variations of thermo-chemical quantities and the relationship of the heat release rate with the curvature and tangential strain rate. This is due to a higher turbulent kinetic energy associated with the wide range of scales of motion at atmospheric pressure. In particular, the magnitude of the correlation of the heat release rate with the curvature and the tangential strain rate tend to decrease and increase, respectively, with an increase in pressure. Furthermore, the statistics of the flame-turbulence interactions at high pressure also show sensitivity to the changes in the characteristic length- and velocity-scale ratios. The results from this study highlight the need to accurately account for the effects of pressure and characteristic scales for improved modeling of such flames.