Large Recirculation Zone Research Articles

Ultralean Combustion Mechanism of Methane–Air Swirling Premixed Flame

ABSTRACT This study investigated a methane – air premixed flame in a partially tapered swirl burner. Experimentally, we succeeded in forming almost steady ultralean flames with an equivalence ratio as lean as 0.465. On the other hand, a numerical simulation of the flame with detailed chemistry revealed that the flame head existed in a large recirculation zone under ultralean conditions. In addition, high-value peaks of temperature, heat-release rate (HRR), and concentrations of OH, O, and H radicals were formed at the flame head, enabling ultralean combustion. Subsequently, to identify the formation mechanism of these peaks, adiabatic swirl burner flames were simulated with and without the effect of thermal-diffusive imbalance (a Lewis number effect). The results show that a Lewis number of less than unity caused a temperature increase in the flame located in the recirculation zone owing to the thermal-diffusive imbalance, which increased the peak heights of the HRR and the three radicals. The results also showed that even in a flame unaffected by thermal-diffusive imbalance, the backward convective transport could accumulate CO and OH at the flame head thus enhancing the chemical reactions at the head. (185 words)

Distinguishing the vertical air-staging low-NOx function of secondary air for suspension combustion within an industrial-scale coal-fired reversal grate furnace

The numerous coal-fired grate furnaces in China, which were designed to partition primary air on the grate and arrange secondary air in the freeboard for air-staging combustion, suffered from poor burnout and high NOx. However, secondary air was conventionally closed or set at marginal openings in real operations and no investigation was reported to explain why. Upon these off-design secondary-air circumstances, industrial-scale tests and modeling investigations were performed for a 75 t/h coal-fired reversal grate furnace at secondary-air ratios of 0 %, 3 %, 6 %, 10 %, and 16 %, respectively. The primary aims were to (i) uncover its combustion and NOx formation characteristics with varying secondary air and (ii) confirm whether secondary air acted on a real air-staging function in reducing NOx. Flow-field deflection and asymmetric combustion were found in the furnace, with the upward gas entirely deflecting towards the rear side. The front-half furnace was filled with a large but weak recirculation zone without effective combustion, acting on a huge dead zone to waste the furnace-space utilization. With increasing the secondary-air ratio, the upward gas deflection deteriorated continuously. Layer combustion worsened while suspension combustion first strengthened and then weakened. NOx emissions displayed an ascent-to-descent trend. Carbon in slag raised while carbon in fly ash decreased. Among the five case setups, the 0 % setting (i.e., fully closing secondary air) exhibited the lowest NOx emissions and burnout loss. Compared with the habitual 6 % setup, fully closing secondary air not only reduced NOx emissions by 22 % but also improved a little burnout. These trends meant that opening secondary air only strengthened suspension combustion for burning well fly ash while raised both NOx and total burnout loss. Secondary air thus failed to role as an air-staging function in reducing NOx.

Study on the fuel/air mixing process in a compact combustor with uneven flame stabilizer wall

Close-coupled injection is an important fuel injection method for future compact design combustors, and improving the mixing of fuel and gas is an important challenge for a wide operating range of the engine. By arranging uneven flame stabilizer walls behind the injection nozzles, the fuel penetration depth and fuel/air mixing of the close-coupled injection can be effectively improved at a low thrust state. In this paper, two kinds of uneven flame stabilizer wall structures were proposed, namely, the forked tail type and the lug-shaped type. The flow characteristics of different uneven flame stabilizer wall structures were investigated by combining numerical simulation and atomization experiments, and the differences in fuel distribution, jet trajectory, and combustion performance between the strut with uneven flame stabilizer wall structures and the U-shaped strut were qualitatively assessed. The results show that both proposed strut structures had a larger low-speed flame stabilization region, a greater jet penetration depth, and better combustion performance than did the common U-shaped strut, but with a greater flow resistance loss. The strut with a forked tail caused the airflow to deflect at the trailing edge of the strut, which in turn deflected the fuel, resulting in a larger recirculation zone and greater jet penetration depth. The lug-shaped strut produced a small recirculation zone behind the lug-shaped structure, and the fuel with a low fuel/air jet momentum flux ratio collided with the lug-shaped structure, thereby increasing the fuel jet penetration depth. The gap lug-shaped structure enhanced fuel diffusion and fuel/air mixing in the radial direction. Numerical studies under real operating conditions showed that the two types of uneven flame stabilizer wall structures were promising for improving the compactness of the combustor. Overall, two types of uneven flame stabilizer wall structures can promote fuel/air mixing during low engine operating states, which is conducive to widening the engine operating range and providing design solutions for the next generation of engine combustor designs with wide operating ranges.

Bubble size distribution and turbulence characterization in a bubbly flow in the presence of surfactant

This study aimed to quantify the less explored complex multiphase hydrodynamics of a bubble swarm in the presence of surfactant which is the key to flotation process widely used in the resources and environmental engineering applications.Experiments were conducted in a rectangular column (cross-section: 100 mm × 100 mm) in batch mode by varying the gas flux (0.02 to and 0.08 cm/s) in the presence of an anionic surfactant sodium dodecyl sulphate (5 to 15 ppm). High-speed imaging was then used to visualise the unsteady bubble plume dispersion behaviour and estimate the bubble size distribution (BSD) which showed a reduction in the mean bubble size with the increasing gas flux and surfactant concentration.Next, particle image velocimetry (PIV) was utilised to measure the instantaneous velocity field which was used to determine the turbulence characteristics. It was shown that the bubble plume contributes to significant anisotropy in the flow field which increased in the higher surfactant concentration and gas flux cases. The energy containing turbulence length scale was characterized by the integral length scale, which was observed to increase linearly with both the gas flux and surfactant concentration. Also, the local turbulence energy dissipation rate exhibited a strong linear correlation with the bubble surface area flux parameter. In the presence of surfactant, the turbulence energy spectrum of the system exhibited a less steep slope in the inertial subrange regime compared to the Kolmogorov −5/3 slope. The spectrum also showed a leftward shift indicating energy addition to the larger turbulence length scales which was reflected in the formation of large recirculation zones around the bubble plume.

Aerodynamic center of a wing at low Reynolds number

The location of the aerodynamic center, with respect to the center of gravity, determines the longitudinal static stability of an aerial vehicle. While most studies in the past focused on high Reynolds number (Re) flows, we consider flows at low Re (100≤Re≤6000) past an end-to-end wing and a finite wing of various semi-aspect ratio (0.25≤sAR≤5). It is shown that two-dimensional computations, even for an end-to-end wing, are inadequate for accurate prediction of aerodynamic coefficients. The chordwise pressure distribution as well as the variation of aerodynamic coefficients with angle of attack (α) at low Re are significantly different from that at high Re. Beyond a certain α, the flow is associated with a large recirculation zone, resulting in a non-linear variation of the aerodynamic coefficients with α. The conventional definition of the aerodynamic center is too restrictive at low Re. Owing to the non-linear variation of the pitching moment with change in α, it is not possible to determine one single location where it is invariant over a large range of α. The aerodynamic center is, therefore, determined over various sub-regimes of α. It varies significantly over the sub-regimes. The wing-tip vortex significantly affects the vortex shedding, flow structures, and pressure distribution on a finite wing. The locations of the aerodynamic center and center of pressure move fore, toward the leading edge, with a decrease in aspect ratio compared to that for an end-to-end wing.

Annular swirling jet reactor for converting hydrocarbons to olefins and aromatics

This paper describes a novel reactor for high temperature thermochemical conversion of ethane to olefins. The central feature is an efficient flow pattern with a large recirculation zone that provides stable and compact 2500 °C flame and a thin conical annular swirling jet that provides fast transfer of mass, momentum, and heat. The jet is directed through a unique converging–diverging nozzle that maintains relatively low wall temperature and consequently reduces the need for external cooling. The hydrocarbons stream is introduced tangentially through volutes in such a way that there is initially heat transfer with minimal mass transfer followed by fast mixing of countercurrent streams. CFD simulations with detailed kinetics were employed to design and optimize this reactor including the order of the volutes to feed ethane, H2 fuel, oxidizer (O2 and N2 mix), and a cooling N2 gas; oxygen enrichment of the air, and also the overall operating conditions of this flexible and robust reactor. This highly intensified reactor was fabricated and tested at a semi-pilot scale with up to 15 kg/hr feed. The optimized reactor has high selectivity to ethylene and acetylene comparable to conventional steam cracker.

Analysis of vortex combustion flow in a chamber of guide ring coupling with swirl flow

To investigate the impact of different vortex structure on the combustion stability in a chamber, this paper evaluates the performance of a vortex combustion chamber coupled with a swirler and a guide ring. The simulated cases include different guide ring angles of 95°, 105°, 115°, 125° and 135°, in addition to a control case without coupling the guide ring. The distributions of velocity, vorticity, temperature, outlet temperature distribution factor ( OTDF), reaction rate, turbulent kinetic energy ( k), synergy angle ( β), and NO emission are numerically analyzed. Results show that the coupling of the swirler and guide ring changes the vortex structure and significantly influences the chamber performance. The increase of the guide ring angles ( α) enhances the interaction between the swirl flow and the direct jet, which intensifies heat and mass transfer, as well as turbulence. The application of the guide ring induces external entrainment of the jet, leading to the formation of a large central recirculation zone compared to that without a guide ring. With increasing the α from 95° to 135°, both average outlet velocity and tangential velocity ( Vt) at the coaxial line position decrease, while the flame length decreases by 286 mm. And the OTDF variation among all cases is less than 0.065. The β of the central section decreases from 76.18° to 70.17°. Furthermore, combustion chamber without the guide ring exhibits higher temperature and OTDF.

Optimizing air distribution in floor and wall burners of an industrial steam cracking firebox: a CFD study

A 3D computational fluid dynamics model for an industrial scale steam cracking firebox is combined with a 1D reactor model for the cracking process within the reactor coil. This framework is validated using industrial tunable diode laser absorption spectroscopy (TDLAS) measurements in a naphtha steam cracking furnace. Industrial O2, H2O, CO and temperature data show good agreement with simulation results at different locations within the firebox. The influence of varying the primary-to-secondary air flow distribution to the floor and wall burners separately is investigated using the validated framework. Altering the primary-to-secondary air flow distribution of the floor burners from 75/25 to 25/75 results in a 66% increase in furnace run length and a 35% decrease in NOx formation. This shift in air ratio ensures a lower rollover height of the flame and a large recirculation zone of the flue gas, spreading the radiative heat flux on the reactor coil more evenly. Altering the primary-to-secondary air flow distribution of the sidewall burners from 50/50 to 100/0 results in a 13% increase in run length and a 30% decrease in NOx formation. More primary air towards the sidewall burners is found to lead to more evenly spread out flames along the wall, resulting in flame interactions between the different burners and a combined flow towards the center of the firebox. These interactions result in a more uniform tube metal temperature profile along the different passes of the coil.

Assessment of a hydrogen-fueled swirling trapped-vortex combustor using large-eddy simulation

In recent years, the study of hydrogen combustion has gained significant interest due to its exceptional lean blow-out (LBO) performance, high efficiencies, and almost negligible greenhouse gas emissions. The present study combines the advantages of hydrogen combustion with the beneficial characteristics of trapped-vortex combustion (TVC) and swirling flows. Large-eddy simulation (LES) is conducted to evaluate the performance of a swirling trapped-vortex combustor. LES sub-gird terms are described using k-equation subgrid model, while, turbulence-combustion interaction is modelled employing the PaSR (Partially Stirred Reactor) closure with a detailed 23-step chemical reaction mechanism for hydrogen/air. A radial vane swirler is incorporated to supply an annular mainstream and generate a wake region in front of the cavity leading edge to enhance cavity flow entrainment. To evaluate the performance of a hydrogen-fueled TVC under non-swirl and swirling conditions with varying swirl numbers (0.1–0.6), several flow and combustion characteristics are analyzed. These includes temperature distribution, vortical structure, combustion efficiency, NO emission, and flame shape. The results show that a large recirculation zone dominates the cavity for the all swirl numbers resulting in a strong fuel–air mixing and high combustion efficiency. It is demonstrated that the swirling motion effectively mitigates the negative impacts of pressure fluctuations arising from the interaction of combustion with cavity oscillations. A stable flame is observed inside the cavity and in the shear layer at the cavity lip in all cases, merging to form a distinctive C-shaped flame. Increasing the swirl number causes the second part of the flame to move to the wake region before the leading edge, enhancing combustion stability and efficiency. Combustion efficiency exceeds 98.5% for all cases, reaching 99.8% for a swirl number of 0.6. The temperature distribution is nearly uniform inside the cavity, with an improved distribution at the TVC’s outlet for higher swirl numbers. Under non-swirling conditions, the corrected NO concentration is below 5 ppm. Increasing the swirl number escalates the outlet NO emission, nevertheless, a maximum NO concentration of 19 ppm is observed for the swirling cases. Comparing these findings with existing data suggests that the swirling TVC offers advantages over other hydrogen-fired TVC’s.

Effect of kerosene injection states on mixing and combustion characteristics in a cavity-based supersonic combustor

It has been found that the static pressure distribution along the axial direction of liquid kerosene is lower than that of the gaseous kerosene under the same flow condition and overall equivalent ratio from previous studies. To further investigate this phenomenon, a compressible two-phase parallel simulation method is utilized to analyze the mixing and combustion characteristics of gaseous and liquid kerosene jets in a cavity-based supersonic combustor. The numerical results are consistent with the experiments and demonstrate that gaseous injection leads to a cavity shear layer that dives deeper into the cavity, forming two recirculation zones in the front and rear of the cavity. In contrast, the cavity shear layer is closer to the mainstream during liquid injection, and only a large recirculation zone is formed in the rear of the cavity. As a result of the cavity shear layer and the recirculating flow, the fuel vapor of gaseous injection accumulates in the front of the cavity, while for the liquid injection, the fuel vapor disperses in the cavity, cavity shear layer, and the region above, and the rear of the cavity has a higher fuel vapor concentration than the front. This unique fuel distribution causes the combustion area to be concentrated in the cavity during the gaseous injection but dispersed inside and downstream of the cavity during the liquid injection. As a result, forming a thermal throat under the same conditions is more challenging during liquid injection, and the generated static pressure distribution is lower than that during the gaseous injection.

Effects of the swirler on the performance of an advanced vortex combustor

Trapped vortex combustors have garnered significant interest due to the cavity vortex's insensitivity to flow disturbances. Swirling combustion is also highly desirable due to its excellent mixing performance. This study explores the potential of combining vortex combustors with swirling combustion under reacting conditions. Steady-state Reynolds-averaged Navier–Stokes equations are solved through three-dimensional numerical simulations. The results demonstrate a significant improvement in combustion efficiency, from 55% in original vortex combustor to over 99% in swirling vortex combustor, and fuel–air mixing effectiveness. The spatial spiraled vortex structure forms in the cavity and behind the blunt body when the swirler is used. The position of the blunt body influences the shape of the cavity vortex, with a round vortex structure forming when the cavity length to the total combustion chamber length ratio is around 0.2. When the radius of the blunt body to the radius of the combustion chamber, varying from 0.2 to 0.7, is 0.6, two main recirculation zones can be created in the model. Increasing the swirling number results in a larger recirculation zone in the cavity, with an elongated recirculation area appearing behind the blunt body at a blade installation angle of 60°, corresponding to the swirling number is 1.251. However, an excessively high swirling number leads to uneven pressure distribution in the outlet. These findings provide novel insights for advanced vortex combustor research and optimization.

CFD simulation and experimental analysis of solid-liquid mixing characteristics in a stirred tank with a self-similarity impeller

BackgroundThe nonlinear geometry could create a longer turbulence production region and higher turbulence intensities compared with conventional geometry. A self-similarity impeller based on Hibert curves was introduced to intensify the solid-liquid chaotic mixing process in this work. MethodsThe hydrodynamics of solid-liquid mixing process in a stirred tank with pitched-blade impeller, self-similarity 1 impeller and self-similarity 2 impeller were studied through experimental analyses and numerical simulation. Significant findingsResults showed that self-similarity impeller could consume less power consumption compared with pitched-blade impeller at the same impeller speed. Self-similarity impeller could improve the largest Lyapunov exponent (LLE) value compared with pitched-blade impeller under the constant power consumption, and the LLE value continued to increase with the increase of the number of self-similar iterations. Meanwhile, self-similarity impeller could improve the solid particle suspension quality compared with pitched-blade impeller under the constant power consumption, and increasingly so with the self-similar iteration number. Moreover, self-similarity impeller could destroy the large tailing vortex and recirculation zone, and enhance the turbulent kinetic energy dissipation rate by the jet flows through the concave-convex edges compared with pitched-blade impeller, and this phenomenon became obvious with the increase of self-similar iteration number.

Improving combustion and lowering NO emissions of an industrial coal swirl burner by optimizing its nozzle structure

• Simulation and comparison are conducted for the burner before and after optimization. • Entrained effect of secondary air on primary air is enhanced with retrofitting ISACS. • Retrofitting ISACS has significant effects on the combustion characteristics. • Optimized burner performs better in NOx reduction, flame stability and burnout rate. To accommodate rapidly growing but highly variable renewable power in the grid and large variations in power demand, coal-fired power plants need to be flexibly operated through fast and wide load changes, still with high efficiency and reliability and low emissions. Towards reliable implementation in a power plant under flexible operation, this paper comprehensively investigates the improvement of an industrial low-NO x swirl burner via numerical simulation and various field tests. Burner aerodynamics, which play a vital role in its performance, are largely affected by the burner nozzle structure. For the low-NO x swirl burner under study, the inner secondary air cone structure is found to have the most significant impact on the burner aerodynamics and the combustion performance. For the coal fired in the power plant, the newly designed inner secondary air cone structure, which is a kind of an extended smooth diverging duct, greatly facilitates the entrainment of the primary air into the surrounding swirl secondary air stream and largely enhances lateral mixing. A large recirculation zone is formed between the primary air and the inner secondary air stream, stabilizing ignition and combustion under flexible operation conditions. This is particularly important for low-load operation. Comparatively, reducing the secondary air duct area for a higher axial momentum of the secondary air stream and adding a new particle separation ring for more dispersed particle distribution are found to have a minor impact on the burner aerodynamics. The optimized burner nozzle structure via numerical simulation has been finally adopted by the power plant for the burner retrofit, on which various field tests have been performed on the retrofitted burner. The tests show that the nozzle structure optimization remarkably improves the burner performance, which is also consistent with the numerical prediction. This study also provides useful guidance for optimizing the swirl burners of the similar type.

Stability and emission characteristics of ammonia-air flames in a lean-lean fuel staging tangential injection combustor

The present work suggests the stability and emission characteristics of an ammonia-air tubular flame in a fuel staging tangential injection combustor. Owing to the development of a large inner recirculation zone induced by a high swirling motion, the ammonia tubular flame exhibited wide operational flexibility at an ambient temperature condition. With ammonia fuel staging, we confirmed that the Thermal DeNOx process significantly mitigates NO emissions when the exit temperature of the primary region is close to the threshold for active NO reduction. In this study, fuel multi-stage technology was used for pure ammonia-air combustion, and the result effectively challenges the existing paradigm of the NO reduction strategy: Rich-Quench-Lean staging configuration. Thus, our results demonstrate the potential use of ammonia as a fuel in a dry low-NOx system.

Three-dimensional numerical investigation on flow behaviors around a diversion dike

Three-dimensional numerical simulations of a flow past a non-submerged diversion dike are performed for various blockage ratios (1/6 ≤ β ≤ 1/3) and Froude numbers (0.152 ≤ Fr ≤ 0.253). The instantaneous and time-averaged flow characteristics, the evolution process of mixing layers, and the bed shear stress of the dike are analyzed. The blockage ratio β plays an important role in the momentum redistribution and turbulent kinetic energy of the diversion dike. As the blockage ratio increases, the flow velocity gradient and the vortex shedding frequency in the mixing layer significantly increase. The Froude number Fr has little effect on the flow field. A large recirculation zone is observed in the wake of the dike, and the recirculation length is more depended on the length of the dike rather than the Froude number. The bed shear stress around the dike increases significantly with the increase in the blockage ratio.

Large eddy simulation of temperature-variation effect of impinging planar lobed synthetic jet on flat plate and the semi-cylindrical concave plate

Temperature-variation becomes increasingly evident when an impinging synthetic jet is restricted and confined excessively by the target plate. However, the confinement effect of the target plate on the temperature-variation effect during impinging heat transfer by a synthetic jet has not been clarified. In this study, impinging heat transfer by a synthetic jet induced by a planar lobed orifice on a flat plate and a semi-cylindrical concave plate is investigated by means of Large Eddy Simulation. The planar lobed synthetic jet is characterized by the geometry of the lobed nozzle, and the velocity along its major diameter is larger than that along its minor diameter. As the mainstream jet moves downstream, the difference between the major and minor diameters decreases gradually. At dimensionless jet-to-surface distance of H/d = 8, the flow field attached to the flat plate along the major diameter is similar to that along the minor diameter. The results show that two types of mechanisms have induced the temperature-variation. For the flat plate, its temperature-variation comes from the suction of the wall jet near the stagnation region confined by the target plate, which becomes more conspicuous particularly at small dimensionless jet-to-surface distance. By contrast, the temperature-variation is less conspicuous at large dimensionless jet-to-surface distance because the inhaling region is distant from the target plate. For the concave plate, its temperature-variation is due to the large recirculation zone along the curvilinear direction. During the suction process of the cycle, the heated wall jet attached to the plate surface is re-inhaled into the cavity through the large recirculation zone, and the temperature of the air inside the cavity increases. During the ejection process of the cycle, the wall jet is entrapped into the jet mainstream and then returns to the target plate. Correspondingly, the mean velocity of the wall jet and the wall shear stress of the target plate are less than those of the flat plate. Time and frequency analyses had revealed the characteristics of inherent aggressive pulsation inside the planar lobed synthetic jet. The velocity in the primary frequency dominates the planar lobed synthetic jet, whereas higher-order velocities are negligible.

Study on Sand-Accumulation Changes of Highway and Formation Mechanism of Sand Damage in Drifting Dunes Areas

After the construction of desert highway, the physiognomy changes caused by surface wind erosion and accumulation not only seriously threaten the stability of road structure, but also have a tremendous impact on the safety of the highway operation and the maintenance work on the highway. The purpose of this paper is to explore the change of sand sedimentation and the law of sand transport along the highway in the moving dune areas, and to clarify the change of sand flow and the formation mechanism of sand damage in the moving dune areas. Taking the test section of Wuhai-Maqin Expressway in the hinterland of Tengger Desert as the research object, the on-site observation of sand accumulation and the recording of wind information by small weather stations were adopted, supplemented by CFD numerical simulation method, in order to provide reference for the construction of sand control system in moving dune areas. The study results show that: (1) Dunes not only obstruct wind-drift sand but are also the sediment source condition for forming road sand. The windward dunes near the road are affected by wind and the deposition of sand will quickly bury the road in the strong wind season. (2) Compared with highways with flat terrain, the existence of dunes affects the flow field structure and the distribution of sand sedimentation on the highway, in which, under the influence of the gathering effect, the flow velocity reaches the maximum at the top of the dune and a large low-speed recirculation zone is formed on the lee side of the dune, easily causing sand accumulation. (3) Sediment accumulates at the windward side of the embankment or dune where sandy air current is easy to saturate. However, with the increase of wind speed, in addition to the grit carried by the sandy air current itself, new sand rolled up on the windward side of the dunes also form deposits on the road surface and the amount of sand-accumulation on highway surface and leeward side tends to increase. As a result, for the highway in drifting sand dunes areas, sediment prevention and control measures should be taken actively. It is necessary to remove sediment from the road in time and reduce the moving speed of sand dunes and the deposition range of wind-sand flow, ultimately for the purpose of reducing the damage wind-sand activity causes to the highway in desert.

Large eddy simulation of soot evolution in turbulent nonpremixed bluff body flames

Large Eddy Simulation (LES) is utilized to investigate soot evolution in a series of turbulent nonpremixed bluff body flames featuring different bluff body diameters. The modeling framework relies on recent development in the soot subfilter Probability Density Function (PDF) model that can correctly account for the distribution of soot with respect to mixture fraction, correcting errors in previous soot subfilter PDF models that significantly overpredict soot oxidation. With the previous soot subfilter PDF model, no soot was observed outside of the recirculation zone in past studies on similar bluff body flames. Results obtained with the current LES modeling approach compare favorably with the experimental measurements of the flow field and the soot volume fraction. Notably, the current LES modeling approach correctly predicts large soot volume fractions in the recirculation zone, a decrease in the soot volume fraction through the high-strain neck region, and then an increase again in the downstream jet-like region. Consistent with the experimental measurements, the larger bluff body diameter, with its larger recirculation zone with longer residence times, generates more soot in the recirculation zone and also more soot in the high-strain neck region. Analysis of the soot volume fraction source terms lead to mechanistic understanding of soot evolution in the entirety of the bluff body flames. Most of the soot generated in the recirculation zone is oxidized but some escapes unoxidized and is passively transported through the neck region. Virtually no new soot forms in the downstream jet-like region, and the increase in the soot volume fraction in the jet-like region is due to acetylene-based surface growth of the soot transported through the neck region. This mechanism could not be predicted with the previous soot subfilter PDF model, with the recent soot subfilter PDF model being critical in the understanding of this basic mechanism.

Hybrid Modeling for Solutions of Sediment Deposition in a Low-Land Reservoir with Multigate Sluice Structure

At the multigate sluice structure on a fluvial river, undesired sediment deposition affects the normal operation of the reservoir in question. Physical and numerical models are hybridized to help explore flow and sedimentation patterns. Field and laboratory investigations show that the deposition is attributable to the formation of large recirculation zones at low and medium discharges. As a potential countermeasure, an array of guide vanes is recommended to cope with the concern. Their attack angle with the flow is a dominant parameter that needs to be evaluated. Tests in the fixed-bed model demonstrate that the vanes bend the reservoir flow towards the sluice and suppress the circulation zones along both banks. The favorable range of attack angle is 15–20°. With the examination of sedimentation of both bed and suspended loads, the numerical modeling indicates that the sediment-removal efficiency increases with an increase in attack angle. By weighing the flushing efficiency and the risk of local scouring at the vanes, the 15° vane layout is recommended. This study is expected to provide a reference for guide-vane design in similar situations.

Design of impeller blades for intensification on fluid mixing process in a stirred tank

BackgroundMany studies reported that the nonlinear geometry could create a longer turbulence production region and higher turbulence intensities compared with conventional geometry. The self-similarity impeller based on Hibert curves was introduced to intensify the fluid mixing process in this work. MethodsThe mixing performance of Rushton turbine (RT) impeller and self-similarity impeller were evaluated using experimental analyzes and computational fluid dynamics (CFD) with detached eddy simulation (DES) approach. The chaotic mixing characteristics, mixing time, power consumption, trailing vortices structures, turbulence intensity, turbulent kinetic energy dissipation rate and velocity magnitude distribution were investigated. Significant findingsResults showed that self-similarity impeller can effectively increase the largest Lyapunov exponent (LLE) and multiscale entropy (MSE) of mixing system, shorten the fluid mixing time and decrease the mixing number (η) compared with RT impeller under the constant power consumption, and an improvement was achieved in mixing efficiency with the increase of self-similar iteration number of self-similarity impeller. Meanwhile, the reductions in the power number (Np) of 34.6% with self-similarity 1 impeller and 51.9% with self-similarity 2 impeller were achieved in a wide Re range compared with RT impeller. Two large recirculation zones and trailing vortices were easy to form at the suction side of rectangular impeller blade, which can be destroyed and decomposed into smaller ones through the jet flows produced by the concave-convex structure of self-similarity impeller. In addition, self-similarity impeller can improve the uniformity of fluid turbulence intensity, enhance the turbulent kinetic energy dissipation rate and increase the axial circulation capacity in the stirred tank, and increasingly so with the self-similar iteration number.