Length Of Recirculation Zone Research Articles

Numerical modelling of oxy-coal combustion to access the influence of swirl strength, combustion environment and gasification reactions on the flow and combustion behaviour

A simplified 2D axisymmetric computational fluid dynamics simulation of oxy-coal combustion is performed to address the influence of swirl strength and the combustion environment on the flow field and combustion characteristics. A quantitative analysis of the internal recirculation zone length and flame length under various operating conditions is studied by employing developed numerical models. The influence of char-CO2 and char-H2O gasification reactions on the species consumption and temperature profile is also studied through numerical modelling. The swirl strength considerably influences both the flow field and temperature profile inside the furnace. At higher swirl strength, radially dispersed shorter flames are obtained due to spreading of the secondary stream. Results showed that the flame shifted towards the burner at higher oxygen content due to rapid devolatilisation at higher flame temperature. The ignition of pulverised coal particles is enhanced at higher oxygen concentration due to increased stoichiometry near the burner. Oxy-35% O2 has 300 K higher flame temperature than oxy-21% O2. Endothermic gasification reactions significantly influence the temperature profile in the furnace. CO2-char gasification reaction has a more pronounced influence than the H2O-char gasification reaction. The consideration of gasification reactions has resulted in an approximately 5% reduction in peak temperature.

Experimental Study on Backflow Patterns Induced by a Bilateral Groin Pair with Different Spacing

Groins are one of the popular manmade structures to modify the hydraulic flow and sediment response in river training. The spacing between groins is a critical consideration to balance the channel-depth and the cost of construction, which is generally determined by the backflow formed downstream from groins. A series of experiments were conducted using Particle Image Velocimetry (PIV) to observe the influence of groin spacing on the backflow pattern of two bilateral groins. The spacing between groins has significant effect on the behavior of the large-scale recirculation cell behind groins. The magnitude of the wake flow induced by a groin was similar to that induced by another groin on the other side, but the flow direction is opposite. The spanwise velocity near the groin tip dictates the recirculation zone width behind the groins due to the strong links between the spanwise velocity and the contraction ratio of channel cross-sections between groins. Based on previous studies and present experimental results, quantitative empirical relationships are proposed to calculate the recirculation zone length behind groins alternately placed at different spacing along riverbanks. This study provides better understanding and a robust formula to assess the backflow extent of alternate groins and identify the optimum groins array configuration.

Numerical Investigation of Active Flow Control on Laminar Forced Convection over a Backward Facing Step Surrounded by Multiple Jets

Laminar, transient forced convection problem over a 2D backward facing step (BFS) at an inlet Reynolds number (Re) of 400 is investigated numerically using OpenFOAM. To increase the Nusselt number (Nu) along the bottom wall, active flow control is applied by zero-net-mass-flux (ZNMF) combinations of suction and injection through three thin slits which are placed on the top, step and the bottom walls in the vicinity of the BFS. The combinations of each jet velocity is determined by jet to inlet mean velocity ratios which are limited to integer numbers between -2 and 2 and satisfying ZNMF condition where negative and positive values indicate suction and injection, respectively. All 19 cases which satisfy these rules are investigated. Average Nusselt number, friction coefficient and recirculation zone lengths are calculated along the bottom wall from time averaged flow fields. Among 19 cases with each having different jet configuration, some cases converged to steady state solution while others indicated temporal effects and converged to periodic solutions. To understand these transient effects, velocity oscillation magnitude and Strouhal number which are monitored at a selected critical point are evaluated. It is shown that temporal interaction of chosen active flow control methodology has significant effect on enhancing mixing which results in an increase of Nusselt number. Among all cases, the best case concerning thermal improvement has an increase of 78.5% in Nu number while the best aerodynamic improvement is achieved for another case with a decrease of 81% in total recirculation zone length compared to the reference case where no control is applied.

Recirculation zone downstream lateral expansions of open channel flow

The present work combines experiments, computational fluid dynamics, and hydraulic models to explain the recirculation length variations downstream of sudden lateral expansions of an open-channel flow. This situation can be compared to the classical turbulent backward facing step situation, but the flow shallowness brings substantial differences. Experiments show that the recirculating zone length relative to the expansion width, L/d, varies in a quite wide range from 2 to 14. The expansion ratio Rb and mostly the bed friction number S influence these variations. Three dimensional numerical simulations confirm these results but also detail the role of a third parameter, the relative water depth h/d. A one dimensional hydraulic model reveals the mechanisms leading to so huge L/d variations. Finally, two dimensional depth-averaged flow equations reveal the prevailing role of bed friction for high S values. Thanks to all these results, the present paper explains the evolution of the recirculation zone length with the different parameters. The shortening of this length is caused by two different mechanisms, corresponding to two asymptotical regimes. For low bed friction numbers S and high relative water depth h/d, large-scale vortices shed from the expansion corner drive the momentum exchanges from the freestream to the recirculation. For high bed friction numbers, their role is negligible: the recirculation zone is accelerated by a velocity leveling due to bed friction, leading to L/d ∝ S−0.7.

Tributary Channel Width Effect on the Flow Behavior in Trapezoidal and Rectangular Channel Confluences

Channel confluences happen commonly in water transport networks and natural rivers. Utilizing a 3D CFD code, a series of numerical simulations were performed using a large eddy simulation turbulence model to investigate the effect of the variations in tributary channel width and the transverse geometrical shape of the main channel on the flow parameters and vertical structure in a T-shape confluence. The code was calibrated using the experimental data from the literature. Flow parameters were considered in ratios of tributary width to the main channel width in trapezoidal and rectangular channels. Results indicate that decreasing the width ratio of the tributary channel to the main channel significantly affects the flow structure in the confluence. Generally, it increases the width and length of the main recirculation zone. It also increases the maximum velocity near the bed, especially in cases with a trapezoidal shape. Besides, it highly affects the structure and formation of the recirculation zone in trapezoidal channels.

Effects of confinement length of the central toroidal recirculation zone partly confined by the small pilot stage chamber on ignition characteristics

Distributed Lean Direct Injection (DLDI) combustor is a new promising low NOx emission combustor. The ignition process independently occurs in the pilot stage chamber, which partly confines the swirled spray at the small confinement ratio. This study investigates ignition characteristics of the partly confined swirled spray experimentally, where effects of confinement length on flame propagation path and ignition probability are mainly considered. Particle image velocimetry, Mie scattering, phase Doppler anemometry and high-speed visualization were used to reveal the mechanisms of flame propagation and stabilization. The ignition probability was firstly counted in three chambers with the length varying from 18 mm to 27 mm, when pressure drop, ΔP, was 3000 Pa, and total equivalence ratio, ϕoverall, varied from 0.67 to 0.73. Results show that reduction of the chamber length increases the ignition probability. To interpret this trend, aerodynamics, droplets characteristics and ignition process in three chambers were analyzed at ΔP=3000 Pa and ϕoverall=0.73. The time-average flow field shows that the modification of chamber length results in changes of the confinement length of the central toroidal recirculation zone (CTRZ). Furthermore, the structure of CTRZ is directly impacted by changes of the confinement length. The reduction of the confinement length makes more air flow back, which means the greater recirculation capability of the CTRZ. More recirculating air can transport more droplets back into the CTRZ before sparking, which can also transport more high-temperature mixture there to help hold flame during ignition process. The high-speed visualization shows that the failed mode of ignition process is that the swirling flame can grow up at the beginning of Phase 3, but will blow out gradually after the extinction of the flame in corner recirculation zone (CRZ). According to characteristics of aerodynamics, droplets characteristics and ignition process, it can be concluded that since significant time is taken for flame to reach the fully burning state, the reduction of the confinement length improves the recirculation capability of the CTRZ which helps the partly confined flame get better stability. In addition, in Phase 2 of ignition process, greater recirculation capability makes the anchored position of the edge flame more upstream, which can make the propagation path shorter.

Numerical simulation of atomization process of nickel-based alloy powders prepared by vacuum induction melting gas atomization

ANSYS Fluent 19.0 software is used to numerically simulate atomization process in the production of nickel-based alloy powders by VIGA. The gas flow field in the atomizing chamber and atomization process of the metal melt is studied. Atomizing air flow is sprayed through the nozzle to form supersonic gas flow. The results show that the recirculation zone is inverted cone, and the length of recirculation zone is 12.28 mm. Under the stagnation point, the velocity direction of the air flow is downward, and the velocity increases gradually (the maximum is 457.43 m/s). In gas-liquid two-phase state, the length of recirculation zone is about 16 mm. The higher the turbulence in the recirculation zone and turbulent layer, the more intense the interaction between primary and secondary atomization.

Experimental study of Partially Premixed-Diffusion Turbulent Flame with Circular Plate Bluff Body

This Study presents a parametric study on flame stabilization of partially premixed-diffusion turbulent flame behind the circular bluff body. The effects of the premixing and equivalence fuel ratios on flame temperature distribution and emission variation along the flame axis are investigated. The results indicate that as the premixing ratio increases the residence time and the recirculation zone length increase while the flame length decreases. The axial temperature distribution enhancements at the recirculation zone, and then a decrease towards the flame tip. As the premixed ratio increases the flame average temperature increase while emission measurements indicate a decrease in CO concentration with a slight increase in NOX production along the flame axis.

Influence of Shear-Thinning Blood Rheology on the Laminar-Turbulent Transition over a Backward Facing Step

Cardiovascular diseases are the leading cause of death globally and there is an unmet need for effective, safer blood-contacting devices, including valves, stents and artificial hearts. In these, recirculation regions promote thrombosis, triggering mechanical failure, neurological dysfunction and infarctions. Transitional flow over a backward facing step is an idealised model of these flow conditions; the aim was to understand the impact of non-Newtonian blood rheology on modelling this flow. Flow simulations of shear-thinning and Newtonian fluids were compared for Reynolds numbers ( R e ) covering the comprehensive range of laminar, transitional and turbulent flow for the first time. Both unsteady Reynolds Averaged Navier–Stokes ( k − ω SST) and Smagorinsky Large Eddy Simulations (LES) were assessed; only LES correctly predicted trends in the recirculation zone length for all R e . Turbulent-transition was assessed by several criteria, revealing a complex picture. Instantaneous turbulent parameters, such as velocity, indicated delayed transition: R e = 1600 versus R e = 2000, for Newtonian and shear-thinning transitions, respectively. Conversely, when using a Re defined on spatially averaged viscosity, the shear-thinning model transitioned below the Newtonian. However, recirculation zone length, a mean flow parameter, did not indicate any difference in the transitional Re between the two. This work shows a shear-thinning rheology can explain the delayed transition for whole blood seen in published experimental data, but this delay is not the full story. The results show that, to accurately model transitional blood flow, and so enable the design of advanced cardiovascular devices, it is essential to incorporate the shear-thinning rheology, and to explicitly model the turbulent eddies.

Computational study of non-reactive swirling flow in tangentially-fired configuration gasifier

This paper presents a computational study of swirling flow in a tangentially-fired air blown entrained flow Mitsubishi Heavy Industries gasifier in a non-reactive environment. Flow fields are obtained by using the Reynolds-averaged Navier Stokes turbulence model. Shear-stress transport k-ω model is used for the modelling the effect of turbulence. Variation in length of the internal recirculation zone with the swirl number for tangentially injected flow is reported. It is observed that for the MHI gasifier the length of the internal recirculation zone in a non-reactive environment increases upon increasing the swirl number at the inlet of the gasifier.

Numerical Study of Laminar Flow Separation in Step Geometry Using Corners Formulation in Lattice Boltzmann Method

In this study, the velocity and temperature distributions of incompressible laminar flow of a step downstream have been investigated using the lattice Boltzmann method. The inlet and outlet flows at internal and external corners of this step have been simulated using the single relaxation time model in various Reynolds number and expansions ratios, two dimensionally. According to Zou and He, constant temperature boundary conditions and open-boundary condition have been utilized at inlet and outlet, respectively. The model results have been validated by experimental data. It was obtained that there are agreements between them. It was resulted that the length of recirculation zone is a function of Reynolds number of inlet flow. Increasing the Prandtl number decays the rate of fluid temperature decrement. Furthermore, in constant Reynolds and Prandtl numbers, decreasing the expansion ratio decreases the Nusselt number of flow. Generally, the ability of lattice Boltzmann method in simulating complex fluid and thermal flows has been shown.

Investigation of Reduced Kinetics Mechanisms for Accurate LES and Scaling of the Dynamics of Premixed Swirling Oxy-Fuel Combustion

ABSTRACT In this paper, we examine the role of reduced chemical kinetics mechanisms in predicting the flow structure using multi-dimensional large-eddy simulations in complex geometries. We investigate the attributes of the kinetics mechanisms required to predict flow structures such as recirculation zones. Premixed methane-oxy combustion is modeled in a swirl-stabilized combustor. Results show that kinetic mechanisms that accurately predict the extinction strain rate of the underlying flames were able to predict the evolution of the flame macrostructure with equivalence ratio and the associated velocity profiles. The recirculation zone length was found to linearly scale with the extinction strain rate, in agreement with previous findings in different fuel-oxidizer mixtures and combustor geometry. The scaling holds regardless of fuel or oxidizer type, Reynold’s number or inlet temperature. Surprisingly the scaling also held well for two different combustor geometries.

Numerical modeling of electrostatic spray painting transfer processes in rotary bell cup for automotive painting

Here we implemented a 3D comprehensive Eulerian-Lagrangian model in order to investigate the electrostatic spray transfer processes in the high-speed rotary bell sprayer. This efficient algorithm contains spray dynamics, airflow, paint droplets tracking and an electrostatic effect to simulate atomization. The algorithm is implemented using the OpenFOAM package. A solver for the particle trajectory was used to illustrate the process of spray transport and also the interaction of the airflow and the particle that is solved by momentum coupling. Creating an initial condition of the particle approach has been proposed that is matched with practical applications. The fluid-dynamics is simulated by solving the unsteady 3D compressible Navier-Stokes equations. Unsteady flow is computed by using a Large eddy simulation (LES) turbulence approach, while the motion of the particles is simulated by tracking the droplet size distribution approach. The model correctly predicts that the bell cup spin forces the paint particles to fall off from the bell surface towards the high-velocity airflow. The present work illustrates a tentative benchmark and contains a systematic analysis of the recirculation zone length, the toroidal vortex, the overspray phenomena and the flowfield characteristics like mean velocity, pressure, turbulent kinetic energy and velocity fluctuation. The results indicate as dominant operating parameter the air-paint flow rate with voltage level deeply affecting the spray shape. A more uniform distribution of the coating is obtained by growing this high-velocity shaping airflow, although the values of the transfer efficiency (TE) are reduced. The distribution of the particle size is very sensitive to changes in the rotational speed. Experimental results obtained in this study put forward a clear link between the shaping air flow rate and the rotation frequency on the aerodynamics and also provide valuable insights to design modern ERBS. The paint spray distribution obtained in the present work is validated against coating experimental results with suitable accuracy.

On the turbulent flow and pollutant emission characteristics of disk stabilized propane-air flames, under inlet mixture stratification and preheat

The reacting, time averaged and fluctuating flow field characteristics of bluff-body stabilized, stratified, propane-air flames have been systematically investigated for a range of conditions, under a combination of inlet mixture stratification and preheat. The burner consists of three concentric disks that form two premixing cavities. Fuel is injected in the first cavity and is partially premixed with air flowing through this cavity system, resulting in a radially stratified equivalence ratio profile at the inlet of the stabilization region. Investigations have been carried out at four levels of preheat temperatures, of the incoming reactants, ranging from 300 to 743 K, for lean and ultra-lean mixtures. The burner set up is capable of anchoring flames of sub-limit mixtures, down to global equivalence ratio values of about 0.13 at 743 K. The time averaged and fluctuating reacting velocity fields, developing in the downstream near wake, have been evaluated with Particle Image Velocimetry (PIV) analysis, indicating a proportionate decrease of the recirculation zone length with increasing fuel levels. OH∗ chemiluminescence emissions revealed a progressive alignment of the reacting front with the flame axis at weaker mixtures, while exhaust gas measurements have been obtained to evaluate the impact of preheat on emissions performance and combustion efficiency.

Influence scope of local loss for pipe flow in plane sudden expansions

A pipe with sudden expansions are used in a wide variety of engineering applications such as hydraulic, civil, and nuclear industries. Flow through a pipe with sudden expansion about its head loss becomes local head loss as the boundary changes. The dynamic water pressure for the rapidly varied flow corresponding to the local head loss has remained inconclusive and inaccurate. It is partly due to the complexity of the vortex involved and partly because of limitations of the traditional experiments. Therefore, in this study, the methods based on theoretical analysis and numerical simulation were employed for the prediction of the influence scope of a plane pipe with sudden expansions. A parametric study is performed for the expansion ratios are 1.5, 2, 3, 4, 5 and in a Reynolds number range of 10 - 100000. On the basis of numerical results, an existing correlation of the influence scope is also extended to take into account the effect of the scale effect additionally. The simulation results show that the influence scope of local head loss is affected greatly by expansion ratio and Reynolds number of the plane pipe, and expansion ratios occurring from the sudden expansions are less sensitive to the Reynolds number. The obtained research results have shown that for the cases when Reynolds number is relatively small (Re < 2000), the recirculation degree (L/D, where L is the length of recirculation zone, D is the diameter of the latter pipe) increases significantly with the increase of the Reynolds number; and for the cases when the Reynolds number is larger (Re > 10000), the recirculation degree changes slowly with the Reynolds number. In general, the length of the influence scope is logarithmically related to the Reynolds number.

A scaling law for the recirculation zone length behind a bluff body in reacting flows

The recirculation zone length behind a bluff body is influenced by the turbulence intensity at the base of the body in isothermal flows and also the heat release and its interaction with turbulence in reacting flows. This relationship is observed to be nonlinear and is controlled by the balance of forces acting on the recirculation zone, which arise from the pressure and turbulence fields. The pressure force is directly influenced by the volumetric expansion resulting from the heat release, whereas the change in the turbulent shear force depends on the nonlinear interaction between turbulence and combustion. This behaviour is elucidated through a control volume analysis. A scaling relation for the recirculation zone length is deduced to relate the turbulence intensity and the amount of heat release. This relation is verified using the large eddy simulation data from 20 computations of isothermal flows and premixed flames that are stabilised behind the bluff body. The application of this scaling to flames in an open environment and behind a backward facing step is also explored. The observations and results are explained on a physical basis.

Simulation of depth and wind effects on the hydraulic efficiency of sedimentation tanks

AbstractIn this research, we aim to investigate the effects of the depth and wind effect on the surface of water on the hydraulic efficiency of the sedimentation tanks in water and wastewater treatments plants. A verified two‐dimensional numerical study was performed to evaluate hydraulic performance of series settling tanks by four different depths of 2.5, 3, 3.5 and 4 (m). Wind velocities of 5 and 7 (m/s) in co‐current and counter‐current direction of water flow in sedimentation tank were applied on the surface of the water. In this study, k–ε turbulent model and passive scalar tracer were used to perform the simulations. The research confirms that wind influence on the surface of water causes recirculation zones and increases the length of recirculation zones. In both windy and normal situation, the Real Hydraulic Retention Time and the effective volume of sedimentation tanks increases widely as the depth of the tank.

Study on the Change of Incident Direction and Incoming Flow Velocity to the Reattachment Point and the Center Position of the Large Vortex

This paper establishes a model with the variation of the standard angle with the height of the step. Numerical simulation is carried out to study the influence of the change of the incident direction and the incoming velocity on the large eddy structure and the length of the recirculation zone in the reattached shear layer under different standard angles. Finding that with the increase of the incident angle β, the length of the recirculation zone tends to increase firstly and then decrease. The large vortex center position Y/H firstly maintains a small range of fluctuations; when the incident angle β reaches a certain value, the large vortex center position suddenly rises sharply to a certain value. The center position of the large vortex decreases again when β continues to increase. When the Reynolds number increases from 150 to 300, the value of Xr/H will suddenly increase sharply and the position of the large vortex will have a significant downward trend. When the Reynolds number is greater than 300, the length of the recirculation zone will remain unchanged from the center of the large vortex. That is the recirculation zone length and the Y/H value of the center position of the large vortex is not affected by the Reynolds number.

Flow separation control over a rounded ramp with spanwise alternating wall actuation

An implicit large-eddy simulation is carried out to study turbulent boundary-layer separation from a backward-facing rounded ramp with active wall actuation control. This method, called spanwise alternating distributed strips control, is imposed onto the flat plate surface upstream of a rounded ramp by alternatively applying out-of-phase control and in-phase control to the wall-normal velocity component in the spanwise direction. As a result, the local turbulence intensity is alternatively suppressed and enhanced, leading to the creation of vertical shear-layers, which is responsible for the presence of large-scale streamwise vortices. These vortices exert a predominant influence on the suppression of the flow separation. The interaction between the large-scale vortices and the downstream recirculation zone and free shear-layer is studied by examining flow statistics. It is found that in comparison with the non-controlled case, the flow separation is delayed, the reattachment point is shifted upstream, and the length of the mean recirculation zone is reduced up to 8.49%. The optimal control case is achieved with narrow in-phase control strips. An in-depth analysis shows that the delay of the flow separation is attributed to the activation of the near-wall turbulence by the in-phase control strips and the improvement of the reattachment location is mainly due to the large-scale streamwise vortices, which enhance the momentum transport between the main flow and separated region.

Comparision of Turbulence Models in Two-dimensional V-gutter Flameholder Flow Field

In this paper, nine common turbulence models are used to numerically simulate the flow field before and after the flame stabilizer. Comparing the flow velocity, normal velocity, vorticity and recirculation zone length between experiments and simulations, it is found that the SA model and the k-kl-ω model have good prediction performance for different parts of the simulation results. At the same time, it is found that the flow in the numerical simulation is more sensitive to the flow field distortion and the separation occurs before the experimental results. The velocity profile obtained by numerical simulation recovers more slowly as the flow develops. Compared with the normal velocity, the simulation results of the streamwise velocity distribution agree well with the experimental results.