Mean Inlet Velocity Research Articles

LES of Nonlinear Saturation in Forced Turbulent Premixed Flames

The mechanisms for nonlinear saturation of a bluff-body stabilised turbulent premixed flame are investigated using LES with the transported flame surface density (TFSD) approach to combustion modelling. The numerical simulation is based on a previous detailed experimental investigation. Results for both the unforced non-reacting and reacting flows are validated against experiment, demonstrating that the fundamental flow features and predicted flame structure are well captured. Key terms in the FSD transport equation are then used to describe the generation and destruction of flame surface area for the unforced reacting flow. In order to investigate the non-linear response of the unsteady heat release rate to acoustic forcing, four harmonically forced flames are considered having the same forcing frequency (160 Hz) but different amplitudes of 10 %, 25 %, 50 % and 64 % of the mean inlet velocity. The flame response is characterised using the Flame Describing Function (FDF). Accurate prediction of the FDF is obtained using the current approach. The computed forced flame structure matches well with the experiment, where effects of shear layer rollup and growth of the vortices on the flame can be clearly observed. Transition to nonlinearity is also observed in the computed FDF. The mechanisms leading to the saturation of the flame response in the higher amplitude case are characterised by inspecting the terms in the FSD transport equation at conditions when the integrated heat release is at its maximum and minimum, respectively. Pinch-off and flame rollup can be seen in snapshots taken at the phase angle of maximum integrated heat release. Conversely, intense vortex shedding and flame-sheet collapse around the shear-layer, as well as small-scale destruction of flame elements in the wake, can be seen in snapshots taken at the phase angle of minimum integrated heat release. The pivotal role of FSD destruction on nonlinear saturation of the flame response is confirmed through the analysis of phase-averaged terms in the FSD transport equation taken at different locations. The phase-averaged subgrid curvature term is found to concentrate in the cusps and downstream regions where flame annihilation is dominant.

Theoretical Results for Utilizing Nozzle Between the Wind-Way and Wind Turbine in Roof of the Buildings - Wind Speed Increase for Wind Turbine to Produce Electricity

In order to increase the electrical power of the wind turbines, the velocity of the wind blowing on the wind turbine, is the most important factor that has to increase. In this paper it has been recommended that Contraction Nozzles could be applied between Wind Turbines and wind-way to provide the wind through themselves with more velocity. For all cases analysed in this paper, a three dimensional contraction nozzle with the same length (3 meters) but different input (in which wind blows through it) and output segment areas is considered. In the presented calculations, the inlet mean velocity is considered a constant value of in a windward area. Numerical solutions and CFD results have been the same for increased wind velocity in each Nozzle outlet or wind turbine’s inlet. Furthermore, the electrical power of related wind turbines has been numerically calculated in the presence and absence of the nozzles between wind turbines and wind flow and it shows a dramatic increase in wind turbines power.

Study on gas-particle heat transfer in oscillating flows

A numerical study of the gas-particle cluster interphase heat transfer in oscillating flows is conducted by using the one-dimensional Eulerian-Eulerian model. The effects of flow oscillation on the two-phase flow, particle concentration and as well the interphase heat transfer rate are investigated. The results show that the heat transfer between the gas and particle clusters can lead to the increases of the average particle concentration and the corresponding oscillation amplitude, but the decrease of the oscillation wavelength, which are closely related to the interphase heat transfer during the particle velocity relaxation process. Convective particle clustering is the primary cause of the oscillation of the two-phase mixture equilibrium temperature. Particle initial velocity and gas inlet velocity have great influences on the particle concentration and temperature. Increases in the gas inlet mean velocity and oscillation frequency are beneficial for the reduction in particle and gas temperature oscillation amplitudes. Particle diameter has an insignificant influence on the averaged values of the two-phase equilibrium mixture parameters but a noted impact on the oscillation amplitude of particle concentration. The increase in particle diameter is helpful for the reduction of the oscillation amplitude of the two-phase mixture equilibrium temperature.

Effects of cylindrical and conical heights on pressure and velocity fields in cyclones

Nine modifications of Stairmand high-efficiency type cyclone with various cylindrical and conical heights were used to investigate their effects on pressure drop and flow field within cyclones. An experimental setup was built and experiments were conducted on various cyclone geometries at inlet velocities ranging from 10 to 18.5m/s. The body heights ranged from D to 2D (D being the diameter of the cyclone body), while the conical heights were between 2D and 3D. The experimental results were used to calibrate CFD (Computational Fluid Dynamics) model. Experimental results showed that the pressure drop ranged from 191 to 235Pa and 690 to 920Pa at the lowest and highest inlet velocities, respectively, and that pressure drop is a function of both cylindrical and conical heights with reduced pressure drops as cylindrical and/or conical heights increase. Maps showing the change in pressure drop with respect to cylindrical and conical heights were prepared and interpreted to determine optimum ratio of conical-to-cylindrical height for reduced pressure drop. Optimum ratio was found experimentally as between 1.67 and 2.5. CFD results agreed well with the experimental data and the results helped gain insights into complicated phenomena taking place in cyclones. CFD results showed that increased axial velocities at the lip of vortex finder distrub radial velocity profiles and that tangential velocities in the stabilized vortex can be as high 1.4 times the mean inlet velocity. Both findings indicate that collection efficiency of the cyclone may be influenced by the height of the cyclone.

Filtration of aerosol particles by cylindrical fibers within a parallel and staggered array

The World Health Organization (WHO) in 2013 reported that more than seven million unexpected losses every year are credited to air contamination. Because of incredible adaptability and expense viability of fibrous filters, they are broadly used for removing particulates from gasses. The influence of appropriate parameters, e.g., the fiber arrangement, solid volume fraction (SVF or ?), fluid flow face velocity (mean inlet velocity), and filter thickness (Ix), on pressure drop and deposition efficiency are researched. Furthermore, to study the effects of variation of the laminar flow regime and fiber's cross-sectional shape on the deposition of particles, only a single square fiber has been placed in a channel. By means of finite volume method (FVM), the 2-D motion of 100---1000 nm particles was investigated numerically. The Lagrangian method has been employed and the Saffman's lift, Drag, and Brownian forces have been considered to affect this motion. Contribution of increasing the Reynolds number to filtration performance increased with smaller fine aerosols to a level of 59.72 %. However, for over 500 nm, the Re = 100 has more efficient results up to 26.97 %. Remarkably, the single square fiber in Re = 200 regime performs similarly to the optimum choice of multi-fibrous filters. It was portrayed the parallel circular multi-fibrous filter with a ratio of horizontal-to-vertical distances between fibers, l/h = 1.143; ? = 0.687, Ix = 116.572, and h/df = 1.0 is the most efficient filter's structure. The increase in the ratio of vertical distances between fibers-to-fiber's diameter (h/df) and decrease in SVF or ?, results in a drastically decrement of the filtration performance of both parallel and staggered structures. The obtained results have been validated with previous research findings.

A fully synthetic turbulent boundary condition with a homogeneous vortex distribution

Temporally and spatially resolved simulations of turbulent flow need realistic inlet velocity fluctuations. The vortex method, described and implemented in this work, adds vortices to the inlet mean flow profile. The vortices are randomly distributed over the inlet and they move across the extent of the inlet with a finite life-time. The initial vortex sizes, strengths, as well as their developing motion and life-times, are determined by the inlet mean velocity and turbulence distributions. In contrast to previous studies of the vortex method, the inlet mean velocity and turbulence distributions are in the present work theoretically determined, and thus form an explicit part of the boundary condition. This makes the method independent of precursor RANS simulations. Special care has been taken to distribute the vortices evenly across the inlet. The tangential velocity of the vortices is randomly initialized and the spatial and temporal correlation is preserved. An edge bouncing mechanism is implemented at solid boundaries.The boundary condition is applied to a DNS simulation of pipe flow at a Reynolds number of 5300, based on pipe diameter, aiming at reaching fully developed conditions at a short distance from the inlet. The results are compared with those using cyclic boundary conditions, and with two DNS results found in the literature. It is shown that a good agreement is reached for the mean velocity profiles five pipe diameters downstream from the inlet. Also the resolved velocity fluctuations at that location are reasonable. It is concluded that the present implementation of the vortex method boundary condition, based on theoretical mean velocity and turbulence profiles, gives an inlet flow that is sufficiently realistic to reach a well-resolved fully developed turbulent pipe flow at five pipe diameters downstream the inlet.

A pressure-gradient mechanism for vortex shedding in constricted channels

Numerical simulations of the unsteady, two-dimensional, incompressible Navier-Stokes equations are performed for a Newtonian fluid in a channel having a symmetric constriction modeled by a two-parameter Gaussian distribution on both channel walls. The Reynolds number based on inlet half-channel height and mean inlet velocity ranges from 1 to 3000. Constriction ratios based on the half-channel height of 0.25, 0.5, and 0.75 are considered. The results show that both the Reynolds number and constriction geometry have a significant effect on the behavior of the post-constriction flow field. The Navier-Stokes solutions are observed to experience a number of bifurcations: steady attached flow, steady separated flow (symmetric and asymmetric), and unsteady vortex shedding downstream of the constriction depending on the Reynolds number and constriction ratio. A sequence of events is described showing how a sustained spatially growing flow instability, reminiscent of a convective instability, leads to the vortex shedding phenomenon via a proposed streamwise pressure-gradient mechanism.

Impact of flame-wall interaction on premixed flame dynamics and transfer function characteristics

In this paper, we numerically investigate the response of a perforated-plate stabilized laminar methane–air premixed flame to imposed inlet velocity perturbations. A flame model using detailed chemical kinetics mechanism is applied and heat exchange between the burner plate and the gas mixture is incorporated. Linear transfer functions, for low mean inlet velocity oscillations, are analyzed for different equivalence ratio, mean inlet velocity, plate thermal conductivity and distance between adjacent holes. The oscillations of the heat exchange rate at the top of the burner surface plays a critical role in driving the growth of the perturbations over a wide range of conditions, including resonance. The flame response to the perturbations at its base takes the form of consumption speed oscillations in this region. Flame stand-off distance increases/decreases when the flame-wall interaction strengthens/weakens, impacting the overall dynamics of the heat release. The convective lag between the perturbations and the flame base response govern the phase of heat release rate oscillations. There is an additional convective lag between the perturbations at the flame base and the flame tip which has a weaker impact on the heat release rate oscillations. At higher frequencies, the flame-wall interaction is weaker and the heat release oscillations are driven by the flame area oscillations. The response of the flame to higher amplitude oscillations are used to gain further insight into the mechanisms.

Performance of Three-Dimensional 180°-Curved Diffuser (Dept.M)

Flow characteristics in AR=2, AS = 2, 180° curved diffuser is investigated experimentally by a 5-hole Pitot tube, The Pitot tube is calibrated by a special orientation mechanism to get a calibration data file to be processed by the multi-probe program. Measurements of the three velocity components (u, v, w), total and static pressures for the inlet and six downstream sections located at 30° intervals from the diffuser inlet, from convex to concave walls and top to bottom walls. The wall static pressures in the inid-span between the top and bottom walls at convex and concave walls were measured. Contours of the third velocity component show the secondary motion clearly induced by the movement of fluid from convex to concave wall and reverse. The maximum transverse velocity is about 20% of inlet mean velocity as a result of low aspect ratio of the diffuser exit. The overall pressure recovery achieved was 25.6% of the inlet dynamic pressure due to separation and recirculation occurred at the convex wall. These measurements can be used for the validation and inprovement of the turbulence models that used to simulate the flow inside the curved diffuser. It was found that the models predict a later

Large-scale tomographic particle image velocimetry using helium-filled soap bubbles

To measure large-scale flow structures in air, a tomographic particle image velocimetry (tomographic PIV) system for measurement volumes of the order of one cubic metre is developed, which employs helium-filled soap bubbles (HFSBs) as tracer particles. The technique has several specific characteristics compared to most conventional tomographic PIV systems, which are usually applied to small measurement volumes. One of them is spot lights on the HFSB tracers, which slightly change their position, when the direction of observation is altered. Further issues are the large particle to voxel ratio and the short focal length of the used camera lenses, which result in a noticeable variation of the magnification factor in volume depth direction. Taking the specific characteristics of the HFSBs into account, the feasibility of our large-scale tomographic PIV system is demonstrated by showing that the calibration errors can be reduced down to 0.1 pixels as required. Further, an accurate and fast implementation of the multiplicative algebraic reconstruction technique, which calculates the weighting coefficients when needed instead of storing them, is discussed. The tomographic PIV system is applied to measure forced convection in a convection cell at a Reynolds number of 530 based on the inlet channel height and the mean inlet velocity. The size of the measurement volume and the interrogation volumes amount to 750 mm × 450 mm × 165 mm and 48 mm × 48 mm × 24 mm, respectively. Validation of the tomographic PIV technique employing HFSBs is further provided by comparing profiles of the mean velocity and of the root mean square velocity fluctuations to respective planar PIV data.

Two-dimensional simulations of steady perforated-plate stabilized premixed flames

The objective of this work is to examine the impact of the operating conditions and the perforated-plate design on the steady, lean premixed flame characteristics. We perform two-dimensional simulations of laminar flames using a reduced chemical kinetics mechanism for methane–air combustion, consisting of 20 species and 79 reactions. We solve the heat conduction problem within the plate, allowing heat exchange between the gas mixture and the solid plate. The physical model is based on a zero-Mach-number formulation of the axisymmetric compressible conservation equations. The results suggest that the flame consumption speed, the flame structure, and the flame surface area depend significantly on the equivalence ratio, mean inlet velocity, the distance between the perforated-plate holes and the plate thermal conductivity. In the case of an adiabatic plate, a conical flame is formed, anchored near the corner of the hole. When the heat exchange between the mixture and the plate is finite, the flame acquires a Gaussian shape stabilizing at a stand-off distance, that grows with the plate conductivity. The flame tip is negatively curved; i.e. concave with respect to the reactants. Downstream of the plate, the flame base is positively curved; i.e. convex with respect to the reactants, stabilizing above a stagnation region established between neighboring holes. As the plate's thermal conductivity increases, the heat flux to the plate decreases, lowering its top surface temperature. As the equivalence ratio increases, the flame moves closer to the plate, raising its temperature, and lowering the flame stand-off distance. As the mean inlet velocity increases, the flame stabilizes further downstream, the flame tip becomes sharper, hence raising the burning rate at that location. The curvature of the flame base depends on the distance between the neighboring holes; and the flame there is characterized by high concentration of intermediates, like carbon monoxide.

The influence of inlet velocity profile and secondary flow on pulsatile flow in a model artery with stenosis

The results of an experimental investigation to determine the influence of two physiologically relevant inlet conditions on the flow physics downstream of an idealized stenosis are presented. The two inlet conditions are an asymmetric mean inlet velocity profile and an asymmetric mean inlet velocity profile plus secondary flow, as found downstream of a bend. The stenosis is modelled as an axisymmetric 75% area reduction occlusion with a length-to-diameter ratio of 2. The flow was forced by a 10-harmonic carotid artery-inspired waveform with mean, maximum and minimum Reynolds numbers of 364, 1424 and 24, respectively, and a Womersley number of 4.6. Laser Doppler velocimetry and particle image velocimetry were used to characterize the spatial and temporal evolution of a baseline case (no disturbances) as well as the two physiologically relevant inlet conditions. The asymmetric inlet velocity profile was found to reduce the region of influence of the stenosis by forcing the stenotic jet towards the tube wall via an induced non-uniform radial pressure gradient, similar to the Coanda effect. Curvature-induced secondary flow was found to play a minor role in the near-stenosis region. Vortex ring formation was relatively unaffected by the mean velocity gradient and secondary flow. Evidence of remnants of the starting vortex ring was observed far downstream in all cases.

CFD and PTV Steady Flow Investigation in an Anatomically Accurate Abdominal Aortic Aneurysm

There is considerable interest in computational and experimental flow investigations within abdominal aortic aneurysms (AAAs). This task stipulates advanced grid generation techniques and cross-validation because of the anatomical complexity. The purpose of this study is to examine the feasibility of velocity measurements by particle tracking velocimetry (PTV) in realistic AAA models. Computed tomography and rapid prototyping were combined to digitize and construct a silicone replica of a patient-specific AAA. Three-dimensional velocity measurements were acquired using PTV under steady averaged resting boundary conditions. Computational fluid dynamics (CFD) simulations were subsequently carried out with identical boundary conditions. The computational grid was created by splitting the luminal volume into manifold and nonmanifold subsections. They were filled with tetrahedral and hexahedral elements, respectively. Grid independency was tested on three successively refined meshes. Velocity differences of about 1% in all three directions existed mainly within the AAA sack. Pressure revealed similar variations, with the sparser mesh predicting larger values. PTV velocity measurements were taken along the abdominal aorta and showed good agreement with the numerical data. The results within the aneurysm neck and sack showed average velocity variations of about 5% of the mean inlet velocity. The corresponding average differences increased for all velocity components downstream the iliac bifurcation to as much as 15%. The two domains differed slightly due to flow-induced forces acting on the silicone model. Velocity quantification through narrow branches was problematic due to decreased signal to noise ratio at the larger local velocities. Computational wall pressure and shear fields are also presented. The agreement between CFD simulations and the PTV experimental data was confirmed by three-dimensional velocity comparisons at several locations within the investigated AAA anatomy indicating the feasibility of this approach.

Large eddy simulation of particle response to turbulence along its trajectory in a backward-facing step turbulent flow

In order to understand particle response to turbulence along its path, properties of the particle phase and gas phase are compared and analyzed for a turbulent flow over a backward-facing step. The turbulent gas phase is simulated numerically using large eddy simulation and the particle phase is modeled by means of Lagrangian methods. The particle Stokes number ranges from 0.01 to 111.18 and the Reynolds number is 18,400, based on the step height and the inlet mean velocity. Particle velocities, fluid velocities and vorticity along particle trajectory as well as particle dispersion are obtained so as to provide information on particle response to turbulence. Results show that particle behavior depends heavily on the local fluid turbulence along its path, especially for small particles. Particles follow a path along which the gas-phase vorticity is small. However, large particles maintain the inertia along their trajectories without responding to the fluid fluctuations.

ANALYSIS OF ENERGY SPECTRA BY THE VORTEX METHOD WITH ARTIFICIAL TURBULENCE

First, in order to use as an inlet condition for turbulent simulation, a method is presented which produces numerically an artificial turbulence, namely, a series of velocity fluctuations of which frequency is Gaussian, and energy spectrum and root mean square correspond to the given ones. Besides, the fluctuation data are determined by the characteristic parameters of turbulent flows such as the inlet mean velocity, the kinematic viscosity, the Kolmogorov scale and the integral time scale. Our examples show excellent accuracy and flexibility of the method. Secondly, the vortex method has been studied to see the ability of the method to deal with turbulent flows. It is found that the energy spectra produced by this agree well with the ones given as the inlet condition, and that the vortex method is able to produce turbulent flows with the given parameters described above.

ANALYSIS OF ENERGY SPECTRA BY THE VORTEX METHOD WITH ARTIFICIAL TURBULENCE

First, in order to use as an inlet condition for turbulent simulation, a method is presented
which produces numerically an artificial turbulence, namely, a series of velocity fluctuations
of which frequency is Gaussian, and energy spectrum and root mean square correspond to
the given ones. Besides, the fluctuation data are determined by the characteristic parameters
of turbulent flows such as the inlet mean velocity, the kinematic viscosity, the Kolmogorov
scale and the integral time scale. Our examples show excellent accuracy and flexibility of
the method. Secondly, the vortex method has been studied to see the ability of the method
to deal with turbulent flows. It is found that the energy spectra produced by this agree well
with the ones given as the inlet condition, and that the vortex method is able to produce
turbulent flows with the given parameters described above.

Assessment of CHF Enhancement Mechanisms in a Curved, Rectangular Channel Subjected to Concave Heating

An experimental study was undertaken to examine the enhancement in critical heat flux (CHF) provided by streamwise curvature. Curved and straight rectangular flow channels were fabricated with identical 5.0 × 2.5 mm cross sections and heated lengths of 101.6 mm in which the heat was applied to only one wall—the concave wall (32.3 mm radius) in the curved channel and a side wall in the straight. Tests were conducted using FC-72 liquid with mean inlet velocity and outlet subcooling of 0.25 to 10 m s−1 and 3 to 29°C, respectively. Centripetal acceleration for curved flow reached 315 times earth’s gravitational acceleration. Critical heat flux was enhanced due to flow curvature at all conditions but the enhancement decreased with increasing subcooling. For near-saturated conditions, the enhancement was approximately 60 percent while for highly subcooled flow it was only 20 percent. The causes for the enhancement were identified as (1) increased pressure on the liquid-vapor interface at wetting fronts, (2) buoyancy forces and (3) increased subcooling at the concave wall. Flow visualization tests were conducted in transparent channels to explore the role of buoyancy forces in enhancing the critical heat flux. These forces were observed to remove vapor from the concave wall and distribute it throughout the cross section. Vapor removal was only effective at near-saturated conditions, yielding the observed substantial enhancement in CHF relative to the straight channel.

An experimental and numerical study of a 90° bifurcation

The flow field of a 90° square duct bifurcation was examined both experimentally and numerically for a Reynolds number of 1200 and equal branch flow rates. Velocity measurements were carried out with a one-component LDV, recording, besides the time mean streamwise velocity component, its time variation due to vortex shedding which took place in the extension of the main duct, as well as secondary flow velocities in the 90° branch. Flow visualization provided additional information about the three-dimensional character of the flow field by recording the paths of neutrally buoyant solid particles. The experimental data were compared with the results of a numerical method, constituting an extension of the well-known SOLA method to a 3-D generalized coordinate system, presented here for the first time. These comparisons were very successful in the vertical branch but less satisfactory in the horizontal branch where unsteadiness was very strong. The characteristic frequency of the velocity fluctuations in the detached shear layer of the horizontal branch was very well predicted but the velocity amplitudes were underestimated. The predicted secondary flow in the vertical branch was compared well with the experimental data presenting maximum values close to the inlet mean velocity.

Two-Dimensional Simulation of Air Flow in the Transfer Channel of Open-End Rotor Spinning Machines

A two-dimensional computational fluid dynamics model is developed to simulate the flow patterns inside the transportation zone of a rotor spinning machine and analyze their effects on yarn properties. The strength and area of recirculation at the inlet of the transfer channel caused by the geometric variation and rotating opening roller are significantly related to fluid flow, and consequently to rotor spun yarn prop erties. Any variation of geometric dimensions of the transfer channel or opening roller velocity affects the air flow pattern, which then alters the configuration of fibers flowing inside the channel. A larger inlet at the transfer channel will lead to a decrease in the inlet mean velocity and will strengthen the primary recirculation zone, which is a potential source for fiber curving and buckling. The ratio of the circumferential velocity of opening roller to mean air flowing velocity is identified as a critical parameter to classify differences in flow patterns. If the ratio is a constant, the flow pattern will not explicitly change, but the drag force will increase with the increase in Reynolds number. Additionally, if the ratio is either too high or too low, the flow is not suitable for maintaining high quality in spun yarns due to the occurrence of strong recirculation at the outer cover side or opening roller side.

Gas and particle flow patterns in cyclones at room and elevated temperatures

AbstractExperimental results are presented for a study of gas and particle flows in a 102 mm diameter conventional cyclone operated at temperatures between 300 and 2000 K. Inlet gas velocities ranged from 3 to 42 m/s. Particle deposition patterns and the measurements of local pressures were used to determine the flow patterns and velocity profiles within the cyclone. A “Reynolds Number” has been defined based on the mean inlet velocity and the hydraulic diameter of the annulus between the cyclone wall and the gas outlet duct. An empirical equation was derived to correlate the ratio of the wall tangential velocity to the mean inlet velocity with this Reynolds Number.