Laminar Flow and Heat Transfer Characteristics in Jackets of Triangular Flow Channels

Laminar flow and heat transfer characteristics of jacketed vessel with triangular flow channels were numerically studied under hydrodynamically and thermally fully developed conditions. Constant heat flux at theheated wall was assumed. The numerical program code interms of vorticity, stream function, axial velocity com ponent and energy equations was written based on a finite volume method. Based on the numerical results, the flow and temperature field were given, and the effects of Dean and Prandtl numbers on flow and heat transfer were ex amined, and the correlations of flow resistance and mean Nusselt number were developed for the jacket. The results show that the structure of secondary flow is steady two vortices in the investigated range of dimensionless curvatureratio and Reynolds number. Two peaks of local Nusselt number increase significantly with Prandtl and Dean num ber increasing, but the local Nusselt numbers near two ends and at the center of the heated wall increase only slightly. The center and two ends of heated wall are the poor positions for heat transfer in the jacket. Compared with the outer half coil jacket at the same area of heated wall, curvature radius, Reynolds number and Prandtl number, e jacket of triangular flow chmnel has lower flow resistance and less mean Nusselt number.

A comprehensive review of the modeling of transport phenomenon in the flow channels of polymer electrolyte membrane fuel cells

Reactant gas and liquid water transport phenomena in the flow channels are complex and critical to the performance and durability of polymer electrolyte membrane fuel cells.The polymer membrane needs water at an optimum level for proton conductivity.Water management involves the prevention of dehydration,waterlogging,and the cell’s subsequent performance decline and degradation.This process requires the study and understanding of internal two-phase flows.Different experimental visualization techniques are used to study two-phase flows in polymer electrolyte membrane fuel cells.However,the experiments have limitations in in situ measurements;they are also expensive and time exhaustive.In contrast,numerical modeling is cheaper and faster,providing insights into the complex multiscale processes occurring across the components of the polymer electrolyte membrane fuel cells.This paper introduces the recent design of flow channels.It reviews the numerical modeling techniques adopted for the transport phenomena therein:the two-fluid,multiphase mixture,volume of fluid,lattice Boltzmann,and pressure drop models.Furthermore,this work describes,compares,and analyses the models’approaches and reviews the representative results of some selected aspects.Finally,the paper summarizes the modeling perspectives,emphasizing future directions with some recommendations.

On the analytical solution of transient friction in channel flows

The transient friction in channel mean flows is the sum of two contributions,i.e.,the underlying laminar flow(ULF)and the purely turbulent component(PTC),and the contributions are analyzed separately by theoretical experiments.It is found that,the transient friction may be higher or remarkably lower than that in equal-Reynolds number steady-state flows.The universal time constant for plane-parallel laminar flows is reported,and the role of the time constant in a turbulent mean flow is examined.It is shown that the time constant is related to the turbulence's frozen time.Finally,a study of the logarithmic layer during the transient flow is accomplished,which shows that the logarithmic layer is destroyed.

Electroosmotic oscillatory flow of micropolar fluid in microchannels:application to dynamics of blood flow in microfluidic devices

The electroosmotic flow of a micropolar fluid in a microchannel bounded by two parallel porous plates undergoing periodic vibration is studied. The equations for conservation of linear and angular momentums and Gauss＇s law of charge distribution are solved within the framework of the Debye-Hückel approximation. The fluid velocity and microrotation are assumed to depend linearly on the Reynolds number. The study shows that the amplitude of microrotation is highly sensitive to the changes in the magnitude of the suction velocity and the width of the microchannel. An increase in the micropolar parameter gives rise to a decrease in the amplitude of microrotation. Numerical estimates reveal that the microrotation of the suspended microelements in blood also plays an important role in controlling the electro-osmotically actuated flow dynamics in microbio-fluidic devices.

The properties of dilute debris flow and hyper-concentrated flow in different flow regimes in open channels

Particle Image Velocimetry(PIV) technique was used to test the analogues of hyperconcentrated flow and dilute debris flow in an open flume. Flow fields, velocity profiles and turbulent parameters were obtained under different conditions. Results show that the flow regime depends on coarse grain concentration. Slurry with high fine grain concentration but lacking of coarse grains behaves as a laminar flow. Dilute debris flows containing coarse grains are generally turbulent flows. Streamlines are parallel and velocity values are large in laminar flows. However, in turbulent flows the velocity diminishes in line with the intense mixing of liquid and eddies occurring. The velocity profiles of laminar flow accord with the parabolic distribution law. When the flow is in a transitional regime, velocity profiles deviate slightly from the parabolic law. Turbulent flow has an approximately uniform distribution of velocity and turbulent kinetic energy. The ratio of turbulent kinetic energy to the kinetic energy of time-averaged flow is the internal cause determining the flow regime: laminar flow(k/K<0.1); transitional flow(0.1< k/K<1); and turbulent flow(k/K>1). Turbulent kinetic energy firstly increases with increasing coarse grain concentration and then decreases owing to the suppression of turbulence by the high concentration of coarse grains. This variation is also influenced by coarse grain size and channel slope. The results contribute to the modeling of debris flow and hyperconcentrated flow.

Artificial neural network-based subgrid-scale models for LES of compressible turbulent channel flow

Fully connected neural networks(FCNNs)have been developed for the closure of subgrid-scale(SGS)stress and SGS heat flux in large-eddy simulations of compressible turbulent channel flow.The FCNNbased SGS model trained using data with Mach number Ma=3.0 and Reynolds number Re=3000 was applied to situations with different Mach numbers and Reynolds numbers.The input variables of the neural network model were the filtered velocity gradients and temperature gradients at a single spatial grid point.The a priori test showed that the FCNN model had a correlation coefficient larger than 0.91 and a relative error smaller than 0.43,with much better reconstructions of SGS unclosed terms than the dynamic Smagorinsky model(DSM).In a posteriori test,the behavior of the FCNN model was marginally better than that of the DSM in predicting the mean velocity profiles,mean temperature profiles,turbulent intensities,total Reynolds stress,total Reynolds heat flux,and mean SGS flux of kinetic energy,and outperformed the Smagorinsky model.

NUMERICAL INVESTIGATION OF THREE-DIMENSIONAL VISCOUS INCOMPRESSIBLE FLOWS IN DIVERGENT CURVED CHANNELS AND TURBULENT MODEL STUDY

In order to make the numerical calculation of viscous flows more convenient for the flows in channel with complicated profile governing equations expressed in the arbitrary curvilinear coordinates were derived by means of Favre density-weighted averaged method, and a turbulent model with effect of curvature modification was also derived. The numerical calculation of laminar and turbulent flown in divergent curved channels was carried out by means of parabolizeil computation method. The calculating results were used to analyze and investigate the aerodynamic performance of talor cascades in compressors preliminarily.

Numerical Analysis of Flow-Induced Vibration and Noise Generation in a Variable Cross-Section Channel

Flow channels with a variable cross-section are important components of piping system and are widely used in variousfields of engineering.Using afinite element method and modal analysis theory,flow-induced noise,mode shapes,and structure-borne noise in such systems are investigated in this study.The results demonstrate that the maximum displacement and equivalent stress are located in the part with variable cross-sectional area.The aver-age excitation force on theflow channel wall increases with theflow velocity.The maximum excitation force occurs in the range of 0–20 Hz,and then it decreases gradually in the range of 20–1000 Hz.Additionally,as theflow velocity rises from 1 to 3 m/s,the overall sound pressure level associated with theflow-induced noise grows from 49.37 to 66.37 dB.Similarly,the overall sound pressure level associated with the structure-borne noise rises from 40.27 to 72.20 dB.When theflow velocity is increased,the increment of the structure-borne noise is higher than that of theflow-induced noise.

On interaction between freely moving bodies and fluid in a channel flow

The interaction between free fast-moving bodies(or particles)and the fluid surrounding them is studied,motivated by applications in different branches of industry,biomedicine,the environment and science such as flying droplets,ice growth,dust,impacts,food grains,sport,complexity and storms.New inviscid-based modelling and results on the behaviour of two interacting bodies inside a channel flow are described.This is followed by discussion of the more-bodies extension with a view to treating arrays of bodies in a rational manner.Significant dependences on initial conditions and on the comparative body masses and moments of inertia are found for the occurrence of body-body impacts as opposed to wall-body impacts and for the associated impact times.

Slip boundary effect on the critical Reynolds number of subcritical transition in channel flow

In this letter,the effect of slip boundary on the origin of subcritical transition in two-dimensional chan-nel flows is studied numerically and theoretically.It is shown that both the positive and the negative slip lengths will increase the critical Reynolds number of localized wave packet and hence postpone the transition.By applying a variable transformation and expanding the variables about a small slip length,it is illustrated that the slip boundary effect only exists in the second and higher order modulations of the no-slip solution,and hence explains the power law found in simulations,i.e.the relative increment of the critical Reynolds number due to the slip boundary is proportional to the square of the slip length.

Hybrid URANS/LES Method of Flow Fields in Axial-flow Compressor Rotor Rotor

Accurate and efficient prediction of the aerodynamic performance and flow details of axial-flow com-pressors is of great engineering application value for the aerodynamic design and flow control of axial-flow compres-sors.In this work,a delayed detached eddy simulation method is developed and applied to numerically simulate the tur-bulent channel flow and the aerodynamic performance of NASA Rotor 35.Several acceleration techniques including parallel implementation are also used to speed up the iteration convergence.The mean velocity distribution and Reyn-olds stress distribution in the boundary layer of turbulent channel flow and the aerodynamic performance curve of NASA Rotor 35 are predicted.The good agreement between the present delayed detached eddy simulation results and the available direct numerical simulation results or experimental data confirms the effectiveness of the developed meth-od in the accurate and efficient prediction of complex flow in turbomachinery.

Utility Impact below Bridge or Culvert Soffit on Open Channel Flow

The background to this research was a flooding incident that occurred in Bridgend, Co. Donegal, Ireland in August 2017. While several properties were flooded, a flooding case study of a single dwelling house adjacent to the Bridgend River at Riverdale, Bunamayne, Co. Donegal, Ireland is used herein. For this study the flooded site shall be referred to as the “Hegarty property”. A structure in the form of a stone arched culvert is located directly adjacent to the two-storey detached dwelling house on the Hegarty Property. While the culvert is referred to locally as a bridge, within this research the word culvert will be used in connection with the structure. The culvert has a concrete surrounded utility (watermain) crossing at a gradient below the culvert soffit on the upstream face of the structure. The utility obstructed flow through the culvert and contributed to the flooding event. Given the implication of climate change and the increased probability of more extreme flooding events, it was decided to explore the case study to ascertain the factors that contribute to flooding events when utilities are positioned at culvert or bridge structures. This work was completed to assist undergraduate students, researchers, and local authorities in a relatively unknown area of flood causation.

Analysis of the Influence of Geometrical Parameters on the Performance of a Proton Exchange Membrane Fuel Cell

A suitable channel structure can lead to efficient gas distribution and significantly improve the power density of fuel cells.In this study,the influence of two channel design parameters is investigated,namely,the ratio of the channel width to the bipolar plate ridge width(i.e.,the channel ridge ratio)and the channel depth.The impact of these parameters is evaluated with respect to the flow pattern,the gas composition distribution,the temperature field and the fuel cell output capability.The results show that a decrease in the channel ridge ratio and an increase in the channel depth can effectively make the distributions of velocity,temperature and concentration more uniform in each channel and improve the output capability of the fuel cell.An increase in the channel ridge ratio and depth obviously reduces the flow resistance and improves the flow characteristics.

Structural ensemble dynamics based closure model for wall-bounded turbulent flow

Wall-bounded turbulent flow involves the development of multi-scale turbulent eddies, as well as a sharply varying boundary layer. Its theoretical descriptions are yet phenomenological. We present here a new framework called structural ensemble dynamics （SED）, which aims at using systematically all relevant statistical properties of turbulent structures for a quantitative description of ensemble means. A new set of closure equations based on the SED approach for a turbulent channel flow is presented. SED order functions are defined, and numerically determined from data of direct numerical simulations （DNS）. Computational results show that the new closure model reproduces accurately the solution of the original Navier-Stokes simulation, including the mean velocity profile, the kinetic energy of the streamwise velocity component, and every term in the energy budget equation. It is suggested that the SED-based studies of turbulent structure builds a bridge between the studies of physical mechanisms of turbulence and the development of accurate model equations for engineering predictions.

Optimization Design of Hydraulic Valve Block and Its Internal Flow Channel Based on Additive Manufacturing

Hydraulic valve block is an important part of the hydraulic system.The traditional hydraulic valve block is made by turning and milling,drilling and boring,which leads to many right-angle bending and closed cavity structure of process holes in its internal flow channel,seriously affecting the flow performance of oil.Based on the new design space provided by additive manufacturing technology,the internal hydraulic flow channel of valve block is optimized by using B-spline curve.Computational fluid dynamics analysis is carried out on the hydraulic flow channel to determine the optimal flow channel structure with the smallest pressure drop.The weight reduction of hydraulic valve block is carried out through topology optimization.According to the results of topology optimization,using the method of selective laser melting(SLM),the printing of the hydraulic valve block is completed.The optimized hydraulic channel reduces the pressure loss by 31.4%compared with the traditional hydraulic channel.Compared with the traditional valve block,the hydraulic valve block manufactured by SLM with topology optimization reduces the weight by 33.9%.Therefore,the proposed flow channel optimization and valve block lightweight method provide a new reference for the performance improvement of the internal flow channel of hydraulic valve block and the overall lightweight design of valve block.

On the structural features of fiber suspensions in converging channel flow

The structural features of fiber suspensions are dependent on the fiber alignment in the flows. In this work the orientation distribution function and orientation tensors for semi-concentrated fiber suspensions in converging channel flow were calculated, and the evolutions of the fiber alignment and the bulk effective vis-cosity were analyzed. The results showed that the bulk stress and the effective viscosity were functions of therate-of-strain tensor and the fiber orientation state ; and that the fiber suspensions evolved to steady alignment and tended to concentrate to some preferred directions close to but not same as the directions of local stream-lines. The bulk effective viscosity depended on the product of Reynolds number and time. The decrease of ef-fective viscosity near the boundary benefited the increase of the rate of flow. Finally when the fiber alignment went into steady state, the structural features of fiber suspensions were not dependent on the Reynolds numberbut on the converging channel angle.

Numerical Research on the Fiber Suspensions in a Turbulent T-shaped Branching Channel Flow

The concentration and orientation of fiber in a turbulent T-shaped branching channel flow are investi-gated numerically. The Reynolds averaged Navier-Stokes equations together with the Reynolds stress turbulent model are solved for the mean flow field and the turbulent kinetic energy. The fluctuating velocities of the fluid are assumed as a random variable with Gaussian distribution whose variance is related to the turbulent kinetic energy. The slender-body theory is used to simulate the fiber motion based on the known mean and fluctuating velocities of the fluid. The results show that at low Reynolds number, fiber concentration is high in the flow separation regions, and fiber orientation throughout the channel is widely distributed with a slight preference of aligning along the horizontal axis. With increasing of Re, the high concentration region disappears, and fiber orientation becomes ho-mogeneous without any preferred direction. At high Reynolds number, fiber concentration increases gradually along the flow direction. The differences in the distribution of concentration and orientation between different fiber aspect ratio are evident only at low Re. Both Re and fiber aspect ratio have small effect on the variance of orientation angle.

Stability and Drag Reduction in Transient Channel Flow of Fibre Suspension

Drag reduction features in the transition regime of channel flow with fibre suspension were analyzed in terms of the linear stability theory. The modified stability equation was obtained based on the slender-body theory and natural closure approximation. Results of the stability analysis show attenuating effects of fibre additives to the flow instability. For the cases leading to transition, drag reduction rate increases with the characteristic parameter H of fibres. The mechanism of drag reduction by fibres is revealed through the variation of velocity profile and the decrease of wall shear stress. The theoretical results are qualitatively consistent with some typical experiments.

Experimental study on influence of boundary on location of maximum velocity in open channel flows

The velocity dip phenomenon may occur in a part of or in the whole flow field of open channel flows due to the secondary flow effect. Based on rectangular flume experiments and the laser Doppler velocimetry, the influence of the distance to the sidewall and the aspect ratio on the velocity dip is investigated. Through application of statistical methods to the experimental results, it is proposed that the flow field may be divided into two regions, the relatively strong sidewall region and the relatively weak sidewall region. In the former region, the distance to the sidewall greatly affects the location of maximum velocity, and, in the latter region, both the distance to the sidewall and the aspect ratio influence the location of the maximum velocity.

Orientation distribution of fibres in a channel flow of fibre suspension

The orientation and concentration distributions of fibres in laminar and turbulent channel flows are investigated numerically. The obtained results are in good agreement with the experimental data. In the laminar flow regime, more fibres orient to the flow direction as the Reynolds number increases. The shear rate of fluid around a fibre plays an important role in determining the orientation distribution of fibres, while the fibre density and the fibre aspect-ratio have marginal influence on the orientation distribution. In the turbulent regime, the orientation distribution of fibres becomes more homogeneous with the increase of Reynolds number, and the concentration profile is flatter than that in the laminar regime. The fluctuating intensity of fibre velocity in the downstream direction is larger than that in the lateral directions.