Direction Of Throughflow Research Articles

LTNE effects on two‐layer configuration with throughflow

AbstractThe effect of throughflow on the beginning of convective motion in a fluid layer overlying a fluid‐saturated porous layer is examined theoretically in this article under the local thermal nonequilibrium model. We considered boundaries to be insulating to temperature perturbations. The normal mode approach is employed to analyze the governing equation for a composite configuration, with the solution sought through the regular perturbation technique to assess linear stability. The critical Rayleigh number, signifying the initiation of stationary convection, has been determined by solving the system of coupled equations. It is found that the system attains stability with increased porosity–conductivity ratios, regardless of the throughflow direction. This stability is consistently observed for depth ratios of 0.1 and 0.2. However, for higher depth ratios (0.5 and 1.0), the system exhibits dual nature stability when throughflow is present. Further, the stability of the system is enhanced by a graphical depiction illustrating how the throughflow parameter, porosity‐scaled conductivity ratio, depth ratio, Darcy number, and Prandtl number collectively influence the onset of convective motion.

Global nonlinear stability of bidispersive porous convection with throughflow and depth-dependent viscosity

The linear instability and the nonlinear stability analyses have been performed to examine the combined impact of a uniform vertical throughflow and a depth-dependent viscosity on bidispersive porous convection using the Darcy theory with a single temperature field. The validity of the principle of exchange of stability is proved. The eigenvalue problems resulting from both linear instability and nonlinear stability analyses with variable coefficients are numerically solved using the Chebyshev pseudo-spectral method. The equivalence of linear instability and nonlinear stability boundaries is established in the absence of throughflow, while in its presence, the subcritical instability is shown to be evident. The stability of the system is independent of the direction of throughflow in the case of constant viscosity, whereas upflow is found to be more stabilizing than downflow when the viscosity is varying with depth. While the viscosity parameter offers a destabilizing influence on the onset of convection in the absence of throughflow, it imparts both stabilizing and destabilizing effects on the same in its presence. The influence of the ratio of permeabilities and the interphase momentum transfer parameter is to make the system more stable. The findings of a mono-disperse porous medium are presented as a specific case within the broader context of this investigation.

Outlook of Density Maximum on the Onset of Forchheimer-Bénard Convection with throughflow

The vertical throughflow effect is investigated on the onset of porous convection by considering a cubic density-temperature relationship and using the Forchheimer-Darcy model. The stability eigenvalue problem is explained numerically using the Galerkin technique. Contrary to the linear density-temperature relationship, the direction of throughflow alters the onset of convection. The throughflow dependent Péclet number is found to stabilize the fluid motion against convection and the upflow is found to be either stabilizing or destabilizing than the downflow depending on the values of thermal condition parameters λ1 and λ2. A destabilizing effect on the onset is observed with increasing λ1 and λ2. The Darcy number Da and the Forchheimer drag co-efficient, cb instability characteristics have been investigated and depicted graphically.

Nonlinear stability analysis of double diffusive convection in a fluid saturated porous layer with variable gravity and throughflow

This paper develops a detailed study of linear and nonlinear stability analyses, for the double diffusive convection problem in a porous medium, when heating is done from below and, salting is done from below as well as from above. The linear analysis is performed using normal mode technique and nonlinear analysis is developed using energy technique. The Chebyshev pseudo-spectral technique is performed to analyze the effect of variable gravity field and throughflow on the behavior of system stability. The results obtained from linear and nonlinear analysis are compared and found that the results are in better agreement for the heated from below and salted from above system however, the comparatively less agreement is obtained for the system when both heating and salting are done from below. The direction of throughflow and gravity field have considerable effect on the stability of system. The behavior of gravity field is invariant for both the type of system heating and salting from below as well as heating from below and salting from above. The solute Rayleigh number stabilizes the system for heating from below and salting from below, however, destabilizes the system for heating from below and salting from above. In the absence of gravity field, a comparatively good agreement between thresholds is seen for Q = 0, however, the agreement is shifted towards positive value of Q or negative value of Q when gravity is found to be present in the system. Thus, effect of gravity field is dominant on the influences of throughflow on the stability of system.

Benchmark solution for the stability of plane Couette flow with net throughflow

This paper investigates the stability of an incompressible viscous fluid flow between relatively moving horizontal parallel plates in the presence of a uniform vertical throughflow. A linear stability analysis has been performed by employing the method of normal modes and the resulting stability equation is solved numerically using the Chebyshev collocation method. Contrary to the stability of plane Couette flow (PCF) to small disturbances for all values of the Reynolds number in the absence of vertical throughflow, it is found that PCF becomes unstable owing to the change in the sign of growth rate depending on the magnitude of throughflow. The critical Reynolds number triggering the instability is computed for different values of throughflow dependent Reynolds number and it is shown that throughflow instills both stabilizing and destabilizing effect on the base flow. It is seen that the direction of throughflow has no influence on the stability of fluid flow. A comparative study between plane Poiseuille flow and PCF has also been carried out and the similarities and differences are highlighted.

Ferroconvection in a porous medium with vertical throughflow

The problem of thermal convection in a horizontal layer of ferrofluid-saturated porous medium is investigated theoretically in the presence of a uniform vertical magnetic field and throughflow. The flow in the porous medium is described by the modified Brinkman equation with fluid viscosity different from effective viscosity. The rigid, however permeable ferromagnetic boundaries are considered to be insulated to temperature perturbations. The resulting eigenvalue problem is solved both numerically using the Galerkin technique and analytically using regular perturbation technique with wave number a as a perturbation parameter and observed that the results complement with each other. The direction of throughflow has no influence on the stability characteristics of the system, and the effect of throughflow-dependent Peclet number Q is to delay the onset of ferroconvection. The Prandtl number Pr(>1) has insignificant while the nonlinearity of fluid magnetization parameter M3 has no influence on the onset of ferroconvection. Besides, an increase in the value of the Darcy number Da and the magnetic number M1 is to hasten, while an increase in the ratio of viscosity parameter λ is to delay the onset of ferroconvection.

On the stability of internally heated natural convection due to the absorption of radiation in a laterally confined fluid layer with a horizontal throughflow

This study focuses on the marginal stability of the natural convection flow induced by the absorption of radiation. Non-uniform internal heating associated with the absorption of radiation following Beer’s law and the absorption and re-emission of residual radiation by the bottom boundary wall lead to a nonlinear base temperature profile that evolves with time. A horizontal throughflow and lateral confinement are incorporated in the analysis, and two forms of convective instability, i.e. longitudinal and transverse rolls, are investigated. It is shown that the formation of longitudinal rolls, with their axes parallel to the throughflow direction, is not affected by the presence of the throughflow. For transverse rolls, with their axes perpendicular to the throughflow direction, however, the throughflow is shown to have a stabilising effect, with the critical Rayleigh number increasing with the increasing throughflow Reynolds number. It follows that longitudinal rolls are the preferred form of instability in laterally unconfined domains for any nonzero Reynolds number and in confined domains if the Reynolds number is greater than a critical value. The value of the critical Reynolds number, which separates the preferred mode of instability, is found to rapidly reduce with increasing lateral extent and approaches zero for a normalised lateral extent greater than 2.0. The lateral confinement is also shown to have a stabilising effect for both longitudinal and transverse rolls, with the critical Rayleigh number increasing with the reducing lateral extent. It is shown that the lateral confinement only substantially affects the wave number of the rolls with their axes parallel to the confining walls but not the rolls with their axes perpendicular to the confining walls.

Convective and shear-induced turbulence in the deep Kane Gap

[1] The boundary layer above a 4569 m deep slope in the near-equatorial N-Atlantic Ocean Kane Gap, a throughflow for Antarctic Bottom Water (AABW), is characterized by two distinct turbulent regimes that differ by an order of magnitude in intensity depending on the direction of throughflow. During southward and downward flow, vertical mixing is vigorous. This is inferred from high-resolution temperature observations between 6 and 132 m above the bottom. For a representative case study of 2 days, average values are found for dissipation rate of e?=?2.1?±?1 × 10-9 W kg-1 and eddy diffusivity of Kz?=?7?±?4 × 10-4 m2 s-1. The mixing is across relatively large vertical overturns. During northward and upward flow, smaller overturns are more horizontal as in stratified shear flow (with representative 2 day mean e?=?6?±?3 × 10-11 W kg-1, Kz?=?4?±?2 × 10-5 m2 s-1). Stratification is approximately the same during both flow directions. Although the different turbulence regimes are partially associated with frictional boundary layers of large-scale flows above sloping topography, but not with those over flat bottoms, and partially with flow across a hill-promontory, internal waves are a dominant process in promoting turbulence. In addition, internal waves are observed to push stratification toward the bottom thereby importantly contributing to the mixing of AABW.

VARIABLE GRAVITY FIELD AND THROUGHFLOW EFFECTS ON PENETRATIVE CONVECTION IN A POROUS LAYER

The effect of vertical throughflow and variable gravity field on the onset of penetrative convection simulated via internal heating in a porous medium is studied. Flow in the porous medium is governed by Forchheimer-extended Darcy equation. The boundaries are considered to be rigid, however permeable, and insulated to temperature perturbations. The eigen value problem is solved using a regular perturbation technique with wave number as a perturbation parameter. The variable gravity parameter, the direction of throughflow and the presence of volumetric internal heat source in a porous layer play a decisive role on the stability characteristics of the system. In addition, the influence of Prandtl number arising due to throughflow is also emphasized on the stability of the system. It is observed that both stabilizing and destabilizing factors can be enhanced due to the simultaneous presence of a volumetric source, gravity field and vertical throughflow so that a more precise control (suppress or augment) of thermal convective instability in a layer of porous medium is possible.

Throughflow Effects on Penetrative Convection in Superposed Fluid and Porous Layers

The effect of vertical throughflow on the onset of penetrative convection simulated via internal heating in a two-layer system in which a layer of fluid overlies and saturates a layer of porous medium is studied. Flow in the porous medium is governed by Forchheimer-extended Darcy equation, and Beavers–Joseph slip condition is applied at the interface between the fluid and the porous layers. The boundaries are considered to be rigid, however permeable, and insulated to temperature perturbations. The eigenvalue problem is solved using a regular perturbation technique with wave number as a perturbation parameter. The ratio of fluid layer thickness to porous layer thickness, ζ, the direction of throughflow, and the presence of volumetric internal heat source in fluid and/or porous layer play a decisive role on the stability characteristics of the system. In addition, the influence of Prandtl number arising due to throughflow is also emphasized on the stability of the system. It is observed that both stabilizing and destabilizing factors can be enhanced because of the simultaneous presence of a volumetric heat source and vertical throughflow so that a more precise control (suppress or augment) of thermal convective instability in a layer of fluid or porous medium is possible.

Heat and fluid flow in cylindrical and conical annular flow-passages with through flow and inner-wall rotation

The flow and heat transfer in cylindrical and conical annular flow-passages with through flow and inner-wall rotation have been numerically simulated by using the large eddy simulation with a Lagrangian dynamic subgrid-scale model. Inlet through-flow Reynolds number was 1000 and the Taylor number was set at 0, 1000, 2000, and 4000. In the conical flow passage, when the inner-wall rotation speed was increased, at first spiral vortices in the downstream region and then much more complicated vortices appeared. The vortices for Ta = 4000 changed the structure in both through-flow and wall-normal directions in the downstream half of the passage. The flow structure and heat transfer of the conical case were completely different from those of the cylindrical case. It was because of the three factors: the expansion of the flow passage, the rotation radius change in the through-flow direction, and the centrifugally driven through-flow. The last factor is due to the acute angle between the centrifugal and through-flow directions.

UVP Measurement of Taylor-Couette Vortex Flow with Through-Flow

For continuing nuclear energy as one of basic power sources, it is necessary to strengthen the supply stability, moreover, build nuclear fuel cycle which reprocesses spent fuel produced from nuclear power plant, and reuses uranium and plutonium collected. In the research field of reprocessing process of this cycle, recently, a new type centrifugal extractor using Taylor-Couette vortex flow is studied for improving the extraction efficiency. By the way, since the unstable vortex formation causes fluctuations in extractor's torque and chemical reaction time, the understanding and control of this kind of vortex flow are necessary. Especially, for the practical usage, it is important to investigate the characteristics of Taylor-Couette vortex flow with a through-flow. In this study, we investigated the effects of through-flow on Taylor-Couette vortex flow with use of an Ultrasonic Velocity Profiler (UVP), which is capable of visualizing the flow dynamic characteristics. Under through-flow condition, we found that vortices are drifting in the direction of through-flow and their motions depend on Reynolds number of through-flow.

High-throughput electrokinetic bioparticle focusing based on a travelling-wave dielectrophoretic field

This study presents a sheathless and portable microfluidic chip that is capable of high-throughput focusing bioparticles based on 3D travelling-wave dielectrophoresis (twDEP). High-throughput focusing is achieved by sustaining a centralized twDEP field normal to the continuous through-flow direction. Two twDEP electrode arrays are formed from upper and lower walls of the microchannel and extend to its center, which induce twDEP forces to provide the lateral displacements in two directions for focusing the bioparticles. Bioparticles can be focused to the center of the microchannel effectively by twDEP conveyance when the characteristic time due to twDEP conveying in the y direction is shorter than the residence time of the particles within twDEP electrode array. Red blood cells can be effectively focused into a narrow particle stream (~10 μm) below a critical flow rate of 10 μl/min (linear flow velocity ~5 mm/s), when under a voltage of 14 Vp–p at a frequency of 500 kHz is applied. Approximately 90% focusing efficiency for red blood cells can be achieved within two 6-mm-long electrode arrays when the flow rate is below 12 μl/min. Blood cells and Candida cells were also focused and sorted successfully based on their different twDEP mobilities. Compared to conventional 3D-paired DEP focusing, velocity is enhanced nearly four folds of magnitude. 3D twDEP provides the lateral displacements of particles and long residence time for migrating particles in a high-speed continuous flow, which breaks through the limitation of many electrokinetic cell manipulation techniques.

The Impact of Gas Modeling in the Numerical Analysis of a Multistage Gas Turbine

In this work a numerical investigation of a four stage heavy-duty gas turbine is presented. Fully three-dimensional, multistage, Navier-Stokes analyses are carried out to predict the overall turbine performance. Coolant injections, cavity purge flows, and leakage flows are included in the turbine modeling by means of suitable wall boundary conditions. The main objective is the evaluation of the impact of gas modeling on the prediction of the stage and turbine performance parameters. To this end, four different gas models were used: three models are based on the perfect gas assumption with different values of constant cp, and the fourth is a real gas model which accounts for thermodynamic gas properties variations with temperature and mean fuel∕air ratio distribution in the through-flow direction. For the real gas computations, a numerical model is used which is based on the use of gas property tables, and exploits a local fitting of gas data to compute thermodynamic properties. Experimental measurements are available for comparison purposes in terms of static pressure values at the inlet∕outlet of each row and total temperature at the turbine exit.

Measurement and analysis of three-dimensional flow in a centrifugal fan volute with large volute width and rectangular cross-section

Among all the elements of a centrifugal fan, the volute is the one that has the lowest flow efficiency. Therefore, improving the performance of the volute is an efficient way to improve the total performance of a centrifugal fan. To contribute a better understanding of the flow structure in the fan volute, the three-dimensional flow in a centrifugal fan volute with a large volute width and rectangular cross-section has been measured in detail by means of five-hole probe at three different flowrates. The time-average swirling and throughflow velocity, static and total pressure distributions on eight cross-sections in the throughflow direction of the volute are presented. The results show the formation and development of the flow in the fan volute of this type, indicate the variation of flow parameters, and discover some peculiar flow phenomena different from the traditional understanding. On the basis of the experimental results, the main hydraulic losses in this kind of fan volute have been preliminarily classified and analysed. The results show that the traditional one-dimensional design method of the volute should be further improved as it is only based on the law of momentum moment conservation and the ideal assumption that the distribution of flow parameters are uniform at the volute inlet.

Direct optimization method for estimation of supersonic flow turbinestator profiles

An iterative technique for generating approximate wall contours of the diverging portions of a supersonic turbine stator flow passage is described. A pattern-search optimization method is used to minimize the difference between desired nozzle exit conditions and those exit conditions calculated with specified polynomial wall contours. Nozzle exit conditions are characterized by wall-to-wall distributions of flow angularity and flow Mach number. Results are compared with so-called perfect nozzle contours calculated with assumed uniform throat mass flux conditions. Calculations are then extented to two simple models of nonuniform throat mass flux conditions, one of which is symmetric about the throat centerline and one of which is skewed toward a passage wall. Good matches of desired exit conditions and calculated exit conditions are obtained for each case. Flow angularity (deviations from the overall throughflow direction) appear to be more sensitive to the optimization procedure than Mach number, and care must be taken to insure sufficient downstream flow angle uniformity.

Full Potential Solution of Transonic Quasi-Three-Dimensional Flow Through a Cascade Using Artificial Compressibility

A reliable method is presented for calculating the flowfield about a cascade of arbitrary two-dimensional airfoils. The method approximates the three-dimensional flow in a turbomachinery blade row by correcting for streamtube convergence and radius change in the throughflow direction. The method is a fully conservative solution of the full potential equation incorporating the finite volume technique on a body-fitted periodic mesh, with an artificial density imposed in the transonic region to ensure stability and the capture of shock waves. Comparison of results for several supercritical blades shows good agreement with their hodograph solutions. Other calculations for these profiles as well as standard NACA blade sections indicate that this is a useful scheme for analyzing both the design and off-design performance of turbomachinery blading.

Analysis of the through-flow relative eddy of mixed-flow turbomachines

Turbomachine rotors viewed relative to rotation experience a relative eddy. The relative eddy component normal to the blade to blade flow, responsible for slip and for coriolis blade forces in mixed-flow turbomachines, is well known. The remaining eddy component which lies in the through-flow direction, is generally ignored in quasi-two-dimensional design methods. This paper presents the first stage of an analytical method for predicting the influence of the through-flow eddy in axial, mixed-flow or radial machines.