Bottom Wall Of Channel Research Articles

Simulation of instability and sound production in a turbulent channel flow with an acoustic liner

Acoustic liners are used to absorb sound waves in ducts with flow. These liners can become unstable and some experiments have shown that in this case they do not damp sound as they should. This situation is addressed numerically. A turbulent plane channel flow at a friction Reynolds number of about 400 is computed using direct numerical simulation of the compressible Navier–Stokes equations. The channel is in spatial development, and the inflow is fully turbulent, non-reflecting, and can inject an incident acoustic wave. The liner is modeled by an acoustic impedance placed on a portion of the channel bottom wall. Several liners are simulated, stable or unstable. Spatial instabilities are observed and well modeled by a linear spatial stability analysis. The scattering of the instability at the liner trailing edge triggers backpropagating acoustic waves, as well as sound emission downstream of the liner. Backpropagating waves make a feedback loop possible on the liner, so that instabilities are observed even in convectively unstable situations without excitation. The effect of an incident acoustic wave is discussed.

Numerical investigation on heat transfer characteristics of supercritical n-decane in a regenerative cooling channel with inverted pyramid trusses

Regenerative cooling is considered to be an efficient method for cooling aircraft engine combustion chambers. A regenerative cooling channel inserted with the inverted pyramid trusses is proposed. The impact of display mode on heat transfer performance is examined through a numerical investigation. The results show that the inverted and A-oriented structures have the best heat transfer and mechanical properties. The temperature of the bottom channel wall may be reduced by 76.3 K through the implementation of inverted pyramid trusses. The application of inverted pyramid trusses can effectively mitigate the deterioration of heat transfer commonly found in smooth channels. The notable enhancement in the heat transfer coefficient is primarily observed in the area encompassing the truss and the downstream expanse of 12 mm trailing the truss. This phenomenon is attributed to flow acceleration and intricate vortices trailing the truss. For the inverted display mode, the influence of the truss pitch, the angle between the truss and the flow direction, and the truss rod diameter of the truss on the heat transfer were investigated. The difference in heat transfer performance when changing the pitch was reflected in the low heat transfer coefficient region downstream of the trusses. When the diameter and angle changed, the heat transfer performance was mainly dependent on the size of the horseshoe vortex and the arch vortex. Based on the simulated data, the fitted relations for both friction coefficient and Nusselt number were developed with fitting errors of less than 5 %.

Influence of novel ogive shape ribs and cavities on local flow dynamics and thermal characteristics of microchannel heat sink

This study proposes using novel ogive-shaped ribs and cavities to enhance the thermal performance of a microchannel heat sink (MCHS). A three-dimensional conjugate numerical model was employed to investigate the impact of these modifications, i.e., ogive shape ribs and cavities, on the hydro-thermal performance of MCHS in laminar flow regime (Reynolds number 100–1000). The results show that flow separation occurs on the trailing edge of ogive ribs, promoting convective heat transfer by disrupting the boundary layer and enhancing fluid mixing on the downside of ogive ribs due to the formation of recirculation zones. Conversely, stagnant zones are created inside the ogive cavity due to very low fluid local velocity which results in a decrease of local Nusselt number. The study reports a 41.34% maximum improvement in Nusselt number for MCHS with ogive ribs on side channel walls and a maximum thermal enhancement factor of 1.42 for MCHS with ogive ribs on the bottom channel wall.

Numerical investigation of microchannel heat sink with novel ogive shape ribs

In this study, 3D numerical conjugate heat transfer modeling is used to investigate the thermal and hydraulic characteristics of the microchannel heat sink (MCHS) with different configurations of novel ogive shape ribs on channel walls. It was found that new proposed MCHS configurations with ogive ribs have a high Nusselt number as compared to the smooth MCHS because ogive ribs enhance the heat dissipation between channel walls and fluid by continuously interrupting the thermal boundary layer development. MCHS configuration with ogive ribs mounted on bottom wall improves the Nusselt number of smooth MCHS by 1.13–1.87 times, while MCHS with ogive ribs mounted on both side walls and MCHS with ogive ribs mounted on bottom channel wall improve Nusselt number by 1.12–1.70 and 1.08–1.59 times, respectively, at Re 100–1000. In terms of thermal enhancement factor criterion, the MCHS with ogive ribs on side walls shows superior performance at Re 100–300 and has the highest thermal enhancement factor. While MCHS with ogive ribs on bottom wall outperformed other configurations in terms of thermal enhancement factor at Reynold number > 300. A maximum thermal enhancement factor of 1.42 is reported for MCHS with bottom wall ribs ogive at Re = 1000.

Heat Transfer Augmentation and Entropy Generation Analysis of Microchannel Heat Sink (MCHS) with Symmetrical Ogive-Shaped Ribs

This study proposes the use of symmetrical ogive-shaped ribs on the walls of microchannel heat sinks (MCHS) to improve their thermal performance with minimal pressure drop. The ribs are arranged in three different configurations: ribs attached to all channel walls (MC-SAWR), ribs attached to side channel walls (MC-SSWR), and ribs attached to the bottom channel wall (MC-SBWR). Numerical investigations are conducted using the laminar conjugate heat transfer model to study the flow and heat transfer characteristics of the MCHS. The augmentation entropy generation number and thermal enhancement factor criterion are used to quantify the overall hydrothermal performance of the MCHS. The results show that the inclusion of symmetrical ogive-shaped ribs improves the Nusselt number of MCHS. The MC-SAWR configuration shows the highest Nusselt number improvement of 13–50% compared to the smooth MCHS over the Reynolds number range of 100–1000. Additionally, the MC-SAWR configuration shows a maximum reduction of 58% in the total entropy generation rate as it has the smallest augmentation entropy generation number value of 0.42. In terms of the thermal enhancement factor criterion, the MC-SSWR configuration shows the highest performance at Reynolds numbers below 400, but the MC-SAWR configuration outperformed the MC-SSWR configuration at Reynolds numbers above 400. Therefore, the MC-SAWR configuration is the best configuration that provides high cooling performance.

Thermo-fluidic transport process in magnetohydrodynamic Couette channel containing hybrid nanofluid

Current paper focusses on the mathematical simulation of thermo-fluidic unsteady flow in a magnetohydrodynamic (MHD) Couette channel containing novel (ZnO–Al2 O3)-water hybrid nanofluid (HNF). The nanoparticles of zinc oxide(ZnO) and aluminium oxide(Al2O3) are supplemented to basefluid water in 1:1 ratio. The bottom wall of channel kept stationary and stretchable, while the top is set in uniform motion with convective cooling. A uniform magnetic field appealed normally to lower plate. The governed nonlinear partial differential equations(PDE) are integrated employing finite difference method (FDM). A comparison of effect of hybrid nanofluid versus nanofluid (NF) is performed. The effects of non-dimensional quantities namely, nanoparticle volume fraction(NVF), magnetic field, stretching parameter, Biot number and Eckert number are reported graphically. The results demonstrate that (ZnO+ Al2O3)-water HNF shows significant impact over ZnO–water NF on velocity and temperature in the Couette channel.

Numerical Investigation of Convective Heat Transfer and Fluid Flow Past a Three Square Cylinders Controlled by a Partition in Channel

This document presents a research article on the control of fluid flow around three heated square cylinders placed side by side in a 2D horizontal channel using a flat plate. The objective of this research is to examine the effect of the position, length and height of a flat plate on fluid flow and heat transfer. For this purpose, numerical simulations are performed by using the Boltzmann double relaxation time multiple network method (DMRT-LBM). The MRT-D2Q9 and MRT-D2Q5 models are used to treat the flow and temperature fields respectively. In contrast to several existing investigations in the literature in this domain which study the passive control of the flow using a horizontal or vertical plate around a single cylinder, this work presents a numerical study on the effect of the position, length and height of a flat plate (horizontal and vertical) on three heated square cylinders on the flow and temperature fields. First, the effect of the position and length of the horizontal flat plate is examined. This study shows that the implementation of a flat plate of length Lp = 4D at a position g=3 behind the central cylinder reduces the amplitude of the Von Karman Street and allows large and regular heat exchange. Thus, in the second part, the effect of the position and height of the vertical flat plate is studied. The results obtained show that the implementation of a flat plate of height h=2D at a position g=3 behind the central cylinder improves the thermal exchange between the incoming fluid and the heated cylinders. This numerical work could lead to the prediction of the cooling of the electronic components: The cooling of the obstacles is all the better when the control plate is arranged at g = 3 and its height h = 2D in the case of the vertical plate or its length Lp equal to 4D in the case where the plate is implemented horizontally

Математическое моделирование нелинейных колебаний стенки канала, взаимодействующей с вибрирующим штампом через слой вязкой жидкости

The article considers the developed mathematical model and investigates the dynamics of the interaction of a channel wall supported by a nonlinear spring with a vibrating opposite wall through a viscous fluid layer filling the channel. A flat slotted channel formed by two absolutely rigid rectangular walls, parallel to each other was investigated. One of the channel dimensions in the plan was much larger than the other, which leads to the transition to a plane problem. The bottom channel wall rested on a spring with a cubic nonlinear characteristic, and the upper wall was a die oscillating according to a given law. The gap between the walls was assumed to be much smaller than the channel longitudinal dimension, and the amplitudes of wall vibrations were much less than the channel gap. The movement of the viscous fluid in the channel was considered to be creeping. The mathematical model of the channel under consideration consisted of an equation of the dynamics of a single-mass system with a spring having a cubic nonlinearity, as well as the Navier --- Stokes and continuity equations, supple-mented by the boundary conditions for fluid nonslip on the channel walls and its free outflow at the ends. The steady-state nonlinear vibrations of the bottom channel wall at the fundamental frequency were studied, and its hydroelastic response was determined. The proposed model can be used to study nonlinear vibrations of elastically fixed elements that are in contact with liquid and are parts of modern devices and assemblies

Measurements of Turbulent Transport in a Square Channel With One Ribbed Wall

Abstract The velocity field of the fully developed turbulent flow in a one-sided ribbed square channel (rib-height-to-channel-height ratio of k/h=0.0667 and rib-pitch-to-rib-height ratio of p/k=9) was measured at Reynolds numbers (based on the channel height h and the mean bulk velocity uB) of Reh=5.0×104 and 1.0×105 by means of laser Doppler anemometry (LDA). Triple velocity correlations differed slightly between both Reynolds numbers when normalized by the bulk velocity and the channel height, similarly to the first- and second-order statistical moments of the velocity. Their near-wall behavior reflected the crucial role of turbulent transport near the rib crest and within the separated shear layer. Sweep events occurred with the elongated flow structures of the flapping shear layer and gained in importance toward the channel bottom wall, while strong ejection events near the rib leading and trailing edges coincided with flow structures bursting away from the wall. Despite the predominant occurrence of sweep events close to the ribbed wall within the inter-rib spacing, ejection events contributed with higher intensity to the Reynolds shear stress. Ejection and sweep events and their underlying transport phenomena contributing to the Reynolds shear stress were almost Reynolds number-insensitive in the resolved flow range. The invariance to the Reynolds number can be of benefit for the use of scale-resolving simulation methods in the design process of rib structures for heat exchange applications.

Experimental Investigation on Flow Past an Isolated Micro Pin Fin Embedded in a Microchannel

ABSTRACT Micro pin fin heat sink is a very attractive cooling technique for high power density microelectronics. Optimization of its cooling performance requires insightful understanding on fundamental physics of flow inside it, especially around a single pin fin. In the present study, an experimental investigation on flow past an isolated low aspect ratio (height-to-diameter ratio) pin fin embedded in a microchannel is conducted using Micro-PIV. The flow field and vorticity distribution at different channel heights under various Reynolds numbers are obtained. Endwall effect is found to play an important role in flow past the pin fin in the microchannel, and the critical Reynolds numbers are larger than that at the conventional scale. Vorticity concentrations are formed on both sides of the pin fin along the shear layer and intensify with the increase in Reynolds number. Flow fields and vorticity distributions at different heights exhibit different characteristics, especially at higher Reynolds numbers, indicating three-dimensionalities of the flow. Viscous resistance of the endwalls leads to lower overall velocity, smaller extent of the recirculation zone and weaker vorticity in flow layer closer to the top and bottom channel walls. Pressure drop and flow resistance characteristics in the microdevice is analyzed. The effect of aspect ratio of the pin fin on the wake flow is also studied, and the results show that three-dimensionalities increase but critical Reynolds numbers decrease with larger aspect ratios. A comparison with flow across micro pin fin arrays is conducted and differences are observed in velocity field and wake flow features.

Characteristics of a buoyant plume in a channel with cross-flow

We present data from direct numerical simulation (DNS) of a buoyant plume in a channel with cross-flow. The plume originates from a line heat source at the bottom channel wall, impinging the top wall and propagates downstream and/or upstream based on the strength of the cross-flow. We study the critical velocity, defined as the minimum cross-flow velocity which prevents negative backlayering length from occurring. For cross-flow velocities higher than the critical velocities, all the plume generated from the source would be forced downstream and the location of the backlayering front would move downstream. We also visualise the structure of the plume for different source Reynolds number, which indicates that the backlayering is thicker at lower source Reynolds numbers. The thicker backlayering acts like as a blockage, which influences the cross-flow. In addition, an investigation into the relationship between critical velocity, source Reynolds number and the heat release rate has been carried out. A one-third power between the heat release rate critical is found, which is in good agreement with previous studies. For a fixed heat release rate, the critical velocity increases with increasing source Reynolds number. However, the critical velocity approaches an asymptotic value when the source Reynolds number is sufficiently high.

Numerical investigation of double-diffusive mixed convection of Fe3O4/Cu/Al2O3-water nanofluid flow through a backward-facing-step channel subjected to magnetic field

PurposeThis study aims to investigate the buoyancy-induced heat and mass transfer phenomena in a backward-facing-step (BFS) channel subjected to applied magnetic field using different types of nanofluid.Design/methodology/approachConservation equations of mass, momentum, energy and concentration are used through velocity-vorticity form of Navier–Stokes equations and solved using Galerkin’s weighted residual finite element method. The density variation is handled by Boussinesq approximation caused by thermo-solutal buoyancy forces evolved at the channel bottom wall having high heat and concentration. Simulations were carried out for the variation of Hartmann number (0 to 100), buoyancy ratio (−10 to +10), three types of water-based nanofluid i.e. Fe3O4, Cu, Al2O3 at χ = 6%, Re = 200 and Ri = 0.1.FindingsThe mutual interaction of magnetic force, inertial force and nature of thermal-solutal buoyancy forces play a significant role in the heat and mass transport phenomena. Results show that the size of the recirculation zone increases at N = 1 for aiding thermo-solutal buoyancy force, whereas the applied magnetic field dampened the fluid-convection process. With an increase in buoyancy ratio, Al2O3 nanoparticle shows a maximum 54% and 67% increase in convective heat and mass transfer, respectively at Ha = 20 followed by Fe3O4 and Cu. However, with increase in Ha the Nuavg and Shavg diminish by maximum 62.33% and 74.56%, respectively, for Fe3O4 nanoparticles at N = 5 followed by Al2O3 and Cu.Originality/valueThis research study numerically examines the sensitivity of Fe3O4, Cu and Al2O3 nanoparticles in a magnetic field for buoyancy-induced mixed convective heat and mass transfer phenomena in a BFS channel, which was not analyzed earlier.

Impacts of elasticity and porosity of the channels on the performance features of thermoelectric module mounted system and efficient computations with multi-proper orthogonal decomposition approach

Effects of wall elasticity and porosity of the channel on the performance characteristics of TEG module integrated system are explored numerically with finite element method. A porous layer in the lower channel is introduced with hybrid nanoparticles in the fluid. Effects of different values of elastic modulus of the top and bottom channel walls (5×103≤E1,E2≤1010), Darcy number (5×10−4≤ Da ≤5×10−1), porous layer height (0.2H ≤ py ≤ 0.8H) and length in flow direction (0.25L ≤ px ≤ 0.85L), Reynolds number (200 ≤ Re ≤ 1000) and hybrid nanoparticle volume fraction (0 ≤ϕ≤ 2%) on the convection and power generation are analyzed. The presence of elastic walls may affect the flow field in local regions but the overall impact on the power variation is slight while 2.8% variation is obtained. The presence of the porous layer altered the power generation features while increasing the permeability and height of the porous layer resulted in higher thermoelectric power generation. The increment amounts are 32% for the highest permeability and 17% for the highest porous layer height. The length of porous layer in the flow direction has slight impact on power generation features while introducing nano-sized particles further enhanced the power by about 15% at the highest ϕ. The computational cost of generated power is drastically reduced from 2.5 h for full coupled model to two minutes by using a multi-POD approach.

Particle actuation by rotating magnetic fields in microchannels: a numerical study.

Magnetic particles confined in microchannels can be actuated to perform translation motion using a rotating magnetic field, but their actuation in such a situation is not yet well understood. Here, the actuation of a ferromagnetic particle confined in square microchannels is studied using immersed-boundary lattice Boltzmann simulations. In wide channels, when a sphere is positioned close to a side wall but away from channel corners, it experiences a modest hydrodynamic actuation force parallel to the channel walls. This force decreases as the sphere is shifted toward the bottom wall but the opposite trend is found when the channel is narrow. When the sphere is positioned midway between the top and bottom channel walls, the actuation force decreases as the channel width decreases and can reverse its direction. These phenomena are elucidated by studying the flow and pressure fields in the channel-particle system and by analyzing the viscous and pressure components of the hydrodynamic force acting on different parts of the sphere.

Geometrically Non-Linear Vibration of a Cantilever Interacting with Rarefied Gas Flow

Abstract The work is devoted to study 2D pressure driven rarefied gas flow in a microchannel having an elastic obstacle. The elastic obstacle is clamped at the bottom channel wall and its length is half of the channel height. The gas flow is simulated by Direct Simulation Monte Carlo (DSMC) method applying the advanced Simplified Bernoulli Trial (SBT) collision scheme. The elastic obstacle is modelled as geometrically nonlinear Euler Bernoulli beam. A reduced 3 modes reduction model of the beam is created. The influence of the gas flow on the beam vibration is studied, considering the linear and nonlinear beam theories.

Harnessing flow-induced vibration of a D-section cylinder for convective heat transfer augmentation in laminar channel flow

Flow-induced vibration (FIV) of a D-section cylinder is computationally studied and utilized to augment convective heat transfer in a heated laminar channel flow. An in-house fluid–structure interaction (FSI) solver, based on a sharp-interface immersed boundary method, is employed to solve the flow and thermal fields. In conjunction, a spring–mass system is utilized to solve for the rigid structural dynamics. Using numerical simulations, we highlight that the oscillations of a D-section cylinder are driven by vortex-induced vibration and galloping. It is observed that as the cylinder vibrates, vortices are shed from the apex of the cylinder due to the separating shear layers. These vortices, categorized using shedding patterns, interact with the heated channel walls. This interaction results in disruption of the thermal boundary layer (TBL), thus leading to heat transfer augmentation. The enhancement in thermal performance has been quantified using time and space-averaged Nusselt numbers at the channel walls. It is observed that the oscillation amplitude of the D-section cylinder and the extent of vortex–TBL interaction are crucial for determining heat transfer augmentation. Both symmetric and asymmetric thermal augmentation at the top and bottom channel walls have been reported. Finally, to assess the effectiveness of heat transfer augmentation, the D-section cylinder FIV is compared to other FSI systems operating under similar conditions.

Modeling on Vibration of Sandwich Plate Possessing a Compressible Core and Interacting with Viscous Fluid

The paper proposed a mathematical model for analyzing the dynamic response of a three-layered channel wall with a compressible core. The narrow channel formed by two parallel walls is considered. The bottom channel wall was a simply supported three-layered plate. The upper channel wall was considered to be absolutely rigid. The channel was filled with a viscous incompressible liquid with pulsating pressure due to a given harmonic law of pressure pulsation at the channel edges. Longitudinal and transverse vibrations of the three-layered channel wall were studied. The dynamic response of the three-layered channel wall was determined on the basis of hydroelastic oscillations problem solution. The frequency-dependent distribution functions of longitudinal and transverse elastic displacements amplitudes of the three-layered channel wall were constructed. These functions make it possible to obtain amplitude-frequency characteristics of the three-layered channel wall in fixed channel cross-sections. The study was supported by Russian Foundation for Basic Research (projects № 18-01-00127-a and № 19-01-00014-a).

Hydroelastic Response of an End Wall Interacting with a Vibrating Stamp via a Viscous Liquid Layer

The mathematical modelling of a vibrating stamp interaction with a flexible restrained end wall (bellows) of a narrow channel via a viscous incompressible fluid is carried out. The narrow channel formed by two parallel walls and filled with a viscous liquid is investigated. The liquid motion in the channel is considered as a laminar one. The bottom channel wall is absolutely rigid, and the top one is the absolutely rigid vibrating stamp. At the right edge of the channel, the end wall with flexible restraint (bellows) is installed, and at the left one, there is a free liquid flow. The problem of longitudinal hydroelastic vibrations of the channel end wall is formulated and solved analytically. The distribution laws of velocities and pressure in the fluid layer along the channel are found. The motion law of the channel end wall is determined. The amplitude frequency and phase frequency responses of the end wall for steady-state harmonic oscillations are constructed. The mathematical modelling has shown the possibility of damping the channel end wall (bellows) vibrations by changing the distance between the channel walls or by changing the liquid viscosity in the channel.

Second law analysis for mixed convection nanofluid flow in an inclined channel with convectively heated walls

AbstractIn this study, entropy generation analysis for Cu–water nanofluid mixed convective flow in an inclined channel occupied with a saturated porous media with Navier slip and convective boundary conditions is explored. The governing equations composed of equations of velocity and temperature are nondimensionalized and then solved utilizing the technique of homotopy analysis. Temperature and velocity profile expressions are acquired, which are then used to calculate the entropy produced in the scheme. The impacts of the corresponding fluid parameters are addressed in‐depth on velocity, temperature, entropy generation, Bejan number, Nusselt number, skin friction, volume flow rate, and heat carried out by the fluid for nanofluid concentration. Entropy has been observed to be minimal in all cases just above the channel center and maximum at the channel's bottom wall. Fluid friction‐generated entropy has been discovered to have a higher influence on entropy generation. We also provide a comparative study with existing literature to validate our current results.

Modelling of mixing and ignition of a cold hydrogen jet in a supersonic hot air flow

The paper presents the results of numerical studies of self-ignition and supersonic combustion of cold hydrogen jet supplied to the M=2 vitiated air flow. The calculations are carried out using the ANSYS Fluent on the basis of the full Favre-averaged Navier-Stokes equations supplemented by the κ–ω SST turbulence model and several kinetic mechanisms including 1, 16 and 38 reactions. The results of calculations of mixing of a cold hydrogen jet and a hot inert mixture are in good qualitative and quantitative agreement with experimental data. The results obtained for the reactive flow show that in the calculation the combustion layer is closer to the channel bottom wall than in the experiment, and the temperature of the reaction product is by 10 % lower.