Bottom Wall Of Channel Research Articles

Numerical study of an impinging jet in cross-flow within and without influence of vortex generator structures on heat transfer

An impinging jet heat transfer in cross-flow within and without influence of a vortex generator pair (VGP) is studied using the unsteady Reynolds averaged Navier-Stokes (URANS) and the large-eddy simulation (LES). The jet Reynolds number is 15,000 and the cross-flow Reynolds number is 30,000. The elliptic-blending Reynolds stress model (EBRSM) is implemented and adapted to capture the effect of the jet close to the wall. A v2 − f model is also implemented to study the ability in predicting such a benchmark. Both models benefit from the elliptic relaxation equation in the entire computational domain. The URANS results are compared with the accurate results of the LES method and also the experimental data. The URANS method successfully presents the flow features of the impinging jet while underpredicts the enhancing heat transfer over the channel bottom wall. The URANS method fails to correctly predict the flow structures forming around the impinging region, because the method is more diffusive than the LES method. When manipulating VGP, a rectangular winglet vortex generator pair is placed in the cross-flow channel and upstream of the jet nozzle to enhance the impinging heat transfer. The VGP increases the Nusselt number at the impingement region. The structures generated by the VGP alter the effects of the cross-flow on the impinging heat transfer. There are Kelvin-Helmholtz instabilities at the shear layer of the jet and the cross-flow in the base flow (the flow without VGP). These instabilities are altered in the flow with VGP. A swirl component is added in the jet to study the effects on the heat transfer. The result shows that for a high or moderate level of swirl, the jet is diffused before the impinging.

Numerical simulation of buckling and asymmetric behavior of flexible filament using temporal second-order immersed boundary method

PurposeThe purpose of this paper is to perform two-dimensional numerical simulation involving fluid-structure interaction of flexible filament. The filament is tethered to the bottom of a rectangular channel with oscillating fluid flow inlet conditions at low Reynolds number. The simulations are performed using a temporal second-order finite volume-based immersed boundary method (IBM). Further, to understand the relation between different aspect ratios i.e. ratio of filament length to channel height (Len/H) and fixed channel geometry ratio, i.e. ratio of channel height to channel length (H/Lc) on mixing and pumping capabilities.Design/methodology/approachThe discretization of governing continuity and Navier–Stokes equation is done by finite-volume method on a staggered Cartesian grid. SIMPLE algorithm is used to solve fluid velocity and pressure terms. Two cases of oscillatory flow conditions are used with the flexible filament tethered at the center of bottom channel wall. The first case is sinusoidal oscillatory flow with phase shift (SOFPS) and second case is sinusoidal oscillatory flow without phase shift (SOF). The simulation results are validated with filament dynamics studies of previous researchers. Further, parametric analysis is carried to study the effect of filament length (aspect ratio), filament bending rigidity and Reynolds number on the complex deformation and behavior of flexible filament interacting with nearby oscillating fluid motion.FindingsIt is found that selection of right filament length and bending rigidity is crucial for fluid mixing scenarios. The phase shift in fluid motion is also found to critically effect filament displacement dynamics, especially for rigid filaments. Aspect ratio, suitable for mixing applications is dependent on channel geometry ratio. Symmetric deformation is observed for filaments subjected to SOFPS condition irrespective of bending rigidity, whereas medium and low rigidity filaments placed in SOF condition show severe asymmetric behavior. Two key findings of this study are: symmetric filament conformity without appreciable bending produces sweeping motion in fluid flow, which is highly suited for mixing application; and asymmetric behavior shown by the filament depicts antiplectic metachronism commonly found in beating cilia. As a result, it is possible to pin point the type of fluid motion governing fluid mixing and fluid pumping. The developed computational model can, thus, successfully demonstrate filament-fluid interaction for a wide variety of similar problems.Originality/valueThe present study uses a temporal second-order finite volume-based IBM to examine flexible filament dynamics for various applications such as fluid mixing. Also, it highlights the relationship between channel geometry ratio and filament aspect ratio and its effect on filament sweep patterns. The study further reports the effect of filament displacement dynamics with or without phase shift for inlet oscillating fluid flow condition.

Effects of heat transfer on separated boundary layer behavior under adverse pressure gradients

Separated boundary layers in a converging-diverging channel under two different wall heat transfer conditions, including the adiabatic wall and the cooled isothermal wall, have been investigated by using a large eddy simulation (LES) code coupled with a wall heat transfer module. The wall temperature of the cooled isothermal wall is 0.8 times of the inflow temperature, which is a typical value in a low-pressure turbine. The pressure distribution of the converging-diverging channel bottom wall is similar to that of a typical low-pressure turbine blade suction surface. The flow conditions are set as the exit isentropic Reynolds number (Re = 60,154) and the exit isentropic Mach number (Ma = 0.402). The results show that the cooled isothermal wall condition can increase the static pressure rise coefficient significantly, accelerate the transition process of the separation boundary layer, and suppress the separation. Furthermore, the cooled isothermal wall condition can decrease the frequency of the large-scale spanwise vortex rolled-up, reducing the influence of the spanwise vortex on the separated shear layer and decreasing the scales of the downstream hairpin vortex. In addition, the instability of the separated shear layer can be regarded as the superposition result of both the Tollmien-Schlichting (T-S) instability mechanism and the Kelvin-Helmholtz (K-H) instability mechanism, while the K-H instability mechanism on the whole is slightly more dominant in the presence of separation and reattachment. This trend is more obvious under the cooled isothermal wall condition.

Transient cooling characteristics of Al2O3-water nanofluid flow in a microchannel subject to a sudden-pulsed heat flux

Transient cooling characteristics of Al2O3-water nanofluid flow in a microchannel subject to sudden-pulsed heat flux are numerically examined in details. The attention is focused on the effects of microencapsulated phase change material (MEPCM) inside top wall of a microchannel on the transient cooling heat transfer behaviour of the microchannel. Two small portion of the bottom channel wall are exposed to a pulse heat flux. The Al2O3- water nanofluid as the working fluid with a fully developed velocity profile flows into the microchannel to cool the channel walls. Transient conjugate heat transfer is studied using a three-dimensional model in the three parts of the microchannel which are the nanofluid flow, the solid walls of the channel, and the MEPCM layer. The partial differential equations governing the energy conservation are presented and transformed into a dimensionless form. The finite volume method is employed to numerically integrate the equations for the temperature distribution in the microchannel. The numerical results are compared with the available works in the literature and found in good agreement. Besides, the predictions show that the presence of the MEPCM layer cannot effectively control the rise of the temperature of the microchannel in the presence of a pulse heat flux. For a typical pulse heat flux, the bulk temperature of the channel can be raised up to 2 °C while the presence of the MEPCM layer would only suppress the temperature rise by 0.2 °C. It is also found that the temperature of the top wall is under the significant influence of the MEPCM layer. The increase of the amplitude of the heat flux pulse results in better cooling effect of the MEPCM layer.

Numerical Study on the Convective Heat Transfer of Nanofluid Flow in Channel with Trapezoidal Baffles

This article presents a numerical study on forced convection of nanofluid flow in a two-dimensional channel with trapezoidal baffles. One baffle mounted on the top wall of channel and another mounted on the bottom wall of channel. The governing continuity, momentum and energy equations in body-fitted coordinates are iteratively solved using finite volume method and SIMPLE technique. In the current study, SiO2-water nanofluid with nanoparticles volume fraction range of 0- 0.04 and nanoparticles diameters of 30 nm is considered for Reynolds number ranging from 100 to 1000. The effect of baffles height and location, nanopar-ticles volume fraction and Reynolds number on the flow and thermal fields are investigated. It is found that the average Nusselt number as well as thermal hydraulic performance increases with increasing nanopartiles volume fraction and baffle height but accompanied by increases the pressure drop. The results also show that the best thermal- hydraulic performance is obtained at baffle height of 0.3 mm, locations of baffles at upper and lower walls of 10 and 15 mm, respectively, and nanoparticles volume fraction of 0.04 over the ranges of Reynolds number.

NUMERICAL ANALYSIS OF A REATTACHED FLOW BEHIND A RIB

Turbulent characteristics of a reattached flow separated from a rib in an air flat channel were numerically analyzed by RANS and LES methods. The both methods showed low pressure regions in corners formed by the rib and the channel walls that caused a vorticity tube to be formed along the rib. Nevertheless in RANS, a reattached flow was identified as two-dimensional with a velocity decreasing near the channel bottom wall. In LES, the low pressure regions caused transverse and normal motions in the separation region that resulted in redistributions between velocity components, Reynolds stresses and the velocity decrease near the channel bottom wall, i.e., the formation of three-dimensional reattached flow.

Highly Sensitive Microfluidic Chip Sensor for Biochemical Detection

Chip calorimetry offers a power tool for fast and high throughput analysis of biochemical process. However, it is challenging to realize an inexpensive, easy to fabricate microfluidic chip-based calorimeter with high sensitivity. This paper describes the design of a novel, highly sensitive, and continuous flow microfluidic chip sensor with an integrated antimony (Sb)–bismuth (Bi) thin-film thermopile heat detection element. The geometry and the design of the microfluidic device facilitate hydrodynamic flow focusing, and the integration and design of the thermopile sensor into the microfluidic device eliminates the need for reference temperature control. The device contains a single flow channel that is $120~\mu \text{m}$ high and 10-mm wide with two fluid inlets and one fluid outlet. An Sb-Bi thin film thermopile is fabricated on the inner surface of the bottom channel wall using thermal evaporation and was passivized with a $3~\mu \text{m}$ SU-8 photoresist layer. The device has been successfully used to measure the dynamic temperature changes resulting from heat generation following the mixing of glycerol and water. The effect of flow rates on the sensor’s response was measured. The sensor can detect dynamic temperature changes in the order of 10-6 K. The limit of detection of heat power of the device was calculated to be 8.8 pW. With the obtained remarkable sensitivity and heat power detection limit, the microfluidic chip sensor can potentially be used to investigate biochemical processes, such as enzyme-catalysed reactions, and metabolic activity of cells.

Electrokinetic instability in microchannel ferrofluid/water co-flows

Electrokinetic instability refers to unstable electric field-driven disturbance to fluid flows, which can be harnessed to promote mixing for various electrokinetic microfluidic applications. This work presents a combined numerical and experimental study of electrokinetic ferrofluid/water co-flows in microchannels of various depths. Instability waves are observed at the ferrofluid and water interface when the applied DC electric field is beyond a threshold value. They are generated by the electric body force that acts on the free charge induced by the mismatch of ferrofluid and water electric conductivities. A nonlinear depth-averaged numerical model is developed to understand and simulate the interfacial electrokinetic behaviors. It considers the top and bottom channel walls’ stabilizing effects on electrokinetic flow through the depth averaging of three-dimensional transport equations in a second-order asymptotic analysis. This model is found accurate to predict both the observed electrokinetic instability patterns and the measured threshold electric fields for ferrofluids of different concentrations in shallow microchannels.

Shock wave interaction with a thermal layer produced by a plasma sheet actuator

This paper explores the phenomena associated with pulsed discharge energy deposition in the near-surface gas layer in front of a shock wave from the flow control perspective. The energy is deposited in 200 ns by a high-current distributed sliding discharge of a ‘plasma sheet’ type. The discharge, covering an area of mm2, is mounted on the top or bottom wall of a shock tube channel. In order to analyse the time scales of the pulsed discharge effect on an unsteady supersonic flow, we consider the propagation of a planar shock wave along the discharge surface area 50–500 μs after the discharge pulse. The processes in the discharge chamber are visualized experimentally using the shadowgraph method and modelled numerically using 2D/3D CFD simulations. The interaction between the planar shock wave and the discharge-induced thermal layer results in the formation of a lambda-shock configuration and the generation of vorticity in the flow behind the shock front. We determine the amount and spatial distribution of the electric energy rapidly transforming into heat by comparing the calculated flow patterns and the experimental shadow images. It is shown that the uniformity of the discharge energy distribution strongly affects the resulting flow dynamics. Regions of turbulent mixing in the near-surface gas are detected when the discharge energy is deposited non-uniformly along the plasma sheet. They account for the increase in the cooling rate of the discharge-induced thermal layer and significantly influence its interaction with an incident shock wave.

Effects of wall suction/blowing on two-dimensional flow past a confined square cylinder.

A numerical simulation is conducted to study the laminar flow past a square cylinder confined in a channel (the ratio of side length of the square to channel width is fixed at 1/4) subjected to a locally uniform blowing/suction speed placed at the top and bottom channel walls. Governing equations with boundary conditions are resolved using a finite volume method in pressure–velocity formulation. The flow patterns relevant to the critical spacing values are investigated. Numerical results show that wall blowing has a stabilizing effect on the flow, and the corresponding critical Reynolds number increases monotonically with increasing blowing velocity. Remarkably, steady asymmetric solutions and hysteretic mode transitions exist in a certain range of parameters (Reynolds number and suction speed) in the case of suction.

Thermo-Magneto-Convective Transport around a Square Cylinder in a Square Duct under Strong Axial Magnetic Field

A quasi two-dimensional numerical study is performed to analyze the thermo-magneto-convective transport of liquid metal around a square cylinder in a square duct subjected to a strong externally imposed axial magnetic field. The channel bottom wall is considered heated while the top wall is maintained at the free stream temperature keeping the cylinder adiabatic. The Reynolds and Hartmann numbers are kept in the range 0 4 with Ha = 2160 is also considered to show the development of a Kelvin-Helmholtz-type instability that substantially affects the heat transfer rate.

Numerical Study Of Turbulent Thermal-Hydraulic Performance Of Al2o3-Water Nanofluid In Channel With Triangular Baffles

In this paper, turbulent forced convection of nanofluid flow in channel with isoscelestriangular baffles is numerically investigated over Reynolds number ranges of 5000-10000. One baffle mounted on the bottom wall of channel and another mounted on the top wall. Al2O3-water nanofluid with nanoparticles volume fraction of 4% and nanoparticles diameters of 25 nm is used. The governing continuity, momentum and energy equations as well as the low Reynolds number k-ε model of Launder and Sharma have been solved using finite volume method. The effect of baffle height, baffle distance as well as Reynolds number on the flow and thermal characteristics have been presented and discussed. It is found that the enhancement ratio of the average Nusselt number as well as the fraction factor increase with increasing in the baffles height. It is also found that the enhancement ratio of the average Nusselt number increases as the distance of top baffle decrease. Furthermore, the best thermal-hydraulic performance of channel with triangular baffles using nanofluid can be obtained at baffle height of 2.5 mm, distance of the top baffle of 40 mm and Reynolds number of 5000.

Transportation and Settling Distribution of Microparticles in Low-Reynolds-Number Poiseuille Flow in Microchannel

In this article, velocity field and settling distribution of microparticles in a dilute suspension in low-Reynolds-number Poiseuille flow in a microchannel is experimentally investigated using microscopic image analysis. An effective technique is applied to manipulate single-particle tracking in order to determine the controlling parameters on transportation and settling of microparticles in microchannels. The results show that the velocities of dispersed phase are affected by the hydrodynamic properties, and this velocity deviation can be significant when the hydrodynamic coupling between particles and channel walls is considerable. Increasing the Reynolds number would result in decrease in total number of particles settled on the bottom wall of channel.

Experimental study on the direct contact condensation of the steam jet in subcooled water flow in a rectangular channel: Flow patterns and flow field

Direct contact condensation (DCC) of steam jet in subcooled water flow in a channel was experimentally studied. The main inlet parameters, including steam mass flux, water mass flux and water temperature were tested in the ranges of 200–600kg/(m2s), 7–18×103kg/(m2s), 288–333K, respectively. Two unstable flow patterns and two stable flow patterns were observed via visualization window by a high speed camera. The flow patterns were determined by steam mass flux, water mass flux and water temperature, and the relationship between flow patterns and flow field parameters was discussed. The results indicated that whether pressure or temperature distributions on the bottom wall of channel could represent different flow patterns. And the position of pressure peak on the bottom wall could almost represent the condensation length. The upper wall pressure distributions were mainly dependent on steam and water mass flux; and the upper wall temperature distributions were affected by the three main inlet parameters. Moreover, the bottom wall pressure and temperature distributions of different unstable flow patterns had similar characteristics while those of stable flow patterns were affected by shock and expansion waves. The underlying cause of transition between different flow patterns under different inlet parameters was reflected and discussed based on pressure distributions.

Study of heat transfer due through rib-groove channel to turbulent How of nanofluids

Nanofluids for improve characteristics flow in a rib-groove channel are investigate. The continuity, momentum and energy equations were solved by FLUENT program. The bottom wall of channel is heated while the upper wall is symmetry, the left side velocity inlet, and the right side is outlet (pressure out). Four different rib-groove shapes are used. Four different types of nanoparticles, Al2O3, CuO, SiO2, and ZnO with different volumes fractions in the range of 1% to 4% and different nanoparticle diameter in the range of 25 nm to 70 nm, are dispersed in the base fluid water are used. In this paper, several parameters such as different Reynolds numbers in the range of 10000 < Re < 40000 are investigated. The numerical results indicate that the trapezoidal with increasing height in the flow direction rib- trapezoidal groove has the best heat transfer and high Nusselt number; the nanofluids with SiO2 have the best behavior. The Nusselt number increases as the volume fraction increases and it decreases as the nanoparticle diameter increases.

MHD flow and heat transfer behind a square cylinder in a duct under strong axial magnetic field

We capture through numerical simulation the MHD (magnetohydrodynamic) vortex dynamics around a square object in a square duct subjected to a strong externally imposed axial magnetic field. A quasi two-dimensional conditionality allows us to follow a two-dimensional modeling approach. The pertinent MHD control parameters such as the Reynolds and Hartmann numbers are kept in the range 0<Re⩽6000 and 0<Ha⩽2160, respectively. The various regimes of the MHD wake are found in-line with those obtained by Dousset and Pothérat under similar conditions using a circular object (Dousset and Pothérat, 2008). Four different regimes are identified out of which the first three regimes are the classical non-MHD 2D cylinder wakes. The transition from one to another regime is controlled by the friction parameter Re/Ha. The fourth regime is characterized by the vortices evolved from the duct side walls due to the boundary layer separation which strongly disturbs the Kármán vortex street. In order to explore the thermal transport phenomena under the action of the axially imposed magnetic field, the channel bottom wall is considered heated while the top wall is maintained at the free stream temperature keeping the cylinder adiabatic. The heat transfer rate from the heated channel wall strongly depends on the imposed magnetic field strength as well as Reynolds number. Additionally, an enhancement in heat transfer is experienced by placing the square cylinder in the channel over the bare channel, however, simultaneously a degradation in heat transfer would occur if the square object is replaced by a same size circular object.

Forced Convection Analysis of Discrete Heated Porous Convergent Channel

This study investigates numerically forced convection heat transfer and flow analyses of a passive heat exchanger for nonporous and partially filled porous channels with varying exit height (1, 0.5, and 0.25). Four discrete heat sources with uniform heat flux are simulated on the channel bottom wall. The partially filled porous channels are tested at two different porous block heights (0.5 and 1). The flow field and thermal analyses inside the channels are investigated across a wide range of Reynolds and Darcy numbers for Prandtl number of 0.71. The results reveal that the porous block and the exit height affect substantially the flow and heat transfer characteristics inside the tested channels. The Nusselt number is enhanced by 20–40% for the partially filled porous convergent channel (exit height = 0.25 and porous block height = 1) compared to the nonporous channel. Consequently, the heat exchanger size can be reduced by 37.5%. Moreover, the overall heat transfer performance parameter is enhanced with further increase in Darcy number at low Reynolds number. As a result, compact heat exchangers that provide superior heat transfer coefficients lead to development of macro- and microelectronic devices.

Thermal Behaviors in a Solar Air Heater Channel with Arc-Shaped Baffle Turbulators

Influences of arc-shaped baffles turbulators (ASB) on thermal behaviors in a solar air heater channel are studied. In the experiment, the ASB with three pitch ratios (P/e) are placed on the absorber plate (bottom channel wall in this case) to generate stronger turbulence intensity in the channel. The distributions of temperature and local Nusselt number contours in the absorber plate fitted with ASB are observed with thermochromic liquid crystal (TLC) sheet. The results reveal that the heat transfer increases with the decrease in P/e due to longitudinal vortex flow effect. The heat transfer from employing the ASB at P/e = 4.0, 6.0 and 8.0 is higher than the smooth channel around 116%, 120% and 127%, respectively. This indicates that the ASB pitch spacing is optimal at 8 times baffle height in the present work.

Synthetic Jet Interaction With Approaching Turbulent Boundary Layer for Heat Transfer Enhancement

An experimental investigation of the interaction between a double-orifice synthetic jet with an approaching turbulent boundary layer developing on the bottom wall of square channel has been carried out. Schlieren visualization and velocity measurement using hotwire anemometry are carried out to characterize the synthetic jet actuators arrangement. Detailed heat transfer measurements are carried out using liquid crystal thermography. Hotwire measurements on the wall surface have been reported to understand the interaction between the approaching boundary layer and synthetic jet. Maximum overall heat transfer enhancement of 55.48% at an actuation voltage of 55 V is observed with entropy generation number equal to 0.649. The amplitude modulation is observed to increase the heat transfer enhancement in comparison to that without amplitude modulation.

Dynamic behavior of a droplet on a moving wall

A two-dimensional immiscible droplet deformation phenomenon on a moving channel bottom wall is simulated using the lattice Boltzmann method. We considered the effect of the initial static contact angle, the capillary number, and the size of the droplet on the dynamic behavior of the moving droplet. When the initial static contact angle is less than 90°, the moving droplet is deformed and stretched, resulting in increasing width and decreasing height of the droplet. This is due to the hydrophilic (wetting) characteristic of the channel’s bottom wall. However, when the initial static contact angle is larger than 90°, the deformed and stretched droplet on the moving channel bottom wall is broken up, and is then pinched off or detached from the moving channel bottom wall, depending on the initial static contact angle and capillary number. This is due to the hydrophobic (non-wetting) characteristic of the wall.