Top Wall Of Channel Research Articles

Effect of diffusion time on the mechanism of deflagration to detonation transition in an inhomogeneous mixture of hydrogen-air

This study aimed to numerically investigate the effect of diffusion time between H2 injection and ignition on mechanisms behind deflagration to detonation transition (DDT) in an inhomogeneous H2/Air mixture. The combustion chamber is a closed, rectangular tube with seven obstructions and a blockage ratio of 30%. From the top wall of the air channel, hydrogen is injected with diffusion times of 3, 5, 7.5, and 10 s. As a result, the inhomogeneous mixtures with different concentrations gradients are formed in the vertical direction of flame propagation. The solution method is the finite-volume method with k−ωSST turbulence model and the Weller flame wrinkling combustion model. The current findings reveal that shortening the diffusion time between H2 injection and ignition, or increasing the mixture's inhomogeneity, causes DDT to occur faster for a lean inhomogeneous H2-Air mixture of 20%. The onset of detonation in a lean mixture is caused by shock waves focusing ahead of the flame front at the channel's top wall, where hydrogen concentration is higher. For a rich inhomogeneous H2/Air mixture of 35%, the concentration gradient weakens flame acceleration and delays the initiation of detonation. In the rich mixture, three different regimes of onset of detonation appear by changing diffusion time. In the first regime, a strong curved incident shock wave forms a Mach stem. Behind this Mach stem, chemical reactions are being activated. Finally, the flame front and the reaction region behind the Mach stem collide, causing DDT to happen. In the second regime, Mach stem does not form; however, DDT occurs in the direct vicinity of the flame where reflected shock waves from the obstacles collide flame front. In the third regime, neither the Mach stem nor the reflected shock waves play a significant role in detonation initiation.

Forced convection heat transfer of Ag–MgO/water micropolar hybrid nanofluid inside a stairway channel

In this paper, the forced convection heat transfer of Ag–MgO/water hybrid micropolar nanofluid in a channel is studied numerically which the top wall of channel is smooth and bottom of it is in stairway form. The lattice Boltzmann method (LBM) is used for solving the fluid flow and heat transfer equations. The effects of Reynolds numbers ([Formula: see text]–400), volume fraction of nanofluid ([Formula: see text], 0.01 and 0.02) and stairway aspect ratio ([Formula: see text], 0.2 and 0.3) on the velocity, temperature and local and average Nusselt number profiles are investigated. The results show that by incrementing the Reynolds number and solid volume fraction and decreasing the [Formula: see text]/[Formula: see text] simultaneously, the average Nusselt number increases.

A microchannel flow with presence of micro-post arrays on channel top wall

A microchannel flow is considered and the influence of the micro-post arrays and air trapped on velocity and temperature increase of the working fluid is examined experimentally and numerically incorporating 3-D simulations. The channel bottom plate is considered to be smooth and hydrophilic, which moves with a constant velocity, while channel top wall is stationary and it has staggered arrangement of micro-post arrays with hydrophobic wetting state. An experiment is carried out to validate velocity and temperature predictions while setting simulation conditions in line with the experiments. It is found that the flow field in top channel wall vicinity is influenced by the micro-post arrays; hence, wavy flow field is generated in this region. Flow circulation in the trapped air in between the micro-post arrays influences the heat transfer rates. The Nusselt number reduces slightly with increasing pitch size of the micro-post arrays. In addition, increasing the microchannel height lowers the Nusselt number. The pressure drop between the channel inlet and exit is influenced significantly by the micro-post arrays. Increasing pitch size of the micro-post arrays reduces the pressure drop; however, further increase of the pitch size beyond 12 μm does not alter the pressure drop across the microchannel.

Carbon Dioxide Bubbles Removal by Capillary Actuation in the Anode Channel of Direct Methanol Fuel Cells

Abstract On the anode side of a direct-methanol fuel cell, carbon dioxide bubbles are generated as a result of the methanol oxidation reaction. The accumulation of such bubbles prevents methanol from reaching the diffusion layer (DL). Hence, a reduction in the reaction rate occurs, which limits the maximum current density of the cell. To keep carbon dioxide bubbles away from the diffusion layer surface, a new design of the anode flow channel besides wall surface treatment is developed. Such a design can introduce capillary actuation, which forces the carbon dioxide bubbles to move away from the diffusion layer due to capillary forces. This can be achieved by using a trapezoidal shape of the flow channel, as well as the combined effect of hydrophilic and hydrophobic surface treatments on the diffusion layer and top wall, respectively. To identify the optimal design of the anode flow channel, a three-dimensional, two-phase flow model is developed. The model is numerically simulated, and the results are validated with available measurements. Results indicated that treating the diffusion layer with a hydrophilic layer increases the area in direct contact with liquid methanol. Besides, the hydrophobic top channel wall makes it easier for the carbon dioxide bubbles to attach and spread out on the top surface. However, super-hydrophobic treatment of the top wall should be avoided, as it can cause difficulty in bubble extraction from the channel. The current findings create a promising opportunity to improve the performance of direct-methanol fuel cells.

Theory of the flow-induced deformation of shallow compliant microchannels with thick walls.

Long, shallow microchannels embedded in thick, soft materials are widely used in microfluidic devices for lab-on-a-chip applications. However, the bulging effect caused by fluid-structure interactions between the internal viscous flow and the soft walls has not been completely understood. Previous models either contain a fitting parameter or are specialized to channels with plate-like walls. This work is a theoretical study of the steady-state response of a compliant microchannel with a thick wall. Using lubrication theory for low-Reynolds-number flows and the theory for linearly elastic isotropic solids, we obtain perturbative solutions for the flow and deformation. Specifically, only the channel's top wall deformation is considered, and the ratio between its thickness t and width w is assumed to be (t/w)2≫1. We show that the deformation at each stream-wise cross section can be considered independently, and that the top wall can be regarded as a simply supported rectangle subject to uniform pressure at its bottom. The stress and displacement fields are found using Fourier series, based on which the channel shape and the hydrodynamic resistance are calculated, yielding a new flow rate-pressure drop relation without fitting parameters. Our results agree favourably with, and thus rationalize, previous experiments.

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.

Heat transfer and flow characteristics of U-shaped cooling channels with novel wavy ribs under stationary and rotating conditions

Wavy rib is a kind of novel turbulator that can effectively improve the ribbed wall heat transfer of cooling channel and produce acceptable friction loss. Meanwhile, it has the advantage of simple structure. In this paper, the effects of wavy ribs on the heat transfer distributions of ribbed walls, side walls, and top wall are numerically studied in stationary and rotating U-shaped channels. The flow characteristics of cooling air are also analyzed. Taking the 45° V-shaped ribs as the reference scheme, the high-performance wavy ribs that have better heat transfer performance on ribbed wall and induce the same friction loss are selected. The channel aspect ratio is 1, with the height of 12.7 mm. The investigated Reynolds number is 25,000, and the rotation number is 0, 0.2, 0.3 and 0.4. Rib height (e-3e), rib round radius (0–5 mm), and rib angle (20–55°) are the three major geometrical parameters of wavy rib. The results show that, the diversion effect of wavy ribs on cooling air is the reason of friction loss suppression. On the one hand, the wavy ribs confine the fluid disturbance on side walls, leading to the heat transfer reduction. On the other hand, the impingement of cooling air on the channel top wall is strengthened, which improves the wall heat transfer. In stationary and rotating conditions, the high-performance wavy ribs increase the heat flux on heated surface corresponding to the ribbed wall by 5–21% and 4–24% respectively, and weaken the effect of Coriolis force on the heat transfer of the first pass. With the increase of rotation number, the friction penalty and heat transfer of leading side ribbed wall decrease, while the heat transfer of trailing side ribbed wall slightly increases, showing that those high-performance wavy ribs have a good versatility at multiple rotation numbers.

Directional Passive Transport of Microdroplets in Oil-Infused Diverging Channels for Effective Condensate Removal.

Condensation widely exists in nature and industry, and its performance heavily relies on the efficiency of condensate removal. Recent advances in micro-/nanoscale surface engineering enable condensing droplet removal from solid surfaces without extra energy cost, but it is still challenging to achieve passive transport of microdroplets over long distances along horizontal surfaces. The mobility of these condensate droplets can be enhanced by lubricant oil infusion on flat surfaces and frequent coalescence, which lead to fast growth but random motion of droplets. In this work, we propose a novel design of diverging microchannels with oil-infused surfaces to achieve controllable, long-distance, and directional transport of condensing droplets on horizontal surfaces. This idea is experimentally demonstrated with diverging copper and silicon microchannels with nanoengineered surfaces. Along these hierarchical surface structures, microdroplets condense on the top channel wall and submerge into microchannels owing to the capillary pressure gradient in infusing oil. Confined by the microchannel walls, the submerged droplets deform and maintain the back-front curvature difference, which enables the motion of droplets along the channel diverging direction. Subsequent droplet coalescences inside the channel further enhance this directional transport. Moreover, fast-moving deformed droplets transfer their momentum to downstream spherical droplets through the infusing oil. As a result, simultaneous passive transport of multiple droplets (20-400 μm) is achieved over long distances (beyond 7 mm). On these oil-infused surfaces, satellite microdroplets can further nucleate and grow on an oil-cloaked droplet, demonstrating an enlarged surface area for condensation. Our findings on passive condensate removal offer great opportunities in condensation enhancement, self-cleaning, and other applications requiring directional droplet transport along horizontal surfaces.

Particle Based Modeling of Electrical Field Flow Fractionation Systems

Electrical Field Flow Fractionation (ElFFF) is a sub method in the field flow fractionation (FFF) family that relies on an applied voltage on the channel walls to effect a separation. ElFFF has fallen behind some of the other FFF methods because of the optimization complexity of its experimental parameters. To enable better optimization, a particle based model of the ElFFF systems has been developed and is presented in this work that allows the optimization of the main separation parameters, such as electric field magnitude, frequency, duty cycle, offset, flow rate and channel dimensions. The developed code allows visualization of individual particles inside the separation channel, generation of realistic fractograms, and observation of the effects of the various parameters on the behavior of the particle cloud. ElFFF fractograms have been generated via simulations and compared with experiments for both normal and cyclical ElFFF. The particle visualizations have been used to verify that high duty cycle voltages are essential to achieve long retention times and high resolution separations. Furthermore, by simulating the particle motions at the channel outlet, it has been demonstrated that the top channel wall should be selected as the accumulation wall for cyclical ElFFF to reduce band broadening and achieve high efficiency separations. While the generated particle based model is a powerful tool to estimate the outcomes of the ElFFF experiments and visualize particle motions, it can also be used to design systems with new geometries which may lead to the design of higher efficiency ElFFF systems. Furthermore, this model can be extended to other FFF techniques by replacing the electrical field component of the model with the fields used in the other FFF techniques.

A thermal performance factor for evaluation of active engine cooling with asymmetric heating

A theoretical expression of thermal performance factor has been derived in this paper for simple engineering evaluation of different cooling channel designs with asymmetric heating, intended for regenerative engine cooling applications in the aerospace propulsion and power-generation systems. As a specific example, numerical study has been conducted for conjugate heat transfer of the cryogenic methane in rectangular engine cooling channels at a supercritical pressure, with a constant heat flux enforced on the top channel wall. The thermal performance factor is used for practical assessment of the various engine cooling channel designs, in terms of both heat transfer enhancement and pressure loss. Results show that the geometric parameters of the cooling channel, in addition to the Nusselt number and friction factor, play a crucial role in determining the overall performance of fluid flows and heat transfer. An optimum cooling channel design could be approximated using the thermal performance factor as a first-step engineering design tool.

Turbulent flow and heat transfer enhancement of forced convection over heated baffles in a channel

PurposeThe purpose of this paper is to examine the turbulent fluid flow and heat transfer characteristics for rectangular channel provided with solid plate baffles which are arranged on the bottom and top channel walls in a periodically staggered way.Design/methodology/approachThe turbulent governing equations are solved by a finite volume method with the second‐order up winding scheme and the k‐ω turbulence model to describe the turbulent structure. The velocity and pressure terms of momentum equations are solved by SIMPLE (semi‐implicit method for pressure‐linked equation) algorithm. The parameters studied include the entrance Reynolds number Re (5.103‐2.104), the baffles height are fixed at (h=0.08 m); whereas three different baffle spacing were considered S1 = D, S2 = D/2 and S3=3D/2 and the working medium is air.FindingsIn this work, it is found that vortex shedding generated by the baffle on the upper wall can additionally enhance heat transfer along the baffle surfaces. The wavy flow significantly changes the recirculating zone behind the last baffle. Finally, changing the baffles spacing seemed to reduce to changing the heat transfer surface between the solid and the fluid in the sense that higher heat transfer is obtained for lower spacing between baffles.Originality/valueThe results of the numerical calculations of the flow field indicate that the flow patterns around the solid baffles depending on the spacing of the baffles and it significantly influences the local heat transfer coefficient distributions. The problem is inversely proportional for the friction factor.

Sidewall and thermal boundary condition effects on the evolution of longitudinal rolls in Rayleigh-Bénard-Poiseuille convection

Experimental and numerical studies of steady longitudinal convection rolls that develop in a Poiseuille air flow in a rectangular channel heated from below and cooled from the top are conducted in the range 3500 ≤ Ra ≤ 6000 and 20 ≤ Re ≤ 200. The effect of the lateral vertical walls on the onset and development of the convection cells is investigated by changing the transverse aspect ratio of the channel from 4.7 to 18.4. The influence of the entrance temperature and adiabatic or conductive thermal boundary conditions at the side and top walls of the channel is also investigated. The scenario of the roll formation is described in details. It results in a symmetric pattern in the form of steady longitudinal rolls with an even number of rolls that depends not only on the aspect ratio but possibly on the inlet temperature of the flow. It is shown that the fully developed pattern is determined by the two rolls nearby each vertical side wall that are triggered just at the entrance of the channel due to the presence of velocity boundary layers adjacent to the walls. It is also shown that the heat conduction in the top horizontal wall of the experimental channel must be taken into account in the numerical simulations so that the experimental wavenumber can be properly depicted.

Computational analysis of transient turbulent flow and conjugate heat transfer characteristics in a solar collector panel with internal, rectangular fins and baffles

The poor thermal exchange between the absorber and the fluid in the solar air flat plate collector, gives the bad performance and the mediocre thermal efficiency. The introduction of obstacles in the dynamic air vein of the solar collector in order to obtain a turbulent flow is a technique that improves the thermal exchange by convection between the air and the absorber. This article present a computational analysis on the turbulent flow and heat transfer in solar air collector with rectangular plate fins absorber and baffles which are arranged on the bottom and top channel walls in a periodically staggered way. To this end we solved numerically, by the finite volumes method, the conservation equations of mass, momentum and energy. The low Reynolds number k-? model was adopted for the taking into account of turbulence. The velocity and pressure terms of momentum equations are solved by the SIMPLE algorithm. The parameters studied include the entrance mass flow rate of air. The influence of the mass flow rate of air on the axial velocity and the efficiency of upward type baffled solar air heaters have been investigated numerically. The results show that the flow and the heat transfer characteristics are strongly dependent on mass-flow rate of air and the presence and/or the absence of the baffles and fins in the solar collector. It was observed that increasing the Reynolds number will increase the efficiency of the solar panel, as expected.

Spatially resolved wall temperature measurements during flow boiling in microchannels

Spatial and temporal variations of channel wall temperature during flow boiling microchannel flows using infrared thermography are presented and analyzed. In particular, the top channel wall temperature in a branching microchannel silicon heat sink is measured non-intrusively. Using this technique, time-averaged temperature measurements, with a spatial resolution of 10 μm, are presented over an 18 mm × 18 mm area of the heat sink. Also presented, within a specific sub-region of the heat sink, are intensity maps that are recorded at a rate of 120 frames per second. Time series data at selected point locations in this sub-region are analyzed for their frequency content, and dominant temperature fluctuations are extracted using proper orthogonal decomposition. Results at low-vapor-quality boiling condition indicate that temperatures can be determined from recorded radiation intensities with a temperature uncertainty varying from 0.9 °C at 25 °C to 1.0 °C at 125 °C. The time series data indicate periodic wall temperature fluctuations of approximately 2 °C that are attributed to the passage of vapor slugs. A dominant band of frequencies around 2–4 Hz is suggested by the frequency analysis. Proper orthogonal decomposition results indicate that first six orthogonal modes account for approximately 90% of the variance in temperature. The first mode reconstruction accounts for temporal variations in the dataset in the sub-region analyzed; however the magnitude of fluctuations and spatial variations in temperature are not accurately captured. A reconstruction using the first 25 modes is considered sufficient to capture both the temporal and spatial variations in the data.

Calculation of turbulent flow and heat transfer in a porous-baffled channel

This study presents the numerical predictions on the turbulent fluid flow and heat transfer characteristics for rectangular channel with porous baffles which are arranged on the bottom and top channel walls in a periodically staggered way. The turbulent governing equations are solved by a control volume-based finite difference method with power-law scheme and the k– ε turbulence model associated with wall function to describe the turbulent structure. The velocity and pressure terms of momentum equations are solved by SIMPLE (semi-implicit method for pressure-linked equation) method. The parameters studied include the entrance Reynolds number Re (1×10 4–5×10 4), the baffle height ( h=10, 20 and 30 mm) and kind of baffles (solid and porous); whereas the baffle spacing S/ H are fixed at 1.0 and the working medium is air. The numerical calculations of the flow field indicate that the flow patterns around the porous- and solid-type baffles are entirely different due to different transport phenomena and it significantly influences the local heat transfer coefficient distributions. Relative to the solid-type baffle channel, the porous-type baffle channel has a lower friction factor due to less channel blockage. Concerning the heat transfer effect, both the solid-type and porous-type baffles walls enhanced the heat transfer relative to the smooth channel. It is further found that at the higher baffle height, the level of heat transfer augmentation is nearly the same for the porous-type baffle, the only difference being the Reynolds number dependence. As expected, the centerline-averaged Nusselt number ratio increases with increasing the baffle height because of the flow acceleration.

Simulation of a vertical reactor for high pressure organometallic chemical vapor deposition

The suitability of a vertical cylindrical reactor with highly constrained radial flow from a central gas injection port past a set of heated substrate wafers that are embedded in the top channel wall has been evaluated in the context of organometallic chemical vapor deposition (OMCVD) at elevated pressure. Numerical simulations showed that, in addition to the limitation on the channel height necessary for preventing buoyancy driven recirculation, negotiating the ninety-degree bend at the inlet is problematic and also constrains the channel height below a critical value, at which the radial flow area after the inlet bend is equal to the cross-sectional area of the central gas injection port. Restricting the channel height poses the danger of heating of the channel wall opposite to the substrate wafers causing potential problems with deposition of decomposition products and competitive polycrystalline film growth at this location. These problems can be avoided by actively cooling the channel wall opposite to the substrate and by keeping the retention time of the source vapor molecules and fragments thereof in the wafer location below a critical value.

Heat Transfer and Friction in a Low‐Aspect‐RatioRectangular Channel with Staggered Slit‐Ribbed Walls

Fully developed heat transfer and friction in a rectangular channel with slit‐ribbed walls are examined experimentally. The slit ribs are transversely arranged on the bottom and top channel walls in a staggered manner. Effects of rib open‐area ratio (β= 24%, 37%, and 46%), rib pitch‐to‐height ratio (Pi/H = 10, 15and20), and Reynolds number (10, 000 ≤ Re≤50, 000) are examined. The rib height‐to‐channel hydraulic diameter ratio is fixed at H/De = 0.081. It is disclosed that the heat transfer coefficient for the slit‐ribbed channel is higher than that for the solid‐ribbed channel, and increases with rib open‐area ratio. Results also show that the friction factor for the slit‐ribbed channel is significantly lower than that for the solid‐ribbed one. Moreover, the ribs with larger open‐area ratios in a higher flow Reynolds number condition could give the better thermal performance under the constant friction power constraint. Roughness functions for friction and heat transfer are further developed in terms of rib and flow parameters.

Turbulent Heat Transfer and Fluid Flow in a Porous-Baffled Channel

Experiments were conducted to examine the effect of porous-type baffles on heat transfer and friction in a baffled channel. The porous-type baffles are arranged on the bottom and top channel walls in a staggered manner. Flowfield, pressure loss, isotherm pattern, and channel centerline heat transfer coefficients are obtained inside the periodic cell formed between the segmented baffles. Results of the conventional solid-type-baffled channel are also obtained for comparison. The void fraction ν o of porous-type baffle and the baffle spacing S/H are fixed at 0.42, and 1.0, respectively; whereas the baffle height ratio h/H varies from 0.25 to 0.75 and the flow Reynolds number Re ranges from 1 x 10 4 to 5 x 10 4 , respectively. It is disclosed that the flow patterns around the porous- and solid-type baffles are entirely different. These different transport phenomena, in conjunction with the change in baffle effectiveness significantly influence the local heat transfer coefficient distributions but negligibly influence the average heat transfer coefficients. Relative to the solid-type-baffled channel, the porous-type-baffled channel has a lower friction loss.