Showing papers on "Laminar flow reactor published in 2004"

Microfluidic synthesis of colloidal silica.

Abstract: We demonstrate the design, fabrication, and operation of microfluidic chemical reactors for the synthesis of colloidal silica particles. Two reactor configurations are examined: laminar flow reactors and segmented flow reactors. We analyze particle sizes and size distributions and examine their change with varying linear flow velocity and mean residence time. Laminar flow reactors are affected by axial dispersion at high linear velocities, thus leading to wide particle size distributions under these conditions. Gas is used to create a segmented flow, consisting liquid plugs separated by inert gas bubbles. The internal recirculation created in the liquid plugs generates mixing, which eliminates the axial dispersion effects associated with laminar flow reactors and produces a narrow size distribution of silica nanoparticles.

An Extension of the Flow Boiling Correlation to Transition, Laminar, and Deep Laminar Flows in Minichannels and Microchannels

Abstract: Flow boiling in mini- and microchannels offer very high heat transfer capabilities and find applications in many emerging technologies, such as electronics cooling and fuel cells. The low flow rate employed in such geometries, coupled with the small flow channels, often results in a laminar flow with all flow as liquid. Since the single-phase flow with all liquid is in the laminar range, the flow boiling correlations developed for conventional tubes with an inner diameter larger than 3 mm and turbulent flow need to be carefully reviewed. In the present work, flow boiling correlation for large diameter tubes developed by Kandlikar [1, 2] is modified for flow boiling in minichannels by using the laminar single-phase heat transfer coefficient for all liquid flow. The correlation is also extended for flow boiling in microchannels using the nucleate boiling as the dominant part of the original correlation. The trends in heat transfer coefficient versus quality are compared in the laminar and deep laminar regio...

Transition from laminar to turbulent flow in liquid filled microtubes

Abstract: The transition to turbulent flow is studied for liquids of different polarities in glass microtubes having diameters between 50 and 247 µm. The onset of transition occurs at Reynolds numbers of ~1,800–2,000, as indicated by greater-than-laminar pressure drop and micro-PIV measurements of mean velocity and rms velocity fluctuations at the centerline. Transition at anomalously low values of Reynolds number was never observed. Additionally, the results of more than 1,500 measurements of pressure drop versus flow rate confirm the macroscopic Poiseuille flow result for laminar flow resistance to within −1% systematic and ±2.5% rms random error for Reynolds numbers less than 1,800.

Chaotic flow and efficient mixing in a microchannel with a polymer solution

Abstract: Microscopic flows are almost universally linear, laminar, and stationary because the Reynolds number, Re, is usually very small. That impedes mixing in microfluidic devices, which sometimes limits their performance. Here, we show that truly chaotic flow can be generated in a smooth microchannel of a uniform width at arbitrarily low Re, if a small amount of flexible polymers is added to the working liquid. The chaotic flow regime is characterized by randomly fluctuating three-dimensional velocity field and significant growth of the flow resistance. Although the size of the polymer molecules extended in the flow may become comparable to the microchannel width, the flow behavior is fully compatible with that in a macroscopic channel in the regime of elastic turbulence. The chaotic flow leads to quite efficient mixing, which is almost diffusion independent. For macromolecules, mixing time in this microscopic flow can be three to four orders of magnitude shorter than due to molecular diffusion.

Ultrasonic agitation in microchannels

Abstract: This paper describes an acoustic method for inducing rotating vortex flows in microchannels. An ultrasonic crystal is used to create an acoustic standing wave field in the channel and thus induce a Rayleigh flow transverse to the laminar flow in the channel. Mixing in microchannels is strictly diffusion-limited because of the laminar flow, a transverse flow will greatly enhance mixing of the reactants. This is especially evident in chemical microsystems in which the chemical reaction is performed on a solid phase and only one reactant is actually diffusing. The method has been evaluated on two different systems, a mixing channel with two parallel flows and a porous silicon micro enzyme reactor for protein digestion. In both systems a significant increase of the mixing ratio is detected in a narrow band of frequency for the actuating ultrasound.

BURNING VELOCITIES AND A HIGH-TEMPERATURE SKELETAL KINETIC MODEL FOR n-DECANE

Abstract: Laminar flame speeds of n-decane/air mixtures were determined experimentally over an extensive range of equivalence ratios at 500 K and at atmospheric pressure. The effect of N2 dilution on the laminar flame speed was also studied at these same conditions. The experiments employed the stagnation jet–wall flame configuration with the flow velocity field determined by particle image velocimetry. Reference laminar flame speeds were obtained using linear extrapolation from low to zero stretch rate. The determined flame speeds are significantly different from those predicted using existing published kinetic models, including a model validated previously against high-temperature data from flow reactor, jet-stirred reactor, shock tube ignition delay, and burner-stabilized flame experiments. A significant update of this model is described that continues to predict the earlier validation experiments as well as the newly acquired laminar flame speed data and other recently published shock-tube ignition del...

Heating effect on steady and unsteady horizontal laminar flow of air past a circular cylinder

Abstract: Extensive numerical experiments were carried out to study the effect of cylinder heating on the characteristics of the flow and heat transfer in a two-dimensional horizontal laminar flow of air past a heated circular cylinder for the range of Reynolds numbers 0.001⩽Re⩽170. The fluid was treated as incompressible (density is independent of the pressure) while the variation of the fluid properties with temperature was taken into account. By including the transient density term of the continuity equation, which was neglected in a previous study by Lange, Durst, and Breuer [Int. J. Heat Mass Transfer 41, 3409 (1998)], we were able to predict correctly the vortex shedding frequency at various overheat ratios using an incompressible flow solver. The effect of dynamic viscosity and density variations on the flow dynamics occurring with the cylinder heating was analyzed separately. Another emphasis of the work was to investigate the physical mechanism behind the “effective Reynolds number” concept widely applied ...

Two-Phase Flow and Heat Transfer in Rectangular Micro-Channels

Abstract: Knowledge of flow pattern and flow pattern transitions is essential to the development of reliable predictive tools for pressure drop and heat transfer in two-phase micro-channel heat sinks. In the present study, experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel having a 0.406 × 2.032 mm cross-section. Superficial velocities of nitrogen and water ranged from 0.08 to 81.92 m/s and 0.04 to 10.24 m/s, respectively. Flow patterns were first identified using high-speed video imaging, and still photos were then taken for representative patterns. Results reveal that the dominant flow patterns are slug and annular, with bubbly flow occurring only occasionally; stratified and churn flow were never observed. A flow pattern map was constructed and compared with previous maps and predictions of flow pattern transition models. Annual flow is identified as the dominant flow pattern for conditions relevant to two-phase micro-channel heat sinks, and forms the basis for development of a theoretical model for both pressure drop and heat transfer in micro-channels. Features unique to two-phase micro-channel flow, such as laminar liquid and gas flows, smooth liquid-gas interface, and strong entrainment and deposition effects are incorporated into the model. The model shows good agreement with experimental data for water-cooled heat sinks.Copyright © 2003 by ASME

Characterization of performances of the wall-type reactor with plate-fin type nickel catalyst prepared by electroless plating, for methanol decomposition

Abstract: A catalytic reactor requires an effective exchange of heat energy, a quick load response and a downsized dimension, as is seen for the reformer system that generates hydrogen for fuel cells. A wall-type reactor, which metallic wall is directly catalyzed, is attracting interest as a reactor that would satisfies such demands. This research studied the performance of reaction and heat transfer of a rectangular wall reactor, for decomposing methanol to hydrogen and carbon monoxide. The reactor consists of alternating reaction channels with a plate-fin type nickel catalyst prepared by electroless plating between heat medium channels. Furthermore, the dynamic response of the reactor was also examined when the flow rate of feed gas was rapidly changed.The temperature profiles in the rectangular wall reactor demonstrated that the reactor effectively supplies heat energy to reaction zone, even under the reaction condition with a large amount of energy consumption. In addition, the performance of the reactor to handle the feeding material depended on the channel height and the shape of the plate fins inserted into the reaction channel. The performance increased as the height decreased and when serrated-type fins were used. The overall coefficient of heat transfer estimated from the temperature distribution in the reactor suggests that the performance increased because the heat conductivity of the reactor improved as a result of changing the channel height. The heat conductivity of effluent gas stabilized within a few seconds from when the flow rate of feed gas was changed instantaneously. The time required for the reaction to attain steady state was short. The results suggest that the constructed reactor responds to load fluctuation quickly.

A study of laminar flow of polar liquids through circular microtubes

Abstract: Recently, the validity of using classical flow theory to describe the laminar flow of polar liquids and electrolytic solutions through microtubes has been questioned for tube diameters as large as 500 μm [Brutin and Tadrist, Phys. Fluids 15, 653 (2003)]. This potential increase in flow resistance, which has been attributed to electrokinetic effects and enhanced surface roughness effects, is critical to the understanding of certain biofluid systems. We seek to characterize this phenomenon for a variety of capillary/liquid systems. Using a numerical solution to the Poisson–Boltzmann equation, we have calculated the electroviscous effect for the systems under consideration. We have also measured the pressure drop as a function of flow rate across well-characterized stainless steel and polyimide microtubes ranging in diameter from 120 μm to 440 μm. Deionized water, tap water, a saline solution, and a variety of glycerol/water mixtures were used. The calculations and measurements suggest that any deviation fro...

Single-Phase Heat Transfer and Fluid Flow in Micropipes

Abstract: The objective of this paper is to provide a general overview of the research carried out so far in single-phase heat transfer and flow in capillary (micro) pipes. Laminar flow and laminar-to-turbulent flow transition are analyzed in detail in order to clarify the discrepancies among the results obtained by different researchers. Experiments performed in the ENEA laboratory indicate that in laminar flow regime, the friction factor is in good agreement with the Hagen-Poiseuille theory for a Reynolds number below 600–800. For higher values of the Reynolds number, experimental data depart from the Hagen-Poiseuille law to the side of higher f values. The transition from laminar-to-turbulent flow occurs for Reynolds number in the range 1800–2500. Heat transfer experiments show that heat transfer correlations in laminar and turbulent regimes, developed for conventional (macro) tubes, are not properly adequate for heat transfer rate prediction in microtubes.

Thermocapillary Deformation of a Locally Heated Liquid Film Moving under the Action of a Gas Flow

Abstract: We propose a two-dimensional model of a steady laminar flow of a liquid film in a channel in the presence of a cocurrent gas flow. An analytical solution for the problem of temperature distribution is obtained for a linear flow velocity profile. The linearized problem of thermocapillary deformation of the film surface caused by local heating at a constant heat flux is solved. It is established that a thermocapillary bump is formed in the region where a thermal boundary layer emerges on the film surface. Additional perturbations, decaying in the upstream direction, can be present on the free surface in front of the bump. A criterion determining this effect is found.

Mass dispersion in intermittent laminar flow

Abstract: Mass dispersion in unsteady laminar flow is common in dead-end regions of water distribution systems. However, dispersion has not been included in network models and this omission can lead to inaccurate predictions of water quality in regions where laminar flow prevails. Incorporating dispersion in network models is a challenge because it requires solving a twodimensional (2-D) governing equation numerically. The present work introduces and verifies a new analytical equation that can estimate the rate of dispersion in unsteady laminar flow. Existing analytical solutions for one-dimensional mass transport in steady flow were investigated. Dispersive transport of non-conservative solutes in pipe networks was found to be important when the dimensionless group m= 2 U / KE 4 1+ > 1.2, where K is reaction rate, E is dispersion rate and U is average velocity. A new analytical expression for the instantaneous rate of dispersion in unsteady laminar flow was developed. The instantaneous rate of dispersion in pulsating laminar flow through a pipe is a dynamic weighted average of two factors: (i) the dispersion memory from previous pulses with the factor of R(t)=exp(-t/τ0) where τ0=a/16D is a Lagrangian time scale reflecting molecular diffusivity D across the pipe radius a and (ii) the nonlinear excitation from the current pulse with the factor of 1-R(t). Analytical predictions of dispersion agree well with the results obtained from the 2-D governing equation for conservative solute transport in a pipe. When dimensionless average pulse duration η ≤ 0.025 ( η = D τ /a where τ = pulse duration), analytical estimates of time-averaged dispersion E in unsteady laminar flow with two or three repeating pulses are within 15% of 0 E in steady flow with the same average velocity. For these pulse conditions, the analytical equation also agrees very well with experimental data. When η > 0.0625, the steady and unsteady estimates of dispersion diverge. As η approaches one, E converges toward its maximum rate of dispersion, ( ) U 0 1 E Θ + where ΘU is the coefficient of variation of velocity. A field example from EPANET demonstrated that addition of dispersion improved prediction of fluoride concentrations in deadend pipes with laminar flow. Water utilities can improve predictions of water quality in the periphery of distribution systems by incorporating dispersive transport in their current network models.

Dynamic calibration of laminar flow sensor for gases

Abstract: Recently, the unsteady flow rate measurement of gases is very important in various industrial fields. In this paper, we examined theoretically and experimentally the dynamic characteristics of a laminar flow sensor for gases which we developed. At first, we examined static characteristics of laminar flow sensor experimentally. We evaluated the influence of inlet region and the linear characteristic between volumetric flow rate and pressure difference. Moreover, dynamic characteristics of this laminar flow sensor have been measured experimentally using an unsteady flow generator with isothermal chamber. As a result, we confirmed the flow sensor can be measured oscillatory flow up to 50 [Hz] and the laminar flow sensor has phase-lead characteristic.

Elution control of microparticles with a coupled acoustic-gravity field and orthogonal laminar flow

Abstract: Particles accumulated in a coupled acoustic-gravity field, in which the base-material of a particle determines the equilibrium aggregation positions, are eluted from a separation channel by an orthogonal laminar flow. Elution times are controllable by changing the intensity and frequency of the ultrasonic waves.

Numerical study of flow mal-distribution on the flow and heat transfer for multi-channel cold-plates

Abstract: This study examines the flow mal-distribution and nonuniformity of temperature field of a cold-plate. The calculated results indicate the mal-distribution is decreased when the number of channels in the cold-plate is increased. For the present impingement flow arrangement, the mal-distribution increases with the flow rate. The cold-plate with a flow guide plate is introduced to reduce the flow mal-distribution and increase the heat transfer performance.

Peak Formation Due to Chemiluminescence Reaction through the Collapse of Laminar Flow Liquid–Liquid Interface in a Microreactor

Abstract: A peak signal due to a chemiluminescence (CL) reaction in a microreactor consisting of a microchannel without sample plug formation is presented. The CL reaction of luminol-hydrogen peroxide-Cu(II) was used in this study.A CL profile, including a peak signal, was observed by the collapse of the liquid-liquid interface based on laminar flow in the microchannel. We examined CL profiles for various flow rates of reagents and detection points in the microreactor. The data obtained were taken into consideration, together with the residence times and diffusion lengths. On the basis of on the measurements of peak height, Cu(ll) was determined over the range of 20 nM-0.1 mM with a detection limit of 20 nM (S/N = 3).