Miniature Channel Research Articles

Employing nonuniform magnetic fields to improve energy transfer of flow after a sudden expansion inside a miniature channel: A hydrothermal study

AbstractThis study aims to use single/multiple dipoles to control the flow field after a two‐dimensional milli channel, including a sudden expansion. The study is done numerically and using a finite volume based software. First, a single dipole is positioned under the lower boundary of the channel located after the step (which is heated), and the influences of the different locations of the dipole under the heat transfer and pressure drop are analyzed. The same analysis is done for a dipole located on the opposite wall. The results show that the best performance is achieved when the dipole is located under the heated wall, where it enhances local heat transfer by 700%. The study also reveals that when multiple dipoles are used in the domain, some enhancement happens in the local Nusselt number, but the resulting pressure loss is so high. For example, when three dipoles are used, the local heat transfer enhances by 15% in specific places, but the pressure drop increases by five times. The higher strength of the dipoles also results in a much higher pressure drop. Also, the study of the inlet flow Reynolds number shows that magnetic effects on the flow decrease as the Reynolds number rises.

Numerical study of entropy generation and forced convection heat transfer of a nanofluid in a channel with different fin cross-sections

PurposeThis study aims to numerically investigate the thermal-hydrodynamic performance of silicon oxide/water nanofluid laminar flow in the heat sink miniature channel with different fin cross-sections. The effect of the fin cross-section including semi-circular, rectangular and quadrant in two directions of flat and curved, and channel substrate materials of steel, aluminum, copper and titanium were examined. Finally, the analysis of thermal and frictional entropy generation in different channels is performed.Design/methodology/approachAccording to the numerical results, the highest heat transfer coefficients belong to the rectangular, quadrant 2, quadrant 1 and semi-circular fins compared to the channel without fin is 38.65%, 29.94%, 27.45% and 17.1%, respectively. Also, the highest performance evaluation criteria belong to the rectangular and quadrant 2 fins, which have 1.35 and 1.29, respectively. Based on the thermal conductivity of the substrate material, the best material is copper. According to the results of entropy analysis, the reduction of thermal irreversibility of the channel with rectangular, quadrant 1, quadrant 2 and semi-circular compared to non-finned channel is equal to 72%, 57%, 63% and 48%, respectively.FindingsThe rectangular and quadrant 2 fins are the best fins and the copper substrate material is the best material to reduce the entropy generation.Originality/valueThe silicon oxide/water nanofluid flow in the heat sink miniature channel with various fin shapes and the curvature angle against the fluid flow was simulated to increase the heat transfer performance. The whole test section is simulated in three-dimensional. Different channel materials have been investigated to find the best channel substrate material.

Measurement Device for Ambient Carbonyl Sulfide by Means of Catalytic Reduction Followed by Wet Scrubbing/Fluorescence Detection.

A portable chemical analysis system for monitoring ambient carbonyl sulfide (COS) was investigated for the first time. COS is paid attention to from the perspectives of photosynthesis tracer, breath diagnosis marker, and new process-use in the manufacture of semiconductors. Recently, the threshold level value of COS was settled at 5 ppm in volume ratio (ppmv) for workplace safety management. In this work, COS was converted to H2S by a small column packed with alumina catalyzer at 65 °C. Then, the H2S produced was collected in a small channel scrubber to react with fluorescein mercuric acetate (FMA), and the resulting fluorescence quenching was monitored using an LED/photodiode-based miniature detector. The miniature channel scrubber was re-examined to determine its robustness and easy fabrication, and conditions of the catalyzer were optimized. When the FMA concentration used was 1 μM, the limit of detection and dynamic range, which were both proportional to the FMA concentration, were 0.07 and 25 ppbv, respectively. Ambient COS in the background level and even contaminated COS in the nitrogen gas cylinder could be detected. If necessary, H2S was removed selectively by reproducible adsorbent columns. COS concentrations of engine exhaust were measured by the proposed method and by cryo-trap-gas chromatography-flame photometric detection, and the results obtained (0.5–5.9 ppbv) by the two methods agreed well (R2 = 0.945, n = 19). COS in ambient air and exhaust gases was successfully measured without any batchwise pretreatment.

Heat Transfer Improvement in a Wavy Vortex Generator Miniature Channel Using Nanofluids

In this article, the thermal and frictional characteristics of interrupted wavy mini-channel are experimentally investigated as the wavy vortex generator miniature channel (WVGMC) using SiO2–H2O and Al2O3–H2O nanofluids. Its performance characteristics are compared to the common configurations of cooling mini-channels, that is, the wavy mini-channel (WMC), straight mini-channel (SMC), and the miniature channel. The effect of the geometric shape of mini-channels, mass flow rate, coolant fluid type, and the concentration of nanoparticles on the thermal and hydraulic performance inside the different geometries are investigated. The range of Reynolds number and weight fraction of nanoparticles are considered to be 500–2000 and 0–0.3 wt%, respectively. The results show that the highest Nusselt number and overall performance are obtained in the WVGMC for all coolant fluids. The Nusselt number is improved about 27.5%, 22%, and 26.5% for the distilled water, SiO2–H2O nanofluid, and Al2O3–H2O nanofluid in the WVGMC compared to WMC, respectively. Also, the friction factor is increased in the WVGMC; for example, this enhancement rate is about 8.2% for the water flow compared to WMC. The SMC show the least Nusselt number for both nanofluids. In all geometries, the thermal performance is increased using the nanofluids than that for the deionized water. Thus, the WVGMC is suggested as an efficient geometry to design the cooling mini-channels.

Effect of different frequency functions on ferrofluid FHD flow

Abstract A finite volume method is used to investigate the unsteady thermomagnetic convection of ferrofluid flow in a miniature channel under the effect of constant and oscillating magnetic fields. A magnetic line dipole as a magnetic source is placed under the lower wall of the two-dimensional channel whose lower and upper walls are subjected to a constant heat flux. To generate the oscillating magnetic field, four different frequency functions including rectangle, sine, triangle and sawtooth functions are implemented in the constant magnetic field. The study parameters include four frequency functions, magnetic field intensity as a magnetic number (Mn = 3.833 × 108, 8.624 × 108, 1.533 × 109), frequencies (f = 0.5–5 Hz) and Reynolds numbers (Re = 20–40). Results revealed that applying the constant magnetic field enhances heat transfer and as the Reynolds number increases, the effectiveness of magnetic field on heat transfer rate diminishes. The oscillating magnetic field, regardless of the type of the applied frequency function, improves heat transfer rate due to thermal boundary layer suppression and increase in velocity field. For all applied functions, results show that there is an optimum frequency where maximum heat transfer happens and this frequency increases for higher Reynolds numbers. For rectangle and sawtooth functions with instant change in the magnetic field, two simultaneous vortexes emerges which leads to better heat transfer than sine and triangle functions. The alteration of the pressure drop in one period is proportionate with the shape of the applied frequency even though the range of the alterations reduces with the increase of the Reynolds number.

Thermal performance enhancement in a miniature channel using different passive methods

In the present work, different passive methods have been numerically investigated to improve the thermal performance in a miniature channel. The heat transfer and pressure drop of 1 vol% Al2O3–H2O nanofluid inside triangular longitudinal pin fin miniature channel and 0.5 vol% SWCNT–H2O nanofluid inside miniature channel with wavy vortex generator with different angles have been numerically investigated. The angles of wavy vortex generator have been considered to be 0°, − 60° and 60°. The nanofluid flow has been simulated by mixture two-phase model. The results show that the thermal performance of Al2O3–H2O nanofluid is increased about 40% compared to that of pure water in miniature channel, while the pressure drop is slightly changed. The overall performance has been improved by the triangular pin fin and wavy vortex generator in miniature channel. The highest value of the overall performance has been observed for Al2O3–H2O nanofluid in triangular longitudinal pin fin miniature channel; the increase rate is about 83% than that of pure water in miniature channel. Also, in all different angles of wavy vortex generators, the thermal and frictional performances have been increased by adding the SWCNT nanoparticles. But, the effect of nanoparticles on the frictional performance was negligible. Also, the heat transfer and pressure drop have been increased when the angle of the wavy vortex generator was 0°. The highest and least values of the overall performance are observed for angles of 60° and − 60°, respectively.

Performance intensification of miniature channel using wavy vortex generator and optimization by response surface methodology: MWCNT-H2O and Al2O3-H2O nanofluids as coolant fluids

The heat transfer and flow characteristics of the wavy vortex generator inside miniature channel as process intensification are numerically investigated. The optimization of wavy vortex generator is performed using the response surface methodology. The studied geometrical parameters are the position angle (i.e. −60°, −30°, 0°, 30° and 60°), wave amplitude (i.e. 0.5, 0.75, 1, 1.25 and 1.5 mm) and wave length (i.e. 5, 10, 15, 20 and 25 mm). The coolant fluids are MWCNT-H2O and Al2O3-H2O nanofluids which are simulated by Eulerian two-phase model. The volume fraction of nanoparticles and the range of Reynolds number are 0–1 vol.% and 200–1000, respectively. To validate the numerical modeling, an experimental investigation is conducted. The numerical results show that the MWCNT-H2O nanofluid has the highest overall performance. The position angle of 60° or 150° has the best overall performance. By increasing of wave amplitude and reducing of wave length, the thermal performance is improved. Also, by increasing the volume fraction of nanoparticles, the thermal and hydraulic performance of nanofluids is increased. Finally, the optimal designs of the wavy vortex generator miniature channel are proposed and also the correlations are provided to predict the Nu number and f of the nanofluids.

New correlations to predict the thermal and hydraulic performance of different longitudinal pin fins as vortex generator in miniature channel: Utilizing MWCNT-water and Al2O3-water nanofluids

In this paper, a parameter as area modification factor is defined for the cross section shape of pin fins. The effect of this parameter on the thermal and hydraulic performance of different coolant fluids inside pin fin miniature channel is numerically investigated. This parameter amount is 90°, 82°, 75°, 68° and 59° that lead to the rectangular, trapezoidal of type (3), (4) and (5) and triangular fin. The MWCNT-water and Al2O3-water nanofluids are working fluids which are simulated by mixture two-phase model. The range of the Reynolds number and nanoparticles volume concentration are 300–2100 and 0–3vol.%, respectively. The results show that the thermal performance is enhanced much folds by using the nanofluid compared to that for the pure water inside the miniature channel with a slight increase in pressure drop penalty. The water flow and 1vol.% Al2O3-water nanofluid has the highest overall performance in the triangular pin fin miniature channel but MWCNT-water nanofluid has the highest and least overall performance in the trapezoidal pin fin of type (3) and triangular pin fin, respectively. New correlations are also suggested to predict the Nu number and friction factor of the different pin fin miniature channels.

The hydrodynamic and heat transfer behavior downstream of a channel obstruction in the laminar flow regime

Micro-fluidic systems have been proposed as potential solutions for the cooling of next-generation Integrated Circuits (ICs) and Photonics Integrated Circuits (PICs). For PICs, integration of micro-fluidics may enable greater laser-bar array densities and, consequently, greater transmission bandwidth. To cool the μm scale hot-spots produced by the laser-bars, a passively actuated structure situated in a micro-channel could regulate temperature by disturbing flow in a target location as required. To this end, a proof-of-concept passive structure was developed from a Shape Memory Alloy (SMA), and demonstrated at the millimeter scale in previous work. The objective of this study is to measure the heat transfer augmentation at a target surface downstream of the obstruction. Two experiments were performed to measure the flow field and heat transfer downstream of the test pieces in a square miniature channel for a range of obstruction Reynolds numbers (Re≈50–170) and opening area ratios (β≈0.2–0.5): using Particle-Image Velocimetry (PIV) and infrared (IR) thermography of a Joule-heated foil respectively. A 50% improvement was observed in the foil area averaged heat transfer coefficient (relative to the unobstructed channel) for Rew=176 and β=0.19. A correlation was developed to interpolate Nusselt number (Nu) from a given Re and β for the studied geometry, and analysis of the correlated exponents showed an improvement in heat transfer relative to a simple pillar obstruction. This was attributed to the wake that formed downstream of the test pieces. The findings of this work are relevant to the modeling and design of practical micro-fluidic systems for targeted hot-spot cooling in future integrated circuit packaging.

The characterization of a low-profile channel–confined jet for targeted hot-spot cooling in microfluidic applications

Photonics Integrated Circuits (PICs) are the backbone of the optical-fiber networks that enable high-speed communication on a global scale. Contemporary devices contain laser-bars (∼μm length-scale) which must be controlled within ±0.1K, and are capable of generating heat fluxes ∼1kW/m2. This represents one of the highest heat fluxes found in nature or engineering applications, and the thermal challenge places a limitation on the density of laser-bar arrays on an individual chip. Chip-integrated μfluidic cooling has been proposed for the thermal management of next-generation PICs to create more energy efficient devices, capable of greater data throughput. Jet impingements are of interest as the primary heat exchangers in this system due to the large heat transfer rates that can be achieved. The objective of this work is to generate a novel, low profile jet impingement within an individual channel suitable for targeting hot-spots in a densely packed circuit, at the low Reynolds numbers prevalent in micro-fluidic applications (Re<500). To this end, two experiments were performed to non-invasively observe the velocity field and local heat transfer in a square miniature channel containing a curved orifice-plate to manipulate the fluid flow. A range of Reynolds numbers (channel Re=100−200) and obstruction opening area ratios (β=0.2−0.5) were investigated through Particle-Image Velocimetry and infrared thermography of a Joule-heated foil. The velocimetry data showed that the curved orifice-plate geometry successfully generated an inclined jet within the channel, and maximum improvements in the area-averaged heat transfer coefficient of 495% (relative to a channel containing no obstruction) were measured. The heat transfer data showed Nu∼Re0.59scaling, similar to that of a micro-fluidic array of normally impinging jets, and this physical relationship is beneficial in the design and modeling of μfluidic cooling systems. The findings illustrate the impact of a channel confined jet on spatial heat transport, and demonstrate the potential for controlled heat transfer enhancement using unconventional obstructions within laminar channel flows.

A visualization of the flow and heat transfer from an oblique impinging jet generated in a square miniature channel

A visualization of the flow and heat transfer from an oblique impinging jet generated in a square miniature channel

On the hydrodynamic characterization of a passive Shape Memory Alloy valve

An attractive approach to the thermal management of next generation photonics devices (heat fluxes > 102 W/cm2) is micro-channel cooling, and micro-valves will be required for refined flow control in the supporting micro-fluidic systems. In this paper, a NiTi Shape Memory Alloy (SMA) micro-valve design for passive flow control and thermal management was prototyped at the macro scale and hydrodynamically characterized. The dynamic behavior of the valve was observed and the loss coefficient (ζv) derived from pressure-flow measurements. The hydrodynamic characterization study is important because ζv is sensitive to Re and geometry in the flow regime of the micro-fluidic system. Static replicas of the SMA valve geometry were tested for low Re (110–220) and a range of opening ratios (β) in a ø1 mm miniature channel. The loss coefficients were found to be sensitive to flow rate and decreased rapidly with an increase in Re. A correlation was developed to interpolate ζv from a given Re and β. The valve loss coefficients obtained in this work are important parameters in the modeling and design of future micro-fluidic cooling systems.

Hydrodynamic characterization of a passive shape memory alloy valve

Next generation high-performance electronics will have large heat fluxes (>102 W/cm2) and an alternative approach to traditional air cooling is required. An attractive solution is micro-channel cooling and micro-valves will be required for refined flow control in the supporting micro-fluidic systems. A NiTi Shape Memory Alloy (SMA) micro-valve design was hydrodynamically characterized in this work to obtain the valve loss coefficient (K) from pressure measurements. The hydrodynamic characterization was important as in the flow regime of the micro-fluidic system K is sensitive to Reynolds number (Re) and geometry. Static replicas of the SMA valve geometry were studied for low Reynolds numbers (110 – 220) in a 1x1 mm CSA miniature channel. The loss coefficients were found to be sensitive to flow rate and decreased rapidly with an increase in Re. The SMA valve was compared to a similar gate micro-valve and loss across both valves was of the same order of magnitude. The valve loss coefficients obtained in this work are important parameters in the modeling and design of micro-fluidic cooling systems.

Mixed Thermocapillary and Forced Convection Heat Transfer Around a Hemispherical Bubble in a Miniature Channel—A 3D Numerical Study

It has been established that for certain conditions, such as microgravity boiling, thermocapillary Marangoni flow has associated with it a significant enhancement of heat transfer. Typically, this phenomenon was investigated for the idealized case of an isolated and stationary bubble resting atop a heated solid that is immersed in a semi-infinite quiescent fluid or within a two-dimensional cavity. This article presents a three-dimensional numerical study that investigates the influence of thermal Marangoni convection on the fluid dynamics and heat transfer around a bubble during laminar flow of water in a minichannel. This mixed thermocapillary and forced convection problem is investigated for channel liquid inlet velocity of 0.01 m/s to 0.03 m/s and Marangoni numbers in the range of 10 to 300 under microgravity conditions. Three-dimensional effects become particularly important on the side and rear regions of the bubble. The thermocapillary forces accelerate the flow along almost the entire bubble interface. The hot core fluid from the heated bottom wall region is forced inward and propelled upward into the thermocapillary jet above the bubble. It can be quantified that the influence of thermocapillary flow on heat transfer enhancement shows an average increase by 40% at the downstream of the bubble and by 60% at the front and rear regions. This heat transfer enhancement depends mainly on the temperature differential as the driving potential for thermocapillary flow and bulk liquid velocity.

Tracing of shallow water flows through buoyant fluorescent particles

In this paper, we investigate the feasibility of tracing shallow water flows in illuminated conditions by using buoyant fluorescent particles. We develop an image analysis-based procedure for automatic detection and real-time tracking of particles based on a computationally inexpensive algorithm implemented on recorded videos. The methodology is validated through experiments conducted in a custom-built reclinable miniature channel that is specifically developed for simulating shallow water conditions. The effect of the fluorescent particles’ dimensions on their ability to trace water flows is theoretically assessed through a parametric study in which the particle response to different flow velocities is estimated through the Basset–Boussinesq–Oseen equation. Particle velocities extracted through image analysis are compared to particle image velocimetry measurements for selected combinations of slopes and water depths. Experimental results support the potential integration of this methodology in field measurement systems for hillslope overland flow velocity.

Characterization of Buoyant Fluorescent Particles for Field Observations of Water Flows

In this paper, the feasibility of off-the-shelf buoyant fluorescent microspheres as particle tracers in turbid water flows is investigated. Microspheres’ fluorescence intensity is experimentally measured and detected in placid aqueous suspensions of increasing concentrations of clay to simulate typical conditions occurring in natural drainage networks. Experiments are conducted in a broad range of clay concentrations and particle immersion depths by using photoconductive cells and image-based sensing technologies. Results obtained with both methodologies exhibit comparable trends and show that the considered particles are fairly detectable in critically turbid water flows. Further information on performance and integration of the studied microspheres in low-cost measurement instrumentation for field observations is obtained through experiments conducted in a custom built miniature water channel. This experimental characterization provides a first assessment of the feasibility of commercially available buoyant fluorescent beads in the analysis of high turbidity surface water flows. The proposed technology may serve as a minimally invasive sensing system for hazardous events, such as pollutant diffusion in natural streams and flash flooding due to extreme rainfall.

Mixed Convective Heat Transfer Due to Forced and Thermocapillary Flow Around Bubbles in a Miniature Channel: A 2D Numerical Study

Marangoni thermocapillary convection and its contribution to heat transfer during boiling has been the subject of some debate in the literature. Currently, for certain conditions, such as microgravity boiling, it has been shown that Marangoni thermocapillary convection has a significant contribution to heat transfer. Typically, this phenomenon is investigated for the idealized case of an isolated and stationary bubble resting on a heated surface, which is immersed in a semi-infinite quiescent fluid or within a two-dimensional cavity. However, little information is available with regard to Marangoni heat transfer in miniature confined channels in the presence of a cross flow. As a result, this article presents a two-dimensional (2D) numerical study that investigates the influence of steady thermal Marangoni convection on the fluid dynamics and heat transfer around a bubble during laminar flow of water in a miniature channel with the view of developing a refined understanding of boiling heat transfer for such a configuration. This mixed convection problem is investigated under microgravity conditions for channel Reynolds numbers in the range of 0 to 500 at liquid inlet velocities between 0.01 m/s and 0.0 5m/s and Marangoni numbers in the range of 0 to 17,114. It is concluded that thermocapillary flow may have a significant impact on heat transfer enhancement. The simulations predict an average increase of 35% in heat flux at the downstream region of the bubble, while an average 60% increase is obtained at the front region of the bubble where mixed convective heat transfer takes place due to forced and thermocapillary flow.

Modeling and Prediction of Two-Phase Microgap Channel Heat Transfer Characteristics

A detailed analysis of microchannel/microgap heat transfer data for two-phase flow of refrigerants and dielectric liquids, gathered from the open literature and sorted by the Taitel and Dukler flow regime mapping methodology, is performed. Annular flow is found to be the dominant regime for this thermal transport configuration and to grow in importance with decreasing channel diameter. A characteristic M-shaped heat transfer coefficient variation with quality (or superficial velocity) for the flow of refrigerants and dielectric liquids in miniature channels is identified. The inflection points in this M-shaped curve are seen to equate approximately with flow regime transitions, including a first maximum at the transition from Bubble to Intermittent flow and a second maximum at moderate qualities in Annular flow, just before local dryout begins. The predictive accuracy of five classical two-phase heat transfer correlations for miniature channel flow is examined. Selecting the best fitting of the classical correlations for each of the flow regime categories is seen to yield predictive agreement with regime-sorted heat transfer coefficients that does not depart significantly from the agreement found in large pipes and channels.

Evaporative Heat Transfer of CO2 in a Smooth and a Micro-Grooved Miniature Channel Tube

Grooved surfaces are widely used to enhance heat transfer. In this study, micro-grooves are applied to miniature channels to improve the evaporative heat transfer of CO2. To evaluate the effect of micro-grooves in miniature channels, heat transfer characteristics are investigated for the smooth multichannel tube with 0.8 mm channel diameter and the grooved multi-channel tube with the same diameter and 8 micro-grooves. The tests were carried out at the evaporation temperatures of 0, 5 and 10°C; mass flux between 400 and 800 kg/m2s; and heat flux of 12–18 kW/m2. At lower qualities (less than about 0.3), heat transfer was considerably enhanced by the grooves, however, the micro-grooves showed negative effect in heat transfer at higher qualities. A significant heat transfer enhancement was obtained at high evaporating temperature. Also, heat transfer was enhanced at lower heat and mass flux. The evaporative heat transfer coefficients measured were compared with several correlations. A new correlation was developed to predict the dry-out quality in miniature channel tubes more accurately.

Analysis of Heat Transfer during Liquid-Vapor Pulsating Flow in a U-Shaped Miniature Channel

Analysis of Heat Transfer during Liquid-Vapor Pulsating Flow in a U-Shaped Miniature Channel