Rectangular Microchannel Research Articles

The flow topology transition of liquid\u2013liquid Taylor flows in square microchannels

This work investigates the change of the flow topology of Taylor flow and qualitatively relates it to the excess velocity. Ensemble-averaged 3D2C-upmu PIV measurements simultaneously resolve the flow field inside and outside the droplets of a liquid–liquid Taylor flow that moves through a rectangular horizontal microchannel. While maintaining a constant Capillary number Ca = 0.005, the Reynolds number (0.52 le {text{Re}} le 2.14), the viscosity ratio (0.24 le lambda le 2.67) and surfactant concentrations of sodium dodecyl sulfate (0–3 CMC) are varied. We experimentally identified the product of the Reynolds number Re and the viscosity ratio lambda to indicate the momentum transport from the continuous phase (slugs) into the droplets (plugs). The position and size of the droplet’s main vortex core as well as the flow topology in the cross section of this vortex core changed with increased momentum transfer. Further, we found that the relative velocity of the Taylor droplet correlates negatively with the evoked topology change. A correlation is proposed to describe the effect quantitatively.Graphical abstract

Pressure Change for Single- and Two-Phase Non-Newtonian Flows through Sudden Contraction in Rectangular Microchannel

The flow characteristics of the single-phase liquid and the gas–liquid two-phase flows including the Newtonian and non-Newtonian liquids were experimentally investigated in a horizontal rectangular micro-channel with a sudden contraction—specifically the pressure change across the contraction. The rectangular cross-sectional dimension has Wu × Hu (width × height) = 0.99 × 0.50 mm2 on the upstream side of the contraction and Wd × Hd = 0.49 × 0.50 mm2 on the downstream side. The resulting contraction ratio, σA (=Wd/Wu), was 0.5. Air was used as the test gas (in the case of the gas–liquid two-phase flow experiment), distilled water and three kinds of aqueous solution, i.e., glycerin 25 wt%, xanthangum 0.1 wt% and polyacrylamide 0.11 wt% were used as the test liquid. The pressure distribution in the flow direction upstream and downstream of the channel was measured. The pressure change and loss at the sudden contraction were determined from the pressure distribution. In addition, the pressure change data were compared with the calculation by several correlations proposed by various researchers as well as a newly developed correlation in this study. From the comparisons, it was found that calculations by the newly developed correlations agreed well with the measured values within the error of 30%.

Simultaneous Optimization of Geometric and Nanofluids Parameters in a Rectangular Microchannel Heat Sink

The use of nanofluids as coolant fluid in the microchannel heat sink (MCHS) is an effective technique for improving the thermal performance of electronic devices. A comparative study is performed with the help of a multi-objective genetic algorithm (MOGA) to find the optimal geometric variables of the MCHS and choose the appropriate nanofluid. For that purpose, four practical nanofluids, including Al2O3-water, Cu-water, SiO2-water, and carbon nanotube (CNT)-water, are thoroughly investigated. Simultaneously minimization of the total thermal resistance and pumping power consumption (POW) are considered as the optimization goal, and the MOGA is employed to achieve the optimal solution. To check the accuracy of the thermal resistance modeling and assessing the optimization algorithm, several case studies with a different number of optimization variables are defined to investigate the capability of the algorithm in finding the soptimal microchannel design variables and choosing the suitable nanofluid. The optimization variables consist of the channel aspect and wall ratios, base thickness, nanoparticles volume concentration, nanoparticles diameter, and the volume flow rate. Compared to other nanofluids, CNT provides better thermal performance. Furthermore, increasing the volume concentration of nanoparticles enhances thermal performance, which can also be achieved through the reduction of nanoparticles diameter.

Modelling corners flow in rectangular microchannel

Rectangular microchannels are most common configuration in microfluidics. They can be used in many industries, for example in lab-on-chip devices. Despite standard fluid dynamics, microfluidics has a significant impact of wall boundary conditions on fluid flow. And in microfluidics, we cannot simply set no-slip boundary conditions if our goal is accurate modeling results. In rectangular microchannels, there is another important moment in modeling that is not present in circular pipes. The velocity profile of the fluid depends on the shear stress at the edges and the velocities at the walls of the microchannel change at different points of the cross-sectional wall of the microchannel. The fluid velocity is lower at the corners of a rectangular microchannel. In this paper, a solution is proposed to find a more accurate way to model the fluid flow in a rectangular microchannel by knowing the friction factor without shear stress distribution.

Experimental study of wavy-annular flow in a rectangular microchannel using LIF method

This article aims at studying gas-liquid flow in a rectangular microchannel with a high aspect ratio (200 × 2045 μm). Liquid and gas phases were 95% ethanol and nitrogen mixture. Experimental flow characteristics are obtained using high-speed visualization and laser-induced fluorescence (LIF) methods. Using the LIF method for wavy-annular flow, the average film thickness, liquid film distribution, and liquid film width were measured. The dependences of the liquid film width and the average film thickness on gas superficial velocity are presented in graphical form and analyzed. An increase in gas superficial velocity causes growth of the liquid film width and thickness of the liquid film, which indicates the process of liquid transfer from the menisci area to the liquid film. For different liquid velocities and the same gas superficial velocities, close values of averaged liquid film thickness were observed for flow with 2D waves and 3D waves on liquid film.

Study of interface gas-liquid flow characteristics in a rectangular microchannel for wavy-annular flow

The aim of this work is an experimental study of a gas-liquid flow in a rectangular slit microchannel with a cross-section of 200 × 2045 μm. Ethanol/water (95/5) mixture and nitrogen are used as working liquid and gas, accordingly. The external T-mixer is used for obtaining of wavy-annular flow pattern. The experimental data on interfacial waves and their characteristics in the meniscus area on the short side of the microchannel are obtained using high-speed visualization for a wide range of gas and liquid superficial velocities. Images are processed using the Python libraries to define the average liquid layer thickness and maximum amplitude of waves. An increase of gas superficial velocity causes decreasing in the average liquid layer thickness and maximal amplitude of the liquid layer thickness. The waves on the liquid layer surface (maximal amplitude) can be three times larger than the average liquid layer thickness for presented liquid and gas velocities. With increasing gas superficial velocities more liquid displace from the meniscus area to the liquid film on the wide side of the microchannel.

An experimental study of flow boiling heat transfer of zeotropic mixture R32/R134a in a microchannel heat exchanger

This paper presents experimental data on heat transfer of a binary zeotropic mixture of refrigerants R32/R134a in a microchannel heat exchanger with a high specific surface within a range of parameters that is practically important for the development of cooling systems for microelectronics and space technology. The experiments were carried out in a horizontal heat exchanger with one-sided heating of a copper microchannel plate 20x40 mm, containing 21 rectangular microchannels with a cross-section of 335x930 μm, within the range of mass fluxes from 80 to 250 kg/m2s, and at an absolute pressure in the system ranged from 12 to 14 bar. A zeotropic mixture of refrigerants R32/R134a with a molar concentration of the initial mixture of 65%/35% was used as a working fluid. Experimental data were compared with model-based calculations that take into account the influence of changes in the concentrations of components in the liquid and gas phases.

Heat Transfer Performance of a Novel Microchannel Embedded with Connected Grooves

To improve the heat transfer performance of microchannels, a novel microchannel embedded with connected grooves crossing two sidewalls and the bottom surface (type A) was designed. A comparative study of heat transfer was conducted regarding the performances of type A microchannels, microchannels embedded with grooves on their bottom (including types B and C), or on the sidewalls (type D) as well as smooth rectangular microchannels (type E) via a three-dimensional numerical simulation and experimental validation (at Reynolds numbers from 118 to 430). Numerical results suggested that the average Nusselt number of types A, B, C, and D microchannels were 106, 73.4, 50.1, and 12.6% higher than that of type E microchannel, respectively. The smallest synergy angle β and entropy generation number Ns,a were determined for type A microchannels based on field synergy and nondimensional entropy analysis, which indicated that type A exhibited the best heat transfer performance. Numerical flow analysis indicated that connected grooves induced fluid to flow along two different temperature gradients, which contributed to enhanced heat transfer performance.

Numerical investigation of flow boiling characteristics in cobweb-shaped microchannel heat sink

In recent years, researchers have conducted extensive studies on the improvement of heat transfer performance of microchannels, however, exploiting biomimetic microchannels to enhance the flow boiling heat transfer performance is still relatively rare. In this study, inspired by the cobweb structures in nature, cobweb-shaped microchannels with horizontal inlet and outlet (CMHS-H) and cobweb-shaped microchannels with inclined inlet and outlet (CMHS-I) are proposed. Flow boiling simulations are carried out with the inlet temperature of 300 K at the bottom heat flux of 75–125 W/cm2 under different mass fluxes by utilizing volume of fluid (VOF) model. The flow boiling characteristics of the CMHS-I and CMHS-H are studied and compared with those of the rectangular microchannel heat sink (RMHS). The results demonstrate that both the CMHS-H and CMHS-I can enhance the heat transfer coefficient and reduce the wall temperature due to the flow disturbance and increased heat transfer area; whereas the CMHS-H is effective to reduce the pressure drop and suppress the flow instability at high mass flux. At high heat flux, the CMHS-H shows the best heat transfer performance and the most stable flow boiling behavior. This study provides a promising approach of using biomimetic microchannels to dissipate high heat flux associated with advanced electronics.

Constructal Optimization of Rectangular Microchannel Heat Sink with Porous Medium for Entropy Generation Minimization

A model of rectangular microchannel heat sink (MCHS) with porous medium (PM) is developed. Aspect ratio of heat sink (HS) cell and length-width ratio of HS are optimized by numerical simulation method for entropy generation minimization (EGM) according to constructal theory. The effects of inlet Reynolds number (Re) of coolant, heat flux on bottom, porosity and volume proportion of PM on dimensionless entropy generation rate (DEGR) are analyzed. From the results, there are optimal aspect ratios to minimize DEGR. Given the initial condition, DEGR is 33.10% lower than its initial value after the aspect ratio is optimized. With the increase of Re, the optimal aspect ratio declines, and the minimum DEGR drops as well. DEGR gets larger and the optimal aspect ratio remains constant with the increasing of heat flux on bottom. For the different volume proportion of PM, the optimal aspect ratios are diverse, but the minimum DEGR almost stays unchanged. The twice minimized DEGR, which results from aspect ratio and length-width ratio optimized simultaneously, is 10.70% lower than the once minimized DEGR. For a rectangular bottom, a lower DEGR can be reached by choosing the proper direction of fluid flow.

Optimization of porous fin location and investigation of porosity and permeability effects on hydro-thermal behavior of rectangular microchannel heat sink

Rectangular microchannel heat sink incorporated with porous fin is studied to improve the thermal performance of conventional solid fin microchannel. Three-dimensional microchannel with fixed aspect ratio and hydraulic diameter is considered and porous fin with three different thickness have been analyzed by mounting at various locations. Copper is used as porous medium with a porosity of 0.4 to 0.8 and permeability of 10−8 - 10−12 m2 are considered. The working fluid is water and the simulations were carried out for the Re range, 500 ≤ Re ≤ 1000. The performance improvement of the porous fin over solid fin is presented with the help of slip length. Pressure reduction efficiency is observed by comparing pressure drop values with and without slip effect. The parametric optimization of fin location has been done with the help of combined hydro-thermal performance. Further, the effect of porous parameters like porosity, permeability and quadratic drag factor on pressure drop and heat transfer coefficient is studied for various fin thicknesses. It is observed that the porous fin with porosity 0.4 and permeability 10−8 m2 has shown better heat transfer characteristics.

Effects of pin fins and vortex generators on thermal performance in a microchannel with Al2O3 nanofluids

This paper performs a comparative analysis to obtain the optimal cross-section shape and parameters of both pin fins and vortex generators. A novel combined structure with pin fins and vortex generators is proposed to enhance thermal performance of an integrated microchannel heat sink. Effects of nanoparticle diameter and volume fraction are investigated using Al2O3 nanofluid and DI-water as working fluid. Pin fins and vortex generators cause enhancements of flow disturbance and heat transfer on microchannel heat sinks. Results indicate that oval pin fins have better improvements of thermal/hydraulic performance compared to round and diamond pin fins. The oval pin fin with 0.4 mm spacing and 0.1 mm height presents the highest overall performance factor in the Reynolds number range of 340–640. Presence of vortices intensifies the mixing of the hot fluid near bottom surface and cold fluid near top surface. The optimal vortex generator with length of 0.08 mm and height of 0.06 mm provides a 30% increase in overall performance factor compared to the rectangular microchannel at Reynolds number of 340. Mechanism of heat transfer enhancement is analyzed by investigating flow velocity, temperature distribution and field synergy angle distribution in microchannels. Based on the field synergy principle, it is found that a small and uniformly distributed synergy angle is achieved in the integrated microchannel. According to comparisons of the overall performance factor and total thermal resistance, the optimal nanoparticle diameter and Al2O3 volume fraction of nanofluids are 20 nm and 4%, respectively.

Numerical investigation of compressibility effects on friction factor in rectangular microchannels

This study numerically investigated the effect of gas compressibility on the friction factor in rectangular microchannels. The numerical model adopted in the study was validated against experimental data obtained by testing rectangular microchannels with a hydraulic diameter of 295 μm. The numerical model was used to evaluate the Reynolds number at which the compressibility effects on the friction factor became significant by analyzing the role of both hydraulic diameter and aspect ratio of the microchannel. To this end, three values of the hydraulic diameter (100, 295, and 500 μm) and five different aspect ratios (from 0.25 to 1) were investigated. The results showed that compressibility effects became increasingly stronger by reducing the hydraulic diameter and that they led to an increase in the average friction factor. For smaller microchannels, the Reynolds numbers at which the compressibility effects became significant tended to reduce and were in the laminar regime. The gas compressibility could not be ignored when friction factors needed to be accurately determined. Moreover, for narrow microchannels (low aspect ratio), compressibility effects became important for higher values of the Reynolds number than those observed for nearly squared microchannels.

Boiling of FC-72 on Surfaces with Open Copper Microchannel

The paper presents the results of experimental research on pool boiling heat transfer of dielectric liquid FC-72. Measurements were made at atmospheric pressure on open surfaces with microchannels. Heat transfer surfaces, in the form of parallel milled microchannels, were made of copper. The rectangular cross-sectional microchannels were 0.2 to 0.5 mm deep and 0.2 to 0.4 mm wide. The surfaces, compared to a smooth flat surface, provided a five-fold increase in the heat transfer coefficient and a two-fold increase in the critical heat flux. The article analyses the influence of the width and height of the microchannel on the heat transfer process. The maximum heat flux was 271.7 kW/m2, and the highest heat transfer coefficient obtained was 25 kW/m2K. Furthermore, the experimental results were compared with selected correlations for the nucleate pool boiling.

Application of the laser-induced fluorescence to study local characteristics of a gas-liquid flow in rectangular microchannel

The Laser-Induced Fluorescence (LIF) method was used to characterize liquid phase distribution in rectangular slit microchannel with cross-section 200×1205 μm for horizontal gas-liquid flow. Ethanol and nitrogen were used as working liquid and gas accordingly. The feature of this study is an application of hydraulic focusing cross-junction mixer for obtaining elongated bubble and transition flows in the microchannel with a high aspect ratio. Using LIF measurements for elongated bubble and transition flows the liquid film distributions were obtained for different distances from the bubble top and average liquid film thickness was compared with the prediction according to Taylor’s law.

A three-dimensional thermal analysis for cooling a square Light Emitting Diode by Multiwalled Carbon Nanotube-nanofluid-filled in a rectangular microchannel

The aim of this paper is to ensure proper thermal management in order to remove and dissipate the heat produced by a square Light Emitting Diode (LED), as well as to ensure stable and safe operation by reducing the junction temperature. For this, we developed a three-dimensional code, time-dependent that solves the systems of equations for the mass, momentum, and energy using Comsol Multiphysics. After validation of this numerical 3D code, the thermal performance of a LED cooling system with three nanofluids such as MWCNT-Water, MWCNT-Ethylene Glycol, and MWCNT-Engine oil is studied numerically into account of aggregation effect. Several parameters such as: the power of the LED lamp, the inlet temperature and velocity of nanofluid, the length of the heat sink, and the length of the microchannel have been varied in order to find an optimal condition allowing a good heat dissipation from the LED chip to the heat sink. It was concluded that the use of MWCNT-Water in the microchannel is the best nanofluid that can cool the heat sink. In addition, the increase of velocity inlet of the coolant in the microchannel, the length of the heat sink, and the microchannel length while the decrease of the inlet temperature of nanofluid in the microchannel are an important factors allowing the decrease of the junction temperature of the square LED lamp.

Low Reynolds numbers convective heat transfer in single/two-phase roughened microchannels

• Low Reynolds numbers Flow boiling with roughened surfaces enhancement was studied. • Axial heat conduction on roughened/smooth microchannels flow boiling was examined. • Heat transfer enhancement in roughened microchannels flow boiling can be up to 38%. • Flow boiling decreases axial heat conduction significantly for roughened surfaces. Convective heat transfer coefficient and friction factor (only for single phase) in single/two-phase roughened rectangular (60 µm × 200 µm; cross-section) microchannels with a length of 25 mm were experimentally studied at low Reynolds numbers of 7 ≤ Re ≤ 22 with deionized water as the cooling fluid. Transverse-rib (100 µm × 200 µm; length × width) roughened surface was applied with three different rib heights of 12 µm, 15 µm and 20 µm at a definite pitch of 150 µm. A constant heat flux of 150 W/cm 2 was mounted on the bottom wall of the channel through the copper surface. Flow velocity distribution was measured via micro particle image velocimetry along the downstream of the single-phase flow for both smooth and roughened plexiglass microchannels. Temperature measurements were conducted via a thermocouple for both single- and two-phase roughened microchannels. The heat transfer coefficient, as well as heat transfer enhancement due to the effect of roughened ribs for single phase and flow boiling are presented and discussed herein. The subcooled flow boiling experiments were only conducted for G = 200 kg/m 2 s with the corresponding inlet Reynolds number of 20. The results indicate that the increase in the critical heat flux can reach up to about 20% and reduce the onset of nucleate boiling by 2 °C as compared to those of smooth channels. Under the same operating conditions, the heat transfer enhancement effect seems greater (38%) due to phase change than that of the single-phase roughened channels, in which there is only about a 16% heat transfer increase as compared to those of smooth channels. In addition, axial heat conduction in the microchannel was found to be so small that the effect on convective heat transfer could neglected especially for flow boiling in roughened microchannels.

Experimental investigation of flow boiling performance in sinusoidal wavy microchannels with secondary channels

Sinusoidal wavy microchannels with secondary channels (SWSC) are developed to integrate advantages of the sinusoidal corrugated structure and the introduction of secondary channels in flow boiling enhancement. Flow boiling investigations of acetone are made with variation in the heat flux and mass flux range of 251–512 kg/(m2·s). The boiling curves, heat transfer coefficients, pressure drops and flow boiling instabilities are systematically investigated. The experimental results show that the SWSC microchannels can achieve better heat removal performance than the conventional rectangular microchannels (R). Besides, the premature ONB and delayed CHF can also be obtained. The increased density of bubble nucleation induced by the enlarged surface area and the liquid stored in the corner regions in the nucleate boiling dominated regime, as well as the continuously developing thin liquid film created by the introduction of secondary channels in the stable liquid film evaporation dominated regime are considered to be the main reasons for the notable enhancement in heat transfer. However, the SWSC microchannels suffer a higher pressure drop penalty. The pressure fluctuations and temperature fluctuations of the SWSC microchannels are also illustrated.

Analytical closed-form solution for fluid flow through microchannel heat sinks including axial conduction

An analytical study on the convective heat transfer through a rectangular microchannel heat sink including solid axial conduction, has been performed. The system has been split into two coupled boundary value problems, each addressing the heat transfer to the fluid through different routes (namely, the sidewall or fin and the base wall or substrate). The results are derived in the form of a net wall and fluid temperature rise. The validation of the proposed analytical model has been done using existing experimental data and a three-dimensional conjugate computational model. The results show excellent agreement in terms of solid and fluid temperature distributions. The parametric studies show an increasingly non-linear behaviour in the solid and fluid temperature profiles in the microchannel, if axial conduction is dominant. The simulated results are consistent with the CFD and experimental outcome. It has also been noted that a uniform heat flux applied at the bottom of the microchannel (substrate) cannot ensure the transfer of heat uniformly to the fluid. If axial conduction is high, a significant fraction of the applied heat influx is transmitted to the fluid near the channel entry region with a corresponding reduction in the flux distribution at the channel exit.

Research on modeling and parameter sensitivity of flow and heat transfer process in typical rectangular microchannels: From a data-driven perspective

With the miniaturized and high-powered development of power equipment and electronic devices, the microchannel heat sink is becoming an important device widely used in the modern cooling field. The heat transfer performance of the microchannel heat sink is affected by many influence factors, and amongst them there exists a complex coupling effect. So far, the mechanism of the microchannel heat sink has not been clearly elucidated. To figure out the dominant characteristics that affect the heat transfer performance of the microchannel heat sink and the interaction mechanism between relevant characteristics, this work carried out numerical simulations and parameter sensitivity analysis to investigate the key factors of the fluid flow and heat transfer performance in rectangular microchannels, and considered variables included key physical parameters of the fluids and dimensional parameters of the microchannels. To satisfy the enormous amount requirement of sample size in the parameter sensitivity analysis, a machine learning model, Gradient Boosting Tree (GBT), was constructed based on computational fluid dynamics (CFD) simulation results; then, the Sobel sensitivity analysis algorithm was coupled in to analyze the multi-parameter contribution of each input parameter. Results show that the number of channels N and Reynolds number Re have a greater impact on heat transfer performance under low Re conditions, and the cross-sectional area A and aspect ratio α have a much higher effect on the pumping power rather than Nusselt numbers Nu; N and A have a positive second-order interaction on Nu. When moving to the conditions with a higher Re, the number of channels N became the dominant factor that influenced both heat transfer and flow performance of the microchannel heat sink. This work also provides effective guidance for the design and optimization of high-efficiency microchannel heat sinks.