Smooth Microchannel Research Articles

Role of gradients and vortexes on suitable location of discrete heat sources on a sinusoidal-wall microchannel

The idea of using the compact device with higher heat transfer potential has encouraged researchers to use microchannels. Creating sinusoidal walls is a technique leading to better effectiveness and smaller size. In this study, the effects of discrete heat sources location on heat transfer and pressure drop are investigated, using graphene nanoplatelets/water inside a sinusoidal microchannel. For this, discrete heat sources are installed in a smooth microchannel (layout A) and compared with two sinusoidal-wall microchannels. In layouts B and C, the heating sources are installed above the convergent/diverging sections, respectively. Since the velocity and temperature gradients are higher in the converging region, the heat exchange and pressure drop for layout B are greater than other ones. In other words, installing heating sources in these regions with high-temperature gradient has a more obvious positive efficacy on heat exchange. For the best layout (B), although the heat exchange compared to the base layout (A) is 37.5% higher, the pressure drop and entropy generation are higher by 79% and 35.2%, respectively. By introducing a new figure of merit (FOM), it is found that layout B is in the desirable zone.

Improving thermal performance of microchannels by combining rectangular pin with chamber

The microchannel heat sink is an effective liquid cooling technique in thermal management of miniature systems. In the current work, to enhance the performance of a microchannel, the combination of rectangular pin with chamber is proposed and studied. A numerical study is carried out, and the governing equations are discretized by the finite volume method using the semi-implicit method for pressure linked equations algorithm. The reliability of the numerical results is verified based on the experimental section of this study. The deviations of the comparison fall in satisfactory ranges (maximum error of 8%). All the complex geometries have the same overall dimensions (height, width, and length) as the smooth microchannel, and a constant heat flux of 100 kW/m2 is applied. Water is used as working fluid, and all cases are examined in the Reynolds number ranging from 423 to 1990. The results illustrate that both the pin and the chamber boost the heat transfer coefficient of the microchannel with a penalty in the pressure drop. It is also found that in the case equipped with the rectangular pin and chamber, there is a 2.3%–53.1% increase in the heat transfer coefficient, and a 3.6%–681.4% increase in the pressure drop, compared with the reference model. Furthermore, the position of the pin and chamber affects the overall performance of the microchannel. It can be concluded that the enlargement in the length of the chamber improves the overall performance of the combined model, while the enlargement in the length of the pin has an inverse impact.

Hotspot thermal management in microchannel heat sinks with vortex generators

Vortex generators (VGs) have been widely applied in the microchannel heat sinks to improve the thermal-hydraulic performance in multicore processors. However, most studies focus on global heat transfer enhancement of microchannel, while the hotspot temperature is often neglected. In this paper, a numerical study of hotspot-targeted thermal management using VGs is performed in a rectangular microchannel with a heat flux of 400 W/cm2 and 50 W/cm2 respectively at the hotspot and background region. With the maximum thermal resistance, pumping power and the overall performance factor as the key metrics, the performances of VGs in four configurations (co-flow-up, co-flow-down, counter-flow-up and counter-flow-down) are compared with a smooth microchannel without VG. Deionized water with temperature-dependent thermo-physical properties is used as the coolant with the Re in the range of 66–330. The results indicate that the adoption of VGs can significantly improve the cooling effect over the hotspot with a lower pressure loss penalty comparing with the smooth microchannel. In addition, the VGs in a “co-flow” configuration lead to the fluid circulation effect and transport the coolant in the core region of the microchannel to the sidewalls. It is also noted that comparing with the smooth microchannel, more than 78.8% pumping power can be saved by using the “co-flow-down” configuration to achieve the same cooling effect to maintain the average temperature at 304.7 K over the hotspot. Geometric parameter analysis reveals that the increase in the height and attack angle of VGs results in more pressure loss, whereas the enhanced heat transfer is insignificant when the VG's height ratio exceeds 0.2 or the attack angle surpasses 45°. Taking full account of the heat transfer performance and pressure loss, VGs with a height ratio of 0.2 and attack angle of 45° are recommended for practical applications.

Experimental Investigation of Microchannel Copper Plate Surfaces on Manufacturing and Wettability

In this study, static contact angle measurement experiments were conducted on smooth copper surface and microchannel structure surfaces under different droplet sizes. The results showed that shallow processing will more easily lead to the deformation of the processed interface. And the wettability of the microchannel surfaces with the same width and different depth were obviously different. Moreover, the contact angles on three microchannel surfaces were almost the same when the size of droplet was 12μl duo to action of gravity. The optimal droplet size is 9μl to measure contact angle in this study.

Boiling of Argon flow in a microchannel by considering the spherical geometry for roughness barriers using molecular dynamics simulation

In this research, the method of molecular dynamic (MD) is used to consider the influences of the spherical roughness barrier on the boiling flow regime of Argon particles, flowing into microchannel with a square cross-section. To prepare boiling conditions, different constant temperatures from 84 K to 133 K are applied on the walls of microchannels. Results show that roughness elements help boiling thermal forces to the distribution of fluid atoms in central layers of a microchannel, especially at high time steps between 750,000 and 1,000,000. This phenomenon is noticeable under applied high wall temperatures of 114 K and 133 K. Also, a summation of velocity values indicated that increasing boundary wall temperatures of rough microchannels results in a small reduction of the velocity of fluid flow, whereas; it is almost unchanged in the smooth microchannel. The statistical approach shows that the presence of spherical roughness does not have a destructive influence on the boiling flow properties of Argon into the microchannel. Also, they are useful to enhance effective surfaces of heat transfer, which empower the boiling process. Therefore, investing in polishing microchannel surfaces and removing the spherical shape of roughness components can even be a serious mistake for some of the practical applications.

Molecular dynamic simulation of Argon boiling flow inside smooth and rough microchannels by considering the effects of cubic barriers

This paper presents the effects of barriers with cubic geometry on the boiling flow behavior of Argon flow with 10 million atoms flowing inside smooth and rough microchannels using molecular dynamic simulation (MDS) method. Effective parameters of this research are boundary temperature of 108 K on the walls of microchannels to prepare boiling condition and driving forces as much as 0.002, 0.01, and 0.02 eV/Å, which is enforced respectively on the fluid at the inlet of the microchannel. For ease of investigation in comparison to results by statistical method, the cubic shape of roughness barriers is simulated on the surfaces of a smooth microchannel with the same dimensions. Results showed that the presence of cubic roughness elements increases oscillations in density profiles due to their influences in arrangements of argon atoms and their consequences in the diminishing role of the boiling process to move atoms from lateral regions to empty region in the middle section of the microchannel. Also, the statistical approach demonstrated that the presence of cubic roughness elements reduces average velocity as much as 1.8 to 13.59%. Moreover, applying external forces in a range of 0.01 to 0.02 eV/Angstrom brings so turbulences in the rough microchannel that destroys stability and normal distribution of atoms under boiling force which is not applicable for practical application.

Numerical study of thermal enhancement in a micro-heat sink with ribbed pin-fin arrays

Targeted at improving comprehensive hydro-thermal performance of microfluidic cooling, a new type of ribbed pin-fin arrays heat sink is proposed. To reveal the mechanism of heat transfer enhancement in inline ribbed pin-fin arrays heat sink (RPHS), the comparisons among smooth microchannel heat sink (MCHS), inline cylinder pin-fin arrays heat sink (CPHS) and RPHS with coolant of deionized water are studied by numerical simulation with laminar flow (360 ≤ Re ≤ 780). Due to the flow disturbance and destroying of boundary layer, RPHS shows the best performance in heat dissipation and uniformity of temperature distribution among the three configurations. The average Nusselt number of RPHS reaches up to 23.1 which is almost twice of CPHS and three times of MCHS when Re number is 780. Meanwhile, the non-dimensional parameter He is proposed to evaluate the temperature distribution of bottom baseplate. RPHS has the lowest total thermal resistance under the same pumping power. Moreover, the entropy generation of RPHS is lower than MCHS and CPHS under the same operating condition. Conclusively, RPHS exerts the best comprehensive hydro-thermal performance.

Delay of subsonic choking in slip regime by structured roughness in microchannel

Roughness is an intrinsic property of a surface. Its presence is recognized at the micro-scale due to the high surface area to volume ratio. In the present experimental work, three-dimensional microchannels with structured roughness in the form of cuboidal protrusions called micro-ridges are fabricated. Ridge fraction (δ) is the ratio of the length of the ridge (s) to the distance between the ridges (L). δ is varied as 0.75, 0.50, 0.25, and 0 to check the occurrence of the choking phenomenon and its impact on the frictional resistance in gaseous slip flow. To this end, mass flow controllers, pressure sensors, and thermocouples are employed to explore the dependence of Poiseuille number (fRe) on Mach number (Ma) in the microchannel. It is demonstrated that the smooth microchannel (δ = 0) and the ridge with the shortest length (δ = 0.25) gets choked subsonically, but the longer ridges (δ = 0.50, 0.75) do not choke under the investigated conditions. Interestingly, fRe (δ = 0.50) > fRe (δ = 0.25) > fRe (δ = 0.75) ≈ fRe (δ = 0). Since choking limits the maximum amount of mass flow rate through a microchannel, its occurrence could be counter-productive or could even be beneficially employed, depending on the specific application.

Hydrothermal performance analysis of various surface roughness configurations in trapezoidal microchannels at slip flow regime

Abstract The effects of various surface roughness geometrical properties including roughness height (5%, 10%, 15%), number (3, 6), and shape (rectangular and triangular) on the flow and heat transfer of slip-flow in trapezoidal microchannels were investigated. The effects of mentioned parameters on the heat transfer coefficient through the microchannel, average Nusselt number and pressure drop for Reynolds number of 5, 10, 15 and 20 were examined. The obtained results showed that increasing the roughness height and number increases the pressure drop due to higher stagnation effects before and after roughness elements and decreases the Nusselt number due to higher recirculation zones effects than obstruction effects. The most reduction in Nusselt number and the most increment in pressure drop occur at the roughness height of 15%, roughness number of 6 and Reynolds number of 20 by about 10.6% and 52.8% than the smooth microchannel respectively.

Prediction of boiling flow characteristics in rough and smooth microchannels using molecular dynamics simulation: Investigation the effects of boundary wall temperatures

Abstract This paper presents a molecular dynamics study on the effects of different boundary wall temperatures in the range of 84 K to 133 K on the flow characteristics of Argon fluid flow. An external force of 0.002 eV/A is enforced on the fluid at the entry of microchannels with roughened and smooth surfaces. Roughness elements are in cubic shape, and fluid atoms are structured inside microchannel in two lateral regions near microchannel walls, while; middle region of the microchannel contains no fluid atom at first. Then, with the passage of computational running, Argon atoms are translocated from two symmetric lateral regions of the microchannel to empty region in the middle segment, under phase change condition. The phase change is prepared by applying different boundary walls temperatures in the order of 84 K, 96 K, 108 K, 114 K, and 133 K. Then, sampling data of density for each case of wall temperatures is done at 4-time steps of 250,000, 500,000,750,000, and 1,000,000. Afterward, the velocity and temperature of Argon fluid flow are reported only at time step 1,000,000. Generally, it is concluded that the roughened surface reduces the intention of Argon atoms to move from lateral layers near the wall to central bins in the middle section of the channel as much as 3% to 5%. Furthermore, adding roughness elements on the smooth surface of microchannel can augmentate density fluctuation. Also, increasing wall temperatures empowers the boiling process, which reduces the effect of roughness. Moreover, roughness can decrease the average velocity of fluid flow values around 1% to 3%.

Experimental investigation of the heat transfer and flow characteristics of microchannels with microribs

The single-phase heat transfer and flow characteristics of microchannels with microribs were investigated experimentally in this paper. Based on a smooth rectangular microchannel (MC-S), rectangular microchannels with rectangular ribs on one side (MC-OSR) and both sides (MC-BSR) were proposed. The effects of the volumetric flow rate and inlet temperature on the Nusselt number and friction factor were analyzed. Five positions were assigned evenly in the flow direction at the wall to measure the temperature and determine the wall temperature distribution. The results showed that the Nusselt number increased as the volumetric flow rate and inlet temperature increase. The friction factor decreased with increasing inlet temperature. The thermal performance index of the MC-OSR was higher than those of the MC-BSR and MC-S, which reflected the superior heat transfer performance of the MC-OSR. The wall temperature was nonlinearly distributed in the flow direction. The amplitude of the temperature difference gradually decreased. Compared to the MC-S, the entrance effects of the MC-OSR and MC-BSR were more obvious mainly due to the presence of ribs.

Molecular dynamic simulation to study the effects of roughness elements with cone geometry on the boiling flow inside a microchannel

Present work studies effects of presence of surface roughness elements with cone geometry on the boiling flow behavior of Argon fluid inside microchannels which are affected by different boundary wall temperatures using molecular dynamic simulation method. Firstly, microchannel is simulated with smooth surfaces under boundary wall temperatures of 84 K, 96 K, 108 K, 114 K and 133 K to prepare boiling condition for Argon atoms. Microchannel surfaces are roughened by cone shape of roughness elements and mentioned temperatures are applied on the roughened microchannel walls to be comparable with smooth one. Also, an external driving force of 0.002 eV/Å is applied on the Argon atoms at the entrance of both rough and smooth microchannels for all cases of wall temperatures. Statistical approach is employed to compare results of both microchannels. It is reported that adding roughness elements on the smooth surfaces of microchannel can extend contact surfaces of energy transfer which empowers boiling process of fluid flow. Presence of cone geometry of roughness elements causes fluid flow velocity reduction as much as 0.5–1.5%. Also, it is observed that roughness elements results in enhancement of temperature profiles of fluid. Moreover, it is found that role of roughness element in low time steps is stronger than high time step. Therefore, evolution of boiling process can reduce consequences of roughness on the flow behavior. Finally, due to ignorable consequences of roughness element on the boiling flow behavior, it is concluded that preparing very smooth surfaces is not economical for practical application such as medical micro probes to destroy abnormal or diseased tissues.

Analysis on heat transfer and pressure drop of a microchannel heat sink with dimples and vortex generators

The combination of vortex generators (VGs) and dimples on a microchannel heat sink is proposed to improve the heat transfer performance on a cooling surface with a constant heat flux of 100 W/cm2. Three pairs of VGs are evenly distributed along flow direction, while the dimples are placed downstream of VGs with the same intervals. Numerical simulations are performed with de-ionized water as working fluid for the Reynolds number in the range from 167 to 834. The result indicates that the adoption of VGs and dimples could improve the heat transfer performance by 23.4–59.8% with a penalty of apparent friction factor increased by 22.1–54.4% compared with smooth microchannel. The intensity of vortices induced by VGs is much stronger than those induced by dimples and the interaction between dimples and VGs is advantageous for heat transfer enhancement. Furthermore, the VGs arranged in a “common flow down” configuration have better heat transfer performance than VGs in a “common flow up” configuration. The parameter study manifests that VGs with a height ratio of h/Hc = 0.6 and attack angle of β = 45° present the best thermal hydraulic performance. The optimized value of R reaches as high as 1.17–1.28 in the present study.

Flow boiling critical heat flux of DI-water and nanofluids inside smooth and nanoporous round microchannels

This work concerns an experimental evaluation of the flow boiling critical heat flux of DI-water and nanofluids inside a 1.1 mm ID channel. Results were obtained for DI-water and solutions of Al2O3 (40–80 nm)/DI-water, Al2O3 (10 nm)/DI-water and SiO2 (80 nm)/DI-water for volumetric concentrations of 0.1% and 0.01%. Additional experiments were performed for pure DI-water on surfaces previously covered with nanoparticles through the flow boiling process. Experiments were performed for mass velocities ranging from 133 to 494 kg/m2s, saturation temperature of 130 °C and vapor qualities at the CHF location ranging from 0.65 to 0.84. The CHF results for DI-water in the tube before covering its surface with nanoparticles were compared with well reputed CHF prediction methods from literature and a reasonable agreement was found. Despite of the large augmentation of the wettability/wickability of the surfaces covered by a layer of nanoparticles, the differences between the CHF values for DI-water for the surfaces with and without the nanoporous covering were within the uncertainty of the CHF measurements. This behavior suggests a negligible effect of the nanoporous layer and, consequently, of wettability/wickability on the saturated CHF in microchannels for high vapor qualities and the range of conditions evaluated in present study. Based on an analysis of the CHF behaviors observed in this study, the saturated CHF mechanism was associated to periodical lumps of liquid that promotes intermittently the surface dryout and its re-wetting. The CHF is established when the time between two consecutives liquid lumps is low enough, so that the surface reaches a temperature for which its re-wetting is not possible.

Effect of interface wettability on the flow characteristics of liquid in smooth microchannels

Rapid advances in microfluidic devices have induced interest in the study of the microscale flow mechanism. However, the experimental results of microscale flow often deviate from the classical theory, and we attribute this deviation to the changing liquid viscosity in the microchannels. Because of the effect of the solid–liquid intermolecular force, the viscosity of the liquid near the walls is different from the bulk viscosity. Based on molecular theory and wetting theory, we propose a modified apparent viscosity model. The apparent viscosity of the liquid in microchannels increases with the increase in wettability and decreases with the increase in distance from the wall and the increase in drive pressure. The apparent viscosity near the hydrophilic wall is higher than the bulk viscosity, which increases the flow friction in the microchannels. To validate this model, we experimentally investigate the frictional characteristic of a deionized water flow in smooth parallel-plate microchannels with different wettabilities and heights of approximately 20 and 50 $$\upmu \hbox {m}$$ . The results indicate that the friction factor is higher than that predicted by the classical theory. Such a difference increases with increasing wettability and decreases with increasing hydraulic diameter and pressure drop, which is consistent with the results of theoretical analysis. The apparent viscosity calculated by the apparent viscosity model notably fit the experimental results, with a relative difference of less than ± 2.1%.

Studies on Gas Flow through Smooth Microchannel Surface – Fabrication, Characterization, Analysis, and Tangential Momentum Accommodation Coefficient Comparison

In the present work, experimental and numerical findings of tangential momentum accommodation coefficient (TMAC) on smooth microchannel surface and comparison with rough surface was performed. A modified silicon micromachining technique was used to achieve smooth surface of the microchannel. About two orders of magnitude decrease in surface roughness was obtained when compared to previous studies. Second order slip model was derived, and a special procedure was developed to simulate the gas flow in microchannel from slip-to-transition regimes. The microchannel system with 20 parallel channels were fabricated on a surface modified silicon surface and flow characteristics were studied by generating accurate and high resolution experimental data with comparison of simulation results. TMAC values were found up to an outlet Knudsen number of 0.851 with nitrogen and the second order slip coefficients were found. When compared to previous TMAC values for rough surfaces, low values were obtained in the present case. This decrease is an indication of the increase in slip velocity due to the reduction of surface roughness.

Enhancement of the cooling capability of a high concentration photovoltaic system using microchannels with forward triangular ribs on sidewalls

Numerical simulations were performed to investigate a microchannel heat sink device as cooling option for a high concentration photovoltaic system. COMSOL Multiphysics 5.1 software is used to solve three-dimensional equations which consider conjugate heat transfer, viscous dissipations, and temperature-dependent-properties. This study investigates the integration of microchannels with complex geometric features on its inner walls into the solar cell structure, to enhance the heat transfer performance of a microchannel heat sink-based active cooling system. Inner sidewall mounted forward triangular ribs are considered in aligned and offset distributions along the microchannel walls. In addition, numerical analysis is developed for a conventional flat plate heat sink integrated to a high concentration photovoltaic system to stablish a baseline solar cell temperature. The numerical results show that a micro-channel heat sink device can control and keep in very low range the solar cell temperature (<301 K). Compared to a smooth microchannel, forward triangular ribs installed on the sidewalls enhance the heat transfer capability. Microchannels with aligned and offset rib distributions increase the Nusselt number between 1.8 and 1.6 times, respectively, and increase the average friction factors between 3.9 and 2.3 times, respectively. The microchannel heat sink device with forward triangular ribs is more efficient and effective at Re ≤ 200, since the pumping power reaches a high percentage of the total power generated by solar cell when Re > 200. At Re = 400, the pumping power reaches 41% and 23% of the total power generated by a multi-junction solar cell in the aligned and offset rib distribution, respectively. The pumping power is greatly reduced while using smooth microchannel, because the maximum pumping power is only 9.5% of the solar cell power at Re = 400, however, the resulting solar cell temperature is slightly higher compared to microchannels with aligned and offset rib configurations. A microchannel heat sink provides a more effective cooling solution compared to a passive flat plate heat sink for a high concentration photovoltaic system. In addition, the possibility of direct integration of a microchannel heat sink into a solar cell structure as proposed in this study, represents an interesting option to feasibly increase thermal performance to a considerable level by maintaining the solar cell temperature in a very low range.

Flow and Heat Transfer in the Tree-Like Branching Microchannel with/without Dimples.

This work displays a numerical and experimental investigation on the flow and heat transfer in tree-like branching microchannels and studies the effects of dimples on the heat transfer enhancement. The numerical approach is certified by a smooth branching microchannel experiment. The verification result shows that the SSG turbulence model can provide a reasonable prediction. Thus, further research on the convective heat transfer in dimpled branching microchannels is conducted with the SSG turbulence model. The results indicate that the dimples can significantly improve the averaged heat transfer performance of branching microchannels, and the heat transfer increment of the branch segment increases with the increase in the branching level. However, the flow dead zones in some dimples at bifurcations and bends suppress the turbulent flow and heat transfer. Furthermore, the Nu number ratio (Nua/Nus) and thermal enhancement factor (η) both monotonously decrease as the Re number increases, while the friction factor ratio (fa/fs) changes nonlinearly. The entropy generation rates of and in all dimpled cases are lower than those in the smooth case, and the dimpled case with the streamwise spacing to diameter ratio s/D = 3 obtains the lowest value of augmentation entropy generation (Ns) under the high Re number conditions. Nua/Nus, fa/fs, and η decline with the increase in the streamwise spacing to diameter ratio (s/D) from 3 to 9; therefore, the dimpled case with s/D = 3 shows the best overall thermal performance.

On form accuracy and surface roughness in micro-ultrasonic machining of silicon microchannels

Accuracy in manufacturing microchannels is important in order to achieve their intended function. Smooth and high aspect ratio microchannels on silicon wafer substrate are needed in the heat removal application in various microelectronic components. Generally, etching techniques are used to fabricate silicon microchannels; however, the maximum achievable limit for the channel depth is a major concern. Micro-ultrasonic machining (micro-USM) is capable of machining high aspect ratio microchannels on hard and brittle material such as silicon, glass, ceramics, etc. However, achieving reasonable form accuracy and surface roughness of the microchannels is challenging. Overcut and edge damage (stray cut) are undesirable for precision machining while surface roughness of the microchannels can be set at an optimized value to attain maximum heat transfer. In the present study, silicon microchannels were fabricated using the micro-USM technique. In order to improve the precision and quality of the fabricated silicon microchannels in terms of surface roughness, overcut and stray cut; viscous fluids with different viscosities were considered for investigation in combination with other machining conditions. The experimental investigation revealed that using low viscous fluids yields better surface roughness compared to high viscous fluid; however, overcut and stray cut were minimized while using high viscous fluids. Machining at higher feed rates could minimize the surface roughness, over cut and stray cut irrespective of the abrasive concentration percentage. Possible interactions between the tool, abrasive and workpiece in the machining zone were analyzed vis-à-vis the experimental results.

Deformation of Red Blood Cells, Air Bubbles, and Droplets in Microfluidic Devices: Flow Visualizations and Measurements.

Techniques, such as micropipette aspiration and optical tweezers, are widely used to measure cell mechanical properties, but are generally labor-intensive and time-consuming, typically involving a difficult process of manipulation. In the past two decades, a large number of microfluidic devices have been developed due to the advantages they offer over other techniques, including transparency for direct optical access, lower cost, reduced space and labor, precise control, and easy manipulation of a small volume of blood samples. This review presents recent advances in the development of microfluidic devices to evaluate the mechanical response of individual red blood cells (RBCs) and microbubbles flowing in constriction microchannels. Visualizations and measurements of the deformation of RBCs flowing through hyperbolic, smooth, and sudden-contraction microchannels were evaluated and compared. In particular, we show the potential of using hyperbolic-shaped microchannels to precisely control and assess small changes in RBC deformability in both physiological and pathological situations. Moreover, deformations of air microbubbles and droplets flowing through a microfluidic constriction were also compared with RBCs deformability.