Heat Transfer In Microchannels Research Articles

Performance Analysis of a Phase Changing Material Based Thermocycler for Nucleic Acid Amplification

Abstract Modeling, simulation, and thermal performance analysis of a thermocycler for the continuous-flow polymerase chain reaction (CF-PCR), with a phase changing material (PCM)-laden annealing flow path, is presented. The incessant threat of microorganisms such as viruses, bacteria, and fungi has fostered effective, quick, and miniature detection devices in order to curtail the wide-spreading of infections. Microfluidics-based CF-PCR systems are compact and ideal for faster response. The thermal cycling process involves a sequential exposure of a given liquid sample to various temperature conditions when it is taken through the continuous-flow path. As a result, a prescribed periodic change of temperature suitable for deoxyribonucleic acid (DNA) amplification is achieved. A rapid temperature reduction and maintenance of isothermal conditions to facilitate the annealing phase of CF-PCR process by a PCM-assisted cooling is envisaged in the present study. Unsteady, two-dimensional, incompressible fluid flow, and internal convection heat transfer in a microchannel annealing path with melting of tetracosane (C24H50) boundary has been simulated using semi-implicit method for pressure linked equations-consistent (SIMPLEC) algorithm based finite volume solver. Solver validation is carried out against the experimental data on internal convection heat transfer in a rectangular microchannel. A detailed numerical study has been performed to assess the spatiotemporal heat transfer characteristics of internal convection in the microfluidic path when the flow triggers the melting of encapsulated PCM. A minimum sample flowrate with PCM encapsulation of less than 600 μm is found to be ideal for achieving desired thermal performance. The present study evidences the swift temperature reduction and management of isothermal conditions congenial for the annealing process in the CF-PCR system for various sample flowrates and PCM masses. The study offers valuable design input for the development of a microfluidic thermocycler for CF-PCR applications.

Transient heat transfer and electro-osmotic flow of Carreau–Yasuda non-Newtonian fluid through a rectangular microchannel

PurposeThe purpose of this study is to investigate heat transfer and electrokinetic non-Newtonian flow in a rectangular microchannel in the developed and transient states.Design/methodology/approachThe Carreau–Yasuda model was considered to capture the non-Newtonian behavior of the fluid. The dimensionless forms of governing equations, including the continuity equation for the Carreau–Yasuda fluid, are numerically solved by considering the volumetric force term of electric current (DC).FindingsThe impact of pertinent parameters such as electrokinetic diameter (R), Brinkman number and Peclet number is examined graphically. It is observed that for increasing R, the bulk velocity decreases. The velocity of the bulk fluid reaches from the minimum to the maximum state across the microchannel over time. At the electrokinetic diameter of 400, the maximum velocity was obtained. Temperature graphs are plotted with changes in the various Brinkman number (0.1 < r < 0.7) at different times, and local Nusselt are compared against changes in the Peclet number (0.1 < ℘e < 0.5). The results of this study show that by increasing the Brinkman number from 0.25 to 0.7, the temperature along the microchannel doubles. It was observed that increasing the Peclet number from 0.3 to 0.5 leads to 200% increment of the Nusselt number along the microchannel in some areas along the microchannel. The maximum temperature occurs at Brinkman number of 0.7 and the maximum value of the local Nusselt number is related to Peclet number 0.5. Over time in the transient mode, the Nusselt number also decreases along the microchannel. By the increasing of time, the temperature increases at given value of Brinkman, which is insignificant at Brinkman number of 0.1. The simulation results have been verified by Newtonian and non-Newtonian flows with adequate accuracy.Originality/valueThis study contributes to discovering the effects of transient flow of electroosmotic flow for non-Newtonian Carreau–Yasuda fluid and transient heat transfer through rectangular microchannel. To the authors’ knowledge, the said investigation is yet not available in existing literature.

Flow periodicity analysis in an additively manufactured metal microchannel heat transfer device using micro-PIV

The flow periodicity in an additively manufactured microchannel heat transfer device is analysed using the micro-particle image velocimetry technique. A metal offset strip fin (OSF) microchannel is additively manufactured using laser powder bed fusion to overcome the limitations of conventional production techniques in manufacturing small complex features. This study demonstrates the capability of measuring the velocity field in an OSF microchannel up to 1 mm deep with a resolution of 10μm and a velocity uncertainty smaller than 0.5% for laminar flows with a Reynolds number below 10. Higher measurement uncertainties are encountered in regions close to the channel walls due to particle deposition. A methodology aiming to evaluate and minimise the uncertainties is presented. The experimentally obtained velocity fields are compared to a numerical simulation of the flow in the offset strip fin microchannel. The results verify that flow periodicity can be observed and the extent of the flow development region can be studied. Nevertheless, in regions with a large velocity gradient, a considerable uncertainty on the velocity measurements exists due to manufacturing imperfections.

Enhanced thermal and mechanical performance of 3D architected micro-channel heat exchangers

Many crystals in nature have simple interatomic microstructures, such as simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC) lattice symmetries, making these structures extremely stable. Inspired by these arrangements, a series of architected micro-channel heat exchangers with rationally designed 3D microstructures were established. A multi-physics mathematical model using thermal-fluid-structure interaction (TFSI) was employed to investigate the coupled heat transfer performance and mechanical properties of these architected heat exchangers. When compared with the corrugated straight plate (CSP) microchannel heat exchanger, the thermal-hydraulic performance factors (TPC) of FCC and BCC microchannel heat transfer were 2.20 and 1.70 times that of SC microchannel heat exchanger, respectively. The micro-channel heat exchanger with FCC architectures could enhance the convective heat transfer performance by 201.0%, while the micro-channel heat exchanger with SC architectures reduced the Von-Mises equivalent (VME) stress by 20.0% when compared with the conventional 2D CSP heat exchanger. The proposed architected micro-channel heat exchangers could find a wide range of potential applications ranging from power electronics in electric vehicles to concentrated solar power systems, where both good convective heat transfer performance and high mechanical strength are simultaneously pursued.

Computer Vision and Machine Learning Methods for Heat Transfer and Fluid Flow in Complex Structural Microchannels: A Review

Heat dissipation in high-heat flux micro-devices has become a pressing issue. One of the most effective methods for removing the high heat load of micro-devices is boiling heat transfer in microchannels. A novel approach to flow pattern and heat transfer recognition in microchannels is provided by the combination of image and machine learning techniques. The support vector machine method in texture characteristics successfully recognizes flow patterns. To determine the bubble dynamics behavior and flow pattern in the micro-device, image features are combined with machine learning algorithms and applied in the recognition of boiling flow patterns. As a result, the relationship between flow pattern evolution and boiling heat transfer is established, and the mechanism of boiling heat transfer is revealed.

Theoretical and experimental research on flow boiling heat transfer in microchannels for IGBT modules

Two-phase microchannel heat sink has received extensive attention due to its small size, high heat transfer efficiency, good temperature uniformity, and low working fluid charge. Heat flux and mass flow rate are important parameters that affect the heat transfer performance of two-phase microchannel heat sink. For different heat fluxes, if the mass flow rate is not reasonable, dry-out may occur, which will significantly reduce the heat transfer performance. In order to further explore the influence of mass flow rate and heat flux on heat transfer characteristics, the flow boiling heat transfer performance and flow regimes under different mass flow rates and heat fluxes were studied for a two-phase microchannel heat sink using R245fa as the working fluid. The results showed that when the mass flow rate was 100 kg/(m2·s), the boiling heat transfer coefficient of R245fa showed a trend of increasing firstly and then decreasing with the increase of the heat flux. At a higher heat flux, the local dry-out in the microchannels began to appear, and the heat transfer performance decreased significantly. And the higher the heat flux was, the more serious the dry-out was. There was no obvious dry-out phenomenon when the mass flow rate was 150–300 kg/(m2·s) and the heat flux was below 500 kW/m2. The research provided a valuable reference for exploring the influence of the mass flow rate and the heat flux on the flow boiling heat transfer characteristics of the microchannel heat sink.

Numerical simulation of flow boiling heat transfer in microchannel with surface roughness

In order to investigate the effect of surface roughness on the thermal-hydraulic performance of flow boiling in the microchannel, the surface with random roughness is generated by the Fourier series method, the VOF model and Lee model are used to simulate the flow boiling process. The results show that with the increase in relative roughness, Nu and fRe first increase and then decrease. The microchannel with a rough surface can achieve a better thermal-hydraulic performance than that with a smooth surface. Within the scope of this study, the thermal-hydraulic performance reaches the best when the relative roughness is 0.304%. Additionally, with the increase in the means width of the roughness elements, the surface undulation becomes gentler, the growth rate of Nu slows down, and the fRe increases monotonously. Besides, with the decrease in inlet velocity, the liquid film at the bottom of bubbles dries up, which results in drought spots at the bottom wall and deteriorates the heat transfer performance.

Enhanced thermal performance by spatial chaotic mixing in a saw-like microchannel

Convection heat transfer in microchannel is an alternative cooling technique for electronic devices. But the long-lasting dominant laminar flow seriously inhibits the further enhancement of single-phase convection. In this work, a saw-like microchannel configuration is proposed to improve convective heat transfer by substantially enhancing spatial fluid mixing and significantly interrupting boundary layer. The heat transfer and hydraulic performance of this configuration are numerically investigated in opposite directions. The diodic performance is also studied by comparing the pressure drop and Nusselt number (Nu) in the forward and backward directions. Parametric study is conducted to investigate the roles of channel width and bifurcation angle on fluid flow and convection. Finally, convective heat transfer in both directions is significantly enhanced in this saw-like microchannel owing to the intensified spatial fluid mixing. Another enhanced factor is the periodic interruption of thermal and fluid boundary layers. Compared to conventional rectangular microchannel configuration, the Nu is dramatically enhanced by about 2.5-fold at Re = 440. The thermal diodicity (Dit) and pressure drop diodicity (Dip) are up to 1.4 and 2.1 at Re = 440, respectively. A comprehensive comparison is presented to show the advantages of our design.

Growth of elongated vapor bubbles during flow boiling heat transfer in wavy microchannels

Flow boiling within microchannels is an efficient heat removal mechanism by utilizing the latent heat of the fluid. Numerical simulations can resolve the small temporal and spatial scales of the vapor bubbles in the flow boiling process, thus accessing many details of the flow which is unattainable experimentally. Most of the previous studies of flow boiling heat transfer used straight microchannels, even though curved microchannels have potential to further enhance the heat transfer performance. In this study, the flow boiling heat transfer in wavy microchannels is studied via numerical simulation by focusing on the growth of the vapor bubbles. Straight and wavy microchannels are considered for comparison and analysis. The numerical simulations are conducted using the finite volume method to solve the flow and the heat transfer, and the volume of fluid (VOF) method to capture the interface dynamics. The effects of key dimensionless numbers on bubble growth rate and wall heat transfer are quantified, including the Weber number, the Capillary number, and the Jakob number. The results indicate that the wavy structure of the microchannel has a strong effect on the bubble growth and the wall heat transfer in microchannels. As the waviness of the microchannel increases, the bubble grows faster because of the bubble deformation by the wavy channel, the bubble moves faster due to the bubble expansion, and the heat transfer is enhanced by the perturbation of the bubble to the flow. With the large velocity perpendicular to the channel flow direction induced by the waviness, the local Nusselt number of the wavy channel could be high up to 2.6 times higher than the straight channel.

Direct liquid cooling heat transfer in microchannel: Experimental results and correlations assessment

In this paper, the hydrothermal characteristics of direct liquid cooling in microchannel were experimentally investigated. The effects of heat flux, inlet temperature, volume flow rate and hydraulic diameter on the wall temperature non-uniformity, the heat transfer coefficient and the pressure drop were investigated. To assess the flow and heat transfer performance of direct liquid cooling in the microchannel, the coefficient of performance was proposed and compared with indirect liquid cooling. The experimental results reveal that the coefficient of performance of direct liquid cooling is 5 times higher than that of indirect liquid cooling. Furthermore, the applicability of the existing empirical correlations was evaluated for microchannel direct liquid cooling. New correlations were developed for predicting Nusselt number and Darcy friction factor, which generated the mean absolute relative error of 11.6% and 22.2%, respectively. The present work could provide valuable guidance for the design of microchannel condenser for engineering application.

Experimental and Numerical Analysis of the Influence of Microchannel Size and Structure on Boiling Heat Transfer

Computational fluid dynamics was used and a numerical simulation analysis of boiling heat transfer in microchannels with three depths and three cross-sectional profiles was conducted. The heat transfer coefficient and bubble generation process of three microchannel structures with a width of 80 μm and a depth of 40, 60, and 80 μm were compared during the boiling process, and the factors influencing bubble generation were studied. A visual test bench was built, and test substrates of different sizes were prepared using a micro-nano laser. During the test, the behavior characteristics of the bubbles on the boiling surface and the temperature change of the heated wall were collected with a high-speed camera and a temperature sensor. It was found that the microchannel with a depth of 80 μm had the largest heat transfer coefficient and shortest bubble growth period, the rectangular channel had a larger peak heat transfer coefficient and a lower frequency of bubble occurrence, while the V-shaped channel had the shortest growth period, i.e., the highest frequency of bubble occurrence, but its heat transfer coefficient was smaller than that of the rectangular channel.

Lattice Boltzmann study of bubble dynamic behaviors and heat transfer performance during flow boiling in a serpentine microchannel

• Mesoscale study of microchannel flow boiling in U bend was conducted. • Detailed bubble dynamics and heat transfer characteristics were presented. • Effect of buoyancy on flow boiling process depends on flow orientation. • The onset of nucleate boiling is delayed as the Re number increases. Understanding the flow boiling process in a serpentine microchannel with U-bends is very important to its design and application in practice. In this study, a hybrid thermal multiphase model consisting of the pseudopotential multiphase lattice Boltzmann model and the finite difference method is employed to investigate the flow boiling heat transfer in a serpentine microchannel. Effects of curvature ratio, flow orientation, heat flux and Reynolds ( Re ) number on the bubble dynamic behaviors and heat transfer performance during flow boiling process are comprehensively evaluated. Bubble behaviors including bubble nucleation, growth, coalescence, departure, recontact, and migration are well captured, and typical flow patterns of bubbly flow and intermittent slug flow are identified. The simulation results show that increasing curvature ratio does not affect the heat transfer performance much, but generates elongated bubbles at the U-bend. Depending on the flow orientation, the buoyancy induced by gravity acceleration has both favorable and unfavorable effects on the bubble dynamic behaviors and local heat transfer characteristics in the serpentine microchannel. In addition, the vapor volume fraction is calculated under different Re numbers and heat fluxes. The lower vapor volume fraction exhibits lower wall superheat and better heat transfer performance.

Transforming Navier‐stokes equation to a fourth order nonlinear differential equation for the study of electrolyte flow in a circular microchannel

AbstractIn this paper, analytical solutions are derived for the study of steady‐state electrolyte flow in a circular microchannel under the simultaneous effects of an external electric field and uniform wall injection. The Navier‐Stokes equations containing an axial linear electric body force are transformed to a fourth‐order nonlinear differential equation and solved analytically using a series solution method. The most important results are presented and discussed in terms of radial Reynolds number and a fundamental non‐dimensional electric field parameter. The Electrical Double Layer theory is employed to model the effects of external electric field. Analytical relations are given for velocity profiles, exit flow rate, and total pressure drop in the microchannel. The results show that a high‐intensity electric field can completely reverse the velocity profile or make it more flat in the presence of wall injection. This electric effect can be used in practical applications to control the shear stress, heat, or mass transfer in microchannels or microelectromechanical systems. In addition, various graphs are presented showing simultaneous effects of electric field and wall injection of the same fluid on the flow physics.

Lattice Boltzmann mesoscopic study of effects of corrosion on flow boiling heat transfer in microchannels

The robustness of boiling heat transfer performance is crucial for the long-term applications, and it is necessary to understand the influence of corrosion on flow boiling heat transfer. In the present study, the corrosion process is first studied and the corrosion morphologies under typical reactive transport conditions and pit geometrical parameters are obtained using the corrosion lattice Boltzmann (LB) model. Then, the flow boiling heat transfer in the corroded microchannels is studied using the hybrid thermal multiphase LB model, and compared with the heat transfer performance and bubble dynamic behaviors of smooth intact microchannel. Effects of corrosion morphology, pit height, and staggered distance between pits on the flow boiling heat transfer characteristics are explored. The corrosion level plays an important role in the heat transfer performance. Although the corroded cavities with various corrosion levels can promote the bubble nucleation, the reduced heat transfer performance occurs at high corrosion level due to inefficient removal of the bubbles from the large corroded cavities. Increasing the pit height and staggered distance between pits can slow down the reduction of heat transfer performance. For the severe corroded microchannels, the heat transfer performance is deteriorated significantly due to the formation of the vapor film covered on the heating surface.

Theoretical Aspects on Heat Transfer and Fluid Flow in Micro Channels

The past ten years have seen a lot of research on the issues of heat transfer and fluid flow in micro-channels. With an emphasis on upcoming research requirements, a critical analysis of the state of research as it stands is offered. Following a brief introduction, the study discusses six themes related to transport phenomena in micro-channels: condensation, cooling of electronics, single-phase gas flow, augmentation of single-phase liquid flow and flow boiling, and micro-scale heat exchangers. In this study, we investigate the three-dimensional heat transfer and water flow properties in a set of rectangular micro-channel heat sinks for advanced electronic systems. Over the past ten years, mini/micro channel type compact heat exchangers have received a lot of attention. The ability to make heat exchangers smaller, lighter, and cheaper than those in use now is the primary motivator. Additionally, emerging applications that call for the cooling of small things, such electronics and micro-electro-mechanical devices, need for heat exchangers with tiny channels. Theoretically, it is examined how a nanofluid moves and transfers heat through a horizontal micro channel while being affected by a magnetic field and an electric double layer (EDL). The flow problem for a micro channel with a large aspect ratio is handled as a two-dimensional nonlinear system. The magnetic field and EDL body force are taken into account while calculating momentum equation

Impact of inlet subcooling on flow boiling in microchannels

Inlet subcooling is of paramount importance for the flow boiling heat transfer in microchannels. This work carries out the experimental investigation of the impact of inlet subcooling on the deionized water flow boiling in silicon microchannels (H = 200 μm, W = 80 μm, L = 10 mm). Experiments are conducted with inlet subcooling degrees of 70, 40 and 20 K, mass fluxes of 446–963 kg/m2·s and base heat fluxes of 17.5–407.2 W/cm2. It is found that the boiling hysteresis with explosive boiling occurs in the boiling inception at a high subcooling degree of 70 K, and boiling hysteresis gradually gets gentle as inlet subcooling decreases, which has not been reported in the previous works. At a higher subcooling degree, more heat is required for the bubble nucleation but higher critical heat flux (CHF) can be achieved. Further, as inlet subcooling decreases, the flow reversal is well mitigated and thus the flow boiling becomes more stable. Based on the above observations, we put forward a new dimensionless number called the boiling utilization to evaluate the flow boiling characteristics at different inlet subcooling conditions. It is found that a larger degree of subcooling results in a smaller boiling utilization; and the copper-based microchannel has a higher boiling utilization than that of the silicon-based microchannel. A new CHF correlation considering the influence of inlet subcooling is proposed, which shows good agreement with experiments, and the mean absolute error between the experimental values and predicted values by the proposed correlation is 7.3 %.

Analytical solutions of slip flow and H1 heat transfer in elliptical microchannels

The slip flow and heat transfer problems inside a micro- or nano-channel are active research topics. Understanding the velocity and temperature distributions in these channels is essential in both theory and applications. In this paper, the analytical solutions of the fully developed slip flow and heat transfer under the H1 boundary condition in elliptical microchannels are obtained with the velocity slip and temperature jump. The separation of variables method is employed to solve the momentum and energy equations, and the coefficient of the velocity profile is calculated using the binomial series twice. Based on the result of the velocity profile, the analytical solution of the temperature profile is also derived. A new model is deduced to predict the Poiseuille number in elliptical microchannels. The close-form solution of the Nusselt number is first obtained and can serve as a benchmark for other approaches.

Measurement of transient liquid film and its effect on flow boiling heat transfer in non-circular microchannels

Micro liquid film plays an important role during flow boiling in microchannels. Current studies on flow boiling in microchannels have mainly focused on circular microchannels. However, microchannels with sharp corners, such as square and rectangular microchannels, are more common in practice. In the present study, flow boiling experiments were performed in square microchannels using ethanol as the working fluid. The mass flux was 26.71 kg (m−2 s−1) and heat flux ranged from 10 to 80 kW m−2. The transient liquid film thickness during flow boiling in square microchannels was measured using a laser confocal displacement meter. The pressure drop between inlet and outlet of the microchannel and the wall temperature were measured synchronously. Experimental results demonstrated that heat transfer performance depended on liquid film thickness and the thickness in square microchannels was smaller than in circular microchannels under the same capillary number. The variation of liquid film thickness during flow boiling in square microchannels was analyzed and a theoretical model was proposed for predicting liquid film thickness. According to variation in wall temperature and liquid film thickness, the flow boiling process in square microchannels was divided into liquid slug, elongated bubble, and dryout zones. The heat transfer mechanism of different zones was analyzed. In addition, the microchannel pressure drop fluctuation and backflow phenomenon were analyzed.

Numerical investigation of flow boiling heat transfer in manifold microchannels

The manifold microchannel (MMC) has drawn much attention due to its excellent convective heat transfer performance. Flow boiling in MMC can reach high heat transfer coefficient, however the research of which is rare in the literature. In this study, transient subcooling flow boiling heat transfer in MMC is numerically studied by a phase change model that renders the phase interfacial temperature converged to the saturation temperature. Effects of heat flux, subcooling, bottom channel height, and outlet/inlet width ratio are investigated in terms of vapor dynamic behaviors, thermal resistance, heat flux with the lowest thermal resistance and pressure drop. A lower subcooling promotes the occurrence of nucleate boiling and leads to lower thermal resistance, however the thermal resistance starts to rise earlier. In addition, too high bottom channel height deteriorates the convective heat transfer while too low bottom channel height leads to worse heat transfer at the upstream corner, and thus there exists an optimal channel height. Moreover, there also exists an optimal outlet/inlet width ratio which avoids the local worse heat transfer at upstream corner or vapor blockage in the outlet channel. Based on the results in the present study as well as in the literature, a new flow-boiling heat transfer correlation is developed for nucleate boiling in MMC. The new correlation agrees well with existing results, with mean absolute deviation (MAD) about 7.87%, much lower than MAD of 200% obtained by existing correlations. The present work is helpful for understanding flow boiling processes and providing flow-boiling heat transfer correlations in MMC.

Effects of surface roughness in multiple microchannels on mixed convective heat transfer

• Effect of process-induced surface roughness on mixed convection was investigated. • Height, width and average roughness parameters were studied on multiple microchannels. • Surface roughness is an important geometric parameter and affects heat transfer. • Mixed convection increases with decreasing cross-section and increasing roughness. The most widely used type of heat transfer in applications is convection heat transfer, and both natural and forced convection modes have an effect in total heat transfer to a certain extent, whether any of these modes is neglected or not. In this study, the contribution of natural and forced convection effects to total heat transfer in rectangular microchannels was investigated experimentally for determining the effect of manufacturing process-induced surface roughness in multiple microchannels fabricated using stainless steel material. The experiments were conducted with microchannel heat sinks having three different surface roughness values of 1.1 µm, 1.8 µm, and 3.0 µm, three microchannel widths of 300 µm, 500 µm and 700 µm and two heights of 300 µm and 450 µm. During the manufacturing process, surface roughness values were controlled by changing electro-erosion fabrication parameters, and detailed surface maps were generated for the microchannel heat sinks. In the experiments using pure water as the working fluid, constant heat flux was applied to the bottom surface of the microchannel heat sink. The experiments were conducted in the range of 10–80 Reynolds numbers to ensure mixed convection conditions. In conclusion, it was determined that surface roughness had a significant effect on mixed convective heat transfer. An increase in surface roughness from 1.1 µm to 1.8 µm in microchannel heat sinks with cross-sections of 300 µm × 300 µm, 700 µm × 300 µm, 300 µm × 450 µm, and 700 µm × 450 µm s led to an increase of about 24 %, 19 %, 17 %, and 13 %, respectively, in the Nusselt number. Likewise, the increase in surface roughness from 1.8 µm to 3.0 µm resulted in an increase of 17 %, 15 %, 14 %, and 10 %, respectively, in the Nusselt number in the same channel cross-sections. In general, an increase in hydraulic diameter led to a reduction in the effect of surface roughness on overall heat transfer. For a constant microchannel width and height, the effect of surface roughness on mixed convection could be observed only with the increase in the Grashof number. Among the eighteen microchannel heat sinks with different geometric parameters, the best heat transfer results were obtained for the smallest microchannel cross-section of 300 µm × 300 µm and the highest roughness value of 3.0 µm.