Heat Transfer In Microchannels Research Articles

Study on Flow and Heat Transfer in Supercritical CO2 in Micro-channel Heat Exchanger

In order to investigate the flow and heat transfer performances of supercritical CO2 in the PCHE, a two-dimensional micro-channel heat transfer model was established. The impacts of the airfoil fin arrangement on flow and heat transfer of supercritical CO2 in a PCHE were studied using Computational Fluid Dynamics software. The results show that the transfer efficiency of PCHE can be effectively improved by reducing the abeam and vertical distances between airfoil fins. However, the pressure drop of supercritical CO2 flow in the microchannel reduces with an increase in abeam and vertical spacing distances.

The bubble merger in rectangular microchannels during boiling processes based on conjugate heat transfer

In recent years, boiling heat transfer in microchannels has become an effective solution for managing high heat flux in electronic devices. Numerical simulations offer details on flow characteristics that experimental methods cannot capture, deepening our understanding of boiling mechanisms within microchannels. In this study, we developed a self-programmed solver, “interMultiRegionFoam,” based on the volume of fluid (VoF) method, phase-change mass transfer models, and conjugate heat transfer. Analyzing the force acting on individual bubbles, a model for two bubbles' merger was constructed, encompassing three stages: bubble bridging, bubble retraction, and bubble contraction. For isolated bubbles, during the restricted growth stage, the dry patch length increases by only 25% with an increase in the Capillary number, while in the elongation growth stage, it grows fivefold. Furthermore, the investigation into four typical bubble merger cases within microchannels demonstrated that the bubble merger model effectively describes the process. During the restricted and free growth stages, bubble merger was observed to enhance heat transfer, and the heat transfer coefficient in the bottom region of the macrolayer increased by a maximum of 16%. However, when bubbles merge in the elongation growth stage, the maximum increase in the heat transfer coefficient at the bottom of the macrolayer is only 7%. Several factors contribute to the enhanced heat transfer during bubble merger, including the evaporation and rewetting of macrolayer liquid, as well as the replenishment of the microlayer. Moreover, the disturbance caused by bubble merger and transient conduction arising from bubble retraction and contraction contributes to the enhancement in heat transfer. With an increase in the contact angle, the peak heat flux gradually decreases, with a maximum reduction of 27%. As the heat flux increases, the Nusselt number gradually increases, reaching a maximum peak increase of 52%. These studies provide theoretical guidance for the design of wall modifications and nucleation in microchannels.

Numerical investigation and data-driven prediction of flow boiling heat transfer in manifold microchannels with slopes

The manifold microchannels (MMC) with high heat transfer coefficient and low pressure drop has received much attention. Flow boiling heat transfer inside MMC is promising for dissipating heat of high-power devices. In this study, flow boiling heat transfer inside MMC is further numerically studied with slopes added in the upstream and downstream corners of MMC. The results show that downstream slope leads to lower local temperature and thermal resistance by removing the upstream vapor more easily, and upstream slope leads to higher heat flux corresponding to the turning point in the flowing boiling curve by restraining the local high temperature at upstream corner. The combination of the upstream and downstream slopes can reduce the thermal resistance by 5.45% and increase the heat flux related to the turning point by 14.28% compared to the MMC without slopes. A new flow boiling heat transfer correlation considering the slopes is developed with prediction accuracy of mean absolute deviation (MAD) of about 0.05%. An artificial neural network (ANN) is further built to predict the vapor saturation inside the MMC based on the outside temperature at the MMC bottom surface, which presents high accuracy of 97.03%.

Force convective heat transfer simulation in a rectangular channel with multiple square obstacles via finite element method

This study investigated fluid flow and forced convective heat transfer in rectangular microchannels with square barriers, as illustrated in Fig.  1 . In the first situation, three obstacles were positioned along the microchannel’s top wall. In the second scenario, obstacles were positioned along the microchannel’s bottom wall. In the final example, three square obstacles are placed symmetrically on either side of the microchannel wall. With the help of the Finite Element Method (FEM), we investigate the physicochemical behavior of the microchannel. The development of computer code within COMSOL multiphysics made it possible to simulate heat transport and fluid flow. The results include the implications of the rarefaction effect on fluid flow and heat transmission and decisions regarding the location of barriers and the shape of obstacles in squares. In addition, with the lowest value in skin friction and a lower Nusselt number, the third example, which has barriers on both sides, provides a valuable method for reducing the fluid temperature at the exit of the microchannel. This is because it has barriers on both sides. In the section under “Results and Discussion,” we provide an in-depth analysis of the numerical data derived from the microchannel. • Objective: This study aims to numerically investigate forced convective heat transfer in a rectangular channel featuring multiple square obstacles. The simulation employs the FEM to model fluid flow and temperature distribution. • Geometry and complexity: The channel geometry incorporates square obstacles, introducing geometric complexity. The FEM is chosen for its capability to handle intricate geometries accurately.

Viscous dissipation effects on slip flow heat transfer in rhombic microchannels

This study aimed to numerically investigate the viscous dissipation effect on forced convection in rhombic microchannels for gases in a slip flow regime. The numerical analysis was carried out by assuming a 2D steady-state flow. The solution of governing equations was obtained by adopting the finite element method and assuming that the fluid is Newtonian, with constant thermophysical properties, and in a fully developed laminar flow regime. The solution of the momentum equation is obtained by considering a first-order boundary condition, while the solution of the energy balance equation is obtained by assuming a constant wall heat flux (H2 boundary condition) and taking into account the wall temperature jump. The validation of the numerical model was carried out using the data available in the scientific literature. The numerical outcomes obtained for several values of the acute angle of the rhombus, the Knudsen number (i.e., rarefaction effects), and the Brinkman number (i.e., viscous dissipation effects) reveal that viscous forces play an opposite role with respect to rarefaction and significantly affect the convective heat transfer coefficient, especially for low Knudsen numbers and for high values of the acute angle. In particular, it was observed that Nu is significantly affected by the Brinkman number for acute angles higher than 50°.

Controlling gas–liquid flow and enhancing heat transfer in a T-junction microchannel by wettability-engineered walls

Characteristics of gas–liquid flow and heat transfer in a cross-flow T-junction microchannel with wettability-engineered walls are numerically investigated in this paper. The validated diffuse interface method is adopted for interface capture. First, the effects of wall wettability on bubble formation and transportation are studied. Three flow patterns are observed due to different combinations of the bottom and the top wall contact angles. On this basis, two methods are proposed to enhance the heat transfer. One is to increase the two-phase interfacial contact area by dividing the microchannel into three functional regions, which can promote the heat exchange at the two-phase interface. The other is to increase the velocity fluctuation intensity by alternating the contact angle along the channel, which can enhance mixing between the hot liquid layer adjacent to the wall and the cool liquid core. These two methods are applicative for steady and unsteady problems, respectively. The flow states, velocity vectors, and streamlines are used to analyze the fluid and thermal mixing mechanism. Meanwhile, a quantitative comparison of the wall temperature is made at a given wall heat flux. The obtained results can provide fresh insights into the gas–liquid flow control and the heat transfer enhancement in a microchannel, which are valuable for the design of microreactors and radiators.

Entropy generation analysis of micropolar fluid flow through a vertical microchannel under the combined effect of Joule heating, viscous dissipation, and thermal radiation with convective boundary conditions

ABSTRACT In this era of modern science, microfluidics has gained much importance in the field of research due to large-scale applications in science and engineering. Transfer of heat via nanofluids in microfluidics is the key concept of research. Therefore, here we have considered micropolar fluid for better heat transfer in vertical microchannel and discussed the influences of radiative heat flux, Joule heating, and uniform heat source/sink with viscous dissipation. The equations that govern the physical model are nondimensionalised by the application of suitable dimensionless parameters. Runge–Kutta–Fehlberg 4th−5th order method is applied for solving the non-dimensional governing ordinary differential equations, and precisely computed microrotation, velocity, and temperature profiles of micropolar fluid. The impacts of various factors on entropy generation and Bejan number are explored and analysed through graphs. It is investigated that a higher coupling number decelerates the fluid flow and reduces the temperature of the fluid. Further, minimum entropy generation can be attained by rising coupling number, Hartmann number, and radiation parameter.

Understanding Inconsistencies in Thermohydraulic Characteristics Between Experimental and Numerical Data for DI Water Flow Through a Rectangular Microchannel

Abstract Facing discrepancies between numerical simulation, experimental measurement, and theory is common in studies of fluid flow and heat transfer in microchannels. The cause of these discrepancies is often linked to the transition from the macroscale to the microscale, where the flow dynamics might be expected to deviate due to possible changes in dominant forces. In this work, an attempt is made to achieve agreement between experiment, numerical simulation, and theoretical description within the usual framework of laminar flow theory. For this purpose, the pressure drop, friction factor, and Poiseuille number under isothermal conditions and the temperature profile, heat transfer coefficient, Nusselt number, and thermal performance index under diabatic conditions (heating power of 10 W) in a heat sink with a stainless steel microchannel with a hydraulic diameter of 850 μm were investigated numerically and experimentally for mass flow rates between 1 and 68 gmin−1. The source of inconsistencies in pressure drop characteristics is found to be linked to the geometrical details of the utilized microchannel, for example, the design of inlet/outlet manifolds, the artifacts of manufacturing technique, and other features of the experimental test rig. For the heat transfer characteristics, it is identified that an appropriate estimation of the outer boundary condition for the numerical simulation remains the crucial challenge to obtain a reasonable agreement. The paper provides a detailed overview of how to account for these details to mitigate the discrepancies and to establish a handshake between experiments, numerical simulations, and theory.

Study on the efficient heat transfer mechanism of microchannel pin-fin arrays under low pumping power

The efficient heat transfer with low pressure loss has long been a critical challenge in the microchannel heat sinks (MCHS) design. Further study is required for the quantitative analysis of the longitudinal vortex heat transfer enhancement mechanism. In this research, a microchannel heat transfer experiment and corresponding simulations are conducted. Based on the common Reynolds number range in engineering applications of MCHS, the underlying causes of heat transfer enhancement and pressure loss increase in MCHS with respect to Reynolds numbers is clarified. The results show that when the Reynolds number is 400, the cylindrical wake reduces the synergistic angle between the velocity and the temperature gradient at the streamwise gap, which greatly enhances the heat transfer performance of MCHS. In addition, when the Reynolds number is 900, the early separation of the cylindrical wake vortex leads to a significant reflux near the center of the spanwise gap, which significantly increases the pressure loss. A Different Reynolds number Efficiency Evaluation Criterion (DEEC) is proposed for evaluating the performance of MCHS. It is found that the flow state of cylindrical wake with Reynolds number of 400 has the best heat transfer efficiency. The concept of introducing longitudinal vortices into the pin–fin array can serve as a key guideline in the heat transfer enhancement optimization design of MCHS.

Heat Transfer and Fluid Flow Characteristics of Microchannel with Discrete Double Inclined Ribs on Sidewalls

The heat transfer and fluid flow characteristics of microchannel with discrete double inclined ribs on sidewalls is performed numerically. A three-dimensional numerical model of the microchannel is constructed and studied. To obtain the optimal microchannel, four structural parameters of the rib length, angle, thickness, and height are changed by the orthogonal test method. In addition, four structural parameters of the rib are investigated separately on the heat transfer and fluid flow characteristics of the microchannel, Nusselt number, friction factor and performance evaluation criterion of the microchannel are calculated with Reynolds number ranging from 200 to 600. Results indicate that the microchannel with discrete double inclined ribs on sidewalls exhibits superior thermal characteristics, which shows a 58.13% reduction in temperature difference between the maximum temperature and the inlet temperature compared to the straight-through channel. Furthermore, all four structural parameters of the rib have varying degrees of effect on the heat transfer and fluid flow characteristics of the microchannel.

FLOW BOILING HEAT TRANSFER IN MICROCHANNEL HEAT EXCHANGERS WITH MICRO POROUS COATING SURFACE

This study experimentally investigated the heat transfer and pressure drop performance of refrigerant HFC-245fa flow boiling in microchannel heat exchangers with and without microporous coating. The flow boiling heat transfer performance at various mass fluxes, heating rate, exit vapor qualities, and surface coating thickness was compared. The test results showed that the 52 μm coating thickness porous surface exhibited 65-148% higher heat transfer performance than the smooth surface, while the 98 μm microporous coating surface showed a lower improvement of 41-90% at various mass fluxes. Partial dry out was observed at high and moderate mass fluxes on both smooth and porous coating channels. It happened at lower exit vapor qualities in microporous coating channels than that in smooth channels. The partial dry-out exit vapor qualities increased with decreasing mass fluxes. For the lowest mass flux, because of the low heat flux and low nucleation suppression, no significant partial dry out was investigated. The pressure drops in microporous coating channels were around 25-47% higher than those in smooth channels. There was no significant influence of microporous coating layer thickness on flow boiling pressures drops.

A comparative study of enhanced thermal performance in Tesla-type microchannels

Enhancing convective heat transfer in microchannels is significant for cooling of micro-electro-mechanical systems (MEMS). The dominant laminar flow in confined domain at microscale normally results in thick thermal boundary layers, seriously deteriorate the heat transfer process because of limited disturbance of flow. In this study, a novel Tesla-type microchannel configuration with significant flow diodicity was proposed to significantly interrupt boundary layers and promote fluid mixing. Hydrodynamic and thermal performances of this as-designed configuration were comprehensively investigated through experimental and numerical studies. Thermal characteristics such as Nusselt number (Nu), thermal resistance (Rth), and performance evaluation criterion (PEC) were computed. Diodic performance was also evaluated by measuring pressure drop diodicity and thermal diodicity in different flow directions. The results demonstrate that convection heat transfer is significantly enhanced because of strong mixing and periodic interruption of liquid flow and thermal boundary layers. Compared to conventional plain-wall microchannel, the average wall temperature is dramatically reduced by about 10 K at Re = 250 because of enhanced mixing effect. Accordingly, Rth is significantly decreased by ∼ 45 %. A PEC of 1.3 is achieved as well. Overall, the Nu is ranging from 10 to 25.2 for Re ranging 100 to 450, about 1.3 ∼ 2.1 times higher than that of plain-wall microchannel. Diodic performances in terms of Dip and Dit are about 1.9 and 1.3 for Dh = 0.4 mm. The new understandings of this study might provide insights for the design of new heat sinks for potential applications in electronics cooling.

Effect of thin obstacles heights on heat transfer and flow characteristics in microchannels

Effect of thin obstacles heights on heat transfer and flow characteristics in microchannels

Enhancing heat transfer in microchannels: A systematic evaluation of crescent-like fin and wall geometries with secondary flow

Recent developments in additive manufacturing processes, coupled with the demands of modern microelectronics, have spurred research in heat sinks at the micro-scale. The present study introduces microchannel designs resembling crescent-like walls and fins inside a coolant-based heat sink to enhance its thermal performance. Three different microchannel designs, namely the crescent-like fin design (CF design), the crescent-like wall design (CW design), and the crescent-like wall design with secondary flow (CWSF design) have been proposed and evaluated for their ability to transfer heat while maintaining a low pressure drop penalty. Furthermore, these geometries were tested with different inlet–outlet configurations. The paper investigates flow profile, pressure drop, and temperature profiles for a wide range of flow rates and explores certain hydrodynamic phenomena, such as the Coanda effect and vortex formation generated by the network of convex and concave surfaces that induce flow disruption, thus increasing the potential for enhanced thermal performance. Velocity and temperature maldistribution analyses have also been included to compare the stress distribution around the microchannel walls, highlighting the relative potential for structural failure. The findings suggest that incorporating secondary flow passages inside the crescent-like walls would result in a 28–51% reduction in pressure drop, indicating the potential to meet the thermal management needs of modern microchips. The main objective of the paper is to study how varying the attributes of crescent-like geometries affects the flow profile and thermal performance of a heat sink. The study aligns with the requirements of modern microchips, such as the 13th Gen Intel® Core™ processor, and serves as a guideline for the manufacturing of microchannel heat sinks with crescent-like morphologies.

Convective Heat Transfer and Entropy Generation Analysis in Elliptic Microchannels

Abstract In this paper, we investigate analytically the first and the second law characteristics of fully developed gaseous slip flow with the H1 boundary condition through elliptical microchannels. The closed-form solution of temperature distribution was obtained with the separation of variables method. Expressions for the Nusselt number, the nondimensional entropy generation rate, and the Bejan number were further deduced. The influences of crucial factors, including viscous dissipation, rarefaction, aspect ratio, and fluid axial heat conduction, have been carefully evaluated. The results indicated that viscous dissipation has a dramatic impact on heat transfer characteristics. But the rarefaction effect was found to significantly reduce the effect of the viscous dissipation on the Nusselt number, and the former may not deteriorate the heat transfer performance when considering the viscous dissipation. The main source of the entropy generation rate is controlled by fluid axial heat conduction when the Peclet number is less than one. The impacts of the viscous dissipation, the rarefaction, and the aspect ratio on entropy generation are magnified when fluid axial conduction dominates the irreversibility. The analytical solutions of the current study will make it possible to compare, evaluate, and optimize alternative elliptical microchannel heat exchanger design options.

Effects of anisotropic permeability on EMHD nanofluid flow and heat transfer in porous microchannel with wavy rough walls

This theoretical study thoroughly examines the complexities of fluid flow and heat transfer within microfluidic devices by employing anisotropic porous media with wavy walls. The effect of anisotropic permeability and electromagnetohydrodynamics (EMHD) on nanofluid transport is examined in a wavy microchannel under a constant pressure gradient. The nonlinear coupled governing equations for electric double-layer potential, velocity, temperature, and nanoparticle volume fraction are solved numerically using shooting techniques with the Runge–Kutta method. The numerical results are validated with the asymptotic analytical solutions. We explore the impact of parameters like the Hartmann number, Darcy number, and joule heating on nanofluidic flow. The interplay between anisotropic permeability ratio and angle significantly influences flow, temperature profiles, and nanoparticle volume fraction. Reduction in nanoparticle concentration is attributed to constrained entry into the porous medium due to elevated anisotropic permeability. Frictional heating, influenced by the Forchheimer inertial effect, enhances temperature and induces slug flow at low Darcy numbers. The Nusselt number peaks at low Darcy numbers with an anisotropic angle of φ=π/2, experiencing a minimum when φ=0. These findings deepen our understanding of the role of anisotropic permeability in shaping fluid flow and heat transfer in microfluidic systems influenced by EMHD effects.

Investigating the Effect of Electroplated Coatings on Single-Phase Fluid Flow and Heat Transfer in Microchannel

Investigating the Effect of Electroplated Coatings on Single-Phase Fluid Flow and Heat Transfer in Microchannel

Effect of viscous dissipative fluid in a slit micro-channel with heated super-hydrophobic surface

The analytical solution of heat transfer and hydro magnetic flow for an incompressible, electrically-conducting, and viscous dissipative fluid traveling vertically through an isothermally heated parallel plate in a slit micro-channel is performed. One channel surface was super-hydrophobic slip and temperature jump, whereas the other did not. This study also examined the effects of super-hydrophobic slip and a temperature jump on characteristic flow of magneto-hydrodynamics (MHD) free convection flow with viscous dissipative fluid. The perturbation technique was used to solve the coupled nonlinear and coupled equations analytically. With the aid of illustrative graphs, the results were presented, and the effects of the relevant controlling parameters namely Brinkman number, magnetic field, velocity slip condition and temperature jump effects were analysed for temperature, velocity, skin friction and Nusselt number distributions when either wall is heated by constant wall temperature. The result demonstrated that the activities of Brinkman number, Br, substantially increase fluid velocity, whereas growing levels of magnetic field effect was seen to slow down the flow movement. In addition, the fluid velocity is higher for greater values of temperature jump and velocity slip effects respectively. Furthermore, this research can prove very useful for designers in improving the performances of mechanical systems when viscous dissipation is involved, as well as heat transfer in micro-channels, as it is in combustion.

Experimental investigation of two-phase heat transfer in saw-tooth copper microchannels

Due to the confinement effect, vapor backflow is ubiquitous in flow boiling process in microscale confined domains. This, in turn, severely deteriorate the flow boiling performances. To solve this dilemma, many microchannel configurations have been exploited to regulate two-phase flows such as inlet orifices and auxiliary channels. However, the inlet orifices would lead to significant increase of flow resistance, scarifying the pumping power. In this study, periodic symmetric saw-tooth structure is designed and fabricated along sidewalls of parallel copper microchannels to effectively suppress the vapor backflow without the cost of pressure drop. The effects of this periodic symmetric saw-tooth structure on suppression of vapor backflow and two-phase flow mixing are studied in both forward and reverse directions. The flow boiling performances of this saw-tooth microchannel configuration was experimentally investigated for total inlet flow rates ranging from 30 to 70 ml/min (mass flux ranging from 183.3 kg/m2·s to 427.7 kg/m2·s). The experimental results and visualization study demonstrate that the successful suppression of vapor backflow in the forward direction and enhancement of two-phase flow mixing in the backward direction can dramatically enhance flow boiling performance. Significant boost on both HTC and CHF is demonstrated. A CHF of 217 W/cm2 is demonstrated at 70 ml/min in the forward direction. Also, significant difference in CHF is observed between the two flow directions as the further increase of flow rates. Compared to conventional plain wall microchannels, the CHF and HTC of this study can be significantly enhanced by ∼77.4 % and ∼250 %, respectively. Note that this improved boiling performance is achieved without sacrificing pressure drops.

Flow and Heat Transfer in Two-Phase Flow Immiscible Droplets in Microchannels

ABSTRACT The enhancement of heat transfer in microchannels without phase change is a significant area of study, primarily driven by the internal fluid recirculation in two-phase flows. This investigation focuses on a circular microchannel, 100 μm in diameter, where mineral oil droplets are introduced into a water flow. The study utilizes the conservative level set method for precise interface tracking and liquid film thickness measurement. This research introduces a modified Nusselt number, specifically tailored to describe the heat transfer characteristics of multiphase flows. The study delves into the effects of varying droplet sizes, from small spheres to a slug. The findings indicate that the most significant heat transfer enhancement occurs with droplets whose volume closely matches that of a sphere fitting within the channel. Moreover, the investigation explores the impact of parameters like inlet velocity, primary-phase slug length, and contact angle. Notably, higher inlet velocities lead to improved heat transfer, resulting in a substantial increase in the Nusselt number compared to single-phase flows. The study underscores the delicate balance between recirculation intensity and droplet heat capacity concerning slug length, as excessive variations can harm thermal performance. It also highlights the pivotal role of surface wettability, showing improved thermal performance on hydrophobic surfaces.