Flat Microchannel Research Articles

Experimental study on thermal performance of a novel serpentine-looped flat microchannel heat pipe

The pulsating heat pipe is an efficient heat transfer element widely used in a variety of applications due to the simple wickless structure and no external power requirements. Increasing the number of pipe turns can enhance the pulsation, but reduce the structural compactness. A new type of serpentine-looped flat microchannel heat pipe (SLFMHP) is proposed with numerous turns by series connection of flow channels and good thermal contact by large flat surface. The thermal performance of SLFMHP is experimentally investigated and compared with that of the copper pulsating heat pipe. Results show that the minimum thermal resistance of SLFMHP is 0.036 K/W, 80 % smaller than 0.19 K/W of the copper pulsating heat pipe. The maximum effective thermal conductivity of SLFMHP reaches 37400 W/(m·K), five times of 7233 W/(m·K) of the copper pulsating heat pipe. The sensitivity to inclination angle has also been reduced with thermal resistance at 0° improved by over 90 %. As the inclination angle increases, the thermal resistance decreases first sharply and then slowly, and the turning point is defined as the critical pulsation angle, about 15° and 8° with the filling ratio of 20 % and 40 %, respectively. The SLFMHP can provide a possible solution for the demand of the thermal control systems with miniaturization, light weight and high heat transfer capability.

Application of discrete symmetry to natural convection in vertical porous microchannels

Abstract This work focuses on the study of natural convection in a flat porous microchannel with asymmetric heating. The novelty of the work lies in the fact that for the first time the method of discrete symmetries was used to analyze the complete system of Navier–Stokes and energy equations in a two-dimensional approximation. Analytical solutions for velocity and temperature profiles have been derived based on symmetry analysis, taking into account boundary conditions such as slip and temperature jump at the channel walls. The effect of Grashof, Knudsen, Darcy, and Prandtl numbers on the flow characteristics in the microchannel and heat transfer coefficients was elucidated. At high Grashof numbers, an ascending flow near the hot wall and a descending flow near the cold wall arise. Increasing the Knudsen number leads to an increase in the velocity, temperature jump at the walls and a decrease in heat transfer coefficients. As the Darcy number increases, velocities amplify in both ascending and descending flows. The temperature jump at the hot wall grows up, while it remains unchanged at the cold wall. In the same time, the heat transfer coefficient at the hot wall decreases.

An experimental and numerical investigation of thermofluidic properties on the novel curved microchannel heat sink with slotted section

The current work proposes the development of a novel curved microchannel heat sink by modifying the existing flat microchannel heat sink for enhancing the heat dissipation in the electronic device without significant pressure drop penalty. The inspiration for the curved channel is taken from a NACA airfoil. The microchannel comprises two sections: a curved section and a slotted straight section. Experimental and numerical investigations are performed on the proposed geometry to study the thermofluidic properties for Reynolds number (Re) ranging from 200 to 500 using DI water as working fluid. Fluid flow and heat transfer are analysed by carrying out a full domain numerical simulation in ANSYS Fluent 19.0. The thermofluidic properties of the curved microchannel heat sink (CMCHS) and straight microchannel heat sink (SMCHS) are compared numerically by keeping the geometrical parameters (hydraulic diameter, foot area and fin area) and inlet parameters (inlet velocity and heat flux) identical. The average heat transfer coefficient (havg) of CMCHS is enhanced over SMCHS due to the formation of vorticities in the slotted region owing to the fluid intermixing and boundary layer disruption. The presence of curved section leads to reducing pressure drop penalty pertaining to gravitational effects. Together, the curved section and slotted section enhance the performance factor of CMCHS over SMCHS. The vortices formed increase as the Reynolds number increases, enabling an increase in the heat transfer coefficient. Therefore, the performance factor also increases as the Reynolds number is increased. An enhancement of ≈ 67.6 % in havg is achieved for Re = 500 in CMCHS compared to SMCHS with DI water as the working fluid.

The effect of a heater thin porous coating on the critical heat flux in flat microchannels with intense localized heating

Experiments were carried out on flow boiling in mini- and microchannels with non-uniform heating (channel width 30 mm, size of centrally located heater 3x3 mm2) with smooth and porous heater surfaces. It has been established that for that surfaces, the dependencies of the critical heat flux and heat transfer coefficients behave similarly, but in the case of porous surface, the critical heat flux increases more strongly with the growth of mass flow, and the heat transfer coefficients are higher due to developed boiling. Instead of a slowly developing boiling crisis with a large bubble constantly sitting on a smooth heater, boiling on porous surface was replaced by intense formation of small bubbles sweeping away large one. As a result, large bubbles either pulsate intensely or do not form at all until the moment of “classical” crisis - the sudden formation of stationary one with a sharp increase in surface temperature. This is true for both types of surfaces. But in the case of a smooth surface, the regime of a pulsating large bubble prevails, and on porous surface it ceases to form until “classical” crisis. The frequency of small bubble formation is significantly greater when boiling on porous surface.

Numerical Investigation of the Effect of Ethylene Glycol with SnFe2O3 Hybrid Nanofluid on Heat Transfer in Flat, Face Step, and Radial Corrugated Microchannels

Nanofluid technology is one of the latest technics aims to enhance the heat exchangers working systems. The nanofluid is used because it has better thermal performance properties than common fluids like water. Tin Oxide (SnO2) and Ferric Oxide (Fe2O3) has used due to their availability and low cost for manufacturing. Flat, face step and radial corrugated microchannels have been studied at the Reynolds number range (4000-7000 Re). In this study, a hybrid nanofluid combined Ferric Oxide and Tin Oxide SnFe2O3 at (0, 5, 15%) of Tin Oxide concentration. The volume concentration of the nanoparticles was from (1-5)% suspended in ethylene glycol as base fluid due to its high thermal performance compared with water. The performance evaluation criteria (PEC) are studied and the results were validated with experimental and numerical results from previous studies. Performance evaluation criteria (PEC) studied the heat transfer in all study steps and found that the 15Sn85Fe2O3 hybrid nanofluid in ethylene glycol at 5% volume concentration in radial corrugated microchannel increased 104% compared with base fluid ethylene glycol in the flat microchannel. The performance evaluation criteria value has increased by 82% from flat microchannel to radial microchannel at 5% of volume concentration of 15Sn85Fe2O3 hybrid nanofluid

Microfluidic-to-macrofluidic: A simple in vitro model of atherosclerosis induced by fluidic stimulation.

Atherosclerosis is the narrowing of the arteries due to the formation of fatty plaques, which is the main cause of myocardial infarction and stroke. It is important to develop an in vitro model that can combine multiple-type cell co-culture, vessel wall-like structure, and fluid condition to simulate the processes of atherosclerosis. Herein, we used a simple microfluidic chip made of three polydimethylsiloxane layers to co-culture endothelial and smooth muscle cells in a flat rectangular microchannel. After being connected with a circulating culture medium driven by a peristaltic pump, the flat microchannel was deformed to a tunnel-like macrochannel. The fluid pressure and shear stress applied on the cells in the deformed macrochannel can be varied by adjusting the circulating flow rate and the thickness of the middle layer. Under three levels of the pressure (65, 131, and 196 mm Hg) or shear stress (0.99, 4.78, and 24 dyne/cm2) conditions, a series of atherosclerosis-related events, including endothelial cell junction, pro-inflammatory cytokine secretion, monocyte adhesion, and lipid accumulation, were investigated. The atherosclerosis-related results showed that the medium pressure or shear stress exhibited a relatively weak pro-atherosclerotic effect in a V-shaped trend. To demonstrate the potential in drug screen, the effects of three well-known anti-atherosclerotic drugs (atorvastatin, tetramethylpyrazine, and high-density lipoprotein) on the lipid accumulation and pro-inflammatory cytokine secretion were evaluated under a strong pro-atherosclerotic fluid condition (65 mm Hg, 0.99 dyne/cm2). This in vitro model of atherosclerosis has shown great potential in drug screen application.

Rapid bubble growth within 200 μs in flat microchannels under “hot spot” heating conditions at ultra-high heat fluxes

The two-phase flow boiling in microchannels is an attractive cooling strategy for high-power density electronic/optoelectronic devices, while the exist of sub-millimeter “hot spots” characterized by ultra-high heat fluxes would degrade the reliability and safety for commercial and consumer applications. In this paper, the bubble nucleation and evolution characteristics of subcooled flow boiling in high aspect ratio flat microchannels were experimentally studied with the aid of high-speed visualization and infrared temperature measurement at localized heat fluxes exceeding 1.5 kW/cm2. Dielectric liquid HFE-7100 was used at mass flow rates ranging from 11.1 to 178.2 kg/(m2·s). Results show that the explosive boiling process was accompanied by an approximately linear bubble growth rate of about 20 m/s at about the first half of growth stage, and the time duration for overall bubble growth was less than 200 μs. The bubble nucleation was suppressed dramatically on the smooth bottom surface of the channel even at a localized “hot spot” heat flux above 2.5 kW/cm2. The introduction of staggered short micro-pin fin arrays at the bottom wall significantly lowered the threshold heat flux and onset temperature of bubble nucleation and reinforced the bubble growth to larger sizes. It is attributed to the reduction in the temperature gradient of thermal boundary layer adjacent to the heated wall, and it would be further reduced by increasing the micro-pin fin denseness. Even at quite high superheats on smooth and micro-structured surfaces, the heterogeneous boiling mechanism was proved to still work.

Influence of heater size on critical heat flux in flat microchannels with intense localized heating

Experiments were carried out on flow boiling in mini- and micro- channels under local heating for different heater size. It was found that with an increase in the channel height in the range of 0.2–3.7 mm, the intensity of heat transfer and the critical heat flux increase significantly. For a given flow rate and channel height, the critical heat flux on the 3x3 mm2 heater exceeds the heat flux on the 10x10 mm2 heater, and the difference increases with increasing channel height. Boiling curves were also obtained.

Concentration performance of solar collector integrated compound parabolic concentrator and flat microchannel tube with tracking system

The compound parabolic concentrator is a non-imaging concentrator that can concentrate solar radiation without tracking system. However, as the concentration ratio increases, the maximum half acceptance angle and the effective working hours decrease. To increase the effective working hours at high concentration ratio, a tracking solar collector is proposed that integrates the compound parabolic concentrator and flat microchannel tube. This new solar collector can track solar radiation by rotating the two reflective surfaces around their respective starting lines. A two-dimensional model is developed for the irradiation concentration of the tracking compound parabolic concentrator. For the geometry of the compound parabolic concentrator and solar irradiant angle, the optimal rotation angles of the reflective surfaces are determined to minimize the irradiation loss. The effects of the rotation angle of the two reflective surfaces on the concentration performance are simulated. The simulation results show that in the tracking mode, the averaged heat flux on the surface of the absorber can reach 7.61 kW m−2 when the concentration ratio and truncation ratio are 8 and 0.5, respectively. Even when the incident angle of solar irradiation is 30°, the averaged heat flux is still 5.12 kW m−2. Compared with other kinds of reflector, the concentration performance of the compound parabolic concentrator is merely influenced by the tracking error. The positive and negative tracking errors lead to different declining rates of normalized optical efficiency and the influence of positive tracking error is more acceptable.

Mixed electroosmotic/pressure-driven flow for a generalized Phan–Thien–Tanner fluid in a microchannel with nonlinear Navier slip at the wall

This work derives an approximate solution of the laminar viscoelastic fluid flow through a parallel flat plate microchannel, driven by electroosmotic and external pressure forces. In contrast to other works published in the past, in the present analysis, the fluid’s rheology obeys the generalized Phan–Thien–Tanner model that has been proposed recently, which uses the Mittag-Leffler function instead of linear or exponential functions of the trace for the stress tensor. The hydrodynamic analysis considers a non-linear Navier slip law at the wall that follows a power-law behavior on the shear stress. For the electric potential in the electric double layer, the Debye–Hückel approximation was used, and it is assumed that the wall’s zeta potentials are symmetric. The solution obtained in this work depends on several dimensionless parameters that allow controlling the flow: ɛWi2 characterizes the fluid’s viscoelasticity, the electrokinetic parameter κ̄, the dimensionless slip coefficient L̄, the slip-power exponent m, and the ratio between pressure and electroosmotic forces, G. Besides, the influence of two parameters of the Mittag-Leffler function, α and β, notably affects the flow’s characteristics. Our results evidence the important consequences of considering the generalized Phan–Thien–Tanner model and the slippage effect on the flow field compared to that described by the Phan–Thien–Tanner model.

Experimental and analytical investigation of CO2/R32 condensation heat transfer in a microchannel

Previous research has shown that CO2 mixtures can improve the thermal performance of CO2 refrigeration systems. Moreover, the use of a microchannel heat exchanger can reduce the refrigerant charge and improve the heat transfer coefficient. Therefore, in this experimental study, we investigated the condensation heat transfer characteristics of CO2/R32 in a porous flat microchannel with a hydraulic diameter of 0.715 mm. According to the results, the convective heat transfer coefficient decreased rapidly then increased slowly with an increase in the CO2 mass fraction. The minimum value was achieved when the CO2 mass fraction was approximately 55%. In addition, the convective heat transfer coefficient decreased with an increase in the condensation temperature. Higher vapour quality and mass flow rate conditions corresponded to higher convective heat transfer coefficients. The prediction correlations applicable to the in-tube condensation heat transfer exhibited low prediction results for CO2/R32, with a heat transfer coefficient prediction error for the newly established prediction model of 16.77%. This study provides theoretical support for analysis of the CO2/R32 condensation heat transfer mechanism in microchannels and the future design of microchannel condensers.

Heat transfer characteristics of thermal energy storage system using single and multi-phase cooled heat sinks: A review

• Passive and active cooling techniques for battery and electronic components cooling • Computational and experimental techniques for battery and electronic components cooling • Single phase coolant investigation • Geometry optimization concerning energy management • Flat plate, pin fin, and microchannel heat sink cooling investigation Heat sinks are considered as heat exchangers employed to cool high-temperature devices such as electronic components. They can significantly improve heat dissipation from the base surface. A wide range of heat sink geometries is categorized into three major types: flat-plate, pin-fin, and microchannel heat sinks. Over the past few decades, to keep up with the rate of electronics components heat flux, extensive examinations carried to enhance heat sinks thermal performance include various fabrication materials, single-phase coolants, geometry optimization, and designing complicated heat sink concepts. Heat sink optimization contributes a significant opportunity to improve thermal management and reduce energy consumption. Hence, developing and reviewing different heat sink research methodologies is fundamental. This paper reviews various research methodologies (using single-phase coolant) to aim for heat sink optimization and thermal performance enhancement in three geometry categories (flat-plate, pin-fin, and microchannel). The reviewed articles focused on experimental, numerical, and computational efforts on energy storage thermal managements utilizing single-phase coolant for flat-plate, pin-fin, and microchannel heat sinks design.

Synchrotron radiation transmission by two coupled flat microchannel plates: new opportunities to control the focal spot characteristics.

An improved theoretical model to calculate the focal spot properties of coherent synchrotron radiation (SR) soft X-ray beams by combining and aligning two microchannel plates (MCPs) is presented. The diffraction patterns of the radiation behind the MCP system are simulated in the framework of the electrodynamical model of the radiation emission from two-dimensional finite antenna arrays. Simulations show that this particular optical device focuses the soft X-ray radiation in a circular central spot with a radius of ∼4 µm. The study points out that such MCP-based devices may achieve micrometre and sub-micrometre spot sizes as required by many applications in the soft X-ray range. Finally, based on experimental and theoretical results of the radiation transmission by this MCP-based device, a new method to characterize the spatial properties of brilliant SR sources is discussed.

Thermodiffusive effect on the local Debye-length in an electroosmotic flow of a viscoelastic fluid in a slit microchannel

This work theoretically studies the influence of the thermodiffusive effect on the local Debye length thickness in a purely electroosmotic flow in a parallel flat plate microchannel. An imposed electric field between the ends of the microchannel interacts with an ionized viscoelastic fluid causing Joule heating, which induces a temperature gradients along the microchannel, affecting the fluid’s physical properties, and in a notable manner, the ionic distribution into the electric double layer (EDL), resulting in thermodiffusion. Consequently, an induced pressure field counterbalances the axial variation of the plug-like electroosmotic velocity to maintain the fluid mass continuity. Also, the ionic distribution and electrical potential based on the non-isothermal Poisson-Boltzmann equation are modified. To estimate the local Debye length thickness, the coupled set of the Poisson-Boltzmann, momentum, and energy equations are solved numerically in the limit of the lubrication approximation theory (LAT). Our results indicate that the thermodiffusion has an important effect on the thickness of the local Debye-length, particularly in the warm zone of the fluid. Besides, the ionic response to thermal fields is given by a positive Soret coefficient, which indicates that the ionic particles move from warm to cold regions in the fluid, giving place to a thinner Debye length and lower ionic concentration around the warming zone; this migration of ions confirms that the dimensionless mass-flow rate is affected with the Soret coefficient compared with the non-thermodiffusion case.

Effect of zeta potential in fractional pulsatile electroosmotic flow of Maxwell fluid

Theoretical analysis of pulsatile electro-osmotic flow (PEOF) asymmetric wall zeta potential in parallel plate micro-channels. The effect of zeta potential on pulsatile electroosmotic flow of Maxwell fluid along with fractional derivative in a parallel flat plate microchannel is elucidated. The governing equations are developed using the Caputo fractional model. The integral Laplace transform is used to determine the exact solution. The impacts of fractional and some physical characteristics of changes in fluid behavior are graphically represented and analyzed.

Multilayer analysis of immiscible power-law fluids under magnetohydrodynamic and pressure-driven effects in a microchannel

In the present study, the combined magnetohydrodynamic and pressure-driven flow of multilayer immiscible fluids into a parallel flat plate microchannel is semi-analytically solved. Due to the handling of complex fluids in various microfluidic platform applications, the fluid transport reviewed here considers the power-law model. The movement of electrically conductive fluid layers is due to Lorentz forces that arise from the interaction between an electric current and a magnetic field. To find a solution for the flow field, the momentum equation and the rheological model for each fluid layer, together with the corresponding boundary conditions at the liquid-liquid and solid-liquid interfaces, are solved simultaneously through a closed system of nonlinear equations. The graphical results show the influence of the dimensionless parameters that arise from the mathematical modeling on the velocity profiles and flow rate. These are the magnetic parameters, the fluid layers thickness, the viscosity coefficients, the ratios between pressure forces and magnetic forces, and the flow behavior indexes. This theoretical work contributes to the design of microfluidic devices for flow-focusing tasks in chemical, clinical, and biological areas.

Experimental study on supercritical heat transfer characteristics of CO2/R41 mixture in microchannel

• Supercritical convective heat transfer characteristics of CO 2 /R41 blends in square porous flat tube microchannel were studied. • The peak values of heat transfer coefficient were analyzed. • Experimental data were compared with the heat transfer correlations. Past researches have shown that CO 2 /HFC mixtures and microchannel heat exchanger could benefit solving the problems of high operating pressure and relatively low cycle efficiency. In this paper, the supercritical heat transfer characteristics of R41, CO 2 /R41 (20.5/79.5) and CO 2 /R41 (51.4/48.6) in square porous flat tube microchannel with the hydraulic diameter of 0.715 mm were experimentally studied. The experimental conditions were as follows: the operating pressure of 6.5–7.5 MPa, the mass flow rate of 150–300 kg/m 2 s, the heat flux of 3–9 kW/m 2 , the fluid temperature of 20–90 °C. The results show that, similar to pure CO 2 , heat transfer coefficient (HTC) of mixtures also appears a peak value near the pseudocritical region. At higher operating pressure, the peak value is less and the change of HTC is gentle. Meanwhile, the inflection point temperature is high. When the bulk temperature is less than the inflection point temperature, HTC is higher at low operating pressure. When the bulk temperature is greater than the inflection point temperature, HTC is higher at high operating pressure. Peak value of HTC decreases and the inflection point temperature of mixtures becomes higher when the CO 2 mass fraction is low. With the increase of R41 mass fraction, the change of HTC is gentle. The width of the peak distance in the pseudocritical region becomes wider. The model of Petrov and Popov is recommended as the prediction model for CO 2 /R41 mixtures. This research can serve as a database for the design of CO 2 /R41 mixtures microchannel heat exchangers.

Viscous micropump of immiscible fluids using magnetohydrodynamic effects and a power-law conducting fluid

Small-scale fluid transport methods have grown significantly in recent years, mainly in applications in microfluidic systems. Therefore, the present study analyzes the movement of two-layers of immiscible fluids within a parallel flat plates microchannel. The fluid layers are composed of a Newtonian fluid and a power-law fluid. The pumping is produced by magnetohydrodynamics effects that act on the non-Newtonian conducting fluid dragging the non-conducting Newtonian fluid by viscous forces. Under the consideration of a laminar, incompressible, and unidirectional flow, the dimensionless mathematical model is established by the momentum equations for each fluid, together with the corresponding boundary conditions at solid-liquid and liquid-liquid interfaces. The problem formulation is semi-analytically solved using the Newton-Raphson method. The results are presented as a function of the velocity profiles and flow rate, showing interesting behaviors that depend on the physical and electrical properties of each fluid and flow conditions via the dimensionless parameters such as the flow behavior index, a magnetic parameter related to Lorenz forces, the fluids viscosity ratios and the dimensionless liquid-liquid interface position. This work contributes to the understanding of the various immiscible non-conducting fluids pumping techniques that can be used in microdevices.

Influence of Liquid Properties on Gas-Liquid Flow Regimes and Pressure Drop in a Flat Microchannel

Influence of Liquid Properties on Gas-Liquid Flow Regimes and Pressure Drop in a Flat Microchannel

Water–copper nanofluid flow in flat and ribbed microchannels: numerical modeling and optimization

PurposeThis paper aims to simulate the nanofluid forced convection in a microchannel. According to the results, at high Reynolds numbers and higher nanofluid volume fractions, an increase in the rib height and slip coefficient further improved the heat transfer rate. The ribs also affect the flow physics depending on the Reynolds number so that the slip velocity decreases with increasing the nanofluid volume fraction and rib height.Design/methodology/approachForced heat transfer of the water–copper nanofluid is numerically studied in a two dimensional microchannel. The effects of the slip coefficient, Reynolds number, nanofluid volume fraction and rib height are investigated on the average Nusselt number, slip velocity on the microchannel wall and the performance evaluation criterion.FindingsIn contrast, the slip velocity increases with increasing the Reynolds number and slip coefficient. Afterwards, a non-parametric function estimation is performed relying on the artificial neural network.Originality/valueFinally, the Genetic Algorithm was used to establish a set of optimal decision parameters for the problem