Convection In Microchannels Research Articles

Investigation of non-Newtonian Fluid Flow and Heat Transfer in Microchannel with Hydrophobic Surface

Forced convection in microchannel for power law fluid was investigated by Lattice Boltzmann method in which term of viscose dissipation and slip condition on the wall are considered. The problem was studied for various Reynolds number, Prandtl number and slip coefficients. In this study, contours of velocity, temperature profile, local friction factor and average Nusselt number are presented. Average Nusselt number with respect to power law index is greater for shear-thinning fluid (n<1) than Newtonian (n=1) and shear-thickening fluid (n>1). For Re=30 enhancing slip coefficient raises the average Nusselt number. The results have shown that the friction factor is decreasing along the channel because of changing in velocity gradient. Slip coefficient improves the convection heat transfer mechanism for Non-Newtonian fluids.

Numerical approach for fluids flow and thermal convection in microchannels

The heat transfer performance of conventional thermal fluids in microchannels is an attractive method for cooling devices such as microelectronic applications. Computational fluid dynamics (CFD) is a very significant research technique in heat transfer studies and validated numerical models of microscale thermal management systems are of utmost importance. In this paper, some literature studies on available numerical and experimental models for single-phase and Newtonian fluids are reviewed and methods to tackle laminar fluid flow through a microchannel are sought. A few case studies are selected, and a numerical simulation is performed to obtain fluid flow behaviour within a microchannel, to test the level of accuracy and understanding of the problem. The numerical results are compared with relevant experimental results from the literature and a proper methodology for numerical investigation of single-phase and Newtonian fluid in laminar flow convection heat transfer in microscale heat exchangers is defined.

Two-and-three-dimensional analysis of Joule and viscous heating effects on MHD nanofluid forced convection in microchannels

This paper aims to study viscous heating, Joule heating and their simultaneous impacts on Al2O3-water nanofluid laminar flow and thermal characteristics in the presence of a magnetic field inside microchannels. The flow field and heat transfer are modeled two- and three-dimensionally by employing the finite volume method. To investigate the influence of viscosity variation with temperature on viscous heating, a temperature-dependent viscosity correlation specifically presented for Al2O3-water is employed. Predicted results indicate that viscosity changes due to temperature changes are negligible and although viscous heating can be reasonably ignored in the absence of the magnetic field, it must be taken into account when the magnetic field is applied. At the Hartmann number of 20, near microchannel walls, the velocity gradient rises up to 165% where the viscous dissipation significantly increases the temperature. In addition, when the magnetic field is applied, the 2D model underestimates the temperature compared to the 3D model where the effects of maximum velocity and velocity gradient on the energy equation and heat dissipation exist. The temperature profiles demonstrate that at the Hartmann number of 20, the effects of Joule heating or viscous heating increase the maximum dimensionless temperature by a factor of about 2.5 and by the rise of the Hartmann number, the dimensionless temperature increases and the effects of Joule and viscous heating become more significant. All in all, it is concluded that the heat generated by viscous and Joule heating can remarkably decrease the cooling performance of microchannels in the presence of magnetic fields.

Large eddy simulations of high-heat-flux supercritical CO2 convection in microchannels: Mixed convection and non-uniform heating

Near-critical-point supercritical fluid convection is a promising solution for emerging high-flux thermal management needs because of the high fluid thermal conductivities and specific heats. Supercritical convection has been extensively studied for large diameter channels with uniform heating (4.1 < D < 22.7 mm, 0.05 < q'' < 330 W cm−2). However, limited information is available on transport processes to guide engineering of high-flux compact supercritical heat transfer equipment, which often have non-uniform heating distributions. To address this need, large eddy simulations (LES) are employed to study supercritical CO2 convection in microchannels (750μm×737μm cross-section). First, the simulation approach is validated with published experimental data. Studies are then conducted for horizontal isothermal heated channels at reduced pressure Pr=1.1, mass fluxes G=100-1000kgm-2s-1 (Re=3000-35,700), average wall heat fluxes q''=24-62Wcm-2, and bulk flow temperatures Tbulk=291-317K (inside and outside pseudocritical range). Results are used to assess the applicability of published supercritical convection correlations for microchannel heat exchangers. All available supercritical correlations are found to under-predict heat transfer coefficients at high heat fluxes (q''=58-62Wcm-2). The bottom-to-top wall heat transfer coefficient (HTC) ratios from these cases are used to determine a new criterion for the onset of significant mixed convection effects. At low mass fluxes (G=100kgm-2s-1), this HTC ratio is found to exceed 2.5×. Simulations indicate that, at these conditions, increased heat fluxes lead to reduced HTCs for low and high mass fluxes, but increased HTCs at intermediate mass flux values. Finally, an illustrative case is evaluated to assess the impact of conjugate heat transfer effects in microscale supercritical convection, and highlight limitations of conventional modeling approaches. This study quantifies the accuracy of convection correlations for high-heat-flux microchannel pseudocritical conditions, provides new criteria for predicting the onset of mixed convection in microchannel supercritical flows, and demonstrates the impact of conjugate heat transfer effects in microchannel supercritical heat exchangers.

Particle Concentration Distribution Control Based on Shear Rate Gradient and Marangoni Convection in Microchannel

The application of the microfluidic chip to the immunoassay has many advantages such as high efficiency, low cost, and high integration. It is of great significance to understand the flow characteristics of fluids in microchannel, which can improve the efficiency and accuracy of detection. In this paper, the effects of microchannel structure on the particle concentration distribution in two-phase flow and the flow behaviour of near-wall layer are studied by microfluidic observation technique. Also, a kind of microchannel structure was designed to increase the particle concentration in the front-end of microfluidic.

Natural convection flow in a vertical micro-annulus with time-periodic thermal boundary conditions

Purpose The purpose of this paper is to investigate the impact of time-periodic thermal boundary conditions on natural convection flow in a vertical micro-annulus. Design/methodology/approach Analytical solution in terms of Bessel’s function and modified Bessel’s function of order 0 and 1 is obtained for velocity, temperature, Nusselt number, skin friction and mass flow rate. Findings It is established that the role of Knudsen number and fluid–wall interaction parameter is to decrease fluid temperature, velocity, Nusselt number and skin friction. Research limitations/implications No laboratory practical or experiment was conducted. Practical implications Cooling device in electronic panels, card and micro-chips is frequently cooled by natural convection. Originality/value In view of the amount of works done on natural convection in microchannel, it becomes interesting to investigate the effect that time-periodic heating has on natural convection flow in a vertical micro-annulus. The purpose of this paper is to examine the impact of time-periodic thermal boundary conditions on natural convection flow in a vertical micro-annulus.

Mixed Convection in Vertical Flat and Circular Porous Microchannels

The paper outlines results of a study of mixed free and forced convection in vertical flat and circular microchannels occupied with porous medium subject to slip boundary conditions. The problem was solved using analytical and numerical methodology including the lattice Boltzmann method. Effects of the Knudsen and Rayleigh numbers and porosity on velocity and temperature profiles, as well as the normalized Nusselt number and friction factor were elucidated. It was revealed that Knudsen number effects dominate in the vicinity of the wall, whereas the Rayleigh number effects are more significant near the channel axis. For high Rayleigh numbers, velocity profiles exhibit M-shapes having a point of minimum at the channel axis. In the same time, temperature profiles become uniform, so that the fluid temperature across the channel is almost equal to the wall temperature. An increase in porosity causes a decrease in the temperature and velocity jumps on the wall. It has been shown that for low Rayleigh numbers a decrease in the Darcy number causes an increase in heat transfer and hydraulic resistance. However, for high Rayleigh numbers, the trend becomes opposite. Comparisons of the quantitative results for the circular and flat channels revealed that the shape of channel cross section makes a significant influence on heat transfer and fluid flow. Results of the analytical studies were validated via comparisons with numerical simulations using the lattice Boltzmann method, which proved to be an accurate and powerful tool able to simulate mixed convection in microchannels, with the deviation from the analytical solution not exceeding 1%.

Electromagnetic Regulation of Electrolyte Solution Heat Convection in Microchannels

With the rapid development of microelectronics and micro-electromechanical system technology, the electronic components have become smaller and the performance has become higher. Under this condition, however, their energy consumption and heat production have also increased continuously, which poses a great challenge to heat dissipation. In this paper, electromagnetic driving technology is applied to drive the electrolyte solution flow within a microchannel to realize efficient heat convection with microchannel walls. By changing the magnitude and direction of electric–magnetic field, the regulation of heat convection performance is studied. The results show that the Nu number of microchannel increases as the Ha number and magnetic direction angle increases, while it decreases as the potential difference increases. According to the average index of the four factors, it was determined that the electrolyte solution had the best heat convection performance with Ha = 0.05, Vb = 0.00006, Pe = 90, and α = 90°. After that, sensitivity analysis of the Ha number, potential difference and magnetic direction angle was used to regulate the heat convection performance. This paper may provide some theoretical support for the design of microelectronics and micro-electromechanical systems.

Mixed convection in a vertical flat microchannel

The paper presents results of an investigation into mixed convection in a vertically oriented microchannel with slip boundary conditions. Solutions of the problem were obtained analytically and using a numerical approach based on the Lattice Boltzmann method (LBM). The solution yields relations, which enable estimating velocity and temperature profiles and the Nusselt number as functions of the Knudsen, Rayleigh and Prandtl numbers. It was shown that Knudsen number effects are prevailing in the vicinity of the wall, whereas near the centerline of the channel effects of the Rayleigh number are stronger. If the Rayleigh numbers are high, velocity profiles demonstrate M-shapes with a point of minimum at the channel centerline, whereas temperature profiles flatten so that the fluid temperature in the channel cross-section tends to the wall temperature. The temperature jump magnitude on the wall is dependent on the Prandtl number value and decreases with the increasing Prandtl numbers. For almost all combinations of the parameters considered in this paper, higher Knudsen numbers entail heat transfer deterioration except for the case of Pr=10 and Ra=100. Increasing the Knudsen number diminishes hydraulic resistance for low Rayleigh numbers, but for high values of the Rayleigh numbers the trend is reversed. It was shown that mixed convection in microchannels can be successfully simulated using the LBM methodology, whose deviation from the analytical solution and is less than 1%.

Heat Transfer Enhancement and Entropy Generation of Nanofluids Laminar Convection in Microchannels with Flow Control Devices

The heat transfer enhancement and entropy generation of Al2O3-water nanofluids laminar convective flow in the microchannels with flow control devices (cylinder, rectangle, protrusion, and v-groove) were investigated in this research. The effects of the geometrical structure of the microchannel, nanofluids concentration φ(0%–3%), and Reynolds number Re (50–300) were comparatively studied by means of performance parameters, as well as the limiting streamlines and temperature contours on the modified heated surfaces. The results reveal that the relative Fanning frictional factor f/f0 of the microchannel with rectangle and protrusion devices are much larger and smaller than others, respectively. As the nanofluids concentration increases, f/f0 increases accordingly. For the microchannel with rectangle ribs, there is a transition Re for obtaining the largest heat transfer. The relative Nusselt number Nu/Nu0 of the cases with larger nanofluids concentration are greater. The microchannels with cylinder and v-groove profiles have better heat transfer performance, especially at larger Re cases, while, the microchannel with the protrusion devices is better from an entropy generation minimization perspective. Furthermore, the variation of the relative entropy generation S′/S′0 are influenced by not only the change of Nu/Nu0 and f/f0, but also the physical parameters of working substances.

Gaseous Slip Flow Forced Convection in Microducts of Arbitrary but Constant Cross Section

This is a theoretical study that extends a classical method of treating the convection heat transfer in complex geometries to gaseous slip flow forced convection in microchannels with H1 thermal boundary condition. Through this line, the momentum and energy equations in cylindrical coordinates are made dimensionless. Afterward, solutions are presented that exactly satisfy the dimensionless differential equations along with the symmetry condition and finiteness of the flow parameter at the origin. The first-order slip boundary conditions are then applied to the solution utilizing the least squares matching method. Though the method is general enough to be applied to almost any arbitrary cross section, four different cross sections of polygonal, trapezoidal, rhombic, and elliptic shape are considered for presentation. The accuracy of the method is proven by comparing the results against available literature data. Finally, after a complete discussion of the results, the tabulated data of Nusselt number are presented for each geometry by covering the entire slip flow range of the Knudsen number.

Effects of High-Order Slip/Jump, Thermal Creep, and Variable Thermophysical Properties on Natural Convection in Microchannels With Constant Wall Heat Fluxes

Natural convection gaseous slip flows in open-ended vertical parallel-plate microchannels with symmetric wall heat fluxes are numerically investigated. A second-order model, including thermal creep effects, is considered for velocity slip and temperature jump boundary conditions with variable thermophysical properties. Simulations are performed for wide range of Rayleigh numbers from 5 × 10− 6 to 5 × 10− 3 in the continuum to slip flow regime. The developing and fully developed solutions are examined by solving the Navier–Stokes and energy equations using a control volume technique. It is found that the second-order effects reduce the temperature jump and the slip velocity, whereas thermal creep strongly increases the slip velocity in both developing and fully developed regions. Moreover, the rarefaction effects increase the flow and heat transfer rates considerably, while decreasing the maximum gas temperature and friction coefficient as compared to the continuum limit. It was also shown that the axial temperature variations of the gas layer adjacent to the wall in the modeling of the thermal creep are of paramount importance and neglecting these variations, which is common in literature, leads to unphysical velocity and temperature distributions.

An experimental study of two-phase convection in micro-channels with impinging FC-72 jets

To improve heat transfer performance for computer cooling, we developed a heat sink with integrated micro-channels, each with multiple jets that sprayed FC-72 dielectric fluid. The heat sink contained 11 micro-channels, each with a variety of pore numbers and configurations. We tested these devices with saturation temperatures of 30 and 50 °C. Depending on the configuration, the volume flow rate was 9.1–73.6 ml/min per channel and the total flow rate was 100–810 ml/min. For single-phase heat transfer, the performance of the heat sink increased with increasing flow rate. However, for two-phase heat transfer, the flow rate influenced heat transfer less. The best heat sink had three nozzles in each channel, each 0.4 mm in diameter and spaced 3.0 mm apart from the other nozzles. At heat inputs of 200–240 W from a 12 mm by 12 mm film heater, this heat sink had the lowest thermal resistance of those we tested, 0.0611 K/W. We developed correlations of the convection heat-transfer coefficient for single-phase and two-phase flow in the micro-channel with impinging jets; these correlations predicted our experimental data within ±22%.

Forced convection in micro-channels filled with porous media in local thermal non-equilibrium conditions

An analytic solution on fully developed forced convection, in parallel plates porous micro-channels is accomplished in Local Thermal Non-Equilibrium (LTNE) condition. The analysis is realized in steady state regime for rarefied gaseous slip flows between two parallel plates with assigned heat flux. The Darcy–Brinkman model is considered in the momentum equation and two energy equations are used to evaluate solid and fluid temperatures. The entropy generation analysis is performed and the total entropy generation number is evaluated as a function of the different dimensionless parameters. The effect of tangential momentum and thermal accommodation coefficients are examined. Results are reported in terms of average Nusselt numbers, dimensionless temperature profiles and total entropy generation number as a function of Biot number (Bi), effective thermal conductive ratio (κ), Darcy number (Da), accommodation coefficients (tangential momentum and thermal) and Knudsen number. Results show that heat transfer increases as Bi increases and reaches asymptotic values. Different trends are observed with respect to tangential momentum accommodation coefficient for assigned thermal accommodation. Total entropy generation number as a function of Da−0.5 presents minimum values with respect to Bi, accommodation coefficients and Brinkman number.

Effect of Flow Pulsation on the Heat Transfer Performance of a Minichannel Heat Sink

Heat sinks with liquid forced convection in microchannels are targeted for cooling electronic devices with a high dissipated power density. Given the inherent stability problems associated with two-phase microchannel heat transfer, this paper investigates experimentally the potential for enhancing single-phase convection cooling rates by applying pulsating flow. To this end, a pulsator device is developed which allows independent continuous control of pulsation amplitude and frequency. For a single minichannel geometry (1.9 mm hydraulic diameter) and a wide range of parameters (steady and pulsating Reynolds number, Womersley number), experimental results are presented for the overall heat transfer enhancement compared to the steady flow case. Enhancement factors up to 40% are observed for the investigated parameter range (Reynolds number between 100 and 650, ratio of pulsating to steady Reynolds number between 0.002 and 3, Womersley number between 6 and 17). Two regimes can be discerned: for low pulsation amplitude (corresponding to a ratio of pulsating to steady Reynolds number below 0.2), a small heat transfer reduction is observed similar to earlier analytical and numerical predictions. For higher amplitudes, a significant heat transfer enhancement is observed with a good correspondence to a power law correlation. This work establishes a reference case for future studies of the effect of flow unsteadiness in small scale heat sinks.

Flow boiling heat transfer of refrigerant FC-72 in microchannels

The design of technological devices and systems, with reduced size but unchanged or increased performances, has been one of the most important scientific challenges faced in the last decade by many fields of the world industry. The capability to remove and exchange heat in more compact components has allowed great advantages in the optimization of all final products as well as in industrial processes. However the never-ending race to miniaturization of appliances has been taking into account problems concerning computational models and design criteria developed in the past for macro-systems. Nowadays most of these methods are no longer reliable for emerging micro-systems because of the substantial differences in transport phenomena and heat exchange, mainly in two-phase flow (scaling effects). This paper aims to provide its contribution to the burgeoning research activity on boiling heat transfer under forced convection in microchannels. The BO.E.MI.A. experimental facility was designed and specifically developed to perform different regimes heat transfer tests on mini and microchannels: a total of 150 tests were performed using a fluorinert fluid FC-72 as coolant. The test section consists of a horizontal 1 mm inner diameter stainless steel tube having a heated length (Joule effect) of approximately 60 mm. The mass flux range is between 1000 and 2000 kg m −2 s −1 while the applied heat flux is between 10 and 150 kW m −2. By means of a preheater we are able to change coolant inlet temperature and by setting the operating pressure in a range between 3 and 7 bar, a broad spectrum of subcooling degree at inlet test section is achieved. The local heat transfer coefficients were evaluated for both subcooled and saturated flow boiling regimes. The experimental data show an increase of local heat transfer coefficient for increasing values of heat flux, with a weak dependence on the vapour quality. The experimental values obtained were compared with those from the adoption of one of the main heat transfer correlations in the literature.

Single-Phase Laminar Forced Convection in Microchannels With Rounded Corners

This work investigates the frictional and heat transfer behavior of laminar, fully developed flow in microchannels with trapezoidal and rectangular cross-section and rounded corners under boundary conditions of uniform heat flux along the channel length and perimeter and uniform temperature on the heated perimeter of each cross section. The equations of momentum and energy are solved numerically using the least square method, and the results are validated with analytical data, when available. The runs have been carried out for different aspect ratios and nondimensional radii of curvature, with either all sides or three sides heated, one short side adiabatic for rectangular geometries and three sides heated, the longest one adiabatic for trapezoidal geometries. The Poiseuille and Nusselt numbers are reported and show, for the rectangular cross-section heated on all sides, a maximum increase for the highest value of the aspect ratio with increments in the Poiseuille and Nusselt numbers of about 11% and 16%, respectively, for values of the nondimensional radius of curvature of 0.5, increasing as the geometry approaches the circular duct (12.5% and 21%). The increase is less pronounced as the aspect ratio decreases and also when only three sides are heated (maximum increase of the Nusselt number around 10% for the latter); in the case of trapezoidal geometry the effects of rounding the corners are almost negligible (a maximum increase in the Nusselt number of around 2%).

Light prototype support using micro-channel technology as high efficiency system for silicon pixel detector cooling

Abstract The development of micro-scale mechanical systems has been moving rapidly, allowing an opportunity to the semiconductor detectors to have ever more power located on the active region. Miniaturization associated with micro-channel technologies allows the design of micro-system structures that are able to cool silicon pixel detectors with power of the order of some W/cm 2 with thickness less than 0.3% of radiation length. We present the design and thermo-hydraulic test results for low material budget support and cooling obtained through forced liquid convection in micro-channels, developed for the innermost layer (Layer 0) of SuperB silicon vertex tracker .

Viscous Dissipation and Rarefaction Effects on Laminar Forced Convection in Microchannels

Fluid flow in microchannels has some characteristics, which one of them is rarefaction effect related with gas flow. In the present work, hydrodynamically and thermally fully developed laminar forced convection heat transfer of a rarefied gas flow in two microgeometries is studied, namely, microannulus and parallel plate microchannel. The rarefaction effects are taken into consideration using first-order slip velocity and temperature jump boundary conditions. Viscous heating is also included for either the wall heating or the wall cooling case. Closed form expressions are obtained for dimensionless temperature distribution and Nusselt number. The results demonstrate that for both geometries, as Brinkman number increases, the Nusselt number decreases. However, the effect of viscous heating on the Nusselt number at greater values of Knudsen number becomes insignificant. In the absence of viscous heating, increasing values of Knudsen number lead to smaller values of Nusselt number. Furthermore, it is observed that viscous heating causes singularities in Nusselt number values. Also, asymmetry causes singularities in Nusselt numbers of both microannulus walls and the parallel plate wall having lower heat flux, even in the absence of viscous heating. For parallel plate microchannel, in the absence of viscous heating, Nusselt number of the wall having larger heat flux is an increasing function of the wall heat fluxes ratio.

Inverse analysis of forced convection in micro-channels with slip flow via integral transforms and Bayesian inference

The present work addresses the direct and inverse problems for convective heat transfer with incompressible laminar gas flow in micro-channels, within the range of validity of the slip-flow regime. The direct problem analysis combines the classical integral transform method and the generalized integral transform technique (GITT), by analytically solving the two-dimensional steady-state convection problem and finding a hybrid numerical-analytical solution for the required eigenvalue problem. The inverse problem analysis makes use of the accuracy and robustness of the direct problem solution and focus on the simultaneous identification of the momentum and thermal accommodation coefficients, related to gas flow and heat transfer within micro-channels, besides the usually unknown boundary condition parameters, here represented by the external Biot number. The inverse analysis is based on the availability solely of temperature measurements at the channel external wall, along its length, as obtained for instance via infrared camera thermography. A Bayesian inference approach is adopted in the solution of the identification problem based on the Monte Carlo Markov Chain method (MCMC) and the Metropolis-Hastings sampling algorithm. A typical example of slip flow in parallel-plates micro-channel is selected to illustrate both the direct and inverse problems solution approaches.