Heat Transfer In Microchannels Research Articles

Enhancement of heat transfer and mixing with two side-by-side freely rotatable cylinders in microchannel

The flow disturbance technique is an effective method to enhance the heat and mass transfer efficiency in microchannels. This paper numerically investigates the influence of two side-by-side freely rotatable square cylinders on microchannel flow, heat transfer, and mass transfer under a Reynolds number (Re) range of 5‒300. Results indicate that the cylinders show five different vortex-induced rotation modes, namely the rotation mode (Re < ∼10), oscillation mode (∼10 < Re < ∼70), stationary mode (∼70 < Re < ∼90), random mode (∼90 < Re < ∼250), and reversal rotation mode (Re > ∼250). The vortices generated by rotations of the cylinders disturb the flow field significantly, thus promoting heat and mass transfer in the microchannel. As Re increases, the average Nusselt number (Nu) of the channel first rises slowly then sharply, specifically, from 3.50 when Re = 5 to 8.35 when Re = 75, and then to 42.45 when Re = 300. Under a small Re, the rotating cylinders only affect the local heat transfer near them, while only when Re > 100 can the cylinders significantly promote the heat transfer in a relatively long distance in the microchannel. This behavior should be attributed to the insufficient vortex disturbance under small Reynolds numbers. Mass transfer is scrutinized by releasing fluorescein at the upper half of the channel entrance, and mixing efficiency (η) is introduced to characterize the mixing situation. We show that when Re = 100, the substances in the channel are almost completely mixed (η = 0.98) compared with η = 0.84 for stationary cylinders; beside, as Re increases, the position where the mixture reaches a steady-state gradually moves forward in the channel, and when R > 100, the distribution of η along the channel seldom changes with Re. Considering fluid pressure loss, heat transfer enhancement index φ and mass transfer enhancement index ϕ are introduced to compare heat and mass transfer capability between microchannels set with rotatable cylinders and stationary cylinders, and phase diagrams relative to Re and x* are drawn. Analysis reveals that the freely rotatable cylinders better improve the heat and mass transfer with a maximum φ of 1.6 at Re ≈ 25, x* ≈ 12 or Re ≈ 100, and maximum ϕ of 1400 at Re ≈ 50, x* ≈ 10‒25.

Investigation on the heat transfer and energy-saving performance of microchannel with cavities and extended surface

Microchannel heat sinks (MCHSs) are widely used in microelectronic systems and are one of the most effective heat sinks in the 21st century. In this work, the fluid flow and heat transfer performance of different extended surfaces (cylindrical fins, rectangular fins, water droplets) of triangular combined cavity, 90° fan-shaped and triangular combined cavity were investigated. The comprehensive performance in microchannels were analyzed and evaluated in detail by different aspects, including flow characteristics, heat transfer characteristics, pressure drop characteristics, comprehensive thermal strengthening performance and thermal strengthening performance aimed at energy saving. The result indicated that the microchannel with the triangular combined cavity exhibited the best heat transfer performance at a low Reynolds number. However, as the Reynolds number gradually increasing, the microchannel with 90° fan-shaped and triangular combined cavity presented more obvious advantages. Furthermore, the microchannel is evaluated by the comprehensive thermal enhancement performance and the enhanced performance evaluation aiming at energy-saving. When Re<144.6, the comprehensive heat transfer performance of the microchannel with triangular combined cavity cylinder fins was the best, and the energy-saving effect was excellent. The microchannel of the 90° fan-shaped and triangular combined cavity cylindrical fins at Re>144.6 had better comprehensive heat transfer performance and energy-saving effects. Because of the combined effect of the boundary layer redevelopment and the cold and heat exchange of the fluid in the channel, the microchannel with cavity and the extended surface has better heat transfer and energy-saving performance than that of the traditional rectangular microchannel.

Thermo-hydraulic and entropy generation investigation of nano-encapsulated phase change material (NEPCM) slurry in hybrid wavy microchannel

PurposeThis study aims to capture the heat transfer and entropy generation characteristics of temperature-dependent nano-encapsulated phase change material (NEPCM) slurry in a hybrid wavy microchannel. In addition, the effect of substrate material combined with NEPCM slurry on conjugate heat transfer condition is captured for different microchannel heat sinks.Design/methodology/approachA novel “hybrid wavy microchannel” is proposed to enhance the overall heat transfer and reduce the pressure drop by combining wavy and raccoon geometry. NEPCM–water slurry is implied in the hybrid wavy, conventional wavy and raccoon microchannel. A user-defined function (UDF) is used to observe the effect of phase-change of paraffin material in thermophysical properties of NEPCM–water nanofluid. All three (hybrid, wavy, raccoon) microchannels are engraved on a rectangular substrate of 1.8 mm width (ωs) and 30 mm length (L), respectively. For hybrid, wavy and raccoon microchannel, waviness (γ) of 0.067 is selected for the investigation.FindingsThe result shows that NEPCM particle presence reduces the fluid domain temperature. The thermal performance of proposed Heat sink 2 is found better than the Heat sink 1. The effect of the geometrical modification, wall thermal conductivity, different volumetric concentrations of nanoparticles (ϕ ∼ 1 – 5%) and Reynolds number (Re ∼ 100 – 500) on thermodynamic irreversibility is also observed. Additionally, the effect of thermal and frictional entropy generation is reduced with a combination of NEPCM slurry and higher conductive material for all heat sinks.Practical implicationsA combination of NEPCM slurry with laminar flow microchannel cooling system emerged as a better alternative over other cooling techniques for higher power density devices such as microprocessors, electronic radar systems, aerospace applications, semiconductors, power electronics in modern electronic vehicles, high power lasers, etc.Originality/valueThe phase-change process of the NEPCM slurry is tracked under conjugate heat transfer in a hybrid wavy microchannel. Furthermore, the phase-change process of NEPCM slurry is captured with different heat sink materials (SS316, silicon and copper) under conjugate heat transfer situation for different heat sinks and concentrations (ϕ ∼ 1–5) of NEPCM.

Microchannel flow and heat transfer enhancement via ribs arrangements

Enhanced heat transfer rates in microchannel flow remain challenging in terms of channel design, proper selection of carrier fluid, and setting the appropriate flow conditions. Introducing extended surfaces, such as ribs, in microchannel design, and the use of nanofluids enable to enhance the thermal performance of the flow system. In the present study, heat transfer characteristics in a microchannel flow incorporating the ribs arrangements on the channel wall are considered and the influence of the ribs location and carrier fluid on the thermal performance of the channel is examined. Temperature and velocity variations in the channel are predicted numerically and the findings of the Nusselt number are compared with that obtained from the experiments. The location of ribs along the channel wall is altered and the resulting variation of the Nusselt number and pressure drop along the microchannel is assessed. Al2O3 and TiO2 nanofluids are incorporated as a working fluid and water are also considered for the comparison. Temperature parameters are introduced to scale the rise of the maximum fluid temperature and temperature difference across the channel length. It is found that locating the ribs in the close region of the channel exit improves the Nusselt number by 25% and increases the pressure drop along the microchannel by 10% as compared to those corresponding to no-ribs arrangements. In addition, nanofluids improve the heat transfer rates and slightly alter the pressure drop across the microchannel.

Research on coupling performance of heat transfer and throttling of microchannel J-T effect cryocoolers

• Printed Circuit Board (PCB) technology is applied to the J-T cryocooler. • The steady-state model for J-T cryocooler is developed and verified. • The coupling performance of heat transfer and throttling in cryocooler is analyzed. • A new parameter η jt is proposed to evaluate the utilization rate of J-T effect. To solve the shortcomings of small cooling capacity of the current Joule-Thomson (J-T) cryocooler, a multi-layer microchannel J-T cryocooler is designed and fabricated by printed circuit board technology. The characteristic of the cryocooler is that there exists heat transfer while throttling. The coupling effect of heat transfer and throttling affects the overall performance of cryocooler. Therefore, the steady-state model was established for each part of the cryocooler. The P-h diagram of N 2 was calculated by the model. Considering the variation of J-T coefficient ( μ jt ) in the process, the coupling performance of heat transfer and throttling in the J-T cryocooler were analyzed. A new parameter, J-T efficiency ( η jt ), was proposed to evaluate the J-T effect. Additionally, according to the dependence of μ jt with temperature and pressure, the refrigeration performance of the cryocooler under different working conditions was simulated. The results show that the heat transfer in the throttle can enhance the throttling effect. When the inlet temperature is 285.0 K and the inlet pressure increases to 18.00 MPa, the N 2 temperature at the cold end reaches 116.3 K, and the gross cooling capacity is 4.55 W. When the inlet pressure is 6.00 MPa and the inlet temperature reduces to 195.0 K, the temperature at the cold end can reach to the saturated temperature of 103.2 K, and the maximum gross cooling capacity is 7.68 W. For the two groups of working conditions, the heat transfer in the throttle can enhance the J-T effect. η jt reaches 95.87% and 94.76%, the proportion of temperature drop caused by J-T effect is 56.10% and 68.03% of the total temperature drops, respectively. Finally, the variation laws of inlet temperature and η jt are analyzed when N 2 reaches saturated temperature under different pressures to guide the selection of inlet parameters.

Experimental Investigation of Artificial Cavities Effect of Single-Phase Fluid Flow and Heat Transfer in Single Microchannel

In this paper, an experimental study has been conducted to investigate the influence of artificial cavities (artificial nucleation sites, ANS) in a single microchannel on the characteristics of flow and heat transfer at a single-phase flow. The experiments were performed with deionized water as a working fluid at 30OC inlet temperature with a range (108.6-2372) of Reynolds numbers. Three models of the straight microchannel (model-1, model-2, and model-3) were manufactured of brass having a rectangular shape with a hydraulic diameter of (0.42 mm). Model-1 has a smooth surface, while model-2 has artificial cavities with a number of 40 ANS located on the base of the microchannel; along a line adjacent to one of the sidewalls. Also, the artificial cavities of model-3 exist at the base of the microchannel; along a line that is nearest to each sidewall for the microchannel. The number of ANS at each sidewall is 40 (i.e. the overall number of artificial nucleations is 80). The results manifested the enhancement of heat transfer by the presence of ANS for model-2 and model-3 as compared to model-1 by 15.53% and 16.67%, respectively. Also, the results proved that the fanning friction factor correlation for laminar and turbulent flow can predict very well the results (MAE=6.6-7.2%) and (MAE=4.1-7.7%), respectively. Also, the Nusselt number increases with increasing Reynolds number. However, the conventional correlation that predicted the experimental results is lower than the correlations (MAE=30.1%, 13.2% and 12.6%) for Model-1, -2 and -3, respectively.

The investigation of the velocity slip and the temperature jump effect on the heat transfer characteristics in a microchannel

This paper presents a study of the influence of various factors on heat transfer in microchannels associated with the use of the velocity slip (including thermal creep) and temperature jump boundary conditions on the walls. These factors can affect heat transfer both individually and together, but their effect on heat transfer in the microchannel has still not been practically studied. An attempt to eliminate this lack was made in this paper. It was shown that as the slip length increases the value of the heat flux density increases by about 40% and the average heat transfer coefficient increases by about 50% compare to no-slip conditions. Wherein, the pressure drop significantly decreased, that is, two positive effects took place simultaneously. The presence of thermal creep also reduced the pressure drop in the channel and increased the average heat transfer coefficient, but this effect was significantly less than the effect of the slip length. On the other hand, the taking into account of a temperature jump on the wall led to a certain increase in the pressure drop, but the heat transfer coefficient increased.

Direct Liquid Cooling Heat Transfer in Microchannel: Experimental Results and Correlations Assessment

Direct Liquid Cooling Heat Transfer in Microchannel: Experimental Results and Correlations Assessment

Experimental Study on Nanofluid Flow and Heat Transfer in Microchannel

Experimental Study on Nanofluid Flow and Heat Transfer in Microchannel

Experimental Study on the Application of Manifold Channels to Enhance Heat Transfer in Microchannels

Experimental Study on the Application of Manifold Channels to Enhance Heat Transfer in Microchannels

Optimization of Convective Heat Transfer in Microchannels Equipped by Porous Ribs

Optimization of Convective Heat Transfer in Microchannels Equipped by Porous Ribs

The Effects of Manufacturing Process-Induced Surface Roughness on Mixed Convective Heat Transfer in Multiple Microchannels

The Effects of Manufacturing Process-Induced Surface Roughness on Mixed Convective Heat Transfer in Multiple Microchannels

Effects of Non-Wetting Fraction and Pitch Distance in Flow Boiling Heat Transfer in a Wettability-Patterned Microchannel

Effects of Non-Wetting Fraction and Pitch Distance in Flow Boiling Heat Transfer in a Wettability-Patterned Microchannel

Measurement of Transient Liquid Film and its Effect on Flow Boiling Heat Transfer in Non-Circular Microchannels

Measurement of Transient Liquid Film and its Effect on Flow Boiling Heat Transfer in Non-Circular Microchannels

Surface Termination of Diamond Microchannel and Single Phase Heat Transfer Performance

Surface Termination of Diamond Microchannel and Single Phase Heat Transfer Performance

Lattice Boltzmann simulation of fluid flow and heat transfer in a micro channel with heat sources located on the walls

This study investigated a numerical study of fluid flow and heat transfer in micro channel with four heat sources that located on upper and lower walls. Four heat sources are located in the upper and lower walls symmetrically respect to centerline. Lattice Boltzmann method is used to solve related equations of flow and temperature of fluids, this method is included two steps such as collision and streaming steps. Reynolds number is varied from 0.1 to 10 and Knudsen number is changed from 0 to 0.1 for air. The slip velocity and temperature jump boundary condition are used for micro channel simulation with Knudsen numbers related to slip velocity flow. Bounce-back boundary conditions were applied on all solid boundaries, which means that incoming boundary populations are equal to out-going populations after the collision. A comparison with other study is done and a good result is gained. The results show that the Knudsen number has important role in heat transfer and the highest mean temperature at outlet occurs at highest Knudsen number and heat transfer convection is more significant for first heat source comparing second heat source.

Pulsed-flow microchannel heat sink: Simulation and experimental validation

Microchannel heat dissipation devices were first conceptualized in 1981 and since then are at the forefront of cooling techniques for a variety of applications, extending from computer chips and turbine blades to lasers and optical systems. However, much of the research is concentrated on steady flow of a cooling fluid through the channels. In this article, transient two-dimensional (2D) simulation for heat transfer in microchannels under a pulsed-flow condition is carried out. For validation of simulation results, a novel heat sink device is designed and fabricated, using milling and micro-electric discharge machining (EDM) technique. The fabricated device is then tested to evaluate the effect of a variable flow rate on the heat transfer characteristics when the flow is pulsating. It is found that the numerical results underpredict slightly as compared to actual experimental results. Results indicate a higher temperature at the outlet of the heat sink device for lower pulse frequency, and as pulse frequency increases, the outlet temperature decreases.

Insight into Newtonian fluid flow and heat transfer in vertical microchannel subject to rhythmic membrane contraction due to pressure gradient and buoyancy forces

The designing of the thermo-controlled micro pumping devices for diagnosis in health care and mixing of the samples at particular temperature are very much essential. Transient analysis for heat and mass transfer in a finite length vertical microchannel is presented in this article. This pumping model aims to analyze the streamlines and isotherms under various parameters at two different phases (contraction and relaxation) of membrane motion. The velocity fields under the effects of Grashof number and heat source parameter are also discussed to examine the velocity profile in the vertical microchannel. Considering the creeping nature of physiological transport phenomena, a lubrication approach has been employed. An analytical approache is adopted to derive the exact solutions, and finite difference method with in-house Matlab code are utilized to simulate the results and validate the anlytical results. Pressure is generated due to the kinematic of membrane motion and controlled by the thermal effects and buoyancy force. The contour of axial velocity enhanced by 28.32% as Grashof number changes from 0 to 2 and heat source parameter changes from 0 to 2 respectively. Volumetric flow rate also varies from 0.08 to 0.12 and 0 to 0.1 with change in Grashof number from 0 to 2 and heat source parameter from 0.01 to 4 respectively.

Heat Transfer and Fluid Flow Characteristics of Microchannel with Oval-Shaped Micro Pin Fins.

A novel microchannel heat sink with oval-shaped micro pin fins (MOPF) is proposed and the characteristics of fluid flow and heat transfer are studied numerically for Reynolds number (Re) ranging from 157 to 668. In order to study the influence of geometry on flow and heat transfer characteristics, three non-dimensional variables are defined, such as the fin axial length ratio (α), width ratio (β), and height ratio (γ). The thermal enhancement factor (η) is adopted as an evaluation criterion to evaluate the best comprehensive thermal-hydraulic performance of MOPF. Results indicate that the oval-shaped pin fins in the microchannel can effectively prevent the rise of heat surface temperature along the flow direction, which improves the temperature distribution uniformity. In addition, results show that for the studied Reynolds number range and microchannel geometries in this paper, the thermal enhancement factor η increases firstly and then decreases with the increase of α and β. In addition, except for Re = 157, η decreases first and then increases with the increase of the fin height ratio γ. The thermal enhancement factor for MOPF with α = 4, β = 0.3, and γ = 0.5 achieves 1.56 at Re = 668. The results can provide a theoretical basis for the design of a microchannel heat exchanger.

Investigation of boiling heat transfer in microchannel of water and isopropanol

Boiling curves of distilled water and isopropanol in 12.5x3x0.2 mm microchannel with rigid inlet and outlet lines are obtained. Critical heat flux is calculated for isopropanol by using the previously obtained correlation for water.