Heat Transfer Enhancement In Microchannels Research Articles

Nanofluid heat transfer enhancement in microchannels: Investigating phases interactions using multiphase Eulerian model

This study employs a robust multiphase Eulerian model to mimic heat transfer enhancement using nanofluids in a circular microchannel. Unlike the Eulerian models reported in the literature, the current model incorporates phase interactions, and its accuracy is significantly improved by integrating collision viscosity, kinetic viscosity, and frictional viscosity. Comparative analysis against other models like mixture and discrete phase models demonstrates superior performance of the current model, particularly for solid particle volume fractions exceeding 1 %. The model evaluates water-based nanofluids (Ag NPs, MgO, ZnO, Al2O3, ZrO2, SiO2) in circular microchannels. Ag NPs/water excels at low volume fractions and high Reynolds numbers, while MgO/water performs better at higher volume fractions. Moreover, the analysis of the Euler number indicates a decreasing trend with increasing Reynolds number across all nanofluids. However, no notable variation in the Euler number is observed when maintaining constant volume fraction and Reynolds number. Ultimately, the model is applied to obtain bulk fluid properties, such as viscosity and thermal conductivity, after suspending the nanoparticles. Subsequently, these properties were incorporated into the single-phase model. Utilizing these derived bulk properties was found to substantially enhance the accuracy of the single-phase model, demonstrating its reliability by yielding results within 13.52 % of experimental data.

Effect of Wavy Wall and Plate Bifurcations on Heat Transfer Enhancement in Microchannel

Abstract Miniaturization of electronic components requires compact and effective cooling techniques to dissipate large heat flux without a significant increase in pumping power. Microchannel heat sink with liquid as working fluid is a suitable technique for the purpose. In this study, heat transfer characteristics in presence of vertical bifurcation placed downstream of the microchannel passage are studied numerically. Six types of bifurcating plates are considered under two categories: (i) thick-plate and (ii) wavy thin-wall. Water is taken as the working fluid and the flow rate has been varied in the Reynolds number range, 100 ≤ Re ≤ 1000. The effect of bifurcations on pressure drop, heat transfer, and the overall thermal resistance are analyzed and compared with those of plane microchannel without bifurcation. The numerical results show that the usage of bifurcation in the microchannel reduces the overall thermal resistance. Field synergy number, entropy generation number, and hydrothermal performance index are calculated to quantify the overall performance improvement in the microchannel with bifurcations. Constant wavy thin-wall bifurcation has been found to improve the overall performance of the microchannel. The detailed geometry of the bifurcation, the resulting convective heat transfer characteristics, and percentage improvement in the performance are reported.

Heat transfer enhancement in microchannels using ribs and secondary flows

PurposeThe purpose of this paper is to study the flow and heat transfer characteristics of microchannel heatsinks with ribs, cavities and secondary channels. The influence of length and width of the ribs on heat transfer enhancement, secondary flows, flow distribution and temperature distribution are examined at different Reynolds numbers. The effectiveness of each heatsink is evaluated using the performance factor.Design/methodology/approachA three-dimensional solid-fluid conjugate heat transfer numerical model is used to study the flow and heat transfer characteristics in microchannels. One symmetrical channel is adopted for the simulation to reduce the computational cost and time. Flow inside the channels is assumed to be single-phase and laminar. The governing equations are solved using finite volume method.FindingsThe numerical results are analyzed in terms of average Nusselt number ratio, average base temperature, friction factor ratio, pressure variation inside the channel, temperature distribution, velocity distribution inside the channel, mass flow rate distribution inside the secondary channels and performance factor of each microchannels. Results indicate that impact of rib width is higher in enhancing the heat transfer when compared with its length but with a penalty on the pressure drop. The combined effects of secondary channels, ribs and cavities helps to lower the temperature of the microchannel heat sink and enhances the heat transfer rate.Practical implicationsThe fabrication of microchannels are complex, but recent advancements in the additive manufacturing techniques makes the fabrication of the design considered in this numerical study feasible.Originality/valueThe proposed microchannel heatsink can be used in practical applications to reduce the thermal resistance, and it augments the heat transfer rate when compared with the baseline design.

Investigation on Heat Transfer Enhancement in Microchannel Using Al2O3/Water Nanofluids

Nowadays, reducing heat generation in electronic devices while using microchannel cooling is used to solve this problem. Because the trend is globally marching toward the compact size, the component’s dimensions get smaller, but the warmth involved within the component increases. Studies of heat transfer rate are conducted to determine the effect of a fully heated microchannel conductor’s heat transfer performance. Experiments are performed using nanofluid Al2O3/water through a concentration percentage of 0.1% and 0.25% and deionized water through a microchannel conductor with 25 rectangular microchannel numbers with a dimension of (0.42×0.42×100) mm3. This present work deals with the effect of nanofluids and their concentration percentages. Finally, it concluded that better heat transfer performance was seen in nanofluids compared to deionized water. The reason is the high viscosity of nanofluid Al2O3/water due to these nanoparticles is deposited on the wall surface of the microchannel and outcomes trendy improvement in the heat transfer. Finally, a high concentration percentage of nanofluids revealed a practical improvement in the transfer of microchannel. As a result, 0.25% of the concentration percentage achieved a satisfactory result compared to the remaining fluids and almost 32.5% and 26% of thermal resistance decrease.

Analysis of Heat Transfer Enhancement in a Micro-Scale Heat Sink Structure

Abstract With the increasing power requirements of electronic devices, high heat flux will cause serious damage to the devices. Based on the basic theory of micro-nano heat transfer, the series and topological microchannel heat sink models are established. The flow field characteristics and temperature distribution in the heat sink are analyzed by numerical calculation. The effects of channel structure on temperature, pressure drop, the Nusselt number, and enhanced heat transfer factor are compared, and the micro-mechanism of heat transfer enhancement in microchannels is clarified. It is found that the Nusselt number of the flow field can be significantly increased by adding the triangular groove in the microchannel, and the enhanced heat transfer factor in the channel can be greatly improved by the topological structure. Further analysis of the factors such as angle α, diameter ratios γ, and topological structures of the triangular groove shows that when α = 70 deg, the Nusselt number of the flow field is 3.1 times of that of the straight channel and the enhanced heat transfer factor is 2.7 times of that of it; compared with the straight channel, the comprehensive heat transfer performance of the microchannel with γ = 1/2 is improved by 31%; when using variable cross-sectional topology microchannel with triangle structure (T.Tr.N.) topology, the convective heat transfer of the microchannel is 2.6 times of that of the straight channel and the comprehensive heat transfer performance is increased by 5.9 times.

Analysis of heat transfer enhancement in microchannel by varying the height of pin fins at upstream and downstream region

In this study, a numerical analysis of a microchannel with the different configuration of varying height of pin fins entrenched at the bottom of the channel base wall has been carried out. Five different configurations of pin fins arrangement which are considered in this study are, Case 1(Full length fins in complete microchannel), Case 2(Full length fins at the upstream), Case 3(Full length fins at the downstream), Case 4(Full length fin at the center of microchannel), Case 5(Full length fins at the inlet and exit of microchannel) and the results of these five cases are compared with the plain rectangular microchannel. In this investigation, deionized ultra-filtered water is used and Reynold’s number is ranging from 150 to 350. Results reveals that the highest Nusselt number is achieved by case 2 at a lesser value of Reynold’s number while by case 5 at higher Reynold’s number and the lowest pressure drop is occurring in case 4. The overall thermal performance of case 2 beats the corresponding cases.

Benefits of spanwise gaps in cylindrical vortex generators for conjugate heat transfer enhancement in micro-channels

Cylindrical vortex generators placed transversely over the span of a micro-channel can enhance heat transfer performance, but adding full-span vortex generators incurs a substantial pressure drop penalty. This paper examines the benefits of introducing various gaps along the length of the vortex generators, both for reducing pressure drop and improving the thermal conductance of the system. Three particular configurations are considered with varied dimensions: symmetrical gaps at each end of the vortex generator, i.e. adjacent to the channel side walls; a single central gap; and a combination of a central and end gaps. The performance is investigated numerically via 3D finite element analysis for Reynolds number in the range 300–2300 and under conditions of a uniform heat flux input relevant to microelectronics cooling. Results demonstrate that having end gaps alone substantially improves heat transfer while reducing the pressure drop. As well as generating longitudinal vortices which draw heat from the adjacent channel side walls, hot fluid passing through the gaps is swept directly upwards and inwards into the bulk flow, where it remains as it flows to the outlet. A thermal-hydraulic performance evaluation index is improved from 0.7 for full-span vortex generators to 1.0 with end gaps present. The central and central-plus-end gap geometries are less effective overall, but do offer localised improvements in heat transfer.

Significant heat transfer enhancement in microchannels with herringbone-inspired microstructures

Herringbone microstructures are a very promising class of flow promoters to passively enhance heat transfer in microchannels by efficiently triggering helicoidal fluid motion. A host of applications are envisioned to benefit from heat transfer enhancement in microchannels, including microfluidic interlayer cooling of 3D electronic chip stacks, or advanced concepts of integrated cooling and electrochemical power delivery. Here we investigate the cooling performance of microchannels with such flow promoters and show that the Nusselt number reaches an average value of Nu=36.6 at a Reynolds number of Re=510 for our best performing design. This result constitutes a fourfold improvement in heat transfer capability compared to a plain microchannel. The fluid temperature is assessed optically using micron-resolution laser induced fluorescence (μLIF), while the wall temperature is measured with on-chip resistance thermometers. In addition, we determine the pressure drop originating from the presence of the herringbone flow promoters. By taking into account both the beneficial heat transfer enhancement and the adverse increase of pressure drop in a non-dimensional figure of merit (FoM), we demonstrate a significant performance enhancement of 220% at Re=350 using herringbone structures for heat transfer augmentation compared to a plain, unstructured reference microchannel.

Numerical investigation of heat transfer in extended surface microchannels

Microchannel heat sinks (MCHS’s) are currently projected as twenty first century cooling solution. In the present numerical study, heat transfer enhancement in microchannel using extended surface has been carried out. Rectangular microchannel and cylindrical microfins are used in current study. Three different configurations of extended surface microchannel; Case I (upstream finned microchannel), Case II (downstream finned microchannel) and Case III (complete finned microchannel) are compared with plain rectangular microchannel. It is found that heat transfer performance of Case I is better than Case II. Case I even performs better than Case III at low Reynolds number. Average surface temperature is also significantly reduced in case of extended surface microchannels. Optimization of extended surface microchannel has also been successively carried out following univariate search method for number of fins, pitch, diameter and height of fins. Average heat transfer enhancement in optimized case is around 160% with acceptable pressure drop penalty.

Numerical study on drag reduction and heat transfer enhancement in microchannels with superhydrophobic surfaces for electronic cooling

Microchannels with superhydrophobic surfaces are a promising candidate for electric cooling with mild frictional penalty. Frictional and thermal performance of laminar liquid-water flow in such microchannels is numerically investigated for various shear-free fractions and Reynolds numbers. The structures on superhydrophobic surfaces include square posts and holes, transverse and longitudinal grooves. Combined frictional and thermal performance of microchannels is evaluated by a goodness factor, and is compared with that of smooth plain channels. It is found that with increasing shear-free fractions, both friction factor and average Nusselt number deteriorate for four surface patterns; however, goodness factor is improved significantly over smooth plain channels. In general, superhydrophobic surfaces containing longitudinal and transverse grooves exhibit the lowest and highest frictional and thermal performance, respectively; however, combined performance of these two are on opposite. Among four surface patterns, longitudinal grooves have the highest goodness factors, except at high shear-free fractions or high Reynolds numbers where overall performance is surpassed by square posts. At very low or high shear-free fractions, frictional and thermal performance of two-dimensional square posts and holes approaches that of one-dimensional longitudinal or transverse grooves. Our study suggests microchannels with superhydrophobic surfaces as promising candidates for efficient cooling devices.

Heat Transfer Enhancement in Microchannel Using Nanofluids

Numerical investigation of heat transfer enhancement in two-dimensional microchannel heat sink (MCHS) using Al2O3-water, CuO-water and TiO2-water was conducted. The effect of different type of nanoparticles at particle volume concentration of 1%, 2% and 5% on the thermal performance in the MCHS was examined. The thermal performance is increased when nanofluids with high thermal conductivity and low dynamic viscosity was used. As the particle volume concentration increases, the heat transfer performance also improved. The result shows that the heat transfer performance of all the nanofluids used in this study was better than that of pure water. Overall, nanofluids with Al2O3-water at 5% particle volume concentration show the best cooling performance.

An Experimental Study of Passive and Active Heat Transfer Enhancement in Microchannels

An experimental study on single-phase heat transfer and fluid flow downstream a single microscale pillar in a microchannel was conducted. A secondary jet flow was issued from slits formed along the pillar. A comparison of the thermal performances of a plain microchannel, a microchannel with a pillar, and a microchannel with a jet issued from a pillar was performed to elucidate the merits of this heat transfer enhancement technique. It was found that the presence of a pillar upstream the heater enhanced the heat transfer; the addition of jet flow issued from a pillar further enhanced the heat transfer. At a Reynolds number of 730, an improvement of spatially averaged Nusselt number of 80% was achieved due to the combined effect of the pillar and the jet compared with the corresponding plain channel. Micro particle image velocimetry (μPIV) measurements provided planar velocity fields at two planes along the channel height, and allowed flow structure visualization. Turbulent kinetic energy (TKE) was used to measure flow mixing and to quantify the hydrodynamic effect of the jet. It was shown that the TKE is closely related to the Nusselt number.

Numerical Study of Heat Transfer Enhancement in Microchannels of the MTPV Systems

Heat transfer and fluid flow in the microchannel cooling passages with three different types of the MTPV systems are numerically investigated. The heat transfer characteristics and thermal performance of the microchannels are analyzed using different Reynolds numbers and hydraulic diameters. Local heat transfer coefficient, total pumping power, heat transferred are obtained from the simulations and the performance is discussed in terms of heat transfer coefficient and thermal efficiency. Results indicated enhanced performance with the smaller hydraulic diameters, and slighter penalty in pumping power. The increase in Reynolds number cause an increase in the heat transfer coefficient at the expense of decreased thermal efficiency. It is also found that the highest heat transfer coefficient is expected in rod bundles microchannels. However, round microchannels have a better thermal efficiency than others.

Heat transfer enhancement in micro-channel with multiple synthetic jets

A three-dimensional computational model has been developed to investigate the effect of multiple synthetic jets interaction with cross-flow in micro-channel on the cooling of microchip. Studies were performed with the use of two micro jets being in-phase and 180° out-of-phase at two different operating frequencies and a fixed diaphragm amplitude. The addition of one synthetic jet was shown to achieve greater mixing of flow in the micro-channel than those with single synthetic jet. Greater heat transfer enhancement was achieved with double synthetic jets. Results showed that greater cooling enhancement could be achieved with the out-of-phase flow configuration at oscillating frequency of 560Hz compared with the in-phase flow configuration. However, the effect of the actuation phase at a frequency of 1120Hz was found to be insignificant. With double synthetic jet actuators operating out-of-phase, a mere 0.1K reduction was achieved in maximum silicon temperature compared with in-phase flow jets. The greater the mixing of flow in the micro-channel in either in-phase or out-of-phase flow configuration due to higher jets Reynolds number resulted in the constraint in further cooling enhancement despite having different phases of the flow configuration.

Large Convective Heat Transfer Enhancement in Microchannels With a Train of Coflowing Immiscible or Colloidal Droplets

We show that heat transfer in microchannels can be considerably augmented by introducing droplets or slugs of an immiscible liquid into the main fluid flow. We numerically investigate the influence of differently shaped colloidal or simply pure immiscible droplets to the main liquid flow on the thermal transport in microchannels. Results of parametric studies on the influence of all major factors connected to microchannel heat transfer are presented. The effect of induced Marangoni flow at the liquid interfaces is also taken into account and quantified. The calculation of the multiphase, multispecies flow problem is performed, applying a front tracking method, extended to account for nanoparticle transport in the suspended phase when relevant. This study reveals that the use of a second suspended liquid (with or without nanoparticles) is an efficient way to significantly increase the thermal performance without unacceptably large pressure losses. In the case of slug-train coflow, the Nusselt number can be increased by as much as 400% compared with single liquid flow.

Heat transfer enhancement in micro-channels caused by vortex promoters

This paper presents a systematic numerical study of the effects of heat transfer and pressure drop produced by vortex promoters of various shapes in a 2D, laminar flow in a micro-channel. The liquid is assumed to be water, with temperature dependent viscosity and thermal conductivity. It is intended to obtain useful design criteria of micro-cooling systems, taking into account that practical solutions should be both thermally efficient and not expensive in terms of the pumping power. Three reference cross sections, namely circular/elliptical, rectangular, and triangular, at various aspect ratios are considered. The effect of the blockage ratio, the Reynolds number, and the relative position and orientation of the obstacle are also studied. Some design guidelines based on two figures of merit (related to thermal efficiency and pressure drop, respectively), which could be used in an engineering environment are provided.

Heat transfer enhancement in microchannels with cross-flow synthetic jets

This paper examines the effectiveness of using a pulsating cross-flow fluid jet for thermal enhancement in a microchannel. The proposed technique uses a novel flow pulsing mechanism termed “synthetic jet” that injects into the microchannel a high-frequency fluid jet with a zero-net-mass flow through the jet orifice. The microchannel flow interacted by the pulsed jet is modelled as a two-dimensional finite volume simulation with unsteady Reynolds-averaged Navier–Stokes equations while using the Shear-Stress-Transport (SST) k– ω turbulence model to account for fluid turbulence. For a range of conditions, the special characteristics of this periodically interrupted flow are identified while predicting the associated convective heat transfer rates. Results indicate that the pulsating jet leads to outstanding thermal performance in the microchannel increasing its heat dissipation by about 4.3 times compared to a channel without jet interaction within the tested parametric range. The degree of enhancement is first seen to grow gently and then rather rapidly beyond a certain flow condition to reach a steady value. The proposed strategy has the unique intrinsic ability to generate outstanding degree of thermal enhancement in a microchannel without increasing its flow pressure drop. The technique is envisaged to have application potential in miniature electronic devices where localised cooling is desired over a base heat dissipation load.