Heat Transfer Characteristics Of Microchannel Research Articles

Experimental study of heat transfer performance in rectangular microchannels enhanced by ultrasound

Ultrasound is recognized as an effective technology to enhance heat transfer, which is widely used in industrial fields such as microelectronics and heat storage. However, the heat transfer characteristics in rectangular microchannels assisted with ultrasound has little revealed in present research. In this work, the external ultrasonic was added in a rectangular microchannel to reveal the influence of ultrasound on the heat transfer characteristics of micro-channels. Hence, the comparative experiments were completed to explore the heat transfer performance with and without ultrasonic. Moreover, a large number of experiments were carried out within rectangular microchannels by varying the velocity of the working fluid and the heat flux at the bottom of the microchannels. The results indicate that the heat transfer performance of rectangular microchannels can be enhanced by ultrasound. But the enhanced heat transfer effect could be limited with the increase of the flow velocity and heat flux in microchannel correspondingly. For rectangular microchannels with adding ultrasound, the lower the flow rate and heat flow density are, the better the enhancement effect will be. During the measurement, when the fluid flow velocity is 0.2 m/s and the heat flux is 408.2 kW/m2, the heat transfer performance of the ultrasound-enhanced microchannel can be improved by a maximum of 12.9 %. Additionally, the tests indicate that the pressure drop mainly comes from the viscous loss along rectangular channel, yet the ultrasound has no negative impact on pressure drop in microchannel. This work could be beneficial to the further application of ultrasonic enhancement technology.

Enhanced heat transfer study of microchannel heat sink with periodically arranged triangular cavities and arc‐shaped ribs

AbstractThe microchannel heat sink with periodically arranged triangular cavities and arc‐shaped ribs (MC‐ARTC) is investigated numerically under the conditions of Reynolds number 147–736. The effects of triangular cavities and arc‐shaped ribs on velocity, pressure, and temperature distribution in the channel are analyzed in comparison with a rectangular microchannel heat sink, microchannel heat sink with arc‐shaped ribs, and microchannel heat sink with triangular cavities. The results show that the triangular cavity and the arc‐shaped rib have important effects on the flow and heat transfer characteristics in the microchannel heat sink. Both rib and cavity can cause a change in fluid flow direction and disturb the development of the flow boundary layer. The fluid flow and heat transfer characteristics of microchannels are improved. Among the four microchannels, the MC‐ARTC has the optimum comprehensive performance for the combined effect of the rib and cavity. Relative cavity width () and relative rib height () are also defined to study the effects of cavity width ( = 89–200 μm) and rib height ( = 8.9–28.9 μm) on the flow and heat transfer characteristics. It is found that the heat transfer of MC‐ARTC is enhanced with increasing and , while lead to higher pressure drop. When and are 252 and 249 μm, respectively, the comprehensive performance factor () of MC‐ARTC reaches 1.76.

A Review of the Complex Flow and Heat Transfer Characteristics in Microchannels.

Continuously improving heat transfer efficiency is one of the important goals in the field of energy. Compact heat exchangers characterized by microscale flow and heat transfer have successfully provided solutions for this purpose. However, as the characteristic scale of the channels decreases, the flow and heat transfer characteristics may differ from those at the conventional scale. When considering the influence of scale effects and changes in special fluid properties, the flow and heat transfer process becomes more complex. The conclusions of the relevant studies have not been unified, and there are even disagreements on some aspects. Therefore, further research is needed to obtain a sufficient understanding of flow structure and heat transfer mechanisms in microchannels. This article systematically reviews the research about microscale flow and heat transfer, focusing on the flow and heat transfer mechanisms in microchannels, which is elaborated in the following two perspectives: one is the microscale single-phase flow and heat transfer that only considers the influence of scale effects, the other is the special heat transfer phenomena brought about by the coupling of microscale flow with special fluids (fluid with phase change (pseudophase change)). The microscale flow and heat transfer mechanisms under the influence of multiple factors, including scale effects (such as rarefaction, surface roughness, axial heat conduction, and compressibility) and special fluids, are investigated, which can meet the specific needs for the design of various microscale heat exchangers.

Investigation of Flow and Heat Transfer Characteristics in Microchannels with Fins

A highly efficient thermal management is imperative to overcome the main challenges associated with heat extraction requirements in electronics. In this study, the flow and heat transfer characteristics of microchannels with various types of fins were numerically analyzed for Re = 0–500 (Re: Reynolds number). Investigation of the aspect ratio, incident angle, and smoothness as well as the flow and heat transfer behaviors revealed the exceptional performance of the optimized fin structure, up to a performance evaluation criterion of 1.53. At large Re values, the fin with a high aspect ratio, small incidence angle, and high smoothness showed the best performance, as it avoids stagnation zones because of flares and sharp corners and simultaneously leads to boundary layer destruction and redevelopment. Interestingly, the microchannel without internal microstructures performed well at small Re values. Among all the designed variables, the influence of the incident angle was superior owing to its ability to generate significant vortices by periodically changing the channel cross-sectional area and flow direction. The conclusions can be innovatively generalized to other microchannels with fins.

Analytical study of fluid flow and thermal characteristics of flow in micro channel heat sinks with working fluid as water and alumina & water nanofluid

The electronic industry is looking at microchannel based MEMS technology to resolve the issue of increasing heat dissipation from electronic circuits. Micro-channels play an important role in development of small scale fluid flow devices. The internal flow configuration represents a convenient geometry for heat transfer between fluids used in chemical processing, environmental control, and energy conversion technologies. In order to fabricate such micro devices effectively, it is extremely important to understand the fundamental mechanisms involved in fluid flow and heat transfer characteristics in microchannels. This paper presents an analytical study of flow mechanism in microchannels, with a focus on effect of geometric dimensions and flow rate on the various flow parameters of a Rectangular microchannel. An attempt is also made to determine optimum microchannel dimensions based on the analysis. Basic flow equations are used for modelling the flow and determining various parameters. Further, the study analyses the effect of using Alumina & Water nanofluid on flow and thermal characteristics as compared to using water alone. The results observed show a deeper and wider microchannel would keep the pressure drop across the microchannel lower. Also using nanofluid as working fluid would result in lower pressure drop.

Effect of fluid distribution along the channel length on the cooling performance of microchannel heat sink

Purpose The purpose of this study is to conduct research on a new kind of division microchannel heat sink (D-MCHS), which can distribute cooling water along the channel-length direction. First, the pressure drops in the D-MCHS with different division region numbers were compared. Then, the cooling performance of the D-MCHS with different division region numbers was also comparatively investigated. Finally, the temperature distribution on the bottom surface of the D-MCHS was analyzed. Design/methodology/approach First, experiments were conducted to investigate the numerical calculation method. Then, a three-dimensional steady, single-phase, laminar flow and solid-fluid conjugate heat transfer numerical model was used to research the flow and heat transfer characteristics in microchannels. Findings The pressure drop in the D-MCHS could be reduced by increasing the number of divided flow regions along the channel-length direction. The bottom average temperature of the D-MCHS could be simultaneously affected by the number of divided flow regions and the water flow rate. The thermal uniformity performance of the D-MCHS could be improved by increasing the number of division flow regions. The number of low-temperature and high-temperature areas on the bottom surface of the D-MCHS is corresponding to the division flow region number. Originality/value The D-MCHS exhibited a positive effect on the pressure drop decrease and thermal uniformity improvement. It not only keeps the electronic module working in a secure temperature environment but also consumes less pump power for a lower pressure drop.

Simultaneously developing flow and heat transfer in circular and parallel-plates microchannels with velocity slip and temperature jump

The entrance effect has an important influence on the heat transfer characteristics in microchannels. It is necessary to determine the influence to design microchannel heat sinks properly. In this work, the simultaneously developing flow and convective heat transfer were investigated numerically inside circular and parallel-plates microchannels. The effects of the Knudsen number (0–0.1) and Peclet number (25–1000) on the local Nusselt number were taken into consideration in the slip flow regime. The numerical results were calculated via an incompressible and steady computational fluid dynamics algorithm under the boundary conditions of the velocity slip and temperature jump. The user-defined functions were used to achieve the velocity slip boundary condition, and a thermal resistance model was first proposed to easily impose the temperature jump boundary condition. The numerical approach was verified using both the analytic solutions of the thermally developing flow and the exact solutions of the fully developed flow, which was then employed to obtain the influences of the axial heat conduction and the rarefaction on the local Nusselt number of the simultaneously developing flow. The results of the apparent friction factor and the heat transfer coefficient were graphically presented as functions of the Knudsen number, Reynolds number or Peclet number, and non-dimensional axial length. It was found that the effect of the axial heat conduction on the entrance effect can be ignored when Pe > 500. The heat transfer performance of the simultaneously developing flow can be predicted by the results of the thermally developing flow when the Knudsen number is large enough. General correlations were also proposed for the apparent friction coefficients, local Nusselt number in circular and parallel-plates microchannels.

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.

Experimental Study and Mechanism Analysis of the Flow Boiling and Heat Transfer Characteristics in Microchannels with Different Surface Wettability.

In this paper experiments have been conducted to investigate the flow boiling and heat transfer characteristics in microchannels with three different surface wettability. Three types of microchannels with a super-hydrophilic surface (θ ≈ 0°), a hydrophilic surface (θ = 43°) and an untreated surface (θ = 70°) were prepared. The results show that the average heat transfer coefficient of a super-hydrophilic surface microchannel is significantly higher than that of an untreated surface microchannel, especially when the mass flux is high. The visualization of the flow patterns states that the number of bubble nucleation generated in the super-hydrophilic microchannel at the beginning of the flow boiling is significantly more than that in the untreated microchannel. Through detailed analysis of the experimental data, flow patterns and microchannel surface SEM images, it can be inferred that the super-hydrophilic surface microchannel has more active nucleation cavities, a high nucleation rate and a large nucleation number, a small bubble departure diameter and a fast departure frequency, thereby promoting the flow and heat transfer in the microchannel. In addition, through the force analysis of the vapor-liquid interface, the mechanism that the super-hydrophilic microchannel without dryout under high heat flux conditions is clarified.

HEAT TRANSFER CHARACTERISTICS IN MICROCHANNELS AT LOW REYNOLDS NUMBERS

HEAT TRANSFER CHARACTERISTICS IN MICROCHANNELS AT LOW REYNOLDS NUMBERS

Numerical study on flow and heat transfer characteristics of microchannel designed using topological optimizations method

Microchannel has demonstrated advantages in the thermal management of integrated chip. In this study, the topology optimization method is applied for designing a topological microchannel to optimize the performances of both heat dissipation and pressure drop. To validate the performance of the topological structure, the flow and heat transfer characteristics of topological microchannel under non-uniform heating flux are numerically studied. The topological structure is designed to cool a heating area of 10 mm × 10 mm with 4 hotspots. Heat flux is 40 W/cm2 in the hotspot area, while it is only 15 W/cm2 in the rest heating area. The results of heat dissipation performance and pressure drop are compared with those of conventional straight microchannel. Numerical result shows that, compared to the straight microchannel, the hotspot temperature and pressure drop of topological microchannel can be reduced by 4 and 0.6 kPa, respectively, under the flow rate of 2.2×10−4 kg/s. The coefficient of performance (COP) of topological microchannel can be 16.1% better than that of straight microchannel, which can be attributed to the effects of optimized bifurcation and confluence structural of topological microchannel.

A NOVEL INVESTIGATION OF HEAT TRANSFER CHARACTERISTICS IN HYBRID MICRO-CHANNEL HEAT SINK STRUCTURE: OPPOSITION-BASED ANTLION OPTIMIZATION

Micro-channel heat sinks have been demonstrated to disperse heat at an extremely higher rate than the traditional devices because of huge heat transfer surface-to-volume ratio. This paper proposed to study the hydrothermal performance of hybrid combination of micro-channel heat sink (HMCHS) with secondary oblique channels in alternating directions and rectangular ribs. To enhance the heat transfer characteristics of the proposed hybrid model, we optimize three geometrical parameters such as secondary channel width, relative rib width and angle between the secondary channels. Primarily, numerical modeling is used to find the ranges of parameters by way of using the continuity, momentum and energy equations. Then the proposed model utilizes opposition-based antlion optimization (OALO) algorithm for selecting the optimal ranges of geometrical parameters to improve the system model. The optimal range will enhance the heat transfer characteristics of micro-channel and reduce the pressure drop when compared to the ones in the existing literature. Further, as a result of higher pressure drop, the micro-channel heat sink with secondary channels and ribs continuously loses its favorable position as an effective heat transfer enhancement technique at higher Reynolds number.

Scale effect of slip boundary condition at solid\u2013liquid interface

Rapid advances in microelectromechanical systems have stimulated the development of compact devices, which require effective cooling technologies (e.g., microchannel cooling). However, the inconsistencies between experimental and classical theoretical predictions for the liquid flow in microchannel remain unclarified. Given the larger surface/volume ratio of microchannel, the surface effects increase as channel scale decreases. Here we show the scale effect of the boundary condition at the solid–liquid interface on single-phase convective heat transfer characteristics in microchannels. We demonstrate that the deviation from classical theory with a reduction in hydraulic diameters is due to the breakdown of the continuum solid–liquid boundary condition. The forced convective heat transfer characteristics of single-phase laminar flow in a parallel-plate microchannel are investigated. Using the theoretical Poiseuille and Nusselt numbers derived under the slip boundary condition at the solid–liquid interface, we estimate the slip length and thermal slip length at the interface.

Experimental Investigation of the Air Flow Behavior and Heat Transfer Characteristics in Microchannels With Different Channel Lengths

This paper attempts to experimentally investigate the influence of channel length on the flow behavior and heat transfer characteristics in circular microchannels. The diameters of the channels were 0.4 mm and the length of them were 5 mm, 10 mm, 15 mm, and 20 mm, respectively. All experiments were performed with air and completed with Reynolds number in the range of 300–2700. Results of the experiments show that the length of microchannels has remarkable effects on the performance of flow behavior and heat transfer characteristics. Both the friction factor and Poiseuille number drop with the increase of channel length, and the experimental values are higher than the theoretical ones. Moreover, the channel length does not influence the value of critical Reynolds number. Nusselt number decrease as the increase of channel length. Larger Nusselt numbers are obtained in shorter channels. The results also indicate that in all cases, the friction factor decreases and the Poiseuille number increases with the increase of the Reynolds number. It is also observed that the value of critical Reynolds number is between 1500 and 1700 in this paper, which is lower than the value of theoretical critical Reynolds number of 2300.

Multiphase fluid flow and heat transfer characteristics in microchannels

The boiling flow or condensation is widely encountered in many industrial applications for both cooling as well as heating processes. Compact heat transfer devices, such as micro-heat exchangers and evaporators, are extensively used for both cooling as well as heating processes over conventional heat exchangers, such as microelectronic circuits, automobile and aerospace industries, due to high surface area to volume ratio and heat transfer rates, compactness and easy thermal control. For better design of micro- or mini-heat exchangers, a detailed specific knowledge of the multiphase flow and its properties such as the flow pattern during flow boiling, critical heat flux (CHF) and stable operation are very important. This paper provides a state of art review on boiling flow in microchannels since year 2000 till date. Flow patterns formed and the parameters influencing flow pattern transitions, during multiphase heat transfer in micro- or mini-channels, have been reviewed in detail. The flow regimes and flow pattern maps, and modeling approaches considered for boiling flow in micro-channels/devices with various challenges have been discussed. A lot of contradiction between the experimental data has been observed for the analysis of flow regimes and flow pattern maps. Further, the effect of hydrodynamics during flow boiling and CHF on heat transfer coefficient has been discussed in detail. Recently, with the advancement in measurement techniques, the heat transfer measurement technologies have been synchronized with the visualization techniques, which helped in understanding the boiling flow physics in micro- and mini-channels. Therefore, an in-depth understanding of flow patterns and regimes under boiling flow conditions in mini- and micro-channels can be used to predict the boiling heat transfer mechanism, which can be further used for developing better heat transfer models for boiling flow. Further, enhancement in heat transfer coefficient for boiling flow in microchannels, either by using complex microchannel configurations or nanocoating on the microchannel surface, have received attention recently, which have been discussed and analyzed in the present review. Both micro- and mini-channels have number of applications in aerospace, refrigeration and computational systems; therefore further attention is needed for more robust and precise design.

An experimental study on fully developed laminar flow and heat transfer in rectangular microchannels

An empirical study has been performed to explore the validity of theoretical correlations based on conventional sized channels for predicting fluid flow and heat transfer characteristics in microchannels. The flow resistance and thermal behavior of laminar flow through 10 different rectangular microchannels with hydraulic diameters of 155–580 µm and aspect ratios of 0.25–3.8 at Reynolds numbers ranging from 30 to 2500 were investigated. The single-phase laminar friction factors in the microchannels conformed to the conventional Poiseuille flow theory. The critical Reynolds number increased from 1700 to 2400 with a decrease in the aspect ratio from 1.0 to 0.25. The existence of two heat transfer regimes was verified. At Re< 180, the experimental Nusselt numbers were lower than the theoretical values and increased rapidly with the Reynolds number. The Nusselt numbers began to exceed the theoretical values for Re> 180; however, its rate of increase with the Reynolds number slowed considerably. In this region, the theoretical value of the Nusselt number was assessed to be reasonable as an estimate for rectangular microchannels when the aspect ratio was greater than 1.0.

Investigation of the roughness effect on flow behavior and heat transfer characteristics in microchannels

Experiments were conducted to investigate the flow behavior and heat transfer characteristics in microchannels with various surface roughnesses. In the experiments, six test pieces with various roughnesses were fabricated in the stainless steel panel and divided into three groups. Each panel includes 30 microchannels in parallel with uniform geometrical parameters. Both width and depth of microchannels are 0.4mm. The relative roughness of surface in the microchannel is from 0.58% to 1.26%. All tests were performed with air. The experiments were completed with the Reynolds number in the ranges of 200–2100. The friction factor, heat transfer coefficient, Nusselt number and thermal performance were analyzed based on the experimental results. Results of experiments showed that the surface roughness in microchannel has a remarkable effect on the performance of flow behavior and heat transfer. Based on the experimental data, the friction factor and the Nusselt number increase with the increasing of the surface relative roughness. At the same time, the effect of roughness in the thermal performances is small within the range of experimental parameters. The results also indicate that the heat flux rarely affects the Nusselt number. In this paper, the corresponding empiric equations for flow resistance and heat transfer characteristics in microchannels were developed.

Twenty first century cooling solution: Microchannel heat sinks

Due to rapid evolution in a wide range of technologies in twentieth century, heat dissipation requirement has increased very rapidly especially from compact systems. There is an urgent need for high-performance heat sinks to ensure the integrity and long life of these petite systems. Use of forced convection cooling has been limited by the requirement of the excessively high flow velocity and associated noise and vibration problems. Microchannel heat sink seems to be most reliable cooling technology due to its superior command over heat carrying capability. Understanding the flow boiling phenomena in microchannel heat sink experimentally and analytically has been topic of intense research in twenty first century. In this review paper, the experimental studies on flow visualization, pressure drop and heat transfer characteristics of microchannels presented by different researchers are summarized. Some different flow patterns observed in microchannel geometry such as bubble nucleation in thin film, periodic variation of flow pattern, flow circulation, bubble suppression and cross-channel flow are explained briefly. The influence of vapour quality, heat flux, mass flux and channel geometry on pressure drop and heat transfer characteristics of microchannel are reported. Different correlations reported for single and two phase heat transfer characteristics are compared. The comparative analysis showed that available single phase and two phase correlations are inconsistence and large variation is observed among these correlations for same channel geometry, fluid and operating condition. Different instabilities associated with microchannels are also briefly presented.

Numerical investigation of fluid flow and heat transfer under high heat flux using rectangular micro-channels

A 3D-conjugate numerical investigation was conducted to predict heat transfer characteristics in a rectangular cross-sectional micro-channel employing simultaneously developing single-phase flows. The numerical code was validated by comparison with previous experimental and numerical results for the same micro-channel dimensions and classical correlations based on conventional sized channels. High heat fluxes up to 130W/cm2 were applied to investigate micro-channel thermal characteristics. The entire computational domain was discretized using a 120×160×100 grid for the micro-channel with an aspect ratio of (α=4.56) and examined for Reynolds numbers in the laminar range (Re 500–2000) using FLUENT. De-ionized water served as the cooling fluid while the micro-channel substrate used was made of copper. Validation results were found to be in good agreement with previous experimental and numerical data [1] with an average deviation of less than 4.2%. As the applied heat flux increased, an increase in heat transfer coefficient values was observed. Also, the Reynolds number required for transition from single-phase fluid to two-phase was found to increase. A correlation is proposed for the results of average Nusselt numbers for the heat transfer characteristics in micro-channels with simultaneously developing, single-phase flows.

Thermal Performance of Microchannels With Wavy Walls for Electronics Cooling

Wavy walls are investigated in this paper as a passive scheme to improve the heat transfer performance of low-Reynolds-number laminar flows in microchannel heat sinks for electronics cooling applications. 3-D laminar fluid flow and heat transfer characteristics in microchannels with wavy walls are numerically studied for a 500-μm hydraulic diameter channel by varying the wavy feature amplitude at different Reynolds numbers (10, 20, 50, and 100). In addition, flow measurements are made using a micrometer-resolution particle image velocimetry technique for understanding the fundamentals of fluid flow in the wavy-walled microchannels for the considered Reynolds numbers. Based on the comparison with straight channels, it was found that wavy channels can provide improved heat transfer performance while keeping the pressure drop within acceptable limits. Accordingly, wavy channels are to found to provide an improvement of up to 26% in the overall performance (which includes the effect of wall waviness on heat transfer, pressure drop, and surface area) compared to microchannels with straight walls for the same pumping power and hence are attractive candidates for cooling of future electronics.