Single-phase Flow In Microchannels Research Articles

A review of single flow, flow boiling, and coating microchannel studies

Abstract The objective of this review article is to provide a comprehensive analysis of the latest research on microchannel heat sinks, with a particular focus on single-phase flow, flow boiling, and coating microchannels. The review aims to highlight the progress made in this field and identify the challenges that need to be addressed to promote the widespread adoption of microchannel heat sinks. The review article examines the research on microchannel heat sinks and analyzes the findings related to single-phase flow, flow boiling, and coating microchannels. The analysis of single-phase flow in microchannels covers the impact of various parameters, including channel size, fluid properties, and flow conditions, on the heat transfer rate. The analysis of flow boiling in microchannels includes the identification of the potential advantages and challenges associated with this technique, as well as the strategies developed to mitigate these issues. The analysis of coating microchannels examines the effects of surface coatings on flow behavior and heat transfer performance. The review finds that microchannel heat sinks have gained significant attention due to their potential for high heat transfer rates in a compact size. The analysis of single-phase flow reveals that the heat transfer rate is proportional to the channel diameter and inversely proportional to the fluid viscosity. Increasing the flow rate results in higher heat transfer rates and pressure drops. The analysis of flow boiling reveals that this technique can significantly increase the heat transfer rate compared to single-phase flow. However, it can also lead to flow instability, wall superheat, and premature drying. The review identifies strategies to mitigate these issues, including using different surface coatings, enhancing nucleation, and optimizing channel geometry. The analysis of coating microchannels shows that coatings can alter the surface energy, wettability, and roughness of the channel walls, affecting the flow behavior and heat transfer performance. Coatings made from materials such as metals, polymers, and self-assembled monolayers have been investigated, and their effects on the flow of boiling heat transfer have been analyzed. The review highlights the need for further research to address the challenges associated with microchannel heat sinks, including flow instability and premature drying. Additionally, the review suggests that there is a need for more comprehensive studies on the effects of different surface coatings on flow behavior and heat transfer performance. Future research can also explore the potential of other materials for coating microchannels and investigate new strategies to optimize channel geometry and enhance nucleation. The review article concludes that the progress made in understanding microchannel heat transfer is significant, but further research is necessary to realize the full potential of microchannel heat sinks.

Experimental Study on Microchannel with Addition of Microinserts Aiming Heat Transfer Performance Improvement

Microchannel technology rapidly established itself as a practicable solution to the problem of the removal of extremely concentrated heat generation in present-day cooling fields. By implementing a better design structure, altering the working fluids and flow conditions, using various materials for fabrication, etc., it is possible to increase the heat transfer performance of microchannels. Two parameters that affect how well a microchannel transfers heat were only recently coupled, and the complicated coupling of the parameter that affects how well a microchannel sink transfers heat is still not well understood. The newest industrial developments, such as micro-electro-mechanical systems, high performance computing systems, high heat density generating future devices, such as 5G/6G devices, fuel cell power plants, etc., all present thermal challenges that require the use of microchannel technology. In this paper, single-phase flow in microchannels of various sizes, with or without microinserts, is described in terms of its thermal-fluid flow properties, including fluid flow characteristics and heat transfer characteristics considering the compound effects of variations of channel size and addition of microinserts. The trials were carried out using distilled water that had thermo-physical characteristics that varied with temperature. A microchannel with microinserts was developed for managing the high heat generation density equipment. The fluid flow and heat transfer characteristics are explored and analyzed for Reynolds numbers ranges from 125 to 4992, for 1 mm channel size, and from 250 to 9985, for 2 mm channel size. The cooling performance criteria are pressure drop characteristics, heat transfer characteristics, and overall performance, whereas the testing parameters were chosen for the variations in channel size and the addition of microinserts. The influence of inserting microinserts on microchannels is discussed. Results suggest that by inserting microinserts, the performance of the heat transfer of microchannels is significantly improved and, also, fluid flow resistance is increased. The criteria of the thermal performance factor are employed to assess the overall performance of the microchannel. Significant intensification of heat transfer is observed with indication that the addition of microinserts to microchannels and reduction in channel sizes exhibited improved overall performance.

Pressure drop of single phase flow in microchannels and its application in characterizing the apparent rheological property of fluids

The aim of this work was to experimentally examine apparent rheological properties of fluids in microchannels based on pressure drop. Pressure drops of Newtonian and non-Newtonian fluids in microchannels were measured using pressure sensors. The effects of flow rate, Reynolds number, and viscosity of fluids on the pressure drop of single phase flow in microchannels were investigated, and the measurements of pressure drop were compared with prediction models. For Newtonian fluids, the friction factor in the laminar regime is f/2 = 26/Re. The results show that the pressure drop of Newtonian fluid is in good agreement with the prediction formula proposed by Cornish. However, the pressure drop of non-Newtonian fluid deviates from the theoretical prediction. A power-law is proposed for the relationship between the apparent viscosity of non-Newtonian fluid and the characteristic shear rate in a microchannel based on the pressure drop measurement. This relationship between the apparent viscosity and the characteristic shear rate in a specific microchannel is completely different from the rheological curve measured by cone-and-plate rheometer, due to the heterogeneous distribution of shear rates at the radial direction for non-Newtonian fluids flowing in a microchannel.

Material Selection for Microchannel Heatsink: Conjugate Heat Transfer Simulation

Heat dissipation during the operation of electronic devices causes rise in temperature, which demands an effective thermal management for their performance, life and reliability. Single phase liquid cooling in microchannels is an effective and proven technology for electronics cooling. However, due to the ongoing trends of miniaturization and developments in the microelectronics technology, the future needs of heat flux dissipation rate are expected to rise to 1 kW/cm2. Air cooled systems are unable to meet this demand. Hence, liquid cooled heatsinks are preferred. This paper presents conjugate heat transfer simulation of single phase flow in microchannels with application to electronic cooling. The numerical model is simulated for different materials: copper, aluminium and silicon as solid and water as liquid coolant. The performances of microchannel heatsink are analysed for mass flow rate range of 20-40 ml/min. The investigation has been carried out on same size of electronic chip and heat flux in order to have comparative study of different materials. This paper is divided into two sections: fabrication techniques and numerical simulation for different materials. In the first part, a brief discussion of fabrication techniques of microchannel heatsink have been presented. The second section presents conjugate heat transfer simulation and parametric investigation for different material microchannel heatsink. The presented study and findings are useful for selection of materials for microchannel heatsink.

A facile pressure drop measurement system and its applications to gas–liquid microflows

A system for measuring the pressure drop of a fluid in a microchannel was developed in this study with measurements ranging from 0 to 7 kPa and an accuracy of 1 Pa for constant pressure drop. This system utilizes commercial pressure sensors, self-made amplifiers and a vibration insulation platform to insure accuracy and reproducibility of the results. Pressure calibrations can be conveniently computed using the manufacturers’ datasheet. This measuring system was firstly tested with the pressure drop measurement of single-phase flow in microchannels and the results showed good agreement with theoretical computations. Oscillating pressure drops in the generation of bubbles in T-junction microchannel were studied using the pressure measurement system and their amplitude relatively to the change of working systems is carefully discussed with the comparison of theoretical models from literatures.

A Theoretical Model for Axial Heat Conduction Effects During Single-Phase Flow in Microchannels

A model is developed to analyze the effect of axial conduction on heat transfer during single-phase flow in microchannels. The axial heat conduction in the wall introduces heat flow toward the inlet section resulting in an increase in the local fluid temperature and a corresponding increase in the wall temperature. Neglecting this effect while reducing the experimental data results in a lower value of the experimental Nusselt number. The model derived in this work takes into account this effect and offers a parameter to estimate the effect introduced by the axial heat conduction effect in the wall.

Screening, Selecting, and Designing Microreactors

Microreactors can be used to carry out hazardous organic reactions like nitration of benzene and toluene safely. A theoretical analysis of the performance of these systems is usually based on using a computational fluid dynamics approach incorporating the effect of reactions. These simulations help us identify different flow regimes of the system as stratified flow, vortex flow, and engulfment flow. Though it provides realistic estimates of the performance this is a computationally intensive approach. In this work we discuss two strategies for modeling, which exploits the inherent features of single phase flows in microchannels. The first is based on a one-dimensional model where lateral mixing is incorporated using a pseudo mass transfer coefficient In the second we use an effective dispersion coefficient to represent lateral mixing in microchannels. In both approaches we assume the fluid flows as a plug, that is, with a uniform velocity across the cross-section. We consider two different sets of reactions, (i) a set of parallel reactions and (ii) a set of series parallel reactions and determine how the flow regime in the channel can be exploited to carry out reactions and simultaneously separate the products. The 1D results are compared with those of the 2D model where the transport in the lateral direction is modeled using a dispersion coefficient. The results of the two simplified models are compared with rigorous two-dimensional simulations using Fluent. The approach proposed here can be used to screen and analyze the performance of different reactions in microchannels and examine if the flow features can be used to carry out reactions and separate the products simultaneously. The promising candidates can then be analyzed rigorously for a realistic evaluation of the performance incorporating the prevailing flow structure, using computational fluid dynamics.

The Role of the Viscous Dissipation in Heated Microchannels

Many experimental works appeared in the last decade in the open literature dealing with forced convection through microchannels. The earliest experimental results on single-phase flows in microchannels evidenced that for channels having a hydraulic diameter less than 1mm the conventional continuum models can no longer be considered as able to accurately predict pressure drop and convective heat transfer coefficients. This conclusion seemed to be valid for both gas and liquid flows. Sometimes the authors justified this conclusion by invoking new micro-effects, e.g., electrostatic interaction between the fluid and the walls or scaling effects (axial heat conduction, viscous forces, conjugate heat transfer, wall roughness, and so on). In this paper the role of the viscous dissipation in liquids flowing through heated microchannels will be analyzed by using the conventional theory. We will present a correlation between the Brinkman number and the Nusselt number for silicon ⟨100⟩ and ⟨110⟩ microchannels. It will be demonstrated that the fluid is of importance in establishing the exact limit of significance of the viscous dissipation in microchannels; a criterion to analyze the significance of the viscous effects will be presented. The role of the cross-section aspect ratio on the viscous dissipation will be highlighted. The main goal of this work is to demonstrate that the problem of heat transfer enhancement in microdevices cannot be solved by indefinitely reducing the microchannel dimensions because the viscous dissipation effects shall offset the gains of high heat transfer coefficients associated with a reduction in the channel size.

Evaluation of Single Phase Flow in Microchannels for High Heat Flux Chip Cooling—Thermohydraulic Performance Enhancement and Fabrication Technology

The increased circuit density on today's computer chips is reaching the heat dissipation limits for air-cooling technology. The direct liquid cooling of chips is being considered as a viable alternative. This paper reviews liquid cooling with internal flow channels in terms of technological options and challenges. The possibilities presented herein indicate a four- to ten-fold increase in heat flux over the air-cooled systems. The roadmap for single-phase cooling technology is presented to identify research opportunities in meeting the cooling demands of future IC chips. The use of three-dimensional microchannels that incorporate either microstructures in the channel or grooves in the channel surfaces may lead to significant enhancements in single-phase cooling. A simplified and well-established fabrication process is described to fabricate both classes of three-dimensional microchannels. Proof-of-concept microchannels are presented to demonstrate the efficacy of the fabrication process in fabricating complex microstructures within a microchannel.

FRICTION AND CONVECTION STUDIES OF R-134a IN MICROCHANNELS WITHIN THE TRANSITION AND TURBULENT FLOW REGIMES

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micromilled in aluminum with hydraulic diameters ranging from approximately 112 to 210 w m and aspect ratios that varied from 1.0 to 1.5. Using overall temperature, flow rate, and pressure drop measurements, friction factors and convective heat transfer coefficients were experimentally determined for steady flow conditions. Effects of Reynolds number, relative roughness, and channel aspect ratio are examined in predicting friction factor and Nusselt number for the experiments. Experiment results indicated that transition from laminar to turbulent flow occurred between a Reynolds number of 2,000 and 4,000. Friction factor results were consistently lower than values predicted by macroscale correlations but exhibited the same trends with Reynolds numbers of macroscale correlations. Nusselt number results also exhibited a similar pattern of lower values obtained in the experiments than those predicted by commonly used macroscale correlations. Nusselt number results also indicated that channel size may suppress turbulent convective heat transfer and surface roughness may affect heat transfer characteristics in the turbulent regime.