Low-Reynolds-number laminar channel flow is used in various heat/mass transfer applications, such as cooling and mixing. A low Reynolds number implies a low intensity of heat/mass transfer processes, since they rely only on the gradient diffusion. To enhance these processes, an active flow control by means of synthetic (zero-net-mass-flux) jets is proposed. This arrangement can be promising foremost in microscale. The present study is experimental in which a Reynolds number range of 200–500 is investigated. Measurement was performed mainly in air as the working fluid by means of hot-wire anemometry and the naphthalene sublimation technique. PIV experiments in water are also discussed. The experiments were performed in macroscale at the channel cross-section (20×100)mm and (40×200)mm in air and water, respectively. The results show that the low Reynolds number channel flow can be actuated by an array of synthetic jets, operating near the resonance frequency. The control effect of actuation and the heat transfer enhancement was quantified. The stagnation Nusselt number was enhanced by 10–30 times in comparison with the non-actuated channel flow. The results indicate that the present arrangement can be a useful tool for heat transfer enhancement in various applications, e.g., cooling and mixing.

The three dimension physico-mathematical model was established for the integrated planar solid oxide fuel cell (IP-SOFC) with the couples of multi components flow of reacting gas, heat transfer and electro-chemical process in order to reveal the inherent multi-scale effect of gas distributing duct and the porous support layer, and also, the microscale effect on the transport process in fuel cell. The mutual influences between heat transfer and chemical components transport were included in the model. In addition, the thermal effect of chemical reactions and its influences on polarizations of fuel cell were considered. And also, besides the Darcy diffusion, the Knudsen diffusion in the sub-microscale structure of the porous support is taken into consideration. Numerical simulation was employed to solve the model, by which, the output performance and polarization characteristics of a single cell were analyzed and compared for electrolyte-supported, anode-supported and cathode-supported SOFC, respectively. The present model was also validated comparing with the experimental data.

We present detailed data on the performance of microstructured geometries for use in the evaporator section of a vapor chamber heat pipe. The central innovation of the geometries is their hierarchical structure, involving the use of large microchannels in order to reduce liquid flow drag while fabricating microscale pin fin arrays whose small pores increase capillary suction. The overall conductance in such a geometry is dependent on the extent of thin liquid film (thickness ∼few microns), which is manipulated by increasing the surface area-to-volume ratio through the use of microstructuring. Experiments were conducted for a heater area of 1cm 2 , with the wick in a vertical orientation. Results are presented for fixed microchannel widths of 30–60 microns, with pin fin diameters ranging from 4 to 32 microns, and pin fin array widths of 150 to 300 microns. The competing effects of increase in surface area due to microstructuring, and the suppression of evaporation due to reduction in pore scale are explored. In the evaporative regime, conductances of the order of 6 W/cm 2 -K are attained at heat fluxes of up to 140 W/cm 2 , until the capillary limit is reached and the wick dries out.

Vapor chambers are often used as spreaders to dissipate high heat fluxes by taking advantage of liquid-vapor phase change. Wicking of the working fluid in vapor chambers is accomplished through capillary action, which is strongly affected by the wick structure. Traditionally, copper meshes with micrometer-scale pore sizes have been used as wicking structures, but it is expected that heat fluxes in the next generation of high-power electronic devices will cause boiling in these devices and lead to dryout with conventional wick materials. With a goal of increasing maximum heat dissipation and reducing thermal resistance, a wick structure composed of both conventional copper mesh and carbon nanotubes has been developed and characterized. The high-permeability mesh provides for a low-resistance bulk flow path while the carbon nanotubes, with their high thermal conductivity and high surface area, modify the wick surface for enhanced capillary action. CNT-enhanced integrated wicks were fabricated by sintering a copper mesh on Cu-Mo-Cu substrates, on which CNTs were grown. A thin layer of copper was evaporated onto the CNTs to improve wicking and wettability with water, the working fluid of interest. Samples grown under varying degrees of positive bias voltage and varying thicknesses of post-CNT-growth copper evaporation were fabricated, so that the surface morphology of the samples could be varied. The resultant boiling curves and associated wick thermal resistances indicate that micro/nano integrated wicks fabricated with higher positive bias voltages during CNT synthesis, and thicker copper coatings, lead to improved thermal performance and lower wick thermal resistance. Notably, heat fluxes at the heater surface of greater than 500 W/cm 2 were observed without reaching a critical heat flux condition.

International attention has been considerably paid for the technology of CO 2 capture and storage (CCS) these days because of global warming. Among the technologies of CCS, the prediction of CO 2 behavior in underground is very crucial for CO 2 storage design. In the present study, a pore-scale level model is developed to mimic real porous structure and by using CFD (computational fluid dynamics) the characteristics of the CO 2 flow and thermal fields are investigated in the pore-level micro porous structure. Varying the CO 2 injection conditions such as flow rate, pressure and temperature, the behavior of CO 2 is calculated. Especially, the physical conditions such as temperature and pressure are set up equivalent to the underground condition at which CO 2 is injected. From the results, the injection condition of CO 2 is very critical to the characteristics of the flow and thermal fields in the micro-scale porous structure.

In recent years considerable attention has been paid to the study of microscale flow and heat transfer with phase change and chemical reactions. This article reviews the patterns of the microscale two-phase gas-liquid flow, the statistical parameters of slug flow and capillary phenomena in annular flow for a rectangular microchannel. The evaporative and condensing heat transfer model for the curved liquid microfilm in microchannel and near contact line is developed and discussed. The influence of forced convection, nucleate boiling and thin film evaporation on microscale flow boiling heat transfer is reviewed and analyzed. The model of forced boiling heat transfer in microchannel is developed and compared with the existing experimental data. The mechanism and patterns of microscale explosive evaporation in the MEMS system is determined at high external heat flux density and the acousto-thermal model of the explosive evaporation is considered. The results of calculations are compared with the experimental data. The peculiarities of heat and mass transfer in a micro channel with surface catalytic reactions producing the hydrogen are presented. The kinetics of sequence of chemical reactions at nanoscale catalyst under conditions of significant nonuniformity of temperature and species concentration fields is considered.

Multiscale simulation is a rapidly evolving area of research that will have a great impact on computational mathematics and numerical modeling in engineering. In this keynote lecture following parts are included. First, what is multiscale problem. In the thermal and fluid science multiscale problems may be classified into two categories: multiscale process and multiscale system. By multiscale process we mean that the overall behavior is governed by processes occur at different length scales. By multiscale system we refer to a system that is characterized by a large variation in length scales. The cooling of an electronic system is such a typical multiscale system. Existing numerical methods for three geometric scales (macro, meso and micro) are briefly mentioned. In the second part the necessity of multiscale simulation is discussed. Examples are provided for multiscale process and multiscale system. In this lecture focus is put on the simulation of multiscale process. In the third section numerical approaches developed for the simulation of multiscale processes are presented. There are two types of simulation approaches. One is the usage of a general governing equation and solving the entire flow field involving a variation of several orders in characteristic geometric scale. The other is the so-called “solving regionally and coupling at the interfaces”. In this approach the processes at different length level is simulated by different numerical methods and then information is exchanged at the interfaces between different regions. The exchange of information should be conducted in a way that is physically meaningful, mathematically stable, and computationally efficient. The key point is the establishment of the reconstruction operator, which transforms the data of few variables of macroscopic computation to large amount of variables of microscale or mesoscale simulation. For different coupling cases the existing methods for such operators are briefly reviewed. In the fourth part, four numerical examples of multiscale simulation are presented: liquid flow in nanochannels with roughness by using MDS and FVM, flow and heat transfer in a micro nozzle by using DSMC in fluid and FVM in solid, flow past a cylinder and natural convection heat transfer in a square cavity by using coupled FVM and LBM. Finally, it is pointed out that we have a long way to go in order to have a successful full multiscale simulation for the complicated engineering problems as transport process in PEMFC and refrigerant condensation process on a enhanced surface. Further researches are highly required to establish robust and quick-convergent numerical solution approaches. Some further research needs are proposed.

Complex macroscale and microscale heat and mass transfer phenomena encountered in several thermal energy storage and transport systems are discussed. Thermal storage and transport systems involving ice slurries and nanoemulsions of phase change materials can be used for either cooling or heating applications or both, which can contribute to the reduced usage of electricity during peak hours. But heat and mass transfer and stability issues are encountered in the production, transport and storage of the heat storage media. Both the heat transfer enhancement effect and detrimental effects such as Ostwald ripening and supercooling will be discussed. Another interesting microscale phenomenon recently encountered in energy transport devices such as heat pipes is the enhancement of heat transport with the use of self-rewetting fluids. Critical heat fluxes in boiling can be enhanced by up to 300% and this helps prevent liquid dryout at high heat fluxes in different types of heat pipes. Both the nature of the enhancement effect and possible mechanisms will be discussed.

Chip microscale liquid-cooling reduces conductive and convective resistance thereby improving the efficiency of datacenters by allowing coolant temperatures above the free cooling limit in all climates. This eliminates the need for chillers and allows the thermal energy to be re-used in cold climates. Replacing the combustion processes for secondary users with recycled heat from the datacenter effectively eliminates carbon dioxide emission during the winter season and reduces operating cost throughout the year. The energy balance of emission-reduced datacenters is compared with a classical air cooled datacenter, a datacenter with free cooling in a cold climate zone, and a datacenter with chiller-mediated energy re-use. Hot water cooled datacenters reduce the effective energy cost by almost a factor of two compared to a current datacenter and reduce the carbon footprint by an even larger factor. Our energy re-use concept has been demonstrated in terms of cost and energy savings in a 60°C liquid cooled supercomputer. Additional alternative energy re-use schemes in hot climates for desalination and adsorption cooling allow close to full use of datacenter heat in all climates and all seasons. Output temperatures for these applications compared to space heating need to be 10–20°C higher which becomes possible through hotspot adapted cooling that eliminates mixing of fluids with different temperatures. In addition, interlayer cooled chip stacks allow double sided hotspot optimized cooling even closer to the heat source with low flow rates and low pumping power. This improves the large efficiency gain that becomes possible through 3D chip stacking.

An experimental study is conducted to investigate the micro-scale heat transfer at an evaporating moving 3-phase contact line. The moving evaporating meniscus is formed by pushing or sucking a liquid column of HFE7100 in a vertical channel of 600 μm width using a syringe pump. The gas atmosphere is pure HFE7100 vapor. This channel is built using two parallel flat plates. A 10 μm thick stainless steel heating foil forms a part of one of the flat plates. Two-dimensional micro-scale temperature field at the back side of the heating foil is observed with a high speed infrared camera with a spatial resolution of 14.8 μm × 14.8 μm and an in-situ calibration procedure is used at each pixel element. A high speed CMOS camera is used to capture the shape of the moving meniscus, the images are post-processed to track the free surface of the meniscus. Local heat fluxes from the heater to the evaporating meniscus are calculated from the measured transient wall temperature distributions using an energy balance for each pixel element. In the vicinity of the 3-phase contact line the heat flux distribution shows a local maximum due to high evaporation rates at this small region. The local maximum heat flux at the 3-phase contact line area is found to be dependent on the input heat flux, the velocity and the direction of the meniscus movement. The results give detailed insight into the specific dynamic micro-scale heat and fluid transport process.

Heat transfer coefficients and pressure drops during condensation of carbon dioxide (CO 2 ) are measured in small quality increments in microchannels of 100 < D h < 200 μm. Channels are fabricated on a copper substrate by electroforming copper onto a mask patterned by X-ray lithography, and sealed by diffusion bonding. The test section is cooled by chilled water circulating at a high flow rate to ensure that the thermal resistance on the condensation heat transfer side dominates. A conjugate heat transfer analysis in conjunction with local pressure drop profiles allows driving temperature differences, heat transfer rates, and condensation heat transfer coefficients to be determined accurately. Heat transfer coefficients are measured for G = 600 kg m −2 s −1 for 0 < x < 1 and multiple saturation temperatures. Preliminary results for a 300 × 100 μm (15 channels) test section are presented. These data are used to evaluate the applicability of correlations developed for larger hydraulic diameters and different fluids for predicting condensation heat transfer and pressure drop of CO 2 .

The recent first-principle model shows a dual-phase-lagging heat conduction in nanofluids at the macroscale. The macroscopic heat-conduction behavior and the thermal conductivity of nanofluids are determined by their molecular physics and microscale physics. We examine numerically effects of particle-fluid thermal conductivity ratio, particle volume fraction, shape, aggregation, and size distribution on macroscale thermal properties for nine types of nanofluids, without considering the interfacial thermal resistance and dynamic processes on particle-fluid interfaces and particle-particle contacting surfaces. The particle radius of gyration and non-dimensional particle-fluid interfacial area in the unit cell are two very important parameters in characterizing the effect of particles’ geometrical structures on thermal conductivity of nanofluids. Nanofluids containing cross-particle networks have conductivity which practically reaches the Hashin-Shtrikman bounds. Moreover, particle aggregation influences the effective thermal conductivity only when the distance between particles is less than the particle dimension. Uniformly-sized particles are desirable for the conductivity enhancement, although to a limited extent.

Study of fluid flow characteristics at microscale level is gaining importance with shrinking device sizes. Better understanding of fluid flow and heat transfer in microchannels will have important implications in biomedical industry, MEMS, electronic chip cooling, heat exchangers, and microfluidic devices. Also, due to short lengths employed in microchannels, entrance header effects can be significant and needs to be investigated. In this work, three dimensional model of microchannels, with aspect ratios (α = a/b) ranging from 0.1 to 10, are numerically simulated using CFD software, FLUENT. Heat transfer effects in the entrance region of microchannel are presented by plotting average Nusselt number as a function of non-dimensional thermal entrance length x*. The numerical simulations with both circumferential and axial uniform heat flux (H2) boundary conditions were performed for four wall, three wall and two wall cases. Large numerical data sets, generated in this work for rectangular cross sectional microchannels for three walls and two walls H2 boundary condition, can provide better understanding and insight into the transport processes in the microchannel.

Pore scale numerical simulation of heat and mass transfer in several foams are realized. 3D geometry is reconstructed from X-Ray tomographic images and fully characterized using the iMorph software. Microscale quantities such as temperature, pressure and velocity fields are computed using commercial software (StarCCM+) based on finite volume method. Macroscale properties are then deduced from numerical data and compared to experimental ones. Impact of foam topology and material as well as fluid nature (Fluid dynamic viscosity, solid thermal conductivity …) on transfer properties are systematically studied. We discuss correlations of these results with geometrical characteristics of the samples by scaling the metal foam in order to change pore diameter.

This paper presents new experimental flow boiling pressure drop results in a microscale tube. The experimental data were obtained under diabatic conditions in a horizontal smooth tube with internal diameter of 2.3 mm. Experiments were performed with R134a as working fluid, mass velocities ranging from 100 to 600 kg/m 2 s, heat flux ranging from 10 to 55 kW/m 2 , saturation temperatures of 31 °C, and exit vapor qualities from 0.20 to 0.99. Flow pattern characterization was also performed from images obtained by high-speed filming. Pressure drops up to 48 kPa/m were measured. These data were carefully analyzed and compared against 13 two-phase frictional pressure drop prediction methods, including both macro- and micro-scale methods. Comparisons against these methods based on the data segregated according to flow patterns were also performed. Overall, the method by Cioncolini et al. [1] provided quite accurate predictions of the present database.

Scale deposition on heat transfer surfaces from water containing dissolved salts reduces the efficiency and performance of heat transfer equipments considerably. Scale deposition could be reduced through physical or chemical methods. In some cases, chemical methods are unacceptable, due to cost, contamination issues, etc. In these cases, physical methods are the only acceptable choices. Surface energy of the heat exchanger has been thought to be one important factor affecting the growth of fouling. Applying low energy surfaces to reduce scaling deposition is one of the effective physical methods. The formation and the characteristics of the calcium carbonate scaling on low energy surfaces have been studied in this paper. Copper and stainless steel surfaces were modified by micro-scale (μm thickness) PTFE (Poly-Tetrofluorethylene) films and nano-scale (nm thickness) thiolate SAMs (Self-Assembly Monolayers). The resulting surface energy of PTFE films and SAMs layers based on copper and stainless steel were significantly reduced compared with the original metal surfaces. To study the formation of the calcium carbonate scale, a recirculation cooling water system was used. The formation of the calcium carbonate scale on PTFE surfaces, SAMs surfaces, polished copper surfaces, and polished stainless steel surfaces were investigated respectively. The rate of calcium carbonate scale formation was decreased and the induction period was prolonged with the decrease of the heat transfer surface energy. The characteristics of the calcium carbonate scale formed on heat transfer surfaces with different surface energies was analyzed with fractal theory after taking photos with SEM (Scanning Electron Microscope). The fractal dimension values of the calcium carbonate scale on different heat transfer surfaces with different surface energies were calculated. The results showed that the fractal dimension values of calcium carbonate scale formed on lower energy PTFE and Cu-SAMs surfaces were greater than those that formed on higher energy Cu and stainless steel surfaces. Results of this study clearly indicated that the formation of calcium carbonate scaling on lower energy heat transfer surfaces is reduced.