In the past decades, the Lattice Boltzmann method has gained much success in variety fields especially in multiphase flow, porous media flow, and other complex flow, and become a promising method for computational fluid dynamic (CFD). The outlet boundary condition (OBC) and its numerical scheme are critical issues in CFD, which may influence the accuracy and stability of the calculation. The common OBCs i.e. Neumann boundary condition (NBC), extrapolation boundary condition (EBC), and convection boundary condition (CBC), which have been widely investigated in single-phase LB model, have rarely been investigated in multiphase LB model. The previous research on the OBCs for two-phase LB model only aims at small density ratio. While in most industrial applications, the density ratio often ranges from a hundred to a thousand, and a large density ratio would bring some problems such as parasitic current and bad stability in LB method. Lee and Fischer have proposed an improved LB model which is suitable for large density ratio two-phase flow. In order to assess the OBCs for large density ratio LB model, the OBCs are investigated. And it is found that the existing OBC numerical scheme cannot be directly applied to the large density ratio LB model. In present study, a novel numerical scheme for the OBCs is proposed assuming that the outlet velocity is gained by the outlet boundary condition instead of the momentum equation which is an improvement of previous scheme, and it can be used in large density ratio LB model. The performance of the proposed OBC scheme is examined for different density ratios. The results show that the proposed OBC scheme could converge in a stable manner. Comparing with the reference flow condition, the CBC scheme shows a better performance than the NBC scheme and the EBC scheme. The NBC scheme would lead a large droplet deformation, large velocity peaks at the outlet, and large errors for both small and large density ratio. And the EBC scheme keeps a good droplet shape, but it would lead large velocity peaks at the outlet and large error when large density ratio is considered. The CBC scheme always shows superior performance including a good droplet shape, smooth outlet velocity profile, and small errors no matter whether the density ratio is small or large. Hence the CBC scheme could be applied in large density ratio LB model for the outlet boundary condition, which has a good accuracy and stability in the calculation.

Self-assembly of sub-micron particles suspended in a water film is investigated numerically. The liquid medium is allowed to evaporate leaving only the sub-micron particles. A coupled CFD-DEM approach is used for the simulation of fluid-particle interaction. Momentum exchange and heat transfer between particles and fluid and among particles are considered. A history dependent contact model is used to compute the contact force among sub-micron particles. Simulation is done using the open source software package CFDEM which basically comprises of two other open source packages OpenFOAM and LIGGGHTS. OpenFOAM is a widely used solver for CFD related problems. LIGGGHTS, a modification of LAMMPS, is used for DEM simulation of granular materials. The final packing structure of the sub-micron particles is discussed in terms of distribution of coordination number and radial distribution function (RDF). The final packing structure shows that particles form clusters and exhibit a definite pattern as water evaporates away.

Water vapor condensing process inside horizontal tube has been studied in the paper. A CFD simulation model has been developed with Fluent software. An experimental investigation has also been conducted with an 8-mm inside diameter horizontal tube. As well as the heat transfer correlations available in literatures, the CFD model has been validated against the experimental results. The discrepancy of the simulation model is within 15%. The effect of wall temperature, inlet vapor velocity and inlet superheating degree on local heat transfer coefficient has been analyzed. The simulation model can be employed in future work to develop a condensation heat transfer correlation in horizontal tube.

Microchannel heat exchangers have become widely employed in modern systems, found within aerospace applications, waste heat recovery, water treatment processes, air conditioning, biomedical treatments and various industrial process applications. The microchannels increase the ratio of heat transfer surface to volume, thus improving the heat transfer performance significantly whilst reducing the overall weight and size. Moreover, by utilizing secondary flow from Dean Vortices induced by curved microfluidic channels, the fluid flow and heat transfer performance can be enhanced even further beyond conventional straight channels. However, since pressure drops found in microchannels are often quite high, channel lengths must be kept relatively short to balance the friction loss and energy consumption. Due to this, the developing region length at the microchannel entrance area has a greater impact than for macroscale channels, in terms of hydrodynamic and thermal performance over the remaining full developed region. The thermo-hydraulic design for heat transfer microchannel surfaces is strongly dependent on several dimensionless performance indicators, namely Nusselt number ‘Nu’ for heat transfer, and Poiseuille number ‘Po’, which is the product of Fanning friction factor ‘f’ and Reynolds number ‘Re’. These parameters are used to characterize and optimize the performance of microchannel surfaces and heat exchangers in general, also can be used to determine both the thermal and hydraulic developing region lengths at the channel entrance area. Whilst many such studies exist for theoretical analysis and experimental verifications, currently there is little literature on the developing region lengths and impacts researched through the method of Computational Fluid Dynamics (CFD). As such, this paper identifies and explores via quantitative analysis the hydraulic and thermal performance changes created by the relevant developing region lengths at the entrance area of spiral microchannels, as well as determinations and comparisons of these effects over straight channels. The numerical results, generated via COMSOL Multiphysics and contrasted with previous literature on the subject, also compared with the effect of the developing region on the effectiveness and efficiency of both spiral and straight microchannels, finding an improved heat transfer performance but an increased impact of hydraulic friction as well for spiral channels against straight counterpart. Furthermore, significant differences between thermal developing region length and hydraulic developing region length can be observed throughout, which illustrates high challenge and the need for compromise in microchannel design. In this way, implications for the configuration and design of industrial microchannels and micro heat exchangers are self-evident. All the key factors given in this paper are dimensionless, and thus the generated results can be utilized for a variety of flow conditions. Hence, this work should permit an increased understanding for and boost the curved microchannel and micro heat exchanger designs subsequently, through reducing the required numbers of tests and experiments and expediting the development for similar applications followed.

In this study, we simulate the flow and heat transfer during hot-wire anemometry and investigate its thermal behavior and physics using the Computational Fluid Dynamics (CFD) tool. In this regard, we use the finite-volume method and solve the compressible Navier-Stokes equations numerically in slightly non-continuum flow fields. We do not use any slip flow model to include the transitional flow physics in our simulations. Using the CFD method, we simulate the flow over hot–wire and evaluate the uncertainty of CFD in thermal simulation of hot-wire in low transitional flow regimes. The domain sizes and the mesh distributions are carefully chosen to avoid boundary condition error appearances. Following the past researches, we do not take into account the conduction heat transfer passing through hot-wire mounting arms in our simulations. Imposing a fixed temperature condition at the face of hot-wire, we simulate the flow over and the heat transfer from hot-wire and calculate the convection heat transfer coefficient and the local Nusselt number values. To be sure of the accuracy of our CFD code, we simulate a number of similar test cases and compare our numerical solutions with the available numerical solutions and/or experimental data.

Absorber is an important component in absorption refrigerating system. Its performance plays a significant role on the overall efficiency of absorption refrigerating system. The nanofluids which can enhance the heat and mass transfer will be utilized to absorber for enhancing the water vapor absorption process and improving the absorber efficiency. The software CFD-FLUENT is used to analyze the falling film absorption process of the nanofluids, which consists of H 2 O/LiBr solution with Fe 3 O 4 nanoparticles in this paper. The results indicate that the enhancing heat and mass transfer of nanofluids is related to the nanoparticle concentration and size. The stronger the nanoparticle concentration, the greater enhancement of heat and mass transfer of falling film; while the smaller the nanoparticle size, the greater enhancement of heat and mass transfer of falling film. It is also found that the enhancement ratio of heat and mass transfer flux reach 1.48 and 1.37, respectively, as the Fe 3 O 4 nanoparticles mass concentration of 0.01wt% and the size of 50nm.

The heat transfer performance of fluid flowing in a microchannel was experimentally studied, to meet the requirement of extremely high heat flux removal of microelectronic devices. There were 10 parallel microchannels with rectangular cross-section in the stainless steel plate, which was covered by a glass plate to observe the fluid flowing behavior, and another heating plate made of aluminum alloy was positioned behind the microchannel. Single phase heat transfer and fluid flow downstream the microchannel experiments were conducted with both deionized water and ethanol. Besides experiments, numerical models were also set up to make a comparison with experimental results. It is found that the pressure drop increases rapidly with enlarging Reynolds number (200), especially for ethanol. With comparison, the flow resistance of pure water is smaller than ethanol. Results also show that the friction factor decreases with Reynolds number smaller than the critical value, while increases the velocity, the friction factor would like to keep little changed. We also find that the water friction factors obtained by CFD simulations in parallel microchannels are much larger than experiment results. With heat flux added to the fluid, the heat transfer performance can be enhanced with larger Re number and the temperature rise could be weaken. Compared against ethanol, water performed much better for heat removal. However, with intensive heat flux, both water and ethanol couldn’t meet the requirement and the temperature at outlet would increase remarkably, extremely for ethanol. These findings would be helpful for thermal management design and optimization.

The Wavy channels are supposed to enhance performance of microchannel heat sink through chaotic advection. The change in boundary layer thickness (thinning) and the macroscopic mixing due to the formation of Dean’s vortices have been found to be main reasons for enhanced heat transfer in wavy microchannel. Present study carries out a detailed numerical investigation for flow and heat transfer in wavy channel. A 3D geometry for a single loop of wavy channel is modeled in GAMBIT and simulated in CFD software FLUENT. The basic dimensions were 0.15 mm width, 0.3 mm height and 1.5 mm length. The formation of Dean vortices are shown. In parametric study, the effect of Re number on the flow and heat transfer performance is shown. Heat transfer was found to be increased with Re. The effect of Aspect ratio is shown. The channel with the aspect ratio of 0.5 is found to be best among the channels studied including wavy and straight microchannels. A novel concept of secondary branches is introduced to wavy microchannel to take advantage of high pressure zone at crust. The branched wavy microchannel encouraged the secondary flow thus enhanced the macroscopic mixing. Due to disrupt of boundary layer development and its re-initialization, an improved thermal performance was achieved.

Washing, separation and concentration of bioparticles are key operations for many biological and chemical analyses. In this study, the simulation of an integrated microfluidic device is studied. The proposed device has the capability to wash the bioparticles (transferring the bioparticles from one buffer solution to another), to separate the particles based on their dielectric properties and to concentrate the bioparticles. Washing and concentration of bioparticles are performed by acoustophoresis and the separation is performed by dielectrophoresis. For simulating the flow within the microchannel, a computational fluid dynamics model using COMSOL Multiphysics software is implemented. In order to simulate the particle trajectories under ultrasonic and electric field, point-particle assumption is chosen using MATLAB software. To account for the size variation of the bioparticles, particles with normal size distributions are used inside the microchannel. The effect of the key design parameters such as flow rate, applied voltage etc. on the performance of the device is discussed.

The efficiency in photovoltaic modules decreases as the cell temperature increases. It is necessary to have adequate thermal management mechanism for a photovoltaic module, especially when combined with building facade. This study aims to investigate the thermal and electrical performance of a photovoltaic module integrated with a microencapsulated phase change material layer under temporal variations of daily solar irradiation and exterior ambient temperature, via computational fluid dynamics simulations. The results show that the melting temperature and aspect ratio of the microencapsulated phase change material layer have significant effects on the thermal and electrical performances of photovoltaic modules.

Experimental investigations are conducted to determine the two-phase frictional pressure drop of refrigerant R134a and the thermal entrance length for flow in a helically coiled mini-tube of 2mm inner diameter. The objective of this work is to study the effect of the tube curvature on the frictional pressure drop in two-phase flow as well as the thermal entrance length in laminar single phase flow. The two-phase frictional pressure drop is investigated through flow boiling of the refrigerant under uniform wall temperature boundary conditions. A generalized correlation is proposed to predict the single phase and two-phase frictional pressure drop through the coiled tube. A large set of experimental data is collected to evaluate the prediction performance of the proposed two-phase correlation. The results of the prediction are fairly good when compared with the measured pressure drop from experiments. A Computational Fluid Dynamics (CFD) analysis is also performed to compare with the thermal entrance length study from experiments. The results from the experimental analyses and CFD are seen to be in good agreement. It is found that the thermal entrance length for the coiled tube is 27–103.3% larger than a straight tube of equal length and cross-section.

This paper describes the CFD analysis of single rectangular microchannel for hydraulic diameter 319 μm. While CFD analysis the Nusselt number observed is 4 to 5 with different Reynolds Number variation for flow rate of 0.001 kg/sec to 0.012 kg/sec. The current work describes CFD analysis of single microchannels for length of 50 mm with water as a fluid medium with laminar flow. Computational Fluid dynamics analysis of Single rectangular microchannel Single rectangular microchannel of 319 μm hydraulic diameter is analyzed to study the flow characteristics in the inlet, microchannel test section and outlet test section with ANSYS CFX-11 for pressure drop, temperature drop, velocity counter of single micro-channel. For analyzing the weather the turbulence is created at inlet part of the microchannel a pressure drop analysis is carried for flow rate of 0.012 kg/sec with heat input 5.33 watt/cm 2 under laminar flow consideration. For analyzing the temperature profile across microchannel a for flow rate of 0.012 kg/sec with heat input 5.33 watt/cm 2 under laminar flow is considered.. For single microchannel the temperature rise of water is in range of 1 °K to 2 °K at center plane of microchannel. It is found that at leading edges or leaving edge the temperature rise in water is higher as compare to entering edge of microchannel. It is due to while entering to leaving of water particles in microchannel it collapse each other and try to increasing friction along each other so at outlet or leading edge the temperature rise is seen higher as compare to in let portion of single microchannel.

Emulsification is an important process in various fields including foods, pharmaceuticals, cosmetics, and chemicals. Emulsification operation is commonly conducted using conventional emulsification devices, such as high-speed blenders, colloid mills, high-pressure homogenizers, and ultrasonic homogenizers. However, these emulsification devices result in the production of polydisperse emulsions with wide droplet size distributions and poor controllability in droplet size and its distribution. In contrast, monodisperse emulsions consisting of monosize droplets have received a great deal of attentions over the past decade due to their high-tech applications, e.g., monosize microparticles as spacers for electronic devices and monosize micro-carriers for drug delivery systems (DDS). Our group proposed microchannel (MC) emulsification as a promising technique to produce monodisperse emulsions in the mid 1990s. Micro/Nanochannel (MNC) emulsification enables generating monosize droplets with the smallest coefficient of variation (CV) of below 5% using MC and nanochannel (NC) arrays of unique geometry. The resultant droplet size, which ranged from 0.5 to 200 μm, can be precisely controlled by channel geometry. Droplet generation for MNC emulsification is very mild and does not require any external shear stress; a dispersed phase that passed through channels is transformed spontaneously into monosize droplets inside a continuous-phase domain. The aim of this paper is to present recent developments in MNC emulsification chips, particularly focusing on asymmetric straight-through MC arrays for large-scale production of monodisperse emulsions. Asymmetric straight-through MC array chips were fabricated using a silicon-on-insulator wafer. Numerous asymmetric straight-through MCs each consisting of a microslot and a narrow MC were positioned in the central region of the chip. Monosize droplets were stably generated via asymmetric straight-through MCs at high production rates. Below a critical droplet production rate, monosize droplets were generated via asymmetric straight-through MCs, with droplet size and size distribution independent of the droplet productivity. The use of a large asymmetric straight-through MC array chip achieved the mass production of monosize tetradecane oil droplets at ∼1 L/h. The simulation results using CFD (computational fluid dynamics) agreed well with the experimental results and provided useful information, such as the movement of the oil-water interface during droplet generation. Monosize submicron droplets were also obtained using NC emulsification chips made of single-crystal silicon.

The study of the natural convection over a very small heat sources is important in the analysis of heat transfer problems in the electronics industry. However, the characteristics of the spatial distribution of the velocity in the near wall region, which is crucial to the mechanisms of heat transfer process in natural convection around a microscale object, is not well understood. In this investigation, the microscale natural convection in the near wall region of a platinum micro heat source was investigated numerically, using FLUENT, a commercially available computational fluid dynamics (CFD) software, and compared with corresponding experimental results. The influence of the nanoparticles on the natural convection was observed using the single-phase or two-phase models available in FLUENT. The temperature and velocity fields were obtained, with which the Brownian diffusion coefficient was deduced. The results indicate that the temperature gradient induced Brownian diffusion and thermophoresis in the near wall region plays an important role in the microscale natural convection in the water/nanoparticle mixture investigated and are in good agreement with the results from a corresponding experimental investigation.

As physical model used to interpret the fluid behavior when it passes through a semi-permeable membrane, Kedem-Ketchalsky’s (K-K) equations [1, 2] were successfully used in a bunch of filtration processes. However, because they were developed and dedicated to normal ultra- and micro-filtration systems, their limitations were obviously observed in some sort of nano-filtration processes that the pore size of the filtration membrane is only several nanometers. This paper analyzed the feasible utilization scopes of K-K equations. And two methods, Molecular Dynamics (MD) method and Computational Fluid Dynamics (CFD) method, used respectively to analyze nano- and ultra-filtration processes are introduced in this paper.

This paper presents computational simulations for internal condensing flows over a range of tube/channel geometries — ranging from one micro-meter to several millimeters in hydraulic diameters. Over the mm-scale, three sets of condensing flow results are presented that are obtained from: (i) full computational fluid dynamics (CFD) based steady simulations, (ii) quasi-1D steady simulations that employ solutions of singular non-linear ordinary differential equations, and (iii) experiments involving partially and fully condensing gravity driven flows of FC-72 vapor. These results are shown to be self-consistent and in agreement with one another. The paper demonstrates the existence of a unique solution for the strictly steady equations for gravity and shear driven flows. This paper also develops useful correlations for shear driven and gravity driven annular stratified internal condensing flows (covering some refrigerants and common operating conditions of interest). A useful map that marks various transitions between gravity and shear dominated annular stratified flows is also presented. For the micro-meter scale condensers, computations indentify a critical diameter condition (in non-dimensional terms), below which the flows are insensitive to the orientation of the gravity vector as the condensate is always shear driven. Large pressure drop, importance of surface tension, and vapor compressibility for μm-scale flows are also discussed. With the help of comparisons with 0g flows, the paper also discusses effects of transverse gravity on the solutions for horizontal channel flows.

The flow and heat transfer characteristics of water in smooth rectangular microchannels had been simulated numerically by using CFD method. Computational results showed that the ratio of the channel length and hydraulic diameter, hydraulic diameter, the ratio of height and width of microchannels, porous ratio had different effects on the flow and heat transfer of microchannels. On the condition of the ratio of the length and hydraulic diameter of channel greater than 70, thereby the entrance effects could be neglected, the effect of hydraulic diameter, the ratio of height and width of microchannels and porous ratio on flow and heat transfer in microchannels had been simulated respectively. The mechanism of flow and heat transfer of each condition had been obtained, which could offer important data to discuss the mechanism of flow and heat transfer in microchannels further and realize the goal of optimizing design of microchannel heat exchanger.

Roughness effects on flow and heat transfer in flat microchannels has been numerically simulated by using CFD with fluid-solid conjugate heat transfer techniques, the surface roughness has been modeled through a series triangular toothed roughness cells. In this paper, the influence for roughness on the entrance length of flow and heat transfer has been emphasized, the influence for relative roughness on transitional Reynolds number has been also analyzed at the same time.

Nanofluid has the promising potential for enhancing the heat transfer performance of conventional fluids. Several experimental and numerical attempts have been made earlier to investigate its important thermo physical properties like thermal conductivity and viscosity. The findings and results are quite disperse instead of reaching a definitive agreement. This paper presents effective viscosity measurements of CuO and ZnO nanofluids experimentally. A Brookfield viscometer model DV-I Prime with a CPE 40 cone has been used to determine the effective viscosity of nanofluids. The measurements have included the effect of volume concentration of nanoparticles and temperature. The experimental results are compared with several experimental and theoretical models available in the existing literature. From the obtained experimental results it can be concluded that the viscosity values of the above mentioned nanofluids has a tendency to increase with increase of nanoparticle concentration and follows a decreasing trend with an increase in temperature. Presented results can be used to define the above mentioned nanofluids within the experimental volume concentration range in CFD software package and hence to predict overall heat transfer performance using these nanofluids.

A valve-less micro-pump was realized with just one diffuser/nozzle element. The pressure-loss in a nozzle is lower than that in a diffuser, and therefore one-way flow may be realized in the nozzle direction. The frequency characteristics and the pump characteristics are measured. Dimensionless numbers are introduced to rearrange the measured data and to understand the physical mechanisms of the micro-pump. Simplified analysis was done for unsteady operation of the pump by considering the channel geometries and pressure-loss coefficients based on Bernoulli’s theorem. The calculated pump characteristics agreed with the measured ones. Numerical calculations were made using the commercial CFD (computational fluid dynamics) code CFX. The calculated flow patterns showed differences between the diffuser and nozzle directions.