A comprehensive literature research was undertaken for the available void fraction correlations and experimental void fraction data during condensation inside tubes. Comparisons between the correlations showed that slip ratio models were probably more suitable for the determination of void fraction at low mass velocity when the slip ratio was chosen appropriately. For high mass velocity, there was no apparent difference for the predictions of various models. In addition, the Froude rate parameter (dimensionless number) can be used for representing the flow characteristics during condensation inside tube and was defined in terms of mass flux and quality. Based on the observations made, a new correlation was developed through the weighted average method without resorting back to very complex expressions. This correlation was a simple model and obtained involving with the Froude rate parameter and slip ratio models. The improved correlation has been shown to be in good agreement with data ranging from low mass velocity up to very high mass velocity.

The von Kármán vortex street is a flow instability that is observed in the wake of a blunt body if a certain (cylinder) Reynolds number is exceeded. It is one of the classical problems in fluid mechanics and a vast amount of research has been dedicated to the investigation of the fundamentals of this phenomenon. The present study is concerned with the numerical simulation of the flow in a microchannel having a cylinder located in its channel center. A pressure driven flow is induced in the channel described by the channel Reynolds number. The cylinder is subjected to an externally-applied electric field that causes electroosmosis in the electrical double layer which is present around the cylinder surface. In this setup, two distinctions to the classical von Kármán vortex street can be noted. On the one hand, the presence of the microchannel walls confines the flow field in lateral direction. On the other hand, the electroosmotic slip velocity impacts the flow topology in the vicinity of the cylinder and, thus, may have an impact on the formation and the periodic nature of the von Kármán vortex street. Various numerical simulations are performed to investigate the influence of the cylinder-diameter-to-channel-width ratio and the direction of the electrical field.

Rarefied gas flow plays an important role in the design and performance analysis of micro-electro-mechanical systems (MEMS) under high-vacuum conditions. The rarefaction can be evaluated by the Knudsen number (K n ), which is the ratio of the molecular mean free path length and the characteristic length. In micro systems, the rarefied gas flow usually stays in the slip- and transition-flow regions (10 −3 < K n < 10), and may even go into the free molecular flow region (K n > 10). As a result, conventional design tools based on continuum Navier-Stokes equation solvers are not applicable to analyzing rarefaction phenomena in MEMS under vacuum conditions. In this paper, we investigate the rarefied gas flow by using the lattice Boltzmann method (LBM), which is suitable for mesoscopic fluid simulation. The gas pressure determines the mean free path length and K n , which further influences the relaxation time in the collision procedure of LBM. Here, we focus on the problem of squeezed film damping caused by an oscillating rigid object in a cavity. We propose an improved LBM with an immersed boundary approach, where an adjustable force term is used to quantify the interaction between the moving object and adjacent fluid, and further determines the slip velocity. With the proposed approach, the rarefied gas flow in MEMS with squeezed film damping is characterized. Different factors that affect the damping coefficient, such as pressure of gas and frequency of oscillation, are investigated in our simulation studies.

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling levels, and mass fluxes are considered in this type of heat sink for use in applications with high fluxes up to 300 W/cm 2 . The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 μm and 1 cm radius with a single inlet and a single outlet. The channel is heated on one side through a copper conducting surface, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. Flow boiling heat transfer and pressure drop data were obtained for this heat sink device using water at near atmospheric pressure as the working fluid for inlet subcooling levels from 20 to 81°C and mean mass flux levels ranging from 184 to 716 kg/m 2 s. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface with water equilibrium contact angles in the range of 14–40°, typical of common metal surfaces. The other was a surface coated with zinc oxide nanostructures that are superhydrophilic with equilibrium contact angles measured below 10°. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter as indicated by reduced temperature oscillations during boiling and achieving higher maximum heat flux without dryout. Reducing inlet subcooling levels was also found to reduce the magnitude of oscillations in the oscillatory boiling regime. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm 2 with an average surface temperature of 134 °C and requiring a pumping power to heat rate ratio of 0.01%. The hydrophilic surface retained wettability after a series of boiling tests. Recommendations for use of this heat sink design in high flux applications is also discussed.

Liquid film thickness data in slug flow in a 320 μm diameter capillary tube have been obtained and are compared with existing data and correlations. Solutions of glycerol in water at varying concentrations between 50 and 70% were injected into the capillary tube along with air, at ambient temperature. The thickness of the liquid film was measured using a laser confocal displacement sensor. Gas slug velocity data were obtained from high speed video images recorded at 40,000 frames per second. As liquid viscosity and hence capillary number was reduced, the film thickness around the gas slugs in the capillary tube decreased as expected. The liquid film thickness data were slightly underpredicted by existing correlations.

Addressing the traditionally contradictory problem of obtaining considerable drag reduction without negatively impacting heat transfer as much is an arduous scientific challenge. In this paper, prior efforts on frictional drag reduction and the associated issues are discussed in relevant detail, and the effectiveness of Conducting-Lubricating (CO-LUB) surfaces as one of the potential options to address this challenge for single phase forced convection of liquids is numerically pursued. CO-LUB surfaces have exceptionally high wetting characteristics, and when saturated with a liquid microlayer, provide remarkable lubrication to bulk liquid flow and simultaneously facilitate heat transfer by conduction through the microlayer. In the simulations, the side walls of a high aspect ratio rectangular channel were assumed as CO-LUB surfaces and flow and heat transfer of bulk liquid flow were modeled using ANSYS FLUENT 14.5. Volume-of-Fluid (VOF) method was used to model the two phases with a free surface interface, with water as the microlayer liquid and oil as the bulk liquid, in a narrow channel of 5 mm width and 50 mm length under laminar flow, constant wall heat flux conditions. The results were compared with a regular channel of the same dimensions (without CO-LUB surfaces) and it was found that pressure drop decreased remarkably by ∼23 times for some cases but without any heat transfer attenuation (actually, improved heat transfer performance was observed) leading to highly energy-efficient convective transport.

Some fundamental issues with respect to turbulent flows through a porous matrix are addressed by analyzing DNS results (DNS: direct numerical simulation, i.e. no turbulence modeling). In a porous matrix with pore sizes of micro or even nano dimensions turbulent flow may occur when the (local) Reynolds number is sufficiently large. An open question, however, is whether turbulence structures are restricted in size by the pore size dimensions or not. This is an important aspect that immediately affects the way turbulence has to be modelled. In order to find out which influence the solid matrix has on the turbulence a generic matrix built from a large number of bars with square cross sections is investigated. Two different DNS approaches are used, a finite volume one and a Lattice-Boltzmann approach. From both DNS calculations detailed flow field information about the influence of the solid matrix on the turbulence structure are obtained. Finally the extension of Darcy’s friction law by the Forchheimer term is investigated with respect to the question whether this extended law may be used in the fully turbulent flow regime.

In this paper, heat transfer enhancement using liquid-liquid Taylor flow is examined. The experiments are conducted in mini-scale tubes with constant wall temperature. The segmented flow is created using several fractions of low viscosity silicone oil (1 cSt) and water for a wide range of flow rates and segment lengths. The variety of liquids and flow rates change the Prandtl, Reynolds, and capillary numbers. The dimensionless mean wall flux and the dimensionless thermal flow length are used to analyze the experimental heat transfer data. The comparison shows the heat transfer rate for Taylor flow is higher than in single-phase flow. The heat transfer enhancement occurs due to internal circulation in the fluid segments.

The stability of thin water films on a variety of gold nanostructures was simulated by molecular dynamics simulations. The critical film thickness to prevent a film break-up was investigated as a function of the characteristic length of the nanostructures. Layering of water molecules adjacent to the gold substrate was observed as a result of the strong van der Waals interactions between the water molecules and the gold atoms. A model for the critical film thickness in the presence of nanostructures is developed based on the stability analysis. The model prediction is compared with molecular dynamics simulations with good agreement.

Controlling thermal energy transport (thermal diode) for the desired direction is crucial to improve the efficiency of thermal energy transport, conversion, and storage systems as electrical diodes significantly impact on modern electronic systems. The degree of thermal rectification is measured by the difference between the heat transfer rate in favorable and unfavorable directions to the heat transfer rate in the unfavorable direction. A gas-filled, nano-gap structure with two different surface coatings is considered to design the thermal rectifier. In such a structure where the characteristic length scale is similar to the order of the mean free path of the fluid particles (Knudsen flow regime), the effective thermal conductivity is dominantly controlled by the gas-surface interaction, i.e., thermal accommodation coefficient. For the thermal rectification, the adsorption-based, nonlinear thermal accommodation coefficient change is a key design parameter. Here, these are examined using the kinetic theory for various pressure and temperature ranges. Optimal material selections are also discussed.

In this paper, single-phase liquid and two-phase gas-liquid pressure drop data through 180° return bends have been obtained for horizontal rectangular micro-channel and mini-channel. To investigate the size effects of the test channels, the hydraulic diameters were 0.25 mm and 3 mm respectively as the micro-channel and the mini-channel. The curvature radii of the bends were 0.500 mm and 0.875 mm for the micro-channel, while 6 mm for the mini-channel. To know liquid properties effects, distilled water, surfactant and glycerin aqueous solutions, ethanol and HFE (hydrofluoroether)-7200 were used as the test liquid, while nitrogen gas and air as the test gas. Pressure distributions upstream and downstream tangents of the bend were measured for the single-phase and the two-phase flows. From the pressure distribution data, the bend pressure loss was determined. By analyzing the present data, the bend loss coefficient for single-phase flow in both micro- and mini-channels could be correlated with Dean number. On the other side, the total bend pressure loss for two-phase flows were correlated by using an approach of Padilla et al., in which the total pressure loss is the sum of two pressure drop components, i.e., frictional pressure drop and singular pressure drop. The approach was found to be applicable to the present data for the micro- and the mini-channels if the frictional pressure drop was calculated by Lockhart-Martinelli method with Mishima & Hibiki’s correlation and Kawahara et al.’s correlation and the singular pressure drop was calculated by a newly developed empirical correlation.

Dielectrophoresis (DEP) has become one of the most popular mechanisms for label free particle manipulations and transport in microfluidics. The efficacy of this mechanism is greatly dependent on the understanding and control of DEP interactive motion among particles. In this study, we performed a systematic investigation to understand the effect of particles size and electrical properties on DC DEP interactions among particles using in-house hybrid immersed boundary – immersed interface numerical method. Immersed boundary method is employed to predict flow field and immersed interface method is used to simulate electric field. The numerical model utilizes Maxwell’s stress tensor to obtain DEP forces, while solving transient Navier-Stokes equation it determines the hydrodynamic interaction between each of the particles and the fluid containing them. By varying the number of particles as well as the particles’ size, electrical properties and initial orientations, a number of possibilities were considered. Results indicate that the particles with similar electrical conductivities attract each other and tend to align themselves parallel to the external electric field regardless of sizes. If electrical conductivity of particles is lower than that of the fluid medium then the particle-particle interactions is caused by the negative DEP. If electrical conductivity of particles is higher than that of the fluid medium then the interactive motions of particle is attributed to the positive DEP. On the other hand, electrically dissimilar particles still attract each other but tend to align perpendicular to the electric field. Both negative and positive DEP contributes in interactions between electrically dissimilar particles. Numerical simulation also shows that the identical sized particles move at the same speed during interaction. In contrast, smaller particles moves faster than the larger particle during the interactions. This study explains the effect of size and electrical properties on DEP interactive motions of particles and can be utilized to design microfluidic devices for DEP particle manipulations.

The equivalent pore radius (i.e. capillary radius) and contact angle determine the capillary pressure generated in a porous medium. The most common method to determine these two parameters is through measurement of the capillary pressure generated by a test liquid and a reference liquid (i.e. a liquid with near-zero contact angle). The rate of rise technique commonly used to determine the capillary pressure results in significant uncertainties. In this study, we utilize our recently developed technique for independent measurement of the capillary pressure and permeability to determine the equivalent capillary radii and contact angle of water within micropillar wick structures. In this method, the experimentally measured dryout threshold of a wick structure at different wicking lengths is fit to Darcy’s law to extract the capillary pressure generated by the test liquid. The equivalent capillary radii of different wick geometries are determined by measuring the capillary pressures generated using n-hexane as the working fluid. It is found that the equivalent capillary radius is dependent on the diameter of pillars as well as the spacing between pillars. The equivalent capillary radii of micropillar wicks determined using the new method are found to be up to 7 times greater than the current geometry-based first order estimates. The contact angle subtended by water at the walls of the micropillars was determined by measuring the capillary pressure generated by water within the arrays and the measured capillary radii for the different geometries. This contact angle was determined to be 52.7°.

In order to investigate the coalescence and out-of-plane jumping of two incompressible droplets on a non-wetting surface surrounded by an incompressible fluid with matched viscosity in the low Ohnesorge number regime, a two-dimensional lattice Boltzmann phase-field model is implemented. An interfacial force of potential form is used to model the internal surface tension force and capture the fluid-surface interaction, viz. the contact-line dynamics. We evaluate the simulated velocity fields and interface shape evolution during coalescence and the subsequent jumping event. We confirm that the coalescence dynamics of the binary droplet system is similar to the case where the outer fluid viscosity is small compared to that of the droplet fluid, as is the case of condensed water droplet jumping on superhydrophobic surfaces in a gaseous ambient. An argument is also developed to demonstrate that the dynamics in 2D, when appropriately scaled, should be approximately equivalent to the corresponding 3D case. A simple drag model is used to capture the rapid velocity decay of the jumping droplet as it moves away from the surface into the viscous fluid. The results suggest the possibility of experimentally observing coalescence-induced droplet jumping in liquid-liquid systems that may be potentially exploited for microfluidic applications.

Spray formation occurring at the outlet of short microchannels/micro orifices due to the cavitation phenomenon is of great importance in biomedical and engineering applications. The spray characteristics are affected dramatically by the flow regime in the micro orifice. If properties of the flow are identified in the outlet of the nozzle, the treatment of the spray can be predicted. These properties can be used as boundary conditions. The experimental investigations show that the cavitation phenomenon occurs in the orifice and strongly affects the spray characteristics. However, visualization of the spray at the outlet of the micro orifice is a challenging task, since the phenomena related to the spray are occurred in very small scale and also the region near to the micro orifice is not clear. Therefore there is an urgent need to new and advanced visualization techniques and measurement equipments. In this study, spray formation and atomization, bubble evolution at the outlet of a short microchannel of an inner diameter of 152 μ m were experimentally studied at different injection pressures with the use of a high speed visualization system. High speed visualization was performed at four different segments to cover ∼15 mm distance beginning from the microchannel outlet to understand the spray formation mechanism. It was observed that cavitating bubbly flow is strongly affected by injection pressure. Up to an injection pressure of 50 bars bigger size droplets form at the outlet, while beyond 50 bar injection pressure, cavitation erosion of intensified cavitation becomes dominant leading to smaller droplet sizes and a more conical spray. The results showed a good agreement with previous studies. This energy could be exploited in several applications, where destructive effects of bubbly cavitating flows are needed.

Surface wettability of materials is important in heat transfer and thermal processes at micro-scale. This paper presents the manipulation of metal surface wettability by nanofluid boiling nanoparticle deposition. As confirmed by microscopy, particles can be deposited on metal surfaces by boiling in nanoparticle suspension, which significantly enhanced the surface wettabiliy relative to that of its original condition. The change in wettability is coupled to boiling conditions, such as nanoparticle concentration, heat flux, boiling duration, substrate roughness and so on. It has been observed that the higher the concentration of nanoparticles in the liquid during the boiling deposition process, the more pronounced the impact on wetting. Hence, surface wettability can be manipulated by controlling the nanoparticle concentration during the nanofluid boiling nanoparticle deposition (NBND) process. Such method can potentially be applied to enhance the heat transfer performance in thermal devices.

A numerical analysis of flow and heat transfer fields in a rough microchannel is carried out using a hybrid solver dynamically coupling kinetic and Navier–Stokes solutions computed in local rarefied and continuum areas of the flow, respectively. The roughness geometry is modeled as a series of triangular obstructions and a relative roughness up to 5% of the channel height is considered. Keeping Mach number low (incompressible flow) while varying Knudsen number allow us to investigate different rarefaction levels of the flow. The competition between roughness, rarefaction and heat transfer effects is discussed in terms of averaged Nusselt and Poiseuille numbers and mass flow rate. Discrepancy between the full Navier–Stokes and hybrid solutions is investigated, assessing the range of applicability of the first order slip boundary condition for rough geometries with and without heat transfer presence.

We present a method to quantify and enhance separation of binary cells mixture in the microfluidic device using high frequency dielectrophoresis (>20 MHz). At these frequencies, the DEP response depends primarily on the dielectric properties of the cytoplasm. In order to achieve efficient separation, there must be a difference in the intrinsic dielectric properties of the populations to be sorted. For algae cells, the shift in high frequency dielectrophoresis response during lipid accumulation can be used as a basis of separation. We defined a separability parameter based on the expected difference in the dielectrophoresis responses of the algae cells. Chlamydomonas reinhardtii cells were cultured in regular media and then the same cells were cultured under nitrogenfree conditions to accumulate neutral (non-polar) lipids. Separability of microalgae cells with different lipid content via high frequency dielectrophoresis were investigated by a thin needle shaped electrodes patterned by standard photolithographic and wet etching procedures. Experimental separability factors were measured by estimation of relative lipid content with BODIPY 505/515 fluorescence dye and calculating the area-weighted intensity average of fluorescent images. Theoretical separability parameter was calculated using analytical analysis of single shell model by MATLAB. Theoretical and experimental separability parameters, as tools to determine the optimal separation method, were calculated for microalgae cells with different lipid content. This objective function was maximized in the range of 35–45 MHz for C. reinhardtii cells after 21 days of lipid accumulation in a static separation. In order to design a continuous cell sorter device, the theoretical separation factor was maximized based on differences in the magnitude or the direction of the DEP force.

Multiphase flow phenomena in single micro- and minichannels have been widely studied. Characteristics of two-phase flow through a large array of microchannels are investigated here. An air-water mixture is used to represent the two phases flowing through a microchannel array representative of those employed in practical applications. Flow distribution of the air and water flow across 52 parallel microchannels of 0.3 mm hydraulic diameter is visually investigated using high speed photography. Two microchannel configurations are studied and compared, with mixing features incorporated into the second configuration. Slug and annular flow regimes are observed in the channels. Void fractions and interfacial areas are calculated for each channel from these observations. The flow distribution is tracked at various lengths along the microchannel array sheets. Statistical distributions of void fraction and interfacial area along the microchannel array are measured. The design with mixing features yields improved flow distribution. Void fraction and interfacial area change along the length of the second configuration, indicating a change in fluid distribution among the channels. The void fraction and interfacial area results are used to predict the performance of different microchannel array configurations for heat and mass transfer applications. Results from this study can help inform the design of compact thermal-fluid energy systems.

It is shown that a droplet will levitate over the liquid surface for 50–700 ms when released from a critical height 1.5–4 times the droplet diameter. While releasing a droplet out of this range will lead to direct submersion. Additionally, it is shown that by applying a temperature difference between the liquid pool and droplet it is possible to elongate the levitation time of that droplet as it pulls the surrounding air molecules between the drop and the pool surface. Lastly, the thickness of the air gap is calculated theoretically for a range of temperatures and compared with experiments. Surprisingly, larger temperature difference between droplet and surface causes an increase in the thickness of the air gap. It is also found that the size of droplet and type of fluid can significantly affect the lifetime of non-coalescent drops.