Lab-on-chip processes often require long dwelling times leading to coiled capillary reactors with laminar flow. These tubular reactors are designed for a reaction time, unfortunately with a wide residence time distribution. This contribution presents a modular concept based on coiled flow inverters (CFI), which achieve high radial mixing with narrow residence time distribution at low Reynolds numbers [1]. The modular design enables quick adaptation to changing residence times and flow rates with low pressure loss. The tube diameters range from capillaries with a few 100 μm to several millimeters for high throughput and long residence time. With the aid of a design space diagram, the required pipe diameters and lengths can be quickly determined based on standardized coil diameters [2]. The modular concept enables various arrangements for different residence time and flow rate requirements with minimum pressure loss. In the laboratory, for example, a chemical process in the throughput range of a few grams per hour can be developed and processed in the simple device. The results can be scaled via the platform concept to higher production rates with constant residence time characteristics. The scale-up concept can easily be displayed and designed graphically in the reactor performance diagram.

In this study, liquid-gas two-phase flow pressure drops were measured in an ex-situ PEM fuel cell test section. Pressure drop signatures were studied for three nominal air flow rates and different water flow rates within a flow channel. The pressure drop signatures showed an increasing trend at the beginning of the experiments which were followed by a drop to lower values before reaching uniform patterns. It was observed that as the water flow rate increased, the time interval at which pressure signatures reached uniform patterns decreased. In addition, a qualitative comparison with Mishima-Hibiki model [13] revealed that this two-phase flow pressure drop model showed the best prediction capability for the medium air flow rate used in this study, ∼300mℓ/min inflow channel, corresponding to ∼220 Reynolds number.

Impinging flows are widely used to enhance convective heat transfer by promoting separation, recirculation and higher rates of local convection. We consider unsteady flow and heat transfer effects in a prototypical T-shaped geometry as an impinging jet. Depending on the relative length scales, the steady laminar flow in this geometry may lose stability and transition to time periodic flow even at a low Reynolds number. A key feature of the periodic structure is the presence of ‘twin’ circulation regions adjacent to the jet column, and separation vortices anchored at the impinging surface in place of the wall jet in steady flow. The separation vortices are located above shear layers lying along the confining plane of the geometry which is flush with the jet exit. Consequently, convective heat transfer is enhanced across this plane. We present calculations to show the effect of the structure of the periodic flow on heat transfer rates across the two parallel surfaces. For a shear thinning fluid the local Nusselt number at the confining surface averaged over a long length scale (∼ 50 times the nozzle width) is more than twice as large compared to that in steady flow, while for the Newtonian fluid the mean Nusselt number increases about 60%. A mild increase in the transport rate across the impinging surface is also observed. Thus flow periodicity due to instability of the steady flow field provides a mechanism to increase the total heat transfer rate across the two surfaces.

In a variety of electronic systems, cooling of various components imposes a significant challenge. A major aspect that inhibits the performance of many cooling solutions is the thermal resistance between the chip package and the cooling structure. Due to its low thermal conductivity, the thermal interface material (TIM) layer imposes a significant thermal resistance on the chip to cooling fluid thermal path. Advanced cooling methods that bypass the TIM have shown great potential in research and some specialty applications, yet have not been adopted widely by industry due to challenges associated with practical implementation and economic constraints. One advanced cooling method that can bypass the TIM is jet impingement. The impingement cooling device investigated in the current study is external to the integrated circuit (IC) package and could be easily retrofitted onto any existing microchip, similar to a standard heatsink. Jet impingement cooling has proven effective in previous studies. However, it has been shown that jet-to-jet interference severely degrades thermal performance of an impinging jet array. The present research addresses this challenge by utilizing a flow path geometry that allows for withdrawal of the impinging fluid immediately adjacent to each jet in the array. In this study, a jet impingement cooling solution for high-performance ICs was developed and tested. The cooling device was fabricated using modern advanced manufacturing techniques and consisted of an array of micro-scale impinging jets. A second array of fluid return paths was overlain across the jet array to allow for direct fluid extraction in the immediate vicinity of each jet, and fluid return passages were oriented in parallel to the impinging jets. The following key geometric parameters were utilized in the device: jet diameter (D = 300μm), distance from jet to impinging surface (H/D = 2.5), spacing between jets (S/D = 8), spacing between fluid returns (S r /D = 8), diameter of fluid returns (D r /D = 5). The device was mounted to a 2cm × 2cm uniformly heated surface which produced up to 165W and the resulting fluid-to-surface temperature difference was measured at a variety of flow rates. For this study, the device was tested using single-phase water. Jet Reynolds number ranged from 300–1500 and an average heat transfer coefficient of 13,100 W m −2 K −1 was achieved at a Reynolds number of only Re d = 305.

Liquid-in-air generation of monodisperse, microscale droplets is an alternative to conventional liquid-in-liquid methods. Previous work has validated the use of a highly inertial gaseous continuous phase in the production of monodisperse droplets in the dripping regime using planar, flow-focusing, PDMS microchannels. The jetting flow regime, characteristic of small droplet size and high generation rates, is studied here in novel microfluidic geometries. The region associated with the jetting regime is characterized using the liquid Weber number ( We l ) and the gas Reynolds number ( Re g ). We explore the effects of microchannel confinement on the development and subsequent breakup of the liquid jet as well as the physical interactions between the jet and continuous gaseous flow. Droplet breakup in the jetting regime is also studied numerically and the influence of different geometrical parameters is investigated. Numerical simulations of the jetting regime include axisymmetric cases where the jet diameter and length are studied. This work represents a vital investigation into the physics of droplet breakup in the jetting regime subject to a confined gaseous co-flow. By understanding the effects that different flow and geometry conditions have on the generation of droplets, the use of this system can be optimized for specific high-demand applications in the aerospace, material, and biological industries.

In this study, a three-dimensional numerical investigation on the thermo-hydrodynamic performance of a newly proposed recharging microchannel (RMC) is carried out. In this new design, a straight microchannel separated into more than one small channels and each small channels having individual inlet and outlet. This design enhances the heat transfer and makes the temperature almost uniform across the length of the substrate. The comparison of fluid flow and heat transfer performance between this recharging microchannel (RMC), interrupted microchannel (IMC) and straight microchannel (SMC) with same hydraulic diameter and substrate length were conducted to explore the effect of geometrical configuration on the heat transfer enhancement. The results reveal that for the recharging microchannel, the average Nusselt number increases by 49–122%, while the total pressure drop increases by 15–89%, compared with the interrupted and straight microchannel for the Reynolds number ranging from 100 to 500. From the result, it is also observed that for the investigated Reynolds number range the recharging microchannel having the highest thermal performance compared to interrupted and straight microchannel with a maximum performance factor of 1.80. The outcome of this study indicates possible use of recharging microchannel heat sinks for high heat flux removal applications such as electronic cooling.

The present work represents a 2-D numerical investigation of forced convection heat transfer over three electronic blocks (silicon chip) in an inline arrangement with elliptical shaped vortex generators (VG-copper made) placed on top of the channel, for a range of Reynolds numbers. The block is prescribed with a 1,000 W/m 2 heat flux due to heating of the electronic components installed in the CPU casing. The results show that, vortex generators could effectively enhance the heat transfer in the channel. Subsequently, the effects of Reynolds number (from 500 to 1050), the number of vortex generators (baseline, 1, 2 and 3), aspect ratio of heated block (0.125, 0.15, 0.22), and aspect ratio of vortex generators (0.3125, 0.4, 0.5) on the heat transfer and fluid flow characteristics are examined. The characteristics of the performance parameters are studied numerically with the aid of computational fluid dynamics (CFD). The 3 VG demonstrates nearly 28.35% enhancement of Nusselt number compared to the 1 VG case at Re = 479. The change in pressure drop is less at low Reynolds number compared to higher Reynolds number respective to other parameters. Increasing the aspect ratio of the block increases the convection coefficient while decreasing aspect ratio of VG increases heat transfer coefficient. This enhancement is less significant for the third block as the cooling effect is predominant close to the channel inlet. Increasing consecutive distance between the blocks, enhances the heat transfer coefficient with the penalty of additional pressure drop. However, parametric studies are conducted for the maximum heat transfer enhancement.

Microscale heat and fluid flow in macro geometries have been made practical in terms of cost and fabrication, by superimposing two macro geometries which are fabricated using readily-available CNC machining methods. Wavy-profile has been proposed to enhance heat transfer performance in the microchannel owing to the simplicity of geometry and feasibility to be fabricated using simple turning process. Experimental studies were conducted on single-phase, forced convective heat transfer using water as the working fluid for the Reynolds number range of 1300 to 4600, for a constant heat flux of 53.0 W/cm 2 . Three sinusoidal waves with different wavelength and same amplitude are studied to examine the effect of the total number of waves on the heat transfer and hydrodynamic performance within constant microchannel length. The maximum performance index, which evaluates heat transfer performance per unit pumping power, is 1.39, achieved by wavy profile with the shortest wavelength at Reynolds number of 2800. The performance index for all the enhanced microchannels peaks at the Reynolds number range of 2500 to 2800. Beyond that, the performance index is not a strong function of the wavelength. At lower Reynolds numbers, profile with the shortest wavelength achieves substantially higher performance indices, as the increment in pressure drop is accompanied by a comparable increment in heat transfer. Future work includes the introduction of correlations for the implementation of such geometries in industrial heat exchangers.

Heat dissipation in modern high-power electronics require high performance cooling, which traditional air-based systems cannot provide. Rather, novel systems using liquids, which have inherently better heat transfer characteristics, must be used. Therefore, these have recently been extensively examined. The present study aims to identify the liquid flow patterns which significantly increase heat transfer, examine them through simulation (transient 2D laminar DNS) and experimentally realize the most promising configuration. Any such flow patterns should target a major inhibitor of heat transfer, namely, the development of the thermal boundary layer. From the literature, it was seen that traveling vortices should meet this demand, due to generation of perpendicular unsteady or periodic flows, and consequently significant disruption of the boundary layer. Traditionally, micro-channels have been widely employed for micro-electronics cooling. However, the generation and persistence of the desired vortices over longer distances, as well as a desired lower pressure drop can be obtained in micro-gaps, which have inherently overall lower wall-fluid friction. The desired vortices can be further enhanced by active methods such as inlet flow pulsation. In the present study, based on numerical simulations (grid-independent and validated against an analytical solution) a suitable micro-gap geometrical configuration was chosen, while the flow rate (Re) and excitation frequency (Strouhal number around the well-known resonance, St = 0.3) with low amplitude, were examined over a wide range. Further examination led to the choice of two methods for vortex generation. The first is a use of bluff bodies as flow obstructers in the micro-gap, whereby vortex shedding (von Karman street) occurs already at low Reynolds numbers (Re>50). A preliminary experimental device was constructed with side and top view capabilities, for flow visualization, as well as the possibility of wall temperature measurement by IR thermography. Preliminary simulations and experiments showed that Vortex shedding onset was only mildly affected in the micro-scale (200 micron obstruction in 600 micron channel), while heat transfer was seen to increase three-fold over obstruction-free gap, with only mild pressure drop increase. The second method has additional advantage of imposed perpendicular flow. The model consists of a row of slot-jets in a micro-gap with cross-flow. Recent experimental and numerical studies employing a similar hybrid cooling scheme, showed significant heat flux dissipation (305 W/cm2). Here too, significant increase of the heat transfer was found, with additional increase associated with flow pulsation. In future experimental work, the intention is to include MEMS based actuators for individual control of the jets’ excitation ability and effective slot width.

This paper investigates water flowfield characteristics inside micro-pipes containing superhydrophobic walls under laminar flow conditions. It also investigates the effects of solid fraction, wall pattern, and Reynolds number on both skin friction drag and flow field characteristics. A transient, incompressible, three-dimensional, volume-of-fluid (VOF) methodology has been employed to continuously track the air–water interface and to visualize the dynamic behavior of the complex flows inside micro-pipes containing different superhydrophobic wall features (square micro-posts and longitudinal micro-ridges). The results of the present simulations show that micro-pipes containing superhydrophobic walls with longitudinal micro-ridges features have a better frictional performance than those having square posts features. The predicted results also show that the frictional performance of micro-pipes is a monotonically decreasing function of Reynolds number for both patterns examined in the present study. In addition, as the solid fraction decreases, the flow enhancement of superhydrophobic micro-pipes increases and it seems, based on the studied cases, to reach an asymptotic value. However, a further study is needed to confirm this latter issue.

This article presents a computational study to investigate the hydrodynamic and thermal characteristics of the flow inside a rectangular microchannel with the dimensions of 5000 × 1500 × 100 μm 3 (l × w × h’) with different inline arrangements of cylindrical micro pin fins. A parametric study is performed on the effect of different geometrical specifications of micro pin fins on the wake-pin fin interaction. Three values of (50, 100 and 200 μm) are considered for the pin fin diameters (D) while the overall height (H) of the system is set to be constant (100 μm). For the first two cases, two longitudinal and vertical pitch ratios (S L /D and S T /D) of 1.5 and 3 are considered while for H/D ratio of 0.5, only S T /D ratio of 1.5 and S L /D ratios of 1.5 and 3 are considered. As a result, a total number of ten different geometries are analyzed in five different Reynolds numbers of 20, 40, 80, 120 and 160. A constant heat flux is applied through the bottom surface of the microchannel as well as the micro pin fins surfaces. All other surfaces are assumed to be thermally isolated. Thermodynamic properties of water are set to vary with temperature and it is assumed that the working flow remains in the liquid form in all operating conditions. ANSYS commercial package v14.5 with an academic license is utilized to generate the 3D models, applying the appropriate grid networks and simulating the flow fields for each configuration. Results show major dependencies of pressure drops, friction factors, Nusselt numbers and Thermal Performance Index values on S T /D ratio and Reynolds number while minor dependencies of these parameters on S L /D and H/D ratios are observed.

Increasing the mixing rate of fluid droplets within modern lab-on-a-chip technology is being increasingly stressed by members of the biological and chemical communities that wish to study reaction kinetics and mechanisms. Faster mixing allows for a more complete analysis of a wider range of chemical and biological reactions, and due to this technologies ability to help better characterize these reactions, there is an increasingly large number of demands for chips that can fully mix droplets in an order of microseconds. High speed mixing like this is difficult to characterize using standard microscopy. This article looks at how to achieve high-speed droplet collisions using flow focusing geometry, and how these collisions are visualized using a novel laser-imaging system. Microchannel droplet mixing is plagued with severe limitations due to the restricted microchannel lengths between elements within the chips, as well as strong surface tension forces involved and low Reynolds numbers that are found in all microchannel flow. These characteristics further reduce the rate of mixing, and thus the rate of diffusion, between the two colliding droplets. Previous attempts at mixing droplets in microchannels stemmed from two methods; hydrodynamic flow focusing, and chaotic advection mixing, this later approach leading to the advent of inertial-based droplet micromixer technology. Both of these techniques rely on rapidly decreasing the diffusion length scales between the two mixing droplets through the creation of striations. Generating these droplets while developing a method for quantifying high-speed diffusion mixing is the main focus of this article. For this study, liquid droplets are formed in a flow-focusing configuration using a high speed gaseous flow and collided at angled Y-junctions in the microchannel fabricated in PDMS. In order to determine the instantaneous rates of mixing within the internal droplet flow during the collision process, an experimental setup using epi-fluorescence microscopy in conjunction with high speed laser-induced fluorescence was developed. This setup is capable of tracking and recording fluorophore concentration within the droplets during the collision and coalescence process in multiple images. The droplets are distinguished using differing concentrations of fluorophore which correspond to differing levels of fluorescent intensity while the colliding droplets remain unmixed, and consistent levels of fluorescent intensity when the colliding droplets are fully mixed. These images are then processed using a statistical analysis software to determine both the diffusion length scale as well as the quasi-instantaneous rate of mixing at all times during the collision process.

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.

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.

The phase change heat transfer is one of the most effective cooling methods. Therefore, investigations for the phase change heat transfer and the two-phase flow have been performed by many researchers in the past. This study provided the frictional drop of single-phase flow and flow boiling heat transfer in microchannels. An internal diameter of the present micro pipes for our research was 161 μm, 86 μm and 54 μm, respectively. Test liquid was commercial pure water. A range of Reynolds number was 20 < Re < 2.7×10 3 : the range of liquid velocity was 0.21 < u < 12 m/s. The correlation between a heat flux and a temperature difference between the wall temperature and the bulk temperature with a 161 μm internal diameter was higher than the conventional correlations for turbulent flow about single phase heat transfer. The correlation between a heat flux and a temperature difference between the wall temperature and the bulk temperature with an 86 μm internal diameter was also higher than the conventional correlations for laminar flow. However, the correlation between a heat flux and a temperature difference between the wall temperature and the bulk temperature with a 54 μm internal diameter was in good agreement with the conventional correlations for laminar flow. CHF was increased with increasing the internal diameter. Moreover, critical heat flux depends on velocity of flow. The CHF in the case of a 161 μm internal diameter in turbulent flow was approximately 20 MW/m 2 ; the CHF in the case of an 86 μm internal diameter in laminar flow was approximately 6.9 MW/m 2 and a 54 μm internal diameter in laminar flow was approximately 3.1 MW/m 2 . As a result, the CHF in case of an 86 μm internal diameter in laminer flow was in good agreement with conventional value calculated by Ivey-Morris equation.

Reducing the electricity consumption from the cooling systems in a data center can cut the energy cost. The development of the high efficiency cooling schemes can effectively save more energy from the computer room air conditioning (CRAC) system. Impingement cooling with air jets can achieve high heat transfer rates and could be used to cool local hot spots in the cabinets. A 5 × 5 jet array was designed utilizing circular and elliptic holes. The cross sectional area of the test region was 250 mm × 25 mm. The jet Reynolds numbers is 3300. Heat transfer coefficients on the target surface were investigated. Results indicated that the impingement cooling provided lower surface temperature than the traditional fan cooling.

Microscale plasma actuators operate at lower voltages than their macroscale counterparts and allow easy integration into microsystems. Field-emission driven microplasma actuators can be applied for gas flow enhancement in microchannels for pumping and microcombustion applications. The present work studies the feasibility of microplasma actuation as a pump for gaseous microchannel flow. We use 2D Particle-In-Cell / Monte Carlo Collisions (PIC/MCC) method to calculate the volumetric force generated by field-emission driven micro dielectric barrier discharge (DBD). The simulations show that the induced volumetric force and heat source scale inversely with the dielectric thickness. A volumetric force of 1000 μN/mm 3 with Joule heat source of 6 W/mm 3 for an input power of 16 mW/m was obtained for a dielectric thickness of 3 μm per DBD. This force couples with the momentum flow in the microchannel in the solution of the Navier-Stokes equations. The flow enhancement increased with the decreasing Reynolds number (Re). In a long microchannel (40 mm) at Re = 73, the actuation lead to 22% increase in mass flow rate. However the vorticity induced by heating reduced this gain by 0.03%. In a short microchannel (1.5 mm) without pressure gradient, the actuator induced flow rate was found to be higher than that of a conventional DBD pump. The inclusion of heat source further enhanced the flow by 0.05% in the short channels.

Laminar forced convection flow of nanofluids in a rectangular micro-channel has been numerically studied. The study is carried out to investigate the flow and heat transfer characteristics of hybrid single walled carbon nanotube (SWCNT) and Copper (Cu) nanofluid in a micro-channel. Hybridization of SWCNT and Cu nanoparticles are varied with different proportions such as 50% - 50%, 70% - 30% and 30% - 70% using sphericity based effective thermal conductivity evaluation. A two-dimensional multiphase mixture model has been developed and the effects of Reynolds number, nanoparticles mixture volume concentration on the flow and heat transfer characteristics of hybrid (SWCNT + Cu) nanofluids are reported. The accuracy of present numerical model has been validated with the experimental and numerical results available in the literature. The results show that the average convective heat transfer coefficient increases with increase in Reynolds number. It is also observed that 1 vol.% hybrid nanofluid (0.7 vol.% SWCNT + 0.3 vol.% Cu) significantly enhances the average convective heat transfer coefficient than that of pure water. Moreover, the multiphase mixture approach showed better enhancement in terms of heat transfer when compared with single phase homogenous model. The study concludes that hybrid nanofluids with suitable volume concentration of carbon (SWCNT) nanoparticles can be used as modern working fluid based on cooling requirement. Further, hybridizing nanoparticles at higher volume concentrations will minimize the working fluid cost and also enhances the heat transfer characteristics in comparison with pure metal based nanofluids.

Valveless micropump based on synthetic jet possesses larger flowrate and more successively outflow than valveless micropump with diffuser/nozzle elements. In order to optimize the critical structure parameters including the height of the pump chamber and the outlet diameter, a valveless piezoelectric micropump based on synthetic jet and for transporting liquid was designed and studied. A three-dimensional numerical simulation was done to obtain the flow feature and the performance of the micropumps with different structure parameters. The velocity boundary condition was applied to simulate the oscillation of the piezoelectric diaphragm and the SST flow model was utilized as the maximum Reynolds number was 800. Based on the simulation, quadratic linear regression was used in the response surface optimization. The response surface illustrates the region of the parameters leading to the largest flowrate. The results reveal that the optimal region is from 4.6 mm to 6.4 mm for the outlet diameter and from 7.2 mm to 8.8 mm for the height of the pump chamber. The maximum flowrate of the micropump can achieve 33.41 mL/min when the maximum Reynolds number is 800. It suggests that the interation exists of the height of the pump chamber and the outlet diameter. And the micropump with a higher pump chamber requires a larger outlet diameter.

We report on a computational model used to study the reversal of flow direction inside the annular region between concentric micro-cylinders filled with an incompressible Newtonian fluid. The flow is induced by boundary deformations on the inner and outer cylinder surfaces due to forward-propagating transverse waves and their reflections. This microfluidic transport mechanism is postulated as a vital pathway for removal of beta-amyloid from the brain along sub-millimeter cerebral arteries, and failure of this clearance is associated with Alzheimer’s disease. We show that the direction of this annular flow depends on superposition of the peristaltic waves and their reflection waves. A control volume analysis is developed to predict the transport characteristics and compared with numerical solutions of the Navier-Stokes equations. The identified set of microfluidic parameters that leads to a net reverse flow will aid biologists in understanding why an aging brain becomes prone to beta-amyloid accumulation.