Validation of a Second-Order Slip Model for Transition-Regime, Gaseous Flows

We discuss and validate a recently proposed second-order slip model for dilute gas flows. Our discussion focuses on the importance of quantitatively accounting for the effect of Knudsen layers close to the walls. This is important, not only for obtaining an accurate slip model but also for interpreting the results of the latter since in transition-regime flows the Knudsen layers penetrate large parts of the flow. Our extensive validation illustrates the above points by comparing direct Monte Carlo solutions to the slip model predictions for an unsteady flow. Excellent agreement is found between simulation and the slip model predictions up to Kn = 0.4, for both the velocity profile and stress at the wall. This demonstrates that the proposed second-order slip model reliably describes arbitrary flowfields (and related stress fields) in a predictive manner at least up to Kn = 0.4 for both steady and transient problems.

Convective Mixing and Its Application to Micro Reactors

This work shows the application of convective fluid flow caused by flow-induced secondary vortices to fluidic single-phase micro mixers. As an example we used simple static T-shaped micro mixers. The convective flow was observed both by simulations and by experiments and is suitable for enhancing the mixing quality. Concerning micro reactors, it is necessary that the mixing is faster than the chemical reaction to be induced so that the creation of unwanted side products is minimized. The mixing model by Bourne is slightly modified for continuous flow reactors and applied to our mixers. Using this model, timescales for the mixing in our micro mixers are calculated. A first test reaction — the iodide-iodate reaction by Villermaux and Dushman — to check the validity of the timescales is outlined. These overall results will help to achieve a deeper understanding of micro reactors.

Experimental Study of Flow Patterns, Pressure Drop and Flow Instabilities in Parallel Rectangular Minichannels

The use of phase change heat transfer in parallel minichannels and microchannels is one of the solutions proposed for cooling high heat flux systems. The increase in pressure drop in a two phase system is one of the problems, that need to be studied in detail before proceeding to any design phase. The pressure drop fluctuations in a network of parallel channels connected by a common head need to be addressed for stable operation of flow boiling systems. The current work focuses on studying the pressure-drop fluctuations and flow instabilities in a set of six parallel rectangular minichannels, each with 333 μm hydraulic diameter. Demonized and degassed water was used for all the experiments. Pressure fluctuations are recorded and signal analysis is performed to find the dominant frequencies and their amplitudes. These pressure fluctuations are then mapped to their corresponding flow patterns observed using a high speed camera. The results help us to relate pressure fluctuations to different flow characteristics, and their effect on flow instability.

Multistage Viscous Micropumps

The viscous micropump consists of a cylinder placed eccentrically inside a microchannel, where the rotor axis is perpendicular to the channel axis. When the cylinder rotates, a net force is transferred to the fluid due to the unequal shear stresses on the upper and lower surfaces of the rotor. Consequently, this causes the surrounding fluid in the channel to displace towards the microchannel outlet. The simplicity of the viscous micropump renders it ideal for micro pumping, however, previous studies have shown that its performance is still less than what is required for various applications. The performance of the viscous micropump, in terms of flow rate, pressure head and efficiency, may be enhanced by implementing more than one rotor into the configuration. The present study will numerically investigate the performance of various configurations of the viscous micropumps with multiple rotors, namely the dual-horizontal rotor, the triple-horizontal rotor, the symmetrical-dual-vertical rotor, and the 8-shaped dual-vertical rotor. The development of drag force with time, as well as the viscous resisting torque on the cylinders were studied. In addition, the corresponding drag and moment coefficients were calculated. Results show that the symmetrical-dual-vertical rotor configuration yields the best efficiency, and generates the highest flow rate. The steady state performance of the single-stage micropump was compared with the available experimental and numerical data, and was found to be in very good agreement. This work provides a foundation for future research on the subject of fluid phenomena in viscous micropumps.

Conjugated Heat Transfer in 2-Dimensional Mini and Micro Channels: Influence of Axial Conduction in the Walls

For small Reynolds numbers, conductive heat transfer in the wall of mini-micro channels can become quite multidimensional: the wall heat flux density does not stay uniform and heat transfer mainly occurs at the entrance of the channels. The use of a ID model to invert measurements designed for estimating the convective heat transfer coefficient can lead to misinterpretations such as a variation of the Nusselt number with the Reynolds number. Three analytical models of conjugated heat transfer in channels are proposed, and the potential inversion of measurements is considered. A non-dimensional number M quantifying the relative part of conductive axial heat transfer in walls is introduced.

Proteinaceous Bubbles and Nano Particle Flows in Microchannel

Proteinaceous bubbles of 185 nm in average diameter were synthesized by a sonochemical treatment of bovine serum albumin in aqueous solution and the nanoparticles (TiO2) solution was made by ultrasonic irradiation. To study the macroscopic flow behavior associated with the changes in the state of microparticles, a flow test of these solutions in microchannels was done. Also the size distributions of the proteinaceous bubbles in solution before and after the flow test were measured by a light scattering method. Test results show that the air-filled proteinaceous bubbles in solution adjust their size to reduce the shear stress encountered in the flow through the microchannel. On the other hand, the flow rate of the solution with nanoparticles suspensions becomes smaller than that of deionized water above the flow rate of 6 cm3/min in the microchannel with a dimension of 100×150 μm2.

MC Simulation of Radiative Heat Transfer Between Planar Semi-Infinite Media and Nano-Spheres With Near Field Effect

Based on the principle of electric dipole radiation and the Planck’s spectral distribution of emissive power, the enhancement of thermal radiation between two planar semi-infinite media or two nano-spheres was studied in this paper by the Monte Carlo method. By this simple method, some parameter’s influence on the radiative heat transfer was investigated, such as the distance between two semi-infinite media, the particle’s radius, the distance between two particles and the difference in temperature between two particles, and so on. This solution is not rigorous but simple. The results show that heat transfer can be enhanced by several orders of magnitude for the near field effect. And the radiative heat transfer is decreasing sharply with the increasing of the distance.

Numerical Simulation on the Electrophoretic Motion of Multiple Particles in Microchannels

This paper considers the electrophoretic motion of multiple spheres in an aqueous electrolyte solution in a straight rectangular microchannel, where the size of the channel is close to that of the particles. This is a complicated 3-D transient process where the electric field, the flow field and the particle motion are coupled together. The objective is to numerically investigate how one particle influences the electric field and the flow field surrounding the other particle and the particle moving velocity. It is also aimed to investigate and demonstrate that the effects of particle size and electrokinetic properties on particle moving velocity. Under the assumption of thin electrical double layers, the electroosmotic flow velocity is used to describe the flow in the inner region. The model governing the electric field and the flow field in the outer region and the particle motion is developed. A direct numerical simulation method using the finite element method is adopted to solve the model. The numerical results show that the presence of one particle influences the electric field and the flow field adjacent to the other particle and the particle motion, and that this influences weaken when the separation distance becomes bigger. The particle motion is dependent on its size, with the smaller particle moving a little faster. In addition, the zeta potential of particle has an effective influence on the particle motion. For a faster particle moving from behind a slower one, numerical results show that the faster moving particle will climb and then pass the slower moving particle then two particles’ centers are not located on a line parallel to the electric field.

Diffusion Modeling in a Microchannel for Separation of Species

Recently there is an increased interest in the design of microfluidic devices for research in biotechnological studies, applied to sample detection and analysis of species. When fluids are confined to small volumes, mixing results almost entirely by diffusion due to low velocities of flow in microchannels. As a result, it is possible to design microfluidic systems in which dissimilar fluids flow along side each other over long distances without significant mixing. The H-filter is a microfluidic device used for the extraction of molecular analytes from liquids containing interfering particles. The principle behind H filter is that small molecules will diffuse quickly from a sample stream to the buffer stream while very large molecules and particles will remain indefinitely in the sample stream because of their much larger size and much decreased diffusion rate. Because the Reynolds number in most microfluidic channels is generally kept well below 1, no turbulent mixing of fluids occurs. The only means by which solvents, solutes and suspended particles move in a direction transverse to the direction of flow is by diffusion. Differences in diffusion coefficients can be used to separate molecules of large particles over time. The time spent in flowing in a channel is proportional to the length of the channel. Before carrying out experiments, it is worthwhile to simulate the diffusion process in a microfluidic device for various properties of species and channel geometry. This paper attempts to model the diffusion process in an H-filter for typical species using CFD-ACE+, a software for solving problems in fluid dynamics with multi-physics capabilities. A module of CFD ACE+, called user-scalar that allows the user to define scalar quantities and boundary conditions for this scalar is used in the simulation. As seen from the studies, the diffusivities of species A and B in the buffer influence their diffusion. Optimization of geometry for a given species can be done with this method and separation can be achieved. The results from such a study will be useful for the design optimization and fabrication of such devices.

Constructal Networks for Efficient Cooling/Heating

Channel networks designed with constructal theory are compared. The efficiency of the networks when used for cooling a uniformly heated surface is compared. Three networks are compared and it is found that the two constructal designs with two and three constructal levels have similar performance. It is shown that for a given pumping power, the constructal designs give a heat transfer coefficient of the surface which is almost a factor of magnitude higher than the one obtained for a parallel channel system.