Flow Characteristics In Microchannels Research Articles

Integrated numerical analysis to evaluate replication characteristics of micro channels in a locally heated mold by selective induction

High-frequency induction heating is an efficient way to heat mold surfaces by electromagnetic induction using a non-contact procedure. Due to its ability to rapidly heat and cool mold surfaces, this method has been applied recently to the injection molding of micro/nano structures. The present study investigates a localized heating method involving the selective use of mold materials to enhance the heating efficiency of high-frequency induction heating. A composite injection mold consisting of ferromagnetic material and paramagnetic material was used for localized induction heating. The feasibility of this localized heating method was investigated through numerical analyses in terms of its heating efficiency for localized mold surfaces and the resulting flow characteristics in micro channels. To take into account the effects of thermal boundary conditions of localized induction heating, a fully integrated numerical analysis effectively connecting electromagnetic field calculation, heat transfer analysis, and injection molding simulation was carried out. The proposed integrated simulation was applied to the injection molding of a rectangular strip containing micro channels, and the resulting mold heating capacity and replication characteristics of the micro channels were compared with experimental findings in order to verify the validity of the proposed simulation.

Computational fluid dynamics modelling on flow characteristics of two-phase flow in micro-channels

Simulated gas-liquid two-phase horizontal flow characteristics were compared with corresponding experimental data taken from the Triplett. For the simulation of the two-phase flow characteristics, the well-known volume of fluid approach co-operated with the piecewise linear interface calculation interface reconstruction method was carried out. First, a variety of two-phase flow patterns were reproduced. The numerical results give good agreement with the flow regimes expected from the Triplett. Next to that, the two-phase pressure drops in micro-channels were analysed numerically by using the same computational fluid dynamics method. Also, the comparison of pressure drop among the numerical simulations, experimental data and the results calculated by homogeneous equilibrium model was presented. The agreement between numerical results and the existing experimental data was found to be satisfactory. So, it can be deduced that the numerical analysis procedure proposed here can achieve a better prediction for two-phase flow characteristics in micro-channels.

Boiling flow characteristics in microchannels with very hydrophobic surface to super-hydrophilic surface

Boiling flow process plays a very important role to affect the heat transfer in a microchannel. Different boiling flow modes have been found in the past which leads to different oscillations in temperatures and pressures. However, a very important issue, i.e. the surface wettability effects on the boiling flow modes, has never been discussed. The current experiments fabricated three different microchannels with identical sizes at 105 × 1000 × 30000 μm but at different wettability. The microchannels were made by plasma etching a trench on a silicon wafer. The surface made by the plasma etch process is hydrophilic and has a contact angle of 36° when measured by dipping a water droplet on the surface. The surface can be made hydrophobic by coating a thin layer of low surface energy material and has a contact angle of 103° after the coating. In addition, a vapor–liquid–solid growth process was adopted to grow nanowire arrays on the wafer so that the surface becomes super-hydrophilic with a contact angle close to 0°. Different boiling flow patterns on a surface with different wettability were found, which leads to large difference in temperature oscillations. Periodic oscillation in temperatures was not found in both the hydrophobic and the super-hydrophilic surface. During the experiments, the heat flux imposed on the wall varies from 230 to 354.9 kW/m 2 and the flow of mass flux into the channel from 50 to 583 kg/m 2s. Detailed flow regimes in terms of heat flux versus mass flux are also obtained.

A study of novel viscous dissipation measurement for polymer melts in microchannels

Studies on the rheological behaviour of polymer melts, flowing through microchannels, are complicated because a large number of factors affect the melt viscosity. One such factor, viscous dissipation, is investigated in the current work through a novel experimental technique that is used in determining the viscous dissipation of a polymer melt flowing through microchannels. Relative tests are conducted using melts of high-density polyethylene (HDPE) extruded through several capillary dies at different temperatures. Experimental results indicate that the temperature rises due to viscous dissipation increase with increasing shear rate. In addition, simulations considering viscous dissipation are carried out. The comparison of the experimental results with those predicted from the simulations at different melt temperatures indicates that the maximum temperature rise deviation is about 15 per cent. Therefore, the measurement method of viscous dissipation is available, which is helpful to better understand the flow characteristics of microchannels.

The entrance effect on gases flow characteristics in micro-tube

Entrance region may have important effect on gases flow characteristics in micro-channels. It’s concluded in the available papers that the entrance effect causes significant difference. An experimental system of single-phase gas flow characteristics in microchannels was set up. Flow characteristics of nitrogen in PEEK polymer micro-tube (hydraulic diameter is 553µm) was studied experimentally. According to the data of nitrogen flow in the micro-tube with the length ranging from 0.1m to1.524m, it is shown that the friction constant becomes higher when the tube becomes shorter. By using pipe cutting methods, it’s confirmed that entrance effect is one of the key factors that cause friction constant higher than conventional theory. It’s found that friction constant of fully developed flow is lower than the value predicted by conventional theory in turbulent region. The result indicates that the flow transition occurs at Reynolds number ranging from 1600–2000. The phenomenon of obvious early transition is not found.

Investigation of variable-property microchannel flows with electro-thermo-hydrodynamic interactions at constant pressure gradient or constant flow rate

In the present paper, a systematic investigation on numerical modeling of variable-property microchannel flows with electro-thermo-hydrodynamic interactions is presented. General variable-property electro-thermal flow model consisting of mass, momentum, energy, electric potential and ions transport equations in dimensionless form is formulated and employed to study transport phenomena in microchannel flows. The major concerns are: (1) evaluation of the model approximations such as equilibrium ion distribution, constant-property and/or small zeta potential for non-overlap electric double layer (EDL); (2) the electro-thermo-hydrodynamic interactions under conditions of strong EDL effects, and (3) differences in microchannel flow characteristics at driving conditions of constant pressure gradient (CPG) and constant flow rate (CFR). Variations of fluid, transport and electric properties with local temperature and/or ion concentration are considered. The present results demonstrate that the variable-property model with Poisson–Boltzmann potential is an appropriate approximation for transport phenomena in microchannel flows. The results also disclose that, compared to those in CPG flows based on the same Reynolds number, effects of temperature non-uniformity and variable-property are relatively more pronounced in the CFR flows. At either CPG or CFR condition, heat transfer performance changes little with the variations of electric and flow parameters in this low Reynolds number ( Re = 0.1) flow; while electric and flow fields can be significantly affected by variation of the governing parameters.

Nucleation characteristics and stability considerations during flow boiling in microchannels

Flow boiling in microchannels and minichannels has received considerable attention in the past decade. The interest stems from the possibility of achieving the extremely high heat fluxes, over 1 kW/cm 2, needed for future generation electronics cooling application. The attention has been focused on obtaining experimental heat transfer and pressure drop data, but the fundamental aspects of nucleation have been largely overlooked. In the present paper, the local wall superheat and bulk liquid subcooling prevailing in the channel at the onset of nucleation are identified as critical in the flow boiling stability. To understand the role of local conditions on nucleation, the available literature on onset of nucleate boiling is critically reviewed and the relationships between the local bulk subcooling and local wall superheat as a function of nucleation cavity diameters are presented. The unique flow characteristics in microchannels and minichannels are further analyzed and their influence on flow boiling stability is investigated experimentally using visual images generated with a high-speed camera.

The effect of channel diameter on adiabatic two-phase flow characteristics in microchannels

The effect of channel diameter on two-phase flow was investigated to identify the phenomena which distinguish microchannels from minichannels. Experiments were conducted with a mixture of nitrogen gas and water in circular channels of 530, 250, 100, and 50 μm diameter. The temperature, pressure and flow rates of the liquid and gas were measured and images of the flow patterns recorded. The two-phase flow was characterized by the flow regime map, void fraction, and frictional pressure drop. In the 530 and 250 μm channels, the two-phase flow characteristics were similar to those typically observed in minichannels of ∼ 1 mm diameter. In the 100 and 50 μm channels, the two-phase flow behaviour departed from that observed in minichannels––the occurrence of slug flow dominated, the void fraction–volumetric quality relationship departed from a linear Armand-type correlation, and mass flux no longer influenced the two-phase frictional multiplier. Clearly, the channel diameter has an effect on two-phase flow in the range of channel diameters investigated. A new slug flow model is also proposed to gain physical insight into the observed flow characteristics in microchannels. The model can predict the two-phase frictional pressure gradient for the 100 and 50 μm channels, if the actual void fraction data are used, substantiating the assumed flow mechanism.

Effect of compressibility on gaseous flows in micro-channels

Two-dimensional compressible momentum and energy equations are solved in a parallel plate channel to obtain the effect of compressibility on gaseous flow characteristics in micro-channels. The numerical methodology is based on arbitrary-Lagrangian–Eulerian method. The computations were performed for a wide range of Reynolds number and Mach number and for both ‘no heat conduction’ and isothermal flow conditions. The results are presented in form of the product of friction factor and Reynolds number ( f· Re) expressed as a function of both Reynolds number and Mach number and was compared with available experimental results. It was found that f· Re is mainly a function of Mach number and is different from the incompressible value of 96 for parallel-plate channels.

Experimental investigation of flow friction for liquid flow in microchannels

In this paper, investigations on the liquid flow in microchannels with different experimental methods are presented. The experiments were carried out in channels with hydraulic diameter ranging from 30 μm to 344 μm at Reynolds number ranging from 20 to 4000. Based on the experimental data collected and those available in the literature, comparisons and analysis have been carried out to evaluate the possible phenomena occurring in the liquid flow in microchannels. Results obtained show that characteristics of flow in microchannels agree with conventional behaviors predicted by Navier-Stokes equations in the region of those dimensions tested. In this paper, the detailed explanations on experimental results are discussed.