Experiments on the Laminar/Turbulent Transition of Liquid Flows in Rectangular Microchannels

A combined experimental and numerical study on the laminar-to-turbulent transition in microchannels using gas flow is presented. The effects of two geometric parameters, namely aspect ratio (height to width) of microchannels and inlet manifold shape, are considered on the value assumed by the critical Reynolds number linked to the laminar-to-turbulent transition. To study the effect of aspect ratio, seven rectangular microchannels having an aspect ratio between 0.25 and 1.04 are micro-milled in PMMA plastic with a constant length of 100 mm. Four rectangular microchannels with different inlet shapes, namely sudden contraction, rounded entrance, V shape and bellmouth, are fabricated to analyze the effects of inlet shape. Pressure loss analyses are then performed for all 11 microchannels by evaluating both average and semi-local friction factors. The Reynolds number in correspondence of which the transition takes place is determined by observing the trend of the friction factor. In parallel, numerical simulations using an intermittency-based transitional turbulence model are also performed and results are compared with the experiments. Experimental and numerical results have demonstrated that both of the investigated geometrical characteristics (aspect ratio and inlet manifold shape) play an important role on the range of the Reynolds number between the onset of transition and the onset of fully turbulent regime for gas microflows. Experimental critical Reynolds numbers show a good agreement with the predictions of the conventional theory and are in the range of 1863–3470 for all the tested microchannels. The role of gas compressibility on the laminar-to-turbulent transition is also discussed.Graphic abstract

A control-volume numerical approach has been used to study rarefaction effects in simultaneously hydrodynamically and thermally developing flow in rectangular microchannels with a prescribed uniform wall heat flux in the slip-flow regime (10 ≤ Kn ≤ 10). The effects of velocity slip and thermal creep on the key flow parameters are examined in detail. Low Reynolds number flows (Re ≤ 1) for different channel aspect ratios (0 ≤ α* ≤ 1) are considered. The effects of rarefaction on the global features of the flow and thermal development in the entrance region are examined. Dramatic reductions in the friction coefficient are observed in the entrance region due to rarefaction effects, which are enhanced by thermal creep. For the fluid heating cases considered here, thermal creep increases slip at the wall and thereby further reduces the friction coefficient and slightly enhances heat transfer at a given Reynolds number. For an identical heat flux applied to the microchannel walls, thermal creep effects become much more pronounced at lower Reynolds numbers since it results in higher axial temperature gradients.

Microscopic particle image velocimetry (microPIV) was used to measure velocities in rectangular microchannels with aspect ratios ranging from 0.97 to 5.69 for 200<Re<3267. Mean velocity profiles, velocity fluctuations, and Reynolds stresses were determined from the microPIV data. Transition to turbulence was observed at Re=1765-2315 for the five aspect ratios studied, agreeing very well with both recent microscale experiments and macroscale duct flow and indicating no evidence of early transition for any of the aspect ratios studied. The onset of fully turbulent flow was observed at Re=2600-3200. For the fully turbulent flow, the ⟨u′⟩∕umax and ⟨v′⟩∕umax fluctuations at the channel centerline were 6% and 3%–3.5% and generally agreed well with macroscale results. As aspect ratio increased, the ⟨u⟩∕umax and ⟨u′⟩∕umax profiles became flatter, with nearly uniform values extending for some distance from the centerline of the channel. This region of uniform ⟨u⟩∕umax and ⟨u′⟩∕umax became larger with increasing aspect ratio. The Reynolds shear stress for fully turbulent flow also displayed a strong dependence on aspect ratio. For the W∕H=0.97 microchannel, ⟨u′v′⟩∕umax2 steadily increased in value moving from the centerline to the wall, but for the higher aspect ratio microchannels, ⟨u′v′⟩∕umax2 remained close to zero in the center region of the microchannel before increasing in value at locations close to the wall, and this region of near zero ⟨u′v′⟩∕umax2 became larger with increasing aspect ratio. This behavior in the Reynolds shear stress is due to the region of uniform velocity and, hence, small mean shear, near the channel centerline of the high aspect ratio microchannels.

A study on fluid flow and heat transfer in rectangular microchannels with various longitudinal vortex generators

Rectangular microchannels are most common configuration in microfluidics. They can be used in many industries, for example in lab-on-chip devices. Despite standard fluid dynamics, microfluidics has a significant impact of wall boundary conditions on fluid flow. And in microfluidics, we cannot simply set no-slip boundary conditions if our goal is accurate modeling results. In rectangular microchannels, there is another important moment in modeling that is not present in circular pipes. The velocity profile of the fluid depends on the shear stress at the edges and the velocities at the walls of the microchannel change at different points of the cross-sectional wall of the microchannel. The fluid velocity is lower at the corners of a rectangular microchannel. In this paper, a solution is proposed to find a more accurate way to model the fluid flow in a rectangular microchannel by knowing the friction factor without shear stress distribution.

An experimental study on fully developed laminar flow and heat transfer in rectangular microchannels

Interfacial evolution and dynamics of liquid bridge during droplet coalescence in rectangular microchannels: Effect of aspect ratio

In the last decade, inertial microfluidics has attracted considerable attention for particle manipulation including particle focus, concentration and separation. To illustrate the detailed underlying mechanism of the particle segregation occurring in inertial microfluidics, dynamics of a single and multiple neutrally buoyant spherical particles in a straight microchannel with a rectangular cross-section under Reynolds numbers ranging from 50 to 200 have been simulated using the Lattice-Boltzmann method with the discrete external boundary force. In a square microchannel, particles are focused to the equilibrium positions, each corner and center of each edge, which is in agreement with the theoretical analysis as well as the experimental observations from the literature. Inertial focusing of multiple spherical particles in rectangular microchannels with different channel aspect ratio over a wide range of Reynolds numbers have also been explored. Trains of particles are formed along the flow direction near the planar equilibrium positions of a single particle. The well-known focusing of the particles at the two centers of the long channel walls occurs at a relatively low Reynolds number, whereas additional stable equilibrium positions emerge close to the short walls with increasing Reynolds number. The equilibrium positions of particles with different sizes have also been studied, and larger particles focus more close to the centerline than the smaller ones. The verified numerical tool provides a new method to simulate the particle segregation and dynamics in inertial microfluidics, uncovers the underyling physics, and provides a design tool for inertial microfluidic devices.

Mean flow structure in high aspect ratio microchannel flows

Electro-osmotic flows are a means of circulating polar fluids through microchannels without resorting to mechanical pumping. The lack of moving parts, of noise and the ease of integration in silicon chips make them an interesting option for microchip cooling and miniaturized total analysis systems. This paper describes an optimization in terms of first- and second-law analysis of the cross-section of a microchannel subject to electro-osmotic flow. Starting from rectangular cross-sections of different aspect ratios, its corners are progressively smoothed and the resulting Poiseuille and Nusselt numbers computed. Performance evaluation criteria are then used to assess the change in, among others, heat transfer rate, temperature difference between wall and bulk fluid, and equivalent pumping power. The entropy number is also computed and the results commented. It is found that the latter criterion highlights a configuration of minimum entropy generation, whereas the trends of the heat transfer and temperature difference are opposite to that of the equivalent pumping power.

Advanced Microsink Design Utilizing Disruptive Structures

Microchannels based heat sinks are considered as potential thermal management solution for electronic devices. The overall thermal performance of a microchannel heat sink depends on the flow characteristics within microchannels as well as within the inlet and outlet plenum and these flow phenomena are influenced by channel aspect ratio, plenum aspect ratio and flow arrangements at the inlet and outlet plenums. In the present research work an experimental investigation has been carried out to understand how the heat transfer and pressure drop attributes vary with different plenum aspect ratio and channel aspect ratio under different flow arrangements. For this purpose microchannel test pieces with two channel aspect ratios, 4.72 and 7.57 and three plenum aspect ratios, 2.5, 3.0 and 3.75 have been tested under three flow arrangements, namely U-, S- and P-types. Test runs were performed by maintaining three constant heat inputs, 125 W, 225 W and 375 W in the range 224.3 ≤ Re ≤ 1121.7. Reduction in channel width (increase in aspect ratio, defined as depth to width of channel) in the present case has shown about 126 to 165% increase in Nusselt number, whereas increase in plenum length (reduction in plenum aspect ratio defined as width to length of plenum) has resulted in 18 to 26% increase in Nusselt number.

<p indent="0mm">In this study, a hybrid heat sink model with a rectangular microchannel and porous fin in a three-dimensional electronic device is taken as the research object. First, constructal design optimization of the hybrid heat sink is conducted using a complex function as the optimization objective. This complex function comprises a linear weighting sum of the entransy dissipation rate and pumping power consumption. The total volumes of the microchannel and porous fin are fixed, and the microchannel aspect ratio is considered as the optimization variable. The effects of the total mass flow rate of cooling water, the length of the hybrid heat sink, and the number of elements on the optimal constructs are analyzed. Furthermore, multi-objective optimization of the hybrid heat sink is conducted using an artificial neural network and the NSGA-II algorithm. The entransy dissipation rate and pumping power consumption are considered as the optimization objectives. The results reveal that when the total mass flow rate of cooling water is <sc>0.002 kg/s,</sc> there exists an optimal aspect ratio of 0.15, which minimizes the complex function to a value of 0.905. This is different from the complex function optimization result of the maximum temperature difference. After optimization, the value of the complex function is reduced by 9.5% compared with its original value. For the multi-objective optimization, the minimum deviation index of LINMAP and TOPSIS decision-making methods is 0.385, which is the optimal result. The corresponding optimal aspect ratio is 0.20, which can be adopted as the optimal constructal design scheme for multi-objective optimization of the hybrid heat sink. The results of this study provide new theoretical guidelines for the heat dissipation designs of hybrid heat sinks of electronic devices.

Conjugate heat transfer in rectangular, trapezoidal and rectangular-trapezoidal combined microchannels is investigated for hydro-dynamically fully developed, single-phase, laminar flow with no-slip conditions. Hydraulic diameter of all geometries is so adjusted so that for given aspect ratio of the rectangular microchannel the other two geometries have same hydraulic diameter. Various physical quantities heat transfer coefficient, Temperature gradient has been examined to deduce the effective cooling in the microchannel. Simulation has also been done for various velocities to have a look on effectiveness of the microchannel.

Analysis of the transition from laminar annular flow to intermittent flow of steam condensation in noncircular microchannels