Noncircular Microchannels Research Articles

Computational study of bubble, thin-film dynamics and heat transfer during flow boiling in non-circular microchannels

Flow boiling in multi-microchannel evaporators is one of the most efficient thermal management solutions for high-power-density applications. However, there is still a lack of understanding of the governing two-phase heat and mass transfer processes that occur in these devices, which has resulted in a limited availability of applicable boiling heat transfer prediction methods based on first principles, and of reliable thermal design tools. This article presents a systematic analysis of the dynamics of bubbles and the surrounding liquid film during flow boiling in three-side-heated non-circular microchannels. The study is performed using a custom version of ESI OpenFOAM v2106 with a geometric volume-of-fluid method to capture the interface dynamics, also incorporating conjugate heat transfer through the evaporator walls. The hydraulic diameter of the channel is fixed to Dh=0.229mm and the range of width-to-height aspect ratios ϵ=0.25−4 is examined. We investigate different fluids, namely water, HFE7100, R1233zd(E), R1234ze(E), and evaporator materials, namely copper, aluminium, silicon, stainless steel, with base heat fluxes in the range qb=50−200kW/m2. The results show that conjugate heat transfer acts to make the temperature distributions around the perimeter of the channel cross-section more uniform, and that the topography of the lubricating film and the extension of the dry vapour patches that develop while the film is depleted both depend on the cross-sectional channel shape and influence the heat transfer performance significantly. For highly wetting conditions, channels with ϵ=0.25 tend to allow enhanced heat transfer rates, with a spatially-averaged Nusselt number that is 50% higher than that obtained for ϵ=1 (square channels) and 10% higher than that for ϵ=4. This arises thanks to an extended evaporating film that covers the vertical walls which, owing to the three-side-heated configuration, contribute twice to the spatially-averaged heat transfer performance. For more hydrophobic conditions, large dry patches develop over the vertical walls for ϵ=0.25 due to the lower evaporator temperatures, leading to reduced heat transfer, with thermal performance weakly dependent on ϵ in the range ϵ=0.5−2.

Boiling heat transfer of binary and ternary mixtures in multiple rectangular microchannels

In this study, the boiling heat transfer of binary and ternary refrigerant mixtures in a horizontal multiport tube with multiple rectangular microchannels was investigated. The influences of the mass velocity, vapor quality, and heat flux on the heat transfer were evaluated for near- and non-azeotropic mixtures of R410A, R32/CF3I, R32/R1123, R32/R1234yf, R1123/R32/R1234yf, and CO2/R32/R1234yf. The obtained results were compared with those of pure refrigerants R1234yf and R32. For R1234yf at low mass velocity, the flow patterns were estimated as intermittent (plug) and slug-annular flows with an extremely thin liquid film. The heat transfer coefficient decreased when the heat flux was increased because dry patch areas formed and expanded on the sides of the noncircular microchannel. The near-azeotropic mixtures with small temperature glides exhibited heat transfer characteristics similar to those of the pure refrigerant. Conversely, the heat transfer coefficients deteriorated slightly under lower mass velocities. The heat transfer coefficients of binary and ternary mixtures with larger temperature glides were significantly different from those of pure refrigerants and near-azeotropic mixtures; they decreased with decreasing mass velocity owing to mass diffusion resistance. The heat transfer coefficients of binary mixtures with large temperature glides decreased monotonically as the mass velocity decreased, and the heat transfer noticeably deteriorated at lower mass velocities. The heat transfer model was developed for rectangular microchannels by considering the heat transfer contributions of thin liquid film evaporation due to surface force, forced convection, and nucleate boiling. Moreover, this model considers the flow pattern of multiple microchannels, heat transfer degradations due to dry patches and mass diffusion resistance, and dryout incipience quality. The results of the present model agree well with the heat transfer coefficients, including those of binary and ternary mixtures, in the pre- and post-dryout regions with a mean percentage error of 3.1%.

Measurement of transient liquid film and its effect on flow boiling heat transfer in non-circular microchannels

Micro liquid film plays an important role during flow boiling in microchannels. Current studies on flow boiling in microchannels have mainly focused on circular microchannels. However, microchannels with sharp corners, such as square and rectangular microchannels, are more common in practice. In the present study, flow boiling experiments were performed in square microchannels using ethanol as the working fluid. The mass flux was 26.71 kg (m−2 s−1) and heat flux ranged from 10 to 80 kW m−2. The transient liquid film thickness during flow boiling in square microchannels was measured using a laser confocal displacement meter. The pressure drop between inlet and outlet of the microchannel and the wall temperature were measured synchronously. Experimental results demonstrated that heat transfer performance depended on liquid film thickness and the thickness in square microchannels was smaller than in circular microchannels under the same capillary number. The variation of liquid film thickness during flow boiling in square microchannels was analyzed and a theoretical model was proposed for predicting liquid film thickness. According to variation in wall temperature and liquid film thickness, the flow boiling process in square microchannels was divided into liquid slug, elongated bubble, and dryout zones. The heat transfer mechanism of different zones was analyzed. In addition, the microchannel pressure drop fluctuation and backflow phenomenon were analyzed.

Conjugate heat transfer effects on flow boiling in microchannels

• Heat distributes nonuniformly around the perimeter of microchannels in evaporators. • In single-phase flow, low aspect-ratio channels yield best heat transfer. • In two-phase flow, the Nusselt number trends vs. aspect-ratio are mixed. • High aspect-ratio channels exhibit larger thermal resistance and higher evaporator temperatures. • Conjugate heat transfer should be included in design models for evaporators. This article presents a computational study of saturated flow boiling in non-circular microchannels. The unit channel of a multi-microchannel evaporator, consisting of the fluidic channel and surrounding evaporator walls, is simulated and the conjugate heat transfer problem is solved. Simulations are performed using OpenFOAM v2106 and the built-in geometric Volume Of Fluid method, augmented with self-developed libraries to include liquid-vapour phase-change and improve the surface tension force calculation. A systematic study is conducted by employing water at atmospheric pressure, a channel hydraulic diameter of D h = 229 µm, a uniform base heat flux of q b = 100 kW / m 2 , and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range ϵ = 0.25 –4 and W f = D h / 8 − D h , respectively. The effects of conjugate heat transfer and channel aspect-ratio on the bubble and evaporative film dynamics, heat transfer, and evaporator temperature are investigated in detail. This study reveals that, when the flow is single-phase, higher Nusselt numbers and lower evaporator base temperatures are achieved for smaller channel aspect-ratios, from Nu ≃ 4 and T b − T s a t ≃ 9 K when ϵ = 4 , to Nu ≃ 6 and T b − T s a t ≃ 2 K when ϵ = 0.25 , for same fin thickness W f = D h / 8 . In the two-phase flow regime, Nusselt numbers in the range Nu = 12 − 36 are achieved. The trends of the Nusselt number versus the aspect-ratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, W f = D h / 8 and W f = D h / 4 , an overall ascending trend of Nu for increasing aspect-ratios is apparent, although in the narrower range ϵ = 0.5 –2 the Nusselt number appears weakly dependent on ϵ . For thicker fins, W f = D h / 2 and W f = D h , the Nusselt number decreases slightly when increasing the aspect-ratio in the range ϵ = 0.5 –2, although this trend is not monotonic when considering the entire range of aspect-ratios investigated. Nonetheless, due to conjugate heat transfer, Nusselt numbers and evaporator base temperatures follow different trends when varying the aspect-ratio, and channels with ϵ < 1 seem to promote lower evaporator temperatures than higher aspect-ratio conduits.

Measurement of Transient Liquid Film and its Effect on Flow Boiling Heat Transfer in Non-Circular Microchannels

Measurement of Transient Liquid Film and its Effect on Flow Boiling Heat Transfer in Non-Circular Microchannels

A Review on the Hydrodynamics of Taylor Flow in Microchannels: Experimental and Computational Studies

Taylor flow is a strategy-aimed flow to transfer conventional single-phase into a more efficient two-phase flow resulting in an enhanced momentum/heat/mass transfer rate, as well as a multitude of other advantages. To date, Taylor flow has focused on the processes involving gas–liquid and liquid–liquid two-phase systems in microchannels over a wide range of applications in biomedical, pharmaceutical, industrial, and commercial sectors. Appropriately micro-structured design is, therefore, a key consideration for equipment dealing with transport phenomena. This review paper highlights the hydrodynamic aspects of gas–liquid and liquid–liquid two-phase flows in microchannels. It covers state-of-the-art experimental and numerical methods in the literature for analyzing and simulating slug flows in circular and non-circular microchannels. The review’s main objective is to identify the considerable opportunity for further development of microflows and provide suggestions for researchers in the field. Available correlations proposed for the transition of flow patterns are presented. A review of the literature of flow regime, slug length, and pressure drop is also carried out.

Interfacial instabilities due to immiscible fluid displacement in circular and non-circular microchannels

Interfacial instabilities caused by the displacement of one fluid by another were studied experimentally in three horizontal channels of different shape of cross section with hydraulic diameters ranging from 100 to 200 µm. Flow instabilities were induced by the displacement of a more viscous fluid (silicone oil) by an immiscible, less viscous fluid (aqueous solutions of glycerol) with viscosity ratios between the two fluids ranging from 20 to 100. In addition, the effect of surfactant was studied by the addition of Sodium Dodecyl Sulfate to the displacing fluid. Flow regime maps were developed for the different types of instability observed, with more complex 3-D instabilities shown to occur as the capillary number increases. Whilst fluid viscosities, channel shape and wall wettability were shown to affect the threshold capillary numbers for instabilities, the addition of SDS did not have a significant impact, which is believed to be a consequence of the long contact time between the two fluids during the whole displacement process. It was found that higher flow rates of the displacing fluid (resulting in more complex interfacial instabilities) did not cause a proportionally faster removal of the displaced fluid, which is an important finding for practitioners.

A Numerical Study of Initiation and Migration of Trapped Oil in Capillaries with Noncircular Cross Sections

To clarify the initiation and migration mechanisms of a discontinuous oil phase in pores, a numerical study was performed to interpret the starting phenomenon and flowing rules of oil trapped in capillaries that have noncircular cross sections. In this study, capillaries with three different cross sections were used to investigate the deformation law of oil and the pressure drop across these microchannels at different displacement velocities by computational fluid dynamics (CFD). The geometrical structure of the microchannels was precisely controlled, and the migration process of the oil, which is too small to be observed by direct experimentation, was assessed and quantitatively analyzed. By analyzing the shape of the trapped oil after reaching a steady state at different velocities, the nonstart and start conditions could be distinguished and the accuracy of the numerical method was verified by a comparison with an analytical method (the MS-P method). Two aspects of oil migration in noncircular microchannels were observed in combination with previous studies: there is a driving force on the cross section of the oil drop and a viscous force at the oil-water interface in the corners, and the more irregular the pore section is, the more easily the trapped oil will migrate. Additionally, the influence of the microchannel cross-sectional shape on the non-Darcy flow of a discontinuous oil phase was clarified. It can be concluded that the presence of the non-Darcy flow in pores arises because trapped oil, as a discontinuous phase, cannot be separated from the capillary wall without reaching critical velocity.

Marangoni effect on the motion of a droplet covered with insoluble surfactant in a square microchannel

Despite its significance in various applications, e.g., droplet microfluidics and chemical enhanced oil recovery, the motion of surfactant-laden droplets in non-circular microchannels remains an unsolved fundamental problem. To facilitate studies in this area, we present a systematic investigation on the motion of a droplet covered with an insoluble surfactant in a square microchannel. This work is realized via our three-dimensional front-tracking finite-difference model with integration of the convection-diffusion equation for surfactant transport on a deforming drop surface. Our results indicate significant effects of the surfactant on steady-state characteristics of droplet motion, especially the droplet-induced additional pressure loss in the channel. More particularly, the surfactant-induced reduction in drop surface tension remarkably lowers the additional pressure loss, but this effect can be fully counteracted by the effect of surface tension gradient induced Marangoni stress (i.e., to enlarge the additional pressure loss). The increasing effect of the Marangoni stress is primarily determined by two surfactant-related dimensionless parameters, i.e., the surface Peclet number and the elasticity number. The additional pressure loss significantly increases with either of them increasing. Besides, the Marangoni effect on the droplet-induced additional pressure loss also strongly depends on three other independent parameters, i.e., it is inhibited by decreasing the size ratio of the drop to the channel, increasing the viscosity ratio of the drop to the surrounding fluid, or increasing the capillary number. Finally, we discuss the mechanism of the Marangoni effect on drop motion via analyzing the distributions of the surfactant concentration and drop surface velocity on a three-dimensional drop surface.

Residual film thickness following immiscible fluid displacement in noncircular microchannels at large capillary number

An experimental study of the displacement of one immiscible fluid by another was performed in microchannels with circular, square and near‐semicircular cross‐sections, with hydraulic diameters from 100 to 200 μm. Experiments were performed over a range of capillary number, Ca, from 0.02 &lt; Ca &lt; 80, with viscosity ratios between the two fluids ranging from 20 to 100. The liquid film left on the channel wall following the advance of the displacing fluid was obtained from visual measurements and a method for the estimation of mean film thickness was shown to be in good agreement with existing correlations. The addition of a surfactant (Sodium Dodecyl Sulfate, SDS) dissolved in the displacing fluid led to a reduction in the thickness of the residual film. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 3456–3466, 2018

Effect of Cross Aspect Ratio on Flow in Diverging and Converging Microchannels

Aspect ratio is an important parameter in the study of flow through noncircular microchannel. In this work, three-dimensional numerical study is carried out to understand the effect of cross aspect ratio (height to width) on flow in diverging and converging microchannels. Three-dimensional models of the diverging and converging microchannels with angle: 2–14 deg, aspect ratio: 0.05–0.58, and Reynolds number: 130–280 are employed in the simulations with water as the working fluid. The effects of aspect ratio on pressure drop in equivalent diverging and converging microchannels are studied in detail and correlated to the underlying flow regime. It is observed that for a given Reynolds number and angle, the pressure drop decreases asymptotically with aspect ratio for both the diverging and converging microchannels. At small aspect ratio and small Reynolds number, the pressure drop remains invariant of angle in both the diverging and converging microchannels; the concept of equivalent hydraulic diameter can be applied to these situations. Onset of flow separation in diverging passage and flow acceleration in converging passage is found to be a strong function of aspect ratio, which has not been shown earlier. The existence of a critical angle with relevance to the concept of equivalent hydraulic diameter is identified and its variation with Reynolds number is discussed. Finally, the effect of aspect ratio on fluidic diodicity is discussed which will be helpful in the design of valveless micropump. These results help in extending the conventional formulae made for uniform cross-sectional channel to that for the diverging and converging microchannels.

Hydrodynamics and mass transfer in liquid-liquid non-circular microchannels: Comparison of two aspect ratios and three junction structures

Hydrodynamics and mass transfer in liquid-liquid slug flow inside a square microchannel (MC-1, aspect ratio 1) and a rectangular microchannel (MC-2, aspect ratio 2) with identical hydraulic diameter (400µm) were investigated experimentally. The results showed that the overall volumetric mass transfer coefficient (KLa) in MC-2 was 16.2% lower than that in MC-1 due to enlargement of the dispersed phase slugs in MC-2, leading to development of stagnant zones within plugs and reduction of wall film renewal. Three different junction configurations (JC) were studied. KLa was enhanced by up to 34.8% using JC-1, compared to JC-2 or JC-3 due to greater movement of the biphasic interface, resulting in higher mixing within slugs using JC-1. The junction arrangement influenced not only the mass transfer during the slug-formation stage, but also the slug-flowing stage due to effects on the vortex patterns along the microchannel.

Experimental and numerical investigations of local condensation heat transfer in a single square microchannel under variable heat flux

This paper presents an experimental investigation on the local and average condensation heat transfer in a single noncircular microchannel. Furthermore, it develops one dimensional model for annular condensation flow in a microchannel under variable heat flux condition. The condensate film thickness is calculated for each location in a microchannel including the effects of capillary number, Boiling number, contact angle, heat flux, vapor pressure, and hydraulic diameter. A comparative study shows that the present model well predicts the experimental data concerning local condensation heat transfer coefficient. The mean deviation between the local predictions of the theoretical model with the measurements for local heat transfer coefficient is 20%. It is found that the correlation of Quan et al. (2008) [19] gives the good predictions of the measurements with maximum deviation of 13% at high Reynolds number.

A three-dimensional computational model of H2–air premixed combustion in non-circular micro-channels for a thermo-photovoltaic (TPV) application

Abstract Wall temperature uniformity and enhancement in a micro combustor for thermo photovoltaic (TPV) applications have attracted considerable attention from researchers in recent years because of their direct impact on efficiency and feasibility of desired energy conversion. In this regard, numerous experimental and numerical studies in micro-combustion application have been conducted and reported. However, most previous studies have been focused on geometrical configurations limited to planar and circular channels. It is therefore of interest to investigate the impact of different channel geometries on wall temperature distribution and energy conversion efficiency. This study addresses flow and flame behavior in a micro-combustor. By utilizing the well-established computational fluid dynamics (CFD) approach, the effect of geometrical parameters on the flow behavior and wall temperature is examined and evaluated. In order to improve the productive capability of the computational model, several steady state Reynolds Average Numerical Simulation (RANS) turbulence models alongside with different reaction rate formulations are evaluated. The results indicate that Reynolds Stress Model (RSM) with Eddy Dissipation Concept (EDC) provide the best quantitative prediction. The developed model is employed to investigate the effect of inlet velocity on flame structure and outer wall temperature. Furthermore, the effect of reactor cross sections, including circular, square, rectangular, triangular and trapezoidal, on the wall temperature is also evaluated. The results show that the wall temperature is increased with an increase in the inlet velocity. Trapezoidal and triangular cross-sections are found to have better performance in terms of Figure of Merit (FoM), a parameter used in this study to gage thermal and hydraulic performance of a micro-combustor.

Reconstructing the 3D shapes of droplets in glass microchannels with application to Bretherton’s problem

The outstanding potential of absorbance imaging for the 3D shape analysis of dynamic scenes in multiphase flows is demonstrated and experimentally confirmed. The experimental procedures and workflows have been extended and applied to the 3D shape analysis of droplets moving in microchannels. Using this technique, the problem of measuring the thickness of the lubrication layer and the droplet geometry with sub-\(\upmu\)m resolution could be solved. The obtained data generally conform to models based on Bretherton’s theory and allow its extension even for non-circular microchannel geometries.

Condensation heat transfer enhancement in a horizontal non-circular microchannel

Numerical investigation of steam condensation in a non-circular microchannel is conducted. The conservation equation of mass, momentum and energy have been solved in both phases for various microchannel hydraulic diameters. Different correlations established for condensation flow heat transfer are evaluated. It is found that the correlation of Dobson et al. and Koyama et al. are the closest to the calculated steam condensation average heat transfer. Results are given for different microchannels shapes, aspect ratio, and for various inlet vapor mass fluxes and contact angle. Reducing the microchannel hydraulic diameter from 250 to 80 μm reduces the condensate film thickness and increases the average heat transfer coefficient up to 39% for the same mass flux. The enhancement factor of the heat transfer coefficient reaches 100% by increasing the contact angle from 6 to 15°. The influence of the microchannel shape on the condensation heat transfer is highlighted too. Thus it is seen that the lowest average Nusselt numbers are obtained for the square microchannel.

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

The condensation liquid was pulled into the corner of the noncircular microchannels due to the effect of surface tension. The thickness in the meniscus region increased along the channel with the decrease of the thin-liquid film region. The condensation annular flow completed when the short side wall of the microchannel was entirely covered by the meniscus region. At this point, the vapor liquid interface presented a circular shape, and the transition flow occurred. The location of transition flow depends on the short side length and the meniscus area ratio (MAR) at the breakup point under the same experimental conditions. The location of the transition flow moves toward the outlet with the increase of the short side length and decrease of the aspect ratio in the rectangular and trapezoidal microchannels. The transition flow occurs further downstream, and the annular flow regime is expanded in the trapezoidal and triangular microchannels compared with the rectangular microchannel with the same hydraulic diameter. The analysis was compared with the experimental data for different cross section shape microchannels. Most predicted values provided good predictions of the experimental data with a MRD of 30.50% and a MAD of 5.15mm. Based on the numerical results, a dimensionless correlation with a new parameter MAR was also introduced to predict the transition flow location at different cross section shape microchannels.

Slug flow heat transfer in square microchannels

While heat transfer in Taylor (slug) flows in circular microtubes has been examined exclusively by researchers, noncircular microchannels have not been widely considered in the literature. This is a large gap in research since noncircular microchannels are common structures in microcooling processes. Square and rectangular microchannels are the most important examples. In the present study the heat transfer process in slug flows in square microchannels has been investigated numerically under constant wall temperature boundary condition. The local heat flux for the moving slugs has been converted to total microchannel heat flux using the integration methods suggested recently by the authors. This leads to microchannel wall average heat flux which is the parameter of interest in heat sink problems. Potential effects of liquid film around bubbles on heat transfer process have been discussed by comparing the numerical results with experimental data. Finally, effects of Reynolds number, contact angle, and slug length on heat transfer in slug flow inside square microchannels have been studied and graphs for prediction of slug flow heat transfer in square microchannels have been provided for slug flows with no liquid film or very thin liquid film around the bubbles.

Blood flow in non-circular microchannel under pulsating condition

With the advent of point-of-care diagnostics systems, investigation into properties of blood at micron scales is gaining fundamental importance. Past research has shown that blood displays significantly different properties at small scales than at conventional scales. This study investigates properties of blood flow in small non-circular passages (hydraulic diameter: 95-960 μm) under pulsatile condition. The observations are compared with flow of water under otherwise similar conditions. Prominent observation includes a more stable response to abrupt mass flow rate fluctuations as compared to water, which is attributed to the presence of deformable cells in blood. The study also reveals that, the pressure drop for blood flow with pulsations is less than for steady condition with the difference increasing with a reduction in microchannel size and flow rate. Such a comparative study facilitates development of models for blood flow at micro-scales, and will eventually aid in the design of future micro-Total ...

Measurement of pressure distribution inside a cross-corrugated heat exchanger using microchannel pressure tappings

This paper suggests a novel method to measure pressure distribution inside a cross-corrugated heat exchanger using microchannel pressure tappings. The microchannel-embedded corrugated sheet is fabricated. In particular, the hydraulic diameter of the non-circular microchannel is measured. The specially designed pressure chamber is made to control the reference pressure. The pressure response is examined theoretically and experimentally in terms of delay time and peak pressure. The delay time shows linear decreases in the length of the microchannel and the peak pressure also decreases with increasing frequency of the reference pressure. The microchannel pressure tapping is applicable to steady pressure measurement but not to transient pressure measurement. Finally, 20 corrugated sheets and 1 microchannel-embedded corrugated sheet are stacked to form the cross-corrugated heat exchanger. The static and stagnation pressures inside the heat exchanger are measured. The measured friction factor is compared with that in previous reports.