Pressure Drop In Microchannels Research Articles

Generalized Two-Phase Pressure Drop and Heat Transfer Correlations in Evaporative Micro/Minichannels

Existing databases and correlations in literature on the microchannel pressure drop and heat transfer are reviewed in this paper. From this review, it is found that none of the existing correlations can cover the wide ranges of working fluids, operational conditions, and different microchannel dimensions. In addition, the importance of the Bond number, which relates the nominal bubble dimension or capillary parameter with the channel size, is revealed in this paper. Using the Bond number, improved correlations of pressure drop and heat transfer are established, which predict the existing data well over a wide range of channel sizes, fluids, and operational conditions.

Measurement and modeling of condensation heat transfer in non-circular microchannels

Heat transfer coefficients in six non-circular horizontal microchannels (0.424 < D h < 0.839 mm) of different shapes during condensation of refrigerant R134a over the mass flux range 150 < G < 750 kg m −2 s −1 were measured in this study. The channels included barrel-shaped, N-shaped, rectangular, square, and triangular extruded tubes, and a channel with a W-shaped corrugated insert that yielded triangular microchannels. The thermal amplification technique developed and reported in earlier work by the authors is used to measure the heat transfer coefficients across the vapor-liquid dome in small increments of vapor quality. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to interpret the results based on the applicable flow regimes. The effect of tube shape was also considered in deciding the applicable flow regime. A modified version of the annular-flow-based heat transfer model proposed recently by the authors for circular microchannels, with the required shear stress being calculated from a non-circular microchannel pressure drop model also reported earlier was found to best correlate the present data for square, rectangular and barrel-shaped microchannels. For the other microchannel shapes with sharp acute-angle corners, a mist-flow-based model from the literature on larger tubes was found to suffice for the prediction of the heat transfer data. These models predict the data significantly better than the other available correlations in the literature.

사각 마이크로채널 내의 2 상 유동 압력강하 상관식의 검증 및 개발

2 상 유동 압력강하에 대한 모델은 균질유동모델과 분리유동모델 두 가지가 있다. 많은 선행 연구자들은 마이크로채널에서의 2 상 유동 압력강하에 대한 상관식을 제시하였고, 대부분은 분리유동모델에 해당하는 Lockhart-Martinelli<SUP>(27)</SUP>의 수정된 상관식에 기초하고 있다. 본 연구에서는 사각 마이크로채널에서의 압력강하에 대한 연구를 위해서 액상의 물과 기상의 질소를 사용하여 사각 마이크로채널에서의 실험을 수행하였다. 2 상 마찰 압력강하는 2 상 유동양식에 큰 연관성을 가지고 있는 결과를 확인할 수 있었다. 6 가지의 2 상 점성 모델을 포함한 균질유동 모델 (Owen<SUP>(21)</SUP>’s, MacAdams<SUP>(22)</SUP>’s, Cicchitti et al.<SUP>(23)</SUP>’s, Dukler et al.,<SUP>(24)</SUP> Beattie and Whalley,<SUP>(25)</SUP> Lin et al.<SUP>(26)</SUP>)과 6 가지의 분리유동 모델 (Lockhart and Martinelli,<SUP>(27)</SUP> Chisholm,<SUP>(31)</SUP> Zhang et al.,<SUP>(15)</SUP> Lee and Lee,<SUP>(5)</SUP> Moriyama and Inue,<SUP>(4)</SUP> Qu and Mudawar<SUP>(8)</SUP>)에 대한 평가를 실험결과와 비교를 통해 수행하였다. 가장 우수한 2 상 점성 모델은 Beattie and Whalley 의 모델이었고, 가장 우수한 분리유동 모델은 Qu and Mudawar 의 상관식이였다. 균질유동모델과 분리유동모델 모두에 대해서 2 상 유동양식에 종속성을 나타내었다. 그러므로, 본 연구에서는 2 상 유동 양식에 기초한 새로운 상관식을 균질유동모델과 분리유동모델에 대해 각각을 제시하였다.

Experimental Investigation of Two-Phase Pressure Drop in a Microchannel

Experimental results of two-phase pressure drop in a horizontal circular microchannel are reported in this paper. A test tube was made of fused silica having an internal diameter of 781 μm with a total length of 261 mm and a heated length of 191 mm. The outer surface of the test tube was coated with an electrically conductive thin layer of ITO (indium tin oxide) for direct heating of the test section. Refrigerants R134a and R245fa were used as the working fluids, and mass flux during the experiments was varied between 100 and 650 kg/m2-s. Experiments were performed at two different system pressures corresponding to saturation temperatures of 25°C and 30°C for R134a and at three different system pressures corresponding to saturation temperatures of 30°C, 35°C, and 40°C for R245fa. Two-phase frictional pressure drop characteristics with variation of mass flux, vapor fraction, saturation temperature, and heat flux were explored in detail. Finally, the prediction capability of some well-known correlations available in the literature, some developed for macrochannels and others especially developed for microchannels, was assessed.

Surface Roughness Effects on Flow Boiling in Microchannels

The influence of surface roughness on flow boiling heat transfer and pressure drop in microchannels is experimentally explored. The microchannel heat sink employed in the study consists of ten parallel, 25.4 mm long channels with nominal dimensions of 500×500 μm2. The channels were produced by saw-cutting. Two of the test piece surfaces were roughened to varying degrees with electrical discharge machining (EDM). The roughness average Ra varied from 1.4 μm for the as-fabricated, saw-cut surface to 3.9 μm and 6.7 μm for the two roughened EDM surfaces. Deionized water was used as the working fluid. The experiments indicate that the surface roughness has little influence on boiling incipience and only a minor impact on saturated boiling heat transfer coefficients at lower heat fluxes. For wall heat fluxes above 1500 kW/m2, the two EDM surfaces (3.9 μm and 6.7 μm) have similar heat transfer coefficients that were 20–35% higher than those measured for the saw-cut surface (1.4 μm). A modified Bertsch et al. [2009, “A Composite Heat Transfer Correlation for Saturated Flow Boiling in Small Channels,” Int. J. Heat Mass Transfer, 52, pp. 2110–2118] correlation was found to provide acceptable predictions of the flow boiling heat transfer coefficient over the range of conditions tested. Analysis of the pressure drop measurements indicates that only the roughest surface (6.7 μm) has an adverse effect on the two-phase pressure drop.

Modeling of velocity distribution among microchannels with triangle manifolds

AbstractA model of velocity distribution among microchannels with triangle manifolds is proposed. According to the flow behaviors analyzed by Fluent, the manifolds are divided into several approximate rectangular channels, and then an equivalent simplified resistance network model is developed to establish the relationships between the velocities and pressure drops in microchannels and approximate rectangular channels. The velocity distributions are calculated under two situations, respectively, considering and ignoring singular losses. The outcomes of the present study are compared with Fluent's simulated results to analyze the effects of singular losses on the velocity distributions. It indicates that the proposed model is suitable for the calculation of velocity distribution among microchannels with obtuse angled or right triangle manifolds under low Reynolds numbers. The premise of ignoring singular losses is that the frictional pressure drops are three times larger than the singular pressure drops in each flow loop. The manifold optimization results indicate that the velocity distribution among microchannels with right triangle manifolds is much more uniform than that of the corresponding one with obtuse angled manifolds. © 2009 American Institute of Chemical Engineers AIChE J, 55: 1969–1982, 2009

Recent Work on Boiling and Condensation in Microchannels

Recent work on boiling of water and condensation of steam in single and parallel microchannels is reviewed in this paper. It is found that the amplitude and frequency of fluctuations of temperature and pressure during the unstable flow-boiling mode depend greatly on the inlet/outlet configurations and the exit vapor quality. By fabricating an inlet restriction on each microchannel or the installation of a throttling valve upstream of the test section, reversed flow of vapor bubbles can be suppressed resulting in a stable flow-boiling mode. Boiling heat transfer coefficient and pressure drop in microchannels under stable flow-boiling conditions are obtained. These data at high vapor qualities are found to be substantially different from the correlations obtained for flow-boiling in macrochannels. Microbubble emission boiling phenomena, which can defer the arrival of critical heat flux, exist in a partially heated Pyrex glass microchannel at sufficiently high heat flux and high inlet subcooling conditions. For condensation in a microchannel, transition from annular flow to slug/bubbly flow is investigated. The occurrence of the injection flow is owing to the instability of the liquid/vapor interface. The location, at which the injection flow occurs, depends on the mass flux and the cooling rate of steam. Increase in steam mass flux, decrease in cooling rate, and microchannel diameter tend to enhance the instability of the condensate film on the wall, resulting in the occurrence of injection flow further downstream at increasingly high frequency. The pressure drop in the condensing flow increases with the increase in mass flux and quality or with decreasing microchannel diameter. The existing correlations for pressure drop and heat transfer of condensing flow in macrochannels overestimate the experimental data in microchannels.

Slip-Flow Pressure Drop in Microchannels of General Cross Section

In the present study, a compact analytical model is developed to determine the pressure drop of fully-developed, incompressible, and constant properties slip-flow through arbitrary cross section microchannels. An averaged first-order Maxwell slip boundary condition is considered. Introducing a relative velocity, the difference between the bulk flow and the boundary velocities, the axial momentum reduces to Poisson’s equation with homogeneous boundary condition. Square root of area is selected as the characteristic length scale. The model of Bahrami et al. (2006, “Pressure Drop of Laminar, Fully Developed Flow in Microchannels of Arbitrary Cross Section,” ASME J. Fluids Eng., 128, pp. 1036–1044), which was developed for no-slip boundary condition, is extended to cover the slip-flow regime in this study. The proposed model for pressure drop is a function of geometrical parameters of the channel: cross sectional area, perimeter, polar moment of inertia, and the Knudsen number. The model is successfully validated against existing numerical and experimental data collected from different sources in literature for several shapes, including circular, rectangular, trapezoidal, and double-trapezoidal cross sections and a variety of gases such as nitrogen, argon, and helium.

A new type of diabatic flow pattern map for boiling heat transfer in microchannels

Flow pattern data and bubble measurements for two small diameter sight glass tubes are used to propose a new type of flow pattern map for evaporating flows in microchannels. Rather than segregating the observations into the traditional flow regimes and an adiabatic map, the new diabatic map classifies flows into three types: (i) the isolated bubble regime, where the bubble generation rate is much larger than the bubble coalescence rate and includes both bubbly and slug flows, (ii) the coalescing bubble regime, where the bubble coalescence rate is much larger than the bubble generation rate and exists up to the end of the coalescence process and (iii) the annular regime, whose extent is limited by the vapor quality at the onset of critical heat flux. This formulation is thought to be more useful for phenomenological modeling of the processes controlling boiling heat transfer and two-phase pressure drops in microchannels, and it also visually defines the feasible operating limit of microchannel heat spreaders at the critical vapor quality corresponding to critical heat flux (CHF). The database covers two refrigerants (R-134a and R-245fa) and two channel diameters (0.509 and 0.790 mm). The micro-evaporator length was varied from 20 to 70 mm, the inlet subcooling from 2 to 15 °C, the mass flux from 200 to 2000 kg m−2 s−1 and heat fluxes up to 597 kW m−2. Three different saturation temperatures were tested: 26, 30 and 35 °C.

Single phase pressure drop in microchannels

This article focuses on investigating fully developed liquid and vapor flow through rectangular microchannels with hydraulic diameters varying from 69.5 to 304.7 μm and with aspect ratios changing from 0.09 to 0.24. R134a liquid and vapor were used as the testing fluids. During the experiments, the Reynolds numbers were varied between 112 and 9180. Pressure drop data are used to characterize the friction factor in the laminar region, the transition region and the turbulent region. When the channel surface roughness was low, both the laminar friction factor and the critical Reynolds number approached the conventional values, even for the smallest channel tested. Hence, there was no indication of deviation from the Navier–Stokes flow theory for rectangular microchannels. The friction factor data in the turbulent region were larger than the predictions from the [Churchill, S.W., 1977, Friction factor equations spans all fluid-flow regimes, Chemical Engineering 45, 91–92] equation for smooth tubes, even for the smoothest channel tested ( R a/ D h = 0.14%). In addition, it was likely that surface roughness was responsible for higher laminar flow friction and earlier transition to turbulent flow in one of the channels tested.

State-of-the-Art Overview of Boiling and Two-Phase Flows in Microchannels

A state-of-the-art overview of recent work on boiling and two-phase flows in microchannels are reviewed, focusing primarily on the high points of recent developments. The topics covered include critical heat fluxes and their prediction in microchannels, new two-phase flow pattern maps, flow regimes and visualization in microchannels, boiling phenomena and flow instabilities in multi-microchannel cooling elements, flow boiling prediction methods specifically for microchannels and their comparison to data, trends in flow boiling data in microchannels and mechanisms that may be responsible for the seemingly divergent trends, and two-phase pressure drops in microchannels.

Adiabatic two-phase frictional pressure drops in microchannels

Two-phase pressure drops were measured over a wide range of experimental test conditions in two sizes of microchannels (sight glass tubes 0.509 and 0.790 mm) for two refrigerants (R-134a and R-245fa). Similar to the classic Moody diagram in single-phase flow, three zones were distinguishable when plotting the variation of the two-phase friction factor versus the two-phase Reynolds number: a laminar regime for Re TP < 2000, a transition regime for 2000 ⩽ Re TP < 8000 and a turbulent regime for Re TP ⩾ 8000. The laminar zone yields a much sharper gradient than in single-phase flow. The transition regime is not predicted well by any of the prediction methods for two-phase frictional pressure drops available in the literature. This is not unexpected since only a few data are available for this region in the literature and most methods ignore this regime, jumping directly from laminar to turbulent flow at Re TP = 2000. The turbulent zone is best predicted by the Müller-Steinhagen and Heck correlation. Also, a new homogeneous two-phase frictional pressure drop has been proposed here with a limited range of application.

Numerical investigation on frictional pressure loss in a perfect square micro channel with roughness and particles

A numerical study is performed to investigate the effect of inner surface roughness and microparticles on adiabatic single phase frictional pressure drop in a perfect square micro channel. With the variation of particles sizes (0.1 to 1 μm) and occupied volume ratio (0.01 to 10%) by particles, the Eulerian multi-phase model is applied to a 100 μm hydraulic diameter perfect square micro channel in laminar flow region. Frictional pressure loss is affected significantly by particle size than occupied volume ratio by particles. The particle properties like density and coefficient of restitution are investigated with various particle materials and the density of particle is found as an influential factor. Roughness effect on pressure drop in the micro channel is investigated with the consideration of roughness height, pitch, and distribution. Additionally, the combination effect by particles and surface roughness are simulated. The pressure loss in microchannel with 2.5% relative roughness surface can be increased more than 20% by the addition of 0.5 μm diameter particles.

Measurement and Modeling of Condensation Heat Transfer Coefficients in Circular Microchannels

A model for predicting heat transfer during condensation of refrigerant R134a in horizontal microchannels is presented. The thermal amplification technique is used to measure condensation heat transfer coefficients accurately over small increments of refrigerant quality across the vapor-liquid dome (0&lt;x&lt;1). A combination of a high flow rate closed loop primary coolant and a low flow rate open loop secondary coolant ensures the accurate measurement of the small heat duties in these microchannels and the deduction of condensation heat transfer coefficients from measured UA values. Measurements were conducted for three circular microchannels (0.506&lt;Dh&lt;1.524mm) over the mass flux range 150&lt;G&lt;750kg∕m2s. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to interpret the results based on the applicable flow regimes. The heat transfer model is based on the approach originally developed by Traviss, D. P., Rohsenow, W. M., and Baron, A. B., 1973, “Forced-Convection Condensation Inside Tubes: A Heat Transfer Equation For Condenser Design,” ASHRAE Trans., 79(1), pp. 157–165 and Moser, K. W., Webb, R. L., and Na, B., 1998, “A New Equivalent Reynolds Number Model for Condensation in Smooth Tubes,” ASME, J. Heat Transfer, 120(2), pp. 410–417. The multiple-flow-regime model of Garimella, S., Agarwal, A., and Killion, J. D., 2005, “Condensation Pressure Drop in Circular Microchannels,” Heat Transfer Eng., 26(3), pp. 1–8 for predicting condensation pressure drops in microchannels is used to predict the pertinent interfacial shear stresses required in this heat transfer model. The resulting heat transfer model predicts 86% of the data within ±20%.

Flow-Regime-Based Model for Pressure Drop Predictions in Microchannels

A flow-regime-based pressure drop model is developed to correlate experimental data in the intermittent and annular flow regime. For the intermittent region, an alternative method for microchannel pressure drop predictions based on the average kinetic energy of the mixture is developed. A parameter based in the Weber number, the Lockhart-Martinelli parameter, and the liquid-to-vapor density ratio is proposed for pressure drop predictions in the annular flow region. The two correlations were built on experimental data of R-410A, R-134a, and air-water mixtures with mass fluxes ranging from 50 to 300 kg/(s·m2), with quality ranging from 0 to 1. Six-port and fourteen-port aluminum microchannel tubes with hydraulic diameters of 1.54 mm and 1.02 mm, respectively, were used for the two-phase pressure drop experiments. Entrance/exit pressure drops were directly measured with an additional experiment. The losses due to the entrance/exit zones have been found to exhibit homogeneous flow characteristics, and were correlated to a homogeneous flow parameter based on the kinetic energy of the flow field.

An experimental investigation of microchannel flow with internal pressure measurements

Experiments have been conducted to investigate discrepancies in previously published data for the pressure drop in microchannels. Straight channel test sections with integrated pressure sensors were developed with channel hydraulic diameters ranging from 25 to 100 μm. Compressible flow results for 6.8 < Re < 18,814 and incompressible flow results for 4.9 < Re < 2068 have been obtained. The results suggest that friction factors for microchannels can be accurately determined from data for standard large channels. The large inconsistencies in previously published data are probably due to instrumentation errors and/or improper accounting for compressibility effects.

자성유체의 온도에 따른 점성 변화를 이용한 미소 유체 소자

This study focused on the charateristic of magnetic fluids, the viscosity deviation of magnetic fluids due to temperature changes, and fabrication of a 'purely' liquid type microvalve. The viscosity of magnetic fluids decreases sharply during increasing of temperature. The viscosity of magnetic fluids is rated 1,000 cP at the room temperature and 25 cP when the temperature reaches <TEX>$100^{\circ}C$</TEX>. Briefly, it is remarkable that the fluid flow can be controlled by the temperature and this characteristic can be adopted to the microfluidics as a microvalve. The fabrication of a liquid type microvalve is more easy than solid state microvalves and which can increase an efficiency of the controlability with respect to the thermo-pneumatic micropump which is studied broadly for many years. When the magnetic fluid used as a sealant for high level sealing, the pressure leakage is less than solid state microvalve. The experimental results show that the pressure drop in microchannel, filled with the magnetic fluid, is significant in the temperature range of <TEX>$20^{\circ}C{\sim}50^{\circ}C$</TEX> and this result explains why the use of magnetic fluids is possible as a microvalve searcher uses this characteristics. Well known thermo-pnumatic.

A microfluidic experimental platform with internal pressure measurements

Large inconsistencies in previous microchannel pressure drop data have motivated the development of a microchannel experimental platform that enables the measurement of pressures inside the microchannel. The system utilizes bulk etched silicon components that integrate pressure-sensing membranes with the microchannel test section. The deflection of the membranes is detected by an off-chip, optical-lever based system that allows the sensors to be calibrated for pressure measurement. The sensor sensitivity can be adjusted after microfabrication is complete, allowing for sensor optimization after fabrication. The sensor can be used in many applications where the fabrication of electrical contacts, piezoresistors and capacitors is problematic. Calibration uncertainties as low as 2% have been obtained during actual use of the system. The presented system resolves many of the issues associated with measuring pressures inside microchannels for the range of hydraulic diameters tested, 25–100 μm. The system has been used to study pressure drop in microchannels and a sample of the results is presented. It has been found that standard models with appropriate assumptions can accurately predict the measured pressure data.

ADIABATIC GAS–LIQUID FLOW IN MICROCHANNELS

This article presents a review of adiabatic two-phase flow in minichannels and microchannels. Differences between them are identified and explained based on this review and our own research. Several channels of decreasing diameter were used in our experiments to determine the effect of the channel size on the two-phase flow of nitrogen gas and water. The effect of channel geometry was examined by characterizing the two-phase flow in a circular and square microchannel of similar size. Only slug flow was observed in the microchannels. Four new sub-classes of slug flow were subsequently defined. A new correlation was developed for the time-averaged void fraction data in the microchannels. The two-phase pressure drop in microchannels was predicted by treating the two phases as being separate with a large velocity difference. Regarding the effect of microchannel geometry, the transition boundaries on the two-phase flow regime maps were shifted for the slug flow subcategories.

マイクロチャネルにおける摩擦圧力損失(S20-2 様々なマイクロ流動現象,S20 マイクロ・ナノスケールの流動現象)

The friction characteristics of water in a sub-millimeter scale channel were investigated experimentally. The friction factors and the critical Reynolds number were measured using water flow through circular tubes and rectangular channels. The experimental results show that the measured friction factor for water agreed well with the conventional Poiseuille (λ=64/Re) and Blasius (λ=0.316 Re^<-0.25>) equations in laminar and turbulent flow regime.