Condensation In Microchannels Research Articles

Numerical investigation on dynamic flow characteristics of methane condensation in microchannels

Microchannel condensers play an essential role in cryogenic two-phase heat management systems due to their efficient heat transfer characteristics. Thus, it is worth conducting an in-depth study on microscale condensation characteristics of cryogenic fluids. This paper delves into the flow condensation process of methane in microchannels. A two-dimensional transient model with high accuracy for cryogenic fluids has been developed by combining a self-defined program for the source term of the phase transition model. The model fully considers the boundary layer thickness and accurately explores the mesh accuracy. The complete condensation flow patterns are captured for various vapor quality, mass flux, and wall subcooling degrees. The injection flow is a unique flow regime for condensation in microchannels. The decrease in wall subcooling degree and increase in mass flux leads to the separation point at the neck of the injected flow moving towards the exit, while the annular flow region is expanding and the flow pattern transition is lagging. The mass flux improves the heat transfer coefficient more significantly at high vapor quality. During injection and bubble flow, the wall shear stress and local heat transfer coefficient are subject to bouncing and oscillations, which may induce fluctuations in the upstream annular flow. The prediction performance of six classical heat transfer correlations is evaluated. The results indicate that the Nie et al. correlation has the highest comprehensive prediction accuracy with MRD and MARD of -5.00 % and 15.83 %, respectively.

Recent advances in condensation heat transfer in mini and micro channels: A comprehensive review

Condensation in mini and microchannels has been investigated to improve utilization of thermal energy and heat transfer in various disciplines and applications. The high heat transfer coefficients accomplished by two-phase condensing flows in microchannels has prompted the use of compact heat transfer equipment and utilization of environmentally friendly heat transfer fluids. This work presents a comprehensive review of condensation studies in microchannels that have been carried out in the last two decades. The review includes experimental investigations, theoretical predictive models of two-phase heat transfer coefficients and pressure drop, as well as computational predictions of condensation in mini and microchannels. A wide range of operating conditions, channel geometries, heat transfer fluids, methods of investigation, and advancements in condensation heat transfer in mini and microchannels are presented. As condensation at the microscale presents a promising path towards heat transfer enhancement for numerous applications, this paper is intended to be a reference for future research targeting advancements in mini and microchannel condensation.

Numerical and experimental studies on flow condensation in hydrophilic microtubes

• A parametric study was performed to investigate the flow condensation in microtubes. • The dynamic contact angle was used to obtain wall temperature and shear stresses. • The effect of diameter, inlet vapor quality, and vapor mass flux was assessed. • A non-dimensional analysis was performed to define flow regime transition criteria. • A flow map for condensing flow in microtubes with 250 μm < D h ≤ 900 μm was obtained. Microchannels have increasingly been used to miniaturize heat transfer equipment, improve energy efficiency, and minimize heat transfer fluid inventory. A fundamental understanding of condensation in microscale will yield far-reaching benefits for the different areas of industry. In this study, microtubes with inner diameters of 250, 500, 600, and 900 µm were used to investigate the effect of microtube diameter, inlet quality, and mass flux on the liquid/vapor interface near the wall boundaries in condensing flow. After validation with the experimental results, a transient numerical model (based on the Volume of Fluid approach) was developed to investigate the hydrothermal properties of condensing such as bubble dynamics, flow map transitions, transient interface shear force, and temperature on flow condensation performance in terms of heat transfer coefficient and pressure drop. The liquid film thickness, slug velocity, and location of transition from annular flow to slug flow inside the microtube were characterized for different microtubes, and the resultant alteration in condensation flow heat transfer and pressure drop is discussed in detail. Using non-dimensional analysis, a flow map was constructed and compared with the available flow maps for flow condensation in microchannels. The obtained results indicated that the interfacial characteristics of condensing flow in microtubes with hydraulic diameters lower than 500 µm are majorly different from those with D > 500 µm.

Experimental investigation of refrigerant condensation in circular and square micro- and mini- channels

Microchannel condensers have the potential to improve system level efficiency by increasing the heat transfer coefficient because of the small hydraulic diameters. In addition, they offer a high surface area-to-volume ratio, reducing the overall system size and fluid inventory. Understanding condensation in microchannels is essential for efficient component design, especially with the need for new low Global Warming Potential (GWP) working fluids. Condensation of refrigerants R134a, R245fa and R1234ze(E) in circular (Dh = 1.55 mm) and square (Dh = 0.98 mm) microchannels over a range of mass fluxes (50 < G < 200 kg m−2 s−1), saturation temperatures (30 < Tsat< 50°C), and wall subcooling (1.7 < ΔT < 5K) is investigated in this study. It is found that at high mass fluxes (G ≥ 150 kg m−2 s−1), R245fa has a higher heat transfer coefficient and pressure gradient than both R1234ze(E) and R134a, which have comparable heat transfer coefficients at similar conditions. In the circular channel, for a mass flux range of 50 < G < 150 kg m−2 s−1 and Tsat = 30°C, the heat transfer coefficients for R245fa, R134a, and R1234ze(E) varied between 2.3 and 6.7 kW m−2 K−1, 2.5 and 4.7 kW m−2 K−1, and 2.2 and 5.4 kW m−2 K−1, respectively.

Lagrangian modeling of intermittent flow condensation in circular micro- and mini-channels

Very few models exist in the literature for predicting phase-change heat transfer in the intermittent flow regime during condensation in microchannels. Intermittent flow is characterized by a transient bubble-slug train, which is modeled here using a transient Lagrangian bubble tracking scheme. The compressibility of the bubbles and local parameters, such as the film thickness, bubble velocity and slug velocity are taken into account. The model also accounts for the phase change due to condensation of vapor in the bubbles into the surrounding liquid. Heat transfer is assumed to occur through conduction in the thin film around the bubble and through convection in the slug. The model is applied to the condensation of R134a in a circular microchannel of hydraulic diameter 1.55 mm at a saturation temperature of 30°C and a wall subcooling of 2.5 K. Hydrodynamic parameters show good agreement with studies in the literature, and condensation heat transfer coefficients are in good agreement with experimental results (AAD = 11%).

A critical review of analytical and numerical models of condensation in microchannels

Understanding condensation in microchannels is critical for designing optimized and compact microchannel condensers, which have the potential to increase system level efficiency and reduce system size. Analytical and numerical models can yield insights into local condensation phenomena. This study provides a review of models of condensation in microchannels. These models are categorized based on the applicable, typically annular and intermittent, flow regimes. Annular-flow-based models are further subdivided into analytical models with closed-form solutions, analytical models with numerical solutions, reduced-order CFD models, and full-fledged CFD models. Intermittent-flow-based models are subdivided into single-bubble, repeated-unit-cell, and overall-bubble-train models. The characteristics, use, and range of applicability of each of these models is highlighted. A criterion for the film Reynolds number is proposed for internal forced convective condensation in microchannels, to assess the validity of the modeling assumptions. Annular flow models using the laminar film assumption appear to be valid for Re δ0 < ~ 360, beyond which turbulent effects must be modeled. Intermittent flow models using the laminar film assumption, in turn, appear to be valid for Re δ0 < ~ 130.

Fundamental issues related to flow boiling and condensation in microchannels – experimental challenges and opportunities

In recent years, application of the microchannels has received considerable attention due to their capability for thermal management of microelectronic equipment, high power devices and development of compact evaporators. The present paper focuses at a discussion of the heat transfer during flow boiling and condensation in microchannels of the heat sink to identify the methods to improve heat transfer performance and select the physically based models for heat transfer prediction. The main purpose of this study is to discuss the mechanisms of heat transfer in microchannels at different aspect ratio for the three-sided heating case using the experimental data were obtained for refrigerant R134a and dielectric fluid.

Experimental study on two-phase flow pressure drop during ethanol–water vapor mixture condensation in microchannels

Ethanol-water vapor mixtures condensation shows different flow patterns in microchannels due to the equivalent surface free energy differences. The transition point of streak flow pattern was introduced in the paper which was found closely related to the frictional pressure drop of ethanol–water vapor mixtures condensation flow in microchannels. Characteristics of two-phase pressure drop for ethanol–water vapor mixtures condensation flow in silicon microchannels of streak and smooth annular flow pattern were investigated. Four types of multi-port trapezoidal and triangular microchannels with hydraulic diameter in the range of 126–155 μm were employed. The pressure drop was determined under steam mass flux between 259.2 and 504.8 kg m−2 s−1 when the inlet ethanol weight concentration varied from 1% to 60%. The experimental pressure drop results were compared with existing correlations of flow condensation. The transition point basing on the streak flow pattern of binary vapor condensation was proposed to replace the traditional Reynolds number criterion of the laminar flow state to turbulent flow state for the frictional factor and the C parameter for Kim and Mudawar’s universal pressure drop model, which is in good agreement with the results the experiment results.

Theoretical and experimental study of wavy flow during R134a condensation flow in symmetrically and asymmetrically cooled microchannels

The paper presents experimental and theoretical studies of wavy flows during R134a condensation in oval parallel microchannels with a hydraulic diameter of 301.6 µm for mass fluxes from 60 to 250 kg/(m2s) and vapor mass qualities from 0.1 to 0.9. The inlet saturation temperature of R134a is 31.3°C. Waves were seen in the thin liquid film and in the channel corners with measurements of their wave lengths and velocities. The results show that both the film wave length and the corner wave length increase with decreasing mass flux and decreasing vapor mass quality. Additionally, the film wave velocity and corner wave velocity both increase with increasing mass flux. The cooling method, either asymmetric cooling or symmetric cooling, only affected the corner wave length and velocity along the channel except near the inlet. A two-dimensional theoretical model was developed to predict the liquid-vapor interface instability conditions during condensation flow by neglecting the liquid inertia and vapor flow viscous terms. The normal modes method was used to analyze the system instability reactions to various perturbation wave lengths. Two modes were obtained with one giving the greatest instability wave length which agrees well with the measured wave length data.

Condensation in Microchannels: Detailed Comparisons of Annular Laminar Flow Theory With Measurements

A relatively simple theory of annular laminar film condensation in microchannels, based on the Nusselt approximations for the condensate film and a theoretically based approximation for the vapor shear stress, has no empirical input and gives the local heat transfer coefficient and local quality for given vapor mass flux and vapor–surface temperature difference distribution along the channel. As well as streamwise vapor shear stress and gravity, the theory includes transverse (to the flow direction) surface tension-driven motion of the condensate film and gives a differential equation for the local (transverse and streamwise) condensate film thickness. As well as four transverse direction boundary conditions due to condensate surface curvature, a streamwise boundary condition is required as in the Nusselt theory. When the vapor is saturated or superheated at inlet, this is provided by the fact that the film thickness is zero around the channel perimeter at the position of onset on condensation. Most experimental investigations have been conducted with quality less than one at inlet and only approximate comparisons, discussed in earlier papers, can be made. The present paper is devoted to comparisons between theory and measurements in investigations where local heat flux and channel surface temperature were measured and the vapor at inlet was superheated. Measured and calculated heat transfer coefficients and their dependence on distance along the channel and on local quality are in surprisingly good agreement and suggest that the mode of condensation is, in fact, annular and laminar, at least where the quality is high.

Numerical simulation of condensation in microchannels between spherical particles

In this article, a numerical investigation of vapor condensation in a two-dimensional ordered microchannel was conducted with computational fluid dynamics software Fluent. A simplified physical model was built up to simulate a rectangular channel filled with particles. A constant wall heat flux was added to the side walls of the rectangular channel. Volume of fluid model was adopted to pursue the interface of the gas and liquid. The results showed that a better heat transfer performance could be obtained with the porous structure. The local heat transfer coefficient obtained from simulation was in good accordance with the former experimental data, which increased with the increase in fluid velocity and decreased along the flow direction. Parametric analyses were conducted concerning the effects of initial vapor velocity u0, initial temperature T0, and wall heat flux qw on local heat transfer coefficient. The velocity u0 played a significant role during the process of condensation. Temperature distributions along the porous channel and side walls were also analyzed. The results showed that the temperature decreased along the flow direction and increased with the increase in fluid velocity.

Experimental study on two-phase flow pressure drop during steam condensation in trapezoidal microchannels

The characteristics of two-phase pressure drop for steam condensation in trapezoidal microchannels with different cross-sectional geometries via novel experimental facility and specific analytical method were investigated in this study. Three types of multi-port trapezoidal microchannels with hydraulic diameter range of 134–166μm and aspect ratio range of 3.1–6.7, were employed. The pressure drop was determined under steam mass flux between 130 and 340kgm−2s−1 accompanied with various cooling water volume flow rate ranging from 20 to 40Lh−1. The results showed that the total frictional, deceleration and contraction pressure drop were more obvious than other local pressure drop. It also presented that not only the steam mass flux, steam quality, cooling water volumetric flow rate and hydraulic diameter but also the aspect ratio had a significant influence on the two-phase frictional pressure drop. The comparison between the experimental data and existing models were finally carried out to verify their predictive accuracy for steam condensing flows in microchannels, and accordingly a modified Friedel model was proposed. The model considering the effect of geometrical parameters of microchannels, the forces acting on fluid and fluid properties, can be available to predict the steam condensing two-phase flow pressure drop multiplier for liquid flowing alone.

A Critical Review on Condensation Heat Transfer in Microchannels and Minichannels

Condensation in microchannels and minichannels is widely used in small devices such as air-cooled condensers for the air-conditioning and automotive industry, in heat pipes, thermosyphons and other applications for system thermal control. Currently, many research centers all over the world are dealing with the structure and operation of compact refrigerating devices. This is in line with the trend of 21st century that is moving towards the use of energy-saving and environmentally friendly technical equipment. In the present study, a critical review on condensation heat transfer in microchannels and minichannels is presented. This review include a wide range of different parameters such as the channel diameter (d), the saturation temperature (Ts), the mass flux (G), the vapor quality (x), different working fluids like steam, CO2 or R744, FC72, R22, R410A, and R407C, various shapes such as circular and noncircular, different orientations like horizontal and vertical, and systems consist of either single or multiple channels. At the end, recommendations for future studies will be given. As a result, this paper cannot only be used as the starting point for the researcher interested in condensation heat transfer in microchannels and minichannels, but it also includes recommendations for future studies on condensation heat transfer in microchannels and minichannels.

Visualization study of flow condensation in hydrophobic microchannels

A visualization study on flow condensation in hydrophobic rectangular silicon microchannels with hydraulic diameter of approximately 150 μm is conducted. Thin Au film with thickness of 200 nm is sputtered on channel surfaces to create a hydrophobic surface with an equilibrium contact angle of approximately 96°. In addition to traditional droplet flow, droplet‐annular compound flow, droplet‐injection compound flow, and droplet‐bubble/slug compound flow are also observed. The results indicate that injection location is postponed, and injection frequency increases with increasing inlet vapor Reynolds number and condensate Weber number. An empirical correlation of the injection location and injection frequency are presented and discussed. In particular, for a larger inlet vapor Reynolds number, the injection flow is closer to the channel outlet and the condensation heat transfer is enhanced. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1182–1192, 2014

Flow regime transitions during condensation in microchannels

Heat transfer and pressure drop for two-phase flow inside tubes are closely related to the corresponding flow mechanisms. The flow patterns formed in microchannels during condensation differ from those observed in conventional tubes. Using an extensive R134a condensation flow-regime database (1 < Dh < 4.91 mm, 150 < G < 750 kg m−2 s−1), new flow regime transition criteria are proposed. The data are used to understand the physical mechanisms and the governing influences for each of the identified flow regimes and develop dimensionless transition criteria. These criteria can be utilized to identify the flow regimes and transitions for various fluids, operating conditions and channel sizes, thereby generalizing their applicability. This mechanistic determination of condensation flow regimes in different operating conditions and geometries will assist in the development of models for predicting condensation heat transfer and pressure drop, enabling the development of optimized microchannel heat exchangers.

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.

Pressure Drop During Condensation in Microchannels

The paper reports calculations of friction pressure gradient for the special case of laminar annular flow condensation in microchannels. This is the only flow regime permitting theoretical solution without having recourse to experimental data. Comparisons are made with correlations based on experimental data for R134a. The correlations differ somewhat among themselves with the ratio of highest to lowest predicted friction pressure gradient typically around 1.4 and nearer to unity at high quality. The friction pressure gradients given by the laminar annular flow solutions are in fair agreement with the correlations at high quality and lower than the correlations at lower quality. Attention is drawn to the fact that the friction pressure gradient cannot be directly observed and its evaluation from measurements requires estimation of the nondissipative momentum or acceleration pressure gradient. Methods used to estimate the nondissipative pressure gradient require quality and void fraction together with equations which relate these and whose accuracy is difficult to quantify. Quality and void fraction can be readily found from the laminar annular flow solutions. Significant differences are found between these and values from approximate equations.

Heat Transfer and Pressure Drop During Laminar Annular Flow Condensation in Micro-Channels

There is wide disagreement between experimental investigations and correlations for heat transfer during condensation in micro-channels. The major problem is the fact that the vapor-side resistance is usually appreciably smaller than that on the coolant side so that methods where only the overall resistance is measured, and the vapor-side heat transfer coefficient obtained by subtraction of resistances, are prone to large uncertainty. A few more recent correlations, based mainly on data for R134a, are in fair agreement when predictions for R134a and for the same conditions are compared. The fact that wide discrepancies are found when the correlations are used for fluids with widely different properties indicates that some or all of the correlations do not capture all of the essential mechanisms. That closely similar predictions are found when the correlations are applied to R134a indicates that the datasets used in the different studies were in essential agreement. Similar comments apply for pressure drop. The special case of annular laminar condensate flow permits wholly theoretical solution without recourse to empirical input. For this mode of condensation and for specified fluid, channel geometry, flow parameters and tube wall temperatures, local heat transfer-coefficient, and local pressure gradient can be calculated as well as local quality and void fraction. The theory is outlined in the article, and recent developments are discussed. Comparisons with the correlations for heat transfer and pressure gradient are given. For the heat transfer coefficient, the results of the annular flow theory are in surprisingly good agreement with the correlations when applied to R134a. For ammonia, the theoretical results lie between the widely spread values obtained from the correlations. Results for pressure gradient given by the annular laminar flow model are generally lower than those given by the correlations.

Numerical Modeling of the Conjugate Heat Transfer Problem for Annular Laminar Film Condensation in Microchannels

This paper presents numerical simulations of annular laminar film condensation heat transfer in microchannels of different internal shapes. The model, which is based on a finite volume formulation of the Navier–Stokes and energy equations for the liquid phase only, importantly accounts for the effects of axial and peripheral wall conduction and nonuniform heat flux not included in other models so far in the literature. The contributions of the surface tension, axial shear stresses, and gravitational forces are included. This model has so far been validated versus various benchmark cases and versus experimental data available in literature, predicting microchannel heat transfer data with an average error of 20% or better. It is well known that the thinning of the condensate film induced by surface tension due to gravity forces and shape of the surface, also known as the “Gregorig” effect, has a strong consequence on the local heat transfer coefficient in condensation. Thus, the present model accounts for these effects on the heat transfer and pressure drop for a wide variety of geometrical shapes, sizes, wall materials, and working fluid properties. In this paper, the conjugate heat transfer problem arising from the coupling between the thin film fluid dynamics, the heat transfer in the condensing fluid, and the heat conduction in the channel wall has been studied. In particular, the work has focused on three external channel wall boundary conditions: a uniform wall temperature, a nonuniform wall heat flux, and single-phase convective cooling are presented. As the scale of the problem is reduced, i.e., when moving from mini- to microchannels, the results show that the axial conduction effects can become very important in the prediction of the wall temperature profile and the magnitude of the heat transfer coefficient and its distribution along the channel.

Flow condensation in parallel micro-channels – Part 2: Heat transfer results and correlation technique

This second part of a two-part study concerns heat transfer characteristics for FC-72 condensing along parallel, square micro-channels with a hydraulic diameter of 1 mm, which were formed in the top surface of a solid copper plate. Heat from the condensing flow was rejected to a counter flow of water through channels brazed to the underside of the copper plate. The FC-72 condensation heat transfer coefficient was highest near the channel inlet, where the annual liquid film is thinnest. The heat transfer coefficient decreased along the micro-channel because of the film thickening and eventual collapse of the annular regime. Notable heat transfer enhancement was observed for annular flow regions of the micro-channel associated with interfacial waves. Comparing the present data to predictions of previous annular condensation heat transfer correlations shows correlations intended for macro-channels generally provide better predictions than correlations intended specifically for mini/micro-channels. A new condensation heat transfer coefficient correlation is proposed for annular condensation heat transfer in mini/micro-channels. The new correlation shows excellent predictive capability based on both the present FC-72 data and a large database for mini/micro-channel flows amassed from eight previous sources.