Heat transfer and fluid dynamics of air–water two-phase flow in micro-channels

Heat transfer, pressure drop, and void fraction were simultaneously measured for upward heated air–water non-boiling two-phase flow in 0.51 mm ID tube to investigate thermo–hydro dynamic characteristics of two-phase flow in micro-channels. At low liquid superficial velocity j_l frictional pressure drop agreed with Mishima–Hibiki’s correlation, whereas agreed with Chisholm–Laird’s correlation at relatively high j_l. Void fraction was lower than the homogeneous model and conventional empirical correlations. To interpret the decrease of void fraction with decrease of tube diameter, a relation among the void fraction, pressure gradient and tube diameter was derived. Heat transfer coefficient fairly agreed with the data for 1.03 and 2.01 mm ID tubes when j_l was relatively high. But it became lower than that for larger diameter tubes when j_l was low. Analogy between heat transfer and frictional pressure drop was proved to hold roughly for the two-phase flow in micro-channel. But satisfactory relation was not obtained under the condition of low liquid superficial velocity.     

One-group interfacial area transport equation and its sink and source terms in narrow rectangular channel

The characteristics of two-phase flow in a narrow rectangular channel are expected to be different from those in other channel geometries, because of the significant restriction of the bubble shape which, consequently, may affect the heat removal by boiling under various operating conditions. The objective of this study is to develop an interfacial area transport equation with the sink and source terms being properly modeled for the gas–liquid two-phase flow in a narrow rectangular channel. By taking into account the crushed characteristics of the bubbles a new one-group interfacial area transport equation was derived for the two-phase flow in a narrow rectangular channel. The random collisions between bubbles and the impacts of turbulent eddies with bubbles were modeled for the bubble coalescence and breakup respectively in the two-phase flow in a narrow rectangular channel. The newly-developed one-group interfacial area transport equation with the derived sink and source terms was evaluated by using the area-averaged flow parameters of vertical upwardly-moving adiabatic air–water two-phase flows measured in a narrow rectangular channel with the gap of 0.993 mm and the width of 40.0 mm. The flow conditions of the data set covered spherical bubbly, crushed pancake bubbly, crushed cap-bubbly and crushed slug flow regimes and their superficial liquid velocity and the void fraction ranged from 0.214 m/s to 2.08 m/s and from 3.92% to 42.6%, respectively. Good agreement with the average relative deviation of 9.98% was obtained between the predicted and measured interfacial area concentrations in this study.     

The effects of channel diameter on flow pattern,void fraction and pressure drop of two-phase air–water flow in circular micro-channels

Two-phase air–water flow characteristics are experimentally investigated in horizontal circular micro-channels. Test sections are made of fused silica. The experiments are conducted based on three different inner diameters of 0.53, 0.22 and 0.15 mm with the corresponding lengths of 320, 120 and 104 mm, respectively. The test runs are done at superficial velocities of gas and liquid ranging between 0.37–42.36 and 0.005–3.04 m/s, respectively. The flow visualisation is facilitated by systems mainly including stereozoom microscope and high-speed camera. The flow regime maps developed from the observed flow patterns are presented. The void fractions are determined based on image analysis. New correlation for two-phase frictional multiplier is also proposed for practical applications.     

Bubbly-to-cap bubbly flow transition in a long-26 m vertical large diameter pipe at low liquid flow rate

The concurrent upward two-phase flow of air and water in a long vertical large diameter pipe with an inner diameter (D) of 200 mm and a height (z) of 26 m (z/D = 130) was investigated experimentally at low superficial liquid velocities from 0.05009 to 0.3121 m/s and the superficial gas velocities from 0.01779 to 0.5069 m/s. The resultant void fractions range from 0.03579 to 0.4059. According to the observations using a high speed video camera, the flow regimes of bubbly, developing cap bubbly and fully-developed cap bubbly flows prevailed in the flows. The developing cap bubbly flow appeared as a flow regime transition from bubbly to fully-developed cap bubble flow in the vertical large diameter pipe. The developing cap bubbly flow changes gradually and lasts for a long time period and a wide axial region in the flow direction, in contrast to a sudden transition from bubbly to slug flows in a small diameter pipe. The analysis in this study showed that the flow regime transition depends not only on the void fraction but also on the axial distance in the flow and the pipe diameter. The axial flow development brings about the transition to happen in a lower void fraction flow and the increase of pipe diameter causes the transition to happen in a higher void fraction flow. The measured void fraction showed an N-shaped axial changing manner that the void fraction increases monotonously with axial position in the bubbly flow, decreases non-monotonously with axial position in the developing cap bubbly flow, and increases monotonously again with axial position in the fully-developed cap bubbly flow. The temporary void fraction decrease phenomenon in the transition region from bubbly to cap bubbly flow can be attributed to the formation of medium to large cap bubbles and their gradual growth into the maximum size of cap bubble and/or cluster of large cap bubbles in the developing cap bubbly flow. In order to predict the N-shaped axial void fraction changing behaviors in the flow regime transition from bubbly to cap bubbly flow, the existing 12 drift flux correlation sets for large diameter pipes are reviewed and their predictabilities are studied against the present experimental data. Although some drift flux correlation sets, such as those of Clark and Flemmer (1986) and Hibiki and Ishii (2003), can predict the present experimental data with reasonable average relative deviations, no drift flux correlation set for distribution parameter and drift velocity can give a reliable prediction for the observed N-shaped axial void fraction changing behaviors in the region from bubbly to cap bubbly flow in a vertical large diameter pipe.     

Two-phase pressure drop and flow visualization of FC-72 in a silicon microchannel heat sink

The rapid development of two-phase microfluidic devices has triggered the demand for a detailed understanding of the flow characteristics inside microchannel heat sinks to advance the cooling process of micro-electronics. The present study focuses on the experimental investigation of pressure drop characteristics and flow visualization of a two-phase flow in a silicon microchannel heat sink. The microchannel heat sink consists of a rectangular silicon chip in which 45 rectangular microchannels were chemically etched with a depth of 276 μm, width of 225 μm, and a length of 16 mm. Experiments are carried out for mass fluxes ranging from 341 to 531 kg/m² s and heat fluxes from 60.4 to 130.6 kW/m² using FC-72 as the working fluid. Bubble growth and flow regimes are observed using high speed visualization. Three major flow regimes are identified: bubbly, slug, and annular. The frictional two-phase pressure drop increases with exit quality for a constant mass flux. An assessment of various pressure drop correlations reported in the literature is conducted for validation. A new general correlation is developed to predict the two-phase pressure drop in microchannel heat sinks for five different refrigerants. The experimental pressure drops for laminar-liquid laminar-vapor and laminar-liquid turbulent-vapor flow conditions are predicted by the new correlation with mean absolute errors of 10.4% and 14.5%, respectively.     

An experimental investigation of two-phase air–water flow through a horizontal circular micro-channel

This paper is a continuation of the authors’ previous work. Two-phase air–water flow experiments are performed in a horizontal circular micro-channel. The test section is made of a fused silica tube with an inner diameter of 0.15 mm and a length of 104 mm. The flow phenomena, which are liquid/unstable annular alternating flow (LUAAF), liquid/annular alternating flow (LAAF), and annular flow, are observed and recorded by a high-speed camera mounted together with a stereozoom microscope. A flow pattern map is presented in terms of the phase superficial velocities and is compared with those of other researchers obtained from different working fluids. Image analysis is performed to determine the void fraction, which increases non-linearly with increasing volumetric quality. It is revealed that the two-phase frictional multiplier data show a dependence on flow pattern rather than mass flux. Based on the present data, a new pressure drop correlation is proposed for practical applications. According to the present study, in general the data for the two-phase air–water flow characteristics are found to comply with those of working fluids other than air–water mixtures.     

Impact of aspect ratio on flow boiling of water in rectangular microchannels

In this paper we focus on the impact of varying the aspect ratio of rectangular microchannels, on the overall pressure drop involving water boiling. An integrated system comprising micro-heaters, sensors and microchannels has been realized on (1 1 0) silicon wafers, following CMOS compatible process steps. Rectangular microchannels were fabricated with varying aspect ratios (width [W] to depth [H]) but constant hydraulic diameter of 142 ± 2 μm and length of 20 mm. The invariant nature of the hydraulic diameter is confirmed through two independent means: physical measurements using profilometer and by measuring the pressure drop in single-phase fluid flow. The experimental results show that the pressure drop for two-phase flow in rectangular microchannels experiences minima at an aspect ratio of about 1.6. The minimum is possibly due to opposing trends of frictional and acceleration pressure drops, with respect to aspect ratio. In a certain heat flux and mass flux range, it is observed that the two-phase pressure drop is lower than the corresponding single-phase value. This is the first study to investigate the effect of aspect ratio in two-phase flow in microchannels, to the best of our knowledge. The results are in qualitative agreement with annular flow model predictions. These results improve the possibility of designing effective heat-sinks based on two-phase fluid flow in microchannels.     

Flow characteristics in hydraulically equilibrium two-phase flows in a vertical 2 × 3 rod bundle channel

In order to increase data on two-phase flow distribution in a multi-subchannel system, being similar to a rod bundle, experiments have been carried out using water and air at ambient pressure and temperature as the working fluids and a newly constructed 2 × 3 rod bundle channel as the test channel. The channel contained six rods in rectangular array and two-kinds of six subchannels, simulating a BWR fuel rod bundle. Experimental data on flow distribution and pressure drop along each subchannel axis were obtained in various single- and two-phase flows under a hydraulic equilibrium flow condition. From the measured pressure drop in the single-phase flow, friction factor data in each subchannel were obtained. The two-phase pressure drop data were compared with calculations by a simple, one-dimensional, one-pressure two-fluid model. In addition, Taylor bubble velocity in each subchannel in slug-churn flows was measured with a double needle contact probe. Using the bubble velocity data, we obtained a subchannel void fraction in each subchannel, and discussed a relationship of the subchannel void fractions between two different subchannels. Results of such experiments and discussions are presented in this paper.     

Experimental investigation of horizontal forced-vibration effect on air-water two-phase flow

In order to investigate the potential seismic vibrations effect on two-phase flow in an annular channel, experimental tests with air-water two-phase flow under horizontal vibrations were carried out. A low-speed eccentric-cam vibration module capable of operating at motor speed of 45–1200 rpm (f = 0.75–20 Hz) was attached to an annular channel, which was scaled down from a prototypic BWR fuel sub-channel with inner and outer diameters of 19.1 mm and 38.1 mm, respectively. The two-phase flow was operated in the ranges of 〈j_f〉 = 0.25–1.00 m/s and 〈j_g〉 = 0.03–1.46 m/s with 27 flow conditions, and the vibration amplitudes controlled by cam eccentricity (E) were designed for the range of 0.8–22.2 mm. Ring-type impedance void meters were utilized to detect the area-averaged time-averaged void fraction under stationary and vibration conditions. A systematic experimental database was built and analyzed with effective maps in terms of flow conditions (〈j_g〉-〈j_f〉) and vibration conditions (E-f and f-a), and the potential effects were expressed by regions on the maps. In the 〈j_g〉-〈j_f〉 maps, the void fraction was found to potentially decrease under vibrations in bubbly flow regime and relatively lower liquid flow conditions, which may be explained by the increase of distribution parameter. Whereas and the void fraction may increase at the region closed to bubbly-to-slug transition boundary under vibrations, which may be explained by the changes of drift velocity due to flow regime change from bubbly to slug flows. No significant change in void fraction was found in slug flow regime under the present test conditions.     

Development of a robust image processing technique for bubbly flow measurement in a narrow rectangular channel

This paper presents a robust image processing technique for bubbly flow measurement over a wide range of void fractions. The proposed algorithm combines geometrical, optical and topological information recorded with high speed cameras to separate and reconstruct the overlapping bubbles. The common difficulties such as overlapping, irregular bubble shape, surface deformation and large clustering in digital image processing are solved by combining different information based on a preset decision table and flow chart. Test with synthetic bubble images is performed to evaluate the reliability of the algorithm and quantify the uncertainty of the data. The result shows that the proposed algorithm can accurately measure bubbly flows with void fraction up to 18% for large bubbles. Four runs of bubbly flow images in a 30 mm  ×  10 mm rectangular channel are then recorded by three high speed cameras. The area-averaged void fraction of these test runs range from 2.4% to 9.1%. The axial and lateral distributions of bubble number density are obtained by the present algorithm for studying the characteristics of these flows.     

Interfacial area concentration in gas–liquid bubbly to churn flow regimes in large diameter pipes

This study performed a survey on existing correlations for interfacial area concentration (IAC) prediction and collected an IAC experimental database of two-phase flows taken under various flow conditions in large diameter pipes. Although some of these existing correlations were developed by partly using the IAC databases taken in the low-void-fraction two-phase flows in large diameter pipes, no correlation can satisfactorily predict the IAC in the two-phase flows changing from bubbly, cap bubbly to churn flow in the collected database of large diameter pipes. So this study presented a systematic way to predict the IAC for the bubbly-to-churn flows in large diameter pipes by categorizing bubbles into two groups (group 1: spherical or distorted bubble, group 2: cap bubble). A correlation was developed to predict the group 1 void fraction by using the void fraction for all bubble. The group 1 bubble IAC and bubble diameter were modeled by using the key parameters such as group 1 void fraction and bubble Reynolds number based on the analysis of Hibiki and Ishii (2001, 2002) using one-dimensional bubble number density and interfacial area transport equations. The correlations of IAC and bubble diameter for group 2 cap bubbles were developed by taking into account the characteristics of the representative bubbles among the group 2 bubbles and the comparison between a newly-derived drift velocity correlation for large diameter pipes and the existing drift velocity correlation of Kataoka and Ishii (1987) for large diameter pipes. The predictions from the newly-developed two-group IAC correlation were compared with the collected experimental data in gas–liquid bubbly to churn flow regimes in large diameter pipes and their mean absolute relative deviations were obtained to be 28.1%, 54.4% and 29.6% for group 1, group 2 and all bubbles respectively.     

Effective property models for homogeneous two-phase flows

Using an analogy between thermal conductivity of porous media and viscosity in two-phase flow, new definitions for two-phase viscosity are proposed. These new definitions satisfy the following two conditions: namely (i) the two-phase viscosity is equal to the liquid viscosity at the mass quality = 0% and (ii) the two-phase viscosity is equal to the gas viscosity at the mass quality = 100%. These new definitions can be used to compute the two-phase frictional pressure gradient using the homogeneous modeling approach. These new models are assessed using published experimental data of two-phase frictional pressure gradient in circular pipes, minichannels and microchannels in the form of Fanning friction factor (f_m) versus Reynolds number (Re_m). The published data include different working fluids such as R-12, R-22, argon (R740), R717, R134a, R410A and propane (R290) at different diameters and different saturation temperatures. Models are assessed on the basis minimizing the root mean square error (eRMS). It is shown that these new definitions of two-phase viscosity can be used to analyze the experimental data of two-phase frictional pressure gradient in circular pipes, minichannels and microchannels using simple friction models.     

Technique for quantitative mapping of three-dimensional liquid–gas phase boundaries in microchannel flows

A diagnostic technique capable of characterizing interfaces between transparent, immiscible fluids is developed and demonstrated by investigating the morphology of liquid–gas interfaces in an adiabatic two-phase flow through a microchannel of 500 μm × 500 μm square cross section. Water seeded with 0.5 μm-diameter fluorescent polystyrene particles is pumped through the channel, and the desired adiabatic two-phase flow regime is achieved through controlled air injection. The diagnostic technique relies on obtaining particle position data through epifluorescent imaging of the flow at excitation and emission wavelengths of 532 nm and 620 nm, respectively. The particle position data are then used to resolve interface locations to within ±1 μm in the focal plane. By mapping the interface within individual focal planes at various depths within the channel, it is possible to determine the complete liquid–gas interface geometry across the channel cross section in a dynamic flow environment. Utilizing this approach, the liquid–gas phase boundaries of annular flows within a microchannel have been successfully characterized.     

Void fraction and pressure drop during external upward two-phase crossflow in tube bundles – part I: Experimental investigation

The present paper is the part I of a broad study concerning void fraction and pressure drop for air-water upward external flow across tube bundles. Experimental results were obtained for liquid and gas superficial velocities ranging from 0.02 to 1.50 m/s and 0.20 to 10.00 m/s, respectively. Void fraction measurements were performed for bubbly flow using a capacitive probe. The test section consisted of a triangular tube bundle counting with 19 mm OD tube and transverse pitch of 24 mm. Initially, the paper describes the test facility and the data regression and experimental procedures. Then, the pressure drop and void fraction measurements are validated based on tests for single-phase flow and quiescent liquid conditions, respectively. Finally, the experimental data are presented and analyzed. In the second part of this study (Part II), a literature review on predictive methods for void fraction and pressure drop is presented. Additionally, these methods are compared with the database presented in Part I and new predictive methods for void fraction and frictional pressure drop are proposed.     

Flow pattern based correlations of two-phase pressure drop in rectangular microchannels

Numerous pressure drop correlations for microchannels have been proposed; most of them can be classified as either a homogeneous flow model (HFM) or a separated flow model (SFM). However, the predictions of these correlations have not been compared directly because they were developed in experiments conducted under a range of conditions, including channel shape, the number of channels, channel material and the working fluid. In this study, single rectangular microchannels with different aspect ratios and hydraulic diameters were fabricated in a photosensitive glass. Adiabatic water-liquid and Nitrogen-gas two-phase flow experiments were conducted using liquid superficial velocities of 0.06–1.0 m/s, gas superficial velocities of 0.06–72 m/s and hydraulic diameters of 141, 143, 304, 322 and 490 μm. A pressure drop in microchannels was directly measured through embedded ports. The flow pattern was visualized using a high-speed camera and a long-distance microscope. A two-phase pressure drop in the microchannel was highly related to the flow pattern. Data were used to assess seven different HFM viscosity models and ten SFM correlations, and new correlations based on flow patterns were proposed for both HFMs and SFMs.     

Flow pattern change in horizontal rectangular laterally ribbed ducts through alteration of the ribs thickness and pitch

Experiments were conducted on two-phase flow in laterally ribbed rectangular ducts. Air–water adiabatic flow at atmospheric pressure and room temperature was driven through a 3.6 m long rectangular ribbed test section with cross-section of 100 × 50 mm. To investigate the effect of rib thickness and pitch on flow pattern diagrams and transition boundaries, nine various rib arrangements were implemented with thicknesses of 2, 4 and 8 mm and pitches of 50, 60, and 80 mm. Unlike non-ribbed rectangular duct, lateral rib arrangement did not allow any stratified flow to occur. However wavy, plug and slug flows were parallel in both flow conditions, rib existence caused explicitly coarser pattern shapes. Increasing the rib thickness, while keeping the pitch constant, results in different flow patterns to occur as well as dramatic changes in boundaries positions and shapes. On the other hand, as pitch shifts up at a constant rib thickness, one can notice the duplication of almost identical flow patterns and their boundaries however, boundary values undergo tangible changes. Consistent attention was paid to conditions under which wavy pattern zone extends while intermittent flow zones were avoided. Studies concerned ribbed duct are of major applicable value to designing and enhancing heat transfer systems.     

Void fraction measurement of bubble and slug flow in a small channel using the multivision technique

Using the multivision technique, a new void fraction measurement method was developed for bubble and slug flow in a small channel. The multivision system was developed to obtain images of the two-phase flow in two perpendicular directions. The obtained images were processed—using image segmentation, image subtraction, Canny edge detection, binarization, and hole filling—to extract the phase boundaries and information about the bubble or slug parameters. With the extracted information, a new void fraction measurement model was developed and used to determine the void fraction of the two-phase flow. The proposed method was validated experimentally in horizontal and vertical channels with different inner diameters of 2.1, 2.9, and 4.0 mm. The proposed method of measuring the void fraction has better performance than the methods that use images acquired in only one direction, with a maximum absolute difference between the measured and reference values of less than 6%.     

Void fraction and pressure drop during external upward two-phase cross flow in tube bundles – part II: Predictive methods

The present paper is the Part II of a broad study concerning void fraction and pressure drop for air-water upward external flow across tube bundles. In the Part I, the experimental facility and the data regression procedures were described and the experimental results are presented and discussed. Initially, Part II presents a literature review concerning void fraction and pressure drop predictive methods available in the open literature for two-phase upward flow across tube bundles. Next, the methods from literature are compared among them and with the database presented in paper Part I. Significant discrepancies are observed among the predictive methods, and deviations as high as two orders of magnitude are verified among the predicted values of pressure drop. Then, a new void fraction predictive method is proposed based on the experimental results and on the minimum kinetic energy principle. This method provides satisfactory predictions of the results described in paper Part I and also of independent data from the literature. A new predictive method for frictional pressure drop during two-phase flow based on two-phase multiplier is also proposed. This method predicted 94% of the experimental data obtained in the present study within an error margin of ± 30%, and also provides accurate predictions of independent results for triangular tube bundles gathered in the open literature.     

Developing structure of two-phase flow in a large diameter pipe at low liquid flow rate

In order to develop the interfacial area transport equation for the interfacial transfer terms in the two-fluid model, accurate data sets on axial development of local parameters such as void fraction, interfacial area concentration, interfacial gas velocity and Sauter mean diameter are indispensable to verify the modeled source and sink terms in the interfacial area transport equation. From this point of view, local measurements of both group 1 spherical/distorted bubbles and group 2 cap/slug bubbles in vertical upward air–water two-phase flow in a large diameter pipe with 200 mm in inner diameter and 26 m in height were performed at three axial locations of z/D = 41.5, 82.8 and 113 as well as 11 radial locations from r/R = 0–0.95 by using four-sensor probe method. Here, z, r, D and R are the axial distance from the inlet, radial distance from the pipe center, pipe diameter and pipe radius, respectively. The liquid flow rate and the void fraction ranged from 0.0505 m/s to 0.312 m/s and from 1.98% to 32.6%, respectively in the present experiment. The flow condition covered extensive region of bubbly flow, cap turbulent flow as well as their transition. The extensive analysis on the radial profiles of local flow parameters and their axial developments demonstrate the development of interfacial structures along the flow direction due to the bubble coalescence and breakup and the gas expansion. The significant decrease in void faction and interfacial area concentration and the increase in Sauter mean diameter and interfacial velocity were observed when the gradual flow regime transition occurred. Finally, the net change in the interfacial area concentration due to the bubble coalescence and breakup was quantitatively investigated in the present paper to reflect the true transfer mechanisms in observed two-phase flows.