Flow pattern,void fraction and pressure drop of refrigerant two-phase flow in a horizontal pipe—I. Experimental data

Experiments with refrigerant two-phase flow in a horizontal pipe have been performed and data on flow pattern, void fraction and pressure drop have been obtained. Refrigerants used were R12 and R22, and the range of saturation pressure was from 5.7 to 19.6 bar.In this paper, the experimental equipment and procedure are described in detail, and the data are both tabulated and presented graphically.     

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.     

Flow pattern,void fraction and pressure drop of refrigerant two-phase flow in a horizontal pipe—II: Analysis of frictional pressure drop

Two-phase flow in horizontal pipe was analyzed with simplified models for annular and stratified flow. The velocity profiles for the liquid and gas phase were described with the Prandtl mixing length. From this analysis, the frictional pressure drop was calculated with the modified Baker map for flow pattern transition. The intermediate region, i.e. wavy flow, was interpolated between annular and stratified flow. Comparison of this analysis with existing experimental data of refrigerants showed good agreement.     

Turbulent air flow in a round pipe at high temperatures

The calculation of the heat exchange and fractional resistance during the turbulent flow of a gas with variable physical properties represents a complicated problem, which can be solved only with the help of a computer (see [1, 2], for example). Fundamentally new devices, the working substance of which is a gas heated to high temperatures, have been undergoing intensive development recently in engineering. The velocity and enthalpy profiles at the entrances of such devices can be nonuniform, not coinciding with the profiles typical of developed turbulent flow; the channel walls can consist of strongly cooled metal sections and weakly cooled ceramic sections; the ceramic materials used may be rough. The enumerated properties additionally complicate the calculation of the flow of a gas with variable physical properties. In the present report we consider two cases: the flow of air heated to high temperatures in a channel with a constant wall temperature under the conditions of strong cooling, and the flow in a pipe consisting of a strongly cooled metal section and a weakly cooled ceramic section. The results are obtained numerically using an implicit finite-difference scheme. They are in satisfactory agreement with the available experimental data.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 162–166, March–April, 1978.In conclusion, the author thanks A. N. Sekundov, I. P. Smirnova, and A. B. Lebedev for useful advice on questions connected with the development of the calculation program and A. B. Vatazhin for constant attention to the work.     

Calibration unit for micro-anemometers at very low air velocities

Commercially available anemometers generally are used at air velocities above the cm/s range. The present contribution deals with a micro-anemometer developed for measuring air velocities in the cm/s range. For the calibration at these low velocities a calibration unit has been built which creates air velocities in this range with an accuracy of the order of one percent.     

Laminar flow in a pipe at low and moderate reynolds numbers

Summary The laminar flow of a homogeneous viscous liquid in the inlet of a pipe is investigated numerically for a range of small and moderate Reynolds numbers where the boundary layer approximation is inapplicable. Velocity profiles and other characteristics of the flow are calculated and the results compared with approximate results obtained by other methods. The limiting case of vanishingly small Reynolds number is also treated analytically.Part of this work was performed while the second author was a summer visitor in the Applied Mathematics Department, Brookhaven National Laboratory, Upton (L.I.), New York.     

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 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 and flow characteristics for counter-current two-phase flow in a vertical round tube with wire-coil inserts

Flow pattern, void fraction and slug rise velocity on counter-current two-phase flow in a vertical round tube with wire-coil inserts are experimentally studied. Flow pattern and slug rise velocity are measured visually with a video camera. The void fraction is measured by the quick-closing valve method. Four kinds of coils with different coil pitches and coil diameters are used as inserts. The presence of wire-coil inserts induces disturbance into gas and liquid flows so that the shape and motion of gas slug or bubbles in a wire-coil inserted tube are quite different from those observed in a smooth tube without insert. The bubbly flow occurs in the low gas superficial velocity region in the wire-coil inserted tube, while the slug or churn/annular flow only appears in the smooth tube without insert over the all test range. The measured slug rise velocity in the wire-coil inserted tube is higher than that in the smooth tube. With modified mean flow velocity calculated with core area, the slug rise velocity in wire-coil tube inserted is in good agreement with Nicklin's correlation. The void fraction in a wire-coil inserted tube is lower than that in a smooth tube in the range of high gas superficial velocities. By introducing a simple assumption on considering the effective flowing area, the measured void fractions in a wire-coil inserted tube are in relatively good agreement with the predicted result based on the drift flux model proposed by others with the correlation for slug rise velocity given by others when the coil pitch is dense.     

Drift-flux correlation for gas-liquid two-phase flow in a horizontal pipe

A drift-flux correlation has been often used to predict void fraction of gas-liquid two-phase flow in a horizontal channel due to its simplicity and practicality. The drift-flux correlation includes two important drift-flux parameters, namely, the distribution parameter and void-fraction-weighted-mean drift velocity. In this study, an extensive literature survey for horizontal two-phase flow is conducted to establish void fraction database and to acquire existing drift-flux correlations. A total of 566 data is collected from 12 data sources and 4 flow-regime-dependent and 1 flow-regime-independent drift-flux correlations are identified. The predictive capability of the existing drift-flux correlations is assessed using the collected data. It is pointed out that the drift velocity determined by a regression analysis may include a significant error due to a compensation error between distribution parameter and drift velocity. In this study, a simple flow-regime-independent drift-flux correlation is developed. In the modeling approach, the void-fraction-weighted mean drift velocity is approximated to be 0 m/s, whereas the distribution parameter is given as a simple function of the ratio of non-dimensional superficial gas velocity to non-dimensional mixture volumetric flux. The newly developed correlation shows an excellent predictive capability of void fraction for horizontal two-phase flow. Mean absolute error (or bias), standard deviation (random error), mean relative deviation and mean absolute relative deviation of the correlation are 0.0487, 0.0985, 0.0758 and 0.206, respectively. The prediction accuracy of the correlation is similar to the correlation of Chexal et al. (1991), which was formulated based on the drift-flux parameters by means of many cascading constitutive relationships with numerous empirical parameters.     

Flow patterns in vertical two-phase flow

This paper is concerned with the flow patterns which occur in upwards gas-liquid two-phase flow in vertical tubes. The basic flow patterns are described and the use of flow pattern maps is discussed. The transition between plug flow and churn flow is modelled under the assumption that flooding of the falling liquid film limits the stability of plug flow. The resulting equation is combined with other flow pattern transition equations to produce theoretical flow pattern maps, which are then tested against experimental flow pattern data. Encouraging agreement is obtained.     

Liquid-phase axial mixing in two-phase horizontal pipe flow

The liquid-phase axial dispersion coefficient and volume-averaged fractional phase hold-ups have been measured in two-phase horizontal pipe flow. Radioactive ^99mTc—technetium-99 metastable—(as an aqueous solution of sodium pertechnate) was used as a tracer. The pulse technique with two-point measurement was employed. Superficial gas (air) and liquid (water) velocities were varied in the range 20–2300 and 30–800mm/s, respectively. The flow regimes covered were bubbly, elongated bubbly, stratified, wavy and slug. Experiments were also performed using single-phase pipe flow. The liquid-phase dispersion coefficient has been shown to depend upon the flow regime and the superficial gas and liquid velocities.     

Simultaneous velocimetry/accelerometry measurements in a turbulent two-phase pipe flow

A two-color digital particle image velocimetry and accelerometry (DPIV and DPIA) measurement technique is described that records the velocity and acceleration fields of both the solid and liquid phases simultaneously. Measurements were taken at turbulent conditions of a vertical pipe flow using glass spheres as the solid phase and fluorescent particles to indicate fluid phase motion. Nd-YAG pulse lasers acted as illumination sources and images were recorded by two monochrome CCD cameras. The two-color aspect of the technique was realized by placing optical filters in front of the cameras to discriminate between the phases. Cross-correlations and auto-correlations were applied to determine velocity and acceleration fields of the two phases. Results showing some of the capabilities of the technique as applied to a two-phase pipe flow experiment are provided. For the condition studied, it was found that there was turbulence suppression due to the solid phase and that the statistics associated with the acceleration probability distribution were different for the solid and fluid phases.     

Experimental investigation of a developing two-phase bubbly flow in horizontal pipe

Experimental results for various water and air superficial velocities in developing adiabatic horizontal two-phase pipe flow are presented. Flow pattern maps derived from videos exhibit a new boundary line in intermittent regime. This transition from water dominant to water–gas coordinated regimes corresponds to a new transition criterion C_T = 2, derived from a generalized representation with the dimensionless coordinates of Taitel and Dukler.Velocity, turbulent kinetic energy and dissipation rate, void fraction and bubble size radial profiles measured at 40 pipe diameters for J_L = 4.42 m/s by hot film velocimetry and optical probes confirm this transition: the gas influence is not continuous but strongly increases beyond J_G = 0.06 m/s. The maximum dissipation rate, derived from spectra, is increased in two-phase flow by a factor 5 with respect to the single phase case.The axial evolution of the bubble intercept length histograms also reveal the flow organization in horizontal layers, driven by buoyancy effects. Bubble coalescence is attested by a maximum bubble intercept evolving from 2.5 to 4.5 mm along the pipe. Turbulence generated by the bubbles is also manifest by the 4-fold increase of the maximum turbulent dissipation rate along the pipe.     

Interfacial structure of air-water two-phase flow in a relatively large pipe

Miniaturized four-sensor conductivity probes are used to study flow structure development in air-water bubbly flow, cap-bubbly flow, and transition to slug flow. The measurements are performed at three different elevations in a vertical round pipe with an inner diameter of 101.6 mm. The time-averaged local void fraction, interfacial velocity, and bubble number frequency are measured by the conductivity probes. Also, the interfacial area concentration and averaged bubble Sauter mean diameter are obtained. A detailed representation of the flow structure is revealed by investigating the acquired data. Furthermore, comparisons of the data at three elevations demonstrate the development of the interfacial structure along the flow direction due to bubble interactions and hydrodynamic effects. This may provide the community with a better knowledge about two-phase flow in a relatively large pipe. In addition, these data can also serve as an experimental database for investigation of the interfacial area transport in large-pipe two-phase flow. Published online: 19 November 2002 This work was performed under the auspices of the U.S. Nuclear Regulatory Commission through the Institute of Thermal-hydraulics.     

Justification of the drift-flux model for two-phase flow in a circular pipe

A flow of a gas-liquid dispersed mixture in a circular pipe with a variable inclination to the horizon, as applied to oil and gas flows in wells, is considered. Within the framework of a multi- fluid approach, the equations of an asymptotic drift-flux model, which contains an algebraic relation between the phase velocities and one momentum equation for the volume-averaged velocity of the mixture, are derived. It is shown that the drift-flux model in this formulation strictly follows from the balance laws under assumption of inertialess velocity slip of the phases in case of validity of one of the following conditions: (i) the dispersed-phase volume fraction is small; (ii) the phase velocity slip may be neglected; or (iii) the flow regime is inertialess and the acceleration of the mixture can be neglected. A numerical algorithm based on the SIMPLE method is implemented for solving the obtained equations of the drift-flux model. The possibility of modeling the gravitational segregation and the pressure buildup in a shut-in well and transient slug flows is demonstrated.     

Flow regime transition criteria for two-phase flow at reduced gravity conditions

Flow regime transition criteria are of practical importance for two-phase flow analyses at reduced gravity conditions. Here, flow regime transition criteria which take the frictional pressure loss effect into account were studied in detail. Criteria at reduced gravity conditions were developed by extending an existing model from normal gravity to reduced gravity conditions. A comparison of the newly developed flow regime transition criteria model with various experimental datasets taken at microgravity conditions showed satisfactory agreement. Sample computations of the model were performed at various gravity conditions, such as 0.196, 1.62, 3.71 and 9.81 m/s² corresponding to micro-gravity and lunar, Martian and Earth surface gravity, respectively. It was found that the effect of gravity on bubbly–slug and slug–annular (churn) transitions in a two-phase flow system was more pronounced at low liquid flow conditions, whereas the gravity effect could be ignored at high mixture volumetric flux conditions. While for the annular flow transitions due to flow reversal and onset of droplet entrainment, higher superficial gas velocity was obtained at higher gravity level.