Return Bend Research Articles

Experimental Study on Gas–Liquid Two-Phase Flow Upstream and Downstream of U-Bends

In this study, the influence of U-bends on the flow and pressure propagation characteristics of a gas–liquid two-phase flow in upstream and downstream straight pipes was investigated experimentally. The superficial velocities of the gas and liquid are 0.18–25.11 m/s and 0.20–1.98 m/s, respectively, covering plug flow, slug flow, and annular flow. The experiments were conducted in U-tubes with inner diameters of 9 mm and 12 mm and with a curvature ratio of 8.33. The U-tube was C-shaped. The pressure fluctuations at the axial measurement points of the straight tubes were measured. Flow images of the distal straight tubes and U-bends were obtained. The disturbance from U-bends in the two-phase flow in the vicinity of the bend is very obvious. The perturbation from U-bends in the fluid in the adjacent straight tubes is highly related to the incoming flow pattern. The slug flow has the most significant influence, whereas the effects of the plug and annular flows are small. Fundamentally, it mainly depends on the weight relationship between the gravity, centrifugal force, and inertial force of the gas–liquid two-phase fluid. The pressure fluctuation propagates in the form of a wave with the same dominant frequency in the straight pipes of the U-tube. The pressure pulsation energy in the straight tubes strengthens with decreasing distance from the 180° return bend. In addition, the pressure fluctuation energy downstream of the U-bend is greater than that upstream of the return bend.

The impact of 180° return bend inclination on pressure drop characteristics and phase distribution during oil-water flow

The present work aims to capture the influence of the inclination of the return bend on flow patterns and pressure drop during oil-water flow. The experiments were carried out for different inclinations (0°, 15°, 30°, and 45°) of return bend for various superficial velocity combinations of oil (kerosene) and water ranging from 0.07 to 0.66 m/s. The experiments showed that pressure drop increases with the increase in inclination. However, the pressure drop at a fixed inclination (say 15°) decreases with the increase in the superficial velocity of the water. Distinct flow patterns observed in the return bend were droplet flow, film inversion, slug flow, plug flow and large slug flow. Droplet flow dominates at the lower range of kerosene (i.e., Usk = 0.07–0.2 m/s) and higher range of water superficial velocity (i.e., Usw = 0.40–0.66 m/s) at all the inclinations considered in this study. Additionally, comparisons between the experimental and numerical simulation results were made. The numerical solution utilized the Euler-Euler approach, considering the different phases as interpenetrating continua. The Volume of Fluid (VOF) model was used within this approach, monitoring the volume fraction of each phase over the domain while calculating one set of momentum equations for each phase. To capture the turbulent effects accurately, the k-ε turbulence model was incorporated. It happened to be found that the numerical findings showed remarkable agreement with the experimental data.

Design of sudden contraction return bend pipe for the measurement of holdup and fouling characteristics: a CFD analysis

Design of sudden contraction return bend pipe for the measurement of holdup and fouling characteristics: a CFD analysis

Numerical Investigation of Pressure Loss and Heat Transfer Characteristics in 180-Degree Bends

The pressure loss and heat transfer characteristics of single-phase water flows and non-boiling air-water flows through a slender 180° return bend with a rectangular cross-section are investigated. Such slender ducts appear in various applications. In comparison to abrupt return bends the unsymmetrical flow field affects a bigger surface. Therefore, there is an interest to quantify the impact of such curvature effects. For the pressure loss consideration, the curvature ratios are 19.9, 40, and 58.1, whereas for heat transfer, the focus is on the latter, the hydraulic diameter and aspect ratio are 3.9 mm and 1.067, respectively. The numerical results are compared to published experimental ones. From the numerical modeling results, where three different approaches of turbulence modeling were tested, it is confirmed that above a curvature ratio of 90 it seems reasonable to neglect curvature effects on pressure drop. The validated conjugate heat transfer models are utilized to develop local and global relationships for the Nusselt number ratios between the convex and concave surface. The two-phase flow modeling is realized via a homogenous flow model approximation. The results show that the modeling provides reasonable results and that the impact of curvature on such flows can be relevant.

Water-soluble polymer-induced drag reduction and flow patterns for oil–water flows in return bend

Water-soluble polymer-induced drag reduction and flow patterns for oil–water flows in return bend

A developmental perspective on measuring stress- and sex hormones in hair

This paper provides a qualitative visual observation of the two-phase flow patterns for HFO-1234yf and R-134a during downward flow in a vertical 6.7 mm inner diameter glass return bend. The different flow regimes observed are: slug, intermittent and annular flows. Bubble and vapor slug dynamical behaviors in downward slug flow are reported for HFO-1234yf. In addition, to determine the perturbation lengths up- and downstream of the return bend, the total pressure drop has been measured at different pressure tap location up- and downstream of the singularity. Furthermore, 285 pressure drop data points measured for two-phase flow of HFO-1234yf, R-134a and R-410A in vertical downward flow return bends are presented. The flow behavior in the return bend, which is subjected to the complex combined actions of gravity and centrifugal force was expressed in terms of the vapor Froude number. This experimental pressure drop database, which is included in the appendix, is compared to four well-known prediction methods available in the literature.

Dynamics of a Viscous Droplet in Return Bends of Microfluidic Channels.

Return bends are frequently encountered in microfluidic systems. In this study, a three-dimensional spectral boundary element method for interfacial dynamics in Stokes flow has been adopted to investigate the dynamics of viscous droplets in rectangular return bends. The droplet trajectory, deformation, and migration velocity are investigated under the influence of various fluid properties and operational conditions, which are depicted by the Capillary number, viscosity ratio, and droplet size, as well as the dimensions of the return bend. While the computational results provide information for the design of return bends in microfluidic systems in general, the computational framework shows potential to guide the design and operation of a droplet-based microfluidic delivery system for cell seeding.

Experimental investigation of single- and two-phase convective heat transfer in slender rectangular 180° return bends

The convective heat transfer characteristics of single-phase water flows and two-phase air-water flows through a slender 180° return bend with a rectangular cross-section are investigated. A moderately curved section is considered with a curvature ratio around 58, a hydraulic diameter of 3.9 mm and an aspect ratio of 1.067. The focus of this work is to investigate if there is a recognizable difference in heat transfer, respectively Nusselt numbers, between the outer (convex) and inner (concave) curved surface of the duct. An approach that considers the conjugated heat transfer is applied to adequately investigate this difference. Additionally the impact of the secondary gas-phase on the heat transfer characteristics is discussed.

Working out of design procedure of sheet metal straightening parameters on roller-type straightening machine in rollers of different diameter

It is installed that non-uniform deviations from planeness of sheet different in magnitude, and also having negative and positive curvature at straightening on the traditional roller-type straightening machine, as rule, are not corrected, as the car has equal diameters of all working rollers which affects on planeness defect of sheet with equal curvature of rollers barrels. Dependences for calculation of roller radius and return bend angle are presented. It is shown that if the return bend angle is equal to initial bend angle of sheet at counter-camber the sheet is completely detected; if the return bend angle is less than initial bend angle of sheet return bend occurs partially, the sheet also is detected partially; if the return bend angle is more than initial bend angle of sheet the sheet is bent towards return bend. Dependences for calculation of powers of the editing spent for alternating bending of sheet, the power spent for friction of rolling motion of rollers on surface of sheet, the power spent for overcoming of frictional force on necks of carrying rollers in roller-type straightening machine with rollers of different diameters. Calculation of power consumption of the main drive electric motor at straightening of sheets from low-alloy steel for 5 —7-roller straightening machines with different diameters of rollers is presented. Difference between calculation results and the experiment data showed, difference makes 10...20 % that testifi es to adequacy of model and about possibility of its practical use for calculations of straightening parameters of sheet products on roller-type straightening machine with different diameters of rollers.

Determination of adjustment positive allowance at straightening of sheet metal on roller-type straightening machine

The designs analysis of roller-type straightening machine for sheet straightening showed that by positive allowance adjustment principle between machine rollers, two types are performed: with parallel and inclined rollers. Dependences for calculations of return bend curvature, adjusting positive allowance for parallel and inclined rollers roller-type straightening machine are presented. Pilot straightening is carrion on 7-roller straightening machine at straightening of sheets from 10 steel with thickness of 16 mm and width of 2400 mm. Comparison of results of the experimental value of positive allowance of 13.1 mm and the calculated value of 12.8 mm showed 4.5 % divergence.

Design of heat exchanger for cooling fluid used on magnetic induction welder

Pengelasan induksi magnetik adalah proses umum dalam produksi pipa. Impeder yang digunakan dalam mesin las perlu didinginkan untuk menjaga suhunya di bawah 35oC. Cairan pendingin adalah air dengan 2% cutting oil dan disebut pendingin. Itu harus memasukkan impeder pada 15oC. Laju aliran yang dibutuhkan adalah sebanyak 30 lt / mnt. Untuk memastikan suhu impeder dijaga lebih rendah dari suhu yang dibutuhkan, maka penukar panas harus dirancang untuk mendinginkan pendingin dari 40oC hingga 15oC. Air chiller berpendingin udara tipe UWAP750AY3 digunakan untuk menghasilkan air dingin yang dibutuhkan oleh penukar panas. Pipa tembaga bersirip 5/8-inci digunakan sebagai tabung di penukar panas. Pendingin dibagi dalam empat tabung yang sama dari header. Untuk memenuhi area perpindahan panas yang dibutuhkan, setiap tabung akan ditekuk untuk membuat 10 lintasan (atau baris). Di akhir baris terakhir, empat aliran akan dikumpulkan di header. Kemudian, pendingin akan diedarkan ke impeder dengan suhu yang dibutuhkan. Simulasi numerik dilakukan untuk mendapatkan vektor kecepatan dan distribusi suhu aliran di dalam tabung, dan juga penurunan tekanan aliran melintasi tabung dan header. Vektor kecepatan aliran di sekitar tikungan balik, aliran masuk dan keluar menarik. Mereka memberikan pemahaman bahwa penurunan tekanan akan lebih tinggi pada aksesori semacam itu daripada pada pipa lurus. Penurunan tekanan cairan pendingin yang mengalir di dalam tabung penukar panas adalah 5143,2 Pa dari solusi analitis dan 4611,4 Pa dari simulasi numerik menggunakan Fluent. Temperatur cairan pendingin di outlet yang didapat dari simulasi numerik hampir sama dengan suhu yang dirancang, yaitu 15oC. Jadi, simulasi numerik memperkuat desain penukar panas.

Numerical evaluation of a compact generator design for steam driven H2O/LiBr absorption chiller application

The objectives of this study are to analyze effects of design parameters of a generator with a steam heat source on condensation heat transfer and flow characteristics inside tubes and to design optimum configuration of the generator with minimum volume that satisfies design constraints for H2O/LiBr absorption chiller application. To that end, the heat transfer and flow characteristics of a straight-pipe tube and a tube with return bends were analyzed through numerical analysis according to the tube diameter, number of tubes, and the diameter of return bends. It was found that the minimum tube volume satisfying the design constraints was obtained for Ntube=2 and Dtube=12mm in the case of the straight tube. The smaller the diameter of the return bend was, the greater the flow separation area, which caused a large local flow velocity. In the case of the return bend tubes, the smallest tube volume that satisfies both the flow and pressure drop constraints was obtained for Ntube=2 and Dturn/Dtube=1.0. In the case Dturn/Dtube=1.5, where the pressure drop was the smallest, the pressure drop was higher by 9% compared to the pressure drop constraint (1 bar), but the tube volume that satisfies the flow constraints decreased by 49%.

Experimental heat transfer, friction factor and effectiveness analysis of Fe3O4 nanofluid flow in a horizontal plain tube with return bend and wire coil inserts

The paper reports on the convective heat transfer, friction factor, effectiveness and number of transfer units (NTU) of Fe3O4/water nanofluids flow in a double pipe U-bend heat exchanger with and without wire coil inserts under turbulent flow conditions. The heat transfer and friction factor experiments were conducted at different Reynolds numbers (16,000–30,000), different particle concentrations (0.005–0.06%) and different wire coil inserts of coil pitch to tube inside diameter (p/d) ratios (1, 1.34 and 1.79). The experimental results indicate that, Nusselt number increases with increasing Reynolds number and particle concentrations, and decreasing p/d ratio of wire coil inserts. The Nusselt number enhancement for 0.06% volume concentration of nanofluid is 14.7% and it further increases to 32.03% for the same 0.06% nanofluid with wire coil inserts of p/d=1 with a friction penalty of 1.162-times at the Reynolds number of 28,954 compared to water data. The experimental data was fitted using regression analysis to estimate the Nusselt number and friction factor of nanofluid as a function of the Reynolds number, Prandtl number, volume concentration, and wire coil helix ratio. The effectiveness of heat exchanger is formulated in terms of number of transfer units (NTU) for the base fluid and nanofluids.

Heat transfer, friction factor and effectiveness analysis of Fe3O4/water nanofluid flow in a double pipe heat exchanger with return bend

The convective heat transfer, friction factor and effectiveness of different volume concentrations of Fe3O4 nanofluid flow in an inner tube of double pipe heat exchanger with return bend has been estimated experimentally and turbulent flow conditions. The test section used in this study is of double pipe type in which the inner tube diameter is 0.019m, the annulus tube diameter is 0.05m and the total length of inner tube is 5m. At a distance of 2.2m from the inlet of the inner tube the return bend is provided. The hot Fe3O4 nanofluid flows through an inner tube, where as the cold water flows through an annulus tube. The volume concentrations of the nanoparticles used in this study are 0.005%, 0.01%, 0.03% and 0.06% with Reynolds number range from 15,000 to 30,000. Based on the results, the Nusselt number enhancement is 14.7% for 0.06% volume concentration of nanofluid flow in an inner tube of heat exchanger at a Reynolds number of 30,000 when compared to base fluid data; the pumping penalty of nanofluid is <10%. The effectiveness of heat exchanger for water and nanofluid flow is explained in terms of number of transfer units (NTU) in order to estimate the overall performance of the double pipe heat exchanger. New correlations for Nusselt number and friction factor have been developed based on the experimental data.

Upward gas–liquid two-phase flow after a U-bend in a large-diameter serpentine pipe

We present an experimental study on the flow behaviour of gas and liquid in the upward section of a vertical pipe system with an internal diameter of 101.6mm and a serpentine geometry. The experimental matrix consists of superficial gas and liquid velocities in ranges of 0.15–30m/s and from 0.07 to 1.5m/s, respectively, which cover bubbly to annular flow. The effects on the flow behaviours downstream of the 180° return bend are significantly reduced when the flow reaches an axial distance of 47 pipe diameters from the U-bend. Therefore, reasonably developed flow is attained at this development length downstream of the bend. Other published measurements for large-diameter film thickness show similar trends with respect to the superficial gas velocity. However, the trends differ from those of small-diameter pipes, with which the film thickness decreases much faster with increasing gas flow. As a result, only a few of the published correlations for small pipe data agreed with the experimental data for large pipe film thickness. We therefore modified one of the best-performing correlations, which produced a better fit. Qualitative and statistical analyses show that the new correlation provides improved predictions for two-phase flow film thickness in large-diameter pipes.

Numerical prediction of flow and heat transfer characteristics of water-fly ash slurry in a 180° return pipe bend

A three-dimensional numerical simulation is performed to predict the thermofluidic transport characteristics of water-fly ash slurry in an 180° return bend. U pipelines of diameter 53 mm with radius ratios of 2.98 and 5.6 are considered that may replicate a shell and tube type heat exchanger. The pressure drop and heat transfer characteristics are predicted and the effects of Dean, Nusselt and Reynolds numbers on the vortex structure formation and heat transfer are studied. The numerical simulation is carried out by deploying the granular Eulerian multiphase model following a finite volume approach. The turbulent transport is addressed using the RNG k−ε turbulence model. The results revealed that the heat transfer coefficient of pipe bends of smaller radius ratio is 53.28% more than the larger radius ratio for the solid concentration of 10% and velocity of 1 m/s. Its value increases with increase in the particle concentration and velocity due to the presence of a secondary flow in the bends. The Dean number increases with decreasing the radius ratio and the average Nusselt number increases with increasing the Reynolds number. With increasing Dean Number, the Nusselt number increases with decreasing the radius of curvature for the same particle concentration. When the particle concentration increases, the average Nusselt number also increases. The average Nusselt number in the return bend appears to be higher than that in the inlet and outlet pipes due to the presence of the secondary flows.

Effect of the bend geometry on the two-phase frictional pressure drop and flow behaviour in the vicinity of the bend

The effect of a sharp return bend on the two-phase flow frictional pressure drop close up-and downstream of the bend is investigated. The measurements are performed for refrigerant R134a at mass fluxes ranging from 200 to 500kg/m2s. Four bend geometries are tested, with inner channel diameters ranging from 4.93mm to 8.1mm and the curvature ratio between 2.55 and 4.43. For both up-and downward oriented flow the main bend effect on the frictional pressure drop is observed close downstream of the return bend. However, for downward oriented flow a decrease in pressure drop is observed and for upward flow an increase is found. To assess the effect the underlying flow behaviour, the void fraction time trace is analysed. The average void fraction and wavelet variance of the void fraction signal can be used to explain the trends in the frictional pressure drop. The correlation between the frictional pressure drop and the flow behaviour indicators is investigated quantitatively for the different geometries.

Numerical Simulation of Non-Newtonian Core Annular Flow through Rectangle Return Bends

The volume of fluid (VOF) model together with the continuum surface stress (CSS) model is proposed to simulate the core annular of non-Newtonian oil and water flow through the rectangle return bends (∏-bends). A comprehensive investigation is conducted to generate the profiles of volume fraction, pressure and velocity. The influences of oil properties, flow direction, and bend geometric parameters on hydrodynamic of nonNewtonian oil and water core annular flow in ∏-bends are discussed. Through computational simulations the proper bend geometric parameters were identified, these results are useful for designing and optimizing the pipefitting system.

Two-phase frictional pressure drop and flow behaviour up- and downstream of a sharp return bend

In this work, the two-phase flow frictional pressure drop of a refrigerant flow up- and downstream of a sharp return bend is studied. The radius of the return bend is 10.2 mm and the channel inner diameter measures 8 mm. The refrigerant is R134a, the mass flux is varied between 200 and 500 kg/m2 s and the vapour quality x is varied between 0 and 1. The bend orientation is vertical. Upward flow as well as downward flow through the bend is studied. To be able to explain the effect of the return bend on the frictional pressure drop, the two-phase flow behaviour is measured at several locations up- and downstream of the return bend by means of a capacitance sensor. From this capacitance signal, the void fraction and wavelet variance are derived. Close downstream of the return bend the measured pressure gradient is lower than the reference for downward oriented flow. At these locations the void fraction is higher compared to the reference and the wavelet variance is lower compared to the reference measurement, which is consistent with a lower pressure gradient. For upward oriented flow an increase in pressure gradient compared to the reference measurement is observed close downstream of the return bend. At these locations an increase in wavelet variance compared to the reference is observed, which is consistent with the increase in frictional pressure gradient. Upstream the affected length is rather small, less than 10 diameters upstream, downstream the bend effect is more pronounced and present for more than 30 diameters.

Developing air–water flow downstream of a vertical 180° return bend

The importance of developing two-phase flow downstream of return bends has been brought about by several authors wishing to quantify irreversible losses in singularities. In a recent work, a method for evaluating such losses in vertical return bends was proposed based on gas holdup and pressure drop measurements in the bend. In the present study, the nature of the developing flow upstream and downstream of a vertical 180° tube bend (curvature ratio of 8.7) is investigated by means of gas holdup and pressure drop measurements at several positions along the 26.4-mm ID tubes connected to the bend. The effect of the bend on the flow characteristics was evaluated for the plug, slug and annular flow regimes in the upward and downward flow directions. The results showed that, although the variations in gas holdup seemed to extend beyond the measuring limits, significant pressure changes were limited to 40 pipe diameters upstream and 60 diameters downstream of the bend.