Return Bend Research Articles

Capacitance sensor measurements of upward and downward two-phase flow in vertical return bends

In this work, the feasibility of using capacitance sensors to assess the two-phase flow behaviour up- and downstream of a sharp return bend is evaluated. A return bend with a radius of 11mm and an inner diameter of 8mm is used. The refrigerant is R134a, the mass flux is varied between 200 and 400kg/m2s 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. Eight capacitance sensors are placed at several locations upstream and downstream of the return bend. The capacitance time traces for these sensors are compared with the aim to determine the presence and the extent of the bend effect. It was concluded that the disturbance due to the return bend stretches out to at least 21.5 tube diameters downstream of the bend. Upstream of the bend, an effect was absent for downward flow, for upward flow a limited effect was observed.

Liquid film thickness behaviour within a large diameter vertical 180° return bend

Experimental results of liquid film thickness distribution of an air–water mixture flowing through a vertical 180° return bend are reported. Measurements of liquid film thickness were achieved using flush mounted pin and parallel wire probes. The bend has a diameter of 127mm and a curvature ratio (R/D) of 3. The superficial velocities of air ranged from 3.5 to 16.1m/s and those for water from 0.02 to 0.2m/s. At these superficial velocity ranges, the flow pattern investigated in this work focused on churn and annular flows. It was found that at liquid and gas superficial velocities of 0.02m/s and 6.2m/s, respectively, the averaged liquid film thickness peak at 90°. At gas superficial velocity of 16.1m/s, the relationship between them is linear due to the shear forces overcoming gravity. Additionally, it was found that deposition of entrained droplets keeps the liquid film on the outside of the bend. The results of polar plots of average liquid film thickness in the bend showed that the distribution of the liquid film is not symmetrical with thicker films on the inside of the bend due to the action of gravity. Experimental results on average liquid film thickness showed good agreement with the simulation data reported in the literature.

Flow Boiling Heat Transfer of Liquid Nitrogen in Heated U-Tubes

The flow boiling heat transfer characteristics of liquid nitrogen in three U-tubes with different curvature ratios were investigated experimentally. The effects of inlet pressure, heat flux, and curvature ratio on heat transfer characteristic are analyzed. The results indicate that the local heat transfer characteristics change obviously as fluid flows through the return bend, especially in the case of high heat flux. The drying out occurs near the inner wall of the return bend under high heat flux. A parameter Rh (down/up), which is defined as the ratio of heat transfer coefficient between the downstream and upstream section of U-bend, is proposed to evaluate the contributions of the curvature ratio to the heat transfer. It is found that the Rh (down/up) increases with the decrease of the curvature ratios. Furthermore, the experiments results of the average heat transfer coefficient are compared with the calculated results of the empirical correlations.

Curvature Ratio Effect on Two-Phase Pressure Drops in Horizontal Return Bends: Experimental Data for R134a

This article presents 154 pressure drop data points measured during two-phase flow of R-134a in horizontal return bends. The tube diameter is constant at 10.85 mm and the curvature ratio is either 7.74 or 5.53. Saturation temperature varies from 15 to 20°C, vapor quality from 0.05 and 0.95, and mass velocity ranges from 300 to 600 kg m−2 s−1. Return bend pressure drops are calculated by subtracting the straight tube pressure drop from the total measured pressure drop along the bend. The perturbations induced up- and downstream of the singularity are taken into account in the measurements. The comparison of the pressure drops for the two configurations (curvature ratio of 5.53 and 7.74) showed that they are greater (about 10%) for the larger curvature ratio. This can be attributed to the effect of the developed length on the pressure drop; on the other side the pressure gradients are larger for the lower curvature ratio, which can be explained by the effect of the centrifugal force and the perturbations up- and downstream of the return bend. The experimental data are compared against four prediction methods available in the literature. The Domanski and Hermès correlation is the best at predicting the present data.

Flow regime visualization and pressure drops of HFO-1234yf, R-134a and R-410A during downward two-phase flow in vertical return bends

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 6.7mm 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.

Two-phase air–water flow through a large diameter vertical 180o return bend

An experimental study of churn-annular flow behaviour and mean film fraction of an air–water mixture flowing through a vertical 180° return bend using an electrical conductance technique is reported. Measurements were made of film fraction using probes placed before, within the bend (45o, 90o, 135o) and after the bend. The bend, made of transparent acrylic resin, has a diameter of 127mm and a curvature ratio (R/D) of 3. The superficial velocities of air ranged from 3.5 to 16.1m/s and those for water from 0.02 to 0.2m/s.Flow patterns were identified using the characteristic signatures of Probability Density Function (PDF) plots of the time series of mean film fraction. The average film fraction is identified to be higher in the straight pipes than in the bends. The study also identified that at low gas superficial velocity, the film fraction for the riser was generally greater than for the downcomer. For low liquid and higher gas flow rates, film break down occurs at the 45o bend due to gravity drainage. The condition for which the liquid goes to the outside or inside of the bend are identified based on a modified form of Froude number based on published material. A comparison between the present work and that of Hills (1973) based on the mean film fraction showed same tendency.

Two-phase flow visualization and pressure drop measurements of HFO-1234yf and R-134a refrigerants in horizontal return bends

Two-phase flow regimes of HFO-1234yf and R-134a in a horizontal 6.7mm inner diameter return bend are investigated. The different flow regimes observed are: slug, intermittent and annular flows. The perturbation lengths up- and downstream of the singularity are experimental investigated and their contribution to the pressure drop is identified. It is shown that the upstream perturbation length is less than 10D and that the downstream perturbation length is less than 20D for the conditions tested in this study. In addition, this paper presents 574 pressure drop data points measured for two-phase flow of HFO-1234yf and R-134a in horizontal return bends of inner diameters (D) from 7.90 to 10.85mm and curvature ratios (2R/D) from 3.68 to 4.05. The experimental pressure drop data in return bends is compared to four well-known prediction methods available in the literature. The whole present pressure drop in return bends experimental database for the next generation HFO-1234yf and for the R-134a refrigerants is presented in the appendix.

Numerical simulation of laminar forced convection heat transfer of Al2O3–water nanofluid in a pipe with return bend

Laminar forced convection heat transfer of the Al2O3–water nanofluid in a pipe with a return bend is analyzed by using a finite element method. The results show that the average Nusselt number increases with increasing Reynolds number and Prandtl number, and the increment of specific heat in the nanofluid contributes to the heat transfer enhancement. The average Nusselt number in the return bend appears higher than that in the inlet and outlet pipes due to the secondary flows. However, the pressure drop in the pipe largely increases with the increment of nanoparticle volume concentration. The empirical correlations for the average Nusselt numbers are obtained as functions of the Dean number and the Prandtl number.

Two-phase pressure drop in return bends: Experimental results for R-410A

This study presents 238 pressure drop data points measured for two-phase flow of R-410A in horizontal return bends. The tube diameter ( D) varies from 7.90 to 10.85 mm and the curvature ratio (2 R/ D) from 3.68 to 4.05. The mass velocity ranges from 179 to 1695 kg m −2 s −1 and the saturation temperatures from 4.6 °C to 20.7 °C. Preliminary tests show that the recovery length necessary for a correct pressure drop measurement downstream of the return bend is less than 20 D, for the experimental conditions covered in this study. The singular pressure drop is determined by subtracting the regular pressure drop in straight tube from the total pressure drop. The experimental data are compared against four available correlations found in the literature. The present experimental database for the return bend pressure drop is presented in the Appendix A.

Effect of Return Bend and Entrance on Heat Transfer in Thermally Developing Laminar Flow in Round Pipes of Some Heat Transfer Fluids With High Prandtl Numbers

Abstract The paper presents experimental results and analysis of heat transfer in a thermally developing region of round pipes for three fluids typically used as low temperature coolants, in the range of 0–−40°C. The experiments were performed at low Re (200–1000) and high Pr (80–140) numbers that are typically found in secondary refrigeration loop conditions. The effect of horizontal U-bend is also presented. It is shown that the positive effect of thermal development (high Nu number) lasts long because of the technically significant length of the thermally developing region. Secondary flows developed in and after the U-bend are so significant that they have almost an identical effect as the thermal development at the pipe entrance. That is a reason for the extremely good performance of the heat exchangers with secondary refrigerants in laminar flow regimes. Experimental data are presented with developed empirical correlations, which show good relationships to several existing correlations.

A visual observation of the air–water two-phase flow in small diameter tubes subject to the influence of vertical return bends

This study provides a visual observation of the two-phase flow patterns of the air–water mixtures inside two 3.0 mm and two 6.9 mm smooth tubes subject to the influence of vertical return bend. The effects of tube size ( D = 6.9 and 3 mm) and curvature ratio (2 R/ D = 3 and 7) are examined. The total mass flux is from 50 to 800 kg/m 2 s. Some unique phenomena such as flow reversal, freezing slug are observed for the vertical return bend having D = 6.9 mm that are absent in previously reported results for horizontal configurations. However, the flow reversal and freezing slug phenomena are hardly seen at a smaller diameter tube ( D = 3 mm) due to the influence of the flow inertia and surface tension. For D = 6.9 mm, the elongated bubble across the return bend reveals a separate and re-merged phenomenon but this phenomenon is nearly gone for D = 3 mm. Unlike that of D = 6.9 mm, for the same mass flux and vapor quality with D = 3.0 mm, there is no great difference of the two-phase flow patterns between the flow entering in the upper tube or the flow entering from the lower tube.

Two-phase slug flow across small diameter tubes with the presence of vertical return bend

This study examines the two-phase slug flow across small diameter tubes with the presence of vertical U-type return bends. The translational velocity of the air slug across return bend usually peaks at an angle of π/2–3 π/4. The increase of translational velocity is especially pronounced when flow enters at the lower tube with a smaller curvature ratio. An approximately twofold increase of the slug velocity is observed at a curvature ratio of 3. Dimensionless correlations for flowing upwards and downwards with a mean deviation of 18.7% and 24.5% are proposed that can describe the variation of translational velocity within the return bend.

On the Use of Five-Hole Probes in the Testing of Industrial Centrifugal Compressors

This paper addresses the use of 5-hole probes in the testing of industrial centrifugal compressors. The 5-hole probes utilized for this work are of the conical-tip type and were used in a non-nulling configuration (i.e., the probes do not need to be rotated or moved in any way during the tests). These 5-hole probes proved to be fairly robust, making them practical for a nonlaboratory setting such as an industrial multistage compressor test stand. A discussion of 5-hole probes and how they function is provided, including an overview of the mathematical formulations and calibrations required to translate the pressure data gathered from the 5 holes into static and total pressures, velocities and flow angles. A method to transform these variables from a probe-based coordinate system to a machine-based coordinate system is also presented and schematics of this process are provided to aid the reader’s understanding. The testing performed on a prototype multistage centrifugal compressor using 5-hole probes is also discussed, showing that the probes provided valuable insight into the flowfield exiting the impellers and at the return bend. The hub-to-shroud velocity profile exiting an impeller was found to be more skewed than expected and was contributing to poor performance in the downstream stationary components. The measured flowfield from one of the tests is also compared against 3-D CFD results and comments are offered regarding the agreement between the analytical and measured results. Advantages and disadvantages of 5-hole probes as compared to more conventional instrumentation are presented. Finally, conclusions are drawn regarding the value of 5-hole probe data in the development and/or troubleshooting of high performance turbomachinery and in the validation/calibration of design and analysis tools.

Measurements and correlations of frictional single-phase and two-phase pressure drops of R-410A flow in small U-type return bends

This study presents single-phase and two-phase frictional data for R-410A in four U-type return bends with tube diameter D=3.3 and 5.07 mm and curvature ratio (2 R/ D) ranged from 3.91 to 8.15, and their frictional performances are investigated. The friction factor f C and two-phase pressure gradient in the return bend considerably increase with the decrease of curvature ratio. For D=5.07 mm and G⩾200 kg m −2 s −1, the ratio of (d p/d z) C/(d p/d z) S is relatively independent of the vapor quality x. However, a considerable rise of this ratio is encountered at x<0.5 and G=100 kg m −2 s −1. The significant increase of this ratio may be attributed to the change of flow pattern from stratified flow to annular flow. In contrast, for D=3.3 mm and G⩾300 kg m −2 s −1, the higher pressure gradient ratio at lower x was not observed. The flow pattern at low vapor quality is slug flow and no dramatic change of flow pattern across the return bend is seen. For the single-phase results, existing correlations give fair agreements with the present f C data. For two-phase results, the Geary correlation shows a better agreement with the data. A modified two-phase friction factor based on the Geary correlation is then proposed. The proposed correlation gives a good agreement to the present R-410A data and Geary’s R-22 data with a mean deviation of 19.1%.

Influence of horizontal return bend on the two-phase flow pattern in small diameter tubes

The influences of return bend on the two-phase flow patterns for air–water mixtures inside 6.9 and 3 mm smooth tubes are investigated in this study. For D=6.9 mm and at a mass flux of 50 kg/m 2 s having a quality less than 0.1, no influence on the flow patterns is seen at a larger curvature ratio of 7.1. However, as the curvature ratio is reduced to 3, the flow pattern in the recovery region is temporarily turned from stratified flow into annular flow. But this phenomenon is suppressed again when the quality is increased to 0.3. The temporarily flow pattern transition phenomenon from stratified flow to annular flow is again suppressed when the tube diameter is reduced to 3 mm. It is likely that this phenomenon is related to the influence of surface tension and the reduction of developing length. Based on the present flow visualization, two flow pattern maps are proposed to describe the effect of return bend of the transition of two-phase flow pattern for 3 and 6.9 mm diameter tubes. An empirical fit of the observed length of the temporary annular flow in the recovery region is proposed that gives good predictions with the present data.

Two-phase flow pattern in small diameter tubes with the presence of horizontal return bend

This study provides a qualitatively visual observation of the two-phase flow patterns for air-water mixtures inside 6.9, 4.95, and 3 mm smooth diameter tubes with the presence of horizontal return bend. The influence of the return bend on the two-phase flow patterns are investigated. For D=6.9 mm and at a mass flux of 50 kg m −2 s −1 having a quality less than 0.1, no influence on the flow patterns is seen at a larger curvature ratio of 7.1. However, were the curvature ratio reduced to 3, the flow pattern in the recovery region is temporarily turned from stratified flow into annular flow. The temporary flow pattern transition phenomenon from stratified flow to annular flow is not so pronounced with the decrease of tube diameter. It is likely that this phenomenon is related to the influence of surface tension and the reduction of developing length of the swirl flow. Based on the present flow visualization, three flow pattern maps are proposed to describe the effect of return bend on the transition of two-phase flow pattern.

Flow and Heat Transfer Characteristics of Ice Slurry in a Bending Flow Channel with Rectangular Cross Section.

The characteristics of both flow and heat transfer were investigated experimentally and analytically for ice slurry flow in a return bend with rectangular cross section. The ice slurry, which is a mixture of fine ice particles and ethylene glycol aqueous binary solution, was utilized as a testing material. In this study, heat transfer coefficient at the heated wall was experimentally determined and ice packing factor distribution in the flow channel was observed in detail. Furthermore, a numerical simulation has been carried out to investigate both velocity distribution and solid fraction distribution in the flow channel. The solutions were obtained numerically by adopting the Algebraic Slip Mixture (ASM) model of a CFD code for number of flow path dimensions and flow parameters. The simulation results indicated that the flow behavior in the curved duct was strongly affected by both buoyancy and centrifugal force, therefore, secondary flow appeared within the parameter range covered in the present study.

Frictional performance of R-22 and R-410A inside a 5.0 mm wavy diameter tube

The influence of return bend on the frictional performance of R-410A and R-22 in a 5-mm diameter tube is examined with a curvature ratio of 6.63. The existing single-phase correlations give fairly good agreements with the present single-phase data, but the existing two-phase correlations of the return bend fail to predict the present two-phase data. For test results of the two-phase flow at G⩾200 kg m −2 s −1, ((d P/d z) c/(d P/d z) s) is approximately equal to 1.8 and is relatively independent of the vapor quality x. However, at a smaller mass flux of 100 kg m −2 s −1, ((d P/d z) c/(d P/d z) s) decreases with x, reaching approximately 5 for x=0.1. The significant increase of this ratio for G increased from 100 to 200 kg m −2 s −1 may be attributed to the change of the two-phase flow pattern.

Influence of horizontal return bend on the two‐phase flow pattern in a 6.9 mm diameter tube

AbstractThe two‐phase flow pattern for air‐water mixtures inside a 6.9 mm U‐tube is reported to have curvature ratios of 3˜7.1. At a lower total mass flux of 50 kg/m2·s and a quality of 0.1, or at a larger curvature ratio of 7.1, no influence on the flow patterns is seen. However, if the curvature ratio is reduced to 3, the flow pattern in the recovery region just after the return bend is temporally turned from stratified flow into annular flow. For a quality larger than 0.4, the annular flow pattern prevails in the entire tube. For G = 400 kg/m2·s and x &lt; 0.01, the size of the plug in the downstream is usually larger than that in the upstream due to the coalesce in the return bend. This coalescence phenomenon continues to further increase the total mass flux at the lower quality region. For a total mass flux above 500 kg/m2·s, the bubbly flow pattern in the upstream region may become intermittent.

Flow visualization of relaminarization phenomena in curved pipes and the related measurements

Flow visualization results for secondary flow phenomena at the exit of 90° and 180° bends and helically coiled pipes (1, 2 and 5 turns), (radius of curvature R_c = 381 mm, inside pipe diameter d=37.5 mm, curvature ratio d/2R_c = 0.049) and in the downstream straight pipe ( l/d = 30 ) are presented to study the stabilizing (relaminarization) effect in curved pipes with fully developed entry turbulent air flow and the destabilizing (re-transition from laminar to turbulent flow) effect in the downstream straight region. The entry Reynolds numbers are Re = 2200, 3200, 4300 and 5300). The related measurement results using a hot-film anemometer are presented for developing profiles of the time-mean streamwise velocity distribution and the axial turbulence intensity field in the 180° return bend and in the downstream straight pipe for Reynolds numbers Re = 3200, 4300, 6300 and 8200. The time traces showing the output of the hot-film sensor are also presented for developing fluctuating velocity field in the 180° bend and in the downstream straight pipe for the same Reynolds number range. Some aspects of the relaminarization phenomena in curved pipes and the re-transition phenomena from laminar to turbulent flow in the downstream straight pipe are clarified by the present experimental investigation.