Stratified Wavy Flow Research Articles

A new diabatic two phase flow pattern transition model of R600a

In this paper, a detailed experimental study was carried out to investigate the two phase flow patterns of R600a in a smooth horizontal tube with an inner diameter of 6 mm. The experiments were performed at conditions covering saturation pressures from 0.215 to 0.415 MPa, mass fluxes from 67 to 194 kg m−2 s−1 and heat fluxes from 10.6 to 75.0 kW m−2. Based on high speed camera, four main flow regimes can be identified: plug flow, stratified-wavy flow, slug flow and annular flow. Intermittent flow (include plug and slug flow) to annular flow transition was detected and plotted on flow pattern map. Comparisons between available intermittent/annular (I/A) transition lines in the literature and the experimental data have been made. The result shows that no transition equations can fit the experimental transition curve well. By taking the surface tension, gravity force, heat flux, viscosity and density into account, four dimensionless number were introduced into the transition equation. Finally, a transition equation of intermittent/annular flow for R600a was proposed and shows good agreements with the experimental data.

Reduced-order description of phase fraction data measured using wire-mesh sensor in two-phase pipe flows

Proper Orthogonal Decomposition (POD) technique has been applied to the three dimensional (cross-sectional and time) void fraction data acquired by the application of wire-mesh sensor (WMS) measurement technique. The measurements are performed in a 76.2 mm ID horizontal pipe with air and water as fluids, and with 16 × 16 WMS located at a distance of 17 m from the inlet (L/D=223). In order to perform the detailed analysis using the POD technique, flow patterns covering stratified-wavy, slug, pseudo-slug, and annular low liquid loading are investigated in the paper. Thanks to the time-resolved data obtained using the WMS, classical POD method is applied in spectral space after converting time-dependence to frequency. The analysis showed that for all the cases considered, substantial part of the original data can be recovered using a couple POD modes and a few frequency modes. The most dominant two phase flow features are captured at near perfect accuracy. The global flow parameters like space- and time-averaged liquid holdup values are compared between the original and reconstructed fields and the agreement is found to be very good. The phase portraits obtained from the random coefficients associated with different POD mode diagrams correlate with the characteristics of the flow pattern, such that it is possible to observe two distinct orbits in slug and pseudo slug flow cases, and single orbit indicating more random behaviour for the stratified wavy and annular flow cases.

Effect of inclination on heat transfer and flow regimes in large flattened-tube steam condensers

An experimental study of convective steam condensation inside a large, inclined, flattened tube used in air-cooled condensers for power plants is presented. This is the fourth of a four-part group of papers. The first three parts (Kang et al., 2017; Davies et al., 2017, 2018) published in the same journal present the facility, pressure drop, void fraction, flow regime and heat transfer results, while this study presents the effects of inclination on heat transfer and flow regimes.The condenser is a flattened steel tube with brazed aluminum fins. The full tube has dimensions 10.72 m × 214 mm × 18 mm. The condenser tube is cut in half lengthwise and covered with a polycarbonate window to perform visualization simultaneously with the heat transfer measurements. The steam is condensed at atmospheric pressure, and cooled by air at a uniform velocity profile. The angle of inclination is varied from horizontal (0°) to 75° downward. Condenser performance is also predicted with a model.The majority of the condenser is found to be in the stratified flow regime for all inclinations tested, with only a short annular section at the inlet of the condenser. The tubes inclined greater than 60° are also found to have stratified-wavy flow near the condenser outlet. Overall condenser U is found to increase with increasing downward inclination angle of the condenser, with a maximum increase of approximately 4% at 75° inclination. This improvement is found to be the result of improved drainage and increased void fraction near the condenser outlet. Mean steam-side heat transfer coefficient1HTC – Heat transfer coefficient1 (HTC) is found to remain constant along the tube, and for the entire condenser, with changes in tube inclination angle. Commonly-used inclined condensation HTC correlations are found to underpredict the magnitude of the experimentally-determined steam-side HTC.

An experimental study on gas/liquid/solid three-phase flow in horizontal coalbed methane production wells

Coal particles caused by well drilling and completion, hydraulic fracturing, and formation pressure decreasing would migrate into the horizontal well carried by fracturing flow-back fluid, formation water and CBM, and then deposit on the low side of pipe leading to the decrease of cross section area of pipe for flow and even blocking of the pipe. The purpose of current research was to obtain the flow pattern while there was stationary solid particle bed on the low side of the pipe and an explicit equation to predict the height of solid particle bed was proposed. In this study, with the coal particle, water, and air as the solid, liquid, and gas phases respectively, gas/liquid/solid three-phase flow experiments in horizontal well were carried out and 4 crucial factors including different water flow rate, gas flow rate, coal particle size, and coal particle mass concentration were considered. Fluid's real apparent velocity was defined and used to analyze the distribution of flow pattern coordinate points. Results show that the flow pattern coordinate points determined by real gas apparent velocity and real water apparent velocity distribute near the flow pattern transition lines of Taitel and Dukler flow pattern map. Besides, coal particle size, water flow rate, and gas flow rate have obvious impact on the height of coal particle bed and the coal particle mass concentration has a negligible effect while the height of interface between gas layer and water layer is only dramatically affected by the gas flow rate. At last, an explicit equation for predicting the cross section area of coal particle bed based on the experimental data was proposed which is crucial to predict the flow pattern and calculate the pressure drop in horizontal well. The predicting cross section area of coal particle bed is in good agreement with the measured cross section area of coal particle bed and the average relative differences are 9.38% and 13.45% for stratified smooth flow and stratified wavy flow respectively. The equation is applicable for a range of coal particle size (8–30 mesh) and water apparent velocity (0.025–0.123 m/s).

A flow regime map for condensation in macro and micro tubes with non-equilibrium effects taken into account

A flow regime map for condensation from superheated vapor is proposed in this paper. The flow regime map takes the non-equilibrium effects into account by following the development of liquid film from the real onset of condensation to the end. It can be applied to both conventional tubes and microchannels. The transition mechanism between annular and stratified-wavy flow is determined to be the force balance between the shear force, gravity and surface tension. The transition mechanism from annular to intermittent flow is found to be the comparison between wave heights and tube diameter. The transition mechanism for stratified-wavy and fully-stratified is kept the same as elaborated by Xiao and Hrnjak (2017). The connections between the dimensionless number We, Fr and Bo and the transition criteria are analyzed. The flow regime map is validated by the experimental data in Xiao and Hrnjak (2017) and some current visualizations, which includes diabatic visualizations of R134a, R1234ze(E), R32, R245fa and R1233zd(E) condensing at 30 and 50 °C in 1, 4 and 6 mm tubes.

Heat transfer measurement and flow regime visualization of two-phase pulsating flow in an evaporator

Heat transfer and flow regime of two-phase R134a pulsating flow in an evaporator have been studied experimentally in this work. Heat transfer coefficient was measured and liquid-vapor two-phase flow regimes was observed under different mass flux, inlet vapor quality and pulsating period. Results show that heat transfer can be enhanced by pulsating flow in short periods (<16 s) with a maximum enhancement of 28% compared with the continuous flow; a deterioration of heat transfer is also observed for flow under long pulsating periods for some conditions, which could be as much as 30%. The effect of mass flux and inlet vapor quality is complex and coupled with pulsation period, which might result from the difference of flow developing between different pulsating periods. The instantaneous flow regime is recorded by a high speed camera and statistically analyzed to explore the mechanism of pulsating flow on heat transfer performance. The statistical analysis of flow regime from the recorded video shows that the flow regime of pulsating flow at on-time remains the same as that of continuous flow with the same instant mass flux, and the flow regime at off-time is mostly stratified, stratified wavy flow depending on the pulsating period. The transition of stratified-wavy flow regime to intermittent, annular flow regime of pulsating flow is the main contribution to the enhancement of heat transfer while dry-out during off-time plays an important role in heat transfer deterioration.

Capacitive Phase Shift Detection for Measuring Water Holdup in Horizontal Oil⁻Water Two-Phase Flow.

In this paper, a phase shift detection system of flow impedance is designed based on a concave capacitance sensor (CCS). The flow impedance of oil–water stratified flow is investigated by establishing an equivalent circuit model and a finite element model. The influence of exciting frequency and sensor geometric parameters on the phase shift output of the CCS is studied to access an optimal phase shift measurement system. An experiment of horizontal oil–water two-phase flows was conducted during which four flow patterns are observed, i.e., stratified flow (ST), stratified wavy flow (SW), dual continuous flow (DC), and dispersed oil-in-water and water flow (DO/W&W). The phase shift responses of the CCS to the water holdup variation are collected. The results indicate that the phase shift response of the CCS presents satisfied sensitivity for ST and SW flow patterns, which is consistent with the predictions of the equivalent circuit model and the finite element model. Although the flow structures of DC and DO/W&W flows are extremely nonuniform, the phase shift response of the CCS still shows better linearity and sensitivity to the water holdup variation. In general, the capacitive phase shift detection technology exhibits advantages for water holdup measurement in horizontal oil–water two-phase flow with nonuniform phase distributions and conductive water.

On the influence of gas–liquid two-phase flow on Lorentz force velocimetry

Lorentz force velocimetry (LFV) is a contactless and non-invasive flow measurement technique for electrical conducting fluids. When the fluid passes a magnetic field, a Lorentz force will be generated. The force is proportional to the electrical conductivity, to the velocity of the fluid, and to the square of the magnetic flux density. It is possible to apply this technique over a range of conductivities varying from high values as for liquid metals to lower values as typically for electrolytes (e.g. salt water). The main challenge is measuring the force, which is six orders of magnitude smaller for electrolytes in comparison to liquid metals. It was shown that LFV is insensitive to the shape of different velocity profiles and especially to strongly asymmetric profiles as long as the volume flow is constant. However, the influence of a second gaseous phase on the force signal remains an open question. This is of fundamental interest especially for chemical and food processing industries. To examine this influence in detail, electrolyte flow with different volume flow fractions was investigated for the current study. By using frits, it is possible to generate a homogeneous distribution of the second (gaseous) phase in the electrolyte. Small electrolyte velocities or flow rates (v < 1.5 m s−1, = 225 l min−1) enable the formation of slug flow and stratified wavy flow, and higher velocities (v > 1.8 m s−1, = 270 l min−1) the formation of bubbly two-phase flow in the used test section. By this means, it is possible to study a wide range of flow patterns in a horizontal duct. The experimental results are in good agreement with the analytical estimations for fully dispersed flow. However, for a nonhomogeneous distribution of air the force signal deviates much stronger. An explanation can be the superposition of Lorentz forces and magnetic forces, caused by differences of the magnetic susceptibilities of water and air, which will be discussed in greater detail.

Experimental study of aqueous foam generation and transport in a horizontal pipe for deliquification purposes

This work presents and analyses the results of experimental activities aimed at a preliminary characterization of foamy flows for pipeline dewatering, in order to assess whether the addition of surfactants may effectively reduce the liquid holdup in horizontal pipelines. Static tests were run to compare the foam cycle (generation and decay) for three commercial surfactants and to choose the most suitable one. Dynamic tests with the selected product were performed in a 20 m long, 60 mm i.d. Plexiglas® pipe, where a 0.3% wt. solution of surfactant in tap water was pumped after mixing with an air flow at nearly atmospheric pressure and temperature. Superficial velocities ranged between 0.03 m/s and 0.05 m/s for water and between 1.5 m/s and 11.5 m/s for air, which would determine stratified/stratified wavy flows in the case of pure water-air flow, i.e. the benchmark case. Due to the presence of the surfactant, foam formed in the mixing section, which implied a significant change in the flow patterns that were photographically recorded and classified into three main types: plug, stratified wavy and stratified with foam entrainment, as far as the air superficial velocity was increased at constant water superficial velocity. The associated pressure drop, linearly distributed along the pipeline, resulted greater than the benchmark value in all the operating conditions, with a dramatic increase (even more than 100%) for plug flows. On the other hand, the percentage relative difference was found to lower with increasing the air superficial velocity, apart for stratified wavy flows where it seemed to keep constant at about 3.3%. Finally, a theoretical model for stratified flows was used to relate the pressure drop to the void fraction in order to get at least an approximate indication of the liquid load reduction due to the surfactant addition, which ranged between 6% and 39%.

Foam generation, characterization and breakup in pipes

Experiments were conducted utilizing a 0.025 m diameter Foam Characterization Rig (FCR) skid. Aqueous foam was used in the experimental program, utilizing both Drill Foam F-450 and SI-403 surfactants to generate the foam. Foam generation and characterization experiments were conducted with different foam generation devices, with the aim of identifying the optimal configuration, which generates the most stable foam. A comparison between the foam quality and inlet gas volume fraction (IGVF) demonstrates that these variables are close to each other at 90% IGVF (vsg = 0.09 m/s) owing to the occurrence of homogenous flow for this condition. The data also confirm that the foam generation configuration with two 125 μm mesh size and 3 mm beads generates the most stable foam.Three different flow patterns were observed in the pipe, namely, stratified, dispersed and slug flow, exhibiting different foam quality. Foam flow in pipes experiments revealed that the gas shear effect on foam break-up depends on the existing flow pattern in the pipe. For stratified wavy flow, increasing the superficial gas velocity can lead to either increasing or decreasing foam breakup efficiency, which depends on the gas velocity. On the other hand, in slug flow, generally increasing the superficial gas velocity results in slightly decrease in the foam breakup efficiency.

Numerical simulation of oil-water non-Newtonian two-phase stratified wavy pipe flow coupled with heat transfer

Oil-water two phase stratified wavy pipe flow widely exits in the petroleum and chemical industries. When the temperature of the oil is below abnormal point, the oil phase acts as non-Newtonian behavior and the flow process becomes more complex. A coupling mathematical model of oil-water non-Newtonian stratified wavy pipe flow under non-isothermal condition is established in bipolar coordinate system with the consideration of the fluctuation at the oil-water interface. The pressure gradients calculated are validated by the results from literature and the relative errors are within 6%. Then the oil-water non-Newtonian two-phase stratified wavy pipe flow process is simulated in this paper. The bulk temperature and pressure drop along the pipeline can be predicted by the models. Temperature and velocity distribution at the different pipe cross sections are simulated and analyzed in detail. The non-Newtonian behavior of oil phase and the effective roughness of oil-water interface can also be obtained from the model. Moreover, the non-dimensional parameter analysis is used to research the mechanisms of flow and heat transfer. Thus, the numerical model can be widely applied to calculate the global fluid properties and other operating parameters which are useful for the management of oil-water mixing transportation pipelines.

Optical void measurement method for stratified wavy two phase flows

We developed a measurement method to determine the phase distribution in a stratified wavy two-phase flow of water and air. It is based on an optical observation of the flow, followed by an image processing and evaluation process. Two-dimensional images were taken of a three-dimensional channel flow. Through averaging in the channel depth and integration over time, a two-dimensional distribution of the liquid and the gaseous phase of the channel flow is obtained. The flow was observed in a rectangular channel with a cross section of 90 × 110 mm2 consisting of a horizontal and a inclined part. The volume flow of air varied between 125 and 180 l/s, and the water flow varied between 25 and 45 l/min. A validation, performed by comparing the measurement results with those obtained by a local electric resistivity probe, showed a good agreement. Deviations between both measurement results are supposed to be based on the different detection of the disperse phase which is underestimated by the resistivity probe and overestimated by the optical method.

Interrogating flow development and phase distribution in vertical and horizontal pipes using advanced instrumentation

The characterization of two-phase flow in both vertical and horizontal pipes is important in oil/gas transportation. As the pipe orientation may change from well to well, it is also important to understand the characteristic responses expected of a two-phase mixture to a change in pipe inclination. This study concerns the changes in the void fraction, structure frequency and structure velocity that occur within an air–silicone oil mixture as a function of the pipe orientation. Experimental data were obtained from the combined use of electrical capacitance tomography (ECT) and wire mesh sensors (WMS), which allow the 3D visualization of the flow patterns. The reported experiments were performed on a 67 mm diameter pipe, in a flow loop, in which a pipe section may be inclined at angles of between −5 or 90° to the horizontal. For this study, the inclined pipe was either set at an angle of 0 or 90 to the horizontal, which correspond to a horizontal or vertical pipe setting, respectively. The results of flow development using the PDF of void fraction obtained at 3 measurement locations; ECT 1, ECT 2 and WMS at 4.4, 4.489 and 4.92 m, respectively, showed that the flow is fully developed and statistically stable for the vertical two-phase flow. While on the other hand, the flow is in rapid development for the horizontal two-phase flow scenario at same liquid and gas superficial velocities. The processed data reveal the differences in flow distribution produced by the pipe inclination. Within the vertical pipe configuration spherical cap bubbles, slug and churn two phase flow patterns were observed, whilst plug, slug and stratified wavy two-phase flows were identified in the horizontal configuration of the pipe. It is concluded that a plot of mixture superficial velocity against average void fraction may be used to provide a qualitative assessment of the flow patterns observed, regardless of pipe inclination. The two-phase flow variables that are concluded to significantly influence the distribution coefficient, C0, and drift velocity, VD, are the pipe orientation and flow patterns.

Comparison of gamma densitometry and electrical capacitance measurements applied to hold-up prediction of oil–water flow patterns in horizontal and slightly inclined pipes

Measurement techniques are vital for the control and operation of multiphase oil–water flow in pipes. The development of such techniques depends on laboratory experiments involving flow visualization, liquid fraction (‘hold-up’), phase slip and pressure drop measurements. They provide valuable information by revealing the physics, spatial and temporal structures of complex multiphase flow phenomena.This paper presents the hold-up measurement of oil–water flow in pipelines using gamma densitometry and electrical capacitance tomography (ECT) sensors. The experiments were carried out with different pipe inclinations from −5° to +6° for selected mixture velocities (0.2–1.5 m s−1), and at selected watercuts (0.05–0.95). Mineral oil (Exxsol D60) and water were used as test fluids. Nine flow patterns were identified including a new pattern called stratified wavy and mixed interface flow. As a third direct method, visual observations and high-speed videos were used for the flow regime and interface identification.ECT and gamma densitometry hold-up measurements show similar trends for changes in pipeline inclinations. Changing the pipe inclination affected the flow mostly at lower mixture velocities and caused a change of flow patterns, allowing the highest change of hold-up. ECT hold-up measurements overpredict the gamma densitometry measurements at higher input water cuts and underpredict at intermediate water cuts. Gamma hold-up results showed good agreement with the literature results, having a maximum deviation of 6%, while it was as high as 22% for ECT in comparison to gamma densitometry.Uncertainty analysis of the measurement techniques was carried out with single-phase oil flow. This shows that the measurement error associated with gamma densitometry is approximately 3.2%, which includes 1.3% statistical error and 2.9% error identified as electromagnetically induced noise in electronics. Thus, gamma densitometry can predict hold-up with a higher accuracy in comparison to ECT when applied to oil–water systems at minimized electromagnetic noise.

Effect of T-junction diameter ratio on stratified-wavy flow separation

Most of the research carried out on reduced diameter ratio T-junctions concluded that partial phase separation can be improved by decreasing the diameter ratio. Despite using very small diameter ratio T-junctions, petroleum industry has always faced the problem of continuous and sometimes very high liquid carryovers. In this regard, the separation of air-water stratified-wavy flow in 1.0, 0.52, 0.67, 0.27 and 0.20 diameter ratio T-junctions was examined to investigate the effect of diameter ratio on phase separation. Phase distribution of each T-junction was evaluated by observing the point at which liquid carryover starts (liquid carryover threshold) and the maximum liquid carryover delivered (peak liquid carryover). For optimum separation it is essential that liquid carryover threshold is high and peak liquid carryover is low, however these parameters were found to decrease with the reduction in diameter ratio. It was observed that in stratified-wavy flow, the drop in liquid carryover threshold was reasonable and thus in this flow regime a decrease in diameter ratio was always found to enhance phase separation. Two correlations were also developed to predict the phase splitting behavior, under a wide range of operating conditions.

An Experimental Study on the Performance of Drag-Reducing Polymers in Single- and Multiphase Horizontal Flow Using Particle Image Velocimetry

This paper presents experimental investigations conducted to understand the influence of water-soluble drag-reducing polymers (DRPs) in single- and two-phase (stratified wavy) flow on flow-field characteristics. These experiments have been presented for water and air–water flowing in a horizontal polyvinyl chloride 22.5-mm ID, 8.33-m long pipe. The effects of liquid flow rates and DRP concentrations on streamlines and the instantaneous velocity were investigated by using particle image velocimetry (PIV) technique. A comparison of the PIV results was performed by comparing them with the computational results obtained by fluent software. One of the comparisons has been done between the PIV results, where a turbulent flow with DRP was examined, and the laminar–computational fluid dynamic (CFD) prediction. An agreement was found in the region near the pipe wall in some cases. The results showed the powerfulness of using the PIV techniques in understanding the mechanism of DRP in single- and two-phase flow especially at the regions near the pipe wall and near the phases interface. The results of this study indicate that an increase in DRP concentrations results in an increase in drag reduction up to 45% in single-phase water flow and up to 42% in air–water stratified flow.

Simulasi CFD Aliran Stratified Air-Udara pada Pipa Horisontal

Two-phase flow can be found in industries, such as petroleum, electricity generation. In geothermal power plants, two-phase flow occurs from a mixture of water and steam which should be avoided in the piping system because it can cause equipment damage in the operating system. Therefore, an operator and engineer need a knowledge of flow patterns, phenomena and characteristics of the two-phase flow. One of the methods to predict the flow pattern is ud\sing the Computational Fluid Dynamic (CFD). This CFD simulation was modeled using Ansys Fluent 15.0 software to determine the changes of Stratified flow characteristics. The model used is Volume of Fluid (VOF). The fluid type is water and air. The variation of superficial water velocity JL of 0.025 m/s - 0.1 m/s while the superficial air velocity (JG) of 0.05 m/s - 1 m/s. The pipe used is an acrylic of 19 mm diameter and length of 1000 mm. The simulation results show that: (1). Stratified flow occurred for the low velocity of the liquid and gas phases. It was shown by the water and air are clearly separated. (2). Flow patterns are not stable, but it can change depending on the superficial velocity of gas and water. (3). The larger of JG causes the wave height because of Bernoulli’s effect so that it will make the stratified wavy or ripple flow pattern. (4). The frequency of stratified-wavy and ripple waves will tend to decrease when the JL increases, while the effect of JG is insignificant. Furthermore, it can be concluded that the speed of gas must be regulated so as not too large to the speed of water.

Hydrodynamics of air and oil–water dispersion/emulsion in horizontal pipe flow with low oil percentage at low fluid velocity

The objective of this paper is to investigate the flow regimes of three phase flow, emerged to be as two phase flow. These two phases are air phase and oil dispersed in water phase, in other words the latter phase can be termed as water–oil emulsion phase or simply emulsion phase. Pipeline emulsions flow is inescapable for upstream oil production system in transporting mixtures of crude oil and water. The mixing, turbulence as well as agitation through wellbores, expansion or contraction, valves, pumps, etc. will emulsify either the oil phase or water phase, depending on the volumetric amount of the phases. This is executed in a flow loop system with varying air and liquid-emulsion velocities. The oil percentages to create the water–oil emulsion are varied from 0% to 24%. Flow pattern maps are developed at low range of fluid velocities (0.01–0.1 ms−1) and it is compared with established maps. Stratified smooth (SS), stratified to stratified-wavy (SSW), stratified wavy (SW) and stratified wavy with ripples (SWR) are the flow patterns, observed in the experiment. The stratified smooth and stratified to stratified-wavy flow regimes are found to be the most dominant. Oil dispersed in the water (emulsion) phase results a clear distinction of the stratified and the stratified-wavy regimes. An increase in flow regime area occupied by stratified/stratified-wavy in the flow pattern map is evident with increasing oil percent in the dispersed liquid phase. The addition of oil to the liquid phase causes a dampening effect on the flow regime transition, most considerably from stratified to nearly non-stratified flow. This might be attributed to the increase in viscosity at high oil percentages. The pressure drop is not significant in the whole flow phenomena and is found to increase with increasing liquid velocity for all oil percentages as the velocity range of the experiment is very low (0.01–0.1 ms−1). Pressure drop and liquid hold up are compared with previous theories and our prediction. Experimental data are predicted with approximately ±10% deviation.

Experiment investigation of two-phase flow pattern transition and frictional pressure drop of R600a in a horizontal tube

With the growing concern of environment problems, there are urgent demands for environment-friendly refrigerants in the refrigeration industry. As a natural refrigerant, R600a not only has zero ozone depletion potential and quite low global warming potential, but also has better cooling performance than other refrigerants. Due to its good cooling performance and environment-friendly characteristics, R600a has been used as an alternative refrigerant in heat transfer applications such as refrigerators, freezers, and heat pumps. It is known that accurate prediction of two phase flow pattern transition and frictional pressure drop of R600a in a horizontal tube play an important role in design and optimization of refrigerators, freezers, and heat pumps. According to this issue, a detailed experimental study was carried out to investigate the two-phase flow pattern transition and frictional pressure drop characteristics of R600a in a smooth horizontal tube with an inner diameter of 6 mm. The experiments were performed at conditions covering saturation temperature from 282.4 to 304 K, mass fluxes from 67 to 194 kg/(m2 s). Based on a high speed camera, four main flow regimes can be observed: plug flow, stratified-wavy flow, slug flow and annular flow. The flow map of R600a has been drawn and comparisons between the experimental data of flow patterns and transition lines in the literature have been made. The results showed that with the increase of mass flux, the inception of the annular flow regime occurs earlier. The intermittent flow regime takes place over a narrow range of vapor quality and annular flow regime occupies a wide range of vapor quality. In addition, the trend of Barbieri and Ong and Thome I / A transitions lines are consistent with the experimental data. However, both of them overestimate the inception of the annular flow regime. After taking the influence of vapor inertia, liquid viscous and surface tension forces into account, a modified Weber number We * has been introduced to the new flow pattern transition model. Based on the relationship of X tt and We *, a new correlation has been proposed and it can well fit with the experimental data of flow pattern. Furthermore, a detailed discuss of frictional pressure drop of annular flow and non-annular flow has been made. The results indicated that frictional pressure drop increases with the increases of vapor quality. The tendency of frictional pressure drop presents different the changing laws in different flow regime. The frictional pressure drop increases slowly in the non-annular flow regime while it increases rapidly when the flow pattern turns into the annular flow regime. The reason is that the vapor-phase pressure drop often dominates the total two-phase frictional pressure drop. The annular flow regime owns higher void fraction than the non-annular flow regime. Nine classic prediction correlations of pressure drop were evaluated and the results showed that the correlation of Gronnerud and Muller-Steinhagen and Heck give the best fit to the experimental frictional pressure drop of annular flow and non-annular flow with a mean absolute relative deviation of 38.5% and 19.7% respectively.

Application of a single wire-mesh sensor in a parabolic trough facility with direct steam generation

The direct steam generation process can contribute to increase the cost-effectiveness of line-focused solar thermal power plants. The current research focuses on the once-through mode as very promising option for large-scale commercial plants. However, the existing pool of knowledge about the steam–water flow in the horizontal receiver pipes reveals knowledge gaps. The present contribution aims to experimentally investigate the two-phase flow in the receiver pipes of the DISS test facility at the Plataforma Solar de Almería, Spain, by an invasive measurement device for the first time. Therefore, a conductance wire-mesh sensor is employed in the low-quality evaporation section of the DISS test facility. This measurement device measures the local instantaneous void fraction that is used to describe the prevailing flow pattern under different pressures and mass flow rates. The results show slug flow, roll waves, wavy flow, and stratified-wavy flow in the horizontal receiver pipes within pressures of 30–80bar. The probability of slug flow or large-amplitude waves decreases with higher pressures. The findings provide a valuable knowledge about the flow structure within the receiver pipes and confirm the proof of function of the wire-mesh sensor under conditions of the direct steam generation process.