Annular Flow Regime Research Articles

Two-phase slug flows in helical pipes: Slug frequency alterations and helicity fluctuations

Air-water numerical simulations in the slug flow regime have been performed in horizontal helical pipes and the effects of geometries on the flow regime have been investigated. Depending on the length of the helix, outlet slug frequencies have been reduced with various degrees of efficiency. Correlations between mean tangential velocity and helicity density fluctuations have been identified and investigated qualitatively. These flow fields show smaller time scales than those obtained in volume fractions fluctuations. Shifts observed in the tangential velocity and mean helicity fluctuations to smaller time scales (high frequencies) are associated with regime transitions. For a slug flow undergoing a continuous transition to the annular flow regime, it is shown that the presence of slower (low frequencies) helicity fluctuations is attributed to the variations in the axial velocity. Finally, the analysis of the helicity at gas-liquid interface confirms the presence of the mixing zone at the slug front.

Assessment of wall friction model in multi-dimensional component of MARS with air–water cross flow experiment

Recently, high precision and high accuracy analysis on multi-dimensional thermal hydraulic phenomena in a nuclear power plant has been considered as state-of-the-art issues. System analysis code, MARS, also adopted a multi-dimensional module to simulate them more accurately. Even though it was applied to represent the multi-dimensional phenomena, but implemented models and correlations in that are one-dimensional empirical ones based on one-dimensional pipe experimental results. Prior to the application of the multi-dimensional simulation tools, however, the constitutive models for a two-phase flow need to be carefully validated, such as the wall friction model. Especially, in a Direct Vessel Injection (DVI) system, the injected emergency core coolant (ECC) on the upper part of the downcomer interacts with the lateral steam flow during the reflood phase in the Large-Break Loss-Of-Coolant-Accident (LBLOCA). The interaction between the falling film and lateral steam flow induces a multi-dimensional two-phase flow. The prediction of ECC flow behavior plays a key role in determining the amount of coolant that can be used as core cooling. Therefore, the wall friction model which is implemented to simulate the multi-dimensional phenomena should be assessed by multidimensional experimental results. In this paper, the air–water cross film flow experiments simulating the multi-dimensional phenomenon in upper part of downcomer as a conceptual problem will be introduced. The two-dimensional local liquid film velocity and thickness data were used as benchmark data for code assessment. And then the previous wall friction model of the MARS-MultiD in the annular flow regime was modified. As a result, the modified MARS-MultiD produced improved calculation result than previous one.

Analytical Modeling of Boiling Nanofluids

The idea of employing nanoparticles in boiling flows has been welcomed by many researchers to increase heat transfer coefficient. In the present study, the effects of adding different nanoparticles to the two-phase vapor–liquid flow of pure water is investigated with the aims of increasing the heat transfer coefficients and increasing dryout length in the annular flow regime of boiling flows. The heat and mass transfer in boiling flows of nanofluids are simulated using an analytical model. The four involved mechanisms of mass transfer (evaporation, deposition, entrainment, and condensation) are comprehensively simulated in the model. Attempts are made to employ the fewest possible simplification assumptions and empirical correlations in the modeling procedure. The model is then verified with experimental models available in the literature. It will be shown that employing nanoparticles in a boiling flow with annular regime considerably increases the heat transfer coefficient. In contrast, adding particles has no significant effect on dryout length.

Phenomenological modelling of CHF in annular flow in annuli using new models of droplet deposition and entrainment

In this paper, we present a phenomenological model to predict the CHF (critical heat flux) for upward annular flow in heated vertical annuli. In present model, a new set of correlations of droplet deposition and entrainment in annuli was used which were verified by comparison with the data of Moeck (1970) for developing liquid films in adiabatic annuli. In the results presented here, these new correlations have been used to predict 2249 independent data on critical heat flux (CHF) obtained both regarding internal heating of the rod as well as simultaneous heating of the rod and the outer tube in six heated vertical annuli under various mass flow rate, pressure and inlet quality and where the conditions were such that (as is most common) the CHF condition occurred in the annular flow regime. The comparisons between the calculated and measured CHFs showed that the present model has good predictive capability in predicting CHF.

Transient force analysis and bubble dynamics during flow boiling in silicon nanowire microchannels

A study on bubble growth mechanisms and underlying physical phenomena of flow boiling in silicon nanowire (SiNW) microchannels has been performed and compared these results with plainwall microchannels. A new approach in studying bubble dynamics and forces acting on liquid–vapor (L–V) interface of growing bubble has been proposed based on theoretical, experimental and visual studies. Bubble size, liquid film thickness, interfacial properties are measured and L–V interfaces are detected from high speed visualization data and results are analyzed by vision-based approach. Force analysis during instantaneous bubble growth from bubble nucleation to formation of annular flow regime has been performed for both the SiNW and plainwall microchannel configurations. Results from force analysis show the dominance of surface tension at L–V interface of growing bubble which resulted higher heat transfer contact area, lower thermal resistance and higher thin film evaporation. Whereas, inertia force is dominant at L–V interface of fully grown bubble and it helps in bubble removal process and rewetting before flow reversal. Significant differences between SiNW and plainwall microchannels have been observed in bubble growth mechanism, heat transfer mode and forces acting on L–V interface.

Prediction of the critical heat flux for saturated upward flow boiling water in vertical narrow rectangular channels

A study, on the theoretical method to predict the critical heat flux (CHF) of saturated upward flow boiling water in vertical narrow rectangular channels, has been conducted. For the assessment of this CHF prediction method, 608 experimental data were selected from the previous researches, in which the heated sections were uniformly heated from both wide surfaces under the high pressure condition over 41bar.For this purpose, representative previous liquid film dryout (LFD) models for circular channels were reviewed by using 6058 points from the KAIST CHF data bank. This shows that it is reasonable to define the initial condition of quality and entrainment fraction at onset of annular flow (OAF) as the transition to annular flow regime and the equilibrium value, respectively, and the prediction error of the LFD model is dependent on the accuracy of the constitutive equations of droplet deposition and entrainment. In the modified Levy model, the CHF data are predicted with standard deviation (SD) of 14.0% and root mean square error (RMSE) of 14.1%. Meanwhile, in the present LFD model, which is based on the constitutive equations developed by Okawa et al., the entire data are calculated with SD of 17.1% and RMSE of 17.3%. Because of its qualitative prediction trend and universal calculation convergence, the present model was finally selected as the best LFD model to predict the CHF for narrow rectangular channels.For the assessment of the present LFD model for narrow rectangular channels, effective 284 data were selected. By using the present LFD model, these data are predicted with RMSE of 22.9% with the dryout criterion of zero-liquid film flow, but RMSE of 18.7% with rivulet formation model. This shows that the prediction error of the present LFD model for narrow rectangular channels is similar with that for circular channels.

Hydrate formation and deposition in a gas-dominant flowloop: Initial studies of the effect of velocity and subcooling

Gas hydrate formation is a critical flow assurance risk in oil and gas production, as remediation of blockages may require weeks of operating downtime and represent a significant safety hazard. While many studies over the past two decades have focused on quantifying hydrate blockage risk in crude oil systems, there is a dearth of information available with which to assess hydrate growth rate or blockage severity in natural gas systems, which typically operate between stratified and annular flow regimes. In this investigation, a single-pass gas-dominant flowloop was used to measure hydrate growth and particle deposition rates with variable liquid holdup (1–10 vol%) and subcooling (1–20 °C). A particular focus of this study was the impact of reducing the gas phase velocity to achieve lower liquid entrainment and, therefore, decrease hydrate formation rate. Reducing the gas velocity from 8.7 to 4.6 m/s at a constant subcooling around 6 °C reduced the total formation rate by a factor of six. At these conditions, the sensitivity of hydrate formation rate to velocity was about 40 times greater than the sensitivity to subcooling. This reduction in gas velocity also halved the estimated rate of hydrate deposition on the pipeline wall. Finally, new observations of hydrate wash-out are reported, whereby significant localized hydrate deposits were effectively removed by modulating the subcooling of the flowloop wall from 6 °C to 3.5 °C. The results provide new insight to inform the next generation of predictive hydrate growth and deposition models for gas-dominant flowlines.

Condensation heat transfer in rectangular microscale geometries

Heat transfer coefficients during condensation of refrigerant R134a in small hydraulic diameter (100<Dh<160μm) rectangular (1<AR<4) channels are presented. A novel technique to accurately determine condensation heat duty and heat transfer coefficient in such microscale geometries at small Δx is used. Models in the literature that were developed for larger tubes are shown to under predict the data. A new model that accounts for the flow mechanisms during condensation at such small scales, and takes into account the effect of G, x, Tsat, Dh and AR, is developed. The model predicts 94% of the data in the intermittent, transition and annular flow regimes within ±25%.

Developing new mechanistic models for predicting pressure gradient in coal bed methane wells

A range of mathematical models and correlations is used to estimate the pressure drop of coal bed methane (CBM) or coal seam gas (CSG) wells. These correlations were originally developed for co-current two-phase flows in conventional wells in the oil and gas industry. However, the upward flow of gas and downward flow of water in the annulus between casing and tubing of a CSG well results in counter-current two-phase flows. The flow regimes developed in counter-current two-phase flows in annuli are noticeably different to co-current two-phase flow regimes in pipes, and thus the existing models used to predict pressure profiles in co-current wells do not adequately describe two phase flows in a CSG well.In this study, we develop new mechanistic models for predicting holdups and pressure gradients of counter-current bubble and slug flows in vertical annuli following the existing models of co-current and counter-current flows in annuli and pipes. A model based on the work of Taitel and Barnea (Taitel and Barnea, 1983) was also developed to predict the transition from slug to annular flow regime in counter-current flows in annuli. Our comparison of the pressure gradients of co-current and counter-current flows in annuli shows that the pressure gradients of counter-current flows are appreciably different to those in co-current flows under the same conditions at high liquid flow rates. This indicates that the models currently employed in typical commercial well flow simulators may considerably overestimate the pressure gradient across a CSG well.

Onset of flow reversal in upflow condensation in an inclinable tube

This work performs an experimental analysis of the onset of flow reversal in upflow condensation of refrigerant 134a in an inclinable 5-mm diameter tube. As the upward unidirectional annular flow regime is established, solitary waves are generated on the liquid substrate and are initially carried upwards by the vapor. With an increase in the condensate flow rate, the liquid film grows thicker and more disturbed, leading to a wider range of film flow structures as well as oscillatory changes in the liquid velocity. These changes mark the onset of flow reversal in the liquid film and the subsequent transition to churn flow. Image processing techniques have been applied to high-speed video sequences of the two-phase flow to identify the onset of film flow reversal and properties of the interfacial waves. A film structure characterized by the existence of the so-called primary and secondary waves was observed to exist near the region of transition to bidirectional flow. Spatio-temporal maps of the liquid film thickness have been generated aiming to quantify the influence of the operating conditions on the wave behavior. Adaptations of existing correlations for the critical vapor velocity associated with the point of flow reversal and wave frequency (Strouhal number) have been proposed.

Waves and instabilities in high quality adiabatic flow in microgap channels

Two-phase flow in microgap channels offers highly potent thermal management capability and is the foundation for the emerging “embedded cooling” paradigm of electronic cooling. While heat transfer and pressure drop in such flows are intimately tied to their distinct forms of vapor–liquid aggregation, insufficient attention has been paid to characterizing the wave patterns and sub-regimes in high-quality microgap channel flow. The present visualization study focuses on two-phase flow in an adiabatic 184μm microgap channel operating at three mass fluxes of FC-72; 220, 420, and 620 kg/m2-s, with flow qualities ranging from approximately 40% to 90%. As predicted by a modified Taitel–Dukler flow regime map, annular flow is found to be the dominant flow regime for the present microgap configuration. Within the annular flow regime, unique 3-D wave patterns are observed at the liquid–vapor interface. The wavelength of these interfacial waves is observed to decrease with increasing flow quality and mass flux. Linear stability analysis of the liquid–vapor interface is found to yield strong agreement in predicted wavelength and wave growth rate distribution with the experimental results.

Condensation heat transfer and pressure drop of R-410A in a 7.0 mm O.D. microfin tube at low mass fluxes

R-410A condensation heat transfer and pressure drop data are provided for a 7.0 mm O.D. microfin tube at low mass fluxes (50–250 kg/m2 s). The heat transfer coefficient of the microfin tube shows a minimum behavior with the mass flux. At a low mass flux, where flow pattern is stratified, condensation induced by surface tension by microfins overwhelms condensation induced by shear, and the heat transfer coefficient decreases as mass flux increases. At a high mass flux, where flow pattern is annular, condensation induced by shear governs the heat transfer, and the heat transfer coefficient increases as mass flux increases. The pressure drop of the microfin tube is larger than that of the smooth tube at the annular flow regime. On the contrary, the pressure drop of the smooth tube is larger than that of the microfin tube at the stratified flow regime.

Numerical simulation of annular flow boiling in microchannels

Flow boiling in microchannels has been researched for decades due to its potential for heat transfer applications in micro structured devices. The large number of existing experimental datasets and fitted correlations show considerable scatter and lack a mechanistic basis. The computational fluid dynamics (CFD) model developed here applies to boiling in the annular flow regime and provides insights into the underlying behaviour. Analysis of the heat transfer augmentation by interfacial waves is studied. Parametric investigations are performed using the model to understand the impact of important flow and system parameters. The calculated heat transfer coefficients are consistent with those from existing correlations, showing no more scatter than the data used to create them.

Numerical simulation of condensation for R410A at a different saturation temperature in mini/micro tubes

ABSTRACTHeat transfer and pressure drop characteristics of condensation for R410A inside horizontal tubes (dh = 0.25, 1, and 4 mm) at saturation temperatures Tsat = 310, 320, and 330 K are investigated numerically. Liquid–vapor interfaces and stream traces are also presented to provide a better understanding of the effect of saturation temperature on the condensation process inside micro tubes. The results indicate that local heat transfer coefficients and pressure drop gradients increase with increasing mass flux and vapor quality and with decreasing tube diameter and saturation temperature. Liquid film thickness also increases with increasing saturation temperature because of the lower surface tension at a higher saturation temperature. When gravity dominates the condensation process, a vortex with its core lying at the bottom of the tube is found in the vapor phase region. For annular flow regime at dh = 0.25 mm, the vortex disappears, and stream traces point from the symmetry plan to the liquid–vapor interface, where the vapor phase turns into the liquid phase. Numerical heat transfer coefficients and pressure drop gradients are compared with available empirical correlations. Two new models for heat transfer coefficients and frictional pressure drop gradients are developed based on the numerical work. A reasonable agreement between predicted and numerical results is obtained.

MODELLING AIR AND WATER TWO-PHASE ANNULAR FLOW IN A SMALL HORIZONTAL PIPE

Numerical simulation using computational fluid dynamics (CFD) has been carried out to study air and water two-phase flow in a small horizontal pipe of an inner diameter of 8.8mm, in order to investigate unsteady flow pattern transition behaviours and underlying physical mechanisms. The surface liquid film thickness distributions, determined by either wavy or full annular flow regime, are shown in reasonable good agreement with available experimental data. It was demonstrated that CFD simulation was able to predict wavy flow structures accurately using two-phase flow sub-models embedded in ANSYS-Fluent solver of Eulerian–Eulerian framework, together with a user defined function subroutine ANWAVER-UDF. The flow transient behaviours from bubbly to annular flow patterns and the liquid film distributions revealed the presence of gas/liquid interferences between air and water film interface. An increase of upper wall liquid film thickness along the pipe was observed for both wavy annular and full annular scenarios. It was found that the liquid wavy front can be further broken down to form the water moisture with liquid droplets penetrating upwards. There are discrepancies between CFD predictions and experimental data on the liquid film thickness determined at the bottom and the upper wall surfaces, and the obtained modelling information can be used to assist further 3D user defined function subroutine development, especially when CFD simulation becomes much more expense to model full 3D two-phase flow transient performance from a wavy annular to a fully developed annular type.

Upscaling modeling using dimensional analysis in gas–liquid annular and stratified flows

Abstract Upscaling of models based on small diameter and low pressure experimental data to large diameter and high pressure conditions is very important in multiphase flow application and research studies. Studies with a large diameter facility and high pressure would significantly improve our understanding (and modeling) of flow characteristics in actual field conditions. Large diameter experiments can be found in literature to certain extent. However, experiments at high pressures are very limited because of the high cost of these experiments. A model based on dimensional analysis to scale up or down pressure drop and liquid holdup data is proposed in this paper. Annular and stratified flow regimes were considered in validating the proposed model. Therefore, our main objective of this paper is to investigate the effect of system pressures on the pressure drop and liquid holdup using dimensional analysis for annular and stratified flow regimes.

Numerical simulation of condensation for R410A at varying saturation temperatures in mini/micro tubes

ABSTRACTHeat transfer and pressure drop characteristics of condensation for R410A inside horizontal tubes (dh = 0.25, 1, and 2 mm) at saturation temperatures Tsat = 310, 320, and 330 K are investigated numerically. The results indicate that local heat transfer coefficients and pressure drop gradients increase with increasing mass flux and vapor quality and with decreasing tube diameter and saturation temperature. Liquid film thickness also increases with increasing saturation temperature because of the lower surface tension at higher saturation temperature. When gravity dominates the condensation process, a vortex with its core lying at the bottom of the tube is found in the vapor phase region. For the annular flow regime, stream traces point from the symmetry plan to the liquid–vapor interface, where the vapor phase becomes the liquid phase. Numerical heat transfer coefficients and pressure drop gradients are compared to available empirical correlations. Two new models for heat transfer coefficients and frictional pressure drop gradients are developed based on the numerical work.

Comparison of Flow Boiling Pressure Drop Correlations for Smooth Macrotubes

This paper investigates two-phase pressure-drop correlations available in the literature and compares them with the experimental data in order to assess their performances in terms of prediction accuracy. Pressure drops for pure refrigerants and mixtures are predicted by comparing 1714 experimental data points with eight two-phase pressure drop correlations. In this context, R22, R407C, R410A, R744, R717, R404A, and R134a are selected as test refrigerants. A flow-pattern-based pressure-drop correlation developed by Moreno Quiben and Thome is also discussed. The Moreno Quiben and Thome model takes the effect of annular to dryout transition, and therefore maximum pressure drop occurs at the end of the annular flow regime. Results refer to experimental works in circular smooth tubes between 3.25 and 14.0 mm, for saturation temperatures between –30.0 and 50.0°C, mass fluxes between 100.0 and 1082.0 kg/m2-s, and heat fluxes from adiabatic to 36.8 kW/m2. The Muller-Steinhagen and Heck correlation shows better performance, predicting 61.7% of experimental data within a ±30.0% error band. It is also seen that the Chawla and Gronnerud correlations agree with experimental results only for R134a and R717.

Experimental Study Structure Of Gas-Liquid Flow In Rectangular Minichannel By Optical Methods

In this paper, using method of two-beam laser scanning and high-speed video was identified patterns of distribution phases in the cross-section of the rectangular channel 0.72 × 1.50 mm with hydraulic diameter of the order to capillary constant. The structure of the gas-liquid flow was studied including flow regimes and statistical characteristics of the two-phase flow in regime with elongated bubbles, transition flow and annular flow regime.

Experimental investigation on flow instability of forced circulation in a mini-rectangular channel under rolling motion

An experimental study was performed in a mini-rectangular channel under rolling condition to investigate the flow instability of forced circulation. In order to analyze the influence of rolling motion on two-phase flow instability, experiments under both static condition and rolling condition were conducted using the same thermodynamic parameters, such as flow rate, system pressure and inlet fluid temperature. During the experiments, typical pressure drop oscillation (PDO) was observed in static condition, while abundant experimental phenomena including single-phase flow fluctuation, trough-type flow oscillation, coupled flow oscillation and two-phase flow fluctuation were found in rolling condition. Single-phase flow fluctuation, which fluctuates in sinusoidal wave, is directly induced by rolling motion, while two-phase flow fluctuation is supposed to be in annular flow regime which is accompanied with fluctuation caused by rolling motion. Trough-type flow oscillation is confirmed as the boiling incipience according to the pressure drop and heat transfer characteristics. As intermittent boiling occurs at the section close to the outlet of the test channel, during trough-type flow oscillation, two wave peaks in one rolling period are found in the pressure drop fluctuation of the outlet section of the channel. The method of Fast Fourier Transform (FFT) is used to analyze the coupled flow oscillation, and two frequencies, including the rolling frequency and PDO frequency, are found in the amplitude spectrum. Thus, the coupled flow oscillation is supposed to be superposition of trough-type flow oscillation and PDO. Besides, the coupling mechanisms of the coupled flow oscillation are discussed and the evolution characteristics of flow instabilities are presented.