Churn Flow Research Articles

Flow pattern effect on two-phase pressure drops in vertical upward flow across a horizontal tube bundle

Shell-side two-phase pressure drops under cross-flow condition play an important role in the operation of heat exchangers. However, no well-verified general correlation for predicting two-phase friction multipliers in horizontal tube bundles is available in the open literatures. In the present paper, based on a combination of Kanizawa and Ribastki flow regime criteria with the modified Mao and Hibiki flow regime criteria, the flow patterns of two-phase vertically upward flow across horizontal tube bundles in the open literatures were identified. The results show that flow patterns have a significant effect on two-phase friction multipliers. For the bubbly flow, finely dispersed bubbly flow, and annular flow, the mass velocity effect on two-phase friction multipliers can be neglected. However, the two-phase friction multipliers of cap bubbly flow and churn flow decrease with an increasing mass velocity. Then, a general dimensionless correlation in terms of the Martinelli parameter and the two-phase Froude number were developed, which is able to accurately predicting gas–liquid two-phase flow resistances of churn flow in staggered horizontal tube bundle under cross-flow condition. New correlations for predicting two-phase friction multipliers of bubbly flow, finely dispersed bubbly flow and annular flow both in staggered and in-line horizontal tube bundles were also developed and can give excellent representations for the existing data in the open literatures.

Dynamics of pressure drop oscillations during flow boiling inside minichannel

Dynamics of pressure drop oscillations during flow boiling inside minichannel with diameter of 1 mm was experimentally investigated. High speed camera was used to identify two-phase flow patterns. The chaotic pressure drop oscillations occurring with a low and high frequencies were observed. The analysis of the dynamics of chaotic pressure oscillations and recorded videos show that a certain dominant sequences of flow patterns can be identified. These sequences depend on the mean value of pressure drop. The map of dynamical properties of two-phase flow patterns was constructed using the windowed recurrence quantification analysis. The clusters on this map identified the four kinds of dominant sequences of flow patterns. These were: liquid flow - small bubbles flow - confined bubbles flow, small bubbles flow - slug flow - reverse slug flow, churn flow - wavy annular flow - reverse slug flow, churn flow - wavy annular flow. There has been shown that each dominant sequence of flow boiling can exist during a certain time period in the non-stationary flow boiling, therefore such sequence can be treated as temporary boiling pattern.

Fluid structure behaviour in gas-oil two-phase flow in a moderately large diameter vertical pipe

Intermittent flows in vertical pipes occur in many industrial settings including power generation and downstream oil-and gas production. This type of flows include cap bubble, slug and churn flow regimes. These regimes are of interest as downstream processes and control may heavily depend on the intermittency of the inflow. There are a number of correlations that predicts the features in such flows in vertical pipes. Most of the correlations were developed for air and water fluid pair for slug flow regime in vertical pipes with 25 to 50 mm inner diameter. In this paper, an attempt has been made to assess the suitability of several of these correlations specific to slug flow regime for a fluid pair that is different to air-water system. In this work, air-silicone oil flow development was experimentally investigated in a vertical pipe with an inner diameter of 68 mm. A Wire Mesh Sensor (WMS) and an Electrical Capacitance Tomography (ECT) sensor were installed in series at four locations (15D, 30D, 45D and 65D) downstream of the mixing section. The flow was visually observed using a high speed camera. The void fraction time series obtained from the WMS and the ECT were used to establish the flow characteristics such as slug length, slug frequency, void fraction in liquid slugs and Taylor bubble velocity. A comparison showed that the void fraction measurements using ECT and WMS are in good agreement. Axial measurements shows that the flow development beyond 45D is minimal. Change in physical properties of the liquid phase is responsible for the deviation associated with the existing slug flow models, particularly those developed to predict the gas holdup in liquid slugs.

Was the Deepwater Horizon Well Discharge Churn Flow? Implications on the Estimation of the Oil Discharge and Droplet Size Distribution

AbstractImproved understanding of the character of an uncontrolled pipeline flow is critical for the estimation of the oil discharge and droplet size distribution both essential for evaluating oil spill impact. Measured oil and gas properties at the wellhead of the Macondo255 and detailed numerical modeling suggested that the flow within the pipe could have been “churn,” whereby oil and gas tumble violently within the pipe and is different from the bubbly flow commonly assumed for that release. The churn flow would have produced 5 times the energy loss in the pipe compared to bubbly flow, and its plume would have entrained 35% more water than that of the bubbly flow. Both findings suggest that the oil discharge in Deepwater Horizon could have been overestimated, by up to 200%. The resulting oil droplet size distribution of churn flow is likely smaller than that of bubbly flow.

Flow patterns and heat transfer of oil-water two-phase upward flow in vertical pipe

In this paper, local heat transfer coefficients (HTC) and different flow patterns of oil-water two-phase flow in a vertical pipe were investigated. The test section was an 11 mm inner diameter (ID) copper pipe with a length to diameter ratio of 145. Water and kerosene (1.49 mPa s viscosity and 780 kg/m3 density) were selected as immiscible liquids and high speed photography technique was used for the flow pattern identification. The superficial Reynolds numbers ranged from 750 to 6000 for oil and 1000 to 11500 for water. The heat transfer experimental data showed a very strong dependency on the flow pattern. In addition, a heat transfer correlation was proposed for churn flow pattern. The experimental data were successfully correlated by the proposed heat transfer correlation with an average deviation of 10.6%, a standard deviation of 5.6%, and a deviation range of −20% to 3%.

Two-Phase Flow Patterns and Evaporation Heat Transfer of R134a in a Vertical Narrow Rectangular Channel

This study experimentally investigated the flow pattern, void fraction, and evaporation heat transfer characteristic of R134a upward flow in a vertical narrow rectangular channel having a hydraulic diameter of 0.99[Formula: see text]mm, resembling a plate heat exchanger. Experiments were performed with mass velocities and vapor quality ranging between 30–200[Formula: see text]kgm[Formula: see text]s[Formula: see text] and 0.05–0.9, respectively, at a saturation temperature of 15[Formula: see text]C. Flow patterns were classified into bubble, slug, churn, and annular flows. The void fraction increased with increasing quality, and decreased with decreasing mass velocity in the low-quality region. Further, the influence of flow inlet/outlet positions remarkably appeared when the superficial gas velocity was low. The observed flow patterns and the measured void fraction were compared to that in previous studies. The effects of mass velocity and heat flux on the evaporation heat transfer coefficient were small, and the heat transfer through the thin liquid film was dominant.

Multiphase flow behavior for acid-gas mixture and drilling fluid flow in vertical wellbore

The transition laws of multiphase flow in a wellbore during acid-gas mixture influx are important for the hydraulic parameters design and wellbore pressure control in acid gasfield drillings. Herein, the phase change of acid-gas mixture in a wellbore was investigated by the experimental analysis. The results indicate that the acid-gas mixture in the supercritical phase exists under conditions of certain wellbore temperature and pressure, which makes its physical properties change abruptly near the critical point. Moreover, considering the phase change and dissolution of acid-gas mixture in drilling fluids, the multiphase flow may go through in turn the single phase flow, supercritical-liquid two phase flow, liquid-liquid two phase flow and gas-liquid two phase flow (including bubbly flow, slug flow, churn flow and annular flow) with the mixed fluids rising from bottom hole to wellhead. Meanwhile, the flow transition criteria, drift flux models and friction factor calculating methods for this type of multiphase flow were proposed. Finally, the multiphase flow characteristics in a wellbore during acid-gas mixture influx were analyzed using the new method with an example. The results suggest that during the acid-gas mixture rising up along the wellbore, the flow pattern transforming from the supercritical-liquid two phase flow or the liquid-liquid two phase flow into gas-liquid two phase flow can cause a large volume expansion, increasing the blowout risk.

Analogous behavior of pseudo-slug and churn flows in high viscosity liquid system and upward inclined pipes

Pseudo-slug (PS) and churn (CH) flows are among the least understood flow patterns for gas-liquid flow in pipes, despite their importance in many engineering applications. Owing to visual differences, they are commonly perceived as different flow patterns. This study presents characterizations of these flow patterns, which suggests their analogous behavior, thus enabling possibility of unified modeling between them.The experiments were conducted using an air-oil (0.213 Pa.s liquid viscosity) system with a 0.0508-m ID, 21.7-m long, upward inclined polycarbonate test section under atmospheric pressure. Flow patterns were identified using the dimensionless voltage time-trace readings of two-wire capacitance probes, supplemented by high-speed camera observations. The capacitance probes were used for slug/ pseudo-slug/ churn flow characterizations. Additionally, capillary-seal differential pressure transmitters for pressure drop measurement and quick-closing-valves section for liquid holdup measurement were used.Experimental results demonstrate that despite their visual differences, PS and CH share several common characteristics. First, both PS and CH exist between slug (SL) and segregated flows (annular or stratified) as superficial gas velocity (vSG) changes in the flow pattern map. Second, non-monotonic relationships between translational and mixture velocities are observed for both flow patterns in contrast to linear behavior for conventional SL flow. Third, both flow patterns exhibit similar signature of capacitance probes time-trace data and its distribution histogram. Fourth, they show similar values of drift-flux flow distribution coefficient (C0). And fifth, a consistent bias tendency is observed when conventional SL flow models are used to predict the pressure gradient of both flow patterns. The visual difference between PS and CH may be attributed to higher frequency, wavier film interface, and more uniform pipe wetting exhibited by CH compared to PS flow due to inclination angle effects.Several SL to CH transition models are modified/ extended to predict SL to PS transition. The combination of Taylor bubble flooding and a unified slug flow model is found to predict the SL to PS transition accurately within the experimental range, further supporting the analogy between PS and CH.

Experimental and numerical evaluation of slugs in a vertical air–water flow

Multiphase flows are often present in chemical processes and they include gas-liquid transport, which may become intermittent depending on the operating conditions. One type of flow structure is the presence of large gas bubbles that can occupy the entire duct diameter, separated by liquid portions. This can result in large pressure fluctuations, which are normally associated with damage to equipment over time. The intermittent nature and the chaotic distribution of the phases in a turbulent flow complicate the design and optimization of the equipment. In this context, computational fluid dynamics is a useful technique since it can provide important information on several flow conditions, but the use of suitable models and boundary conditions is required. In this study, an air and water flow with the presence of slugs is evaluated in a system consisting of horizontal and vertical ducts, with an inner diameter of 94 mm. Experimental data on the pressure and void fraction were analyzed and compared to the predictions obtained from numerical simulations, obtained considering the VOF approach. It was observed that the use of the geometrical reconstruction scheme to simulate a compressible flow resulted in a better agreement with the experimental results. Using this setup, the average volume fraction and pressure values showed the expected behavior. The frequency of slugs, established using three different methods, was around 1.5 Hz for the evaluated conditions evaluated. The pressure probability density function (PDF) indicated good agreement between numerical simulations and experimental data, while the volume fraction PDF indicated that the flows evaluated have the characteristics of a churn flow regime. Moreover, it was found that an increase in the water flow rate can lead an increase or a decrease in the average velocity.

Similarities and differences in churn and annular flow regimes in steady-state and oscillatory flows in a long vertical tube

This study investigates the effects of induced-low-frequency flow oscillations on gas–liquid two-phase flows using a 42-m long, 0.048-m ID vertical pipe system. This work uses the experimental data set from a previous study on the effect of forced flow oscillations on churn and annular flow. However, in this previous study, the similarities and differences between flows in steady-state and under induced-low-frequency oscillations were not discussed. The time variation of liquid holdup, frequency of large liquid structures, and pressure gradient were analyzed under two superficial liquid velocities (0.017 and 0.3 m/s), and superficial gas velocities ranging from 4 to 21 m/s. The impact of flow oscillations in two-phase flow regime transitions is also investigated. The observations of flow regimes under oscillatory flow showed significant differences compared to the ones expected in steady-state flow under similar conditions. For higher pressures and oscillatory flow, churn flow regime was observed, when annular flow regime is expected in steady-state flow. Annular flow regime was not observed for higher superficial liquid velocities and high inlet pressure for oscillatory conditions. Surprisingly, the pressure gradient for oscillatory flow is significantly lower than in steady-state flow. However, this lower pressure gradient effect is only observed for low superficial gas and liquid velocities at the pipe inlet and outlet. This experimental observation is attributed to the formation of large liquid waves at the pipe inlet, which is a consequence of the induced-low-frequency flow oscillations.

Using artificial intelligence to improve identification of nanofluid gas–liquid two-phase flow pattern in mini-channel

This work combines fuzzy logic and a support vector machine (SVM) with a principal component analysis (PCA) to create an artificial-intelligence system that identifies nanofluid gas-liquid two-phase flow states in a vertical mini-channel. Flow-pattern recognition requires finding the operational details of the process and doing computer simulations and image processing can be used to automate the description of flow patterns in nanofluid gas-liquid two-phase flow. This work uses fuzzy logic and a SVM with PCA to improve the accuracy with which the flow pattern of a nanofluid gas-liquid two-phase flow is identified. To acquire images of nanofluid gas-liquid two-phase flow patterns of flow boiling, a high-speed digital camera was used to record four different types of flow-pattern images, namely annular flow, bubbly flow, churn flow, and slug flow. The textural features extracted by processing the images of nanofluid gas–liquid two-phase flow patterns are used as inputs to various identification schemes such as fuzzy logic, SVM, and SVM with PCA to identify the type of flow pattern. The results indicate that the SVM with reduced characteristics of PCA provides the best identification accuracy and requires less calculation time than the other two schemes. The data reported herein should be very useful for the design and operation of industrial applications.

Local Void Fractions and Bubble Velocity in Vertical Air-Water Two-Phase Flows Measured by Needle-Contact Capacitance Probe

Multiphase flow measurements have become increasingly important in a wide range of industrial fields. In the present study, a dual needle-contact capacitance probe was newly designed to measure local void fractions and bubble velocity in a vertical channel, which was verified by digital high-speed camera system. The theoretical analyses and experiments show that the needle-contact capacitance probe can reliably measure void fractions with the readings almost independent of temperature and salinity for the experimental conditions. In addition, the trigger-level method was chosen as the signal processing method for the void fraction measurement, with a minimum relative error of −4.59%. The bubble velocity was accurately measured within a relative error of 10%. Meanwhile, dynamic response of the dual needle-contact capacitance probe was analyzed in detail. The probe was then used to obtain raw signals for vertical pipe flow regimes, including plug flow, slug flow, churn flow, and bubbly flow. Further experiments indicate that the time series of the output signals vary as the different flow regimes and are consistent with each flow structure.

Investigation on the frictional pressure drop of gas liquid two-phase flows in vertical downward tubes

Experimental studies on the frictional pressure drop of air-water two-phase flows in vertical downward tubes are conducted in this paper. Experiments are performed at room temperature with the outlet pressure of experimental tube ranging from 0.17MPa to 0.28MPa, and four tubes are used with the tube inner diameter being 15mm, 25mm, 40mm and 65mm respectively. Based on the 978 data obtained in the experiments, prediction performances of 19 existing correlations, which have been developed for predicting the frictional pressure drop of gas-liquid two-phase flow in different tubes, are evaluated to assess the possibility and rationality of using these correlations to predict the frictional pressure drop of gas-liquid two-phase flow in vertical downward tubes as studied in the present paper. The results show that the prediction accuracy of the existing correlations for the frictional pressure drop of air-water two-phase flow in vertical downward tubes decreases remarkably with the increase in tube diameters, and the prediction values are far less than the experiment results under conditions with slug flow and churn flow patterns. It is also found that the buoyancy of bubbles can lead to the increase in the frictional pressure drop of the gas-liquid two-phase flow in vertical downward tubes, especially when big bubbles exist in the two-phase flow. A new model is then established in this paper by introducing a new parameter B into the separated flow model to consider the effect of buoyancy. It is found that in comparison with the existing correlations, the new model can well predict the frictional pressure drop of gas-liquid two-phase flows in vertical downward tubes with higher accuracy.

Churn flow in high viscosity oils and large diameter columns

Churn flow is an important intermediate flow regime occurring in between slug and annular flow patterns in two-phase flow, with profound implications in chemical and petroleum industry. The majority of studies to date in churn flow have been carried out mainly using water or liquids of low viscosities and limited information exists regarding the behaviour of high viscosity liquids which resemble realistic process conditions. In this paper, a study that investigated churn flow and its characteristics in high viscosity oils (360 and 330 Pa.s) and large diameter columns (240 and 290 mm) is presented for a first time. Transition to churn flow regime starts when the structure velocity, length and frequency of the liquid bridges, which appear at the end of slug flow, increase. In churn flow, gas flows at the core of the oil column with a wavy passage, leaving the top surface open to atmosphere with a possibility of creating a very long bubble. The average length of the bubbles seen to decrease with increasing the gas flow rate. While, no considerable change is observed in void fraction, structure velocity and film thickness at this flow pattern.

Visualization and flow regime identification of downward air–water flow through a 12 mm diameter vertical tube using image analysis

Downward co-current air–water flow through a 12 mm diameter tube has been investigated in an indigenously developed test facility having a typical fluid injection and mixing section. The flow has been characterized and the different regimes of the flow have been identified based on extensive analysis of high speed images. Some unique methods have been proposed for processing the images to establish objective ways for identifying the flow regimes considering different scales of the spatial co-ordinates. Several new flow regimes with intermittent behaviour have been observed, while the absence of churn flow is noted. A unique flow regime map pertinent to the present study has also been proposed.

Measurement of gas phase characteristics in vertical oil-gas-water slug and churn flows

In the present study, gas phase characteristics of oil-gas-water slug and churn flows in a vertical upward pipe with 20 mm inner diameter (ID) are experimentally investigated. We firstly measure the fluctuating signals of a traversable bi-optical fiber probe at different radial positions. The gas phase flow parameter distributions (local gas velocity, local gas holdup) are obtained and the flow structure is uncovered by calculating the series of gas bubble chord lengths at different radial positions. Additionally, in order to describe the flow structure of slug flow, the relative length of the liquid slug, the profile distributions of gas holdup and bubble size in liquid slug are investigated. To understand the nonlinear dynamic characteristics of slug and churn flows, multi-scale cross entropy (MSCE) algorithm is applied to analyze the signals of a high-resolution half-ring conductance sensor and bi-optical fiber probe. The result indicates that multi-scale cross entropy can be an effective tool for validating the measurement of gas phase characteristics by using the traversable bi-optical fiber probe.

Experimental investigation on flow boiling heat transfer and flow patterns in a single micro-channel with large mass velocity

In this paper, a new single micro-channel heat sink with 0.55 × 0.55 mm cross-section and 78 mm length based on R134a flow boiling is established to investigate the heat transfer coefficient and heat transfer mechanism with different inlet vapor quality. The experiment is conducted under the condition of heat flux in 0.918–6.993 kW/m2, mass velocity in 1821–4115 kg/(m2 s) and original saturation temperature in 20 °C. In the study, the relationship between various inlet vapor quality and average heat transfer coefficient is discussed, and the heat transfer mechanism is illustrated by the flow patterns captured by the high-speed camera with microscope lenses. Four major flow patterns are found in the experiment: bubbly, slug, churn and annular/wavy-annular flow, they appear in different inlet vapor quality regions. The result shows that the heat transfer coefficient increases with increasing inlet quality in low inlet quality dominated by nucleate boiling, but decreases in high inlet quality dominated by convective boiling. In the middle inlet quality, the heat transfer coefficient starts to decrease. Interestingly the forming process of churn flow is clearly seen in the experiment, it is formed by two steps: the bubble formation is restricted by the liquid film of a long vapor slug; the bubble tears apart the liquid film. It results the heat transfer mechanism of churn flow is a fierce confrontation of nucleate boiling and convective boiling. The effects of heat flux and mass velocity during different inlet quality region are also discussed.

Visualization of R134a flow boiling in micro-channels to establish a novel bubbly-slug flow transition criterion

In this study, the fundamental flow pattern characteristics of R134a flow boiling in a micro-channel heat sink were investigated based on high-speed flow visualization. Visual experiments were operated with R134a as the working fluid. The test section was composed with 20 parallel rectangular channels 500 μm wide, 500 μm deep, and 60 mm long. Mass flux and heat flux were variable from 164 to 573 kg/m2 s and 1 kW/m2 to 200 kW/m2, respectively, at the inlet temperature of 22 °C. Flow images were recorded with speed up to 10,000 frames/s. Bubbly flow, slug flow, churn flow, liquid lump flow, and annular flow were observed experimentally and the results plotted onto an x-G flow pattern map. For “M”-shaped heat transfer coefficient curve, five observed flow patterns were combined into one curve to reflect the heat transfer characteristics of flow boiling in micro-channels. Mechanisms of transition regions (bubbly-slug, slug-churn, churn-liquid lump, and liquid lump-annular) were established accordingly. The Active Nucleation Site Density Na was found to be the most significant parameter in regards to the bubbly-slug and slug-churn transition mechanisms. To verify the effect of the Active Nucleation Site Density (Na) on the bubbly-slug transition, the relations of Na and Boiling number at bubbly and slug flow regions were compared; the resulting slope features made the relations of Na and Boiling number easily distinguishable. Classification between the bubbly flow and slug flow of R134a was proposed based on these distinctions. The proposed criterion classified the transition region between the bubbly and slug flow. The proposed transition criterion was proven effective and feasible by comparison against previous transition criteria and datasets.

Modelling of upwards gas-liquid annular and churn flow with surfactants in vertical pipes

Based on our earlier experimental work on the effect of surfactants on air-water flow in vertical pipes with internal diameters of 34 mm, 50 mm, and 80 mm, we create a mechanistic annular flow model for the pressure gradient. The major effect of the addition of surfactants is the formation of foam. We model the formation of foam and its impact on the flow. In the model we consider a gas core and a film at the wall, which consists of a layer of liquid at the wall and a layer of foam between the liquid layer and the gas core. We do not consider entrainment in the model. We developed four closure relations in order to solve the model: (i) for the density of the foam, (ii) for the viscosity of the foam, (iii) for the interfacial friction between the gas and the film, and (iv) for the thickness of the liquid layer at the wall. Subsequently, we solve for the film thickness that yields the imposed liquid flow rate. Comparing the experimental results for the pressure gradient to the results from the model, we observe that in most cases the model can predict the pressure gradient within 25%. Furthermore, the model is able to predict the onset of downwards flow in the film. Therefore, it can predict the transition between annular flow and churn flow. We show that the effect of five different surfactants on the flow is equal, apart from a scaling factor of the concentration, which means that the model can be applied for many different types of surfactants. The scaling factor is an input parameter to the model, which needs to be determined in a small scale experiment.

The concept of liquid inlet model and its effect on the flooding wave frequency in vertical air-water churn flow

In upward air-water churn flow in a vertical tube, the liquid fed at the inlet is entrained upwards by the gas flow in the core, resulting in a wavy film flow. The present work considers the influence of the modelling of the liquid inlet on the wave frequencies, with the final purpose to understand the causes and effects in an actual flow. The perforated wall liquid inlet section, commonly used in experiments, is modelled in simulations as a simple inlet boundary condition on a short section of a pipe wall. At a given liquid mass flow rate in the experiment, the magnitude of the wall normal velocity component is controlled by the inlet area, which is used as a modelling parameter in the present study. The study shows that the calculated wave frequency is proportional to the imposed wall normal velocity at the liquid inlet. The velocity profile at the perforated wall liquid inlet section was not measured in available experiments and presents a major source of uncertainty in simulations. The parametric study revealed that a suitable value for the magnitude of the wall normal velocity can be determined over a range of flow conditions, leading to good agreement of simulated and measured wave frequencies. This finding suggests, that the wave frequency in actual churn flow is not a property depending only on geometric and flow conditions, but on the liquid inlet flow as well. Simulations were performed in three-dimensional tube section using the multiphase solver interFoam from the open-source code OpenFOAM.