Two-phase Flow Regime Transitions Research Articles

Spatio-temporal void fraction visualization in air-water two-phase flow regime transitions by combination of convolutional neural network and long short-term memory implemented into multiple current-voltage (MCV-CNN_LSTM)

Spatio-temporal void fraction in air-water two-phase flow regime transitions has been visualized by combination of convolutional neural network and long short-term memory implemented into multiple -current-voltage (MCV-CNN_LSTM). The MCV-ML is composed of two components, which are MCV-CNN_LSTM training and evaluation. In the first component, four steps are carried out as 1) true void fraction α measurement was conducted by wire-mesh sensor (WMS) as objective variables, 2) simulated MCV voltage U calculation by simulation of MCV measurement for explanatory variable, 3) CNN_LSTM training instance generation, and 4) CNN_LSTM model training and testing. In the second component, two additional steps are experimentally conducted which are 5) actual MCV voltage V measurement by MCV for input of trained ML, and 6) spatio-temporal void fraction α prediction. For the dataset generation, the experiment was conducted on air-water two phase flow regime transitions in vertical pipe with an inner diameter of 25 mm and a length of 350 mm from the elbow. The superficial velocity of liquid and gas phase are varied to present the flow regime transition of bubbly to slug flow. As a result, the estimated spatio-temporal void fraction αˆ by MCV-CNN_LSTM enables the visualization of detailed gas distribution in vertical upward two-phase flow. The spatial averaged instantaneous void fraction measured by WMS and estimated by MCV-CNN_LSTM show qualitative agreement in normalized cross correlation (INCC) of 0.364 on its temporal variation.

Experimental heat transfer results and flow visualization of horizontal Near-Saturated liquid nitrogen flow boiling in uniformly heated circular tube under Earth gravity

This study investigates 1-ge horizontal flow boiling of liquid nitrogen with a near-saturated inlet along a circular tube of dimensions 8.5-mm inner diameter and 680-mm heated length. Experiments are conducted using a payload developed for eventual parabolic flight experiments. The operating parameters varied are mass velocity of 406.76–1572.77 kg/m2s, inlet quality of −0.05 to −0.01, and inlet pressure of 336.29–493.07 kPa. High speed video recordings are presented to explain two-phase flow structure and regime transitions which are visualized through a transparent tube in a visualization section situated downstream of the heated tube. Recognized flow patterns are bubbly, plug, slug, stratified annular, and annular. Heat transfer results are presented and discussed in terms of flow boiling curve trends, streamwise wall temperature profiles, streamwise heat transfer coefficient (HTC) profiles, and average HTCs. Previous HTC correlations are evaluated against the measured HTC data, of which two are identified for superior accuracy in predicting cryogen data. Finally, this study confirms the reliability and readiness of the payload for subsequent parabolic flight experiments intended for acquisition of microgravity data.

Experimental investigation of flow orientation effects on cryogenic flow boiling

Cryogenic flow boiling physics is of paramount importance for future cryogenic space applications employing Cryogenic Fluid Management (CFM) technologies. This experimental study investigates the two-phase flow and heat transfer performance of LN2 flow boiling across five distinct flow orientations: vertical upflow, vertical downflow, horizontal flow, 45° inclined upflow, and 45° inclined downflow. The study employed the steady-state heating method within a circular heated tube featuring an 8.5-mm inner diameter and a 680-mm heated length. High-speed video recordings were utilized to capture two-phase flow patterns and interfacial behaviors across various flow orientations. The experiments covered a wide range of operating conditions, including mass velocities ranging from 351.80 to 1572.77 kg/m²s and inlet pressures from 297.14 to 1032.97 kPa, primarily with near-saturated inlet subcooling. Distinct two-phase flow patterns and regime transitions were identified for each flow orientation. Symmetrical flow patterns were evident in vertical orientations, whereas non-vertical orientations exhibited asymmetric flow stratifications, primarily influenced by the buoyancy force in a terrestrial gravity environment. Bubble dynamic parameters were quantified, and bubble collision and dispersion phenomena were visualized. Analyzing heat transfer performance based on local flow boiling curves and variations in heat transfer coefficients (HTCs), it was observed that vertical upflow demonstrated the most enhanced heat transfer performance, while vertical downflow exhibited the lowest. As mass velocity exceeded 830 kg/m²s, the differences in heat transfer among orientations became less distinct, emphasizing the role of flow inertia in mitigating the influence of flow orientation. A direct comparison of microgravity HTC data from a recent study by the authors against the 1-ge HTC data indicated enhanced heat transfer performance in microgravity for flow orientation configurations in terrestrial gravity. However, this enhancement progressively diminished as the heat flux increased. Distinct HTC trends were observed as mass velocity increased for different flow orientations and gravity levels.

Euler–Euler Multi-Scale Simulations of Internal Boiling Flow with Conjugated Heat Transfer

A numerical approach was implemented, to study a boiling flow in a horizontal serpentine tube. A NEPTUNE_CFD two-fluid model was used, to study the behavior of the refrigerant R141b in diabatic cases. The model was based on the Euler–Euler formalism of the Navier–Stokes equations, in which governing equations are solved for both phases of the fluid at each time step. The conjugate heat transfer—between the tube wall and the fluid—was considered via a coupling with the SYRTHES 4.3 software, which solves solid conduction in three dimensions. A mesh convergence study was carried out, which found that a resolution of 40 meshes per diameter was necessary for our case. The approach was validated by comparison with an experimental study of the literature, based on the faithful reproduction of the positions of two-phase flow regime transitions in the domain. Original post-processing was used, to unravel the flow characteristics. The mean and RMS fields of void fraction, temperatures and stream wise velocities in several sections were analyzed, when statistical convergence was reached. A thermal equilibrium was reached in the saturated liquid, but not in the vapor phase, due to the flow dynamic and possibly the presence of droplets. Finally, a thermal analysis of the configuration was proposed. It demonstrated the strong coupling between the temperature distribution in the solid, and the two-phase flow regimes at stake in the fluid domain.

Identification of high-pressure two-phase flow regime transition using image processing and deep learning

The commercial production process of methane gas from methane hydrate has been developed by the MH21-S R&D consortium (MH21-S) in recent years. In order to establish the gas production method, an accurate gas-liquid two-phase flow regime identification under high-pressure conditions is indispensable. In the present study, a 28 m long test section with an inner diameter of 52.7 mm was newly constructed, and a nitrogen-water two-phase flow experiment was conducted at 3.0 MPa G. Two-phase flow images were successfully acquired using the high-speed camera from the visualization port. The sequential image of upward gas-liquid two-phase flow was merged into a single image by the time strip method. Then, flow regime identification using the convolutional neural network (CNN) trained under sequential images was carried out. The proposed method has shown an excellent capability to identify flow regime images obtained at the current high-pressure conditions. In addition, for the slug flow regime, it was found that the Taylor bubble length highly influences the identification performance, and the model proposed by Street and Tek showed good agreement for the present dataset. Furthermore, the bubbly-slug transition region was successfully evaluated and quantified through the present model. The current methods have shown a good correlation with the existing model. These results indicate that the gas void fraction at the transition region to be larger than the theoretical value for the elevated pressure system.

On the mechanism of onset of significant void in subcooled flow boiling

Theoretical investigation was carried out for a possible mechanism of the onset of significant void (OSV) or net vapor generation (NVG) in subcooled flow boiling. Based on available knowledge on the two-phase flow regime transition from bubbly to slug flows, it was postulated that bubble coalescence is intensified substantially when the local void fraction near the heated wall exceeds a critical value. It was discussed that this can be a main cause of a rapid increase in the cross-sectional area-averaged void faction since the condensation rate decreases suddenly due to a sudden decrease in the interfacial area of vapor bubbles. It was demonstrated that if the vaporization rate on the heat transfer surface is evaluated using a reliable wall heat transfer correlation, the OSV conditions calculated by the present model agree with those by a widely-used empirical correlation well.

Identification of Upward Two-Phase Flow Regime Transition Region under High-Pressure Condition Using Deep Learning

Currently, the MH21-S R&D consortium (MH21-S), supported by the Ministry of Economy, Trade, and Industry (METI), is developing the commercial production process of methane gas from methane hydrate. In order to develop a gas production method, the objective flow regime identification technique of gas-liquid two-phase flow under high-pressure conditions would be useful. Therefore, in the present research, a new method for identifying the flow regime by a convolutional neural network (CNN) using high-speed images obtained at a narrow view was proposed, and its performance was evaluated. Specifically, the upward gas-liquid two-phase flow images were recorded using a high-speed camera, and they were merged into a single image containing information from entire frames. The CNN was created and trained with these newly merged images, and it was utilized to identify flow regimes of unlabeled test images. As a result, the proposed method using the CNN model was capable of identifying the flow regime with high accuracy. Thus, the present study has shown that the CNN model with the proposed image conversion method can properly identify the upward gas-liquid two-phase flow regime. In addition, by utilizing the developed network, the bubbly-slug transition region, which has only been studied through visual observation or void fraction signals in the past, was successfully evaluated quantitatively.

Evaluation of Length Scales and Meshing Requirements for Resolving Two-Phase Flow Regime Transitions Using the Level Set Method

Abstract New criteria for fully resolving two-phase flow regime transitions using direct numerical simulation with the level set method for interface capturing are proposed. A series of flows chosen to capture small scale interface phenomena are simulated at different grid refinements. These cases include droplet deformation and breakup in a simple shear field, the thin film around a Taylor bubble, and the rise of a bubble toward a free surface. These cases cover the major small scale phenomena observed in two-phase flows: internal recirculation, interface curvature, interface snapping, flow of liquid in thin films, and drainage/snapping of thin films. The results from these simulations and their associated grid studies were used to develop new meshing requirements for simulation of two-phase flow using interface capturing methods, in particular the level set method. When applicable, the code used in this work, PHASTA, was compared to experiments in order to contribute to the ongoing validation process of the code. Results show that when the solver meets these criteria, with the exception of resolving the nanometer scale liquid film between coalescing bubbles, the code is capable of accurately simulating interface topology changes.

Slug-to-churn vertical two-phase flow regime transition study using an interface tracking approach

Direct numerical simulation (DNS) coupled with an interface tracking method (ITM) is used to demonstrate the applicability of DNS to the study of vertical two-phase flow regime transition. The study focuses on the slug flow to churn-turbulent regime transition. PHASTA, a finite element based flow solver which utilizes the level set method for interface tracking, has been used to perform the presented simulations. A domain size study has been conducted in order to find pipe dimensions that match the natural wavelength of the periodic slug flow. A mesh resolution study has been completed. The results show that using DNS to simulate the slug-to-churn transition is within the capabilities of the code as the simulations agree well with experimental data and empirical knowledge. The simulations were analyzed to find flow features such as the velocity profile in the wake of a Taylor bubble and the bubble interface shape evolution during breakdown. An understanding of such features could help identify the driving physics behind the transition phenomenon.

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.

The effect of mesh-type bubble breaker on two-phase vertical co-flow

The influence of a mesh-type bubble breaker in a two-phase vertical co-flow is experimentally investigated using high-speed imaging. A range of liquid and gas flow rates were considered that correspond to the gas-to-liquid flow rates ratios (GLRs) from 0.05 to 39.6 covering bubbly, slug and churn flow regimes. The mesh-type bubble breaker was found to be effective in reducing the size of nozzle-generated bubbles in two-phase co-current vertical flow. The bubble size reduction by 50%-70% is observed in the presence of the mesh-type bubble breaker compared to that without a bubble breaker. The results also show that at a given GLR, the mesh-type bubble breaker delayed the flow regime transition compared to that without a bubble breaker. A Froude number correlation was proposed to predict the mean bubble size downstream of the bubble breaker for the bubbly flow regime, which accurately predicted the bubble size over a range of bubble breaker parameters. It is observed that the geometry of the bubble breaker influence the two-phase flow regime transition. At low liquid flow rates, small pore size of the breaker was found to be effective. This effect diminishes at higher liquid flow rate due to the elongation of the bubbles by the liquid inertia. The results also show that a longer bubble breaker located closer to the bubble generating nozzle is the most effective in producing bubbly flow over a longer range of gas-liquid flowrates ratios.

Flow regime transition criteria for upward two-phase flow in vertical rod bundles

The demand of accurate prediction for two-phase flow behavior in a nuclear reactor core composed of fuel rod bundles or a heat exchanger composed of heat transfer tube bundles requires comprehensive understanding of flow regime, void fraction, heat transfer and pressure drop. In comparison with the great success in developing the two-phase flow regime transition criteria for simple geometries such as pipes, annulus and rectangular channels, limited researches have been performed for developing the flow regime transition criteria of upward two-phase flow in vertical rod bundles. In the vertical rod bundles, slug bubbles spanning the bundle casing cannot exist due to their surface instability and the two-phase flow characteristics in the vertical rod bundles are different from those in pipes, annulus or rectangular channels whose channel size are smaller than the length scale of the surface instability. This study has proposed a new flow regime transition criteria model based on the analysis on the underlying physics of the upward two-phase flow behavior in the vertical rod bundles. A reliable drift-flux correlation to predict void fraction in the vertical rod bundle developed recently has been used in modeling the flow regime transition criteria. This study has classified the flow regime into 6 distinct flow regime such as bubbly, finely dispersed bubbly, cap-bubbly, cap-turbulent, churn and annular flows. The newly developed flow regime transition criteria have been compared with existing 4 flow regime maps obtained in vertical rod bundles. The fluid systems include air-water and steam-water. A fairly good agreement with some discrepancies has been obtained between the newly developed transition criteria and the measured flow regime maps.

Effect of forced flow oscillations on churn and annular flow in a long vertical tube

An experimental study has been carried out to investigate the effects of periodic forced flow oscillations on gas-liquid two-phase flows using a 42m long, 0.05m ID vertical pipe system. From the knowledge of the authors, there is no experimental work available in the literature on the forced flow oscillations for churn flow regime. Therefore, the main objective of this paper is to experimentally characterize churn and annular flows in a long vertical tube. The time variation of liquid holdup, pressure drop and pressure gradient were analyzed under two superficial liquid velocities (0.017 and 0.3m/s), and superficial gas velocities oscillations ranging from 4m/s to 21m/s. The impact of flow oscillations in two-phase flow regime transitions is also investigated.The observations of flow regimes under oscillatory conditions showed significant differences compared to the ones expected in steady-state flow under the same conditions. Under oscillatory conditions, the local liquid holdup and the amplitude of its waves decreased with the distance from the inlet. For the experimental runs with oscillatory gas flow and low superficial liquid velocities (Uls=0.017m/s), the liquid holdup at the bottom of the tube decreased while the gas flow rate was lowered, whereas in steady-state conditions the liquid holdup should increase while reducing superficial gas velocities. This process is described in this study by the formation of large waves in the bottom of the pipe as a consequence of the flow oscillations.

Two-phase CFD validation: TOPFLOW-PTS steady-state steam-water tests 3–16, 3–17, 3–18 and 3–19

For the Pressurized Thermal Shock (PTS) issue, the NEPTUNE_CFD code solves the Eulerian two-fluid model with specific models for liquid/gas interfaces, which are much larger than the computational cells size. The CFD validation database dedicated to PTS includes the TOPFLOW-PTS experiment, which represents condensation phenomena in a PWR cold leg, with the Emergency Core Cooling system (ECC) and a downcomer. The present study deals with NEPTUNE_CFD calculations of steady-state steam-water tests 3–16, 3–17, 3–18 and 3–19, which differ each other by the ECC liquid inlet flow rate (m˙L,ECC). So the liquid turbulence, which is the main input of the condensation models, is changed from one test to the other, firstly in the ECC region. The direct and first order effect of the ECC flow on the liquid temperature is shown with sensitive two-phase flow regime transitions, which require a careful meshing. This condition being fulfilled, satisfactory NEPTUNE_CFD results mesh-independency on refinement is shown. Present CFD is able to calculate the effect of m˙L,ECC on condensation and temperatures. CFD versions with k-ɛ and Rij-ɛ SSG turbulence modeling are compared and the improvement brought by the new version with Rij-ɛ SSG is shown.

Evaluating cavitation regimes in an internal orifice at different temperatures using frequency analysis and visualization

Experiments on a water cavitating orifice were conducted to investigate the influence of pressure and temperature on flow regime transition due to cavitation. The thermal effects could be important in cases with cryogenic cavitation or hot fluid injection. The investigations were based on CCD observations and a pressure fluctuations frequency analysis.The high-speed photographic recordings were used to analyze the cavitation evolution and individuate the frequency content of the two-phase flow by processing the pixel-intensity time-series data.The cavitating structures showed different behaviors and characteristics with variations in operating conditions, as the pressure inside the orifice and the flow temperature .The flow regime map for the cavitating flow was obtained using experimental observations to analyze the occurrence of the different two-phase flow regime transitions at various operating conditions.As the pressure at the orifice inlet increased, at the same downstream pressure, cavitation inception occurred. The decrease of the cavitation number brought a significant increase in cavitation zone extension. As the pressure drop inside the orifice increased, the cavitation was characterized by an evident increase in cavitation zone length to the outlet of the orifice. With a further cavitation number decrease, the transition to jet cavitation was evident.The temperature influenced both the cavitation intensity and the cavitation number at which different two-phase flow regime transitions occurred, which tended to increase with temperature.The vapor fraction was estimated using an image processing algorithm.The frequency content given by the pressure fluctuations was analyzed and compared with the frequency spectra obtained from the visual observations. The behavior of the different cavitating flows could be correlated to the frequency spectrum of the pressure fluctuations measured upstream and downstream of the orifice. The cavitation number reduction and consequent increase in cavitating area width were related to a corresponding significant increase in the amplitude of typical frequency components. The transition to jet cavitation was characterized by a significant increase in the first peak in the frequency spectrum; weaker spectral peaks were also present at high cavitation numbers.

CO2 and R410A: Two-phase flow visualizations and flow boiling measurements at medium (0.50) reduced pressure

The conversion to more environmentally friendly refrigerants is of prime importance for refrigeration and air-conditioning industries. A first option for the substitution of the fluids which use was banned could be natural refrigerants, such as CO2, or HFC mixtures with low global warming and zero ozone depletion impacts. To determine the heat transfer characteristics during flow boiling of CO2 a remarkable amount of literature on flow boiling of CO2 has been published, especially during the last decade. The experiments on this fluid pointed out that, at medium reduced pressure, the heat transfer coefficients trends versus vapor quality are quite different than those commonly encountered with other refrigerants at ordinary temperatures and several interpretations for this were proposed.In this work tests for CO2 and R410A are carried out to study the behaviour of the local heat transfer during evaporation at medium reduced pressure (0.50). The tests were run on a smooth, horizontal, stainless steel tube with an inner diameter equal to 6.00 mm, varying the mass flux between 200 and 350 kg m−2 s−1 and the heat flux between 5.0 and 20.0 kW m−2 over the whole range of vapor qualities.It was verified by experiments that for R410A at a medium reduced pressure the peripheral local heat transfer coefficient trends, as the two-phase flow structures and flow regime transitions, are similar to those for CO2, at the same reduced pressure.

Flow Boiling of R134a in Circular Microtubes—Part II: Study of Critical Heat Flux Condition

A detailed experimental study was carried out on the critical heat flux (CHF) condition for flow boiling of R134a in single circular microtubes. The test sections had inner diameters (ID) of 0.50 mm, 0.96 mm, and 1.60 mm. Experiments were conducted over a large range of mass flux, inlet subcooling, saturation pressure, and vapor quality. CHF occurred under saturated conditions at high qualities and increased with increasing mass fluxes, tube diameters, and inlet subcoolings. CHF generally, but not always, decreases with increasing saturation pressures and vapor qualities. The experimental data were mapped to the flow pattern maps developed by Hasan [2005, “Two-Phase Flow Regime Transitions in Microchannels: A Comparative Experimental Study,” Nanoscale Microscale Thermophys. Eng., 9, pp. 165–182] and Revellin and Thome [2007, “A New Type of Diabatic Flow Pattern Map for Boiling Heat Transfer in Microchannels,” J. Micromech. Microeng., 17, pp. 788–796]. Based on these maps, CHF mainly occurred in the annular flow regime in the larger tubes. The flow pattern for the 0.50 mm ID tube was not conclusively identified. Four correlations—the Bowring correlation, the Katto-Ohno correlation, the Thome correlation, and the Zhang correlation—were used to predict the experimental data. The correlations predicted the correct experimental trend, but the mean absolute error (MAE) was high (>15%) A new correlation was developed to fit the experimental data with a MAE of 10%.

Probabilistic mapping of adiabatic horizontal two-phase flow by capacitance signal feature clustering

In order to better quantify two-phase flow regime transitions, a sensor is developed which measures the electric capacitance of two-phase flows. A large number of experiments are done with air–water flow in a 9 mm ID horizontal tube. Based on a multivariate analysis, the most suitable sensor signal parameters are selected for building a flow regime classifier. This classifier is based on a fuzzy c-means clustering algorithm together with a regression technique. The output of the algorithm is used to create a probabilistic flow regime map. A comparison between a visual classification based on high speed camera images and the outcome of the flow regime classifier shows a remarkable agreement. The flow regime transitions are further quantified and discussed based on the probabilistic information and the sensor signal characterization. Probabilistic mapping makes it possible to combine flow regime dependent correlations in the two-phase flow models for heat transfer and pressure drop with smooth and appropriately quantified transitions from one flow regime to another.

Modélisation dynamique des structures d’interface via l’équation de transport de l’interface

The interfacial area transport equation dynamically models the two-phase flow regime transitions and predicts continuous change of the interfacial area concentration along the flow field. Hence, when employed in the numerical thermal-hydraulic system analysis codes, it eliminates artificial bifurcations stemming from the use of the static flow regime transition criteria. Accounting for the substantial differences in the transport phenomena of various sizes of bubbles, the two-group interfacial area transport equations have been developed. The group 1 equation describes the transport of small-dispersed bubbles that are either distorted or spherical in shapes, and the group 2 equation describes the transport of large cap, slug or churn-turbulent bubbles. The source and sink terms in the right-hand-side of the transport equations have been established by mechanistically modeling the creation and destruction of bubbles due to major bubble interaction mechanisms. In the present paper, the interfacial area transport equations currently available are reviewed to address the feasibility and reliability of the model along with extensive experimental results. These include the data from adiabatic upward air-water two-phase flow in round tubes of various sizes, from a rectangular duct, and from adiabatic co-current downward air-water two-phase flow in round pipes of two sizes.

Two-Phase Flow Regime Transitions in Microchannels: A Comparative Experimental Study

In recent years, micro-technologies have become very important to cutting edge industries such as aerospace and biotechnology. In the fluidic research area, two-phase flow study in microchannels has been an emerging topic in the past few years. Characteristics of two-phase flow in microchannels such as flow regimes, pressure drop, void fraction, and heat transfer are now being extensively studied by numerous groups. The ultimate goal of this research ranges from compact heat exchangers to small-sized refrigeration systems. One of the major drawbacks in this area to date is the lack of a universal flow regime map enabling the prediction of flow regimes in microchannels. In the present study, a new test rig was designed and constructed to extend the range of the existing data on flow regimes. Several flow regime maps, for a hydraulic diameter of less than 1.0 mm obtained from a comprehensive literature review, are tested and compared. A total of approximately 1475 experimental data points from present and previous studies performed in channels with Dh ≤ 1.0 mm were used for comparison. Two universal flow regime maps were created, one each for horizontal and vertical channels with a hydraulic diameter ranging between 0.1 mm and 1.0 mm. The resulting universal maps presented here are based on flow regimes observed in all the studies on two-phase flow in microchannels. We suggest that the use of these flow regimes could help diminish the amount of discord present among research groups pertaining to the definition of flow regimes. Based on the comparisons between the universal maps and different experimental flow regime maps, appropriate conclusions on the effects of channel orientation and geometry are suggested.