Pipe Wall Research Articles

EFFECT OF AN INWARD-FACING BAFFLE ON LAMINAR FORCED CONVECTION HEATING ALONG A CYLINDRICAL HORIZONTAL PIPE FOR DIFFERENT NANOFLUIDS

Improving heat exchange intensity is a major goal in the heat exchanger industry. The use of baffles is one of the techniques employed to achieve this goal. In this numerical work, the effect of an inward-facing baffle placed on the wall of a cylindrical horizontal pipe is treated for the case of nanofluid. A sequential analysis is offered to better understand the different effects and their consequences, particularly on the average exchange rate, in addition to somewhat filling the gap identified in the literature for the case of nanofluid with various shapes of the baffle. The study, divided into three parts, begins for 10 ≤ Re ≤ 250 with the case of pipe without baffle, where the water-based nanofluid effect is treated. Three types of nanoparticles (Cu, Al<sub>2</sub>O<sub>3</sub>, and TiO<sub>3</sub>) at volume concentration 0 ≤ φ ≤ 10% are considered. An insulated primary pipe is placed to ensure dynamic establishment at the entrance to the heating pipe assumed to be under imposed temperature. The results showed the clear effects of modifying the kinematic viscosity and thermal diffusivity on the dynamic and thermal lengths, respectively, with the addition of nanoparticles compared to the base fluid. Correlations are proposed for their determination. A heat exchange rate that improves as the volume concentration increases is recorded, particularly for nanoparticles with high thermal conductivity. In the second part, a rectangular baffle is assumed in the heated pipe, where the effects of its position, length and width are analyzed respectively. The results showed a greater interest in placing the baffle close to the entrance, especially if it is longer. In the last part of the work, three other shapes of the baffle are proposed (trapezoidal, triangular, and elliptical). The results confirm that the non-smooth shape of the baffle creates more disturbances in the dynamic and thermal fields, and therefore a greater improvement in the heat exchange rate. For the last two parts, the nanofluid effect remains similar to that recorded for pipe without baffle.

Sound Pressure Slopes of Turbulent and Cavitating Liquids, and a Method to Predict Such Levels

Abstract This paper presents an author-created, unique method called the ABC method to predict sound pressure of turbulent and cavitating liquids according to ISA Standard 75.17 and is measured 1 m from the outer wall of a downstream pipe, attached to the outlet of a valve [Baumann, 2023, “Method to Establish Sound and Acceleration Levels of High Pressure Reducing Valves,” ASME Open J. Eng. 2, p. 001000-1]. The purpose of this paper is to explain how such sound pressure levels are estimated. It is accomplished by adding the results of equations in sub-sectors A, B, and C, all in dBA. This gives the sound pressure of a given valve handling water and installed in a Schedule 40 steel pipe. Additional modifiers are shown to modify the ABC method results for other pipes or fluids. The method is open and entirely based on known fluid-mechanic and acoustic laws such as Newton’s and Lighthill’s. It is claimed that such methods will give prediction results superior to those expected using the current International Electrical Commission (IEC) Standard 60534-8-4 [Baumann, 1970, “On the Prediction of Aerodynamically Created Sound Pressure Levels of Control Valves,” ASME Paper WA/FE—28, Presented at the Annual ASME Winter Meeting, November]. A table is shown presenting test data which are compared to ABC method calculated values. Additional graphical information supporting such great prediction accuracy is also shown.

EFFECTS OF PIPE DIAMETER ON INTERFACIAL AND WALL FRICTION FACTORS OF SWIRLING ANNULAR FLOWS IN A VERTICAL PIPE

Interfacial and wall friction factors, <i>f<sub>i</sub></i> and <i>f<sub>w</sub></i>, of swirling annular flows in a vertical pipe of 20 mm in diameter D were measured for comparison with those in a vertical pipe of <i>D </i>= 30 mm. A three-fluid model was applied to evaluate the friction factors. Measurements were carried out in two sections, one near the swirler and the other far downstream the swirler. The swirl intensity of the flow was evaluated by the interfacial swirl number si defined by the ratio of the azimuthal to the axial components of interfacial wave velocity. The interfacial swirl numbers were smaller in the smaller pipe. The ratio of <i>f<sub>i</sub></i> in swirling annular flows to that in non-swirling annular flows decreased to unity with decreasing <i>s<sub>i</sub></i>* which is the arithmetic mean value of <i>s<sub>i</sub></i> for 0 ≤ <i>z</i>* (= <i>z/D</i>) ≤ 3.8, where <i>z</i> is the axial coordinate in the test section, and is higher in <i>D </i>= 20 mm than in <i>D </i>= 30 mm. The ratio of <i>f<sub>w</sub></i> in swirling annular flows to that in non-swirling annular flows decreased to less than unity with decreasing <i>s<sub>i</sub></i>* and is higher in <i>D </i>= mm than in <i>D </i>=30 mm.

МОДЕЛИРОВАНИЕ ТУРБУЛЕНТНОЙ ДИНАМИКИ И ФАЗОВЫХ ПЕРЕХОДОВ В ПОТОКЕ СМЕСИ ИЗ ДВУХ КОМПОНЕНТОВ В ТРУБЕ КРУГЛОГО СЕЧЕНИЯ

Modeling mass transfer processes occurring in gas (vapor) - liquid systems is widely used in the chemical industry. The mathematical representation of the mass transfer process becomes complex due to the presence of hydrodynamic conditions and other subtle factors in real physical processes, which are not always considered in calculations or require significant computational resources to account for these variables. However, in the presence of a second component in the process, these theories already yield inaccurate output data, while in laminar flow, mass transfer occurs through molecular and convective diffusion. At present, there is no universal method that would accurately calculate multicomponent mixtures for various equipment designs and phase interactions. In the scope of this work, modeling of the mass transfer process associated with the condensation of a water-alcohol solution under conditions of turbulent flow in a pipe with a circular cross-section has been conducted. When formulating the problem, parameters were defined, and boundary conditions were established, including the velocity and temperature of the steam inlet, the temperature, and material of the pipe wall. Additionally, the wall temperature was maintained below the vapor saturation temperature during the condensation process modeling, and conversely, above the saturation temperature during the boiling process modeling in the same pipe. The pressure was measured at the outlet of the pipe, and no minimum excess pressure was observed. The study demonstrates that the accuracy of the calculations is sufficiently high, and the obtained results are confirmed by numerous numerical experiments. It is worth noting the well-developed mechanism for calculating hydrodynamics in the Ansys Fluent software package, which significantly influences mass transfer processes.

Improvement of Channel Capacity of MIMO Communication Using Yagi-Uda Planar Antennas with a Propagation Path through a PVC Pipe Wall

Improvement of Channel Capacity of MIMO Communication Using Yagi-Uda Planar Antennas with a Propagation Path through a PVC Pipe Wall

Non-linear AUV Controller Design Using Logic-Based Switching PID Control

The inspection of pipes without stopping pipeline operations is of industrial interest due to the inherent economic benefits. A pipe inspection robot design is proposed together with a suitable motion controller. The proposed robot is an Autonomous Underwater Vehicle (AUV) architecture that does not require shunting, draining or unearthing pipework for inspection purposes. This posed the challenge of controlling the AUV through a narrow pipe where manouvers are restricted. A set of general non-linear equations of motion were identified and refined using existing research and strip theory. The non-linear analytical model was implemented in Simulink to enable real-time monitoring and controller tuning. A non-linear controller based on a combination of classical PID theory and switching logic was developed to control the platform. The controller was transplanted into a Hardware-in-Loop (HIL) testing model developed with the Arduino and Simulink software suites. Pool tests measuring the parameters pitch and yaw angles showed that when operated independently with a 5° input, the maximum overshoot was 0.5° or 10% of the command value with a maximum angular velocity of 1.25°/s. When operated simultaneously, the overshoot rose to 30% with a constant error in the region of 1° over the target. Distance readings conducted with ultrasonic sensors to the pipe wall showed constant Sway-Heave bias errors from the centerline as −5 mm, and 4 mm, respectively, with an error range of 4 mm / −7 mm for Sway and 6 mm/ −4 mm for Heave. Pitch and heave motions were up to 18% faster than yaw and sway motions due to actuator orientation, with speeds of 6.25°/s and 40.57 mm/s, respectively. Despite the turbulence present during tests, the controller successfully drove the AUV to target positions through active flow and presented a reasonable basis for further refinement.

Estimated heat load and proposed cooling system in the FCC-ee interaction region

We discuss the beam power loss related to the heating of the beam pipe walls of the FCC-ee interaction region. We analyse the excitation of trapped modes, which can accumulate electromagnetic energy and determine the locations of these modes. We study the unavoidable resistive-wall wake field, which is responsible for the direct beam pipe walls heating. We show the distribution of the heat load along the central part of the interaction region. We also present the cooling system design and results for temperature distribution in interaction region in the operational mode.

Three-dimensional CFD simulation of geyser boiling in high-temperature sodium heat pipe

A deep understanding of the characteristics and mechanism of geyser boiling and capillary pumping is necessary to optimize a high-temperature sodium heat pipe. In this work, the Volume of Fluid (VOF) two-phase model and the capillary force model in the mesh wick were used to model the complex phase change and fluid flow in the heat pipe. Computational Fluid Dynamics (CFD) simulations successfully predicted the process of bubble nucleation, growth, aggregation, and detachment from the wall in the liquid pool of the evaporation section of the heat pipe in horizontal and tilted states, as well as the reflux phenomenon of capillary suction within the wick. The accuracy and stability of the capillary force model within the wick were verified. In addition, the causes of geyser boiling in heat pipes were analyzed by extracting the oscillation distribution of heat pipe wall temperature. The results show that adding the capillary force model within the wick structure can reasonably simulate the liquid backflow phenomenon at the condensation; Under the horizontal and inclined operating conditions of the heat pipe, the phenomenon of local dry-out will occur, resulting in a sharp increase in local temperature. The speed of bubble detachment and the timely reflux of liquid sodium (condensate) replenishment in the wick play a vital role in the geyser temperature oscillation of the tube wall. The numerical simulation method and the results of this study are anticipated to provide a good reference for the investigation of geyser boiling in high-temperature heat pipes.

Filament manufacturing via external grooving of an HDPE pipe wall: RSM optimization and mechanical tests validation

In order to explore the possibilities of manufacturing and testing specimens from extruded HDPE pipes to retrace material inherent properties, continuous filaments are circumferentially machined by grooving. The proposed protocol imposes to keep to a strict minimum damage effects since semi-crystalline polyethylene is sensitive to deformation and heat. A Taguchi plan is adopted with inputs (cutting speed; feed rate; depth of cut). The modeled performance characteristics are roughness criteria (Rt; Ra) and temperature (T°). Using ANOVA and response surface methodology, the optimized values are 137.0 m/min, 0.4 mm/rev and 4.0 mm respectively for Vc, f and ap. At the highest desirability, the values of Rt (1.100 μm), Ra (0.223 μm) and T (36.44 °C) are satisfactory compared to turning data. Tensile tests on specimens from outer, middle and inner pipe show that (σ–ε) curves are reproducible with a pronounced drawing zone, especially for the inner pipe layers. Practically, the elasticity modulus is increased by 43% from outer to inner layers while the utmost difference in the elastic limit is ~ 5%. Concerning failure strain, the increase is 47% meaning that the material shows a great predisposition to ductility. This behavior is related to the higher crystallinity in internal pipe layers.

Inspection of Welded Joints of Boiler Tubes Using Visual and Penetration NDT Methods

The article discusses weld joint defects that were observed during boiler overhaul during shutdown. Visual methods as well as penetration methods were used in the project to check welds. These are mainly the welds of membrane wall pipes and also the welds of pipes for the supply of combustion air. Found weld defects are described and also their photo documentation is included. The aim of this article is to point out recurring defects in welds, especially when replacing membrane wall panels in boilers, and thereby contribute to the prevention of these inhomogeneities.

基于CFD-DEM耦合中小型气吸红枣捡拾机物料输送仿真试验与验证

To improve the quality of air suction jujube picker, the CFD-DEM coupled method is used to numerically simulate the conveying process of jujube particles and explore the motion state, particle collision and energy loss of jujube particles in the pipeline. The erosion rate is selected as an evaluation index to discuss the influence of wind velocity, bending angle and diameter on the conveying process. The simulation results show that the collisions between jujubes, as well as the collisions between jujubes and pipe wall have an impact on the conveying performance, and the latter is more significant. The erosion rate is positively correlated with the wind velocity, negatively correlated with the pipe diameter, and the bending angle first decreases and then increases. The influence degree of factors on the erosion rate is: wind velocity > bending angle > pipeline diameter. The optimal parameter combination is a wind velocity of 28 m•s-1, a bending angle of 105 °, and a pipe diameter of 130 mm. At this time, the value of erosion rate is the lowest and the number of collisions and energy loss between jujube particles are reduced by 37.3 % and 13.87 % year-on-year, and those between jujube particles and pipe wall are reduced by 17.45 % and 17.61 % year-on-year, respectively. The test results show that the conveying pipe with optimized structural parameters can reduce the jujube skin damage rate by 2.06 %. The results of this study can provide a reference for the design and optimization of the air suction jujube picker.

STATIONARY MODE OF FLUID FLOW IN THE PIPELINE IN THE PRESENCE OF ASSOCIATED SAMPLING AND PUMPING

Pipeline transportation of oil and petroleum products plays a significant role in the transport system of the Russian Federation. When operating oil pipelines, reliability and energy efficiency are the most important criteria. To achieve these indicators, it is necessary to perform optimal technological modes. When constructing models of technological regimes, a methodology is used in which the movement of oil and petroleum products is considered as steady. Based on this assumption, the throughput, pump operating modes, strength characteristics and other parameters are determined. During the operation of the equipment, almost every day it is necessary to switch from one mode to another, which entails unsteady fluid movement for some time. At the same time, the unsteady mode makes up a small part of the total operating time of the system.The beginning of studies of unsteady fluid processes in pipes dates back to the last quarter of the 19th century. N. E. Zhukovsky made a fundamental contribution to the study of this problem. His classic work on water hammer in water pipes was the beginning for the creation of a large number of works in pipe hydraulics. The problem was solved for an ideal elastic fluid, i.e. without taking into account the friction forces against the pipe walls. The resulting formula, according to which the pressure increment is proportional to the velocity, has received numerous experimental confirmations for cases when it is possible to neglect pressure losses to overcome hydraulic resistances. N. E. Zhukovsky also obtained a formula for determining the sound velocity of a liquid droplet in a pipe with elastic walls. It is proved that this speed depends on the pipe material and its thickness.In the future, a large number of works were carried out to study the flow of liquid and gas in round pipes. The fundamental works of I. A. Charny, who obtained a mathematical model in the form of a system of partial differential equations of hyperbolic type, proved fruitful in this direction. Linearization of the equation, taking into account viscosity and hydraulic resistance allowed the researcher to obtain solutions to a number of engineering problems.The introduction of the Dirac function in the description of selectionpumping at specified points made it possible to describe processes in complex pipelines with one partial differential equation.This article discusses a horizontal section of the pipeline with sampling and pumping points. A stationary process for a drip liquid is considered. The problem is solved using the sine-Fourier transform.

APPLICATION OF INTERNAL COATINGS IN ORDER TO INCREASE THE EFFICIENCY OF NATURAL GAS TRANSPORTATION AND REDUCE CORROSION DAMAGE TO THE PIPELINE WALL

With the development of the fuel and energy complex of the Russian Federation, new problems and challenges are emerging that require new scientific and technical solutions. At the moment, the length of pipelines through which various types of hydrocarbons are transported is growing. At the same time, the number of easily recoverable hydrocarbon reserves decreases every year, and at the same time their quality characteristics deteriorate, there is a need for a higher degree of their preparation for further transportation and additional purification. As a result, the corrosion effect on the pipe wall increases, leading to a decrease in the service life of the pipeline. Along with this, the share of heavy components in hydrocarbon mixtures transported through pipeline systems increases, which leads to an increase in energy costs for their transportation. These factors lead to a qualitative change in transportation conditions and, accordingly, to the need for additional technical solutions to protect pipes from corrosion and ensure the efficient operation of pipeline systems due to the increase in pressure losses along the length of the pipeline. As a result, it is necessary to achieve a reduction in energy costs for the transportation of natural gas. The problems of reducing pressure losses during natural gas transportation, as well as preventing corrosion wear of the internal cavity of the main gas pipelines are very relevant and require serious research to solve them.One of the effective ways to reduce energy loss during natural gas transportation and protect the inner wall of the pipeline from corrosion damage is the use of internal smooth polymer coatings of steel pipelines. At the same time, due to the growing influence of the negative factors listed earlier, there is a need for a critical analysis of existing technical solutions for their compliance with the current state and prospects for the development of the oil and gas complex.The article discusses existing and promising developments in the field of application of internal coatings in order to increase the efficiency of natural gas transportation and reduce the amount of corrosion damage to the pipeline wall.

Application of fiberglass pipes in geothermal circulation systems

The results of calculating the coefficient of thermal conductivity of the developed construction of the pipeline from multilayer composite spatially oriented fiberglass are given. A method of calculating the coefficient of thermal conductivity of a pipe wall with a complex structure of reinforcement has been developed. The fiberglass pipe is made by the continuous method of oblique-layer longitudinal-transverse winding on a self-feeding frame of reinforcing pseudo-tape moistened with a binder in the form of a rarefied mesh. With each rotation of the mandrel, it shifts by the amount of hardening of the obtained two-component material under the influence of temperature. A comparison of calculated and experimental data was made. When comparing various known calculated dependencies and experimental values, the accepted physical model and the real structure of the material are determined. The known dependences obtained for a medium with infinite long parallel cylindrical inclusions for unidirectional and orthogonally reinforced materials and therefore for materials with a complex structure and scheme of reinforcements give errors and deviations. The work assumes that fiberglass consists of two components - a reinforcing filler and a binder with pores, the reinforcing fibers are endless and have a circular cross-section, the reinforcing fibers are evenly distributed in the matrix of the binder, there is an ideal thermal contact at the boundaries of the distribution between the reinforcement and the binder. Experimental data on determining the transverse thermal conductivity of unidirectional fiberglass rods with reinforcement based on aluminoborosilicate or magnesium-aluminosilicate glass and epoxy binders showed the smallest error in comparison with the calculated ones (8-10%). They investigated the fact that the coefficient of thermal conductivity of a fiberglass pipe is 100 times lower than that of a steel pipe. It was determined that it is possible to replace a steel pipe with a diameter of 219 mm with a fiberglass pipe with a diameter of 146 mm.

Thermophysical evolution during different decompression of N2‐containing S‐CO2 pipelines

AbstractPipelines transporting impure supercritical carbon dioxide in the carbon capture, utilization, and storage (CCUS) chain exhibit varying decompression characteristics due to engineered emissions or accidental leakage, resulting in diverse temperature drops and heat transfer mechanisms in the media and pipe walls. Therefore, studying heat transfer characteristics during slow and instantaneous decompression is crucial to investigating pipeline operational risks. In this work, supercritical CO2 pipeline valve release and rupture disc release experiments were performed with a 1.5% molar ratio of N2 content in an experimental pipeline (16 m long, 100 mm inner diameter). The evolution of the medium and pipe wall's physical properties was measured and discussed. Two methods of depressurization were employed to analyze the phase changes and heat transfer processes in the pipe. The instantaneous decompression process has a shorter decompression time and undergoes fluctuating and stable decompression stages. The slow decompression process has a slower temperature drop rate, but the wall during the process can reach a lower minimum temperature. Both release methods cause a larger temperature drop and Nusselt number at the bottom of the pipe wall due to evaporation heat transfer compared to the middle and top. The slow decompression process demonstrates a higher peak Nusselt number at the bottom, resulting in superior heat transfer efficiency compared to the instantaneous decompression process. © 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.

CALCULATION OF PIPELINES STRENGTH DUE TO MECHANICAL INFLUENCE OF ICE

Oil and gas pipeline systems contain structures in which ice expansion forces act on the pipes. The force of ice expansion on pipes can be internal and external. The internal force creates a hoop tensile stress in the pipe wall, the external force leads to the appearance of a compressive hoop stress and can lead to loss of stability of the pipe wall. To ensure the environmental safety of pipeline system operation in the oil and gas industry, it is necessary to determine the external and internal pressure on the pipes and evaluate their strength under the influence of ice expansion forces.Aim of research: ensuring environmental safety based on assessing the strength and stability of the pipe wall due to expansion forces of ice.Objects of study: environmental safety, pipeline or its sections under mechanical influence of ice.Research methods: theoretical studies of the stress-strain state of pipes due to external and internal expansion forces of ice.Analytical expressions were obtained to determine the pressure on the pipe surface due to ice formation in the interpipe space, as well as in pipeline cavity. An assessment of the influence of the properties of ice and soil in contact with the pipeline on the pressure values from the ice on the pipeline was made.

Comparative investigation on high-pressure flow accelerated corrosion at gradual contraction and gradual expansion pipes

In current study, the flow accelerated corrosion behaviors of X65 steel gradual contraction and gradual expansion pipes under high-pressure conditions were comparatively investigated by establishing dynamic in situ electrochemical tests under high CO2 partial pressure conditions in a loop apparatus. The findings indicate that the corrosion rate is reduced along the flowing direction of gradual contraction pipe. Nevertheless, the corrosion rate firstly descends and subsequently ascends along the flowing direction of gradual expansion pipe, and the lowest corrosion rate is presented in inclination segment close to the straight pipe with large diameter. The corrosion rate at the top wall of gradual contraction pipe is symmetrical to that at the bottom wall, while it is asymmetrically distributed at the top wall and the bottom wall of inclination segment in gradual expansion pipe. The distribution of corrosion rate is correlated with the flow fields at gradual contraction and gradual expansion pipes. Furthermore, the special corrosion rate distribution at inclination segment of gradual expansion pipe is ascribed to the formation of secondary flow and vortexes.

Study on the mechanical properties and acoustic emission signal characteristics of freezing pipe

The increase in freezing depth requires thicker, stronger, and colder freezing walls, and various complex factors in deep strata greatly increase the risk of freezing pipe fracture. To address the phenomenon of freezing pipe fracture, this paper designs freezing pipe and joint mechanical performance experiments based on acoustic emission (AE) technology, mainly testing the deformation of freezing steel pipes and composite joints at normal and low temperatures, changes in load bearing capacity, and corresponding AE characteristics of the process. Additionally, the associated AE characteristics throughout the process will be analyzed. The ultimate goal is to establish a discriminative pattern for identifying the critical fracture of freezing pipes based on the analysis of AE signal characteristics in conjunction with mechanical properties. The sensitivity of the AE system under low-temperature conditions and the reliability of the test were tested through pencil lead break experiments, and saltwater noise detection experiments were conducted to prevent noise interference from low-temperature saltwater flow and pipe wall friction in the identification of crucial signals for freezing pipe fracture. This study provides a basis for identifying the deformation mechanics and fracture warning of freezing pipes through dynamic analysis of AE monitoring information.

ANALYSIS OF THE CAUSES OF DAMAGE TO A LONG-OPERATED PIPELINE OF WET HYDROGEN SULFIDE-CONTAINING NATURAL GAS

The results of ultrasonic inspection of discontinuities in the metal of the pipe walls, long time operation under high pressure of wet hydrogen sulfide-containing natural gas, and analysis of the data characterizing them are presented. The criteria for selecting characteristic sections of the pipe with defects and sample cutting for fractographic research are substantiated. Based on the results of the analysis of ultrasound inspection and fractographic investigation, the causes and factors determining the process and mechanisms of sulfide cracking of the pipe walls have been established. The pipeline damage was assessed.

Axial and azimuthal development of disturbance waves in annular flow in a horizontal pipe

Transformation of gas-liquid flow in a horizontal pipe is investigated during the transition from stratified to annular flow pattern. Using Brightness-Based Laser-Induced Fluorescence technique, spatiotemporal evolution of liquid film thickness is analyzed over the downstream distance range of about 900 mm (45 pipe diameters), starting from the inlet. The measurements are carried out for three values of azimuthal angle θ: 0 (pipe bottom), 90°, and 180°, to track the circumferential spreading of liquid film and disturbance waves. At large gas velocities, thin liquid film is dragged upwards before the formation of large waves. The disturbance waves are created at the pipe bottom and spread circumferentially as they propagate downstream, over the already-wetted pipe walls. At large enough liquid flow rates, the spreading disturbance waves reach the pipe ceiling and form full rings around the circumference. The frequency and velocity of the disturbance waves eventually become the same around the pipe circumference; the disturbance wave amplitude and the base film thickness decrease with θ. The base film is more uniform around the circumference compared to the wave amplitude. At lower liquid flow rates, the disturbance waves cover only a part of pipe circumference. Their edges demonstrate oscillatory circumferential spreading, which ends by deceleration and decay of the edges. The upper part of the pipe may be wetted by a thin base film layer, covered only with ripples, or remain dry. At low gas speeds and large liquid flow rates, occasional splashing of large waves over the pipe ceiling without pre-wetting by the thin film is possible; however, the remaining film drains downward and no stable annular film is maintained. Liquid droplets, entrained from the pipe bottom and depositing in the upper parts of the pipe, are gathered in trains of creeping pendant droplets at the very top part of the pipe; no continuous wetting is achieved due to droplet deposition.