A growth model for dynamic slugs in gas–liquid horizontal pipes

Long liquid slugs, with sizes reaching 500 pipe diameters or more, may form in gas–liquid horizontal pipe flow at intermediate liquid loadings. Such slugs cause serious operational upsets due to the strong fluctuations in flow supply and pressure. Therefore, predicting the transition from short (hydrodynamic) to long slug flow regimes may play a significant role in preventing or reducing the negative effects caused by the long slugs.     

Experimental research on transient critical heat flux in vertical tube under oscillatory flow condition

The transient critical heat flux (CHF) experiments with forced sinusoidal inlet flow oscillation (oscillation period in 1–11 s, normalized amplitude of inlet flow oscillation in 0–3.0) were conducted in a vertical tube under low pressure condition. To analyze the triggering mechanism and aftermath of periodic dryout, the wall temperature fluctuation characteristics at the onset of periodic dryout and during post-periodic dryout were investigated. Under inlet flow oscillation condition, periodic dryout would be triggered at the wave trough of liquid film oscillation as wall heat flux far below the stable-flow CHF. The transient periodic dryout would give rise to temperature fluctuations on the tube wall, the amplitude of which increased with oscillation period and heat flux. The large wall temperature fluctuation during long-playing periodic dryout could significantly pre-trigger continuous dryout. The changing trends of the periodic dryout heat flux show a reasonable agreement with Okawa’s theoretical model, in which the liquid film oscillation was supposed be weakened by the axial mixing of liquid film. Moreover, the droplet entrainment at the oscillatory interface also has noticeable influence on the oscillation characteristics of liquid film. Based on the analysis of parameter effects on periodic dryout, a semi-empirical correlation was proposed to predict the periodic dryout heat flux under inlet flow oscillation condition.     

Two-phase flow split at T junctions: Effect of side arm orientation and downstream geometry

The effect of flow pattern and geometry on the phase split of gas/liquid flows at T junctions has been examined for a horizontal main tube and horizontal and vertically upwards side arms. Important phenomena which control this split in annular and wavy stratified flow have been identified. The capability of current models to predict the split are discussed. In particular, the effect of geometry in the downstream leg of the main pipe was studied. The configurations studied had no effect in annular flow but influenced the amount of liquid taken off at high take off when stratified flow approached the junction.     

Oscillating heat pipe simulation considering dryout phenomena

In heat transport devices such as oscillating heat pipe (OHP), dryout phenomena is very important and avoided in order to give the optimum performance. However, from the previous studies (including our studies), the dryout phenomena in OHP and its mechanism are still unclear. In our studies of OHP (Senjaya and Inoue in Appl Thermal Eng 60:251–255, 2013; Int J Heat Mass Transfer 60:816–824, 2013; Int J Heat Mass Transfer 60:825–835, 2013), we introduced the importance and roles of liquid film in the operating principle of OHP. In our previous simulation, the thickness of liquid film was assumed to be uniform along a vapor plug. Then, dryout never occurred because there was the liquid transfer from the liquid film in the cooling section to that in the heating section. In this research, the liquid film is not treated uniformly but it is meshed similarly with the vapor plugs and liquid slugs. All governing equations are also solved in each control volume of liquid film. The simulation results show that dryout occurs in the simulation without bubble generation and growth. Dryout is started in the middle of vapor plug, because the liquid supply from the left and right liquid slugs cannot reach until the liquid film in the middle of vapor plug, and propagates to the left and right sides of a vapor plug. By inserting the bubble generation and growth phenomena, dryout does not occur because the wall of heating section is always wetted during the bubble growth and the thickness of liquid film is almost constant. The effects of meshing size of liquid film and wall temperature of heating section are also investigated. The results show that the smaller meshing size, the smaller liquid transfer rate and the faster of dryout propagation. In the OHP with higher wall temperature of heating section, dryout and its propagation also occur faster.     

Thermal-hydraulic Phenomena Relevant to Global Dryout in a Hemispherical Narrow Gap

A series of experimental investigations on the cooling mechanism in hemispherical narrow gaps has been carried out. A visualization experiment, VISU-II, was done as the first step to get a visual observation of the flow behaviour inside a hemispherical gap and to understand the mechanism inducing global dryout. It was observed that the counter-current flow limitation (CCFL) phenomenon prevented water from wetting the heater surface and induced dryout. The CHFG test was performed to measure the critical power corresponding to global dryout and to investigate the inherent cooling mechanism in hemispherical narrow gaps. Temperature measurements over the heater surface show that the two-phase flow behaviour inside the gaps could be quite different from the other usual CHF experiments. The measured values of critical power are lower than the predictions by existing empirical CHF correlations based on the data measured with small-scale horizontal plates and vertical annulus. Received on 14 April 1998     

Resistance of a circular pipe with pulsatory variation of the flow rate

The results of an experimental investigation of the hydraulic resistance of a circular pipe for turbulent flow with periodic flow rate fluctuations are presented. The presence of resonance phenomena in the pipe is revealed. It is established that, for hydrodynamic nonstationarity, the pipe resistance is a nonmonotonous function of the frequency of the imposed flow rate fluctuations and differs from the pipe resistance in the stationary flow regime. Under the conditions considered, to find the pipe resistance it is necessary to take into account the variation of the flow kinetic energy with respect to the phase of the imposed flow rate fluctuations due to the deformation of the velocity profile.     

The formation of hydrodynamic slugs by the interaction of waves in gas–liquid two-phase pipe flow

This paper examines the effects of wave interaction on the formation of hydrodynamic slugs in two-phase pipe flow at relatively low gas and liquid superficial velocities. The experiments were conducted using a horizontal 31 m long, D = 10 cm internal diameter transparent pipe at atmospheric pressure. High resolution photography allowed the location of the gas–liquid interface to be measured with a high degree of accuracy at 5 Hz. Image analysis allowed individual waves to be tracked over a 14D section of the pipe. Regular waves having similar properties such as speed, amplitude and length were seen far from the region of slug formation. However, near the transition region, where hydrodynamic slugs were formed, significant differences between wave properties were observed which resulted in wave interaction leading to a type of sub-harmonic resonance and slug formation. The formation of hydrodynamic slugs due to wave interaction differs from predictions for slug formation using long wavelength stability theory. The properties of the waves were quantified which gave detailed information on the resonance mechanism found near the transition to slugging.     

Stratification of boiling two-phase flow in a single horizontal channel

The objective of this study is to investigate experimentally the stratification phenomena of boiling two-phase flow in a uniformly heated horizontal channel. Two-phase flow stratification due to gravity effects, and consequently its thermal and hydrodynamic behavior, under steady state conditions, have been determined by measuring 16 top and 16 bottom wall temperatures. Six distinct wall temperature profiles are found, and the corresponding flow patterns are discussed. A dimensionless number has been formulated for the prediction of the occurrence of different flow patterns.     

Experimental investigation on the slip between oil and water in horizontal pipes

This work is devoted to study of the slip phenomenon between phases in water–oil two-phase flow in horizontal pipes. The emphasis is placed on the effects of input fluids flow rates, pipe diameter and viscosities of oil phase on the slip. Experiments were conducted to measure the holdup in two horizontal pipes with 0.05 m diameter and 0.025 m diameter, respectively, using two different viscosities of white oil and tap water as liquid phases. Results showed that the ratios of in situ oil to water velocity at the pipe of small diameter are higher than those at the pipe of big diameter when having same input flow rates. At low input water flow rate, there is a large deviation on the holdup between two flow systems with different oil viscosities and the deviation becomes gradually smaller with further increased input water flow rate.     

Flow pattern and boiling heat transfer of CO2 in horizontal small-bore tubes

Increasing attention has been focused on carbon dioxide (CO₂) heat pump system where the temperature level is rather low, while the operating pressure is rather high. In this system, the density difference between vapor and liquid becomes rather small, which significantly affects flow patterns. Low surface tension and latent heat also have significant influence on two-phase flow patterns and heat transfer. This paper describes experimental and numerical investigation on flow patterns and heat transfer characteristics of boiling flow CO₂ at high pressure in horizontal small-bore tubes ranging from 1.0 mm to 3.0 mm I.D. Even though the density difference is rather small at high pressure, phase stratification takes place, which leads to the intermittent dryout at the upper wall. So far developed discrete bubble model by the authors for vertical flows is modified so as to include horizontal flow mechanisms. The predicted flow patterns with this new model agree on the whole with the experimental observation.     

The analysis of a mechanism of liquid replenishment and draining in horizontal two-phase flow

There is a regime of two-phase flow in which large waves or surges pass rapidly along a horizontal tube accompanied by splashing, wave-breaking and entrainment with the result that water is thrown to the upper surface of the tube. Between surges the film on the top surface is depleted by draining under gravity and by evaporation if the tube is heated. If the interval between surges is sufficiently long a dry patch may begin to form. In this paper, theory is given for the calculation of the film thickness left behind on the top surface and for the calculation of the time to dryout. The theory includes both the effect of the boundary layer developement during replenishment of the film and also the effect of the axial deceleration of the film at the point where the liquid replenishment ceases. Finally, the predicted variation of film thickness is compared with experimental film thickness traces obtained in this type of horizontal two-phase flow. The agreement is found to be very satisfactory. This analysis is of interest in connection with the prevention of permanent and intermittent dryout at low qualities in nuclear power station evaporators.     

Air–water counter-current slug flow data in vertical-to-horizontal pipes containing orifice type obstructions

This paper presents experimental counter-current air–water flow data on the onset of flooding and slugging, the slug propagation velocity, the predominant slug frequency and the average void fraction collected by using different size orifices installed at two locations in a horizontal pipe. For the flow conditions covered during these experiments, it was observed that there is no significant difference between the onset of flooding and the onset of slugging when an orifice is installed in the horizontal run. However, a difference was observed for the experiments carried out without orifices. Furthermore, the position of the orifice with respect to the elbow does not affect the onset of flooding and slugging. When an orifice is installed in the horizontal run, it was observed that slugs occur due to the mutual interaction (constructive interference) of two waves traveling in opposite directions. This means that a completely different mechanism seems to govern the formation of slugs in counter-current two-phase flows in horizontal partially blocked pipes. This is in contrast to that described for the slugging phenomena in co-current flow, where wave instability seems to be the principal mechanisms responsible of bridging the pipe. The mutual interaction of waves traveling in opposite directions seems to control the behaviour of the slug propagation velocity, the slug frequency and average void fraction with increasing the gas superficial velocity.     

Studies on two-phase co-current air/non-Newtonian shear-thinning fluid flows in inclined smooth pipes

In this work, co-current flow characteristics of air/non-Newtonian liquid systems in inclined smooth pipes are studied experimentally and theoretically using transparent tubes of 20, 40 and 60 mm in diameter. Each tube includes two 10 m long pipe branches connected by a U-bend that is capable of being inclined to any angle, from a completely horizontal to a fully vertical position. The flow rate of each phase is varied over a wide range. The studied flow phenomena are bubbly flow, stratified flow, plug flow, slug flow, churn flow and annular flow. These are observed and recorded by a high-speed camera over a wide range of operating conditions. The effects of the liquid phase properties, the inclination angle and the pipe diameter on two-phase flow characteristics are systematically studied. The Heywood–Charles model for horizontal flow was modified to accommodate stratified flow in inclined pipes, taking into account the average void fraction and pressure drop of the mixture flow of a gas/non-Newtonian liquid. The pressure drop gradient model of Taitel and Barnea for a gas/Newtonian liquid slug flow was extended to include liquids possessing shear-thinning flow behaviour in inclined pipes. The comparison of the predicted values with the experimental data shows that the models presented here provide a reasonable estimate of the average void fraction and the corresponding pressure drop for the mixture flow of a gas/non-Newtonian liquid.     

Experimental study on the transition between stratified and non-stratified horizontal oil-water flow

The effect of oil and water velocities, pipe diameter and oil viscosity on the transition from stratified to non-stratified patterns was studied experimentally in horizontal oil-water flow. The investigations were carried out in a horizontal acrylic test section with 25.4 and 19 mm ID with water and two oil viscosities (6.4 and 12 cP) as test fluids. A high-speed video camera was used to study the flow structures and the transition. At certain oil velocity, stratified flow was found to transform into bubbly and dual continuous flows as superficial water velocity increased for both pipe diameters using the 12 cP oil viscosity. The transition to bubbly flow was found to disappear when the 6.4 cP oil viscosity was used in the 25.4 mm pipe. This was due to the low E?tv?s number. Transition to dual continuous flow occurred at lower water velocity for oil velocity up 0.21 m/s when 6.4 cP oil was used in the 25.4 mm ID pipe, while for U_so > 0.21 m/s, the transition appeared at lower water velocity with the 12 cP oil.The effect of pipe diameter was also found to influence the transition between stratified and non-stratified flows. At certain superficial oil velocity, the water velocity required to form bubbly flow increased as the pipe diameter increased while the water velocity required for drop formation decreased as the pipe diameter increased. The maximum wave amplitude was found to grow exponentially with respect to the mixture velocity. The experimental maximum amplitudes at the transition to non-stratified flow agreed reasonably well with the critical amplitude model. Finally, it was found that none of the available models were able to predict the present experimental data at the transition from stratified to non-stratified flow.     

Dryout in serpentine evaporators

Waterside on-load corrosion failures in power station evaporators have resulted from dryout along the upper surfaces of the tubes in serpentine arrangements.

It is shown that this type of dryout can occur at intermediate qualities over a wide range of flow rates and pressures for heat fluxes much less than in vertical tubes.

A type of flow which is here called “surge flow” has been identified as being particularly likely to lead to dryout, because the top surfaces are wetted only intermittently by waves or plugs of frothy liquid.

A method is being developed for assessing the likelihood of dryout under surge flow conditions. It correctly predicts the dryout boundaries in a pressurized steam/water facility.     

Use of hydrodynamic cavitation for volatile removal compound

Hydrodynamic cavitation and its feasibility for volatile compound removal in enclosed channels is discussed in this paper. Very high Reynolds numbers are needed to rupture liquid by decreasing its pressure below its saturated vapour pressure. Hence, a simple stratified flow, at which the two phases separate, is precluded in vertical and horizontal tubes, where turbulence stresses will be much larger than the buoyant forces. The most probable flow regime at this high turbulence regime is a bubble- or annular flow, where the volatile matter tends to concentrate in the centre of the pipe because of the lift force resulting from the unequal flow of the viscous liquid around the bubbles in the presence of the pipe wall. Therefore, boiling the volatile matter for volatile compound removal is not enough if hydrodynamic cavitation is pursued. The attainable efficiency must also be assessed. An expression for the volatile removal efficiency and the main parameters affecting this efficiency were derived by utilising a simplified geometrical and physical model. The efficiency was found to approximate a power law as a function of the volatile concentration and its strong dependence on the size of the volatile bubble reasonably well. This result implied the need of bubble growth and the limitation of the process for highly concentrate compounds to a few percent concentrations. With regard to energetic requirements, both thermal and hydrodynamic cavitations are quantitatively similar. Furthermore, the choice of one or another corresponds more to the kind of energy source available.     

Lagrangian slug flow modeling and sensitivity on hydrodynamic slug initiation methods in a severe slugging case

Severe slugging is a dynamic two-phase flow phenomenon with regular liquid accumulation and blow-out in flow-line riser geometries. This paper discusses the applicability of a slug tracking model on a case where hydrodynamic slug initiation in a horizontal part of the pipeline upstream the riser base affects the severe slugging cycle period. The given experimental case is from the Shell laboratories in Amsterdam: air–water flow in a 100 m long pipe (65 m horizontal and 35 m −2.54° downwards) followed by a 15 m long vertical riser.A Lagrangian slug and bubble tracking model is described. A two-fluid model is applied in the bubble region and the slug region is treated as incompressible flow, with an integral momentum equation. Slug initiation from unstable stratified flow can be captured directly by solving the two-fluid model on a fine grid (a hybrid capturing and tracking scheme). Alternatively, slug initiation can be made from sub grid models, allowing for larger grid sizes. The sub grid models are based on the two established flow regime transition criteria derived from the stability of stratified flow and from the limiting solution of the unit cell slug flow model.Sensitivity studies on hydrodynamic slug initiation models on the severe slugging characteristics are presented. No hydrodynamic slug initiation (e.g. large grid size in the capturing scheme) overestimates the severe slug period compared with the experiments. Slug capturing and sub grid initiation models both give good predictions for small grid sizes (provided the detailed inlet configuration is included in the capturing case). Good predictions are also shown for larger grid sizes (factor of 50) and sub grid initiation models.The numerical tests show that correct prediction of the severe slugging cycle is sensitive to the initiation of upstream hydrodynamic slugs, but less sensitive to the local structure of the slug flow (frequencies and lengths) in the upstream region.     

Wave regimes of ascending flow of a thin layer of a viscous liquid in contact with a gas

The flow of a liquid in thin layers is one of the hydrodynamic problems of chemistry and heat engineering. The large surface area of films and their small thickness make it possible to accelerate thermal, diffusive, and chemical processes at the gas-liquid boundary.Theoretical studies of liquid flow in a vertical descending thin layer are presented in [1–4]. In this paper we study ascending wave flows of a liquid in a thin vertical layer in contact with a gas, i.e., flows in the direction opposite the action of the force due to gravity, with account for the action of the gas on the liquid surface. Such motions are encountered when oil is extracted from strata that are saturated with gas. At some distance from the stratum the oil and gas separate: the gas travels at high velocity inside the pipe, occupying a considerable portion of the pipe, and the liquid is displaced toward the pipe walls, forming a thin film. In certain cases a wave-like interface develops between the oil and gas that travels with a velocity greater than that of the liquid but less than the average gas velocity. Similar phenomena are observed in high velocity mass exchangers.We examine the effect of the gas for both laminar and turbulent flow.Studies that neglect the effect of the gas flow on the liquid show that for waves on the film surface whose lengths are considerably longer than the average thickness of the layer, the liquid motion in the film is described by boundary layer equations in which account is taken of the mass force, i.e., the force due to gravity. With some approximation, we can assume that in accounting for the effect of the gas on the liquid the liquid flow is described by these same equations.     

Effect of velocity and pipeline configuration on dispersion in turbulent hydrocarbon-water flow using laser image processing

Drop size distribution and concentration profile data for hydrocarbon-water mixtures are obtained in a 8.2 cm dia pipe at a range of velocities for a straight horizontal pipe, horizontal and vertical flow after one bend and vertical flow after three bends. The laser image processing technique employed in this project is proven reliable.

The maximum drop size (d₉₉), is more dependent on the number of upstream interactive bends than on the velocity. The drop size distributions follow a Rosin-Rammler power law. The values of Rosin-Rammler exponents, based on this work, are on average 2.1 for all the configurations studied.

The concentration profiles as a function of velocity for straight horizontal flow are obtained and show the transition from stratified to adequately dispersed flow at about 2.3 m/s velocity. The concentration profiles for horizontal or vertical flow after one bend show dispersed flow in some cases; however, in other cases swirling makes representative sampling more difficult.

Vertical downflow after three interactive bends breaks the droplets to a finer size, and concentration profiles obtained in this location are more uniform than the other configurations studied. Representative sampling can be accomplished in this location even at 0.7–1.0 m/s velocity, in a 8.2 cm pipe.     

Lubrication-type analysis of thermo-hydraulic transport in a model grooved heat pipe

The objective of this work is to achieve a rigorous resolution of the coupled hydraulic and thermal problems (including the solid) within a single triangular axial groove of a heat pipe in microgravity conditions. This is done by modeling the phenomena of transport of matter, momentum and heat, starting from the equations of continuity, Navier-Stokes and energy conservation. By combining the lubrication approximation and the one-sided approach, our approach leads to a 2nd-order differential equation for the scaled height of liquid. Through solving numerically this equation for a given heat flux input distribution, we can establish the profiles along the heat pipe for the scaled height of liquid and for the scaled averaged axial velocity of the flow. The equation of energy conservation can then also be solved, for the scaled liquid height distribution just determined. It leads to the scaled temperature field in each section of the heat pipe, and hence to the 3D temperature field, both in the solid and in the liquid. The proposed approach also enables determining the maximum heat flux that can be applied in the evaporator section without reaching dryout. Finally, the parametric sensitivity of this maximum heat flux to both geometrical and thermodynamical parameters is also analyzed.