Flow visualization of a recirculating flow by rheoscopic liquid and liquid crystal techniques

Experiments were conducted in a three-dimensional lid-driven cavity flow to study the behavior of longitudinal Taylor-Görtler-like vortices. Flow visualization was accomplished by use of a rheoscopic liquid and of liquid crystals, together with laser-light and white-light sheets, respectively. Photographs of the lighted planes in the flow confirmed the existence of the vortices for a wide range of Reynolds numbers and for stable, neutrally-buoyant and buoyant global flow conditions. As usual the flow visualization revealed flow patterns not deducible by in situ measurements; the liquid crystal photographs give both flow pathlines and temperature distribution on any lighted plane.     

Non-isothermal flow diagnostics using microencapsulated cholesteric particles

The temperature and the flow field of thermo-convective liquid flows are visualized using cholesteric liquid crystal material as tracer particles. This type of tracers offers the scientifically valuable feature of measuring the flow and the temperature field simultaneously. Three thermoconvective flow configurations have been investigated successfully using liquid crystals. The results are discussed in some detail. It turns out that the liquid crystal technique is a valuable tool for thermo-convective liquid flow analysis.     

Effect of gap and flow orientation on two-phase flow in an oil-wet gap: Relative permeability curves and flow structures

Naturally fractured reservoirs contain about 25–30% of the world supply of oil. In these reservoirs, fractures are the dominant flow path. Therefore, a good understanding of transfer parameters such as relative permeability as well as flow regimes occurring in a fracture plays an important role in developing and improving oil production from such complex systems. However, in contrast with gas–liquid flow in a single fracture, the flow of heavy oil and water has received less attention. In this research, a Hele-Shaw apparatus was built to study the flow of water in presence of heavy oil and display different flow patterns under different flow rates and analyze the effect of fracture orientations on relative permeability curves as well as flow regimes. The phase flow rates versus phase saturation results were converted to experimental relative permeability curves. The results of the experiments demonstrate that, depending on fracture and flow orientation, there could be a significant interference between the phases flowing through the fracture. The results also reveal that both phases can flow in both continuous and discontinuous forms. The relative permeability curves show that the oil–water relative permeability not only depends on fluid saturations and flow patterns but also fracture orientation.     

Oil–water flows through sudden contraction and expansion in a horizontal pipe – Phase distribution and pressure drop

In this paper, change of flow patterns during the simultaneous flow of high viscous oil and water through the sudden contraction and expansion in a horizontal conduit has been studied. It is noted that these sudden changes in cross-section have a significant influence on the downstream phase distribution of lube oil–water flow. The observation suggests a simple technique to establish core flow as well as a way to prevent pipe wall fouling during the transportation of such oil. A number of interesting differences have been noted during low viscous oil–water flow through the same test rigs. While several types of core annular flow are observed for the former case, a wider variety of interfacial distribution characterizes kerosene–water systems. The pressure profiles during the simultaneous flow of lube oil and water through the sudden contraction and expansion are also studied and compared with low viscous oil–water flows. The pressure profiles are found to be independent of liquid viscosity and the loss coefficients are observed to be independent of flow patterns in both the cases.     

垂直下降管中两相流的流型以及液相粘度对流型的影响

对空气－油在垂直下降管中的流型进行了实验研究，采用的管径为２９ｍｍ，油和空气的折算流速分别达到４ｍ／ｓ和２０ｍ／ｓ，并借助于压降脉动分析和目测观察相结合的方法来进行流型的识别。研究表明，油气两相流的流型不同于低粘液体的两相流流动，通过实验研究并结合前人的研究成果，给出了液相粘度对流型转变的影响趋势。     

The rivulet flow pattern during oil–water horizontal flow through a 12 mm pipe

The present study reports the hydrodynamics of the rivulet pattern during oil–water flow through a 12 mm horizontal acrylic pipe. The interfacial distribution has been observed visually and characterized from signals obtained from an optical probe as well as by isokinetic sampling. The probability density function (PDF) and fast Fourier transform (FFT) of the signals have provided an understanding of the flow configuration. The experiments have revealed that although rivulet flow is a typical separated flow pattern, it has different characteristics as compared to the stratified and annular flow patterns. The holdup and pressure drop under such conditions have been compared with the drift flux model for horizontal flow as well as the two-fluid model as proposed by Brauner and Maron [9] for liquid–liquid flows.     

Phase split of nitrogen/non-Newtonian fluid two-phase flow at a micro-T-junction

An experimental investigation has been undertaken to understand the phase split of nitrogen gas/non-Newtonian liquid two-phase flow passing through a 0.5 mm T-junction that oriented horizontally. Four different liquids, including water and aqueous solutions of carboxymethyl cellulose (CMC) with different mass concentrations of 0.1, 0.2 and 0.3 wt%, were employed. Rheology experiments showed that different from water, CMC solutions in this study are pseudoplastic non-Newtonian fluid whose viscosity decreases with increasing the shear rate. The inlet flow patterns were observed to be slug flow, slug–annular flow and annular flow. The fraction of liquid taken off at the side arm for nitrogen gas/non-Newtonian liquid systems is found to be higher than that for nitrogen gas/Newtonian liquid systems in all inlet flow patterns. In addition, with increasing the pseudoplasticity of the liquid phase, the side arm liquid taken off increases, but the increasing degree varies with each flow pattern. For annular flow, the increasing degree is much greater than those for slug and slug–annular flows.     

Viscous oil–water flows in a microchannel initially saturated with oil: Flow patterns and pressure drop characteristics

Immiscible viscous liquid–liquid two-phase flow patterns and pressure drop characteristics in a circular microchannel have been investigated. Water and silicone oil with a dynamic viscosity of 863 mPa s were injected into a fused silica microchannel with an inner diameter of 250 μm. As the microchannel was initially filled with the silicone oil, an oil film was found to always form and remain on the microchannel wall. Different flow patterns were observed and classified over a wide range of water and oil flow rates. A flow pattern map is presented in terms of Re, Ca, and We numbers. Two-phase pressure drop data have also been collected and analyzed to develop a simple correlation for slug, annular and annular-droplet flow patterns in terms of superficial water and oil velocities.     

Heating characteristics of blunt swept fin-induced shock wave turbulent boundary layer interaction

An experimental study was conducted on shock wave turbulent boundary layer interactions caused by a blunt swept fin-plate configuration at Mach numbers of 5.0, 7.8, 9.9 for a Reynolds number range of (1.0∼4.7)×10⁷/m. Detailed heat transfer and pressure distributions were measured at fin deflection angles of up to 30° for a sweepback angle of 67.6°. Surface oil flow patterns and liquid crystal thermograms as well as schlieren pictures of fin shock shape were taken. The study shows that the flow was separated at deflection of 10° and secondary separation were detected at deflection of ϑ≥20°. The heat transfer and pressure distributions on flat plate showed an extensive plateau region followed by a distinct dip and local peak close to the fin foot. Measurements of the plateau pressure and heat transfer were in good agreement with existing prediction methods, but pressure and heating peak measurements atM≥6 were significantly lower than predicted by the simple prediction techniques at lower Mach numbers. The project supported by China Academy of Launch Vehicle Technology     

Visualization of water and surfactant floods in oil-saturated porous media

The flow of water and 1% surfactant solution displacing oil through homogeneous and non-homogeneous porous media have been studied experimentally. The results are shown by taking the photographs of unstable interface at regular interval of time. These photographs suggest that the spreading of the moving interface in lateral direction is more for 1% surfactant solution displacing oil than for water displacing oil and also the interface is more stable for surfactant flooding in the bed. The wavelengths of viscous fingers measured from the experiments are found to be in good agreement with the theoretical predictions for homogeneous bed. The percentage oil recovery at breakthrough is improved considerably with the use of surfactant solution. Effect of flow rate on recovery and breakthrough time has also been studied. Finally, the effect of non-homogeneous packing on the growth of fingers has been studied by creating non-homogeneous medium in an otherwise homogeneous porous medium.     

Prediction of the slug-to-churn flow transition in vertical two-phase flow

An assessment is made of the various viewpoints on the slug-to-churn flow transition in vertical upward flow in the light of recent experimental results obtained at Harwell Laboratory. It is found that the flooding model of McQuillan & Whalley and the bubble entrainment model of Barnea & Brauner give satisfactory results at low and high liquid flow rates, respectively. An improved model for flooding, which takes account of the effect of the falling film, has been proposed. It is shown that this new model is in good agreement with experimental results at both low and high liquid flow rates.     

Automatic liquid crystal thermography for transient heat transfer measurements in hypersonic flow

A technique has been developed to measure surface heat transfer on windtunnel models in hypersonic flow based on the colour response of encapsulated thermochromic liquid crystals. The method supplies results of a superior spatial resolution at experimental uncertainties comparable to traditional methods. The approach is different from other liquid crystal applications in several key areas. It combines the calibration of the liquid crystal coating with the actual mesurement and therefore allows for an efficient experiment. The method is automated in most steps involved. Results are shown for the flow over an axisymmetric compression corner at Mach 5 and compared with surface thermocouple measurements.     

Two-phase flow patterns in a 90° bend at microgravity

Bends are widely used in pipelines carrying single-and two-phase fluids in both ground and space applications. In particular, they play more important role in space applications due to the extreme spatial constraints. In the present study, a set of experimental data of two-phase flow patterns and their transitions in a 90° bend with inner diameter of 12.7 mm and curvature radius of 76.5 mm at microgravity conditions are reported. Gas and liquid superficial velocities are found to range from (1.0≈23.6) m/s for gas and (0.09≈0.5) m/s for liquid, respectively. Three major flow patterns, namely slug, slug-annular transitional, and annular flows, are observed in this study. Focusing on the differences between flow patterns in bends and their counterparts in straight pipes, detailed analyses of their characteristics are made. The transitions between adjoining flow patterns are found to be more or less the same as those in straight pipes, and can be predicted using Weber number models satisfactorily. The reasons for such agreement are carefully examined. The project supported by the Canadian Space Agency (CSA) and the visiting scholar program of the Chinese Academy of Sciences (CAS)     

Experimental determination of the flow transport coefficients in the coupled equations of two-phase flow in porous media

The transport coefficients in the coupled equations of two-phase flow are defined if the pressure gradient in one of the two flowing fluids is equal to zero. This definition has been used in experiments with oil and water in a sandpack and the four transport coefficients have been measured over wide water saturation ranges. The values of the cross coefficients were found to be significant as they ranged from 10 to 35% of the value of the effective permeability to water and from 5 to 15% of the effective permeability to oil, respectively.     

Studying disturbance waves in vertical annular flow with high-speed video

In many annular two-phase gas–liquid flows, large disturbance waves propagate liquid mass. These waves are important for modeling of gas-to-liquid momentum transfer and liquid film behavior. High-speed videos of vertical upflow have been analyzed to extract individual and average wave data. Two types of structures, coherent waves and piece waves, have been identified in these flows. Velocities, lengths, and temporal spacings of individual waves and average velocities, lengths, frequencies, and intermittencies have been studied as functions of both gas and liquid flow rates. Velocity and frequency increase with liquid and gas flow rates, length decreases with increasing gas flow and increases with increasing liquid flow, and intermittency is predominantly an increasing function of liquid flow.     

气液混输悬链线立管系统两相流特性实验

严重段塞流是海洋工程气液混输管线--立管系统中常见的一种特殊有害流动现象, 采用水平--下倾--悬链线立管气液混输组合管道系统, 通过系列实验在悬链线立管中获得了严重段塞流、间歇流和震荡流等流型, 阐述了这些流动现象的形成机理, 提出了能够产生严重段塞流的判定准则. 结果表明, 悬链线立管严重段塞流具有明显周期性, 在一个周期内的流动特征可分为液塞形成、液体出流、液气喷发及液体回流等4个阶段, 进而给出了各阶段中相关流动参数的变化规律. 在实验中同时还对悬链线与垂直立管中严重段塞流形成机理进行了比较分析, 发现两者在液塞形成阶段有显著差别. 其中, 在悬链线立管中液塞形成之前首先需要经历一个气液混合液塞形成过程, 而垂直立管则没有这个过程.      

Abnormal rotation of a deformed liquid crystal droplet immersed in an isotropic fluid after transient flow

The morphology evolution of liquid crystal droplets immersed in an isotropic fluid in flow field is found to be different from flexible polymer droplets. In this paper, we investigated the retraction of a liquid crystal droplet after transient flow. It is found that the liquid crystal droplet will rotate during the shape recovery, which has never been observed for an isotropic droplet. The factors that influence the rotational angle of a single liquid crystal droplet during retraction progress were studied, including the temperature, the dimension of the droplets, the time of shear flow, the shear rate, the flow type, and the properties of liquid crystal molecules. The rotation of liquid crystal droplet during shape recovery is ascribed to both the bulk elasticity of liquid crystal droplets and the anisotropic properties of the interface between liquid crystal and isotropic fluid.     

Two-phase flow patterns and void fractions in downward flow Part I: Steady-state flow patterns

Observations of void fractions and flow patterns have been made during steady-state, co-current, downward flow of liquid refrigerant 113 and its vapor. The new data on flow pattern transitions, plus the available downward flow data in the literature, have been compared with available predictions. It was found that the flow pattern map previously developed for horizontal and upward flow can be extended to downward flow with only minor modifications. Part II of this paper will report on the void fraction measurements and the observations of flow pattern transitions during flow transients.     

Detection of flow separation and reattachment using shear-sensitive liquid crystals

Coatings of pure chiral nematic liquid crystals are known to change colour under different levels of surface shear stress. In this study, the liquid crystal was used to provide information about flow separation and reattachment on both a two-dimensional aerofoil and a delta wing. The tests were carried out at a free-stream velocity of 28 m/s and a number of incidence angles. The Reynolds numbers based on the central chord length of the models were 200,000 and 270,000 for the aerofoil and delta wing models, respectively. The study showed that locations of boundary layer separation and reattachment can be identified from spatial variations in the surface colour; the agreement between the results and those obtained using surface oil flow was good. Issues relating to interpretation of the crystal colour pattern and the limitation of this technique in detection of flow separation were also discussed.     

A theoretical model for core-annular flow of a very viscous oil core and a water annulus through a horizontal pipe

A theoretical model has been developed for core-annular flow of a very viscous oil core and a water annulus through a horizontal pipe. Special attention was paid to understanding how the buoyancy force on the core, resulting from any density difference between the oil and water, is counterbalanced. This problem was simplified by assuming the oil viscosity to be so high that any flow inside the core may be neglected and hence that there is no variation of the profile of the oil-water interface with time. In the model the core is assumed to be solid and the interface to be a solid/liquid interface.By means of the hydrodynamic lubrication theory it has been shown that the ripples on the interface moving with respect to the pipe wall can generate pressure variations in the annular layer. These result in a force acting perpendicularly on the core, which can counterbalance the buoyancy effect.To check the validity of the model, oil-water core-annular flow experiments have been carried out in a 5.08 cm and an 20.32-cm pipeline. Pressure drops measured have been compared with those calculated with the aid of the model. The agreement is satisfactory.