Effect of gas properties on the onset of flooding in small diameter inclined tubes

This work concerns the effect of gas phase properties on incipient flooding in counter-current gas–liquid two-phase flow in small diameter inclined tubes (D < 10 mm). The aim is to propose generalized correlations that can be used to predict incipient flooding in small diameter tubes. Experiments were performed in a 7 mm glass tube and at two inclination angles, i.e. 30 and 60° from the horizontal, using water as liquid phase and atmospheric air, He and CO₂ as gas phase. Previously proposed correlations for flooding prediction ( Pantzali et al., 2008) were suitably adjusted to incorporate the effect of gas phase properties. It has been proved that the flooding curves calculated using the proposed correlations are in good agreement with both the results of the present study and relevant literature data.     

Falling film and flooding phenomena in small diameter vertical tubes: The influence of liquid properties

The effect of liquid properties on flooding in small diameter vertical tubes is studied for various liquids with the aim to contribute to the interpretation of flooding mechanisms in such geometries. Data on free falling film characteristics for the various liquids used are acquired using a photographic technique from which mean layer thickness and its statistical quantities are calculated. The experimental data confirm previous observations that in almost all cases the dominant mechanism is wave growth and upward dragging by the gas and that consequently the onset of flooding is strongly affected by the liquid film structure. The results also confirm the influence of the liquid properties on the interfacial wave evolution and film characteristics. New correlations based on dimensionless groups for the prediction of flooding in narrow passages are proposed and found to be in good agreement with available data.     

Liquid layer characteristics in gas-liquid flow in slightly inclined pipes: Effect of non-ionic surfactant additives

The influence of non-ionic surfactant additives on the interfacial structure during both co-current gas-liquid upflow and downflow in slightly inclined pipes is investigated. Experiments are conducted in a 24 mm i.d. pipe for various gas-liquid flow rates and pipe inclination angles (β=±1° and ±3° from the horizontal position). Dilute aqueous solutions of the non-ionic surfactant Triton X-100 are employed, in order to study the effect of the additive on the flow pattern and the liquid layer characteristics (i.e. mean layer thickness, amplitude and frequency of the interfacial waves). In particular, the new data of the liquid layer characteristics indicate how the interfacial structure and the transition between the various flow regimes, during co-current gas-liquid flow in slightly inclined pipes, are affected by the addition of small amounts of the non-ionic surfactant.     

Computational fluid dynamic simulations on liquid film behaviors at flooding in an inclined pipe☆

The complex liquid film behaviors at flooding in an inclined pipe were investigated with computational fluid dynamic (CFD) approaches. The liquid film behaviors included the dynamic wave characteristics before flooding and the transition of flow pattern when flooding happened. The influences of the surface tension and liquid viscosity were specially analyzed. Comparisons of the calculated velocity at the onset of flooding with the available experimental results showed a good agreement. The calculations verify that the fluctuation frequency and the liquid film thickness are almost unaffected by the superficial gas velocity until the flooding is triggered due to the Kelvin–Helmholtz instability. When flooding triggered at the superficial liquid velocity larger than 0.15 m·s?1, the interfacial wave developed to slug flow, while it developed to entrainment flow when it was smaller than 0.08 m·s?1. The interfacial waves were more easily torn into tiny droplets with smaller surface tension, eventual y evolving into the mist flow. When the liquid viscosity increases, the liquid film has a thicker holdup with more intensive fluctuations, and more likely developed to the slug flow.     

Flooding in inclined pipes — Effect of entrance section

Flooding in countercurrent flow of air and water in inclined tubes has been investigated. Data on flooding inception in the whole range of inclinations have been collected and predictive models for calculating the flooding conditions as a function of the flow rates and pipe inclination were proposed. Special attention is placed on the effect of the liquid injection mode. It has been shown that the porous injection system causes a local disturbance and enhances flooding at relatively shallow inclinations and high liquid flow rates.     

The hydrodynamics or gas-liquid contacting in towers with fluidised packings

A model is developed of gas liquid contactors in which packing pieces are maintained in the fluidized state. The model proposes that the column can exist in either of two modes, namely fluidisation without flooding and fluidisation due to incipient flooding. The mode of operation established depends primarily on the packing density and to a lesser extent on the packing size, liquid flow and liquid properties. Equations predicting the transition from one mode to the next are developed. The inherent advantages of operating in the “incipient flooding” mode are stressed. It is argued that the pressure build up associated with flooding will be sufficient to support the weight of packing pieces and the liquid in the column. At this stage, rather than reject counter current flow (true flooding) the bed expands to maintain a state of incipient flooding. In this way counter current flow can be maintained in a situation well above the normal flooding point. This explains the high performance of such units.     

垂直及倾斜上升管内气液两相弹状流壁面切应力的模拟

用VOF模型对垂直及倾斜上升管内弹状流壁面切应力进行数值模拟。计算结果表明,垂直上升流动时,液膜厚度始终小于对应位置倾斜上升弹状流的液膜厚度,壁面切应力从气弹头部至尾部逐渐增大至恒定不变,在尾流区呈杂乱无章状态。倾斜上升流动时,气泡头部顶点偏向管中心线上方,倾角越小,相同轴向位置处测得的液膜厚度越大。当FrTB较小时,倾斜管内弹状流上管壁面的切应力曲线在液膜区有明显波动,而下管壁面在对应区域的切应力分布则比较光滑。随着FrTB的增大,上下壁面切应力分布曲线越来越靠近。     

水平管外水膜流型判据与特征长度

建立了蒸发式冷凝器水平管外水膜流动测试装置。观测发现水膜流动指示剂扩张角小于30°时,水膜湍流明显;扩张角大于50°时,水膜层流明显。研究了水平管外水膜流型随管径及喷淋量的变化,归纳出管外水膜层流临界Reynolds数阈值为15,湍流临界Reynolds数阈值为25。喷淋量增加的过程中,粗管比细管更容易达到湍流。管外水膜流动Reynolds数中的特征长度与水膜厚度一个量级。水膜越厚,特征长度越大;且管径越大,特征长度越大。     

烧结型多孔表面管外池沸腾传热特性

实验研究了热通量为0.1~160 kW·m^-2时,去离子水在光管及烧结型多孔表面管管外的池沸腾传热特性,分析了换热管布置方式(垂直与水平)、管径大小(20、25和32 mm)与多孔层颗粒尺寸(30~105 μm)对池沸腾传热特性的影响规律。结果表明:去离子水在多孔管表面的起始沸腾过热度小于光管,比光管低3 K左右;多孔表面管可明显强化核态沸腾传热,其沸腾传热系数可达光管的3~4.5倍;大热通量下,换热管水平布置时的传热效果较垂直布置佳,且布置方式对多孔管换热效果的影响比对光管的影响小;随管径增大,光管与多孔表面管的沸腾传热系数降低;大颗粒尺寸多孔层的强化效果优于小颗粒尺寸多孔层。     

Liquid crystal engineering of carbon nanofibers and nanotubes

Four high-aspect-ratio carbon nanomaterials were fabricated by template-directed liquid crystal assembly and covalent capture. By selecting from two different liquid crystal precursors (thermotropic AR mesophase, and lyotropic indanthrone disulfonate) and two different nanochannel template wall materials (alumina and pyrolytic carbon) both the shape of the nanocarbon and the graphene layer arrangement can be systematically engineered. The combination of AR mesophase and alumina channel walls gives platelet-symmetry nanofibers, whose basic crystal symmetry is maintained and perfected upon heat treatment at 2500 °C. In contrast, AR infiltration into carbon-lined nanochannels produces unique C/C-composite nanofibers whose graphene planes lie parallel to the fiber axis. The transverse section of these composite nanofibers shows a planar polar structure with line defects, whose existence had been previously predicted from liquid crystal theory. Use of solvated AR fractions or indanthrone disulfonate produces platelet-symmetry tubes, which are either cellular or fully hollow depending on solution concentration. The use of barium salt solutions to force precipitation of indanthrone disulfonate within the nanochannels yields continuous nanoribbons rather than tubes. Overall the results demonstrate that liquid crystal synthesis routes provide molecular control over graphene layer alignment in nanocarbons with a power and flexibility that rivals the much better known catalytic routes.     

Effect of surfactant additives on co-current gas-liquid downflow

The influence of a surfactant additive on the interfacial structure and on the transition from the smooth to the wavy stratified flow regime in inclined pipes is investigated for various gas-liquid flow rates. The effect of a surfactant on the gas-liquid stratified downflow patterns is studied using a dilute aqueous solution of a non-ionic surfactant (Tween^®80), which is found to have a dramatic effect on two-phase flow characteristics. A conventional solution having a surface tension similar to that of the surfactant solution is also employed and this verifies that the atypical behavior of the surfactant solution is ascribed to its special chemical structure and not to its low surface tension. Liquid layer thickness time records are acquired using a parallel-wire conductance technique from which the statistics of the layer thickness, as well as wave celerities, are calculated. Laser doppler anemometry (LDA) is employed to investigate the flow structure in the thin liquid layer both with and without interfacial shear induced by a co-current gas flow. A differential pressure transducer is used for pressure drop measurements in the liquid phase. The interpretation of the results verifies that the “delayed” appearance of the interfacial waves should be associated with the turbulence retardation within the liquid layer and it facilitates the clarification of the mechanisms in which the surfactant interacts with the turbulent structures and causes significant drag reduction.     

EXPERIMENTAL STUDY OF GRAVITY-DRIVEN FILM FLOW OF NON-NEWTONIAN FLUIDS

In this research the problem of a thin layer of a power law liquid falling down an inclined plate was studied experimentally. Three different carboxymethyl cellulose (CMC) solution concentrations (1.1%, 1.5%, and 2%), which are extensively used in industry, have been selected as the operating fluid, and their rheology, surface tension, and contact angle have been determined. Dynamics of the falling film has been studied by image acquisition techniques, and by using image processing methods the velocity of falling film, film thickness, and the shape of the falling film have been investigated. The inclined plate with different inclination angles φ (0 < φ < π/2), and nonidentical surfaces (ceramic, aluminum, and glass) were used to study the effect of inclination, rheological properties, and contact angle on the vital parameters mentioned earlier. These variables are embedded in dimensionless groups, Weber (We), Reynolds (Re), and Froude (Fr) numbers, and some correlations were devised to relate dimensionless velocity distribution parameters and film thickness to these dimensionless groups.     

The structure of a cyanobiphenyl side chain liquid crystalline poly(silylenemethylene)

The structure of a side chain liquid crystalline poly(silylenemethylene) (-(SiCH₃R-CH₂)-: R=O(CH₂)₁₁O-Ph-Ph-CN, Ph=phenyl) (CN-11) has been studied by X-ray diffraction and differential scanning calorimetry (DSC). The DSC results showed that CN-11 has transitions at ∼92 °C (T₂) and ∼147 °C (T₁) during both cooling and immediate heating. A third transition occurred at ∼50 °C (T₃) during heating after annealing at room temperature. The X-ray fiber pattern of the CN-11 annealed at room temperature showed several wide and small angle reflections which were indexed by a monoclinic unit cell with parameters a=16.8 Å, b=7.42 Å, c=43.6 Å and β=102.1° (b: fiber direction), representing a crystal structure with layer thickness of ∼43 Å. Upon heating at T₃, the crystal structure became less ordered (but somewhat more ordered than smectic A (S_A) and smectic C (S_C)). This was followed by S_A (or S_C) phase at T₂, and ultimately an isotropic state (I) at T₁. The observed layer thickness (∼43 Å) is about ∼1.5 times the most extended side chain length, indicating a double-layer structure with tilted or interdigitated side chains. The X-ray fiber pattern had a four-point pattern at d=4.52 Å, suggesting that the side chains in the crystal are likely to be tilted by 56° from the polymer fiber axis.     

Hydrothermal Synthesis of Novel Smectite-Like Mesoporous Materials

Hydrothermal reaction of slurries made from Si-Mg hydrous oxide and water yielded trioctahedral smectite-like mesoporous materials (SMMs). The OH anion had a marked effect on the properties of SMMs under hydrothermal condition whereas Na cation did not influence SMMs properties. With the increase in synthesis pH at 200°C, pore volume, specific surface area and average pore diameter decreased whereas methylene blue adsorption increased in the pH range of 8.5 to 11.4. Therefore, the SMMs having different layer charges could be synthesized. Both micropore and macropore formation rates decreased with increasing synthesis temperature in the range of 125–200°C and the formation of latter increased over 200°C. As a result, the SMMs synthesized at 200°C from slurries of pH 9.0–9.3 revealed the maximum mesopore formation rate: 0.98 and gave a narrow pore size distribution curve of 3–5 nm. The SMMs were characterized by specific surface areas of 243–679 m² g^-1, pore volumes of 0.20–0.48 cm³ g^-1, average pore diameters of 2.2–5.4 nm and methylene blue adsorption values of 0.16–0.96 meq · g^-1.     

垂直管中的液泛

本文较系统地研究了垂直管中的液泛现象,从液泛机理出发,明确定义两种不同的液泛——爬膜液泛和架桥液泛。并侧重研究了管子直径、管口几何形状对液泛的影响。提出平切口管,斜切口管的液泛关联式。指出提高液泛速度的途径。通过实验对所提论点及关联式进行了验证。     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

Formation of gold nanoparticles on multiwalled carbon nanotubes by thermal evaporation

Multiwalled carbon nanotubes (MWCNTs) were grown on W substrates by chemical vapor deposition and modified with Au nanoparticles by thermal evaporation. The resulting hybrid structures were investigated by TEM to determine the effects of evaporation rate, nominal film thickness, and substrate temperature on the nanoparticle size and distribution. The results demonstrate that as-grown MWCNTs can be used as a support for well distributed Au nanoparticles, with the size and distribution on the carbon nanotubes being primarily influenced by the nominal film thickness. The observed structures ranged from small 4 nm diameter spherical particles to 150 nm long wire-like structures. Depositions with substrates at 25 °C and 400 °C resulted in similar particle structures, except for the highest amount of deposited Au.     

The effect of powder sintering on the palladium-catalyzed formation of carbon nanofibers from ethylene-oxygen mixtures

Carbon nanofiber growth on palladium particles from ethylene-oxygen mixtures was investigated with respect to thermal history. Electron microscopy, combined with focused ion beam cross-sectioning show particles sinter quickly, but can be stabilized by the addition of a short carbon deposition step at a temperature below the general reaction temperature. This step generates a thin layer of carbon on the catalyst which reduces sintering once the temperature is raised to the optimal reaction temperature. For example, high temperature (e.g. 500 °C) catalyst pre-treatment leads to catalyst particle sintering, and subsequent fiber growth produces large diameter fibers. In contrast, small diameter fibers form on catalyst particles pretreated at low temperature (ca. 350 °C), even if the fibers are grown at a temperature at which deposition rates are faster (e.g. 550 °C). These results led to the development of unique multiple temperature fiber growth protocols that produce smaller diameter fibers while improving the deposition rate.     

倾角及充液率对并联式脉动热管传热性能的影响

以超纯水为工质,在不同的充液率（35%、50%、70%）和倾角（60°、90°）下,对并联式脉动热管的传热特性进行实验研究,分析在不同加热功率下充液率和倾角对并联式脉动热管壁面温度、加热端与冷凝端温差、传热量及传热热阻的影响,并对传热热阻进行不确定度计算。实验结果表明,充液率和倾角对脉动热管的传热特性影响显著。倾角为90°时,3种充液率条件下,充液率为35%和50%时的壁面温度脉动稳定性要优于充液率为70%的情况,并且充液率为50%的传热热阻最小,加热端与冷凝端温差最小,充液率为70%的传热热阻最大,加热端与冷凝端温差最大。充液率为50%,倾角为60°和90°时,在相同的加热功率下,倾角为60°的壁面温度的脉动幅度较大,传热热阻较高,传热极限变窄,传热效果明显差于倾角为90°的情况。     

Hydrocracking of petroleum gas oil over NiW/MCM-48-USY composite catalyst

MCM-48-USY composite materials were prepared by coating USY zeolite by a layer of MCM-48 mesoporous material at different meso/microporous ratios (SiO₂/USY ratios of 0.1, 0.2, 0.3, 0.4, 0.5) and used as support for nickel and tungsten. The NiW/MCM-48-USY catalysts were prepared using the incipient wetness method. The prepared catalysts were characterized by TPD-TGA acidity, TGA thermal stability, BET surface area, pore volume, pore size, XRD, SEM and TEM and then tested for hydrocracking of petroleum gas oil at reaction temperature of 450 °C, contact time of 90 min and catalyst to gas oil ratio of 0.04. In all prepared samples, the catalyst activity and properties were improved with increasing SiO₂/USY ratio and found that maximum values of a total conversion and liquid product (total distillate fuels) were obtained at SiO₂/USY ratio of 0.5. Finally, the obtained results from hydrocracking of gas oil over composite MCM-48-USY catalysts were compared with those obtained over physically mixed USY and MCM-48 catalysts.