Influence of cavitation phenomenon on primary break-up and spray behavior at stationary conditions

In this paper, a special technique for visualizing the first 1.5 mm of the spray has been applied to examine the link between cavitation phenomenon inside the nozzle and spray behavior in the near-nozzle field. For this purpose, a Diesel axi-symmetric nozzle has been analyzed. Firstly, the nozzle has been geometrically and hydraulically characterized. Mass flow measurements at stationary conditions have allowed the detection of the pressure conditions for mass flow choking, usually related with cavitation inception in the literature. Nevertheless, with the objective to get a deeper knowledge of cavitation phenomenon, near-nozzle field visualization technique has been used to detect cavitation bubbles injected in a chamber pressurized with liquid fuel. Using backlight illumination, the differences in terms of density and refractive index have allowed the distinction between vapour and liquid fuel phases. From these visualization results, two important conclusions can be established: on the one hand, cavitation bubbles have been detected at the nozzle exit for pressure drop conditions at which mass flow was not choked yet. On the other hand, it could be seen that the jet formed by cavitation bubbles spread as pressure drop conditions became stronger. Finally, spray visualization in a nitrogen pressurized chamber has been developed at stationary conditions. In order to analyze cavitation influence on spray characteristics, pressure drop has been modified near the values at which cavitation bubbles have been detected out of the nozzle. Two different test strategies have been used for this purpose: fixing injection pressure, which implied a change in chamber density for each test point, or fixing chamber pressure. Both kinds of measurements revealed a noticeable increment of spray cone angle and spray contour irregularities related with the presence of cavitation bubbles at the orifice outlet. This fact can be assumed as an indicator of atomization improvement induced by the collapse of cavitation bubbles at the nozzle exit.     

Mass transfer characteristics of liquid films flowing down a vertical wire in a counter current gas flow

The wetted-wire packing, mainly consisting of a bundle of vertical parallel wires, is a promising concept for the use in separation columns. To investigate the multiphase flow inside the packing in detail and to estimate the performance of the packing, experiments on liquid films on a single vertical wire in a counter current gas flow were carried out. To get information about the interfacial area, an optical measurement of the film thickness was carried out with a digital high speed camera and image recognition tools. By measuring the evaporation of water and aqueous polyvinylpyrrolidone solutions into air, the gas-side mass transfer was determined. The liquid-side mass transfer was examined by measuring the desorption of CO₂ from water into air. The results show that the mass transfer coefficients are comparable to those appearing in common structured packings. When assuming a sufficiently high wire packing density, a specific interfacial area similar to corrugated sheet structured packings can be reached. Previous studies predicted a low pressure drop per packing height and extended capacity limits compared to common packings. In consideration of these results, the wetted wire packing therefore is shown to be suitable especially for absorption processes where a low pressure drop is favourable.     

Two-phase flow and mass transfer in scrubbers

Experimental investigations were carried out in spray scrubbers of different sizes with cocurrent flow of gas and liquid. Of special interest were the local processes in the mass transfer zone. The scrubber was operated with warm water/air system (cooling tower) to obtain detailed information about mass transfer. Air is being humidified with water vapour, which in turn leads to a temperature drop in the liquid. The liquid temperatures are relatively easy to measure and are shown as liquid isotherms. In the case of plug flow, the liquid isotherms should be straight horizontal lines. In reality, significant deviations from plug flow are caused by the transfer of liquid to the walls. A large part of the liquid forms a film flow at the wall. Furthermore, nearly all the mass transfer is completed in the zone of liquid atomization immediately beneath the nozzle. The number of measured transfer units was between 0.5 and 2.0 and was significantly influenced by the liquid flow rate. Based on the improved knowledge of the proceses inside the scrubber, a simplified model has been developed. Since the model simulates all the essential processes inside the mass transfer zone, liquid distribution and mass transfer efficiency can be approximately predicted.     

Pressure drop and bubble-liquid interfacial shear stress in a modified gas non-Newtonian liquid downflow bubble column

A functional form of equation for predicting pressure drop in a modified non-Newtonian downflow bubble column has been formulated. The equation has been developed based on the bubble formation, drag at interface and the wettability effect of the liquid. Also the bubble-liquid interfacial shear stress in two-phase flow is analyzed and correlated with the dynamic, geometric and physical variables. The functional form of equation appears to predict the pressure drop satisfactorily for two-phase dispersed flow in the co-current modified downflow bubble column with carboxy methyl cellulose (CMC) solution in water with different concentrations.     

Experimental study of liquid distribution in a column with a structured packing

Reflective fiber optic sensors are used to measure the thickness of a liquid film on geometrically complex surfaces inside a distillation column with a structured packing and countercurrent flow of gas and liquid. Distributions of the liquid-film thickness as a function of flow-rate parameters for an elementary region (cell) of a geometrically repeated pattern in a structured packing consisted of Koch 1Y corrugated plates and for the column cross section are presented. It is shown that the flow conditions characterized by the detachment of liquid films from the plate by air flow (until the complete column “flooding”) is accompanied by a sharp increase in the pressure drop in the column. Inside the cell, the liquid forms menisci in the zone of plate contact points. The countercurrent air flow reduces the fluctuations of the film thickness by 1.5 to 2 times on the edge and side downslopes of the corrugation.     

Mass transfer and pressure drop in a cocurrent reciprocating plate extraction column

A 5-cm-diameter reciprocating plate extraction column has been operated in cocurrent flow. The pressure drop for water flow, and the local and average mass transfer products for the system acetic acid/kerosene/aqueous sodium hydroxide have been measured. Under well-agitated conditions, the mass average transfer product is predictable by a model, which is also applicable to data reported earlier by Karr for a 2.54-cm-diameter column.     

PRESSURE DROP AND MASS TRANSFER COEFFICIENTS IN CONCURRENT SCREEN PLATE BUBBLE COLUMN

Pressure drop and oxygen desorption from water and four other aqueous CMC solutions were determined in a 5 cm diameter multistage bubble column with and without plate reciprocation. The plates were made from stainless steel wire screens of porosities greater than 0.62. The column pressure loss was found to increase with plate agitation, phase velocities and screen mesh number. A modified Reynolds number was proposed to permit a reasonable prediction of the pressure loss, based on the model of Noh and Baird (1984). At a specific power consumption, the present volumetric mass transfer coefficients arc considerably larger than those reported in earlier studies with sieve plates. The coefficients were correlated with specific power input, phase flow velocities and system physical properties.     

液相介质对水平气液间歇流动的压降影响

研究了水平管内不同液相介质(水、油和不同浓度的CMC溶液)对气液两相间歇流动压降的影响. 实验管道为内径50 mm的透明有机玻璃管,从入口到分离器长约30 m,实验段由2个长3 m的水平管组成. 共记录了320组不同表观流速下的压降信号：油相0.17~1.85 m/s,水相0.17~2.48 m/s,CMC溶液0.17~1.42 m/s,气相0.06~3.40 m/s. 结果表明,液相为牛顿流体(油或水)的气液流动,随着表观气相流速的增大,压降呈增加趋势;非牛顿幂率流体(不同浓度的CMC溶液)的管道流动,当流动指数低于一定值时,压降随气相流量的增加呈降低趋势,并且低于单液相流动的压降. Lockhart-Martinelli模型过高地预测了气-非牛顿幂率流体两相的压降.     

Experimental investigation and model improvement on the atomization performance of single-hole Y-jet nozzle with high liquid flow rate

Y-jet nozzle, as an efficient multi-hole internal-mixing twin-fluid atomizer, has been widely used for liquid fuel spray in many industrial processes. However, single-hole Y-jet nozzle with high liquid flow rate is indispensable in some confined situations due to a small spray cone angle. In this paper, the atomization performance of single-hole Y-jet nozzles with high liquid mass flow rates ranging from 400 to 1500 kg/h for practical semidry flue gas desulfurization processes was investigated by the laser particle size analyzer, and the effects of spray water pressure, atomizing air pressure and air to liquid mass flow ratio on the liquid mass flow rate and the droplet size distribution were analyzed. Moreover, the secondary atomization model was modified on the basis of previous random atomization model of Y-jet nozzle. The predicted results agreed well with the experimental ones, and the improved atomization model of Y-jet nozzle was well validated to design the nozzle geometry and to predict the droplet size distributions for single-hole Y-jet nozzle with high liquid mass flow rate.     

Studies on frictional pressure drop of gas-non-Newtonian two-phase flow in a cocurrent downflow bubble column

An exclusive study has been done on experimental investigation of the two-phase frictional pressure drop with air-non-Newtonian liquid (CMC solutions) system in cocurrent downflow bubble column. The effects of gas and liquid flowrate on two-phase frictional pressure drop have been illustrated. An attempt has been made to fit the experimental two-phase frictional pressure drop data by modified Lockhart and Martinelli correlation and Aoki correlation. In another approach, friction factor method was adopted to correlate the experimental results in terms of dimensionless groups of the operating and system variables and the predicted values were found to be in good agreement with the experimental result. The experiments were performed in the bubbly flow regime because of its stability and uniformity.     

Prediction of Two‐Phase Frictional Pressure Drop for the Concurrent Gas and Newtonian Liquids Upflow through Packed Beds

Correlations were developed to predict frictional pressure drop for concurrent gas‐liquid upflow through packed beds covering all the three identified flow regimes, i.e. bubble flow, pulse flow and spray flow. The observation that the gas and liquid flow rates have different influences on the two‐phase pressure drop in different flow regimes, was taken into consideration in the development of these correlations. More than 600 experimental pressure drop data from the present study and literature covering a wide range in gas‐liquid systems, flow rates and column packing were used.     

Experimental investigation on flue gas purification and gas-liquid multiphase flow in a spraying column with perforated plate

The synergetic removal of sulphur dioxide and particulate matters (PM) in flue gas was investigated in a spraying column equipped with flow pattern controlling (FPC) internals. The performance of the novel column was compared with that of the traditional open column by monitoring the removal efficiency and system pressure drop. To understand the gas-liquid multiphase flow field and mass transfer process inside the column with different structures, image processing based on the Otsu method and particle image velocimetry (PIV) technology were used. It was found that the FPC column with a section size of 200 mm × 200 mm, hole diameter of 4 mm, and aperture ratio of 0.65 demonstrated the best performance. As clearly observed from the flow filed, bubble flow was formed in FPC column, while the dispersive and falling-film flow was developed in the open column. The liquid holdup in FPC unit was much more than in the open spraying column, which enhanced the mass transfer process. According to the PIV results, the reverse flow of the liquid, caused by the gas jet ascribed to the perforated plate and the confined space in the FPC unit, was the primary reason for bubble generation and the gas-liquid flow pattern in the column.     

导向筛板流体力学性能的研究

对导向筛板的流体力学性能进行了实验研究,实验是以空气和水为介质,在矩形塔中进行的。为了研究表面张力和液体重度对塔板流体力学性能的影响,在部分实验中加入了表面活性剂,而在另一部分实验中以饱和食盐水代替清水。 文中详细报导了导向筛板的干板压降,液层压降,雾沫夹带以及漏液点的实验结果,并提出了可供设计应用的关联式及各种图表。由于在工业设计和生产核算中获得了满意的结果,其可靠性得到了验证。 在广泛的实验基础上标绘了导向筛板的充气系数β对于孔动能因子F_o和计算清液深度h_L的关联图。 必须强调指出,基于空气-水系统的β图在用于低表面张力系统的计算时将会产生显著误差。为此,特别提出了对充气系数β的校正式:该式亦可用于普通筛板及其它类型塔板。 鉴于各种有关报导中所取漏液点的基准不一,本文根据漏液对效率的影响和漏液量随气速而变化的规律综合考虑,认为确定相对漏液量q=10%为漏液点是适宜的。 文中还报导了清液高度和泡沫高度的实验结果和关联式。     

Application of an Air‐Operated Impinging Streams Contactor in Liquid‐Liquid Extraction

An experimental study was conducted on the liquid‐liquid extraction of isobutyric acid by means of cumene from its aqueous solutions in an air‐operated impinging streams contactor (AOISC) with spray nozzles. Aqueous and organic streams may or may not contact one another inside the spray nozzles depending on the modes of operation. The overall volumetric mass transfer coefficient, K_La, determined enabled us to evaluate the performance capability of the contactor. The effects of air and solution flow rates, contactor length and diameter, impingement zone as well as modes of operation have been investigated. These experimental results verify the capability of the method of impinging streams in liquid‐liquid extraction.     

Optimization of the Heat Transfer Rate and Pressure Drop of Inside Profiled Tubes – Part 1

Nine inside profiled tubes were developed and investigated for optimization of the heat transfer rate and pressure drop behavior. The results of this work are presented in two parts. This part describes the comparative investigation of five tubes with different inside profiles to simulate the heat transfer and friction loss of fired tubular heaters in petrochemical processes. In part 2, a further four tubes with different inside profiles will be compared. To test the efficiency of the new profiles a test rig was modified. Using the similarity laws by Reynolds, the Reynolds number calculated for the gas flow in the heater tubes was converted into the flow rate, as well as the pressure and temperature of a distilled water system. Axial and peripheral velocities were measured using a Laser‐Doppler‐Velocimeter (LDV). Friction pressure drop and heat transfer were measured to determine the efficiency of each tested tube under the constant conditions of the distilled water system. The results of the investigations on these five inside profiles showed that profiles with eight flat and symmetrically distributed straight fins (tube III) or with a twist angle of 30° to the tube axis (tube IV), produced heat transfer rates higher that that of the bare tube by 120 % and 156 %, respectively, with increases in pressure drop only 46 % and 76 %, respectively.     

Hydrodynamic Characteristics in the Counter‐Flow Total Spray Tray

The capacity of a column is limited by the distribution of gas and liquid, especially in case of large‐diameter cross‐flow trays. In order to solve the bottleneck, a new counter‐flow tray named total spray tray (TST) was put forward. Taking air/water as medium, the hydrodynamic behavior, including the pressure drop, weeping, entrainment, and clear liquid height, was investigated, in comparison with the CTST, which had a higher capacity. Based on experimental data, the correlations of the TST pressure drop were established by regression analysis method. The experimental results show that the TST has a lower wet pressure drop and less weeping. More interestingly, its clear liquid height can be self‐adjusting with the variation of the gas kinetic energy factor, which is beneficial to improving the capacity.     

Behavior of an organic solvent drop during the supercritical extraction of emulsions

The behavior of a drop of dichloromethane in water in contact with CO₂ at high pressure has been investigated with the purpose of analyzing the phenomena that takes place during the supercritical fluid extraction of emulsions process. Experiments have been performed with and without a solute (β‐carotene) and a surfactant (n‐octenylsuccinic anhydride‐modified starch) dissolved in the drop, and the evolution of the drop volume as well as of the interfacial tension between the drop and the aqueous phase has been measured. Additionally, a mathematical model has been developed that allows describing the mass transfer. Results show that the drop undergoes swelling and shrinking processes due to diffusion of CO₂ into the drop and dichloromethane out of the drop. CO₂ concentration in the drop can be as high as 0.9 (molar fraction). Emulsion drops behave as miniature gas antisolvent precipitators and many particles are formed inside the drop. The interfacial tension between the drop and the aqueous phase increases during the process, therefore destabilizing the emulsion. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

关于双层不锈钢网波填料的流体力学和传质特性的探讨

网波填料是一种新型、高效的规则填料。使用双层不锈钢网具有良好的可润湿性和整个填料表面均匀的液体分布,因而即使对于像水之类表面张力大的液体,在低的液体负荷下,分离效率也能显著增加。 在内径10cm的塔,填料高度200cm和塔顶压力约125Torr的情况下,进行了D_2O/H_2O体系真空精馏试验。本文使用测得的数据,获得了该填料压力降和传质的良好关联。     

Experimental investigation of maldistribution in vertical plate falling film tower

Falling film towers are popular in process industries due to moderate air side pressure drop, less energy ingesting liquid distribution and easy cleanup features. Two major limitations of falling film towers are the uneven distribution of liquid across solid surfaces (maldistribution) and contraction of falling liquid film on the solid surface (poor wetting). Maldistribution of liquid is primarily dependent on the type of liquid distributor and scope of liquid flow (slit opening) across solid surfaces. In the current communication; the influence of liquid distributor on maldistribution problem for vertical plate falling film tower has been experimentally investigated. Four types of liquid distributors: plain tube, spray nozzle, perforated plate, and branch tube distributor have been tested. Effect of parameters: slit opening, liquid flow rate, and the number of plates on maldistribution are also explored. It is found that branch tube distributor is best suited for vertical plate falling film tower application.     

Influence of liquid viscosity and bed porosity on two‐phase pressure drop in concurrent gas‐liquid upflow through packed beds

It was observed in the experimental investigations that the concurrent upflow of air‐Monoethanol amine system through the packed bed gave higher pressure drop in bubble flow regime than the air‐water system. But when the flow regime changed to spray flow, air‐water system showed higher pressure drop than the other. This phenomenon was observed for the two column packing used in this study. An attempt is made to explain this phenomenon.