水平管内多孔板后的气液两相流型可视化实验

多孔板后是否形成均匀分散的泡状流流型是影响多孔板废气吸收装置吸收效果的关键因素。以空气和水作为两相介质,对气液两相混合物在水平管内流经多孔板后形成的流型进行实验。通过孔径分别为2、3、4、5 mm的4只多孔板在内径98.5 mm水平有机玻璃管内的可视化流动及高速摄像,研究了孔径大小、气相流量变化及液相流量变化对多孔板后流型的影响规律。实验结果表明:水平管内插入多孔板后,分层/塞状流转变边界向液相流量增大方向推移,塞状/泡状流转变边界向液相流量减小方向推移;随气相流量减小或液相流量增大,多孔板后流型趋于形成泡状流;孔径大小对多孔板后流型具有重要影响,减小孔径使塞状/泡状流转变边界移向更大气相流量和更小液相流量,即形成泡状流的两相流量范围增大;随孔径减小,孔板后流型趋于由分层流直接过渡至泡状流,塞状流趋于消失。为保证多孔板吸收装置的良好流型和吸收效果,建议多孔板孔径不大于3 mm。     

起伏振动水平管气液两相流型及转变机理

起伏振动下气液两相流型的准确判定对漂浮核电站参数设计有重要意义。对起伏振动水平管气液两相流型特性进行实验研究,定义了起伏振动新流型并得到了气液相分布变化规律,揭示了管径、振频和振幅对流型转变界限的影响规律,建立了泡状流-间歇流和间歇流/分层流-环状流的转变关系式。结果表明,在起伏振动下水平管内出现泡状流、间歇流、分层流以及环状流四种流型,其中间歇流包括泡-弹间歇和弹-塞间歇两种,且气液相分布随振动位置的变化呈规律性改变。在气相折算速度不变的条件下,管径、振频、振幅增加会使泡状流-间歇流边界向上移动,间歇流-分层流边界向下移动。在液相折算速度一定的条件下,管径、振频、振幅增加会使间歇流/分层流-环状流边界向右移动。考虑了振动的影响,建立适用于低频高幅起伏振动的间歇流向泡状流与环状流转变模型,预测和实验结果的相对误差绝对值的平均值分别为7.62%和12.68%。     

基于流型的文丘里管二相流量测量方法

为了准确测量气液二相流流量,将James流量公式作为典型公式,通过对其测量结果分析,发现二相流动分为2大类,一类为泡状流、塞状流和弥散泡状流,另一类为弹状流和环状流。在流型识别的基础上,得到针对流型的流量测量修正关系式,修正后测量误差的均方根由修正前的0.174减小为0.042。试验结果表明,针对流型的流量测量可以有效提高测量精度,同时二相流波动系数与流量测量精度有关,随着二相流波动系数的增大,流量测量精度下降。     

矩形截面螺旋管内气液两相流型转换数值模拟

采用CLSVOF(coupled level set and volume of fluid method)方法,以空气和水为介质对矩形截面螺旋管内气液两相流动进行数值模拟,气相折算速度U_G为0.1～2.5 m·s^-1,液相折算速度U_L为0.09～4.5 m·s^-1.研究螺旋直径、螺旋升角对流型转换边界的影响,并绘制了不同螺旋直径、不同螺旋升角下的流型图.数值结果表明,与传统VOF方法相比,CLSVOF可以得到更精确的相界面;随着螺旋升角的增加,塞状流向泡状流的转换边界向U_L减小的方向进行,但是幅度很小,塞状流向弹状流的转换边界向U_L减小的方向进行;随着螺旋直径的增加,塞状流向泡状流的转换边界向U_L减小的方向进行,塞状流向弹状流的转换边界向U_L减小、U_G增大的方向进行;与Murai流型图相比,流型转换边界有所差异.     

微通道内伴有化学吸收的气液二相流型

采用高速摄像仪对T型进口的矩形微通道内气液二相流型进行了实验研究,实验物系采用单乙醇胺(MEA)水溶液-N2和单乙醇胺水溶液-CO2。对于无相间传质的单乙醇胺水溶液-N2二相流动过程,观测到了泡状流、弹状流、弹状-环状流和液环流;对于伴有化学吸收的单乙醇胺水溶液-CO2二相流动过程,未观测到泡状流,而观测到弹状-泡状流。在实验范围内,随着深宽比减小,无论是否伴有化学吸收,弹状流区域均减小;对伴有化学吸收的气液二相流,随化学反应速率的增大,流型转换线向右移动。以化学反应速率为控制参数,分别给出流型转换判别式,预测结果与实验数据吻合良好。在弹状-泡状流型中,随着气相表观流速的下降和液相表观流速、深宽比以及化学反应速率的上升,微通道内临界泡状距离减小。     

壁面润湿性对微通道内二氧化碳-水两相流流动及传质性能的影响

在1 mm×1 mm矩形截面下微通道内,以二氧化碳-水为工作流体,研究壁面润湿性和气液表观流速对气-液两相流型和气液传质的影响,并研究了气、液表观流速对弹状流流体力学性质的影响。在亲水微通道中观测到了泡状流、泡状-弹状流、弹状流;在疏水微通道中观测到了非对称弹状流、拉长的非对称弹状流、分层流。实验表明亲水微通道中弹状流区域下气泡长度大体上随气相表观流速的增大而增大,随液相表观流速的增大而减小;液弹长度大体上随气相表观流速的增大而减小,随液相表观流速的增大先增大后减小;液侧体积传质系数k_La均随气、液相表观流速的增大而增大,随通道壁面润湿性的增强而增大。     

高宽比对矩形通道内蒸汽冷凝传热特性及流型影响的研究

通过可视化流型观测及实验测量,研究了单边换热矩形小通道的高宽比对两相流型、平均冷凝传热系数及两相流动阻力的影响,采用高速摄像机拍摄了环状流、环波状流、波状流、弹状流、塞状流及泡状流6种典型流型。研究表明,高宽比为1：2及1：3的通道内出现流型种类较为全面,而在高宽比为1：5通道内基本无弹状流、塞状流及泡状流。1：5通道内环状流所占区域较1：2及1：3通道大,随入口蒸汽质量流速的增加,差值逐步减小。随着通道高宽比的减小,平均冷凝传热系数逐渐增大;不同高宽比通道中平均冷凝传热系数间的差值随入口蒸汽质量流速的增加而减小;蒸汽质量流速足够高时,高宽比对传热系数基本无影响。通道内两相流压降随通道高宽比的减小而增大;随着入口蒸汽质量流速的增加,不同比例通道内的两相流动压降差距逐渐增大。     

微通道内醇胺水溶液吸收CO2的压力降研究

采用高速摄像仪对400μm×400μm T型微通道内N-甲基二乙醇胺(MDEA)与单乙醇胺(MEA)混合醇胺水溶液(含0.2%SDS)吸收CO_2过程的气液两相流压力降进行了实验研究。观测到了泡状流、泡状-弹状流、弹状流、弹状-环状流流型。考察了弹状流型下气液两相流流量、醇胺溶液浓度、气液雷诺数与增强因子对压力降的影响。结果表明:压力降随着气液两相流量、气液雷诺数及增强因子的增大而增大。当Q_G﹥80 mL/h时,压力降随醇胺浓度的增大而减小。以分相流模型为基础,提出了微通道内伴有化学吸收的气液两相流压力降预测模型,平均偏差为8.46%,模型计算值与实验值吻合良好。     

微细通道中R32流动沸腾换热的数值模拟

利用VOF多相流模型对R32在1、2 mm水平光管内流动沸腾换热进行了二维非稳态数值模拟。模拟的工况为：质量流速100 kg·m²·s^-1,热通量12 kW·m^-2,饱和温度15℃。模拟结果显示：2 mm通道内工质的流动沸腾过程依次经历了液相单相流、泡状流、弹状流;1 mm通道内工质的流动沸腾过程依次经历了液相单相流、泡状流、受限泡状流、弹状流。利用模拟所得气相体积分数分布、温度分布,分析了R32管内流动沸腾过程中的基本规律和气泡运动特点,以及管径对流动沸腾换热过程流型的影响。利用数值模拟结果与实验结果进行对比,显示较好的一致性。     

高宽比对矩形通道内蒸汽冷凝传热特性及流型影响的研究

通过可视化流型观测及实验测量,研究了单边换热矩形小通道的高宽比对两相流型、平均冷凝传热系数及两相流动阻力的影响,采用高速摄像机拍摄了环状流、环波状流、波状流、弹状流、塞状流及泡状流6种典型流型。研究表明,高宽比为1:2及1:3的通道内出现流型种类较为全面,而在高宽比为1:5通道内基本无弹状流、塞状流及泡状流。1:5通道内环状流所占区域较1:2及1:3通道大,随入口蒸汽质量流速的增加,差值逐步减小。随着通道高宽比的减小,平均冷凝传热系数逐渐增大;不同高宽比通道中平均冷凝传热系数间的差值随入口蒸汽质量流速的增加而减小;蒸汽质量流速足够高时,高宽比对传热系数基本无影响。通道内两相流压降随通道高宽比的减小而增大;随着入口蒸汽质量流速的增加,不同比例通道内的两相流动压降差距逐渐增大。     

多孔板压降特性实验与关联式比较

多孔板作为节流元件具有整流、低噪声、低压损及压差更稳定等优点,然而多孔板压降特性的预测方法目前仍不清晰。以常温水为实验介质,在Re数为4×10⁴~1.6×10⁵范围内,对孔数为172~744、等效直径比为0.544~0.666的多孔板进行了压降特性实验研究,并将实验结果与文献中相关的关联式预测结果进行了比较。结果表明：在相同开孔直径比和流动条件下与单孔板相比,多孔板后涡流引起的黏性耗散较小,因而其压损系数明显低于单孔板;计算多孔板压损系数时,若采用等面积折算所得等效直径比按单孔板进行计算,会使得预测结果明显偏大;Holt关联式的压损系数预测结果与实验结果最为相符,表明同时考虑开孔直径比和孔板相对厚度影响的压损系数关联式预测精度较优。建议通过进一步研究建立完善的多孔板压降预测方法。     

孔数与孔厚对多孔板压损系数的影响机理

多孔板作为管道内节流降压或流量测量元件，在减小管道振动、流动噪声及流动压损等方面较之传统的单孔板具有明显优势。影响多孔板压损系数的结构因素主要有开孔等效直径比、孔数及孔板厚度，其中等效直径比的影响规律已为人熟知，而孔数与孔板厚度的影响机理仍不清晰。本文以均匀布孔的多孔板为研究对象，通过数值模拟方法研究了孔数与孔板厚度对多孔板稳定区压损系数的影响机理。结果表明：在相同的孔板厚度下，压损系数随孔数增多而先快速减小后趋于不变，使压损系数达到稳定的临界孔数随管径减小而减少，但临界孔数对应的孔板相对厚度均接近0.5；在相同的孔数下，随孔板相对厚度增大压损系数先快速降低后趋于不变，使压损系数达到稳定的临界相对厚度为0.5~1.0，临界相对厚度随孔数增多而减小；在相同的相对厚度下，压损系数在薄孔板中随孔数增多先减小后增大，而在厚孔板中随孔数增多而单调增大。这说明除等效直径比外，孔数与孔厚均对多孔板压损系数有影响，应在压损系数关联式模型中予以考虑。     

水平与微倾斜管内间歇流中长气泡的形态特征

利用双平行探针技术和摄像方法对水平和近水平微倾斜管内长气泡的形态特征进行了实验研究.实验结果表明气泡头部以及气泡体的形态特征取决于气液混合Froude数和管道倾角,而尾部特征还与气泡长度有关;小气泡通过单纯水跃面的长气泡尾部向液塞区弥散,而具有阶梯状尾部结构的长气泡并不向液塞区弥散小气泡,所以气泡尾部结构特征的变化决定了弹状流向段塞流的转变;管道倾角对长气泡形态特征有显著的影响,下倾管内的长气泡在低Froude数时出现头尾倒置现象,同时下倾管内的长气泡比上倾管更易保持阶梯状的尾部结构,所以下倾管的弹状流区比上倾管宽.     

The effect of multi-orifice plate configuration on bubble detachment volume

The multi-orifice plate gas sparger, mainly composed of a multi-orifice plate and a gas chamber, is one of the most common sparger facilities. The aeration performance of multi-orifice plate has a close relation with the multi-orifice plate configuration. In addition, the weeping phenomenon has a considerable influence on the gas chamber condition which affects the bubble detachment volume directly. This paper conducts a set of visual experiments to study the influence of multi-orifice configuration and gas chamber condition on the aeration performance of gas sparger. For multi-orifice plate, an improved theoretical model is proposed which considers the wave effect of the previous bubbles generated from adjacent orifices and the variance of the number of active bubbling orifice. A parameter is proposed to evaluate the aeration performance in order to overcome the difficulty caused by the randomness of bubble formation process. The experimental results suggest that the gas chamber filled with water is in favor of large bubble formation. The influence of the pitch of orifice on aeration performance can only be observed in high-restricted case. According to the theoretical model and experimental results, the influences of gas flow rate and the number of open orifices on the aeration performance are analyzed and a design criterion for the number of open orifice is proposed.     

Hydrodynamics and liquid phase axial mixing in orifice pipe reactor

Two phase flow in a horizontal pipe, with orifice plates placed at regular intervals as obstructions, was studied for the effect of phase velocities on flow patterns, fractional phase hold-ups, pressure drop and liquid phase axial dispersion. Radioactive technetium-99m (as an aqueous solution of sodium pertechnatate) was used as tracer. A pulse injection technique with two point measurements was employed. Three different orifice diameters were used (8 mm, 16 mm, and 20 mm) in a pipe diameter of 32 mm. The orifice spacing was 500 mm in all cases. Superficial gas (air) velocity was varied over a range from 0.02 m/s to 1.0 m/s and superficial liquid (water) velocity from 0.03 m/s to 0.85 m/s. Different flow patterns under different flow conditions were identified and a generalised flow map is presented. Variations in hold-ups and pressure drop with flow patterns have been explained. Rational correlations have been developed for fractional phase hold-ups and pressure drop. A preliminary comparison of two phase gas-liquid flow in a horizontal pipe with orifice obstructions (to be called orifice pipe reactor), as a gas-liquid contacting device, is made with a conventional bubble column reactor. Recommendations have been made for future work.     

Adiabatic two-phase flow in narrow channels between two flat plates

Adiabatic cocurrent flow of air and water through a narrow passage between two flat plates 240 mm long and 99mm wide with gap-widths of 0.778 mm and 1.465 mm was investigated for six different orientations: Vertically upward and downward, 45^° inclined upward and downward, and horizontal flows between horizontal plates and between vertical plates. Except for horizontal flow between vertical plates, the effects of gap width and flow channel orientation on flow pattern, void fraction and friction pressure drop were found to be small in narrow channels. The void fraction and two-phase friction multiplier data could also be reasonably correlated in terms of the Martinelli parameter. For horizontal flow between vertical plates, both the void fraction and friction multiplier data showed strong mass velocity effects. Several friction pressure drop correlations were tested for applicability to the narrow channels including a separated flow model proposed in this work.     

Characteristics of a countercurrent reciprocating plate bubble column. I. Holdup,pressure drop and bubble diameter

The hydrodynamics of countercurrent air/water flow in a 5 cm diameter reciprocating plate bubble column have been studied; the plates contained 14 mm diameter perforations and had a fractional open area of 0.57. The ranges of superficial velocities of air and water were respectively 0-0.99 cm/s and 0-3.95 cm/s. The stroke was in most cases 4.5 cm and the reciprocation frequency was in the range 0–5 Hz. The pressure drops were measured in the absence of reciprocation for single phase and two phase flow conditions. Pressure fluctuations and time-averaged pressure drops were measured with plate reciprocation under single and two-phase conditions. The results were described in terms of the simple quasi-steady state model; the effective orifice coefficients of the perforations were within the range 0.4 to 0.97 depending on the reciprocation conditions. The Sauter mean diameters of the bubbles decreased with agitation; they were about twice the values predicted from an earlier correlation developed for liquid-liquid systems. The gas holdups were also substantially greater than predicted from correlations based on liquid-liquid systems. Both these effects were explained as due to the tendency for bubbles to cluster in the plate region.     

Hydrodynamics and droplet size distribution of liquid–liquid flow in a packed bed reactor with orifice plates

A packed bed reactor with orifice plates (PBR@OP) was designed by adding orifice plates periodically in packed beds. Hydrodynamics and droplet size distribution in PBR@OP were experimentally investigated using fatty acid methyl esters (FAME)/water as the model liquid–liquid system. In PBR@OP, the flow pattern was close to plug flow. Droplets with Sauter mean diameter (d₃₂) of 150–550 μm were generated. The pressure drop of orifice, flow velocity and plate spacing were key parameters to control the droplet size. The reactor performance was evaluated by analyzing a FAME epoxidation process. At the same d₃₂ and residence time, the length and total pressure drop of PBR@OP were about 1/3 and 1/4 of those of PBR without orifice plates, respectively. Furthermore, a semi-empirical correlation describing the d₃₂ change in PBR@OP was developed, revealing a relative mean deviation of 8.64%. PBR@OP presents a cost-effective option for the intensification of liquid–liquid medium rate reactions.     

The weeping characteristic of submerged multi-orifice plate

Weeping is an adverse phenomenon which results in higher pressure drop and poorer aeration performance. Visual experiments have been conducted to study the mechanism by which weeping impairs the work performance of multi-orifice plate. A theoretical model is improved to analyze the weeping phenomenon of multiorifice plate based on potential flow theory. The relations of different bubbling conditions and weeping rate are analyzed. Weeping condition and average weeping rate have relation with the driven pressure differential and dynamic variation of gas chamber pressure. In addition, a set of experiments are designed to study the influence of various factors on weeping rate. The bubble coalescence during bubble formation is a fatal factor determining weeping rate, so the relation between weeping rate and gas flow rate is concerned with the pitch of orifices and orifice diameter. There is a critical plate thickness which is in favor of weeping.     

Characteristics of a countercurrent reciprocating plate bubble column. II. Axial mixing and mass transfer

Oxygen absorption from air into water and axial dispersion in the aqueous phase have been measured in a 5 cm diameter reciprocating plate bubble column. The volumetric mass transfer coefficients in semi-batch conditions were found to increase with agitation and were correlated with the specific power input and air flow rate. Under countercurrent conditions, it was found that axial mixing had little effect and conditions approached plug flow. The volumetric mass transfer coefficients were correlated with specific power input, air and water flow rates. Mass transfer coefficients were estimated using holdup and bubble diameter results. Comparison of the coefficients with the literature values indicated that the bubble surfaces were partially mobile.