Yuling Lü

Investigation on Transient Oscillation of Droplet Deformation before Conical Breakup under Alternating Current Electric Field.

In this paper, the conical breakup of a water droplet suspended in oil under the alternating current (ac) electric field was experimentally studied with the help of a high-speed video camera. We observed three stages of transient oscillation of deformation characterized by deformation degree l* before the conical breakup that were described in detail. Then a theoretical model was developed to find out the dynamic mechanisms of that behavior. Despite a very small discrepancy, good agreement between model predictions and experimental observations of the evolution of the droplet deformation was observed, and the possible reasons for the discrepancy were discussed as well. Finally, the stresses on the interface were calculated with the theoretical model and their influence on the dynamic behavior before the breakup was obtained. The differences between the droplet breakup mode of ac and direct current electric field are also discussed in our paper.

An investigation into the deformation, movement and coalescence characteristics of water-in-oil droplets in an AC electric field

Drop-drop coalescence is important in electric dehydrators used for oil-water separation in the oil industry. The deformation degree, angle between the electric field and the center line of two drops, effects of intensities and frequencies of the electric field have been studied by analyzing droplet images. However, seldom have people investigated the movement and the relative velocity in the process of drop-drop coalescence. In this paper forces acting on a single droplet and horizontal water droplets in an AC electric field were analyzed, and experiments were carried out to investigate the deformation, movement and coalescence characteristics of droplets with white oil and water. With a micro high-speed camera system and image processing technology, the droplet images were collected and analyzed. The results indicate that the deformation is mainly affected by the electric field intensity, frequency, droplet diameter and the oil viscosity. High field strength and large diameter facilitate deformation of drops in the electric field. The effect of frequency and oil viscosity is not obvious. Higher frequency and higher oil viscosity will lead to smaller oscillation amplitude. The effect of electric field intensity and droplet diameter on oscillation amplitude is not obvious. When the center-to-center distance between droplets is large, the forces acting on droplets in the horizontal direction are mainly dipole-dipole attraction and drag forces. There is also the film-thinning force when droplets get closer. The forces are simplified and derived. Based on force analysis and Newton’s second law, the relative movement is analyzed in different parts, and the relationship of center-to-center distance and time is in accordance with an explinear function at different stages. According to experimental data, the movement of 145 μm double droplets before coalescence can be fitted well with an explinear function at two stages. In addition, the whole movement process is investigated and can be estimated with a fourth order polynomial curve, from which the relative velocity of droplet movement can also be obtained. With an increases in electric field intensity and droplet diameter and a decrease in oil viscosity, the relative velocity increases. Only when the oil-water interfacial tension is obviously high, can it influence the relative movement significantly. The coalescence is mainly dipole coalescence and chain coalescence under influence of the AC electric field.

Influence of separator control on the characteristics of severe slugging flow

Due to the special structure of offshore multiphase pipes, it is easy for severe slugging to occur in the riser at low gas-liquid velocity. Violent pressure fluctuations and dramatic changes of flow rate are the main characteristics of severe slugging, leading to the risk of serious damage. In this paper, the separator control is adopted to accurately control the separator liquid level and pressure under severe slugging flow conditions. This indicates that the separator liquid level control alone does not have a significant impact on the upstream flow, but it is beneficial for normal operation and pressure control of the separator. As the separator pressure increases, the peak pressure in the riser apparently diminishes, and the amplitude of pressure fluctuation gradually decreases, which means that severe slugging is inhibited. During the slug blowing out, the gas/liquid slipping in the riser intensifies. The long gas plug quickly flows through the riser, and then tends to morph into short and slowly flowing gas bubbles. The elimination effect of the pressure control strategy on severe slugging is related to the relative rate of the superficial gas/liquid flow.

Fluctuation characteristics of gas‐liquid two‐phase slug flow in horizontal pipeline

Slug flow was a flow of long bubbles and liquid slugs alternating in space and time, and under flowing conditions, it is intermittent and unstable. The fluctuation characteristics of liquid holdup, pressure and differential pressure of slug flow were investigated in a 40 m long, 50 mm I.D. horizontal pipeline. At the same superficial liquid velocity, the liquid holdup of liquid film both decreased rapidly at first and then decreased slowly with the increase of superficial gas velocity, while the liquid holdup of liquid slug decreased slowly at first and then decreased rapidly. At the same superficial gas velocity, the liquid holdup of liquid slug and liquid film all increased with the increase of superficial liquid velocity. By statistical analysis, it was found that the probability density distribution of the liquid holdup was bimodal distribution. The high liquid holdup peak was in correspondence with liquid holdup of the slug body and the low liquid holdup peak with liquid holdup of the film. The liquid holdup which was in correspondence with the respective peaks of probability density function was consistent with the mean liquid holdup of smooth stratified film and liquid slug. Moreover, the distribution of pressure was unimodal distribution or bimodal distribution, depending on the number of slug units, and the differential pressure distribution was unimodal distribution.Slug flow was a flow of long bubbles and liquid slugs alternating in space and time, and under flowing conditions, it is intermittent and unstable. The fluctuation characteristics of liquid holdup, pressure and differential pressure of slug flow were investigated in a 40 m long, 50 mm I.D. horizontal pipeline. At the same superficial liquid velocity, the liquid holdup of liquid film both decreased rapidly at first and then decreased slowly with the increase of superficial gas velocity, while the liquid holdup of liquid slug decreased slowly at first and then decreased rapidly. At the same superficial gas velocity, the liquid holdup of liquid slug and liquid film all increased with the increase of superficial liquid velocity. By statistical analysis, it was found that the probability density distribution of the liquid holdup was bimodal distribution. The high liquid holdup peak was in correspondence with liquid holdup of the slug body and the low liquid holdup peak with liquid holdup of the film. The liqui...

Experimental study on eliminating severe slugging by choking in pipeline‐riser systems

In order to eliminate severe slugging in pipeline‐riser systems, two methods of top choking and bottom choking have been used in this experimental investigation. Flow patterns and pressure profiles before and after choking have been comparatively analyzed by statistic method. The results indicate that the region of severe slugging is remarkably reduced after choking. When gas and liquid superficial velocities are constant, by using these two methods properly, severe slugging is virtually eliminated and a stable bubble flow is formed. However, if the extent of top choking is not strong enough, this method not only cannot eliminate severe slugging, but leads to a longer period and larger range of pressure fluctuation. If the extent of bottom choking is not strong enough, severe slugging cannot be completely eliminated, but the period becomes shorter and the range becomes smaller. These two methods can both cause obvious increase of pressure in the upstream pipelines, but the backpressure caused by bottom ch...