| The gas-liquid flow has been commonly found in daily life and industrial applications,in which behavior of the gas-liquid interface and the interaction between gas and liquid phases are of great significance for both academia and industry.To deepen the understanding of the transport of the gas-liquid mixture and the multiphase flow mechanism,geometric characteristics and dynamic behavior of bubbles are expected to be further plumbed.Under the financial support of National Natural Science Foundation of China(No.51676087),experimental and numerical studies were carried out in the presented thesis.The research covers several aspects,namely characteristics of a single bubble rising in quiescent fluid,characteristics of a single bubble passing through the liquid-liquid interface,characteristics of the bubbles generated through submerged gas injection in co-current flow,and the distribution characteristics of the bubbles produced through injecting gas into the wake of a submerged cylinder.A comprehensive discussion was performed.Furthermore,the deep learning method was employed to identify the flow regimes of the gasliquid flow in a horizontal pipe.Main work and conclusions obtained are as follows:For rising characteristics of a single bubble in quiescent liquid,the rising processes of bubbles of various initial sizes in deionized water and white oil were observed using the high-speed photography.Variations of bubble parameters in the two liquids were compared and analyzed through numerical simulation.The results show that bubble deformation and lateral bubble motion are suppressed by the high viscosity liquid.Bubble motion in white oil is highly ordered,which is featured by spherical or elliptical shape and nearly straight ascending trajectory.Bubble shapes in deionized water are diversified and the ascending trajectory exhibits zigzag pattern,which is related to the vortex structure in the bubble wake.Small bubbles are insensitive to the viscous force and the surface tension.The modified correlation between the bubble aspect ratio and the E?tv?s number(Eo)is proposed,which is applicable to the bubble in quiescent liquid with high viscosity.A modification of the correlation between the drag coefficient and Reynolds number(Re)proposed by Turton et al.was implemented,and a higher applicability to bubbles in liquids with different viscosities was proved.Regarding the characteristics of a single bubble passing through the liquid-liquid interface,the development of the bubble involved in the water-oil system was observed.The equivalent diameter and the rising velocity of the bubble were calculated and compared with those in a single liquid.Distinct phenomena associated with the bubble passing through the water-oil interface were described.Geometric characteristics and dynamic behavior of the bubble of different sizes near the water-oil interface were compared.It is demonstrated that the rising process of a single bubble in the water-oil system can be divided into three stages according to the distance between the bubble and the water-oil interface.When the bubble is far away from the interface,bubble geometry and behavior are nearly unaffected.As the bubble is released near the interface,the development of the rising velocity is remarkably limited,and the bubble shape is regular.In addition,when the bubble passes through the interface,a layer of water film wrapping the bubble is gradually formed.In the collapse and falling off process of the water film,an upward waterjet is generated.Both the time of water film collapse and the aggressivity of the waterjet depends on bubble size.For characteristics of the bubbles produced through injecting gas into co-current flow,a nozzle with an outlet diameter of 3.8 mm was used for gas injection upstream of the test section of the horizontal water tunnel.Flow patterns were observed at different gas-liquid velocity ratios.Instantaneous flow images were captured using the high-speed camera.The streamwise length and the pinch-off frequency of the upstream gas cavity were obtained through image processing.The results show that the gas-liquid flow pattern varies considerably with the gas-liquid velocity ratio.When the gas-liquid velocity ratio is extremely low,typical attached gas cavity cannot take shape at the nozzle outlet.When the gas-liquid velocity is relatively low,the upstream gas cavity will not pinch off.When the gas-liquid velocity is relatively high,the coalescence phenomenon between air cavities and bubbles may occur,and the coalescence phenomenon is more distinct when the liquid velocity is lower.When the gas-liquid velocity ratio remains constant,the pinchoff frequency of the upstream gas cavity increases with increasing liquid velocity.The higher the pinch-off frequency,the higher the instability of the gas-liquid interface and the more irregular the gas cavity shape in the flow.When the liquid velocity is kept constant,the pinch-off frequency of the upstream gas cavity decreases firstly and then increases with the gas injection flow rate.With respect to the distribution characteristics of the bubbles generated through injecting gas into the wake of the circular cylinder,several gas injection holes were set upstream of the test section of the water tunnel,and a cylinder was installed downstream of the holes.The particle image velocimetry(PIV)was used to measure the carrier flow field under the condition of no gas injection,and instantaneous bubble images under the condition of gas injection were captured using the shadow imaging technology.The equivalent diameter and velocity distribution of bubbles in the wake were obtained through image processing,and compared with corresponding parameters in the carrier flow field.The results show that under the condition of no gas injection,the time-averaged wake field has symmetrical vorticity and velocity distributions,and streamwise liquid velocity near the wake centerline is lower than that at the wake edge.Under the condition of gas injection,there is a region with low bubble velocity near the centerline of the wake.As Reynolds number remains constant,the longitudinal range of such a region extends with increasing gas injection flow rate.On the same longitudinal section,the equivalent diameter of bubbles in this region is relatively small.The overall distribution of bubble velocity is consistent with that of the liquid velocity in the carrier flow.At the same position,the bubble velocity is slightly lower than that of the carrier flow.The fluctuation amplitude of the bubble velocity is positively correlated with the gas injection flow rate and Reynolds number.When the gas flow rate is certain,the gas void fraction of the data acquisition area increases with increasing Reynolds number and gas injection flow rate.Flow regime identification of the gas-liquid two-phase flow in the horizontal pipe was implemented.Instantaneous gas-liquid structures were divided into seven sub-regimes based on an observation of flow patterns under different operating conditions.Velocity of gas-liquid twophase flow was obtained using the ultrasonic Doppler velocimetry,and the data was converted into image signals.Through a simplified convolutional neural network(CNN)model,a real-time identification model of the sub-regimes was established,which can be identified once every 0.096 seconds.The raw data and denoised data were separately used as input data to construct the identification model,and the identification accuracies were compared.The results show that the denoised data can significantly improve the identification accuracy of the model.After testing,the overall identification accuracy of the denoised model attains 96.5% for the dataset under the trained condition,and 92.7% for the dataset under the untrained condition.Moreover,based on the data obtained using the ultrasonic Doppler velocimetry and the deep learning model,a new method of constructing the flow regime identification model was proposed. |
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