The Study On The Characteristics Of Air Bubble Growth And Departure At The Orifice Of Capillary Tubes

Air bubble dynamics is a branch discipline which gradually developed in the modern times. It mainly studies the rule and the condition of the air bubble growth and departure process. From studying single air bubble formation, growth and departure process to obtain the heat transfer, mass transfer processes and the mechanism of the two-phase flows in the process of air bubble movement. The main parameters used in the mathematic model to describe the air bubble movement are the bubble departure diameter and the period of bubble detachment. The air bubble dynamics research has the vital significance for studying the heat transfer and mass transfer mechanism of the two-phase flows in the porous material. The bubble growth and departure at the tip of capillary tubes under different condition was experimentally investigated in this thesis by using the high-speed visual system. The mainly works and conclusions would be described as follows:(1) In the static liquid environment, the effects of the capillary diameter, the hydrophilic characteristic of the capillary , the gas flux, the air chamber volume and the slope angle of capillary on the air bubble growth and detachment were investigated through the visible experimental study. A sphere model according to the air bubble dynamic theory was introduced, and the variational curve of the contact angle was calculated when the air bubble growth in the fast area.The visual experiments showed that the bubble growth experienced three typical stages: the gradual growth stage, the speedup growth stage, and the speed-down growth stage accompanied by the corresponding variation of contact angle. In the air bubble growth initial period, the contact angle slowly reduces, after enters the fast growth stage, the contact angle rapidly reduces, afterwards the air bubble starts to be departed from the capillary tube orifice, the contact angle rapidly increases until the air bubble is break away. According to the conservation of mass, ideal-gas state equation and laplace equation, studied the variety rule of the contact angle when the air bubble was in the fast growth stage. The calculated result showed that the contact angle rapidly minished with the bubble growth.The bubble departure diameter was increased and the period of bubble growth and departure was decreased with an increase in the internal diameter of capillary tube. A larger gas flow rate led to a larger bubble departure diameter and a smaller period of bubble growth and departure. The volume of gas injection chamber has almost no influence on the bubble departure diameter while the period of bubble growth and departure increased with the increase of the volume of gas injection chamber.The result data of the experiment, which studied the effect of the different soakage property on the air bubble growth and departure on the tip of capillary tube, showed that the processes of the air bubble growth and departure on the hydrophilic capillary and hydrophobic capillary were different. When the air bubble growed up on the hydrophobic capillary orifice, the interface between the gas phase, liquid phase and solid phase would move outward with the air bubble growth. In addition, the departure volume of the air bubble departed from the hydrophobic capillary was less than the the departure volume of the air bubble departed from the hydrophilic capillary.The experiment which investigated the effect of the glass capillary slope angle on the air bubble movement showed that, the capillary incline would induce the air bubble does not grow up symmetrical on the tip of glass capillary with the action of the buoyancy. With the accretion of the capillary slope angle, the air bubble departure volume and diameter would be reduced.Moreover, along with the increase of the capillary slope angle, after the air bubble is separated from the capillary orifice, the refluent liquid into the capacity would be reduced.(2) Under the flowing liquid condition, a series of visual experiments were done to investigate the process of air bubble growth and departure on the capillary with different soakage property, and study the effcet of the velocity of liquid flow on the air bubble movement under the dynamic environment.The experimental result indicated that, the departure period and diameter of the air bubble formed on different material capillary would be reduced with the increasing velocity of the liquid flow. Otherwise, under the dynamic environment, the effect of the different soakage property on the air bubble growth and departure was relatively diminished. So it is known that the transverse force brings with the liquid movement was a important influence on the air bubble growth and departure in the dynamic environment.The experiments, which were taked place in the same expreimental condition with different capillary material, were done to study the air bubble growth and departure movement. The experiment result showed that with the increasing velocity of the liquid flow, the departure period of air bubble would reduce both on hydrophilic capillary and hydrophobic capillary. In addition, with the increasing velocity of liquid flow, the reduce extent of the bubble departure diameter on the hydrophobic capillary was more big than the reduce extent of the bubble departure diameter on the hydrophilic capillary.(3) With the varying gas flux condtion, the air bubbles movement and the interaction between the bubbles formed on the tip of two capillary tubes joined with a gas chamber under was studied by using the high-speed visual system. The bubbles growth and departure at the open tip of two capillary tubes joined with a gas chamber was investigated by using a high-speed video imaging system.The experimental results showed that the capillary tube with larger size and hydrophilic property acted as the liquid flow channel while the one with smaller size and hydrophobic property as the gas flow channel when liquid submerged the tip of the two capillary tubes without internal gas flow. In contrast, the larger capillary tube became a gas flow channel and the smaller one a liquid flow channel when liquid submerged the tip of the two capillary tubes with internal gas flow. The bubble departure period significantly decreased and the bubble departure diameter kept in gentle increase with the increase of the flow rate of gas injected into the gas chamber. Furthermore, the liquid flow rate in test section has a significant effect on the bubble growth and departure. A larger liquid flow rate led to a higher bubble departure frequency and a larger bubble departure diameter. At a larger liquid flow rate, the bubble growth and departure at the tip of the upstream capillary became faster, which resulted in the liquid return at the tip of the downstream capillary tube.