| Multiphase flow refers to a fluid with two or more different phase states or different components coexisting and having a clear interface.It is widely present in natural and industrial environments.Among them,the gas-liquid two-phase flow occurs most widely.The study of multiphase flow is of great significance to many fields,such as the design and operation of engineering equipment,energy development and utilization,etc.It is very important to study the technology of multiphase flow measurement.And Many techniques are used in the measurement of fluid characteristic parameters.Due to the inherent advantages of optical fibers,optical fibers have received extensive attention in the measurement of gas-liquid two-phase flow.And a variety of optical fiber two-phase flow sensors have been developed.The fiber-optic two-phase flow probe with wedge-shaped tip is simple to manufacture and easy to operate,so it has become more common in recent years.But the existence of controversial special signals has been discovered in practical applications.There are different opinions on whether it is noise or useful signal.Studies have shown that the generation of this signal is related to the light reflected on the bubble surface,and the signal processing algorithm developed using this signal can improve the measurement accuracy.In this thesis,the mechanism of the signal generation and their relationship with the angle of the wedge-shaped tip are systematically studied.In this thesis,9 wedge-shaped plastic optical fiber probes with different angles were fabricated through an improved optical fiber polishing device.And these 9 probes were tested in the bubble flow.Four types of signal curves and three types of spikes were found from the experimental results.A simple signal processing process is performed on the signals obtained in the experiment,and the level values of each probe in the air and water,as well as the number of bubbles and the transit time can be obtained.In order to study the cause of the spike signal,the fiber model and fiber-bubble model were established by using the ray tracing method.The characteristics of the output intensity distribution of wedge-shaped fiber are obtained.The fiber-bubble model with typical signal characteristics was simulated,and the intact signal,the reflected light signal from the fiber end face,and the reflected light signal on the bubble surface(the front and rear surface)were obtained.The reason for the peak in the curve of the reflected light signal on the bubble surface is illustrated,and the relationship with the tip angle is explained.The type of spike in the intact signal is affected by the peak size and peak position of the reflected light signal on the front and rear surfaces of the bubble which is related to the tip angle.Finally,suggestions on the selection of the probe tip angle are given. |
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