Capture and recognition of interfacial waves in annular flow based on image analysis technology

Abstract

<p indent=0mm>Annular flow is often encountered in many industrial applications, which features a circumferential liquid film at the pipe wall and a continuous gas core with entrained droplets in the tube center. One of the most remarkable features of annular flow is the distribution of various scales of interfacial waves in wavelength and amplitude on the liquid film. It is generally known that the generation, movement, and breakup of interfacial waves play an essential role in the heat and mass transfer in annular flow. Thus, complete knowledge of the interfacial waves is of great importance for the characterization of mass, momentum and energy exchange process of the unit process and the system. So far, extensive research has been carried out to investigate the characteristics of interfacial waves. There are two major groups of measurement methods for the capture of the interfacial waves, i.e., the contact measurement method and the non-contact measurement method. The conductivity probe is widely used as a typical contact measurement method to investigate the properties of the interfacial waves. However, it is only suitable for local measurement of the conductive liquid film, and the measured liquid film thickness is the average of the thickness of the liquid film around the probe. It is also worth noting that the accuracy of the probe is restricted to the interference to the flow field, the rod-climbing effect of the fluids, the aging of the probe, and the deposition of impurities. The laser-induced fluorescence technology has become one of the most promising non-contact measurement methods due to its high temporal and spatial resolution and fast response speed. However, strict optical environmental conditions, complex and expensive test systems limit its wide application. Therefore, it is important to develop a simple and economical non-contact measurement method to capture and identify interfacial waves. As a common optical measurement method, high-speed photography technology is often employed to analyze the macroscopic characteristics of flow patterns and their transitions. Due to the superimposition, absorption, and separation of the interfacial waves during their movement, it is almost impossible to extract and distinguish waves from the original images. In the present study, a delicate approach for interfacial wave capture and recognition is developed through analyzing the temporal and spatial evolution of liquid film thickness in the annular flow. According to the difference in size and life span, three main types of interfacial waves are recognized as ripples, disturbance wave, and huge wave in gas-liquid two-phase flow. Compared with the laser-induced fluorescence technology, the proposed image processing method is qualitatively demonstrated to have effective identification abilities. Additionally, the velocity and frequency of the disturbance waves are obtained for quantitative verification. Compared with the experimental data in the literature, the proposed wave capture method is proved to implement a high resolution tracking for the interfacial wave at the millisecond time scale and micron level. Although the proposed image processing method offers a relative dearth of detailed information about interfacial waves compared with the laser-induced fluorescence technology, it provides a simple operation and non-contact measurement for interfacial waves in annular flow.