Simultaneous measurement of velocity profile and liquid film thickness in horizontal gas–liquid slug flow by using ultrasonic Doppler method

Abstract

Horizontal gas–liquid two-phase flows widely exist in chemical engineering, oil/gas production and other important industrial processes. Slug flow pattern is the main form of horizontal gas–liquid flows and characterized by intermittent motion of film region and slug region. This work aims to develop the ultrasonic Doppler method to realize the simultaneous measurement of the velocity profile and liquid film thickness of slug flow. A single-frequency single-channel transducer is adopted in the design of the field-programmable gate array based ultrasonic Doppler system. A multiple echo repetition technology is used to improve the temporal–spatial resolution for the velocity profile. An experiment of horizontal gas–liquid two-phase flow is implemented in an acrylic pipe with an inner diameter of 20 mm. Considering the aerated characteristics of the liquid slug, slug flow is divided into low-aerated slug flow, high-aerated slug flow and pseudo slug flow. The temporal–spatial velocity distributions of the three kinds of slug flows are reconstructed by using the ultrasonic velocity profile measurement. The evolution characteristics of the average velocity profile in slug flows are investigated. A novel method is proposed to derive the liquid film thickness based on the instantaneous velocity profile. The liquid film thickness can be effectively measured by detecting the position and the size of the bubbles nearly below the elongated gas bubble. Compared with the time of flight method, the film thickness measured by the Doppler system shows a higher accuracy as a bubble layer occurs in the film region. The effect of the gas distribution on the film thickness is uncovered in three kinds of slug flows.