Experimental study on the fundamental parameters of two-phase flow in small channels using confocal chromatic microscopy

• Ano de defesa: 2025
• Autor(a) principal: Santos, Fernando Neves Quintino dos
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: air-water; ar-água; confocal chromatic microscopy; escoamento bifásico; flow pattern; gradiente de perda de pressão; microscopia cromática confocal; padrão de escoamento; pressure drop gradient; two-phase flow
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/18/18164/tde-22072025-133055/

Two-phase flows have several applications, such as in the oil and gas industries, refrigeration and power generation. They are characterized by an arrangement of the phases, called flow patterns, that can be classified as separated or dispersed flows. Separated flows are either intermittent (plug, slug, churn flows), in which long gas bubbles with a thin liquid film below are followed by a liquid slug; or continuous (stratified, wavy, annular flows), where a continuous gas core flows separated from the liquid film. The measurement of the liquid film thickness is important, as it governs heat transfer, pressure drop and instability. Despite its importance, accurate measurement is often difficult, as the film can be very thin. Several reliable measurement techniques exists for large-diameter channels, such as ultrasound pulse echo or conductance probes; however, their applicability is limited in small channels where the film thickness can be in the order of microns. New, non-invasive optical techniques are developed with varying degree of success. This thesis presents a new method called confocal chromatic microscopy, which is a highly accurate, non-invasive, optical technique, often used in life sciences. It measures continuously the thickness of transparent materials and here it was used to measure the flow profile in air-water flows in a squared channel with 8mm internal side. 164 different conditions were evaluated, with the flow pattern for each being classified using a high-speed camera; and the two-phase pressure drop measured. Aside from the major parameters, the 22 conditions with better signal response were evaluated in details, showing flow profile, probability density function of the liquid film thickness, and wave frequency. Results points that the technique is appropriate especially for plug and slug flows, being able to accurately measure the liquid film thickness and wave frequency. Also, there is a strong difference in the shape of the probability density function for each different flow pattern. A new flow pattern map is presented, coherent with works in similar conditions. The pressure drop for each condition was compared with several correlations, pointing that even in the same correlation there is a varying performance for each different type of flow pattern. In general bubbly flows have the smallest errors, while annular flows have the highest.