Analysis of non-boiling gas-liquid two-phase flow through flow components: Experimental investigation and numerical modelling

Abstract

Today, gas-liquid two-phase flows are encountered in many different micro and macro applications used in industry, such as nuclear reactors and applications, power generation processes, chemical processes, oil and gas transport lines, heat exchangers, and air conditioning systems. The components of these industrial applications where the flow takes place, which we can call flow components, consist of different and complex geometries such as straight pipes, elbows, fittings, tees, junctions, orifices, collectors, etc. Understanding and modeling the two-phase flows that develop in these complex geometries is a challenging engineering problem that is still up to date, as it is directly related to the system parameters. Precise determination of the characteristics of a flow element, and hence of the system and flow characteristics, is critical both from a design and operational point of view. Since the system parameters vary with the flow regimes in two-phase flows, it is essential to determine the flow regime and, in particular, the local characteristics of the phases. The aim of this study is to establish a comprehensive investigation of air-water two-phase flows through flow components, employing both an intense experimental investigation and advanced numerical modeling to provide insight into this complex but critical fluid dynamics problem. In this context, air-water two-phase flow was experimentally investigated in the experimental setup located in the Hydraulics Laboratory at Istanbul Technical University, Faculty of Mechanical Engineering. In the setup, the flow takes place in approximately 15 m long acrylic pipes with an inner diameter of 40 mm as a cycle. The piping system was investigated under four different sections, which are injector, upstream, test (U-bend), and downstream sections. Two-phase flow conditions were established through the 40 mm diameter pipe with 180 l/min water and 30, 35 and 40 l/min air flow rates by the designed circular injector. The air is injected into the water flow 8.5 m after the pump outlet at the injector section, which ensures the single-phase water is fully developed up to this point. The upstream section after the injector section is a 1.5-m-long horizontal section where the air-water flow development is observed before the test section. The test section consists of a unique geometry that is a combination of two upward and downward 90-degree elbows and one vertical 180-degree return bend. All experimental sections have specific measuring points and ports to measure the selected two-phase characteristics and differential pressure values. In this regard, an advanced optical probe with sapphire tips was used to measure two-phase flow characteristics, such as local void fractions and the number of bubbles detected per unit time. For pressure, differential pressure measurements were made employing calibrated pressure transducers in the pre-defined sections for the given experimental conditions. The calibration, measuring ranges, and uncertainties of all devices are elaborately investigated and addressed under the experimental setup section. The experimental results are presented for each section separately. Initially, the results of the differential pressure measurements are interpreted parametrically for the concerning airflow rates. Then, the local two-phase flow characteristics are visualized by plotting highly refined mapped graphs along the cross-section to explain better the measurements taken. The study also involves a detailed development and validation of a numerical model, followed by an in-depth analysis of the defined two-phase characteristics. The numerical model was developed in ANSYS Fluent 2019 R3. The Eulerian-Eulerian approach was used in the computational model, which employs individual equations for each phase. Sub-models for phase and interphase interactions are also employed regarding drag and non-drag force models. The numerical model developed for simulating two-phase flow in the complex geometry was rigorously validated against experimental data. The validation indicated high agreement between the numerical model and experimental results, with deviations within acceptable uncertainty levels. With experimental and numerical investigation, this comprehensive study provides valuable insights into two-phase air-water flow dynamics, with a particular focus on a U-bend section within the flow system. The findings not only confirm the complexity of two-phase flows in such geometries but also highlight the limitations of existing models in accurately predicting flow behaviors. In such complex two-phase applications, it has been observed that modeling requires more than simply analyzing the bend or determining flow regimes in terms of superficial velocities from flow regime maps. Instead, a holistic approach must be employed to identify local flow regime modifications caused by geometry, resulting in accurate and complete flow regime determination. The presented results provide implications for future applications and assist further studies that can address the findings for different piping systems and elbows commonly used in practical applications in the industry under various design conditions.