Characterization of the stratified-wavy and mist flow patterns during phase change processes in horizontal tubes

• Grant number: 21/03634-7
• Support type: Scholarships in Brazil - Scientific Initiation
• Effective date (Start): July 01, 2021
• Effective date (End): June 30, 2022
• Field of knowledge: Engineering - Mechanical Engineering - Transport Phenomena
• Principal researcher: Gherhardt Ribatski
• Grantee: Ana Luiza Guereschi Kohlmann
• Home Institution: Escola de Engenharia de São Carlos (EESC). Universidade de São Paulo (USP). São Carlos , SP, Brazil

Abstract

In vapor-liquid flows, the phase distribution along the cross section varies according to the operational conditions. Such distributions present typical geometries, named in literature as flow patterns, which significantly affect the heat transfer and pressure drop performances. The stratified-wavy and mist flows are among the flow patterns in horizontal ducts occurring in heat exchangers used in commercial and industrial refrigeration and air conditioning sectors. The first is characterized by the segregation of the liquid and vapor phases due to the gravitational effects, with the liquid flowing in the lower region and the vapor in the upper region of the duct. Moreover, the degree of waviness of the liquid/vapor interface affects the heat transfer coefficient, the pressure drop and the transition from stratified-wavy to intermittent and annular flow patterns. For mist flow, the liquid phase flows as dispersed droplets within the vapor flow. The heat transfer rate and the pressure drop during mist flow are related to the characteristics of the liquid entrainment. In this context, the present research proposal concerns the characterization of the amplitude, speed, and frequency of the interfacial waves during the stratified-wavy flow. Besides, the characterization of the density and periodicity of the liquid entrainment during mist flow is also focus of this study. A programs in MatLab (2015) will be developed for the analysis of flow images obtained for these flow patterns in a previous study (Proc. FAPESP n° 2016 / 16849-3) using a high-speed camera performed by the Heat Transfer Research Group of EESC-USP. Such images were gathered for in-tube convective condensation and flow boiling for R134a, R600a, R290 and R1270 and, therefore, involving relative wide ranges of reduced pressures. Based on the obtained data, new correlations based on dimensionless parameters will be proposed for the amplitude, speed, and frequency of the interfacial waves during the stratified-wavy flow and the density and periodicity of the liquid entrainment during mist flow, such as which. These correlations will be suitable of being incorporated into heat transfer and pressure drop models for design and optimization of heat exchangers. (AU)