CFD Analysis of Two-Phase Flow Characteristics in a 90 Degree Elbow

Abstract

Computational fluid dynamics (CFD) analysis was performed for a two-phase air-water flow through a horizontal to vertical 90 0 elbow with a 12.7 mm pipe diameter. Three different air velocities of 15.24, 30.48, and 45.72 m/sec along with three different water velocities of 0.1, 1.0, and 10.0 m/sec were used in this study. To analyze the flow behavior in the elbow, pressure and velocity profiles at six different upstream and downstream locations of the elbow were compared. Computational fluid dynamics (CFD) analysis was performed for 9 different cases using FLUENT commercial code. A mixture model was used to account for different gas and liquid velocities to solve continuity, momentum and energy equations. CFD analysis results showed a decrease in pressure as fluid leaves the elbow in addition to a larger pressure drop at higher air velocities. No significant change in pressure was observed when water velocity was increased from 0.1 to 1.0 m/sec compared to water velocity change from 1.0 to 10.0 m/sec. The normalized pressure drop was larger at lower air velocities compared to higher water velocities. CFD analysis results were compared with available experimental data showing a reasonably good agreement. 1. INTRODUCTION Two-phase flow phenomenon occurs in various industrial applications such as aerospace, automotive, nuclear, oil and gas applications, etc. Due to the unavailability of classical equations to determine the complex flow behaviors associated with two-phase flow, it is important to predict the characteristic behavior of these types of flows. The complexity increases when flow direction changes due to varying interfacial stresses and unstable phase distributions in the flow domain. Among different applications, curved pipes or elbows are commonly used in heating and refrigeration system to transfer heat from one fluid to another. The pressure and velocity distributions at different spatial locations of the piping system are critical for effective heat transfer, mixing and circulation. Single-phase pressure drop can be predicted with reasonable accuracy for curved pipes [1]. Prediction of two-phase pressure drop in curved pipes and bends is quite complex, due to differences in fluid densities, viscosities, phase transformations and their interactions with the surrounding wall. Two-phase flow is a special type of multiphase flow, where fundamental principles of fluid mechanics can be used by adding characteristic equations. The frictional pressure drop in the bend is primarily due to the resistance to flow imposed by the walls of the tubes on the respective phases and the interaction between the phases. Often, one or more of these parameters are not known, due to the difficulty in fully describing the respective flow mechanisms at the different operating conditions [2]. The work presented here will use the computational fluid dynamics (CFD) approach to simulate the pressure drop and velocity distributions in a 90 degree elbow for air-water two phase flow. Detailed studies of two-phase pressure drop have l...

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