Abstract

The mass transfer coefficient plays an important role in predicting corrosion rates. Using similarities between heat and mass transfer mechanisms, a mechanistic model is proposed to predict heat and mass transfer coefficients for two-phase flow in vertical pipes. The mechanistic model is evaluated by using water-air heat transfer experimental data obtained from the literature. The mechanistic model is also compared with commonly used empirical correlations. In comparison with available heat transfer correlations, the mechanistic model performs very well for vertical annular flow, bubbly flow and slug or intermittent flow that were considered. The mechanistic model is based on physics of two-phase flow and thus is expected to be more general than empirical correlations.

Introduction

Corrosion resulting from chemical reactions between a metal alloy and fluid is a widespread and costly problem to several industries involved in the transport of multi-phase fluids. When the reactions are strongly controlled by the mass transfer component, being able to accurately predict the mass transfer coefficient is important for predicting corrosion rates. Because of various complexities involved in mass transfer measurements in multiphase flow, most previous studies of mass transfer rely on heat transfer data to develop correlations for mass transfer coefficients. This is possible because of the analogies between heat and mass transfer. It is well known that there are similarities in the transport of momentum, mass, heat and energy. In turbulent flows, the transport of momentum, mass, heat and energy strongly depend on turbulent diffusion due to turbulent eddies. The main engineering application of transport analogy is that measurements can be conducted in one system, and then applied to another. For example, heat transfer data can be applied to a mass transfer problem where experiments may be more difficult to conduct. The governing equations for mass and heat transfer are similar if they are compared in dimensionless form. It can be shown that the Sherwood number (Sh) and the Schmidt number (Sc) in mass transfer are analogous respectively to the Nusselt number (Nu) and the Prandtl number (Pr) in heat transfer when dimensionless forms of the governing equations are considered. Definitions of these parameters are listed in Table 1.

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