## Basic Parameters of Two‐phase Fluid Flow

In this section we will consider the simultaneous flow of** gas (or vapor) and liquid water** (as encountered in steam generators and condensers) in concurrent flow through a duct with cross-sectional area A. The subscripts **“v”** and** “ℓ”** indicate the **vapor** and **liquid** phase, respectively. Fundamental parameters that characterize this flow are:

## Slip Ratio – Velocity Ratio

In two-phase fluid flow it is convenient to use the **slip ratio**. **The slip ratio** (or **velocity ratio**) in two-phase flow is defined as the ratio of the velocity of the vapor phase to the velocity of the liquid phase. The slip ratio in a two-phase fluid flow is defined as:

In the **homogeneous equilibrium model (HEM)** of two-phase flow, the slip ratio is by definition assumed to be unity (there is no slip). However, most industrial two-phase flows have different velocity of the gas and liquid phases, these can differ significantly. The models that account for the existence of the slip are called **separated flow models**.

**x, α, and S**can be deducted and the result is:

The reason for defining the **void fraction** and the **slip ratio** is that they also make it possible to calculate the **pressure drop** of the two-phase flow. Several correlations for calculation the slip ratio, S, and the void fraction are presented in literature. The following correlations are given in order of increasing accuracy.