## 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:

## Void Fraction

**The void fraction, α**, is one of the most important parameters used to characterize two-phase fluid flow, especially the gas-liquid flow.

Various geometric definitions are used for specifying this parameter. The void fraction in a two-phase fluid flow may be defined as:

- The fraction of the
**channel volume**that is occupied by the gas phase. This void fraction is known as the**volumetric void fraction.** - The fraction of the
**channel cross-sectional area**that is occupied by the gas phase. This void fraction is known as the**cross-sectional void fraction**. - The
**local void fraction**refers to that at one single point or very small volume. Therefore it takes the values of 1 or 0.

For further purposes, we will assume a void fraction to be the fraction of the **channel cross-sectional area** that is occupied by the gas phase (i.e., cross-sectional void fraction) defined as:

The void fraction is important in the two-phase flow because it influences key physical parameters, such as **viscosity**, **pressure drop,** and **heat transfer**.

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