(void_fraction)=
# Void fraction

```{admonition} Summary
* **Subject** Gas volume fraction in two-phase flow
* **Main conclusion:** $\langle \alpha_{G} \rangle = \langle j_{G} \rangle / (C_{0} \langle j \rangle + V_{Gj}) $
```


*Void fraction* is a commonly used term for the gas volume fraction in two-phase flows. In chemical engineering, it is usually referred to as *gas holdup*. Since it gives a direct measure of how much amount of gas we have in the two-phase flow, prediction of the void fraction is important in designing/operating two-phase flow systems. We first define some basic quantities, and then the drift-flux model is introduced. 


```{toctree} 
:maxdepth: 1

void_fraction/averaged_quantities
void_fraction/drift_flux
```