An Experimental and Theoretical £ Investigation of Annular Steam ...

- 6 -
The experimental part of this report (chapter 3) is an
attempt to rectify this lack of a consistent set of film flow
data achieved with steam-water at high pressures. More than 200
film flow measurements are presented, and they are accompanied
by experimental values of pressure gradients, film thicknesses,
wave frequencies and velocities, and burnout heat fluxes. The
experiments were carried out under both adiabatic and diabatic
conditions in one annular and two tubular geometries.
On the basis of these data, a film-flow model for the prediction
of burnout in tubes and annuli is developed in the analytical
part of the report (chapter 4). Finally, the capability
of the model is shown by comparisons of predicted and experimental
values.
2. ANNULAR TWO-PHASE FLOW
The flow regime, which is characterized by the presence of
a liquid film adjacent to the channel wall, is called the annular
two-phase flow regime. This chapter only deals with the
general aspects of annular flow in a very superficial way. A
detailed description of this topic (especially regarding airwater
systems) is given by Hewitt and Hall-Taylor (1970). The
terminology used in the present report follows to a great
extent that used in chis recommendable textbook.
2.1. Flow Regimes
The evolution of the annular flow pattern is illustrated in
conceptual form in fig. 2.1. Here a tube is shown with a heated
wall, where the liquid is introduced at the bottom.
The first generation of steam takes place by nucleation at
the wall, producing steam bubbles. As the fluid proceeds up the
tube, further generation of steam takes place, and the small
bubbles coalesce into bullet-shaped bubbles called slugs. After
the slug flow regime, a transition region called the churn flow
regime is reached. This flow pattern is a result of instabilities
in the slugs due to the increase of the steam velocity.
It is characterized by a "churning" or oscillatory action.
After this transition region, the annular flow regime is
established. It is seen that the liquid film is strongly agi-