This thesis is dedicated to the study of flow boiling of single component fluids and multicomponent
mixtures in vertical pipes at high qualities. Both theoretical and experimental investigations
were carried out with the objective of improving fundamental knowledge of hydrodynamics and
phase change heat transfer in annular flow.
The diabatic experiments had the objective of studying the nature of nucleate boiling in upward
steam-water annular flow. A specially constructed, electrically heated, annulus visualisation test
section (deq = 12.9 mm) was used to observe directly the interaction between disturbance waves
and bubble nucleation through the analysis of high-speed video recordings. It was found that
disturbance waves seem to locally trigger off the activity of nucleation sites as they travel along
the channel. Local measurements of the heater wall temperature, and hence of heat transfer
coefficient, were carried out using a radiation equilibrium thermocouple which could be traversed
along the heated section. An analysis of the Onset of Nucleate Boiling (ONB) in climbing films
was also implemented.
Adiabatic experiments were carried out to investigate the behaviour of the fraction of the liquid
entrained as droplets in the region of transition between the churn and annular flow regimes.
An isokinetic probe was used, which enabled the simultaneous measurement of the gas and entrained
liquid mass fluxes. The campaign was performed in a 10.8 m long, 31.8 mm internal
diameter test section (LOTUS facility) over a wide range of flow conditions. The profiles of
local droplet concentration characterise churn flow as a region in which the radial gradients of
concentration tend to disappear with increasing gas flowrate. As annular flow takes place, the
local concentration is virtually constant with respect to radial position and gas flowrate. As far
as the determination of the fraction of liquid entrained as droplets at the onset of annular flow
is concerned, the experimental results are of particular interest to annular flow computer simulation
codes as it provides a basis for calculating the initial condition for the mass conservation
equations. With that objective in mind, an empirical correlation was developed to predict the
fraction of liquid entrained as droplets at the onset of annular flow. The modelling work on heat transfer deals with phase change of multicomponent mixtures at
high qualities. A differential phenomenological model of the annular flow regime is proposed
so as to describe the deterioration of the heat transfer coefficient observed by Kandlbinder
(1997) for boiling of binary and ternary hydrocarbon mixtures in a 25.4 mm internal diameter,
vertical pipe. The set of correlations for droplet entrainment and deposition by Govan (1990)
was extended to cope with the so-called mixture effects. Use was also made of a Colburn-Drew type formulation for calculation of interfacial parameters (mass fluxes, compositions and
temperature). The formulation gives a very good prediction of bulk and wall temperatures and
of heat transfer coefficients determined experimentally.
A simple mathematical model to describe the formation of waves characteristic of the churn flow
regime is also proposed. Previous work (Govan, 1990), in which high-speed video recordings
were carried out, used a test section with a specially constructed transparent liquid inlet and
showed clearly the process of wave formation. The model, which is based on integral mass
and momentum conservation principles, predicts the wave velocity and distance travelled by the
waves. It also provides estimates of wave parameters, such as critical amplitude, length and flow
rates that are consistent with the experimental observations.