Phase change of single component fluids and mixtures in annular flow
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.