| Various literature models of flow regimes and transitions are contrasted and compared to reported observations. The annular flow regime is modelled by average velocities and distributions of the three "fluids"--liquid film, drops and vapor--in the radial and circumferential directions. Accurate empirical correlations giving rates of interfluid mass and momentum transfer are found to be crucial in predicting the location of tube wall dryout.; A possible direct effect of the heat flux on the droplet entrainment rate is hypothesized. Physically the model involves bubble nucleation at the heated tube wall, migration through the turbulent film and rupture at the film-vapor interface accompanied by a spattering of droplets. From considerations of the energies of the surging liquid jet following the bubble rupture, as well as the bubble size and density, the rate of liquid mass entrainment is predicted.; Since in several industrial applications the heat flux results from light originating at a single azimuth, a model to calculate the transmitted fraction of this radiation is developed. A finite element analysis of heat conduction within the tube cross-section is utilized. Boundary conditions include the incident heat flux profile and wall temperature dependent radiative and convective losses at the outer tube surface, as well as nucleative and convective heat transfer at the liquid-coated inner surface.; Finally a computer code is written to compare predictions of the above models to experimental data available in the literature. The accuracy of the heat flux induced entrainment model could not be determined from the burnout data since large amounts of error in the empirical deposition rate correlations cloud the effect. However the pipe conduction model was evaluated independently by simulation of solar boiler tests in which the overall heat flux, entrainment and deposition rates were low. Predictions of the inner and outer wall temperatures at the most strongly heated azimuth were fairly good, with discrepancies explained by the omission of circumferential variations in film thickness and velocity in the fluid flow model. |
