Steam bubble condensation in sub-cooled water in case of co-current vertical pipe flow

The structure of a steam-water flow in a vertical pipe of 195.3 mm inner diameter was studied using novel wire-mesh sensors for high-pressure/high-temperature operation (max 7 MPa/286 °C). Tests were carried out at pressures of 1 and 2 MPa under nearly adiabatic conditions as well as with slightly sub-cooled water (6 K at max). Steam was injected into sub-cooled water and condensed during the upwards flow. The evolution of radial gas fraction profiles and bubble size distributions along the pipe in a high-pressure steam-water flow was measured for the first time. The experimental data allow correlating the intensity of steam condensation in contact with sub-cooled water with the structure of the interfacial area and the bubble size distribution, which is very important for the model development. The data were used to test the complex interaction of local bubble distributions, bubble size distributions and local heat and mass transfer. The model considers a large number of bubble classes (50). This allows the investigation of the influence of the bubble size distribution. The results of the simulations show a good agreement with the experimental data. The condensation process is clearly slower, if the injection nozzle diameter is increased (from 1 to 4 mm orifices). Also bubble break-up has a strong influence on the condensation process because of the change of the interfacial area. Some modelling errors arise from the uncertainty of the interfacial area for large bubbles and the heat transfer coefficient.