CFD-simulations and experiments on steam condensation in polydisperse bubbly flows

Bubble condensation in sub-cooled water is a complex process, to which various phenomena contribute. Since the condensation rate depends on the interfacial area density, bubble size distribution changes caused by breakup and coalescence play a crucial role.
To investigate the involved phenomena and their complex interplay, new experiments on steam bubble condensation in vertical co-current steam/water flows have been have been carried out in the TOPFLOW test facility in Dresden, which consists of an 8m long vertical DN200 pipe (inner diameter: 195mm). Steam is injected into the pipe and the development of the bubbly flow is measured at different height levels with a wire mesh sensor. By varying the steam nozzle diameter (1mm or 4 mm) the initial bubble size can be influenced. Larger bubbles come along with a lower interfacial area density and therefore they condensate slower (see figure). In addition to previous experiments (Lucas & Prasser, 2007) also the steam velocity is measured by correlating the signals of two wire mesh sensors installed in a small distance to each other. In the new experiment also the pressure drop along the pipe is measured as well as the temperature at selected points (Lucas et al., 2009). The additional sensors allow for choosing a defined gas inflow pressure as well as a distinct sub-cooling temperature at the injection location. Here steam pressures between 1-2 MPa and sub-cooling temperatures from 2 to 4 Kelvin were applied. Due to the pressure drop along the pipe, the saturation temperature falls towards the upper pipe end. This affects the sub-cooling temperature and can even cause re-evaporation in the upper part of the test section.
In second part of the present contribution, the new TOPFLOW condensation experiments are compared with simulations using an extended MUSIG approach. This approach has been developed in cooperation with ANSYS-CFX for the computation of condensation in polydispersed bubbly flows with CFD. This extended MUSIG approach includes the transition of bubbles to smaller size groups due to condensation as well as the shift of the bubble size distribution due to pressure changes. In the second part of the present contribution, simulations with the extended MUSIG approach are compared with the new TOPFLOW condensation experiments. The new CFD approach is able to catch all relevant phenomena qualitatively, such as bubble condensation and evaporation (if the saturation temperature falls below the water temperature) and radial bubble size segregation. Crucial for the condensation simulations is the modeling of coalescence and breakup, which still needs to be improved. The presented condensation experiments and the extended MUSIG approach are a useful basis for validating models for polydispersed bubbly flows.