Moving Boundary Models for Dynamic Simulations of Two-Phase Flows

Two-phase flows are commonly found in components in energy systems such as evaporators and boilers. The performance of these components depends among oth- ers on the controller. Transient models describing the evaporation process are important tools for determin- ing control parameters, and fast low order models are needed for this purpose. This article describes a gen- eral moving boundary (MB) model for modeling of two-phase flows. The new model is numerically fast compared to dis- cretized models and very robust to sudden changes in the boundary conditions. The model is a 7 th order model (7 state variables), which is a suitable order for control design. The model is also well suited for open loop simulations for systems design and optimization. It is shown that the average void fraction has a signifi- cant influence on the system response. A new method to calculate the average void fraction including the in- fluence of the slip ratio is given. The average void fraction is calculated as a symbolic solution to the in- tegral of the liquid fraction profile.