Abstract:
Computational Multi-Fluid Dynamics (CMFD) for multidimensional gas-liquid flows is an emerging field. Due to the complexity and diversity of two-phase gas-liquid flow conditions, further development of two- phase flow modeling, closure laws and numerical methods is needed in order to achieve the general purpose and efficient CMFD methods, which will be applicable to a wide variety of technical and technological conditions.
An original approach to the various aspects of CMFD modeling is presented. It is based on the multi-fluid modeling approach, development of necessary closure laws and derivation of appropriate numerical methods for efficient governing equations solution. Mass, momentum and energy conservation equations are written for several fluid streams, depending on the investigated flow conditions. Great differences of phase densities and other thermo-physical properties in two-phase flows impose strict requirements to the stability and accuracy of applied numerical method of solution. The numerical method should be applicable to interpenetrating multiphase flows, as well as to separated structures with interface movement. Velocity and pressure fields are solved with the SIMPLE type pressure-corrector method developed for the multiphase flow conditions. For the solution of scalar parameters transport equations both implicit and explicit methods are presented. The implicit method is suitable for steady state, slow transients and problems without the sharp front propagation. The explicit method, based on the fluid particle tracking, is a third order accurate numerical scheme. It is developed in order to predict scalar parameters front propagations, as well as phase interface tracking problems.
The developed method possibilities are demonstrated by solving standard benchmark tests, as well as real engineering problems. The solving of the benchmark problems includes the transient boiling boundary prediction, interface tracking problems such as the broken dam problem and the initially homogeneous two-phase mixture separation in the vessel, the dispersed gas phase-droplets and liquid film flows around spacers, and the burn-out in pool boiling. Solved macro domain engineering problems are steady-state and transient two-phase flows in steam generators and kettle reboilers, headers of heat exchangers for refrigerant evaporation and the steam condensation in non-vented pipes in the presence of non-condensables. Obtained numerical results are compared and verified with experimental data or available analytical solutions. The presented numerical results show that the developed CMFD method for multiphase flow is a useful and robust tool for the simulation and analyses of fluid flow in equipment of different geometry and in different thermal-hydraulic processes. Obtained results are a support to equipment design, plant operational diagnostics and safety analyses in energy, chemical and process industry.