Multi-Mode Physical Modelling of a Drum Boiler

Multi-mode modelling and simulation is the ability to switch between states that describe different operating conditions, for instance: turning subsystems on or off, phase appearance or disappearance (e.g. vapor within liquid), switching to a dysfunctional mode (e.g. pump cavitation or breakage), switching controllers, etc. Cyber-physical systems exhibit frequent mode switching. Models with multiple modes may exhibit varying structure since modes may have different levels of detail, hence different numbers of degrees of freedom or state variables. When switching modes, state variables may appear or disappear dynamically. The simulation of such systems is a fundamental difficulty that has been successfully addressed recently within the European MODRIO project by designing and implementing a new Modelica hybrid-state machine in the modelling and simulation tool Dymola. This new hybrid-state machine has been used to model phase appearance and disappearance in thermal-hydraulic volumes. The fundamental physical equations for the one-phase and two-phase modes and the conditions for transition between modes are presented and simulation results are given for the industrial test-case of a drum boiler. Thermal-hydraulic volumes are fundamental components in the staggered grid scheme used to model thermal-hydraulic systems, so the result solves the general problem of phase appearance and disappearance in thermal-hydraulic systems.