On Model Libraries for Thermo-hydraulic Applications

University dissertation from Department of Automatic Control, Box 118, SE-221 00 LUND

Abstract: Mathematical modelling and simulation are important tools when dealing with engineering systems that today are becoming increasingly more complex. Tightly integrated production and process optimization are trends that give rise to heterogeneous systems, which are difficult to handle without expertise in several engineering disciplines. Model libraries provide an excellent way to package engineering knowledge of systems and units to be reused by non experts. Many commercial simulation packages provide good model libraries, but they are usually domain specific and closed. Heterogeneous, multi-domain systems requires open model libraries written in general purpose modelling languages. Modelica(TM) is such an open standard for an object-oriented modelling language for dynamic systems. The thesis describes principles for object-oriented equation-based model libraries. The main topic is modelling of thermo-hydraulic applications. Two different model libraries are presented, the Omola model library K2 for thermal power plants and the Modelica base library ThermoFlow for general thermo-hydraulic applications. The models are based on first principles. Lumped or 1D-discretized control volumes contain the thermodynamic balance equations. The base library is built for flexibility; the control volume can have different medium descriptions, single- or multi-component, and the momentum dynamics can be replaced by static descriptions. Some applications of the libraries are also described: a heat recovery steam generator, a drum-boiler model and a model of evaporating two-phase flow in a pipe. The thesis consists of four articles. Papers I and II describe the two model libraries. Paper III covers the two-phase flow application and gives a simplified physical analysis that shows under what conditions there will be pressure-drop oscillations in an evaporating pipe. Model libraries for industrial use must be validated against measured data. Paper IV describes how parameter estimation methods can be used for model structure validation. The thesis also has a short discussion on other model validation methods.

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