Dynamic modeling of thermo-fluid systems : : with focus on evaporators for refrigeration

Mathematical modeling and simulation are important tools in the optimization of design, production and maintenance of engineering systems. Modelica"T"M is an open standard language for modeling of multi-domain systems featuring the important concept of acausal modeling and object-oriented modeling. Component models are arranged in model libraries facilitating easy relise of models.

The features available in Modelica have not yet been fully explored within the domain of thermo-fluid systems and the present work is one attempt in this direction. This thesis describes the development of a general model library called ThermoTwoPhase for dynamic modeling of thermo-fluid systems. Dynamic modeling of two-phase flows found in evaporators has received particular attention and various types of twophase flow models are implemented giving a selection of models on different levels of complexities.

The experiences from designing the ThermoTwoPhase library are summarized in a new modeling procedure called the Modeling V, which describes how to build a well-structured model library using a standardized modeling methodology. Within control and optimization of evaporators, the location of the liquid dry-out front is of particular interest.

Moving boundary (MB) models is a low order two-phase model type, where the boundaries of the flow regions are computed using lumped control volumes with variable boundaries. A new general MB-model is proposed, which includes all three flow regions (slib-cooled, two-phase and superheated). Various reduction techniques are applied on the MB-model of a dry-expansion evaporator resulting in a variety of new reduced order models.

Simulation results are compared to experimental data showing very good agreement in the location of the liquid dry-out front. The reduced MB-models are seen to capture the essential dynamics of the system and are thus important special cases.