Thermo-fluid systems such as air conditioning and refrigeration systems are widely used in residential, commercial, and industrial applications. By the end of the 20th century, nearly 70 percent of U.S. households had air conditioning, and 99.5 percent had at least one refrigerator. Advanced control and monitoring of the thermo-fluid systems has become an inevitable but challenging issue in this century as the energy efficiency has become increasingly important. Air conditioning and refrigeration systems transport heat by means of refrigerant, repeating phase transition, which is a highly nonlinear process. Mixture of vapor and liquid refrigerant distributed in heat exchangers makes the process dynamics highly complex, including many state variables that are important for stringent control but are not directly measurable. In this thesis, nonlinear observers for estimating those inaccessible state variables are designed and are applied to the control and monitoring of an air conditioning system. First heat exchanger dynamics including complex phase transition phenomena are analyzed. It is shown that the order of the system varies dynamically depending on the distribution of vapor and liquid, which makes the system a varying-order switched system.
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