Thermodynamics Model Based Optimal Control For Liquid-solid Material In Microwave Heating Process

As a novel, green and environmental-friendly energy source, microwave has been found great values in both domestic and industrial applications for thermal purposes such as food heating, chemical reaction assisting. The local overheating problems, i.e., hot spots, overheating and thermal runaway, constitute the major challenges for the development of microwave heating of liquid-solid material process. It is significant for avoiding the local overheating problem by designing the accuracy control strategy. However, the strong nonlinearity, the big time variation of the material's properties and the non-uniformity of the material temperature increase the difficulty to design the real-time accuracy control strategy for microwave heating process. Therefore, designing efficient temperature control strategy, which considers the characteristics of the microwave heating of liquid-solid material process, has become an important topic of the microwave heating research field.This dissertation is supported by the National Basic Research Program of China(973 Program) “Basic Research on Novel Microwave Power Source for Supporting the Efficient Microwave Energy Industrial Application”. This dissertation mainly focuses on two important branches of microwave heating process, namely, microwave heating of liquid material process and microwave drying of liquid-solid material process. Firstly, the “grey-box” thermodynamics model of the microwave heating of liquid material process is built. Secondly, based on the “grey-box” model, real-time temperature control strategies are proposed from the single-input single-output(SISO) case to the multi-input multi-output(MIMO) case. Thirdly, the heat transfer of the microwave drying process is analyzed to design the optimal control strategy from the single temperature tracking case to global temperature tracking case to multi-objective case. Finally, focusing on the actual microwave drying of lignite process, an efficient multi-mode hybrid switching control strategy is proposed. The main work of this dissertation includes five parts:(1)For the microwave heating of liquid material process, the effect of heat transfer of the heating process on material temperature is analyzed. By using the finite volume method(FVM), the energy balance partial differential equation is transformed into the difference equation with the formulation of spatial and temporal discretization representation. A set of microwave power coefficients is proposed to describe the absorbed microwave energy of each finite volume of material. Further, the “grey-box” thermodynamics model is built for the microwave heating of liquid material process.(2)Based on the “grey-box” thermodynamics model of the microwave heating of liquid material process, its space state representation is formulated. Then, a sliding mode variable structure real-time temperature tracking control strategy is designed for the SISO case. For the MIMO case, a model reference adaptive control(MRAC) strategy is presented. Then, by analyzing the properties of the actual microwave heating of liquid material process, the advanced MRAC strategy is proposed by using the expert control(EC). The results show the good temperature tracking performance of each volume of material. The local overheating phenomenon and the system input regulation can be efficiently improved.(3)Focusing on the microwave drying of moist porous material process, the effect of the moisture evaporation of the microwave drying process on the heat transfer of material is analyzed. Then, the energy balance equation, which considers the heat of vaporization, is proposed. Further, based on the FVM and the Lambert's law, the thermodynamics model for each volume of material is built.(4)Based on the thermodynamics model of the microwave drying of moist porous material process, the single point temperature optimal control(SPTOC) strategy and the global optimal temperature control(GOTC) strategy are presented, respectively. Then, a more advanced multi-objective optimal control(MOOC) strategy is proposed by embedding the tracking accuracy, the change intensity of microwave power and the energy consumption. By analyzing the regulation characteristics of the microwave power, an exhaustive search approach is designed to solve those optimal problems. Simulation results indicate that SPTOC has good robustness in dealing with the big variation of the model parameters. The simulation with GOTC results in advantages of the uniformity of the global temperature of material. Finally, MOOC is tested that it can significantly increase the overall process efficiency and inhibit the violent oscillation of control increment to extend the life time of the microwave power source.(5)Aiming at the lignite burning problem of the microwave drying of lignite process, the control objectives and the empirical moisture fraction model of lignite are obtained. And the multi-mode switching hybrid control strategy is proposed based on the expert rule. The control strategy has three control modes, which are proportional-integral-differential(PID) control mode, optimal control mode and zero mode. And the expert rule base is built, which involves the switching rules of the control modes and the assisting control rules. Experiments demonstrate that the proposed control strategy has good performance in avoiding the lignite burning problem and guaranteeing that the lignite can be dried to the objective of moisture content.