UNIVERSITÀ DEGLI STUDI DI PADOVA FACOLTÀ DI INGEGNERIA CORSO DI LAUREA MAGISTRALE IN INGEGNERIA ELETTRICA Measurements and numerical models for the evaluation of performance in microwave ovens Relatore: Ch.mo Prof. Ing. Fabrizio Dughiero Correlatori: Ing. Marco Bullo Ing. Fernando Bressan Laureando: Fabio Pullano Colao ANNO ACCADEMICO 2011/2012 1 INDEX SUMMARY ........................................................................................................................................... 4 SOMMARIO ......................................................................................................................................... 6 INTRODUCTION ................................................................................................................................. 8 1. GENERAL BACKGROUND ....................................................................................................... 10 1.1 Introduction to microwave engineering ........................................................................................ 10 1.2 The history of microwave oven ...................................................................................................... 12 1.3 Maxwell’s equations ...................................................................................................................... 14 1.4 Wave equations ............................................................................................................................ 15 1.5 Energy and power ......................................................................................................................... 17 1.6 Propagation of microwaves in different media ............................................................................ 17 1.6.1 Free space ............................................................................................................................ 17 1.6.2 Lossless dielectric media ...................................................................................................... 18 1.6.3 Lossy dielectric media .......................................................................................................... 18 1.6.4 Good conductor .................................................................................................................... 20 1.7 Propagation of electromagnetic wave between two media .......................................................... 20 1.8 Standing waves ............................................................................................................................. 21 2. THE MICROWAVE OVEN ........................................................................................................ 24 2.1 Magnetron ..................................................................................................................................... 24 2.1.1 Introduction .......................................................................................................................... 24 2.1.2 Motion of electrons in static electric and magnetic fields .................................................... 24 2.1.3 Critical magnetic field .......................................................................................................... 27 2.1.4 Cavity magnetron ................................................................................................................. 28 2.1.5 Overview of operation .......................................................................................................... 29 2.1.6 The oscillatory ignition ........................................................................................................ 29 2.1.7 The preservation of self- oscillation and the electronic clouds ............................................ 30 2.2 Waveguides ................................................................................................................................... 33 2.2.1 Introduction .......................................................................................................................... 33 2.2.2 Transmission of the waveguide ............................................................................................ 33 2.2.3 Propagation in rectangular waveguide ................................................................................ 35 2.2.4 “Modes” of propagation ...................................................................................................... 37 2.2.5 Losses in the waveguide ....................................................................................................... 39 2.3 Resonant cavities ........................................................................................................................... 41 2.3.1 Introduction .......................................................................................................................... 41 2.3.2 Quality coefficient ................................................................................................................ 43 2.3.3 Dielectric losses heating in the resonant cavity ................................................................... 44 2.3.4 Polarization process ............................................................................................................. 44 2.3.5 Power transformed into heat in the material to be heated .................................................. 44 3. MICROWAVE HEATING TEMPERATURE DISTRIBUTION AND UNIFORMITY ....... 48 3.1 Introduction ................................................................................................................................... 48 3.2 Dielectric and thermal properties ................................................................................................. 48 3.3 Heating phenomena which influence heating performance and uniformity .................................. 49 3.3.1 Concentration effects ............................................................................................................ 49 3.3.2 Run-away heating phenomenon ........................................................................................... 49 3.3.3 Edge and corner overheating ............................................................................................... 50 3.3.4 Centre overheating ............................................................................................................... 50 2 4. EXPERIMENTAL TESTS ........................................................................................................... 52 4.1 Introduction ................................................................................................................................... 52 4.2 Microwave ovens ............................................................................................................................ 52 4.3 Equipment for temperature measuring ........................................................................................... 54 4.4 Agar-gel samples ............................................................................................................................ 56 4.5 Tests with agar-gel samples (using thermocouples) ....................................................................... 57 4.5.1 Efficiency calculation ............................................................................................................. 58 4.5.2 First test with MWO 1 ........................................................................................................... 58 4.5.3 Second test with MWO 2 ....................................................................................................... 61 4.5.4 Third test with MWO 3 .......................................................................................................... 64 4.6 Tests with agar-gel samples (using infrared camera) ..................................................................... 67 4.6.1 Efficiency calculation ............................................................................................................. 68 4.6.2 First test with MWO 1 ........................................................................................................... 68 4.6.3 Second test with MWO 2 ........................................................................................................ 71 4.6.4 Third test with MWO 3 ........................................................................................................... 74 4.7 Comparison of experimental results ............................................................................................... 77 5. NUMERICAL MODELS ............................................................................................................. 82 5.1 Introduction of Finite Element Method (FEM) ............................................................................... 82 5.2 Software: COMSOL Multi-physics .................................................................................................. 82 5.3 Simulation results ............................................................................................................................ 90 6. CONCLUSIONS ......................................................................................................................... 104 6.1 Comparison between numerical models and experimental tests ................................................... 104 6.2 Outlooks .......................................................................................................................................