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Modeling Infrared and Combination Infrared-Microwave Heating of Foods in an Oven

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Modeling Infrared and Combination Infrared-Microwave Heating of Foods in an Oven MODELING INFRARED AND COMBINATION INFRARED-MICROWAVE HEATING OF FOODS IN AN OVEN A Dissertation Presented to the Faculty of the Graduate School of Cornell University in Partial Ful¯llment of the Requirements for the Degree of Doctor of Philosophy by Marialuci Frangipani Almeida January 2005 °c Marialuci Frangipani Almeida 2005 ALL RIGHTS RESERVED MODELING INFRARED AND COMBINATION INFRARED-MICROWAVE HEATING OF FOODS IN AN OVEN Marialuci Frangipani Almeida, Ph.D. Cornell University 2005 A quantitative, model-based understanding of heat exchange in infrared and com- bined infrared-microwave heating of food inside an oven is developed. The research is divided into three parts: measurement of optical properties, radiative heat transfer analysis and combined microwave-radiative heat transfer analysis. Optical proper- ties of reflectance, absorptance and transmittance in a potato tissue are measured as a function of wavelength, using a spectroradiometer. Penetration of energy is higher for halogen lamps that emit in the near- and mid-infrared range, compared to ceramic rods that emit mostly in the far infrared range. Reflectance in the near infrared range increases with moisture content of the food, thus decreasing the en- ergy coupled. Surface structure has signi¯cant influence on the optical properties. A 3-D radiative heat exchange model of an oven-food system is developed using a commercial ¯nite-element package. The air in the oven is assumed transparent to the radiation. Heat conduction is assumed in the entire oven (food and air) for the short duration. The wavelength dependence of emissivity (non-gray surface) is found to signi¯cantly a®ect the surface radiative flux and the use of a non-gray model is recommended for such materials, although simpli¯cation of the emissiv- ity variation is required to keep the computation time reasonable. Lowering food surface emissivity reduces the radiative flux that is absorbed by the food surface. Reducing oven wall emissivities increase the radiative flux on the food surface. The location of the radiative heat source in the oven as well as placement of the food relative to the heat source were found to have signi¯cant influence on the radiative heat flux over the food surface. To add microwave heating, Maxwell's equations of electromagnetics were solved for the same cavity using separate ¯nite element software and the volumetric heat generation, in the food, obtained from this model was input to the radiative heat transfer model, thus coupling them. Using mea- sures such as mean temperature rise and the standard deviation of temperatures, it was demonstrated that combination heating leads to more uniform heating, without compromising the speed of heating. BIOGRAPHICAL SKETCH The author was born in S~aoPaulo, Brazil. In 1982, she was admitted to UNICAMP, the State University of S~aoPaulo, Campinas campus. She earned her Bachelor's and Master's degrees in 1987 and 1999, respectively, both from the Department of Food Engineering, at UNICAMP. She had experiences in the meat, spices and food service industry while still in Brazil. In December of 1998, she received a fellowship from the United States Department of Agriculture to pursue her Ph.D. in the subject of Food Engineering, at Cornell University. She joined the ¯eld of Food Science and Technology in September 1999, having Professor Ashim K. Datta, as her advisor. In March of 2003, she began to work in the Bunge Oil Center of Excellence as a Senior Innovation Research Scientist while continuing to work on her dissertation. iii To my love, partner and friend, Cesar, for your wonderful presence in my life To George, Louise and Thomas, for your purest smiles To my beloved father, in memoriam, and mother, for your superb lessons of dedication iv ACKNOWLEDGEMENTS I would like to express my sincere gratitude to Professor Ashim K. Datta. For the past few years, he taught me technical knowledge, showed me the way of scienti¯c thinking, helped me in life and gave me important instructions on career. I would also like to thank Professor Kenneth E. Torrance. His time, patience and encourage- ment are greatly appreciated. My sincere gratitude also goes to Professor William L. Olbricht, for his questioning and input. I want to express special thanks to Professor Kenneth E. Torrance and all the Rhodes Hall sta® for the access to the Light Instrumentation Laboratory, at Cornell University. Professors Dennis Miller and Joseph Hotchkiss showed their support on my study, life and career. Finally I would like to thank my friends Srikanth Geed- ipalli (also co-author in my last chapter), Valeria Acquarone, Natalia dos Santos, Jifeng Zhang, and Seung-Hwan Lee, as well as the departmental sta® from the Riley-Robb and Stocking Halls for their help and friendship. v Table of Contents 1 GENERAL INTRODUCTION 1 1.1 Outline . 1 1.2 Motivation . 2 1.3 Thesis Objectives . 6 2 MEASUREMENT OF OPTICAL PROPERTIES OF FOODS IN NEAR AND MID-INFRARED RADIATION 7 2.1 Introduction . 7 2.2 De¯nition of Terms . 11 2.3 Objectives . 14 2.4 Methodology . 14 2.4.1 Spectroradiometer . 16 2.4.2 Measurement Process . 19 2.4.3 Sample Preparation . 20 2.4.4 Calculation of Penetration Depth . 24 2.5 Results and Discussion . 25 2.5.1 Detector Sensitivity: Signal to Noise Ratio in the 0.85-1.15 ¹m Range . 25 2.5.2 E®ect of the Source of Radiation (Ceramic Rod and Halogen Lamp) . 26 2.5.3 Variation of Spectral Reflectance with Moisture Content . 27 2.5.4 Spectral Variation of Penetration Depth with Moisture Content 28 2.5.5 Penetration Depth for Near-Infrared Heating and Microwave Heating Compared . 30 2.6 Conclusions . 32 3 RADIATIVE HEAT TRANSPORT MODELING INSIDE AN OVEN: PROBLEM FORMULATION AND EXPERIMENTAL SETUP 34 3.1 Introduction . 37 3.2 Objectives . 40 vi 3.3 Problem Formulation . 41 3.3.1 Assumptions for the Enclosure Model . 42 3.3.2 Radiative Heat Transfer Equation . 42 3.3.3 Governing Equations and Boundary Conditions for Heat Con- duction . 45 3.4 Methodology . 49 3.4.1 Numerical Solution . 49 3.4.2 The Macro-surface Concept for Radiation Calculations . 49 3.4.3 Numerical Implementation of the Radiative Exchange in an Enclosure . 51 3.4.4 Solution Parameters . 53 3.4.5 Input Parameters . 54 3.4.6 Cycling Boundary Condition for Infrared Source . 57 3.4.7 Surface Convection Coe±cient . 63 3.4.8 Temperature Measurements . 68 3.4.9 Heat Flux Measurements . 70 4 RADIATIVE HEAT TRANSPORT MODELING INSIDE AN OVEN: E®ect of Oven and Food Parameters 74 4.1 Introduction . 74 4.2 Objectives . 78 4.3 Problem Schematic, Solution and Input Parameters . 78 4.4 Results and Discussion . 82 4.4.1 Mesh Convergence . 82 4.4.2 Gray versus Non-gray Behavior of Food Surfaces . 87 4.4.3 Comparison with Experiments . 89 4.4.4 Global Energy Balance in the Oven . 96 4.4.5 E®ect of Di®erent Infrared Power Levels . 98 4.4.6 E®ect of Di®erent Food Emissivities . 99 4.4.7 E®ect of Di®erent Wall Surface Emissivities . 100 4.4.8 E®ect of Di®erent Food Positions . 102 4.4.9 E®ect of Changing Lamp Positions . 102 4.5 Conclusions . 105 5 COMBINED MICROWAVE AND INFRARED HEATING OF FOODS IN AN OVEN 107 5.1 Introduction . 111 5.2 Literature Studies Modeling of Combination Microwave and Infrared Heating . 112 5.3 Objectives . 113 vii 5.4 Problem Formulation . 113 5.4.1 Governing Equations, Boundary Conditions and Input Pa- rameters for Microwave Heating . 115 5.4.2 Governing Equations, Boundary Conditions and Input Pa- rameters for Infrared Heat Exchange . 120 5.4.3 Governing Equations, Boundary Conditions and Input Pa- rameters for Conduction Heating in the Food . 127 5.5 Methodology . 129 5.5.1 Numerical Solution of the Electromagnetics and Heat Transfer Equations . 129 5.5.2 Coupling of Electromagnetics and Heat Transfer Simulations 130 5.5.3 Experimental Set-up . 131 5.6 Results . 133 5.6.1 Experimental Results . 133 5.6.2 Experimental Validation of Model . 133 5.6.3 Uniformity of Infrared, Microwave and Combination Heating, Described Using Contour Plots . 134 5.6.4 Statistical Analysis of the Uniformity of the Three Modes of Heating . 139 5.6.5 E®ect of Di®erent Microwave Power Levels on the Uniformity of Combination Heating . 145 5.6.6 Manipulation of Surface Heating Using Combination Heating 148 5.7 Conclusions . 149 6 CONCLUSIONS 151 A 155 Bibliography 161 viii List of Tables 3.1 Input Parameters . 56 3.2 Parameter Values Used in Calculating Heat Transfer Coe±cient, hc 66 3.3 Nusselt numbers and heat transfer coe±cients calculated using Eq. 3.16 67 5.1 Input Parameters . 125 5.2 Non-uniformity in Temperature Distribution . 144 5.3 Non-uniformity in Temperature Distribution . 147 5.4 Non-uniformity in Temperature Distribution . 148 5.5 Non-uniformity in Temperature Distribution . 150 A.1 Results from least square approximations: Penetration Depth (mm), 2 T0 and R - 82 and 77% moisture content samples . 156 A.2 Results from least square approximations: Penetration Depth (mm), 2 T0 and R - 72 and 70% moisture content samples . 157 A.3 Results from least square approximations: Penetration Depth (mm), 2 T0 and R - 67% moisture content samples . 158 ix List of Figures 2.1 Direct transmitted fraction concept . 13 2.2 Di®use reflectance and transmittance measurement system layout (Top view) [20]. 16 2.3 An schematic view of the integrating sphere attachment to the spec- troradiometer [20]. 17 2.4 Spectral emissive power (W=(m:m2)).
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