Feyissa, Aberham Hailu

I dette ph.d. studium er der fokuseret på samspillet mellem energi- og massetransport ved tilberedning af faste fødevareprodukter såsom bage- og stegeprocesser. Modellering af energi- og massetransport er en væsentlig videnskabelig udfordring. Under bagning og stegning ændres fødevarens mikrostrukturelle og fysiske egenskaber. Energi- og vandtransport i fødevaren er komplekst koblet, og for nogle fødevarer endnu ikke fuldt beskrevet. Et typisk eksempel på dette er stegning af kød i en konvektionsovn, hvor mekanismen for transport af vand er uklar. Etablering af robuste matematiske modeller der troværdigt beskrivelser de grundlæggende mekanismer vil være af stor betydning. En kvantitativ beskrivelse af energi- og massetransport under tilberedning af faste fødevareprodukter, i form af matematiske ligninger, implementering af løsningsteknikkerne, samt værdien af input parametrene involverer usikkerheder. Målet med denne afhandling har været at udvikle robust modeller til beskrivelse af energi- og massetransport i faste fødevareprodukter. Studiet består i formulering af de mekanistiske modeller, løsning af modellerne vha. en Finite Element (FEM) metode, kalibrering og validering af modellerne med eksperimentel data, samt evaluering af modellerne ved en usikkerhed- og sensitivitetsanalyse. I studiet er kontaktbagning og stegning af kød i en konvektionsovn udvalgt som repræsentative eksempler. For begge eksempler, er fysiske eksperimenter udført og relevant data opsamlet, såsom produkttemperatur, vægttab og procesindstillinger. Vandtransportmekasnismerne i kød under stegning i en konvektionsovn, blev undersøgt. En teoretisk afdækning blev udført for ændringen i struktur, vandbindingsevne og skrumpning. Mekanismerne bag vandtransport blev testet ved måling af lokale vandindhold. For stegeprocessen, blev 2D og 3D mekanistiske kvantitative modeller udviklet til beskrivelse af energi- og massetransport. De grundlæggende ligninger er baseret på princippet om energi- og massebevarelse. Ydermere er Darcys ligning brugt til beskrivelse af trykdrevet transport af vand under stegning af kød. Ændringer af det elastiske modul, fordampning og bevægelige grænseflader blev inkorporeret i de anvendte modelligninger. Den arbitrære Lagrangian-Eulerian (ALE) metode blev brugt til at udtrykke de bevægelige grænser under stegeprocessen. Ligningerne for koblet energi- og massetransport blev løst ved brug af FEM (COMSOL). For kontaktbagning blev en 1D matematisk model udviklet til beskrivelse af energi- og massetransport. Den udviklede model medtager energitransport, lokal fordampning og multifase vandtransport (væske og damp). Ligningerne blev implementeret i et COMSOL-MATLAB miljø med de følgende funktioner: parameterestimering modelvalidering samt usikkerheds- og sensitivitetsanalyse. De ukendte parametre i modellen blev estimeret ved sammenligning mellem målte og simulerede temperaturdata, ved brug af mindste kvadraters metode. Ydermere blev modellen valideret ved brug af eksperimentelt data. En rimelig overensstemmelse imellem de målte og simulerede data blev opnået. Usikkerheds- og global sensitivitetsanalyse blev indarbejdet i modellen til beskrivelse af koblet energi- og massetransport. Modellens usikkerhed, grundet usikkerheder i input parametre såsom termo-fysiske egenskaber, varme- og masseovergangstal, faseovergang, samt start- og grænsebetingelser blev undersøgt. Der blev anvendt en Monte Carlo baseret metode til usikkerheds- og global sensitivitetsanalyse. Sensitivitetsanalysen blev udført for at bestemme den relative effekt af de undersøgte parametre på modellens prædiktion. Parametrenes relative effekt på modellens output blev identificerede og deres indflydelse rangordnet for hvert at modellens output. Generelt giver den udviklede matematiske model til beskrivelse af energi- og massetransport bedre indsigt i de undersøgte processer. Den anvendte indgangsvinkel i form af robust modellering, har vist sig at være et brugbart værktøj til modelopbygning og til at håndtere forskellige udfordringer ved modellering af energi- og massetransport under tilberedning af faste fødevareprodukter. Potentialet for at bruge netop denne indgangsvinkel er især stort for stege- og bageprocesser.The study is focused on combined heat and mass transfer during processing of solid foods such as baking and frying processes. Modelling of heat and mass transfer during baking and frying is a significant scientific challenge. During baking and frying, the food undergoes several changes in microstructure and other physical properties of the food matrix. The heat and water transport inside the food is coupled in a complex way, which for some food systems it is not yet fully understood. A typical example of the latter is roasting of meat in convection oven, where the mechanism of water transport is unclear. Establishing the robust mathematical models describing the main mechanisms reliably is of great concern. A quantitative description of the heat and mass transfer during the solid food processing, in the form of mathematical equations, implementation of the solution techniques, and the value of the input parameters involves uncertainty. The objective of this thesis is to develop robust models of heat and mass transfer during processing of solid foods. The study consists of formulating the mechanistic models, solving the models by the Finite Element method (FEM), calibrating and validating the models by experimental data, evaluating the models by an uncertainty and sensitivity analysis. In the study, contact baking and roasting of meat in convection oven were chosen as representative case studies. For both representative cases, the experiments were performed and the relevant data such as product temperature, mass loss, and other process conditions were obtained. For roasting of meat in convection oven, the mechanism of water transport during roasting was studied; a theoretical assessment was made on the change in structure, water holding capacity and shrinkage. The mechanism of water transport was tested by measuring the local water content. For the roasting process, 3D and 2D mechanistic quantitative models describing the coupled heat and mass transfer were developed. The governing model equations are based on the conservation of energy and mass. Further, Darcy’s equation was used to describe the pressure driven transport of water in meat during roasting. The change in elastic modulus, evaporation, and moving boundary were incorporated into the model equations. The arbitrary Lagrangian–Eulerian (ALE) method was implemented to capture the moving boundary during the roasting process. The model equations for coupled heat and mass transfer were solved using the FEM (COMSOL). For the contact baking process, a 1D mathematical model of the coupled heat and mass transfer was developed. The model developed for the contact baking process considered the heat transfer, local evaporation, and multiphase water transport (liquid and vapour). The model equations were implemented in the COMSOL-MATLAB computing environment with the following features: parameter estimations, model validations and uncertainty and sensitivity analysis. The unknown parameters in the model were estimated by comparing the measured and simulated data – using the least square method by comparing the measured temperature against the simulated temperature. Further, the model was validated using the experimental data and a reasonably agreement between the simulated and experimental data were obtained. The uncertainty and global sensitivity analysis method were incorporated for the model of coupled heat and mass transfer. The uncertainty of model predictions due to the uncertainty in input parameters such as thermo-physical properties, heat and mass transfer coefficients, phase change initial and boundary conditions parameters were studied. A Monte Carlo based method of the uncertainty and global sensitivity analysis was used. The sensitivity analysis was performed to determine the relative effect of the different parameters on the model prediction. The relative effects of parameters on the model prediction were indentified, and their relative impact on each model output was ranked. Generally, the developed mathematical models of heat and mass transfer provide better insights about the processes. The proposed robust modelling approach was found to be a useful tool in the model building that help to cope up with different challenges in modelling of heat and mass transfer during processing of solid foods and the potential of using the approach is particularly great for frying and baking operations

Online Research Database In Technology

Robust Modelling of Heat and Mass Transfer in Processing of Solid Foods

http://orbit.dtu.dk/ws/files/5796903/Phdthesis_Aberham%20Hailu%20Feyissa_2011_finalver.pdf

Robust Modelling of Heat and Mass Transfer in Processing of Solid Foods

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