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Cilj vseh proizvajalcev in uporabnikov pečic je imeti kar se da enakomerno zapečenost različnih jedi. To pa proizvajalci v zadnjih časih zaradi novih trendov in vse krajših razvojnih časov izdelkov težko dosegajo. Zato se je za potrebe numeričnega napovedovanja funkcionalnih lastnosti pečniškega prostora razvil časovno odvisen tridimenzionalni (3D) numerični model, ki se je preverjal z eksperimentalnimi meritvami. V modelu je obravnavan sistem vročega zraka, v katerem delujeta okroglo grelo in ventilator, prevladujoča sta sevanje in prisilna konvekcija. Zaradi obravnave prisilne konvekcije je zelo pomembna ustrezna izbira modela turbulence. Predstavljena je uporaba različnih modelov turbulence. Za obravnavano pečico se je kot najprimernejši izkazal SST-model turbulence. Ugotovljeno je bilo, da je v numerični model nujno treba vključiti proces izparevanja. Numerični model se je preverjal z eksperimentalnimi meritvami temperature in s funkcionalnimi preizkusi pečenja peciva. Po končanju preizkusa pečenja se je piškotom z metodo določevanja barvnih kontrastov, ki temelji na CIE L*a*b barvnem prostoru, določila stopnja porjavelosti Ry. Glede na rezultate se je predlagal linearni model, ki bo omogočal napovedovanje rezultatov porjavelosti s pomočjo numeričnih izračunov. Dodatno preverjanje modela se je izvedlo na področju optimizacije oziroma izboljšave pečenja pečice. S pomočjo numeričnih izračunov se je preverilo šest različnih izvedb pokrova ventilatorja. Na podlagi numeričnih izračunov se je določila numerična stopnja porjavelosti. Numerični rezultati so se preverjali s pomočjo eksperimentalnih preizkusov, ki so se izvedli na starem in novem pokrovu ventilatorja. Tako numerični kot eksperimentalni rezultati so pokazali, da izboljšan pokrov ventilatorja zagotovi boljše rezultate enakomernosti pečenja. The objective of all manufacturers and users of ovens is to achieve uniform browning of various baked foods. In recent years, manufacturers have found it difficult to achieve this, due to the rapid appearance of new trends and due to progressively shorter development times. In this paper, we present the development and validation of a time-dependent 3D computational fluid dynamics model, which enables the numerical prediction of the baking performance and grade of browning of a forced convection oven. Flow and heat transfer of hot air in an oven, where a round heating element and a fan are both operating, are simulated. Radiative and convective heat transfer are taken into account. Because of forced convection, the use of appropriate turbulence model in complex engineering problems is very important. The comparison of different turbulence models in presented. For the oven cavity, the SST turbulence model is the most appropriate. We found, that it is necessary to include water evaporation in the numerical model. The numerical model was validated by performing experimental measurements of temperature and by performing baking tests of shortbread. After baking, the grade of browning was measured for the shortbread. To determine the grade of browning, the method of identification of colour contrasts was used, based on the colour space CIE L*a*b. Based on the results, we proposed a linear model, which enabled the prediction of the grade of browning based on the results of the fluid dynamics simulation. The additional validation of numerical methodology was done on improvement of baking performance of a forced convection oven. Six different designs of oven fan cover were compared with numerical simulations. Follow results were used in a linear model to estimate the grade of browning. The results were validated with experimental measurements of baked shortbread using the old and improved oven design. The results show that the improved fan cover performs better than the existing fan cover. |
