Possible explanation of the strange temperature

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Possible explanation of the strange temperature phenomena based upon the concept of compact crust, porous crumb and flowing core (CCC model), was presented previously, Žitný et al. (2002a,b). The CCC model predicted nonuniform pressure profile along the sample surface (assuming atmospheric pressure at a perimeter) and also non-uniform temperature profiles, closely related to pressure of escaping steam. New experiments make this hypothesis questionable, Žitný et al. (2006), because measured longitudinal temperature profiles were almost uniform during the second stage of free water evaporation. Therefore the rather complicated CCC-model was replaced by a simple integral model, calculating only the mean temperature, mean pressure, and the mean water content as a function of time. Basic assumption that the temperature is controlled by pressure of steam was preserved, but the strict relationship between the pressure and the mass flowrate of escaping steam, taking into account porosity and thickness of crumb evolution, was relaxed. Thus it is possible to take into account also another sources of hydraulic resistance, first of all additional flow resistance of vents filled by the expanding sample. The previous integral model is useful for scale up, for temperature and pressure estimation, but not for description of expansion. Similar model of bread baking was suggested by Zanoni et al.(1994), (1995). The model assumed a moving crust/crumb boundary; nevertheless, the expansion of the sample was described by quite empirical equations without temperature history consideration. Gan et al. (1995) and Fan et al. (1999) used the concept of expanding foam. The foam is formed by starch matrix, liquid film and gas bubbles and dough expansion was considered to be related to bubbles expansion at the early phase of baking. Zhang et al. (2005) also described bread baking, but used a complicated viscoelastic rheological constitutive equation necessitating purely numerical 3D solution. Baking takes place at atmospheric pressure and this is the fundamental difference from the thermal pressure forming in a closed mold. An attempt to model the expansion of sample at elevated pressure was presented by Žitný et al. (2006) assuming a squeezing flow of suspension in the liquid core (the concept of CCC structure was also adopted). Disadvantage of this approach is prediction of strongly nonuniform lateral pressure profile and also prediction of porous structure that does not conform to observations. 2. Integral model It seems to be obvious, that the hypothesis of temperature controlled by pressure inside the mould is in contradiction with observations (uniform longitudinal temperature profile). This is not quite so. The discrepancies concerning more or less uniform longitudinal temperature profile were explained by the effect of vents blocked by expanded material. The blocking effect was manifested by significant pressure peaks recorded soon after the moment, when the expanding sample arrived to the venting holes. In this case the pressure (and
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Skočilas Jan et al.
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