Some recent results on probabilistic models of extreme wind speeds

This paper deals with the determination of the relationship between probability of occurrence and wind speed at a given site, on the basis of maximum annual wind speed series.Previous contributions in the area are reviewed. The arguments in favour of each of the extreme-value distributions in use, as well as methods of parameter and confidence limit estimation are discussed.The difficulties inherent in the approach are pointed out and the effects of several factors, such as type of storm, wind averaging time and ground surface roughness are discussed in detail. Finally, a two-parameter model for mixed populations of winds due to extratropical and thunderstorms is suggested.     

DRYING OF SOLIDS: THE INFINITE SLAB CONDITION

ABSTRACT

Fourier's second law was solved using convective boundary conditions without considering the shrinkage of the solid. The solutions for a finite and an infinite slab were compared to determine the dimensions for a slab to be considered as infinite. The solutions obtained for Bi = 0.1 and Bi = 100 correspond to heat and mass transfer-controlled processes, respectively, during drying. The results show that the finite slab cannot be considered as infinite, even for R²/R¹ > 20. The relative error obtained when the finite slab was assumed to be infinite was not significant for small Fourier numbers, but it increased as the Fourier number increased; errors were also higher for higher Biot numbers. When the numerical solution of a drying model was obtained for finite and infinite slabs, significant differences in drying kinetics and temperature evolution were observed.     

PREDICTION OF DEGRADING REACTIONS DURING DRYING OF SOLID FOODSTUFFS.

Nutrient loss during drying of a shrinking product was determined. A diffusion model that takes into account the shrinkage of the product in the mass balance equation was used, together with a simplified energy balance with no thermal gradients, and convective boundary conditions for both balances. Nutrient loss prediction was obtained, for first and second order kinetics. Finally, a new methodology was proposed, to determine the activation energy and the preexponential of the deterioration reaction, even when the differential equations that rules the drying process are unknown, and only the thermal history of the product is available. This approach is also valid for sterilization calculations, if temperature gradients can be neglected, i.e. when Biot number for heat transfer is small.     

PREDICTION OF DEGRADING REACTIONS DURING DRYING OF SOLID FOODSTUFFS.

Abstract

Nutrient loss during drying of a shrinking product was determined. A diffusion model that takes into account the shrinkage of the product in the mass balance equation was used, together with a simplified energy balance with no thermal gradients, and convective boundary conditions for both balances. Nutrient loss prediction was obtained, for first and second order kinetics. Finally, a new methodology was proposed, to determine the activation energy and the preexponential of the deterioration reaction, even when the differential equations that rules the drying process are unknown, and only the thermal history of the product is available. This approach is also valid for sterilization calculations, if temperature gradients can be neglected, i.e. when Biot number for heat transfer is small.     

Drying Simulation of a Solid Slab with Three Dimensional Shrinkage

ABSTRACT

A mathematical model is developed to simulate the drying of a hygroscopic porous solid. The model, based on the gradient of moisture concentration per unit volume as driving force, takes into account the migration of water within the solid by diffusion and the evaporation at the interface. A mathematical equation for diffusion in a slab with three dimensional shrinkage has been derived, assuming that the magnitude of shrinkage is equal to the volume of water evaporated. The resulting diffusion equation and the heat balance eauation for infinite thermal conductivitv were solved n;merically with temperature dependent diffusion coefficient and convective boundary conditions. The deDendence of the desorption isotherm with temperature is-also considered. corndination of all these factors in a single model provides a tool that is effective in predictinq dryinq behavior and also useful in exploring and understanding the impact of important variables on the drying process.     

Equilibrium sorption isotherms and thermodynamic properties of starch and gluten

A modification of the GAB isotherm (Guggenheim–Anderson–De Boer) is proposed in order to correlate the sorption data for water activities higher than 0.9. The proposed isotherm retains the desirable properties of the GAB isotherm, i.e. good fitting in the range of a_w between 0.05 and 0.80, and also provides a noticeable improvement in the fitting quality for high values of a_w, by introducing a new term with an additional constant. Sorption data for native potato starch and gluten at different temperatures (2°C, 20°C, 40°C and 67°C) were well correlated for the whole a_w range. For starch at 67°C, the values of the constants of the proposed equation do not follow the same tendency obtained for lower temperatures, suggesting that the structure of the material could be changed due to the high temperature. The proposed isotherm can be of interest in the area of drying given that there are few isotherms that accurately represent sorption data at different temperatures in the zone of high a_w. Also, it can be useful to predict other thermodynamic functions. In addition, a new procedure is proposed to determine the isosteric heat by using a second order polynomial for representing the variation of moisture as a function of temperature at a fixed water activity.     

Viscoelastic Behavior of Melon Tissue as Influenced by Blanching and Osmotic Dehydration

ABSTRACT: Viscoelastic properties of melon tissue exposed to steam blanching and osmotic dehydration (atmospheric or under vacuum) in glucose aqueous solutions were analyzed in a dynamic rheometer using oscillatory shear and creep tests. The storage modulus (G') greatly exceeded the viscous modulus (G') for raw and treated melon tissues, but the elastic component was partially lost because of processing. Blanching caused less than a 1 -fold decrease in G', whereas osmotic dehydration resulted in approximately a 3-fold decrease in G'relative to the untreated melon. Both moduli showed a weak dependence on frequency, with greater slope of the G'frequency lines for treated samples. A mechanical model, consisting of a spring in series with 2 Voigt elements and a dashpot element, properly predicted the creep compliance response for all melon samples. In general, overall compliance significantly increased by the treatments. The instantaneous elastic compliance and the viscoelastic compliances were the most sensitive viscoelastic parameters for distinguishing the differences in cell wall structure among osmotically dehydrated sample at atmospheric pressure and the tissues exposed to the other osmotic treatments. In contrast, G'modulus did not show an ability to evidence these structural differences because there were no significant differences in the G'values of osmotically dehydrated tissues at atmospheric pressure or in vacuum, with or without calcium addition.     

Heats and Entropies of Sorption of Cereal Grains : A Comparison Between Integral and Differential Quantities

Several thermodynamic properties for maize, rough rice and wheat has been calculated using desorption isotherms available in the literature. It was obtained an analytical expression to predict the differential heat (isosteric) as function of moisture content using a three parameters equilibrium model, based on enthalpy-entropy compensation effect, which takes into account the effet of temperature. Two integral heats of sorption were calculated, the first from the slopes of the iso-spreading pressure lines and the second from integration of the differential heat. This last one was used to estimate the energy requirement to remove water from initial moisture content to different final moisture levels, which is useful in drying area. Finally it was calculated the differential and integral entropies of sorption as function of moisture content.