Propriedades térmicas de grãos de trigo: determinação e modelagem

Abstract

RESUMOObjetivou-se com o presente trabalho, avaliar as propriedades térmicas dos grãos de trigo, de uma variedade nacional (Aliança), durante o processo de secagem em estufa e ajustar modelos matemáticos que representem satisfatoriamente as variáveis estudadas em função do teor de água. Foram utilizados grãos de trigo com teor de água inicial de 26 %b.s. secos até o teor final de 11 %b.s. em estufa com ventilação forçada a 40 ºC e umidade relativa de 45%. As variáveis estudadas foram o calor específico, obtido pelo método das misturas; a condutividade térmica, obtida por meio de um cilindro teoricamente infinito considerando a hipótese da condução unidimensional na direção radial; e a difusividade térmica, determinada indiretamente utilizando-se os valores experimentais da massa específica aparente, calor específico e condutividade térmica do trigo. Com base nos resultados obtidos, pode-se concluir que o calor específico e a condutividade térmica do trigo aumentam, enquanto a difusividade térmica apresenta redução dos seus valores com a elevação do teor de água e que estas propriedades térmicas do trigo variam linearmente com o teor de água, sendo satisfatoriamente representadas pelo modelo linear simples, para a faixa de umidade estudada. Termos para indexação:Calor específico, condutividade térmica, difusividade térmica. ABSTRACTThe objective of this work was to evaluate the thermal properties of wheat grain, of a national variety, during the drying process in oven and to adjust mathematical models which represent satisfactorily these variables. Wheat grains with initial moisture content of 26 %d.b. were dried in air forced oven up to 11 %d.b. as the final moisture content. The specific heat was obtained by the method of the mixtures and the thermal conductivity by the utilization of a theoretically infinite cylinder considering the hypothesis of the conduction unidimensional in the radial direction. For thermal diffusivity was certain indirectly using itself the experimental values of the bulk density, specific heat and thermal conductivity of the wheat. Based on the results, it can be concluded that moisture content reduction promotes the specific heat and the thermal conductivity reduction but thermal diffusivity increase and which these thermal properties of the wheat vary lineally with water content, for the interval of studied humidity.

Determining and modeling the physical, thermal and aerodynamic properties of Pinto beans with different water contents

“…It has been observed that as water content decreases, the value for thermal diffusivity diminishes. The data found in this study for thermal diffusivity, differ from the ones observed by Corrêa et al (2004) for millet and birdseed; Ribeiro et al (2007) for wheat grain and Jian et al (2013) with canola, in which thermal diffusivity increased with water content decrease in the grains. In other works, a great variant in thermal diffusivity values for the mass of different variety of grains, in function of water content can be verified.…”

“…It can be verified, according to Table 2, that thermal conductivity, specific heat and thermal diffusivity decreased by 22.7; 12.7; 14.3%, respectively, alongside water content drop-off from 32.9 to 13.6% d.b. The same behaviors were also observed by Legrand et al (2007) Thermal conductivity and specific heat presented the same tendency of other products, such as: quinoa grain (Nunes, 2009); wheat grain (Ribeiro et al, 2007); corn grain (Andrade et al, 2004); millet and birdseed grain (Corrêa et al, 2004); soy grain (Ito, 2003); cherry coffee (Borém et al, 2002). The fact that thermal conductivity presents a higher average value for a higher water content, is directly associated with the increase in porosity, once water content in the product increases, because, according to Incropera and Witt (1996), thermal conductivity of a material is the measure of its capacity to conduct heat; for foods, it depends on, mainly, of composition, but also of the presence of empty spaces and its homogeneity.…”

“…In other works, a great variant in thermal diffusivity values for the mass of different variety of grains, in function of water content can be verified. Ribeiro et al (2007) verified that various factors influence heat quantity that goes through granular mass, this way, thermal diffusivity values can vary among products and varieties due to, mainly, its composition, specific mass, porosity and water content., the statistical parameters and the adjusted equations used in the determination of thermal properties, in the various water contents for Pinto beans, cultivar BRS Estilo is shown in Table 4.…”

Determination of thermophysical properties of cupuassu (Theobroma grandiflorum) dry almonds

“…The dried CUA were manually peeled, then they were crushed in a domestic coffee grinder (CADENCE) and used for the determinations described in the following topics (Ribeiro et al, 2007;Sasseron, 1984). These determinations applied the principles of the dynamics of heat transfer between a body and the environment, which are based on Newton's Law of Cooling: ( ⁄ ) = (ℎ • ) • ( − ) and Fourier's Law: = −( . )…”

“…The first stage of the experiment consisted of calibrating the calorimeter (triplicate) by inserting 50 g of water (mc = 0.05 kg) at room temperature (Tc = 25 -27 °C) into the calorimetric vessel and then mixing 100 g of water (mh = 0.10 kg) previously heated (Th = 50 -52 °C) to finally note the equilibrium temperature of the system (Teq = 41 -43 °C). Since the specific heat of water (cp.w) is known as 4.187 kJ/(kg.°C), the calorific capacity of the calorimeter (Cp.cal) was calculated as 0.014 ± 0.002 kJ/°C based on the relation also applied by Ribeiro et al (2007):…”