An analytical model was developed to determine mass transfer parameters during fruit drying. The model considers radius shrinkage during the drying process. With the mathematical model the external mass transfer coefficient k m and the water diffusivity D AB can be simultaneously evaluated. To test the model, experimental data of water mass M A versus drying time t, and the initial radius R 0 and the radius at the end of the process R f were determined. The values obtained for D AB and k m were of the same order of magnitude as those calculated by a numerical method from the literature. The proposed methodology, that considers volume shrinkage, can be used to estimate drying parameters with acceptable accuracy for engineering calculations.
RESUMOO presente trabalho teve por objetivo monitorar as condições meteorológicas nos ambientes interior e exterior de uma casa de vegetação, construída em terreno pertencente ao CEFET-RN. Através dos registros das medidas da temperatura e umidade relativa do ar e da temperatura da superfície do solo, nos dois ambientes, avaliou-se a eficiência do uso da cobertura morta na retenção de umidade no solo e diminuição na temperatura na superfície do solo. Foi também determinada a taxa de evaporação nesses ambientes a partir da medida da perda de massa de água em mini-tanque de evaporação. O monitoramento desses parâmetros foi realizado no período compreendido entre 04/11 e 11/12/2005. O uso de proteção do substrato com folhas secas proporcionou redução significativa na taxa de evaporação da água do solo (p<0,01).Palavra chave: Casa de vegetação, evaporação e umidade. EFFICIENCY OF THE MULCH ON SOIL HUMIDITY RETENTION SUMMARYThis work had the goal of monitoring the internal and external weather conditions of a greenhouse at CEFET/RN school, Brazil. The experiment was carried out to evaluate the efficiency of mulch in retaining the soil humidity, and decreasing the temperature on the soil surface through the measuring records of temperature and relative humidity in both environments. Moreover, the rate of evaporation inside and outside the greenhouse was determined by the measurement of the loss of water mass in Class A evaporation pans. The monitoring for these parameters took place from November 4 to December 11, 2005. The use of the protection for the soil with dry leaves showed a reduction on the rate of the soil evapotranspiration (p<0,01).
ResumenSe presenta la construcción de un secador solar en el que se realizaron ensayos de secado de frutos tropicales y se desarrolla el modelado matemático simplificado del proceso. Se empleó un sistema de adquisición de datos y control de temperatura de secador, usando energía de convección forzada de aire y radiación solar. El sistema es complementado, para emergencia, con resistencias eléctricas. Se realizaron experimentos de secado de bananas en forma de placa, obteniéndose sus pesos y contenidos acuosos en función del tiempo. Se realizó el modelado matemático del proceso de secado, con la ecuación de transferencia de materia, obteniéndose la difusividad del agua en los frutos. Los valores obtenidos concuerdan con datos de la literatura y pueden ser empleados en el diseño preliminar del secador. El sistema estudiado entrega información para calcular la cantidad de agua evaporada del alimento para una determinada temperatura promedio del secador solar, la potencia y el tiempo necesarios para el secado. Palabras clave: secado solar, frutos tropicales, energía solar, difusión de agua Drying Solar System for Tropical Fruits AbstractThe construction of a solar dryer is presented in this work. Experimental runs for the drying of tropical fruits were performed and a simplified model of the drying process. A data acquisition system was used and the drying temperature was controlled by means of an air stream heated by a simultaneous process of forced convection and solar radiation. The system also included electric resistances, for emergency situations. Drying experiments were performed with banana slabs, monitoring their weight and water content as a function of time. A simplified mathematical model was developed considering only the mass transfer equation, estimating the water diffusivity on the fruits. The values of these diffusivities are in agreement with other values reported in the literature, so they can be used in a preliminary design of solar dryer. The system studied provides a way of calculating the amount of water to be evaporated in the dryer at a particular mean temperature, the work power to be supplied and the required drying time.
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