Models to predict the thermal state of rice stored in aerated vertical silos

The thermal state of the rice mass in steel vertical silo with an aeration system, located in a rice storage facility in Rio Grande do Sul State, Brazil, was studied. Data obtained by thermometry system were compared with the predicted data obtained from four one-dimensional models. The first model was based on the hypothetical division of the deep bed into a limited number of thin layers with identical temperatures of the grain and the passing air. The second model presented the system of two partial differential equations, describing the heat transfer and conservation of energy for air and for grain mass. The third model used the generalised dependence between dimensionless temperature and homochronous number and was adapted to conditions when both a variable temperature of the ventilation air inlet and an initial non-uniformity of the grain mass temperature existed. The fourth model was intermediate between the thermal state problem of a body made up of two different materials (with fixed boundary) and the Stefan problem in which a moving boundary separates the different phase domains. The velocity of the moving boundary and thermal diffusivities of each domain were obtained experimentally. Each of the proposed models satisfactorily described the thermal state of the studied silo. The use of the last data reading obtained as a new initial condition increased simulation accuracy. In the case of a significant transverse temperature inhomogeneity, a hypothetical division of the silo into independent vertical cylinders centred on the respective thermometry cables increased accuracy.