EXPERIMENTAL AND NUMERICAL STUDY OF CONVECTIVE HOT AIR DRYING OF CASSAVA PELLETS

SOURCE:

Faculty: Engineering
Department: Mechanical Engineering

CONTRIBUTORS:

Azaka, O. A.
Enibe, S.O.
Achebe, C. H.

ABSTRACT:

A 2-Dimensional numerical model of coupled heat and mass transfer equations of convective drying for rectangular and cylindrical Cassava slices was developed. The experiment was conducted in the convective drier with controlled air temperature, and a digital mass measuring scale. The developed unsteady-state partial differential equations were solved by means of the finite element method by employing Galerkin weighted residual approach. The state variables, surface temperature and moisture content of the samples, as well as the shrinkage, and drying rate were determined using the formulated model. The numerical model was used to predict the effect of fixed and moving boundaries on the drying curve. The relative drying curve, drying rate and surface temperature of the numerical simulation were in good agreement with the experimental data analyzed. The results showed that the model best described the drying process of cassava pellets under the conditions tested. The diffusivity of the rectangular samples were determined to be in range of 1.4260 x 109 - 2.3304 x 109 m2s-1 and described using a 3rd order polynomial equation
with the coefficient of regression R² = 0.8989. The diffusivity plots clearly explains that the moisture diffusivity of cassava pellets can be expressed as a function of temperature using a 3rd order polynomial equation. The quadratic models which described the convective mass transfer coefficients explain that convective mass transfer coefficient varies with time. The R2 values and correlation coefficients (r) are mostly above 0.9 indicates that the convective mass transfer coefficient of the cassava samples is adequately modeled as a quadratic function of drying time for each air temperature. In finite elements analysis of the samples, solutions improved by mesh refinement. Hence, the predictions generated for a 99-element model, a 367-element model, a 1395-element model and a 5447-element model show that the solutions progressively fell within the experimental range of moisture content as number of elements improved. The R2, RMSE and r for the predictions are respectively 0.9340, 0.1924 and 0.9912 for the 1395-element model while the prediction accuracy improved to 0.9958, 0.0483 and 0.9984 for the 5447-element model. Marked improvement in accuracy is recorded on mesh refinement. Using the 5447-element model, FE predictions at the experimental intervals were compared with the measured values and the corresponding statistical goodness of fit values. The R2 values were seen to be above 90% where majority were up to 99% indicating high accuracy of FE prediction of cassava drying.