Alfa Oumar Dissa

An Effective Moisture Diffusivity Model Deduced from Experiment and Numerical Solution of Mass Transfer Equations for a Shrinkable Drying Slab of Microalgae Spirulina

From experimental data, Spirulina effective moisture diffusivity was analytically estimated by considering two diffusion regions and the product shrinkage. Then, the moisture diffusivity was deduced from the numerical solutions of mass transfer equations by minimizing the difference between experimental and simulated drying curves and by taking into account the slab thickness variation. The range of moisture diffusivity used for simulations was estimated from minimal and maximal values of experimental effective diffusivities and calculation started with the mean value of experimental effective diffusivities. Identified effective diffusivities ranged from 1.79 × 10−10 to 6.73 × 10−10 m2/s. These diffusivities increased strongly with drying temperature and decreased slightly with moisture content. A suitable model correlating effective diffusivity, temperature, and moisture content was then established. Effective diffusivities given by this model were very close to experimental ones with a relative difference ranging from 0.5 to 24%.

Impact of Fruit Ripeness on Physicochemical Properties and Convective Drying Characteristics of Kent Mango (Mangifera indica L. cv. 'Kent')

Impact of ripeness on drying characteristics of mango was studied by considering different zones on the fruit. For each zone, ripeness was estimated by total soluble solids/acidity ratio, colour and texture of fruit flesh. For each state of ripeness, drying curves and time-temperature curves were established both in forced and natural convection. Mass diffusivity (estimated by considering two diffusion regions), thermal diffusivity and drying rates were deduced from these drying curves by considering product shrinkage. Results showed that the time required to reduce moisture content to any given level depended on the ripeness state, being highest for unripe samples and lowest for ripe samples. At each drying moment, temperature of ripe sample was higher than that of unripe sample. Mass diffusivity, thermal diffusivity and drying rates strongly increased with ripeness state. At 60°C, unripe and ripe fruit mass diffusivities ranged respectively from 1.69x10-10 to 9.87x10-10 m²/s and 3.38x10-10 to 1.77x10-9 m²/s. Thermal diffusivities ranged from 2.12 x10-11 to 6.44x10-10 m²/s and 2.74x10-10 to 8.05 x10-10 m²/s respectively for unripe and ripe samples. In natural convection, drying rates reached maximal values of 0.16 kg m-2 s for unripe sample and 0.47 kg m-2 s for ripe sample whereas in forced convection they reached respectively 0.43 and 0.67 kg m-2 s. Product shrinkage decreased with ripeness and was almost ideal for the major part of the drying process. Constants of suitable fitting models also varied considerably with fruit ripeness. This work showed that ripeness state influences strongly drying characteristics of mango fruit.

Numerical study of cooling water via an evaporatively cooled immersed porous tube

SYNOPSIS We present a numerical study of the cooling of a volume of water through the wall of a cylindrical terracotta tube, the pores of which are saturated with water. The internal surface of the wall carries a water film which evaporates by forced convection of hot and dry air. Taking into account the physical properties of the material and that the aspect ratio of the tube used has a maximum value (D/L = 0.32), variations of the thermophysical properties of the air between the entry and the exit of the tube are ignored. The mathematical model describing the physical processes is based on the Darcy equation of fluid flow through a saturated porous material, the equation of Laplace related to pressure and the equation of heat in polar coordinates. One can then describe the space-time evolution of the field of temperature inside the wall as well as the temporal evolution of the average temperature of water.