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Implementation of the first period of convective drying in a commercially available CFD package

Żabski J.

Transactions of the Institute of Fluid-Flow Machinery

2015

nr 128

139--157

The issue of the drying of woodchips is investigated in this paper. The most widespread type of the solid drying is a packed bed convective drying process with hot air as a working medium that is considered here. A comprehensive survey has proven a severe shortage of 3D drying-oriented computational fluid dynamics (CDF) packages for handling packed beds. This work was carried out by means of User Defined Procedures (UDF) – self written codes in C implemented in a commercially available CFD package – Ansys Fluent. A strongly flattened fixed bed of woodchips was investigated whose dimensions equaled to 1.5 m×1.5 m×0.2 m in length, width and height, respectively. In theoretical modeling woodchips were assumed as spheres of unique size settled in a cubic layout. The first period of drying was taken into consideration with the inlet air temperature 60 ◦C and humidity 0%. The temperature of the packed bed was set to the wet bulb temperature 21 ◦C. The vapor flux was implemented as a source term in the continuity equation. The core part of the UDF was a DEFINE_SOURCE macro that comprised the source term for evaporating water and partially the liquid water storage. As a result the drying air was humidified from initial 0%, to 22% along the bed, at a constant air enthalpy. The air temperature decreased from 60 ◦C to 38 ◦C, and the drying rate fell from 0.22 to 0.10 kg/m3s.