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ABSTRACT
A research program was undertaken with the goal of developing dehumidifier solar wood dryers and wood thermal phytosanitary treatment cells. The first step of the program, was dedicated to the development of the numerical tools that would assist in the design and optimization of the dryers. These were based on an approach that combines micro (at the level of the wood board) and macro (at the level of the entire dryer and its solar heating system) modellings, and that uses typical year weather data, to perform the simulations. A finite difference numerical code was employed to predict the time evolution of the wood and air properties during the thermal phytosanitary treatment of wood. These predictions are employed as input to the TRNSYS© software to model the retained solar phytosanitary treatment installation configuration. In this system, the air leaving the treatment cell is recycled. But prior to injecting it back, it is mixed, in the right proportions, with outside air in order to adjust its humidity. The recycled air temperature is then risen to the operating value in an air-water heat exchanger fed from the storage tank of the solar water loop. The latter includes solar evacuated collectors, two pumps, a storage tank and a heat exchanger. The concept technical feasibility was prooved. The sytem optimization showed that, for a cell to be installed in Tunis, a 22 m2 evacuated tube collector and a 1.5 m3 water reservoir are needed for the treatment of 1 m3 wood stack i.e. 0.7 m3 of wood (reduced to 0.42 m3 in the winter season). A prototype of the optimized system was constructed and preliminary experimental tests are presented.
Nomenclature
| A(m2) | = | Area |
| = | Specific heat | |
| H(W/m²K) | = | Convection heat transfer coefficient |
| = | Mass transfer coefficient | |
| = | Mass flow rate per unit surface area | |
| P(Pa) | = | Pressure |
| = | Heat flow rate | |
| = | Gas constant | |
| T(K) | = | Temperature |
| T(s) | = | Time |
| V(m3) | = | Air control volume |
| V(m/s) | = | Air velocity |
| X(kgwa/kgdw) | = | Wood moisture content |
| Y(kgwa/kgdw) | = | Air humidity ratio |
| cond | = | Conduction |
| cs | = | Cross section |
| dw | = | Dry wood |
| da | = | Dry air |
| evap | = | Evaporation |
| ex | = | Exchange surface |
| ha | = | Humid air |
| hd | = | Hydraulic diameters |
| hw | = | Humid wood |
| l | = | Liquid |
| sur | = | Wood surface |
| v | = | Vapor |
| wa | = | Water |
| wv | = | Water vapor |
| Greek Symbols | = | |
| Ρ(kg/ m3) | = | Density |
| Subscripts | = | |
| A | = | Air |
| a,in | = | Inlet air |
| a,out | = | Outlet air |
| Conv | = | Convection |
| Exponents | = | |
| i,n | = | Iteration number |
| Non-dimensional Numbers | = | |
| Le | = | Lewis number |
| Nu | = | Nusselt number |
| Pr | = | Prandtl number |
| Re | = | Reynolds number |
Acknowledgment
The authors would like to acknowledge the financial support of the National Program of Research and Innovation (PNRI) of the Tunisian Ministry of Industry. Acknowledgments are also due to the Tunisian Wood and Furnishing Industry Technical Center (CETIBA), the project coordinator, and the Alternative Energy System (AES) Company, the project industrial partner.
Disclosure statement
No potential conflict of interest was reported by the author(s).