Influence of improved structure of drying kiln on the uniformity of wind velocity flow field

Abstract: A uniform flow field means the fluid flowing into a wood drying kiln with equal wind velocity per unit area. The uniformity of the wind flow field can determine the drying quality and efficiency of sawn timber in piles. However, there is often uneven distribution of flow field of wind speed in the top drying kiln. In this study, an improved design was presented by the combined Algorithm for Inventive-Problem Solving (ARIZ) and Computational Fluid Dynamics (CFD) in a wood drying kiln. The ARIZ was applied to modify the structure of the kiln, and the CFD technology was to analyze the improved model. The optimal model of wood drying kiln was obtained after a comprehensive evaluation. The better performance of wood drying equipment was achieved to clarify the influence of the structural parameters on the uniformity of flow field of wind speed in the kiln. In the experiment, a conventional drying was adopted, where the type of drying kiln was a small top air one. In the drying intermediate stage, the moisture was removed after the combined water inside the wood was transformed into steam, according to the airflow characteristics in the kiln. In a two-dimensional CFD simulation, the inlet and exhaust ports were considered to be fully open, where the initial wind speed was 3 m/s at the air inlet, and the exhaust port was free air. The simulation data were then compared with the measured to verify the model. Three aspects were applied to improve the design of the kiln under the ARIZ: to optimize the kiln structure, to adjust the gap between sawdust, and improve the diversion mode. Four schemes were designed for each improved direction, further to obtain a total of 12 geometric models. The CFD simulation was then used to analyze the flow field of wind speed of different models. Two directions were determined to adjust the gap between saw blades and adding deflector in the kiln. Four evaluation indexes were selected, including the cloud diagram for the flow field of wind speed, the difference of wind speed between detection points, the average speed, and the coefficient of speed unevenness. The wind speed data of 84 monitoring points were evaluated to iterate an optimal scheme. It was found that the difference of wind speed at the monitoring point decreased from 1.098 m/s to -0.058 m/s, approaching 0 in the modified optimum scheme. There was a small difference in wind speed between the top and bottom of the kiln, where the average speed increased by 15.60%. The coefficient of speed unevenness was reduced by 72.70%. As such, the performance indexes all met the requirements of national standards. Consequently, the iterative design of structure using ARIZ can effectively improve the non-uniform distribution of flow field of wind speed inside the kiln. The numerical simulation can provide a strong reference to design the drying kiln with a reasonable structure for energy saving and higher production.