Experimental study and numerical simulation of flow pattern and heat - PDF document

‘The Future of Quality Control for Wood & Wood Products’, 4-7 th May 2010, Edinburgh The Final Conference of COST Action E53 Experimental study and numerical simulation of flow pattern and heat transfer during steam drying wood J. Bara ń ski 1 , M. A. Wierzbowski 2 , J. A. Stasiek 3 Abstract The high cost of fossil fuel and soaring consumer interest have encouraged people in the wood industry to look for faster and more energy-efficient methods to dry lumber. In this paper results of experimental study and numerical simulation of flow pattern and heat transfer during steam wood drying are presented. Wood species, namely oak ( Quercus L. ) and pine ( Pinus L. ), were subject of steam drying process in a laboratory kiln especially arranged for that reason (Wierzbowski et al. , 2008). Main focus of those tests was to shorten the time of drying process and afterward to check properties of wood. As results of mechanical properties checking are presented in separate paper, here authors focused on numerical predictions of uniform velocity and temperature profiles through the drying kiln, which is of great importance for drying and also for energy saving. Predicted velocities were used in the laboratory kiln for tests. Satisfactory results were obtained as the time of drying process was significantly reduced. The kiln is equipped with heat exchanger supplied by exhaust gases from furnace allowing spread water to evaporate on its surface. Generated steam, by circulation fan, is distributed between the wood stake. The dryer is for all timber species in terms of final moisture content to 6 % in the high temperature of up to 150 o C. Model of drying chamber was created consistent with existing experimental rig. The flow pattern around and through an array of in-line truncated boards of oak and pine have been simulated numerically. The Renormalization Group k- ε turbulence model and model for the near-wall treatments have been used for simulation. Vortex shedding from in-line boards separated by small gaps have also been numerically investigated. Small gaps between in-line boards have an insignificant effect on the mass transfer. 1 Introduction Reduction of energy consumption and drying processing time are currently two important objectives of timber industry, as drying is one of the most costly consuming steps in terms of energy and time. Extensive researches have been done and are still in progress to determine the optimal drying strategy to achieve the required timber quality at minimum cost. However, most of 1 Senior Research Fellow, jbaransk@pg.gda.pl Mechanical Engineering Faculty, Gdansk University of Technology, Poland 2 Senior Research Fellow, rwierzbo@pg.gda.pl Mechanical Engineering Faculty, Gdansk University of Technology, Poland 3 Professor, jstasiek@pg.gda.pl Mechanical Engineering Faculty, Gdansk University of Technology, Poland http://cte.napier.ac.uk/e53

‘The Future of Quality Control for Wood & Wood Products’, 4-7 th May 2010, Edinburgh The Final Conference of COST Action E53 experiments and modeling predictions focus on heat and mass transport within the boards, while, in practice, local drying conditions in the kiln strongly interact with heat and mass transport inside wood. Especially these are essential for High Temperature Drying (HTD) schedules with short drying times and uniform drying conditions (air temperature, humidity and velocity) in the kiln. The instrumentation and equipment in the kiln provides, together with a control system, environment, which is apparently different from the conventional drying. One important application of mass and heat transfer coefficients, over in-line boards stacked in an array, is analysis of the external convective transfer processes in drying of timber. Accurate evaluation of the external heat and mass transfer coefficients determines proper design of wood drying kilns and optimum operation of drying processes (Sun et al ., 2000). Consequently, it is necessary to revisit this investigation of external transfer over in-line boards. From the other hand thanks to development of the wood drying techniques using saturated or superheated steam and gas-steam mixture flow, waiting time for wood material industry can be reduced and brings economic benefits, such as protection of wood against fungi and fracture, which extends its life. In this paper results of experimental study and numerical simulation of flow pattern and heat transfer during steam drying of wood is presented. Wood species, namely oak ( Quercus L. ) and pine ( Pinus L. ) were subject of steam drying process in a laboratory kiln specially designed for the purposes of the research. 2 Experimental background Drying in superheated steam is economically justified because of the shorter processing time and reduced energy consumption in comparison to drying in hot air. Evaporation of free water does not change wood shape and main dimensions during process of wood drying. With the loss of water evaporation zone moves deeper into the wood. The proper conduct of the drying process allows faster extraction of water (Gard et al ., 2008). In the initial stage of drying hot water was supplied to the chamber to increase humidity and temperature throughout the material. Exhaust gases flow through the heat exchanger to raise the temperature of the mixture in the chamber. As far as humidity and temperature grow, we start with the drying process. The process of drying the material continues to achieve the assumed wood humidity of 10 % EMC (equilibrium moisture content). The next process was the conditioning of wood - slowly cooled chamber with getting hot water to remove the stress in the material which emerged during the whole process of drying. During drying process of great importance are: • physical properties of drying agent, • evaporation of water from both timber and free surface, • hygroscopic properties of wood (depending on the species), • hygroscopic equilibrium of wood, • changes inside wood during evaporation. Wood species, namely oak ( Quercus L. ) and pine ( Pinus L. ), were subject of steam drying process in a laboratory kiln. The kiln is equipped with heat http://cte.napier.ac.uk/e53

‘The Future of Quality Control for Wood & Wood Products’, 4-7 th May 2010, Edinburgh The Final Conference of COST Action E53 exchanger supplied by exhaust gases from furnace. Water, spread from two nozzles, evaporates on exchanger’s surface. Generated steam, is distributed between the wood stake by circulating fan. The dryer is dedicated for all timber species of final moisture content to 6 % in the high temperature of up to 150 o C. Detailed description of laboratory kiln was presented previously (Wierzbowski et al. , 2009, Wierzbowski et al. , 2008). Figure 1. View of the oak ( Quercus L. ) pile of boards inside the kiln. Figure 2. View of the pine ( Pinus L. ) stack inside the kiln. inlet temperature measurement outlet temperature measurement moisture and temperature measurement temperature measurement Figure 3. Model of the drying kiln with measurement points and air supply area. http://cte.napier.ac.uk/e53

‘The Future of Quality Control for Wood & Wood Products’, 4-7 th May 2010, Edinburgh The Final Conference of COST Action E53 Figure 4. Dimensions of stack of boards and location of probes for measuring temperature and moisture content during experiment. Figure 5. Dimensions of stack of timber and location of probes for measuring temperature and moisture content during experiment. 3 Numerical modelling For separated flows and recirculating flows around the blunt boards in a stack, the renormalization group (RNG) k- ε turbulence model (Yakhot et al. , 1986) has been used to solve the turbulent momentum and species transport equations in a three-dimensional geometry. The model equations in their RNG form are similar to those for the standard k- ε model. The RNG k- ε model employs a differential form of the relation for the effective viscosity, yielding an accurate description of how the effective turbulent transport varies with the effective http://cte.napier.ac.uk/e53