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This thesis presents a numerical simulation study for the heat and mass transfer in- and out-side individual boards of a stack during kiln drying of wood and on the effect of side gaps between the boards. The objective is to optimize the drying process for efficiency and high quality products. A literature survey in the area is presented. The importance of the correct link between the transport processes in wood, and heat/mass transfer and fluid flow in the surrounding drying air is emphasized. Objectives, motivations and needs for the present study are also presented. This is followed by a detailed account of the governing equations, description of models, physical properties, and discretization and solution procedures used in the present study. A sample stack of planks has been used to evaluate the performance of various turbulence models and upwinding schemes of the CFX software developed for predicting the transport parameters in air. Given a typical stack set up for drying 105 x 105 mm western hemlock lumber, the effects of side gaps on surface coefficients are studied for different air velocities. An optimum gap size for maximum heat/mass transfer is suggested. The model developed for heat and mass transfer inside the wood is validated for a one dimensional case by comparing the numerical results with published results. The improved performance using a newly proposed relationship for the diffusivity of bound water has been demonstrated. Also a new relationship for the mass transfer boundary condition at the surfaces was proposed to incorporate the effect of the surface resistance. Relative effects of model unknowns in predicting the average moisture content and board center temperatures are discussed. A Fortran program was developed to solve the two-dimensional coupled heat and mass transfer equations inside the wood during the drying process. The model considers the changes in air temperature and humidity due to heat and mass transfer to and from the boards. The iterative SOR (Successive Over Relaxation) method was modified to increase accuracy and stability. Predictions for average and local moisture content are in good agreement with experiments. The effect of side gap on the drying process inside the wood was also examined. A plot of standard deviation of each board versus the board average moisture content is suggested for the judgement about the uniformity of the products. Results indicate that without using the extra gap size there exists a considerable difference between the maximum and minimum final average moisture content of the boards in each row. The first and last boards are usually over-dried. By using the previously suggested gap size the maximum difference of the final average moisture content is almost half the case without the extra gap. Average diffusion and surface coefficients are extracted from the experimental data of drying a stack without side gap. A software tool has been developed to solve the simple unsteady one-dimensional diffusion problem. Results are compared with experiments. The introduced method can be used to obtain the average diffusion and surface coefficients. / Graduate

Theoretical expressions for the transport coefficients of a single component gas with a nonzero but small local angular momentum density are obtained from a modified Boltzmann equation which takes into account the presence of degenerate internal states (specifically, rotational states). As is to be expected, a number of Onsager reciprocal relations are found connecting the transport coefficients. A linearization of the Boltzmann equation is carried out by means of a perturbation expansion about a local equilibrium state which is characterized by a local temperature, stream velocity and angular momentum density. This perturbation is expressed as a linear combination of the macroscopic gradients of the system, whose coefficients, being tensors, are expanded in terms of irreducible Cartesian tensors made up of the angular momentum pseudovector operator J and the reduced velocity vector W. The transport coefficients are then given by combinations of certain scalar expansion coefficients. Expressions for these expansion coefficients in terms of square bracket integrals are obtained with the aid of an iterative variational procedure based on a scalar product which allows for the lack of time reversal symmetry of the Boltzmann collision operator. Finally, the square bracket integrals are reduced to relative and center-of-mass coordinates and expressed in terms of generalized collision cross sections. The techniques developed for the rotating gas with a nonzero local angular momentum density are utilized to obtain an expression for the change in the thermal conductivity of a gas when placed in a magnetic field. It is shown that at saturation the ratio of the changes in the thermal conductivity with the magnetic field (a) parallel to, and (b) perpendicular to, the temperature gradient is 2/3. This value agrees with the experimental result for paramagnetic gases. / Science, Faculty of / Chemistry, Department of / Graduate

The response of a polyatomic gas to microwave radiation including both steady state (pressure broadening) and time dependent (coherence transients) effects - is described theoretically. The treatment is based on solutions of a quantum mechanical Boltzmahn equation and employs kinetic theory methods which have previously been used in the explanation of the field dependence of transport phenomena (Senftleben-Beenakker effects). Much of the recent theoretical work of pressure broadening and coherence transient phenomena is based on a two (energy) state model for the gas molecules. This model, when developed from a density operator point of view, results in a coupled set of three equations which are mathematically equivalent to the Bloch equations of NMR. The present work reexamines this description, and replaces it with a two level model for the gas system. Here, the term "level" implies explicit consideration of the rotational (magnetic) degeneracy associated with each energy state. This model gives a more appropriate representation of the interaction of microwave radiation with a real molecular system. In particular, a more complete set of coupled equations result from this description and involve quantities in addition to the three moments used in a two state approach. The most important of these latter effects are represented by spherical harmonics Ƴ(q) (J) in he angular momentum J of the relevant energy levels. An analogous treatment of rotational effects has previously been used in Senftleben-Beenakker studies. Specific molecular types of interest in microwave spectroscopy -diamagnetic diatonics and linear polyatomics, symmetric tops, and inverting symmetric tops - are treated separately by this two level approach. The vector (and tensor) nature of the motions are emphasized throughout. The number of rotational polarizations that arise in the general two level case is often quite large. The simplest example of a two level system is the j=0 to j=l transition of a diamagnetic diatomic. This is studied in some detail. Here, the scalar component Ƴ⁽²⁾⁰(J) is the only rotational polarization affected by linearly polarized radiation in the usual experiments. The effect of this quantity on both steady state and transient phenomena is described, and a new "combination" experiment is suggested as the best way to detect the presence of this additional polarization. The Doppler effect is treated by appropriately including the effects of translational motion in the quantum Boltzmann equation. A more general set of coupled moment equations then results, and the manner in which the macroscopic velocity polarizations arise is thereby established. A model method solution of the quantum Boltzmann equation, emphasizing the parity invariance of the collision super-operator, is given for a steady state absorption experiment in the absence of saturation but including Doppler effects. Throughout this thesis, the relaxation rates, are related to kinetic theory collision cross sections by solving the quantum Boltzmann equation. Extensive use is made of rotational invariance to reduce the number of independent collision integrals, and their approximate evaluation is accomplished within the context of the distorted wave Born approximation. All collision integrals for the pure internal state polarizations are found to-be expressible in terms of one translational factor, which is itself further approximated by a modified Born approximation. Correspondingly, the translational factor which arises in the relaxation of macroscopic velocity polarizations is completely specified by relating it to the Ω(ℓ,s) integrals of traditional kinetic theory. / Science, Faculty of / Chemistry, Department of / Graduate

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Thesis (M. S.)--Nuclear and Radiological Engineering and Medical Physics, Georgia Institute of Technology, 2010. / Committee Chair: Rahnema, Farzad; Committee Member: Wang, Chris; Committee Member: Zhang, Dingkang. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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