摘要:

Pine sapwood was dried in an air convection kiln at temperatures between 60-80 °C. Temperature and weight measurements were used to calculate the position of the evaporation front beneath the surface. It was assumed that the drying during a first regime is controlled by the heat transfer to the evaporation front until irreducible saturation occurs. Comparisons were made with CT-scanned density pictures of the dry shell formation during initial stages of drying of boards.

ISSN:0737-3937    

DOI:10.1080/07373930008917804

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this work a comprehensive set of experimental results are used as an excellent means to understand the coupling that exists between the material and die electromagnetic fields in a specific industrial microwave applicator. The analysis of the infrared images allows an accurate map of the power and temperature distributions within the wood sample to be determined. This map, together with the simulation results of a previously developed computational electromagnetic model, can provide a detailed understanding of the design features of the microwave applicator. In particular, it is possible to locate the occurrence of localised hot spots and to examine the uniformity of the heat distribution throughout the sample. The simulation results provide the evolution of the electromagnetic fields inside the entire applicator and the sample. The coupling of theory and practice is the best way to proceed in optimising the design and for proposing new applicator geometry that can heat the material more effectively.

ISSN:0737-3937    

DOI:10.1080/07373930008917805

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A one-dimensional stress model was proposed for drying of radiata pine lumber, which has considered wood moisture shrinkage, instantaneous stress-strain relationships, mechano-sorptive creep, time-induced creep and temperature effects. In addition, wood hardening behaviour in the plastic region and differences between stress increase and decrease have been taken into account. The proposed Stress model can predict stress development and relief in a drying cycle once the required wood mechanical and Theological properties have been quantified.

ISSN:0737-3937    

DOI:10.1080/07373930008917806

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A method based on the concept of solving inverse heat and mass transport problems was proposed to identify wood physical properties, with the use of empirical data, a mathematical model corresponding to the direct problem, and an optimization procedure. The computer software was developed to solve transient, three-dimensional, quasi-linear direct and inverse problems of heat and mass transport in wood as an anisotropic body together with initial and boundary conditions of any kind. The software was adapted to data parallel processing environment of high performance computers. Identification of thermal conductivity was performed for beech wood in three principal anatomic directions, and accuracy of temperature distribution predictions was significantly increased (the global relative error of prediction was reduced to 1.0 - 1.9%).

ISSN:0737-3937    

DOI:10.1080/07373930008917807

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The dynamic characteristics of the wood drying process were represented by an actuator network and an internal model. These two subsystems are composed of a set of ordinary differential, partial differential and algebraic equations with adjusting operating functions. The formulation of the operating functions is based on interaction between air and the lumber board in a semi-industrial drying kiln. Using this internal model, it was possible to establish a state observer, a state predictor, and a reference model for which the boundary conditions were generated by the actuator network. Simulation results were obtained with a C++ software package. In order to implement the optimization techniques to the wood drying process, the two-level control structure was proposed and tested for a minimum drying time as a performance criterion. Numerical results confirm that the internal model and its operating functions were properly chosen.

ISSN:0737-3937    

DOI:10.1080/07373930008917808

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A two-dimensional mathematical model for vacuum-contact drying of wood is presented. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady state conservation equation of dry air. Most of the model parameters were determined during independent experiments. The set of equations is then solved in a coupled form using the finite element method. The validation of the model is performed using experimental results obtained during vacuum-contact drying of sugar maple sapwood. The experimental and calculated data are in good agreement. Nevertheless, some discrepancies are observed which can be attributed to the boundary conditions used and to the fact that heat transfer by convection was neglected.

ISSN:0737-3937    

DOI:10.1080/07373930008917809

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Nonlinear programming methods are used to create effective drying schedules for boards. A two-dimensional, orthotropic drying model is connected to an optimization routine that solves convex sub-problems. The iterative optimization program calculates the drying schedule, i.e. the variation of temperature and humidity with time, that gives the shortest drying time for a board with prescribed upper and lower level of the moisture content. To demonstrate the capability of the technique, numerical results are presented.

ISSN:0737-3937    

DOI:10.1080/07373930008917810

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The amount of biological variability in timber creates considerable problems in producing timber of adequate and reproducible quality with a predictable amount of variability in final moisture contents. The development of optimised drying schedules for addressing these problems is therefore desirable. Previous methods (largely of a stochastic type) are reviewed in this work and their limitations assessed. The physical parameters which have the greatest impact on the stress levels (and hence quality) of the timber have been assessed using a diffusion model for the drying of Australian hardwood timber. This deterministic model is then used, together with statistical methods for quantifying the confidence regions for the variability in the physical parameters with the greatest impact, in a systematic technique to develop a new optimised schedule for grey ironbark timber (Eucalyptus paniculaia). This new schedule is then compared with a previous optimised schedule, which did not take this variability into account. The productivity (amount of good quality timber divided by drying time) appears to be maximised when the schedule is such that 90% of the timber produced is good quality.

ISSN:0737-3937    

DOI:10.1080/07373930008917811

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Transferring the necessary heat of evaporation to the stack is the bottleneck in convective vacuum drying of wood. Higher gas velocities are applied to compensate for the lower gas density and to obtain similar heat and mass transfer characteristics as under normal pressure. Like in conventional kiln drying the region with the most unfavorable drying conditions determines drying time and product quality. To use the full potential of the meanwhile established superheated steam vacuum drying technology, it is therefore necessary to work on an improved uniformity of process conditions in the kiln.

ISSN:0737-3937    

DOI:10.1080/07373930008917812

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Nearly eighty percent of the lumber manufactured in the United States is softwood dimension lumber used for the manufacture of housing. The principal quality related problem with softwood dimension lumber is that it is subject to warp related degradation. Much of the degradation occurs during the initial drying process, however, residual stresses and moisture content changes that occur after kiln drying can either reduce or increase the amount of warp. The magnitude of post kilning warp change, here termed delayed warp, has not been well quantified. Measurements of twist, bow, crook and moisture content were taken from eastern spruce (Picea spp) studs immediately after drying and planing. The wood was allowed to sit without restraint for a period of two weeks after which the measurements were repeated. Of the 1224 studs measured, over 76 percent experienced changes in twist, over 84 percent experienced changes in bow and over 73 percent experienced changes in crook. Increases in warp generally exceeded decreases, although a surprising percentage of the studs showed decreases in warp. Attention was focused on finding a correlation between moisture content and the changes in warp, but the correlation coefficients were low for both the increases and the decreases. The most likely cause of the warp was the high percentage of juvenile wood present in the raw material. The most likely causes for the changes in warp during the two-week period were stress relaxation aggravated by a change in the moisture gradient.

ISSN:0737-3937    

DOI:10.1080/07373930008917813

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor