Mass transfer from in-line slabs : application to high-temperature kiln drying of softwoods timber boards.

Type of content
Theses / Dissertations
Publisher's DOI/URI
Thesis discipline
Chemical Engineering
Degree name
Doctor of Philosophy
Publisher
University of Canterbury
Journal Title
Journal ISSN
Volume Title
Language
English
Date
1993
Authors
Kho, Peter Chin Seng
Abstract

The transfer of moisture vapour from the surface of a timber board in a kiln stack has been modelled by investigating the sublimation of naphthalene from a coated slab. The variation of local mass-transfer coefficients over a series of in-line, blunt slabs in the streamwise direction similar to that of a timber stack has been obtained over a velocity range of 3 to 7m s-1. The knowledge of these coefficients will be useful in the design of new kilns for drying specific timbers or in appraising new schedules for existing kilns.

The local mass-transfer coefficients over these boards in the experimental kiln were found to be approximately 60% higher than those predicted from data obtained for a sharp flat-plate at very low turbulence levels. The mass-transfer coefficients varied as the 0.8 power of the velocity, indicating that the transfer was through the turbulent boundary layers. The higher level of free-stream turbulence found in the experimental kiln than those in the wind tunnels used to obtained earlier data agreed with the conclusions of Sugawara et al. [1988], who found that the coefficient in the correlation for heat/mass-transfer is dependent on the freeĀ­ stream turbulence intensity.

Small gaps of up to 1.0 mm wide between the adjacent boards in a layer are inevitable owing to the way boards are arranged to form a stack. Discontinuities in the mass-transfer coefficient profile were observed to occur over these minute gaps as a result of greatly enhanced coefficients in these regions. From flow visualization studies done, a recurring and periodic disturbance, having three distinct phases of motion, was observed to distort constantly the streamlines in the region near to the leading edge of the following board, causing the local mass-transfer coefficients over these regions to be enhanced. Turbulence intensity measurements done by Wu [1989] in a wind tunnel confirmed that turbulence intensity were higher over these gaps than those above the boards. The disturbance had periods of between 0.5 to 1.5 seconds which was in good agreement with the 2 seconds predicted by Langrish et al. [1992] from numerical simulation studies, using a K-B model of turbulence and computational fluid dynamics (CFD) methods. The corresponding Strouhal number, the dimensionless vortex-shedding frequency in the gap was calculated to be 0.05 at a Reynolds number of approximately 800. Sublimation tests and flow visualization studies confirmed that interlayer flow was not the cause of this enhancement.

Minor board irregularities in timber stacks are the consequence of inaccurate sawing, poor stacking and/or changes in dimensions of the boards (shrinking) during the drying process were found to enhance the coefficients and are favourable for drying permeable boards at high temperatures where external conditions in the kiln are relatively more important.

Surface roughness has relatively insignificant effect on the coefficients, confirming the current practice of machining the boards after drying because of added dimensional stability.

Thinner stickers of 15 mm in thickness can be used to increase kiln capacity as sticker thickness over the range 15 to 25 mm was found to have negligible effect on the local massĀ­ transfer coefficients.

The coefficients of the board 3 metres downstream was found to reach an asymptote value at similar levels, suggesting the possibility of using wider stacks to increase kiln capacity. However, drying may be hindered towards the end of the wide stack where the moisture vapour concentration gradient between the board and the bulk air stream may be approaching zero.

Description
Citation
Keywords
Ngā upoko tukutuku/Māori subject headings
ANZSRC fields of research
Rights
All Right Reserved