Joule heating of green Pinus radiata logs for phytosanitary purposes : an in-depth investigation by experimentation and computational modelling. (2018)
Type of ContentTheses / Dissertations
Thesis DisciplineChemical Engineering
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
AuthorsNursultanov, Nurzhanshow all
Pinus radiata is one of New Zealand’s main export products, and much of it is exported as whole green logs. Untreated logsmay be infested by pests, which could be potentially harmful to the flora and fauna of an importing country. Therefore, in order to prevent pests from spreading, the logs must be treated. One of the main current wood treatment technologies is methyl bromide fumigation. The use of this chemical has been restricted due to its ozone depleting properties and, from October 2020, its release to the atmosphere will be prohibited in New Zealand. This research seeks to examine the technical feasibility of an alternative non-chemical method, Joule heating, to sterilize P. radiata green logs, using experimental and computational approaches.
The initial experiments started on green P. radiata boards and showed that for output power of 500Wwith electric field limit of 5000 V¢m¡1 the average Joule heating rate of the pure sapwood was about 11 times higher than that of the heartwood boards. In transitional boards, with coexisting sapwood and heartwood parts, the heartwood was barely heated by the Joule heating effect, as the electric current flowed preferentially through the more conductive sapwood.
Later on, electrical conductivity of green P. radiata sapwood, with moisture content in the range of 100-200%, was studied over a 20-90±C temperature range. The results showed that the conductivity predominantly depends on temperature and grain orientation, and is independent of moisture content and basic density. The electrical conductivity in the longitudinal, radial, and tangential directions increased by factors of 3, 4, and 6, between 20 and 90±C, respectively. Using the linear mixed effectmodel, three empirical equations of electrical conductivity as a function of temperature and grain orientation were derived.
These equations were used in two computational fluid dynamics (CFD) models of Joule heating of export size P. radiata logs: three-dimensional (3D), built using ANSYS CFX, and onedimensional (1D), custom built inMATLAB. In the 1D model, the sapwood of a log was assumed to be isotropic and its electrical conductivity was described by the derived equation of longitudinal electrical conductivity. In the 3D model, the sapwood was assumed to be either isotropic or anisotropic, described by all three derived electrical conductivity equations. These models showed a close agreement; however, as the 1Dmodel had a significantly shorter calculation time, it was used in the further verification and validation steps.
The 1D model was verified and validated using log-heating experiments and its accuracy was evaluated. Although the model cannot accurately predict local temperature distribution, it showed an acceptable agreement with the average global temperature distribution and the total electrical resistance, with the average error around 10%. The 1D model helped to control the heating process and was used to develop a heating sequence that allowed a more uniform temperature distributionwithin the treated logs. This study concluded that, using the CFDmodel and following the heating sequence, Joule heating can be successfully used in the phytosanitary treatment of export logs.