Wood microwave dielectric heating and measurement of material properties

Rimbi, Morris (2004-12)

Thesis (PhD)--University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: Microwave heating technology is extensively used in households, is well-established in certain industries and is being explored in others. lts attraction is that it is quick, energy efficient, clean and can be used to heat materials of both high and low thermal conductivities. Successful application of the technology requires knowledge of both material properties and microwave equipment. Commercial fixtures for measuring dielectric properties are not widely available and are usually created in-house. The domestic oven magnetron is easily available and can be used in prototype microwave systems but applicators for specific applications need development. This thesis covers three main areas, namely; dielectric measurement fixtures, microwave applicators and chokes, and drying of wood. The first part of the thesis presents four dielectric measurement fixtures, one commercial and the others in-house. The design and calibration of the in-house fixtures is presented. An intercomparison of the performance of the four fixtures is carried out by measuring the permittivities of well characterised dielectrics: teflon and perspex. The most convenient fixture is used to measure the dielectric properties of wood at different moisture contents. The second part covers the design of a slotted waveguide fed microwave applicator and a choke. The design of the slotted feed is carried out analytically with and without mutual coupling between the slots. A semi-empirical design using a finite element package is done independent of the analytical approach and the results are compared. The choke is rigorously characterised using a procedure which avoids de-embedding. The third and last section reports on the wood drying experiments carried out in the applicator, simulated and mapped heating patterns of drying wood, and a mock-up industrial wood drying facility.

AFRIKAANSE OPSOMMING: Mikrogolfverhittingstegnologie word wyd gebruik in huishoudings, is goed gevestig in sekere nywerhede en moontlike gebruike daarvan op ander gebiede word gedurig ondersoek. Die vernaamste voordele is dat dit vinnig, energiedoeltreffend en skoon is en dat dit gebruik kan word om materiale van beide hoë en lae termiese geleidingsvermoë te verhit. Suksesvolle toevoeging van die tegnologie vereis kennis van beide materiale-eienskappe en mikrogolftoerusting. Kommersiële apparaat vir die meet van diëlektriese eienskappe is nie wyd beskikbaar nie en word gewoonlik "binnenshuis" ontwikkel. Die huishoudelike mikrogolfoond magnetron is geredelik beskikbaar en kan in prototipe mikrogolfstelsels gebruik word, maar toevoegers geskik vir spesifieke gebruike moet ontwikkel word. Hierdie proefskrif dek drie hoofgebiede, naamlik: diëlektriese metingsapparaat, mikrogolftoevoegers en smoorders, en die droging van hout. Die eerste deel van die proefskrif handeloor vier diëlektriese metingsapparate, een kommersiëel en die ander "binnenshuis". Die ontwerp en kalibrasie van die "binnenshuis" apparate word aangebied. 'n Tussenvergelyking van die verrigtinge van die vier apparate word gedoen deur die meting van baie bekende diëlektrikums: teflon en perspex. Die mees gerieflike apparaat word gebruik vir die meet van die diëlektriese eienskappe van hout teen verskillend voginhoude. Die tweede deel dek die ontwerp van 'n smoorder en 'n mikrogolf toevoeger wat deur 'n gegleufde golfgeleier gevoer word. Die ontwerp van die gegleufde golfgeleier word beide met en sonder wedersydse koppeling tussen die gleuwe, analities gedoen. 'n Semi-empiriese ontwerp wat van 'n eindige-element pakket gebruik maak, is onafhanklik van die analitiese benadering gedoen. Die resultate word dan vergelyk. Die smoorder word deur 'n prosedure wat "de-embedding" vermy, streng gekarakteriseer. Die derde en laaste afdeling behandel die eksperimentele droging van hout binne die toevoeger. Simulasies en verhittingspatrone van die hout wat gedroog word, sowel as die "mock-up(model van die voorgestelde)" nywerheids houtdroogkamer, word gegee.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/50231
This item appears in the following collections: