H. Resch

A review of basic aspects of heat transfer under impinging air jets

The literature on the heat transfer occurring from impinging hot air jets, now employed in many driers is reviewed. Variations in average and local heat transfer coefficients and other parameters are discussed in respect to jet air velocity, distance from jet to the impinged surface, and jet turbulence level. Areas which require additional research are pointed out, in addition to a discussion on elementary heat transfer and a brief review of research on mass transfer occurring under impinging jets.

Über die Holztrocknung mit Mikrowellen

ZusammenfassungHolz kann in einem hochfrequenten, elektrischen Wechselfeld erwärmt und getrocknet werden. Die Erwärmung je Leistungseinheit ist mit Mikrowellen stärker als mit Radiofrequenzenergie, da die Energiedichte mit der Frequenz wächst. Da auch dünne Materialschichten leicht zu behandeln sind und der Wirkungsgrad der Mikrowellenröhren verhältnismäßig hoch ist, hat man in den letzten Jahren Trocknungssysteme mit 2450 MHz- und 915 MHz-Wellen versuchsweise auch an Holz und Papier verwendet.Die Erwärmung kann im stehenden oder fortschreitenden Mikrowellenfeld erfolgen. Sie ist zum Teil von den dielektrischen Eigenschaften des Holzstoffes abhängig, welche besonders bei Feuchtigkeitsänderungen variieren. Das gesichtete Schrifttum und neuere Versuche weisen darauf hin, daß Mikrowellen besonders für die Trocknung von dünneren Holzerzeugnissen eingesetzt werden können. So ließ sich die Feuchtigkeit in ungleichmäßig oder ungenügend getrockneten Furnieren ausgleichen und 25 mm dickes Schnittholz im Feuchtigkeitsbereich unterhalb des Fasersättigungspunktes sehr schnell trocknen. Hauptvorteile der Anwendung von Mikrowellen sind die Möglichkeit einer sehr schnellen Erwärmung, der selektive Energienmsatz in feuchteren Zonen verbunden mit deren bevorzugter Trocknung, und das Auftreten von Temperatur- und Feuchtigkeitsgradienten, welche der Verschalung entgegenwirken und die Trocknung beschleunigen.SummaryWood may be heated and dried in an at high frequency alternating electric field. Heating with microwaves can be more intensive than with radio-frequency current, because the density of electromagnetic energy increases with frequency. During the past few years, drying systems employing 2450 MHz and 915 MHz waves have been used experimentally to dry wood and paper, mainly because microwave applicators lend themselves readily for the rapid heating of thin materials and because microwave tubes are of high efficiency.Heating may be accomplished in a static or a progressive microwave field and depends partly on the dielectric properties of wood products which vary with moisture content. A literature review and recent experimentation indicate strongly that microwaves can be employed successfully especially for the drying of thin wood products. In unequally or insufficiently dried veneers the moisture content could be equalized and one inch thick lumber rapidly dried in the moisture content range below the fiber saturation point. The main advantages of the application of microwaves are: rapid heating; greater energy transfer to regions of higher moisture content resulting in selective drying; and the development of temperature and moisture gradients which prevent casehardening and excessive drying of the surface thus increasing the drying rate.

High-Frequency Energy–Assisted Vacuum Drying of Fresh Eucalyptus globulus

Worldwide, eucalyptus tree plantations have been established in appropriate climates because of fast growth and wood qualities suitable mainly for pulp. A potential exists of converting eucalyptus trees into lumber that may be of higher value than pulp. Conventional drying of lumber of Eucalyptus globulus is often difficult because of the occurrence of drying stresses, leading to collapse and checking. The special method of vacuum drying while heating the wood with high-frequency energy (75–77 mbar, 46–51°C) was used to obtain short drying times (5–13 days from green state to 10% final moisture content) and low crack amount.

Heat transfer from impinging slot jets of air

SummaryThe heat transfer of two-dimensional impinging air jets was examined. Jet widths from 0.0625 to 0.3750 inches (1.59 to 9.53 mm) operating at exit velocities from 2,000 to 10,000 ft/min (10.2 to 50.8 in/sec) were used. Average and local heat transfer coefficients were measured for jet arrays having a variety of different vertical and horizontal spacings. Predictive equations describing average heat transfer data were obtained which explain approximately 95 percent of the measured variation. Jet exit velocity and the lateral distance between the centerline of adjacent jets had the greatest influence on heat transfer.

Heat transfer from impinging slot jets of air part 1: Aerodynamic characteristics of free jets

Aerodynamic data on the axial velocity and turbulence development of free slot jets is presented. Jets varying from 0.0635 to 0.375 inches (1.59 to 9.53 mm) in width and with air velocities between 2,000 and 10,000 ft/min (10.2 to 50.8 m/sec) were examined. This study was the first phase of a research program aimed at developing empirical data describing impinging slot jet heat transfer commonly used in veneer drying.

Research and Development Goals for Wood Technology in Austria

Austrians have been cultivating and using their forests for centuries on the basis of professional knowledge, tradition and love for nature. The idea of sustainable use was born centuries ago and cast in forest law. The latest version of the Austrian forest law dates back to 1975 with an amendment in 1987. It aims at protecting the forest for its 4 functions of multiple use, i.e. commercial production, protection for human settlements, protection of the environment and use for recreation.

Über die nicht-stationäre Bewegung von Luft und Butandampf durch Holz

ZusammenfassungGesetzmäßigkeiten der nicht-stationären Gasströmung durch douglasienholz wurden an Hand von Versuchen mit Luft und Butandampf mittels einer kleinen Cellon-Imprägnieranlage untersucht. Um Grenzbedingungen für theoretische Berechnungen des Strömungsverlaufes zu erhalten, wurden Druckänderungen an der Oberfläche und mit Hilfe einer elektrischen Meßrürcke auch im Innern von Rundhölzern gemessen. Für die Analyse der nicht-stationären Strömung diente als theoretisches Schema eine partielle Differentialgleichung, die auf der Strömungsformel von Darcy beruht.Die aus theoretischen Berechungen resultierenden. Ergebnisse zeigten für die Strömung von Luft eine gute Übereinstimmung mit exprimentellen Meßwerten. Obwohl der Verlauf der Druckabfallskurven beim Entfernen von Butandampf denjenigen von Luft in ihrer Form glichen, vollzog sich der Druckabfall doch langsamer als errechnet. Als Grund dafür dürfte eine Verminderung der Permeabilität des Douglasienholzes durch die Imprägnierung mit verflüssigtem Butan anzuführen sein.Auf Grudn der Ergebnisse wurden einigen theoretische überlegungen hinsichtlich des Druckverlaufes im Rundholz bei der Evakuierung von Luft angestellt.SummaryNon-steady state flow of air and butane vapor through wood of Douglast-fir was investigated using a small Cellon pilot plant. In order to obtain boundary condtions for theoretical calculations, pressure changes were measured at the surface of round logs and inside by means of an electronic pressure transducer. A non-linear partial differential equation based on Darcys law served as a theoretical model for the description of the flow. The solution of this equation was in good agreeement with experimentally measured values as far as the movement of air was concerned. The shape of the pressure reduction curves obtained during the removal of butane vapor was similar to that obtained with air. However, the pressure reduction was somewhat slower than predicted by the theory. The reduction of the wood permeability caused by the impregnantion with liquid butane seems to be an explanation.Based on the experimental results, the pressur distribution in round logs during their evacuation from air is discussed.