The objective of this research based on both the theory and experimentation was to prove that the total pressure difference is the primary driving force during the vacuum drying. The theoretical drying rates of diffusion, free water bulk flow and water vapor bulk flow were calculated and compared. The concept of equilibrium moisture content under the vacuum was developed. The theoretical maximum moisture content drop in one cycle was calculated using energy balance. The model was developed for the vacuum drying to understand the mechanism of the vacuum drying including the boiling front and its movement. To evaluate the effect of the sample size on the drying rate, four different thicknesses (1, 1.5, 2, 2.5 inches) and three different lengths (5, 10, 15 inches) were used. In the cyclic drying, the specimens were heated to the 60 C. The vacuum was pulled to about 18 mm Hg. The vacuum pump was kept running for 140 minutes. It was found that in cyclic vacuum drying, drying rate was not affected by the thickness. However, it was affected by the length. The cyclic drying curve consisted of two distinct parts. The fast drying period lasted about 10 to 20 minutes. The slow drying period occurred next when the pressure inside wood got close to the ambient pressure. In end grain vacuum drying, the specimens were coated with wax, wrapped in the plastic film and inserted into a rubber tube to prevent the moisture loss from the side surfaces during drying. The specimen size was 1×1×10 inches. Red oak and white oak were sealed and dried in both cyclic and continuous vacuum drying. The results showed that sealed specimens dried almost as fast as unsealed specimen. There was little moisture loss from the side surfaces. There was a moisture gradient along the length in both cyclic drying and continuous vacuum drying. Red oak specimens of 2.5×1.5×10 inches were used to study the boiling front in the vacuum drying. In order to detect the boiling phenomenon, the saturation pressures were calculated and were compared with the pressures at the same time and the same location. Boiling occurred during drying and the boiling front retreated to the center of wood as drying proceeded. The retreating speed depended on the heat supply and the permeability. Vacuum drying at room temperature was investigated. The specimens were dried at 20 C and pressure near 18 mm Hg. The results showed that wood can be vacuum dried at room temperature with little or no degrade at a reasonable drying rate. All experimental results support the objective of this study that the primary driving force is the total pressure difference. / Ph. D.

This thesis is about fundamental interaction mechanisms of laser remote fusion cutting, RFC, which is based on the formation of a quasi-stationary laser-induced boiling front that causes drop ejection, preferably downwards. Laser cutting of metals, invented in 1967, has developed from a niche to a well established high quality cutting technique in the manufacturing industry. Usually a gas jet is employed concentric to the laser beam, to eject the molten metal. One technique option, interesting though hardly applied yet because of usually low quality and speed, is remote laser cutting. Two techniques are distinguished, remote ablation cutting, grooving down through a sheet, layer-by-layer, and the here addressed remote fusion cutting, by a single pass through the sheet. For the latter, the ablation pressure from laser-induced boiling at the cutting front continuously accelerates and ejects the melt downwards. Advantages of remote laser cutting, facilitated by high brilliance lasers during the last decade, are the possibility of a larger working distance along with the avoidance of cutting gas and of a gas jet nozzle.   The review paper of the thesis surveys different laser remote cutting techniques, including their modelling, as well as the transition to keyhole welding, owing to similarities particularly from the boiling front and from root spatter ejection. The six Papers I-VI that compose the thesis address fundamental mechanisms of laser remote fusion cutting, theoretically and experimentally. In Paper I a simplified mathematical model of the RFC cutting front enables to estimate the geometrical and energetic conditions of the process. By evidence and post-modelling from high speed imaging, HSI, the simplified smooth cutting front model is developed further to a wavy topology in Paper III, for more sophisticated absorption analysis. As a systematic support, Paper II categorizes and analyses for the first time the different wavy topologies observed at the front, from HSI. The melt dynamics induced by a pulsed laser beam was studied in Paper IV, again from HSI. Apart from other interesting transient melt phenomena it was demonstrated that the ablation pressure can push the melt to a certain pending position during the laser pulse while the melt retreats by surface tension during the pulse break. To engage remote fusion cutting with additive manufacturing, Paper V introduces a novel technique where the drops ejected from RFC are transferred to a substrate, about a centimetre underneath, on which a continuous track forms. This technique can even be applied as an efficient recycling approach. In Paper VI a variant of the technique is presented, to develop a boiling front along the edge of a metal sheet from which the drop transfer takes place, in a different manner. This enables to systematically machine-off the entire sheet, which can be converted to a new shape and product.   Summarizing, the thesis provides a variety of analysis of fundamental mechanisms of a laser-induced boiling front that bear a certain simplicity and in turn controllability, of interest for established as well as for new applications, in manufacturing and in other sectors, including remote fusion cutting.

La vaporisation explosive, ou flashing, par dépressurisation rapide du C6F14 au travers d'une vanne à boisseau sphérique est analysée expérimentalement sur une grande plage de surchauffe. Les visualisations rapides montrent un jet s’ouvrant très largement en aval, preuve de l'existence d'un fort gradient de pression. Des mesures locales par sonde optique et par vélocimétrie phase Doppler ont permis de caractériser tailles, vitesses, concentration et flux numérique des gouttes en fonction de la surchauffe. La faible influence du degré de surchauffe sur la remontée en pression et sur les tailles et flux de gouttes suggère que le champ de pression s’adapte, et que par conséquent le liquide est soumis à une surchauffe locale bien plus faible que la surchauffe globale imposée. Différents scenarii sont discutés pour expliquer les observations, dont le fait que les tailles de gouttes sont peu sensibles à la surchauffe. Celui basé sur l’existence d’un front d’ébullition est le plus probable. / Depressurization (flashing) experiments through a ball valve were conducted with C6F14 for a large range of superheat. High-speed imaging shows a rapid and wide expansion of the jet, which evidences strong pressure gradients. Local measurements with phase detection optical probes and phase Doppler velocimetry were used to characterize size, speed, concentration and volumetric flux of drops as a function of superheat. The level of superheat has little influence on the vaporized fraction and on drop size and flux: this suggests that due to strong pressure gradients the liquid sees a much weaker level of superheat than the global superheat imposed on the system. Different scenarii are discussed to explain observations, in particular the fact that drop size remains approximately constant independent of the superheat. A scenario based on the existence of a boiling front seems to be the most consistent.