Ph. Bournot

Experimental study of CO2‐laser‐induced air breakdown over long distances

Results of an experimental study on air breakdown produced by radiation from a high‐power CO2 laser are presented. From these measurements, the breakdown threshold over a flux range (1.5×108 <φ<7×108 W cm−2) is obtained as a function of the focal length varying from 1 to 55 m. The first of these experiments was carried out in the laboratory atmosphere (10<f/D<100), whereas the second part (f/D=135) was done outdoors under typical atmosphere conditions. The temporal evolution of the energy transmitted through the breakdown region for different laser parameters (5<Ei<220 J) was found.

Influence of a Coflowing Ambient Stream on a Turbulent Axisymmetric Buoyant Jet

This paper reports numerical results on turbulent buoyant axisymmetric jets in a coflowing ambient stream. The objective of this study is to compare the performance of the Reynolds stress algebraic model (ASM) with that of the k-e turbulence model in predicting the flow field. A finite difference method has been used to solve a system of coupled partial differential equations. A comparison has been carried out between the numerical results obtained in the present work and experimental and numerical data reported in the literature. It has been found that the two investigated models reasonably predict the mean flow properties of the flow field. Nevertheless, the ASM proves to be better than the k-e method to predict the effects of buoyancy and the turbulence structure. It has been found that the increase of the coflow can slow the development of the jet to the state of similarity of mean characteristic profiles. A jet with a ratio of coflow velocity Ū ∞ to jet discharge velocity Ū 0 less than 0.05 has developed to closely approximate a free jet in a stagnant medium while a jet with higher Ū ∞ /Ū 0 ratio never reaches a similarity state. In buoyant jets, only a flow with u ∝ /u 0 ≤ 0.05 reaches a similarity state. Buoyancy ensures that the similarity region begins at a distance closer to the nozzle exit than if the medium is stagnant.

Aerothermic characterization of a high-power fast axial flow CO2 laser

Abstract Because the structure of the flow in glow discharge tubes for high-power CO 2 lasers is one of the main physical processes to control in order to avoid arcing, an experimental procedure for studying the velocity and temperature fields in such tubes was performed. Two kinds of injection system have been studied. For the first one, the gas mixture is injected by a single sonic nozzle located on the axis of the tube. Because the power of the laser may be increased by associating several tubes in line, the other injection system consists of a series of 12 holes radially arranged in order to move injectors away from the axis. For both cases, the axial mean time velocity profile was measured (without electric discharge) using a laser Doppler anemometry technique in several sections of the discharge tube. The temperature field was investigated during the discharge by using a special thermofluorescent probe. It is found that turbulence facilitates the homogenization of the discharge.