<title>Experimental And Analytical Investigations Of The Effect Of Wall Roughness And A Curved Elbow Inlet On Thermal Blooming In Axial Pipe Flow</title>

Abstract

Abstract An experimental and theoretical study of thermal blooming phenomena in a laser beam propagating in an axial turbulent pipe flow is presented. The experiments were conducted by using a 250 watt CW C02 laser as the heat source and a mixture of C02 and N2 as the absorbing medium. The optical path difference (OPD) caused by the heating of the gas by the beam were measured by holographic interferometry, using a krypton laser at 0.5309 yrri. The pipe wall roughness was varied by inserting an inner lining made of porous material. Two different inlet conditions to the pipe were considered in the experiments, namely, a straight inlet and a curved elbow. The theory was developed by modifying an existing numerical code in order to include the volumetric heating due to absorption of the laser radiation by the flowing gas. The theoretical OPDs were calculated directly from the temperature profiles, and comparison with the experimental values for the case with a straight inlet showed good agreement. The effects of a curved elbow inlet and pipe wall roughness on OPD were determined by comparison of the results with those for a straight inlet and smooth wall respectively.IntroductionAs a laser beam propagates through a gas medium, distortion of the beamoccurs due to absorption of the laser radiation by the gas, a phenomenon called thermal blooming. A beam tube is generally required to house the high power optics such as relay mirrors, which transport the laser beam from the laser generator to the pointing/tracking system. In order to reduce thermal blooming effects on propagating laser beam, a flow of low absorbing gas, such as nitrogen or argon, either along or across the beam path in the beam tube is necessary. An experimental and analytical study to Investigate the thermal blooming phenomenon In a beam tube conditioned by a turbulent axial flow was performed. The description of the apparatus and the results obtained for a straight inlet to a smooth wall pipe were reported previously. In this paper, the optical setup used to perform the holographic interferometry Is illustrated and discussed in detail, and additional experimental data for a rough wall pipe and a curved elbow Inlet with various flow velocities and pipe wall-gas temperature mismatches are presented. The curved elbow inlet was tested, because of Its practical use in the high energy beam control system. The original theory on the axial flow thermal blooming was extended to account for the wall roughness. The experimental results for the rough wall pipe with the straight inlet are compared to the theoretical predictions with good agreement. The theory was, however, not extended to include the secondary flow effect caused by the curved elbow and hence no attempt was made to compare the experimental data on the curved elbow case with the theoretical predictions. The experimental results were, however, compared among themselves, In order to extract the effect of curved elbow and pipe wall rougness on thermal blooming.Experimental ApparatusThe apparatus used In the experiments was described in detail In Ref. 1. A general view of this apparatus is shown in Figure 1. Basically It used a blowdown flow system as the flow source, and the gas flowed through a 6 meter long pipe with 5.08 cm Inner diameter. The flow velocity in the pipe was varied from 5 m/sec to 15 m/sec to provide optimum thermal blooming information. A 250 watt CW C02 laser was used to provide the heating source, and the desirable thermal blooming condition of a laser was simulated by using a gas mixture of 3% C02 and 91% N2. Absorption of the laser radiation by the flowing gas caused radial density gradients which in turn resulted In OPDs (optical path differences) across the laser beam. The OPDs were measured by holographic Interferometry using a krypton laser at 0.5309 ym. The pipe wall roughness was varied by Inserting a sleeve made of porous material Into the pipe. Static pressure taps located along the pipe were used to measure the axial pressure gradient. The configurations used for the straight Inlet and for the curved elbow are shown In Fig. 2. The radius of curvature for the curved elbow was chosen to equal the pipe diameter.