Qualification of oil-based tracer particles for heated Ludwieg tubes

Abstract

The generation, insertion, pressurization and use of oil-based tracer particles is qualified for the application in heated flow facilities, typically hypersonic facilities such as Ludwieg tubes. The operative challenges are to ensure a sub-critical amount of seeding material in the heated part, to qualify the methods that are used to generate the seeding, pressurize it to storage tube pressure, as well as to test specific oil types. The mass of the seeding material is held below the lower explosion limit such that operation is safe. The basis for the tracers is qualified in off-situ particle size measurements. In the main part different methods and operational procedures are tested with respect to their ability to generate a suitable amount of seeding in the test section. For the best method the relaxation time of the tracers is qualified by the oblique shock wave test. The results show that the use of a special temperature resistant lubricant oil “Plantfluid” is feasible under the conditions of a Mach-6 Ludwieg tube with heated storage tube. The method gives high-quality tracers with high seeding densities. Although the experimental results of the oblique shock wave test differ from theoretical predictions of relaxation time, still the relaxation time of 3.2 μs under the more dense tunnel conditions with 18 bar storage tube pressure is low enough to allow the use of the seeding for meaningful particle image velocimetry studies.