Laser-induced thermal acoustics : simultaneous velocimetry and thermometry for the study of compressible flows

Air-breathing propulsion concepts, such as scramjets, provide a promising alternative to conventional systems for a faster and economically as well as ecologically more efficient transportation of passengers and cargo to any destination on the globe. Furthermore, scramjets are an important supplement to existing rocket-based systems to increase the payload and reduce operational costs of space transportation systems. The development of a scramjet engine is, however, challenging and involves the knowledge of many disciplines. One of the most critical problems is a stable and reliable combustion. The flows relevant to this thesis are therefore characterized by high speeds, high temperatures and chemical reactions. Obtaining quantitative data of such a flow field, sufficiently resolved in time and space, is a difficult task for any measurement technique. However, the continuous study of the occurring flow phenomena as well as the requirement to validate advanced numerical simulations demand the development of new diagnostic methods to provide more sophisticated experimental data sets. The focus of this thesis is the development, evaluation and application of Laser-Induced Thermal Acoustics (LITA) as a promising diagnostic tool for the study of compressible flows. LITA allows non-intrusive and remote measurements of speed of sound, flow velocity, Mach number and temperature – resolved both spatially and temporally. A thorough validation of the setup was conducted for reference cases at flow conditions comparable to the intended application verifying that very accurate and detailed data sets can be obtained with LITA. Three different applications are investigated in this thesis. In the first case, time-resolved speed of sound, flow velocity and Mach number measurements were conducted in a hydrogen/air free jet flame. Flow profiles were obtained at different axial positions showing the evolution of the combustion zone. The second application is dedicated to the flow field inside scramjet combustor models. Detailed experimental data sets were provided for the validation of complementary CFD simulations. In addition, a precise reconstruction of the flow field and the shock system resulting from a jet injected into the supersonic cross flow was possible. These results motivated to use LITA in a shock tube facility. Measurements were successfully conducted behind the incident and reflected shock wave proving the technique’s potential for shock-heated flows