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Due to current and future environmental and safety issues in space propulsion, typical propellants for upper stage or satellite rocket engines such as the toxic hydrazine are going to be replaced by green propellants like the combination of liquid oxygen and hydrogen or methane. The injection of that kind of cryogenic fluids into the vacuum atmosphere of space leads to a superheated state, which results in a sudden and eruptive atomization due to flash boiling. For a detailed experimental investigation of superheated cryogenic fluids, the new cryogenic test bench M3.3 with a temperature controlled injection system was built at DLR Lampoldshausen. After a first test campaign with high-speed shadowgraphy of flash boiling liquid nitrogen sprays, a laser-based Phase Doppler system was set-up to determine the spatial distributions of droplet velocities and diameters in highly superheated sprays. The spatial distributions revealed a core region with high mean velocities close to the injector orifice. With increasing distance from the injector orifice, the sprays develop a more and more monodisperse pattern. These distributions also showed that atomization due to flash boiling generates finer sprays with growing degrees of superheat. In certain spray regions, two droplet populations varying in their direction of motion, velocity and diameter due to possible recirculation zones were observed. The experimental data of flash boiling liquid nitrogen generated within this study provide a comprehensive data base for the validation of numerical models and further numerical investigations., Deutsche Forschungsgemeinschaft, Projekt DEAL

The development of new upper orbit thrusters using cryogenic propellants requires an improved understanding of the dynamics of oxidizer and fuel injection at near vacuum conditions before ignition. Due to the low ambient pressure, the propellants enter a superheated state and flash evaporation occurs. Flash boiling of cryogenic liquid nitrogen is studied experimentally on the newly developed test bench at DLR Lampoldshausen and numerically with a newly developed OpenFOAM© solver. Here, a one-fluid approach is selected where phase properties, such as density, enthalpy and saturation conditions are determined with the thermodynamic library CoolProp and tabulated before runtime. The phase change is modeled by the homogeneous relaxation model. For highly superheated jets the flow becomes supersonic and forms a shock after the injector outlet. The solver is validated with the aid of flashing acetone spray experiments where the shock structures are more clearly visible. The results show that the developed solver is capable to predict the all important gas dynamics by matching shock structure and spray angle to the experiment. The experiments using cryogenic liquid, however, do not reveal any shock structures but regions with low negative axial velocities can be identified on the jet centerline. A comparison with the simulations now demonstrates that shocks continue to persist but the respective shadowgraph signals may be obscured in these flows. The joint experimental and numerical study thus provides a consistent understanding of the observed flow features that govern the fluid dynamics and jet breakup of cryogenic flashing flows.

Coherent anti-Stokes Raman scattering (CARS) spectroscopy has been used to investigate cryogenic liquid oxygen/gaseous methane (LOX/CH4) flames on a medium-size test facility at a pressure of 0.24 MPa and mass flow of 0.025 kg/s. Single-shot, broadband CARS spectra with simultaneous detection of the Q-branches of hydrogen and water molecules were recorded with good signal-to-noise ratio. Temperature was deduced from the H2 and H2O CARS profiles. The spatial temperature distribution in a comparatively harsh environment has been measured successfully. The measurements took place in the windowed combustion chamber of the DLR M3 test facility, aiming to provide data for validation of rocket combustor modeling.

To prepare for the application of CARS thermometry to LOX/methane spray ames CARS spectra have been recorded in a O2/CH4-burner at pressure up to 11 bar. Based on the experience obtained, potential probe molecules have been selected for the application of CARS to a LOX/CH4 spray ame. A CARS system allowing simultaneously the thermometry of the two probe molecules H2 and H2O was then used to map the temperature eld in a cryogenic rocket model combustor. The suitability of H 2-CARS thermometry in a LOX/CH4 ame is demonstrated and the results provide a quantitative data base for the validation of simulation tools.

The European Commission co-funded research project LAPCAT I - Long-Term Advanced Propulsion Concepts and Technologies - was performed within the timeframe spring 2005 until spring 2008. The project was coordinated by the European Space Research and Technology Center, ESA-ESTEC and the consortium consisted of 11 partners from industry, research institutions and universities. The objectives of LAPCAT I were to identify and assess critical propulsion technologies required to reduce long-distance flights, e.g. from Brussels to Sydney, to less than 4 hours. Achieving this goal intrinsically requires a new flight regime for commercial transport with Mach numbers ranging from 4 to 8. At these high speeds, classical turbo-jet engines are not feasible and need to be replaced by advanced airbreathing engines. Different combined cycles, i.e. turbine based combined cycles (TBCC) and rocket based combined cycles (RBCC) were evaluated both on fundamental research level and in the framework of vehicle system studies. The research activities of the project were structured in six main technical work packages. In the present paper, a synthesis of the work package related to combustion experiments is given. The research focused on supersonic combustion experiments in a connected tube facility, investigations of a generic supersonic combustion configuration consisting of intake, combustor and exhaust in a free jet facility, shock tube studies of the disintegration process of hydro carbons in high pressure combustion and the investigation of high pressure combustion of liquid oxygen / hydro carbons in a micro combustor.

Studies on the ignition and flame anchoring of a coaxial gaseous methane-oxygen jet have been performed. The experiment has been set up to record by high-speed visualization techniques the start-up and ignition transients. Results from image processing and data recording are evaluated for correlation of injection and stationary combustion conditions. The capability to perform reliable ignition of a gaseous methane/oxygen jet has been highlighted. Several phases of ignition (“expanding” and “stabilizing”) have been found with specific phenomenology, depending mainly on the ignition time. The influence of the mass flow rate and momentum flux ratio on ignition process is also discussed. Finally the flame stabilization (anchoring or detached flame) is investigated, showing important dependency with the jet injection characteristics, especially the propellants velocity ratio.

The combustion processes of solid fuel slabs with and without boron particle addition were investigated in a planar step combustor. The study was performed under conditions of air inlet velocity and temperature relevant to ramjet applications. The highly turbulent, multiphase combustion process has been analyzed with various intrusive and non-intrusive diagnostic techniques. Gas phase temperature distributions and particle velocities as well as concentrations of various stable reaction products have been determined in order to show the movement and the combustion of the reacting particle phase in the recirculation zone and the downstream boundary layer with the embedded reaction zone. Large scale, coherent vortex structures were observed. These structures exist predominantly in the region above the hydrocarbon diffusion flame located inside the developing boundary layer and show an intense and highly turbulent mixing process. The results show dependencies of the energy release on the flow field related to the presence of reacting or non-reacting particles.

In order to get a better insight in solid fuel ramjet combustion processes the combustion behavior of solid fuel slabs with and without boron particle addition had been investigated in a planar step combustor under ramjet relevant conditions concerning the air inlet temperature. Various intrusive and non-intrusive diagnostic techniques had been used to analyze the highly turbulent multiphase combustion process. Large scale vortical structures can be seen in the outer zone above the diffusion flame in the developing boundary layer behind the recirculation zone with Colour Schlieren as well as with PIV. Distributions of the gas phase temperature, velocities of reacting particles as well as concentrations of stable reaction products have been determined and are presented here, too.

Results obtained from investigations of the transient ignition process of a cryogenic GH2/LOX spray under vacuum conditions with a model rocket combustion chamber are reported. Ignition of the propellants has been initiated with a laser pulse. Transient spray and flame behavior have been analyzed using high-speed visualization methods. Jet spreading angles as well as flame front positions are determined by image processing. Cold flow LOX jet angle is found to be dependant on initial combustor pressure level. Ignition behaviour is compared to that observed under atmospheric conditions.