Dynamisches Verhalten dimerer 1,3,2,4 lambda2-Diazasilastannetidine und -plumbetidine in Lösung - eine Multikern-NMR-Studie

1,3-diisopropyl-2,2-dimethyl-1,3,2,4?2-diazasilastannetidine (1b) and -plumbetidine (2b) are dimeric [(1b)2, (2b)2] in solution. At room temperature the structure of (1b)2 is fluxional. The dynamic behaviour is interpreted - on the basis of 1H, 13C, 29Si and 119Sn NMR data - as an intramolecular process in which the four-membered rings keep their identity. Such a process involves either concerted opening of the two coordinative Sn-N bonds and mutual slippage of the two rings, or consecutive cleavage of one of the coordinative Sn-N bonds and rotation about the other one. At room temperature the dimer (2b)2 is in equilibrium with its monomer 2b, whereas at low temperature the dynamic process corresponds to that established for (1b)2. In solutions which contain a mixture of the dimers (1b)2 and (2b)2, the presence of the mixed dimer 1b/2b can be proved unambiguously by consistent 29Si, 119Sn and 207Pb NMR data

REST HF-10 Test Case: Numerical Simulation of a Single Coaxial LOX-CH4 Injector with Forced Mass Flow Oscillations Using the DLR TAU-Code

Combustion instabilities are high-pressure acoustic oscillations that often develop spontaneously and may lead to catastrophic failure of a liquid rocket engine. In order to investigate this phenomenon, the Rocket Engine Stability initiative (REST) was founded by CNES, ONERA, DLR and CNRS. Supporting current and future engine developments, the REST community has worked towards a better understanding of highfrequency instability phenomena. The advancement of numerical simulation methods created new opportunities to simulate combustion instabilities and allowed for a deeper understanding of processes in rocket engines that are difficult to observe experimentally. Numerical simulations were used to investigate the coupling between different internal processes like pressure oscillations and heat release fluctuations. With Prometheus as the engine of a future European launcher vehicle, predicting instabilities and flame dynamics in methane-oxygen (CH4-O2) combustion became an important step in developing reliable, efficient and reusable rocket engines. In order to support these developments numerically, the REST community proposed a representative single injector test case designed for fundamental research. The test case consists of a hexagonal combustion chamber with periodic boundary conditions and a representative injector geometry. The injector was designed within the frame of the "Sonderforschungsbereich Transregio 40", a research program founded by the German Research Foundation (DFG), and is intended to be suitable both for LOx-CH4 and LOx-H2 rocket engines. The chamber pressure is set to be 100 bar and instabilities are introduced by modulating the inlet mass flows for methane and the oxygen injector up to +-10 % of their nominal values at different frequencies. The main goal of this test case is to compare the different numerical codes and modeling approaches (different turbulence modeling, different combustion modeling) between the members of the REST community and various codes. This paper presents the current status of the DLR contribution to the test case. All simulations are conducted with the DLR in-house Code TAU. The combustion is modeled using a real-gas flamelet model whereas the turbulence is modeled using a 2-layer k-epsilon RANS model. In addition to the URANS simulations, Detached-Eddy simulation (DES) results for all test case load points are presented and compared to the URANS results. It is shown that the URANS simulations greatly overestimates the dense LOx core lengths while the DES results give more reasonable values. Investigations of the flame response to the longitudinal mass flux oscillations showed only a small effect for an excitation of the O2 inflow at 5 kHz while there is a stronger flame response for the other excited cases. We also present results for numerical grid resolution sensor and a grid convergence study indicating that the chosen mesh resolution allows for grid-converged results for this test case

Influence of numerical model setup on the response of acoustically forced LOx/H2 flames

The current work aims to study the influence of the numerical setup on a URANS model of a single co-axial injection element under acoustic forcing. The single injector model is representative of an experimental rocket combustor with multiple injection elements, named BKH and operated at DLR Institute of Space Propulsion. The configurations presented in this work are fully 3D for the 1L and 1T mode excitation and compared with previous configurations.14, 34The flame zone is studied with an unstructured and hybrid mesh. An influence of the numerical setup is visible

Efficient handling of cryogenic equation of state for the simulation of rocket combustion chambers

The simulation of cryogenic flows in rocket combustion chambers is challenging because we have to consider a reactive mixture over a wide temperature and density range. This necessitates the use of more advanced fluid models that impose additional computation overhead. In this study we compare two equation of state (EOS) mixture approximation approaches for cryogenic flows in rocket combustion chambers and present a computationally efficient implementation within a Reynolds Averaged Navier Stokes (RANS) context. The numerical study is validated with experimental results of a lab-scale rocket combustion chamber with optical access

Numerical simulation of cryogenic flows in rocket combustion chambers

The accurate numerical simulation of combustion and atomization processes in rocket combustion chambers is a key element for the design of future space transportation vehicles. The main challenges include the consistent approximation of thermodynamic effects within a broad range from cryogenic temperatures at the injection to high temperature within the flame. This broad temperature range combined with high-pressure conditions is challenging for the stability of the numerical method. Another numerical challenge is to resolve and/or model the disimilar length scales: First to mention is the accurate prediction of the jet breakup of the propellants (e.g. cryogenic oxygen and/or cryogenic hydrogen) and the cryogenic mixture. Second to mention are disimilar length scales between the liquid injection conditions (e.g. liquid oxygen) and the gaseous gases in the flame. In literature the numerical prediction of cryogenic injection processes is an open question how cryogenic sub- and supercritical injection process can be modeled (see e.\,g. the discussion in Oefelein and Bellan) and which physical processes are important to characterize the breakup and the evaporation processes correctly. At these conditions, as found e.\,g. in rocket combustion chambers, all fluid properties have a strong sensitivity to changes in the (non-linear) equation of state. One major challenge is the strong discontinuity in the flow field due to the phase boundary. Another open question is which are the main physical processes that have to be modeled for Reynolds Averaged Navier-Stokes (RANS) based ap proaches, similar to the single-phase turbulence modeling. These turbulence models are of practical importance as they are used to design and assure rocket combustion chambers with many design-type calculations. Detailed numerical simulations (Large Eddy or Direct Numerical Simulations) are still too expensive for those simulations and are used to understand the fundamental flow physics in detail, even the computational resources increased significantly in the last years. In this talk we discuss the numerical methodology to handle cryogenic flows in combustion chambers using a RANS based method. The numerical development is based on the DLR flow solver TAU for which some extensions are proposed that handle the mixture of cryogenic fluids. This includes the numerical framework for an efficient evaluation of cryogenic mixture states as well as state of the art mixture rules for viscosity and heat transfer coefficients. A double-f lux based numerical method is included to prevent spurious numerical oscillations in cryogenic mixture states as proposed by Ma et al. for trans-critical flows. The framework is validated with experimental results for a lab-scale combustion chamber experiment conducted at the DLR branch in Lampoldshausen. During the BKC test campaign a single-injector cryogenic combustion chamber was fired at several combustion chamber pressures ranging from sub- to supercritical conditions. The combustion chamber was fully equipped with OH* and shadowgraphic imaging aiming to provide extensive validation data for numerical meth ods. In addition wall temperature and pressure measurements on the outer combustion chamber walls are available