Influence of non-linear mixing effects onto flow and heat transfer in rocket combustion chambers

Kurzfassung

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. This includes the numerical prediction of mixing and re algas effects at the cryogenic operation conditions: First to mention is the accurate prediction of the jet behavior at sub- and supercritical conditions of propellants (e.g. cryogenic oxygen and/or cryogenic hydrogen). This breakup influences the whole combustion process as well as t he flame structure and flame length. Another challenge for the numerical method are the dissimilar length scales and material properties for the liquid and gaseous parts in combination with the chemical reactions for the hydrogen-ox ygen combustion. Another open research topic is the influence of non-linear mixture effects onto the flow field due to non-linearity in the equation of state (EOS) mixture description (non-linear mixing models). Due to the high-pres sure operating conditions an ideal fluid behavior is not sufficient to describe the behavior of the fuel and oxidizer at these operating points. In this contribution we want to focus on the influence of non-linear mixture effects onto the flow inside the combustion chamber. To access these influences we want to compare the results for the linear Multi-Fluid mixing model [1] to established mixing rules for cubic EOS (e.g. van der Waals mixing rules) (see e.g. in [2]). The first mixing method can be obtained from non-linear mixing rules by neglecting the off-diagonal mixing coefficients. For the application of stationary diffusion flames in rocket combustion chambers it was shown by Direct Numerical Simulation and Large Eddy Simulation investigations [3,4] that the flame acts as flow barrier and prevents a binary mixing of oxidizer and fuel. Thus, it might be justified to use for these case a simplified mixture description that has considerably lower computational costs and complexity. Using the compressible in-house flow solver TAU we investigate these effec ts based on a realistic rocket combustion chamber setting for a supercritical load case at approximately 60bar. For the investigation we use a standard Spalart-Alamaras turbulence model. In this numerical study we compare these two mixing descriptions for the case of a stationary hydrogen-oxygen diffusion flame within a rocket combustion chamber. For both mixing approaches we assume a cubic EOS description for the cryogenic fluids (here Soave-Redlich-Kwong EOS). The numerical results are compared to experimental results performed at German Aerospace Center Lampoldshausen within the single-injector BKC chamber at supercritical conditions. This combustion chamber is well suited for validation as it features full optical access allowing a detailed validation of the numerical results. In this numerical study the influence of non-linear mixing rules for the EOS description is investigated.