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The investigation of high frequency combustion instabilities is still an ongoing challenge, despite it has been addresses since the early developments of rocket science [1]. The development of CFD tools and their improvement in the last decades have brought further insights into the phenomenon, but not enough to represent a self-sufficient instrument for investigation. Experimental data are still required also for validation of the numerical models. This can be done only with comparison between experimental data and CFD simulations results, but also this task is difficult. In fact, the extreme environment of rocket combustion chamber makes experimental measurement of the relevant physical variables for the study of combustion instabilities difficult. A physical quantity which has been widely used for the identification of the combustion zone in hydrogen-oxygen combustion systems is the OH* radiation emitted from the flame [2,3]. Several numerical studies have been performed in order to understand if the OH* radiation emission could be an indicator of combustion instabilities in rocket combustor flames and could be related to the heat release in combustion chambers [4,5,6], but an unambiguous answer has not yet been found. It is therefore of crucial importance to develop methods which allow a direct comparison of OH* radiation measured in the experiments with the results of CFD simulations. Ray-tracing algorithms have been applied in several fields [7] to track the rays along their path and their interaction with objects they encounter along their path. The same logic is followed to study the interaction of OH* radicals with the light coming from an optical system. A method was developed at DLR Institute of Space Propulsion which relies on the spectral modelling of the radiance emitted by OH* molecules and on a ray-tracing algorithm. This method has been applied to a test case in the frame of the 4th REST Modelling Workshop in Lampoldshausen. The test case described is an experimental combustor, named BKD, operated with oxygen-hydrogen propellants. The goal is to assess tools for the individuation of high frequency combustion instabilities in BKD and subsequently perform an analysis of the radiation emitted during combustion which is captured by an optic fiber. As a first step, a 2D RANS simulation was performed to get the axial distribution of the relevant physical variables to perform an acoustic simulation. The 1T mode of the chamber is then calculated, and a pressure perturbation with the frequency of the calculated 1T mode is imposed when performing the 2D URANS simulation. Then, the solutions at steady-state fluctuations are interpolated on a 3D cylindrical geometry. A model for the optical probe has been developed and a spectral model together with a reverse ray-tracing algorithm have been applied to capture the intensity fluctuations of the OH* radiation during flame oscillations. In the present work, the method and its application are presented, together with the main results which show that the signal extracted from simulations has a peak at the excitation frequency of the pressure field. Shear layer dynamics causes both lateral and axial scattering of the rays which results in higher frequency responses in the integrated radiation intensity signal. A comparison between OH* simulation images from the CFD results for the 3D single injector configuration and the experimental images acquired from a 2D window of the chamber [8] are then presented to further assess the effectiveness of the method. |
