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An experimental combustor, designated BKH, is operated at DLR Lampoldshausen to investigate high frequency combustion instability phenomena. The combustor operates with liquid oxygen and gaseous or liquid hydrogen propellants at supercritical conditions analogous to real rocket engines. An externally imposed acoustic disturbance interacts with a se...
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This paper describes experimental research into the initiation and propagation of rotating detonation for liquid Nitrogen TetrOxide (NTO) and liquid MonoMethylHydrazine (MMH). An annular rocket-type combustor without nozzle was designed to investigate detonation combustion. The propellants were injected through unlike impingement injectors. The com...
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... The relative amplitudes measured at each sensor location, together with relative phase information also provided by the Hilbert-Huang transform, can be used to produce a twodimensional reconstruction of the acoustic pressure distribution in the combustion chamber according to the procedure described by Beinke et al. [24,25]. The reconstructed pressure field in the combustion chamber is displayed for several frequencies from below to above f R1L_exp and f R1T_exp . ...
Oscillatory combustion representative of thermo-acoustic instability in liquid rockets is simulated by experiment and LES calculation to investigate the flame behavior in detail. In particular, we focus on how the velocity and pressure fluctuations affect the behavior of the dense oxygen jet, or ‘LOx core’. The test case investigated is a high pressure, multi-injector, oxygen-hydrogen combustor with a siren for acoustic excitation. First, the LES calculation is validated by the resonant frequencies and average flame topology. A precise frequency correction is conducted to compare experiment with LES. Then an unforced case, a pressure fluctuation case, and a velocity fluctuation case are investigated. LES can quantitatively reproduce the LOx core shortening and flattening that occurs under transverse velocity excitation as it is observed in the experiments. On the other hand, the core behavior under pressure excitation is almost equal to the unforced case, and little shortening of the core occurs. The LOx core flattening is explained by the pressure drop around an elliptical cylinder using the unsteady Bernoulli equation. Finally, it is shown that the shortening of the LOx core occurs because the flattening enhances combustion by mixing and increase of the flame surface area.
... Numerically oriented research activities of the last few years on LOX/H 2 propellants have improved understanding of the thermoand fluid dynamic effects in highly turbulent, reactive flow fields at elevated pressures prevalent in LREs [1][2][3][4][5][6][7]. However, such works also highlight the continuing need for more experimental data suitable for validating models of injection and combustion at these conditions. ...
This work presents results of an effort to create an extended experimental database for the validation of numerical tools for high pressure oxygen-hydrogen rocket combustion. A sub-scale thrust chamber has been operated at nine load points covering both sub- and supercritical chamber pressures with respect to the thermodynamic critical pressure of oxygen. Liquid oxygen and gaseous hydrogen were injected through a single, shear coaxial injector element at temperatures of around 120 K and 130 K, respectively. High-speed optical diagnostics were implemented to visualise the flow field along the full length of the combustion chamber. This work presents the analysis of shadowgraph imaging for characterising the disintegration of the liquid oxygen jet. The large imaging data sets are reduced to polynomial profiles of shadowgraph intensity which are intended to provide a more direct means of comparison with similarly reduced numerical results. Comparing half-lengths of these profiles across operating conditions show clear groupings of load points by combustion chamber pressure and mixture ratio. All load points appear to collapse to an inverse dependence of length on impulse flux ratio.
... The DLR TAU code has been validated for various space propulsion cases, including high-pressure gaseous combustion [17], cryogenic transcritical combustion [2], combustion instabilities [3], and scramjets [20,22]. Detailed descriptions of the solver can be found in Gerhold [11], Schwamborn et al. [31], Mack and ...
We investigate the flow in planar microscale nozzles and find that design and analysis paradigms based on the assumption of a dominant isentropic core with moderate viscosity corrections are not valid. Instead, the flow downstream of the throat is dominated by boundary layers that may choke the flow to subsonic velocities. The geometrical expansion ratio is found to be essentially irrelevant, instead, the length from throat to exit plane is found to be a much more important design parameter. Full 3D simulations are required to predict the flow topology; thermophysical modeling of the expanding gas has a noticeable impact on predicted performance. An analytical estimation of the Knudsen number in the expanding flow is given, allowing to determine its values from the expansion pressure ratio. An axial thrust analysis suggest truncation of the nozzle, resulting in a predicted 20% increase in thrust and 30% increase in specific impulse compared to the baseline configuration. The work has been carried out within the European Commission co-funded PRECISE project which was focused on designing and testing a micro chemical propulsion system thruster prototype using catalytically decomposed hydrazine as propellant.
... The goal of these modelling efforts is to investigate flame-acoustic interaction with higher fidelity and complete access to all locations within the flame. By simulating the BKH configuration and comparing with experimental results, the ability of the model to capture high amplitude acoustics and flame response phenomena is being examined (Beinke et al., , 2015. A validated model could in future be used to predict the stability of real engine configurations. ...
... For all operating conditions the calculated time lags of the cross-correlation were between 0.6 and 1.1 ms, corresponding to 7-11 acoustic cycles. This is in agreement with experimental observations in BKH [38], where about 5 cycles have been observed experimentally and 15 cycles numerically by Beinke et al. [18,39]. ...
The cryogenic LOX/H2 multi-injector research combustor “BKD” allows the investigation of high-frequency combustion instabilities under realistic conditions. Two different types of self-excited instabilities were observed. For one instability the underlying coupling mechanism was already identified as LOX injection-driven. The second type of combustion instability was experienced for different operating conditions and is characterized by higher amplitudes, more than 75% of the static chamber pressure. Analysis of the pressure data showed that amplitude and frequency of the acoustic field vary strongly over time, which complicates interpretation of the coupling mechanism. A normalization of the shifting frequency shows that the increase of oscillation frequency depends on transverse acoustic velocity. This is an effect that has also been noticed in other experiments and simulations and is explained by improved mixing leading to a reduced length of the combustion zone. This observation suggests that during the large amplitude pressure oscillations the mode shape remains a first tangential (1T) mode with shifting frequency. By using highly resolved information of the acoustic field in the time domain, both from the combustion chamber and the injector volumes, in combination with acoustic modelling of the injector elements, insights into the coupling mechanism could be gained. For periods of lower amplitudes the pressure oscillations are LOX injector-driven, similar to the first type of instability. With increasing amplitude also the frequency of the unstable mode increases and shifts into a region, where interaction with the hydrogen injector 1L mode becomes possible.
... The shadowgraph images presented here were collected using an optical filter which blocks wavelenghts under 850 nm in order to suppress combustion light but not the near-infra-red light emitted by a Xenon lamp used as back-lighting source. Beinke conducted a DMD analysis on the optical data relative to the 1L mode samples was made in order to identify coherent structures and motions at the excitation frequency [4,36]. The deformation on the shear layer numerically predicted (Fig. 13)is compared with the results of the DMD reconstruction shown in Fig. 14. ...
... The local acoustic fluctuations are estimated by reconstructing the acoustic field from the dynamic pressure sensor data extracted from the DMD modes. The acoustic field reconstruction uses a complex acoustic amplitude formulation and has been previously described in [19,23,24]. The multi-variable DMD method is illustrated in Figure 3. ...
... Initial modelling of BKH experiments has focused on models of single injection elements subjected to a representative acoustic disturbance [19,23]. This approach has now been applied to investigate the flame response over a range of excitation amplitudes. ...
... Previous work [19,23] presented initial results and described the retraction, flattening, and motion of the LOx core and flame under continuous acoustic excitation. This work has been repeated to investigate the flame response to a range of acoustic excitation amplitudes. ...
... For all operating conditions the calculated time lags of the cross-correlation were between 0.6 and 1.1 ms, corresponding to 7-11 acoustic cycles. This is in agreement with experimental observations in BKH [38], where about 5 cycles have been observed experimentally and 15 cycles numerically by Beinke et al. [18,39]. ...
... Modelling of BKH cold flow experiments were used study the BKH acoustic excitation system and how to reproduce the acoustic disturbance numerically [10]. Other methods for acoustically exciting a representative sub-domain containing single or multiple BKH injection elements were later developed and used to investigate the unsteady flame response to longitudinal and transverse acoustic disturbances [7]. Models of single BKH injection elements have been used to investigate the flame response to acoustic excitation in detail. ...
... The acoustic field structure for the 1T mode can be reconstructed from p′ measurements taken along the upper and lower chamber walls. 13) The velocity and pressure distributions for peak 1T amplitude, reconstructed through interpolating between p′ sensor locations, are shown in Fig. 3. The 1T mode has a pressure nodal line aligned approximately with the main (horizontal) axis of the chamber. ...
Experimental measurement of flame response to acoustics under conditions relevant to industrial engines is challenging and so the scope of such measurements is often limited. High fidelity CFD can be used to model the interaction of acoustic waves with cryogenic flames, and modelling an experimental test case can not only serve as a code validation exercise but also be useful in better characterising the experimental results. This work explores this potential by extending the interpretation of high-speed imaging of representative rocket flames based on comparison with a large eddy simulation of the experiment.
