Dense core response to forced acoustic fields in oxygen-hydrogen rocket flames

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

Fructooligosacharides Reduce Pseudomonas aeruginosa PAO1 Pathogenicity through Distinct Mechanisms

Pseudomonas aeruginosa is ubiquitously present in the environment and acts as an opportunistic pathogen on humans, animals and plants. We report here the effects of the prebiotic polysaccharide inulin and its hydrolysed form FOS on this bacterium. FOS was found to inhibit bacterial growth of strain PAO1, while inulin did not affect growth rate or yield in a significant manner. Inulin stimulated biofilm formation, whereas a dramatic reduction of the biofilm formation was observed in the presence of FOS. Similar opposing effects were observed for bacterial motility, where FOS inhibited the swarming and twitching behaviour whereas inulin caused its stimulation. In co-cultures with eukaryotic cells (macrophages) FOS and, to a lesser extent, inulin reduced the secretion of the inflammatory cytokines IL-6, IL-10 and TNF- a . Western blot experiments indicated that the effects mediated by FOS in macrophages are associated with a decreased activation of the NF- k B pathway. Since FOS and inulin stimulate pathway activation in the absence of bacteria, the FOS mediated effect is likely to be of indirect nature, such as via a reduction of bacterial virulence. Further, this modulatory effect is observed also with the highly virulent ptxS mutated strain. Co-culture experiments of P. aeruginosa with IEC18 eukaryotic cells showed that FOS reduces the concentration of the major virulence factor, exotoxin A, suggesting that this is a possible mechanism for the reduction of pathogenicity. The potential of these compounds as components of antibacterial and anti-inflammatory cocktails is discussed.The authors acknowledge financial support from FEDER funds and Fondo Social Europeo through grants from the Spanish Ministry of Economy and Competitiveness (grants SAF2011-22922, SAF2011-22812) the Andalusian regional government Junta de Andalucía (grant CVI-7335) and the Centre of Networked Biomedical Research on Hepatic and Digestive Diseases (CIBERehd) which is funded by the Carlos III Health Institute and the Ramón Areces Foundation, Spain

Analyses of flame response to acoustic forcing in a rocket combustor

High frequency combustion instabilities in liquid propellant rocket engines are spontaneously occurring pressure fluctuations that are coupled with unsteady combustion processes. Under the right conditions the unsteady fluctuations can grow to a point where they affect the operation of the combustion chamber. The cause of combustion instabilities, including which processes are responsible and under what conditions they arise, are not yet fully understood. The ability to predict and prevent combustion instabilities during the design of new combustion chambers, through better understanding, would dramatically reduce the uncertainty and risk in the development of new engines. An experimental combustor, designated BKH, is used to conduct high frequency combustion instability experiments. BKH operates with liquid oxygen and gaseous hydrogen propellants at supercritical conditions analogous to real rocket engines. The chamber features an acoustic excitation system that imposes an acoustic disturbance representative of a high frequency instability upon a cluster of five coaxial injection elements in the center of the chamber. The response of the elements to the imposed acoustic disturbance is observed using high speed optical diagnostics. The main aim of this project is to develop methods for predicting the flame response to high frequency acoustic forcing representative of combustion instability phenomena. BKH is employed as an experimental and numerical test case for investigating the flame response. Modelling and complementary data analysis methods are developed and applied to model the chamber flow field, identify and predict the excited acoustic disturbance, identify the flame response using optical data, and to predict the flame response numerically. The BKH experiments are first characterised by modelling the chamber numerically and determining the local acoustic disturbance acting upon the flame. A steady state chamber model with supercritical oxygen-hydrogen combustion was computed using a specialised CFD code. The model results indicate the secondary injection in BKH has a strong influence on the resulting flame distribution. A method for reconstructing the acoustic field from dynamic pressure sensor data was developed to determine the local acoustic disturbance acting upon the combustion zone over a range of excitation frequencies. A low-order acoustic modelling approach is also shown to predict the resonant mode frequencies and the evolution of the acoustic field. The flame response to the imposed acoustic disturbance is identified by analysing optical data from BKH experiments and unsteady CFD modelling. Multi-variable dynamic mode decomposition (DMD) analysis is used to isolate the flame response to the imposed acoustic disturbance in shadowgraph and OH* imaging data. Wave-like structures propagating along the surface of the liquid oxygen (LOx) jet and a phase difference of 45° between acoustic pressure and observed intensity fluctuations were identified. An unsteady model of an injection element subjected to representative acoustic forcing is used to predict the flame response for a range of excitation amplitudes. Velocity ratio fluctuations caused by acoustic coupling with the oxidiser post in a pressure antinode are identified. The trend of exponential decay of the length of the LOx core with increasing transverse acoustic amplitude excitation is reproduced numerically and the flattening and flapping motion of the flame was further investigated using the numerical results.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2017

Modelling Acoustic Excitation for the Simulation of Combustion Instability Experiments

An experimental combustor, designated BKH, has been developed at DLR Lampoldshausen to investigate combustion instability. The combustor operates with cryogenic liquid oxygen and hydrogen propellants at supercritical pressure conditions analogous to real rocket engines. It features an excitation system for driving acoustic disturbances in order to study combustion instability phenomena. BKH experimental results are being used to develop and validate numerical models. Methods of simulating the influence of the excitation system on the BKH chamber have been examined and the results compared with BKH experiments under cold flow operating conditions

Interpretation of the response of cryogenic rocket flames to forced acoustics using large eddy simulation

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

Coupling Behaviour of LOX/H2 Flames to Longitudinal and Transverse Acoustic Instabilities

A rectangular combustor with acoustic forcing was used to study flame–acoustic interaction under injection conditions that are representative of liquid propellant rocket engines. Hot-fire tests using liquid oxygen and gaseous hydrogen were conducted at pressures of 40 and 60 bar, which are sub- and supercritical conditions, respectively, for oxygen. Examined samples of hydroxyl-radical emission imaging, collected using a high-speed camera during periods of forced acoustic resonance, showed significant response in the multi-injection element flame. Fluctuating acoustic pressure causes in-phase fluctuation of the emission intensity, producing response factor values of around 0.8. Transverse acoustic velocity causes shortening of the flame, concentrating heat release near the injection plane. The flame is also convectively displaced with transverse acoustic velocity, a process believed by many to be responsible for driving transverse mode high-frequency combustion instabilities. The analysis in this work was extended to detect the response of internal flame processes to the transverse acoustic disturbance, namely, pressure response and enhanced transport and mixing of non-premixed propellants. A flame-tracking technique was implemented to isolate the response of these processes from that of collective convective displacement. The results show a significant contribution to overall flame response from the response of internal processes; however, further work is required to quantify the relative contributions of pressure or propellant transport.Justin S. Hardi, Scott K. Beinke, Michael Oschwald, and Bassam B. Dall