Combustion Dynamics Interaction of acoustics and combustion in rocket engines Dr Justin Hardi and the Combustion Instabilities Group at the DLR Institute of Space Propulsion in Lampoldshausen have focused their studies on improving the understanding of the thermoacoustic processes in liquid rocket engines for over one and a half decades. As rocket engines operate at extremely high temperatures and pressures, being able to observe the combustion phenomena poses a huge challenge. Pushing sophisticated high-speed imaging techniques to their limits, the team can visualise thermoacoustic phenomena in liquid propellant rocket engines. Their research not only deepens our understanding of flame-acoustic interactions but also aims to prevent combustion instabilities. ombustion instability is a densities, thus rendering it nearly impossible high priority problem in the to place instruments into the flow. The team construction of rocket engines. sometimes manages to get thermocouples Particularly dangerous high close to the flame while having them survive frequency (HF) combustion a test run, but when traversing the flow, 1 mm instabilities in rocket propulsion can lead distance can mean the difference between 45 Cto massive damage of the combustion and 3600 K, and rocket flames are extremely chamber and surrounding components. turbulent, so it’s difficult to predict where/if Indeed, this was an issue for the later stages such instrumentation will survive. of development and testing of the Aestus engines, which contributed to the rise of The teams’ study of thermoacoustic A view through the nozzle to the faceplate and injectors of the experimental combustor model ‘H’ Franco-German research programme Rocket interactions involves the investigation of (abbreviated as ‘BKH’). The optical access windows Engine Stability iniTiative (REST), aimed at events accompanying unstable operational can be seen either side of the central injectors. better understanding instability phenomena. modes of a rocket engine. Heat release Participating in the REST programme, Dr oscillations arising from flame fluctuations Hardi who heads the Combustion Instabilities affect the combustion chamber acoustics. Group at the DLR Institute of Space The combustion process can drive acoustic can observe the response of flames to but can regulate propellant supply with high Propulsion investigates the thermoacoustic oscillations to very high amplitudes which acoustic disturbances for a wide range of precision and fast reaction times. This allows phenomena in liquid propellant rocket increases the transfer of heat to the inner artificially forced or self-excited acoustic test sequences to be performed with 12 or engines. Dr Hardi and his team specialise surfaces of the combustion chamber. The amplitudes. Experimental combustors imitate more combinations of varying operating in visualising combustion instabilities. engine itself can burn from the inside out, conditions present in real rocket engines, parameters, and with excellent test-to-test Their primary experimental approach is to ending in disaster. The same thermoacoustic with combustion chamber pressures up to 80 repeatability. High-speed cameras register observe these phenomena under realistic process is recreated with the use of robust atmospheres and temperatures up to 3600 K, the processes in the combustion chamber conditions. Not the simplest task: rocket experimental research combustors. In but importantly researchers have visual access and have captured images at impressive rates: engines run with extremely high energy carefully controlled conditions, researchers to the chamber for their optical diagnostics. up to 60,000 frames per second. Back-lit shadowgraph imaging allows visualisation PUSHING BOUNDARIES of the movement of dense liquid oxygen The team runs their experiments with the within the core of the flame. A second camera The team are pushing the boundaries help of the capable test bench team at the collects flame luminosity in the UV. The of what is possible for visualising P8 test facility, DLR Lampoldshausen. The pictures are analysed to identify coherent facility is not only powerful enough to realise structures and motion in response to the BKH during a test at the P8 test facility, with siren forcing of the internal combustor acoustics and high-speed dynamic processes in rocket engines conditions representative of real engines, local acoustic field, which is reconstructed imaging of the flame response through the windows in the side walls. 94 www.researchfeatures.com www.researchfeatures.com 95 Combustion Dynamics Detail RESEARCH OBJECTIVES Dr Hardi’s research explores What is the most challenging problem a failure these days. The problem is that thermoacoustic instabilities in cryogenic that you are currently trying to solve? these methods are largely based on trial rocket engines. His team specialises in While our visualisation data is extremely and error, and therefore represent an visualising thermoacoustic phenomena valuable for validating numerical modelling immense cost to engine development with the aim of examining the participating of rocket flame dynamics, it is not directly programmes. The predictive capabilities dynamic processes under realistic compatible with the results typically of models are currently not good enough conditions. produced by such simulations. For example, to ensure stability at the design stage, but the flame luminosity captured by our any improvement would result in reduced FUNDING camera cannot be easily related to the testing requirements and therefore • The German Aerospace Center (DLR) chemiluminescence of a particular reacting development costs. We support the • The European Space Agency (ESA) species whose concentration is estimated in gradual improvement of the models with • Deutsche Forschungsgemeinschaft (DFG) A conceptual illustration of the inside of the BKH experimental combustor with a rectangular cross-section, siren A conceptual illustration of the inside of the a CFD simulation. We are working on ways our experiments and numerical tools. in the upper wall for acoustic forcing, and optical access windows. On the right is a numerical simulation of the experimental combustor model ‘D’ (‘BKD’ for short). cryogenic flames and flow field inside BKH. This kind of result is compared to experimental data from BKH to BKD is a conventional, cylindrical thrust chamber and to interface our experimental data with COLLABORATORS validate the simulation method. experiences self-excited combustion instabilities numerical results, both our own and those What is the most interesting part of your Members of REST: of partners, so that they can be compared research? ArianeGroup, Ottobrunn; ArianeGroup, INJECTION CONDITIONS more directly and provide quantitative Test firing rocket combustors! The Vernon; EM2C, CentraleSupélec, Paris; INFLUENCE ACOUSTIC MODES validation of the models. adrenaline rush in the seconds before DLR Institute of Aerodynamics and Flow One particular aim of the teams’ research is ignition where we are about to release 90 Technology, Göttingen; CNES; CNRS; to examine the behaviour of the cryogenic What do you think will be the fuel of MW of thermal power in a small tube… CORIA; IMFT; ONERA; Technical University spray flames when experiencing self-excited future space rockets? with a window in it! And the results are of Munich instabilities. The experimental setup reflects Methane is currently very popular as also spectacular. I am always fascinated Other collaborators: common features of frequently used types of a potential fuel for future launchers. by the behaviour of liquid oxygen jets as JAXA; The University of Adelaide; liquid propellant rocket engines. Combustion Theoretically, it has some economic they break up and are consumed in the The University of Rome (La Sapienza); instabilities have long been known to advantages over other high-performance surrounding flame, and how they react Universität der Bundeswehr München; adversely affect the proper functioning propellants like hydrogen – the current to the acoustic waves we send through ETH Zurich of liquid rocket engines, at times leading staple in Europe. All over the world, them. Watching replays of our high-speed to sudden catastrophic failures. Famous teams are studying methane as a fuel imaging of these processes is mesmerising! BIO examples include the catastrophic in-flight and developing engines to demonstrate Justin Hardi graduated with a double loss of the second launch of the Ariane family its performance. We are also shifting our How does your work integrate with the degree in Aerospace Engineering and of rockets in 1980, caused by the occurrence research focus accordingly to support the work of rocket engine designers? Physics from the University of Adelaide of instabilities in one of its Viking engines European efforts. I see our role as to fill the rocket engine in South Australia, and then moved to during first stage burn. designer’s textbook with information they Germany to work on the topic of high BKD during a test at the P8 test facility, where self-excited flame dynamics are measured with fibre-optical probes. Will the models you’re developing can use to make design decisions. We frequency combustion instabilities A modified version of BKD with an optical access window is currently in testing. The team's findings include the elucidation completely eliminate the possibility of work closely with industry to make in cryogenic engines at the of mechanisms responsible