Investigation of an Intake Injected Scramjet with a Hot Wall

A supersonic combustion ramjet, or scramjet, is an air breathing engine that is designed to operate at hypersonic speeds. At these high speeds, scramjet engines use the forward motion of the vehicle to force air ingestion without the need for moving parts. The flow through a scramjet engine remains supersonic throughout causing the residence time of the fuel and air mixture in the combustion chamber to be very limited. This results in the need for long combustion chambers making it difficult for the thrust produced by the engine to overcome the drag on the vehicle. In order to potentially decrease the length of the combustion chamber, a method has been developed where the fuel is injected on the intake of a scramjet allowing it to mix with air in an environment that does not promote ignition. On entering the combustion chamber the mixture is ignited and the combustion process should proceed more efficiently than would be the case if no pre mixing had been allowed. A possible hazard of this method is that if the fuel ignites on the intake it could result in an increased wall pressure on the intake with a corresponding reduction in net thrust. In this study, hydrogen fuel is injected through a porthole in the intake wall of the model. This injection method has been widely used for injection into a combustion chamber. Injection is most commonly angled perpendicular to the flow through the engine in order to promote maximum jet penetration. The flow structure around the normal jet also contains high circulation regions that are conducive to ignition due to the high residence times in these zones. This is not a desirable feature for intake injection where combustion is ideally suppressed until the flow reaches the combustion chamber. Consequently, the fuel jets in this study are angled 45 degrees downstream to the cross flow which results in reduced recirculation zone size. The angled jet has the additional advantage that some of the momentum of the fuel injection is in the direction of motion of the vehicle and can therefore aide in propulsion. It is also thought that penetration depth is not greatly compromised [54]. Previous testing of angled intake injection on a two dimensional model scramjet in the T4 shock tunnel at the University of Queensland has observed no signs of premature combustion [34]. Impulse testing facilities, such at the T4 shock tunnel, provide a useful and cost effective method for hypersonic testing. One major difference between impulse testing and continuous flight is the surface heating experienced by the vehicle or model. The surface temperature is considerably higher in extended flight due to skin friction drag. The implications of high surface temperatures are significant in the case of intake injection because the higher intake wall temperatures increase the likelihood of premature combustion. For this study Computational Fluid Dynamics was used to predict the temperature distribution above the surface of the intake for wall temperatures of up to 800K. It was found that the effect of elevated wall temperature was localised within a few millimetres distance from the wall. This highlighted the importance of understanding the fuel injection trajectory. If the fuel jet penetrates well into the freestream and resides outside the hot boundary layer, it would be unlikely that it would combust on the intake. For this reason the models developed for this study were specifically designed to accommodate features necessary to utilise optical observation techniques. The aim of this investigation was to determine the effect of a heated wall on intake fuel injection. The project is consequently divided into two distinct tasks. The first of which is the development of a heated model that is capable of withstanding the harsh conditions experienced by the shock tunnel test flow. This task was accomplished by initially incorporating a heating element in the wall of an existing two dimensional scramjet. This allowed a wall temperature range of 300K to 500K to be investigated and comprised the first experiments completed with a heated model in the T4 shock tunnel. A second model was then created and various techniques were investigated for heating the surface of an intake ramp. Electrical resistance heating eventuated as the simplest and most robust heating method for the second model. Experiments were performed and temperatures of up to 750K were found to show no signs of combustion on the intake. The second task in this study was to investigate angled fuel injection on the intake using optical techniques. The majority of optical investigations of fuel injection trajectories have observed the maximum height of the jet. This study investigated techniques capable of providing more extensive, three dimensional information about the jet trajectory including observation of both upper and lower jet boundaries. For this reason two optical techniques were utilised for this study. The first was Holographic Interferometry where the density change across the flow was recorded and used to estimate the location of the majority of the fuel and the amount of mixing that is taking place. Secondly, a Planar Laser Induced Florescence apparatus was developed and used to observe nitric oxide (NO) or hydroxyl (OH) molecules. The NO images allow for the structure of the fuel injection to be observed. OH was also targeted because this is a species produced in the hydrogen air reaction scheme and can therefore be used to indicate if combustion is taking place. Flow visualisation using NO PLIF was used to view angled fuel injection allowing both upper and lower fuel boundaries to be seen for jet momentum flux ratios ranging from 0.8 to 2.4. Curve fitting of experimental results allowed correlations estimating the jet penetration of the upper and lower boundaries to be determined. Sideways interaction of adjacent jets was seen to substantially influence the jet trajectory causing the boundaries to diverge from the determined correlations. Finally, OH levels were observed using the PLIF technique. Observations of a butane burner clearly showed that OH could be detected using the apparatus. Images of the flow, however, showed no signs of fluorescence. In conclusion, wall pressure measurements combined with optical visualisation results showed no detectable sign of premature combustion using intake injection onto a hot wall for conditions with total enthalpies ranging from (3.42+/-0.24) MJ/kg to (6.61+/-0.46)MJ/kg