Results and deliverables

EXPECTED RESULTS

      In order for the presented technology to become a viable solution for commercial flight in the second half of this century, several aspects need to be investigated and clarified. The main question that, in the author's view, arises is the issue of operating frequency. The typical detonation frequency in PDEs is of the order 10 Hz [15]. As discussed earlier, high frequency PDC have several important advantages, and the efforts towards increasing it are needed. Obviously, mechanical limitations of the rotational speed of the PDC in the case proposed here must also be considered, and the impact of these supplementary limitations on the engine performance must be assessed and minimized.
    On a related topic, the selection of a valved, or valveless solution must also be addressed. To ensure the correct air flow through the combustor, the classical PDE design proposes a set of valves that open and close the admission of the air, or air-fuel mixture, in the combustor. The main problem in this approach is the high wear experienced by the valves, even more so at high frequencies. Additionally, the valves are subject to very high operating temperatures, and will induce pressure losses in the flow. Valveless designs, based on carefully timed pressure gradients in the flow, are an obvious goal for the future PDC development. Also, the use of a high frequency supersonic jet at the combustor inlet may play the role of an aerodynamic valve, and the effect of this jet on the combustion wave remains an open research topic.
    Nonetheless important is the fuel supply system, which needs to provide the fuel in close correlation with the PDC operating frequency. For this, the fuel will be provided continuously through the disk supporting the rotating PDCs. The disk will also include the ignition system. The approach has also the advantage of providing sufficient space to premix the air and fuel, and to achieve vaporization and possibly preheating in the case of liquid fuels. By monitoring the pressure inside the combustor, feedback signal can be provided to the fuel injection and ignition automation system, to ensure the correct synchronization. A rotating sealing system will have to be designed in order to avoid fuel leakage.
    Maybe the most important outstanding issue is the initiation of the detonation wave, strongly dependent on the inlet conditions. One possible solution is the use of a very energetic spark, but further research is needed into assessing how practical for an aircraft engine this solution is. Another possibility is to use the so-called deflagration-to-detonation transition (DDT) [61]. In this approach, a spark initiates a high-energy deflagration that is further accelerated to become a detonation. The process is not trivial, due to resistance encountered by the wave front, and to the long acceleration ducts that may render the solution inappropriate for aircraft engines.
    Another problem that needs investigation is the optimal geometry of the PDC exit nozzle. Considering the fact that, in order to increase the overall TIDE engine efficiency, the combustors must provide the highest possible impulse on a direction other than along the combustor axis, the optimal direction with respect to the engine performances and constructive solution, as well as the minimization of pressure losses due to the deflection of the flow have to be studied. The addition of ejectors on the PDC nozzle should also be considered. The internal design of the PDC flow path is critical for the optimal detonation wave travel. A design with multiple detonation chambers of increasing wave intensity, serially connected, may provide further engine efficiency increase. The stability and completeness of the supersonic combustion, strongly dependent on the combustor geometry, must also be optimized. Of special interest is the production and emission of NOx. The high temperatures in the PDC favors the NOx production, while the very short residence time in the combustor will tend to decrease the effect, hence a quantitative approach will have to be used to determine NOx emissions. Related to this issue, it must also be noted that supersonic combustion models for finite rate kinetics numerical simulations, as well as the limitations of the existing models applied to detonations are still an open research topic.
    Also important is the definition of the optimal compressor geometry, taking into account the rotation of the downstream combustor, and the discontinuous discharge of the combustor shroud. A high frequency PDC will alleviate the problem, but the effect on compressor stability remains to be assessed. Furthermore, since the combustion process is supersonic, the need to decelerate the flow upstream of the combustor disappears. Therefore, the presence of the compressor stator vanes may no longer be required, thus reducing the pressure losses during the passage between the rotating and fixed blading.
The noise generated by the TIDE engine is also an issue that needs to be studied, and solutions for noise damping must be sought, as the noise levels of the new technology may be expected to be high. The effect of opposite phase pairs of PDCs and the optimal interference of the resulting sound waves needs to be investigated. Also, the presence of detonation waves inside PDCs raises questions on the vibration levels of the new engine, which need further evaluation, understanding and solution finding.
    Finally, the optimal fuel selection for the PDCs such as to allow the reliable initiation of the detonation wave, is also an open research field. Most of the research studies carried out up to the present focus on gaseous fuels [2], mostly Hydrogen, methane, and propane. However, experimental studies conducted on PDCs using liquid fuels (kerosene) have been reported in recent years [2, 62]. The main problems when considering liquid fuels for PDC are the increased difficulty to initiate the detonation and the required very high mixing velocity of the air and fuel to be supplied to the combustor.
The specific fuel consumption of the engine is lower than in the case of a PJE. By reducing the overall engine weight, by optimizing the supersonic combustion process, and by maximizing the engine power, the overall fuel consumption can be further minimized, but the engine cycle design has to be carefully centered on this issue.
    Novel materials, able to withstand the high temperatures in the PDC, together with the mechanical solicitations (mainly centrifugal load and vibrations) for a number of cycles large enough to provide an engine resource economically viable need to be developed in order to bring the engine concept from a breakthrough technology to a market ready product.