Antonio Agresta

Preliminary analysis of strategies for NOx reduction

Although the LAPCAT II project theoretically demonstrated that the chance for future long range high speed commercial transport ( i.e. Bruxelles-Sydney in less than 2 hrs and 30 min) is real, the impact of these vehicles on the environment is a key point to be investigated. In fact, nevertheless hydrogen has been chosen as fuel and no particulate, CO and CO2 emissions are foreseen, the high temperature within the scramjet combustor rises the issue of the NOx, water vapor and OH emissions. Further, the higher altitudes associated with hyper/supersonic flight make the emission of NOx a critical point for the ozone layer depletion. In fact, at current subsonic flight altitudes (troposphere and lower stratosphere), NOx emissions are associated with ozone production whereas, at the high altitudes of 20000-30000 m, which correspond closely to the maximum ozone density, NOx can catalyze ozone destruction. Thus reduction of the NOx Emission Index (grams of emission produced per kilogram of fuel consumed) remains of primary concern, being responsible for the ozone layer depletion. This study has shown that high level of NOx emissions occurs when burning fuel near stoichiometric air-to-fuel ratios. Hence, a strategy in reducing NOx production could be to burn fuel-rich or lean; also the combustor initial pressure has an impact on the combustor emissions. In this paper the impact of the combustor pressure and equivalence ratio on the ozone NOx emissions has been investigated.

Low-Fuel Consumption Gas Turbine Engines for Extended-Range UAVs

In the last years the interest in Unmanned Air Vehicle systems has greatly raised, thanks to their extended operation capabilities. Among the reasons of this growing interest there is the development of very efficient and reliable guiding systems, that allow a thorough remote vehicle control. The endurance capability is certainly one of the most important UAV characteristics, because some missions are supposed to last even more than one day. For this reason, the engine fuel consumption becomes a fundamental aspect. The turboprop engine is commonly used in this kind of missions, with powers in the range between 300-900 kW. To extend the flight range and the endurance characteristics, it is important to have low-fuel consumption propulsion systems. At this purpose, a turboprop engine with intercooling and regeneration has been studied. A thermodynamic numeric program that simulates the behavior of a turboprop with regeneration and intercooling at different engine and operating conditions has been developed. The program allows to compute the thermodynamic working cycle and hence the main engine performances, as specific power, thermal efficiency and specific fuel consumption. An off-design analysis is performed to evaluate the engine behavior when operating at different conditions respect to the design point. An example case, showing the saved fuel for a particular mission profile, is then reported.

Air vitiation effects in scramjet engines

Vitiation is one of the major sources of uncertainty in testing combustors. In fact, in real flight conditions, there is no water in the air entering the combustion chamber, whereas the air entering the test model contains water and radicals. This makes extrapolation to flight difficult. As the total pressure required for engine testing is high, typically 10 MPa for Mach 8 flight conditions, a vitiation air heater burning fuels is required to increase the air enthalpy. Since the arc heaters become unstable under high pressure operations, they are not suitable for heating air for supersonic engine wind tunnels. In spite of such limitations, these vitiation air heaters are the only options available for driving engine wind tunnels. In this work, the chemical inhibition effect on flame temperature and ignition due to OH and H2O has been numerically investigated by means of the 2D RANS of the LAPCAT 2 European MR2 scramjet. Finally, theoretically laws have been proposed for the ground to flight data extrapolation.

Feasibility of high speed atmospheric flight on Venus

Exploring Venus is difficult, so far, probes have managed to survive only few hours. However, Venus’ atmosphere, composed of 96.5 % CO2 might provide an unique opportunity for a airbreathing vehicle. In this context, this paper intends to study the feasibility of a scramjet/ramjet engine for flying in Venus’ atmosphere at an altitude of ~70 km. The first part of the work focuses on the choice of fuel. The most promising candidates to burn in CO2 are metals and their hydrides. In this context, the analysis of a wider range of these candidates including lithium (Li), beryllium (Be), magnesium (Mg), aluminum (Al), silicon (Si), beryllium hydride (BeH2), magnesium hydride (MgH2) and silane (SiH4) has been done. Once the fuel with better performance was chosen, weight and volume of a supersonic aircraft capable of carrying 200 kg of payload and operating range of 1000 km were estimated. The results showed that with a total weight of 995 kg it is possible to keep the size of the aircraft within the limits imposed by modern launchers, leaving enough room and mass weights for an orbital module to be attached to a future venus sound spacecraft.

Electro-chemical propulsion for space exploration

This paper wants to explore the feasibility and convenience to couple an electromagnetic with a chemical thruster in order to exploit the modest propellant consumption of the first and the high thrust of the second. Coupling the advantages of electric propulsion and chemical allows an electric thruster to perform manoeuvers where higher thrust is required. In fact, in case of a electrochemical thruster that uses hydrogen as working fluid can be reacted with oxygen to get larger thrust increase, of course, for a time limited by mass of LOX (or GOX) on board. While in principle this is feasible, in practice, several points must be considered: in particular, those related to chemical propulsion, that is, mixing between two fluids, reactivity at the low feed pressure, and of course the performance achievable. For this purpose, in order to preliminary estimate performance of this electro-chemical thruster in chemical mode when burning H2 with oxygen and at its same operative conditions of pressure and temperature, a theoretical analysis has been performed. This has shown that, combustion of H2/Ox at typical electric thruster pressures is possible with significant thrust increase, coupling the advantages of electric propulsion.

RQL Combustion as an Effective Strategy to NOx Reduction in Gas Turbine Engines

The growing need to drastically reduce aircrafts CO2 emissions has led engineers and scientists in the last years to develop a clean, renewable and sustainable energy system. Hydrogen as a fuel in aviation has shown to be a good choice, since it has an energy release much higher than common and long chain hydrocarbons (119.96 MJ/kg vs ∽42.8 MJ/kg, respectively), a wide flammability limits, a high diffusivity and a very short ignition time. Its thermal conductivity, the highest among all gases, its high heat capacity and its very low dynamic viscosity provide superior cooling properties for operation at high flight speeds and at combustor high temperatures. Furthermore, its low molecular weight makes it the fuel with the higher specific impulse (ISP), ∼450 s: this means that burning 1 kg/s of hydrogen with oxygen produces a thrust of 450 kg-force.However, for its high combustion temperature, it has been demonstrated to be disadvantageous in terms of NOx production. Although NOx pollution is a relatively small part of global human pollution (less than 4%), a particular feature of air transport is that pollutants from air traffic are emitted at high altitudes, in the upper troposphere/lower stratosphere (8 to 12 km), where they are of greater influence than those emitted at ground level. Moreover, further emission reductions need to be achieved by the air transport community, since air traffic has a growth (3% to 5% per year) which exceeds the technology improvement rate.Emissions may be controlled by operating at lean or very lean equivalence ratios (thanks to the wider flammability limits of the hydrogen-air flames compared to kerosene-air flames), or reducing the combustor length (thanks to the higher flame speed of hydrogen compared to other fuels), or via innovative strategies. In this paper, the RQL (Rich-Quench-Lean) strategy for the NOx abatement will be proposed for a high speed hydrogen fuelled vehicle.© 2014 ASME

Performance of a Turboprop Engine with Heat Recovery in Off-Design Conditions

Abstract The research for fuel consumption and pollution reduction in new generation aero engines has indicated intercooling and regeneration as very effective methods for this purpose. Hence, different countries have joined their efforts in common research programs, to develop new gas turbine engines able to reduce considerably the fuel consumption and the ambient impact by means of these two techniques. To study their effects on the engine performance and characteristics, a thermodynamic numerical program that simulates the behavior of a turboprop engine with intercooling and regeneration in different operating conditions has been developed. After the parametric study, and the definition of the design conditions, the off-design analysis is carried on, comparing the main characteristics of the intercooled-regenerated turboprop with those of a conventional engine. Then, once a particular mission profile was fixed, the engine performance, in particular the equivalent power, the fuel consumption and the heat exchanger weight were discussed.

Off-Design Characteristics of Low-Fuel Consumption Gas Turbine Engine for Long-Range UAV

UAVs are becoming of always greater importance, both for civil and military uses. Some UAV missions require a flight length of many hours, sometimes lasting more than two days. In these cases, the engine fuel consumption characteristics become of primary importance. In this sense, it is interesting to study the effects of some techniques, as intercooling and regeneration, on the engine power and fuel consumption, in particular in a turboprop engine, commonly used for this kind of missions. These effects have been studied by means of a thermodynamic numeric program that simulates the behavior of a turboprop engine with intercooling and regeneration, or with regeneration only. The program allows to do a parametric study, computing the main engine characteristics, as thermal efficiency, specific power and specific fuel consumption, for different operating conditions. After the parametric study, an off-design analysis has been performed, to see how the main engine parameters change. Nomenclature T : temperature S : entropy cp : constant pressure specific heat cv : constant volume specific heat γ : specific heat ratio Pshaft : shaft output power v∞ : flight velocity Fn : exhaust nozzle thrust π : compressor pressure ratio R : regeneration effectiveness I : intercooling effectiveness Q : heat amount a m& : air mass flow rate