Optimization of Propulsion Systems

• Research Area: Optimization of Propulsion Systems

  NASA's goal is to reduce NO_x emissions of future aircraft by 70% within 10 years and by 80% within 25 years. Further, CO₂ emissions are to be reduced by 25% and 50% in the same timeframes. Toward this objective, this proposed project will explore the usage of controlled mixing in lean combustion. For turbine engines operating on natural gas, lean premixed combustion in combustors provides a means to reduce pollutant formation, e.g. NO_x and CO₂, and increase combustion efficiency. In addition, NOx can be reduced by adjusting the degree of mixing, i.e. unmixedness, of fuel and air prior to combustion in a lean premixed reaction. Research shows that concentrations of NO_x and CO₂ decrease as the fuel-to-air ratio, f, decreases. However, the NO_x concentration increases for the non-premixed case while it decreases for the premixed case.

  As Figure 1 indicates, concentrations of NO_x and CO₂ decrease as the fuel-to-air ratio, f decreases. However, the NO_x concentration increases for the non-premixed case while it decreases for the premixed case. The term unmixedness is used to indicate either the temporal or spatial variation in the concentration of one gas, e.g. fuel, in another gas, e.g. air. Lyons has developed analytical expressions for spatial unmixedness while Fric has developed expressions for temporal fluctuations in the fuel concentration. The unmixedness depends on the mixing time, the integral length scale of the flow, the intensity, and other turbulent flow properties. These flow properties depend on the method of injection, the length of the premixer, and any other devices that are placed in the flow path that modify the turbulent properties. The research in this area will be organized in terms of the activities described below:

  

• Research Activity:  Mixture Control  

  "For premixed combustion, the production of NO_x and CO₂ decreases as the fuel and air mixture becomes more lean (i.e., decreases in f). However, as the fuel and air mixture becomes more lean, the combustion process approaches a point where there is insufficient fuel to maintain ignition (i.e., the combustion process experiences blow-out). For an aircraft engine to be safe and stable, a uniform fuel-and-air mixture must remain above this point. The hypothesis of this proposed study is that a high degree of intentional, spatial unmixedness may be able to achieve stable combustion below the critical point observed with no unmixedness.

  

   

  Figure 2 shows two parcels of fuel and air mixtures. In the diagram that shows a high degree of unmixedness, the dark regions represent fuel, and the light regions represent air. The diagram that shows a low degree of unmixedness has the same overall average f, but the regions of fuel (dark) and the regions of air (light) have been reduced in size giving the diagram a uniform gray appearance; it is approaching a state of being completely mixed.

  Thus, the goal of this study is to use numerical analysis, supported by experimental data, to determine the combination of the minimum f and the optimal spatial unmixedness necessary to minimize NO_x and CO₂ formation while maintaining a stable reaction. These numerical results will complement the work currently being performed for the UEET (Ultra-Efficient Engine Technology) at the NASA Glenn Research Center (Lewis Field). Once the ideal combination is determined, different techniques of atomization and mixing can be proposed to apply these findings to actual hardware application. The experimental data will be collected in a model gas turbine combustor that has been developed in the MFDC lab. This experiment fixture allows control over the stoichiometric ratio, the swirl number, the location and amount of secondary air injection, and is designed to function with both gaseous and liquid fuels.

   

  

   

  

   

  This experiment fixture allows control over the stoichiometric ratio, the swirl number, the location and amount of secondary air injection, and is designed to function with both gaseous and liquid fuels. 

  As part of the laboratory’s goal to evaluate and validate CFD software, a CFD model of this combustor has been developed. Preliminary numerical results from this model show promise in accurately predicting the behavior of the physical model.  Figure 4 shows an example preliminary data that correlates well with the experimental data.

   

• Research Activity: Measurement of Particulate Emissions

  The study of particulate emissions is a growing field in the area of air pollution. These emissions can contain many harmful substances such as Polycyclic Aromatic Hydrocarbons which are formed during the combustion of organic materials (carbon and hydrogen) and are suspected carcinogens and mutagens.  Because particle emission measurements are a relatively new concern, techniques for collection and quantification are currently being developed.  The MFDC Laboratory has performed some preliminary means of collecting data on these particles (Figure 5).  The goal of this study is to improve these data collection techniques and to further study these Polycyclic Aromatic Hydrocarbons particles.

   

  

   

• Research Activity: Supersonic Combustion

  Another goal of the proposed research is to create a supersonic combustion platform. Many facilities and universities are investigating high speed flight in the attempt to create an air vehicle capable of reaching hypersonic speeds. In hypersonic flight, many optimizing factors such as premixing, ignition, and blow-out need to be considered. Hypersonic flight is viable, but there are many problems that arise that will be predicted, examined, and solved in this proposed research activity. The most challenging goal is maintaining supersonic combustion. To address this challenge, the MFDC Laboratory is proposing to perform both experimental and computational studies in combustion at sonic speeds. A supersonic combustion test bed is currently being designed and fabricated where flow will enter this fixture through a De Laval converging-diverging nozzle to produce supersonic air speeds while fuel is injected mid-stream with a coaxial fuel injector. Preliminary Fluent modeling (Figure 6) supports the concern that sonic combustion is difficult to achieve. The model predicts that combustion can be achieved at Mach 1.8 but not at Mach 3.