Air-breathing hypersonic vehicles such as ramjets and scramjets inject fuel into a heated and compressed airstream to ignite a combustion reaction that provides continuous thrust for the vehicle. However, at high speeds the stability of the flame is compromised and becomes susceptible to blowout. In this work, applicable correlations for flame blowout theory are defined and compared to measured limits. Using the MASIV flow model for a hypersonic vehicle with specified geometry and gas properties, early trend analysis on the flame stability with varying inlet conditions is presented. It was found that operating at low combustion pressure leads to overall lower combustion efficiency. Additionally, higher inlet temperature and a larger number of fuel injector ports correlated with improved combustion efficiency. NomenclatureS b = burning velocity [m/s] U g = incoming gas velocity [m/s] U F = fuel jet velocity [m/s] f s = stoichiometric mixture fraction S T = turbulent burning velocity [m/s] S L = stretched laminar burning velocity [m/s] S L /S L0 = non-dimensional stretch factor Ka = Karlovitz number α 0 = thermal diffusivity u g = turbulence level of gas at flame base h = liftoff height [m] d F = fuel jet diameter [m] p 3 = pressure at combustor inlet [Pa] T 3 = temperature at combustor inlet [K] U 3 = velocity entering the combustor [m/s] Da = Damkohler number
As a hypersonic vehicle travels upward along an ascent trajectory, the static pressure (p 3 ) in the scramjet combustor will decrease, which can lead to engine flameout. At low pressures the chemical reactions between the fuel and air become excessively slow. However, during the ascent the flight Mach number is increasing; this increases the stagnation temperature and the static temperature (T 3 ) at the combustor entrance so it tends to prevent flameout. To investigate this tradeoff, a general method to understand how the flameout limit varies during ascent was developed. The method consists of two parts; first the static temperature and pressure at the entrance to the combustor (T 3 , p 3 ) are computed as a function of the vehicle altitude; this is done using a reduced-order propulsion model called MASIV. In the second part the values of (T 3 , p 3 ) are inserted into an empirical relation for the critical Damkohler number in order to determine if flameout occurs or not at each altitude. The empirical flameout relation is based on previous ground-based measurements made at AFRL and elsewhere.
Computations were performed to understand propulsion tradeoffs that occur when a hypersonic vehicle travels along an ascent trajectory. Operability limits are plotted that define allowable flight corridors on an altitude versus flight Mach number performance map. Two operability limits are set by requirements that combustion efficiency exceeds 0.90 and that flameout be avoided. Ambient gas pressure decreases during ascent, which for a fixed waverider inlet (compressor) design slows finite rate chemistry in the combustor. However, this can be offset by increases in flight Mach number and gas temperature in the combustor. New aspects of the work are that operability limits are computed for a waverider trimmed at each altitude. The University of Michigan-U.S. Air Force Research Laboratory scramjet in vehicle waverider model includes finite rate chemistry, three-dimensional mixing, ram-scram transition, and an empirical value of the flameout Damköhler number. A reduced-order modeling approach is justified (instead of computational fluid dynamics results) because all vehicle forces are computed over 1800 times to generate multidimensional performance maps. Trajectories were optimized to achieve highest combustion efficiency and avoid flameout limits.
Ways to maximize the lift-to-drag ([Formula: see text]) and the thrust-to-drag ([Formula: see text]) ratios of hypersonic vehicles are computed using the reduced-order model Michigan-AFRL Scramjet in Vehicle (MASIV). The 84 geometries considered are variations of a generic X-43 for seven chord lengths and three span lengths of the two horizontal stabilizers, and four engine widths. For all cases the vehicle is trimmed for cruise at Mach 8. Computed for each geometry were [Formula: see text], angle of attack, deflection angle of the horizontal stabilizer, specific impulse, and engine equivalence ratio. It was found that the “lifting body” design (with small horizontal stabilizers) has a larger [Formula: see text] than a wing–body design. Large horizontal stabilizers may not be desirable at hypersonic speeds because, while they reduce the angle of attack and reduce the wave drag, the added length of leading edges increases viscous drag. In addition, the optimum acceleration history was computed that minimizes the fuel required for an ascent, for different geometries. Selecting a large dynamic pressure trajectory was found to significantly minimize the fuel required. The scope of the work is limited to aerodynamics, and it does not consider either vehicle stability or control.
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