Analysis of Combustion Studies in Shock Expansion

The e ect of initial nonequilibrium dissociated air constituents on the combustion of hydrogen in highspeed ows for a simulated Mach 17 ight condition was investigated by analyzing the results of comparative combustion experiments performed in a re ected shock tunnel test gas and in a shock expansion tunnel test gas. The results were analyzed and interpreted with a one-dimensional quasi-three-stream combustor code that includes nite rate combustion chemistry. The results of this study indicate that the combustion process is kinetically controlled in the experiments in both tunnels and that the presence of the nonequilibrium partially dissociated oxygen in the re ected shock tunnel enhances the combustion. Methods of compensating for the e ect of dissociated oxygen are discussed. Introduction Currently, re ected shock tunnels and shock expansion tunnels are the only facilities that can generate high enthalpy conditions for simulation of scramjet combustion at ight Mach numbers greater than 12 (refs. 1 and 2). Even though each facility can simulate the enthalpies and combustor inlet Mach numbers representative of high ight Mach number conditions, the composition of the test gas or simulated air produced is signi cantly di erent in each facility. The re ected shock tunnel test gas contains signi cant amounts of atomic oxygen and nitric oxide because the test gas is brought to a stagnant condition prior to expansion to the desired high-energy ow conditions. Chemical kinetic e ects prevent the recombination of signi cant amounts of the atomic oxygen and the reduction of nitric oxide to nitrogen and oxygen. In the shock expansion tunnel, the test gas is accelerated to the test condition by an unsteady expansion and is never brought to a stagnant condition. The highly dissociated test gas produced in a re ected shock tunnel is of special concern when used for combustion tests, which must be interpreted in terms of combustion in real air. The presence of atomic oxygen in the test gas can a ect combustion tests in several ways. Atomic oxygen reacts very rapidly with molecular hydrogen. If su cient amounts of the atomic oxygen are present in the test gas, the test gas can be expected to be more reactive than real air. Also, the presence of atomic oxygen will increase the heat release by adding the heat of formation to the fuel heat content. In addition, under certain conditions the increased rate of hydrogen reaction in the presence of atomic oxygen can a ect mixing (ref. 2). Recently, a series of comparative experiments were performed at essentially identical conditions in a shock expansion tunnel (the NASAHYPULSE tunnel located at General Applied Science Laboratories (GASL)) and in a re ected shock tunnel (the University of Queensland T4 tunnel) (ref. 2). The purpose of the experiments was to determine the e ects of test gas composition on combustion in high-speed ows. Each facility simulated combustor inlet conditions for a Mach 17 ight condition. The experiments were carried out with identical combustors|a constant area, axisymmetric combustor with an overall length-to-diameter ratio of 24. The fuel was injected through a singular annular slot at Mach 1.9 at an angle of 15 from the combustor axis. The purpose of this study was to analyze and interpret the results obtained from the comparative experiments by using a one-dimensional quasi-threestream combustor code that includes a nite rate chemistry description of the combustion process. An attempt was made to examine the role of chemical kinetics on the combustion of hydrogen in the two facilities, with special emphasis on the e ect of dissociated air compared with nondissociated air. Symbols d combustor internal diameter Hs stagnation enthalpy, MJ/kg P static pressure, kPa Po average tare pressure from fuel-o tests, kPa T static temperature, K Ts stagnation temperature, K U velocity, m/s x axial location along combustor, cm fuel-to-total-oxygen equivalence ratio Results of Comparative Experiments The fuel-to-oxygen equivalence ratio was 3 in all the comparative experiments. The test conditions that were selected for the comparative experiments are listed in table I, where the test gas composition (air) is given as mole fraction. The results of the comparative experiments (shock expansion tunnel air and re ected shock tunnel air a) are given in gures 1 and 2. The results are expressed in terms of measured static pressure along the combustor normalized by the facility tare pressure Po. The tare pressure is the average pressure in the combustor during fuel-o tests. The result shown in gure 1 is a comparison of the measured pressure distributions 1 Table I. Test Flow Conditions and Test Gas Composition (Mole Fraction) Shock expansion Re ected shock tunnel test gas tunnel test gas a b c Hs, MJ/kg . . . . . . . . 15.3 15.7 15.7 15.7 Ts, K . . . . . . . . . . 8355 7880 8350 8180 T , K . . . . . . . . . . 2088 2065 212