Flight Enthalpy Research Articles

Design and characterization of the Sandia free-piston reflected shock tunnel

A new reflected shock tunnel capable of generating hypersonic environments at realistic flight enthalpies has been commissioned at Sandia. The tunnel uses an existing free-piston driver and shock tube coupled to a conical nozzle to accelerate the flow to approximately Mach 9. The facility design process is outlined and compared to other ground test facilities. A representative flight-enthalpy condition is designed using an in-house state-to-state solver and piston dynamics model and evaluated using quasi-1D modeling with the University of Queensland L1d code. This condition is demonstrated using canonical models and a calibration rake. A 25-cm core flow with 4.6-MJ/kg total enthalpy is achieved over an approximately 1-ms test time. The condition was refined using analysis and a heavier piston, leading to an increase in test time. A novel high-speed molecular tagging velocimetry method is applied using in situ nitric oxide to measure the freestream velocity of approximately 3016 m/s. Companion simulation data show good agreement in exit velocity, pitot pressure, and core flow size.

Hypersonic shock impingement on a heated flat plate at Mach 7 flight enthalpy

Abstract

Production of High-Mach-Number Scramjet Flow Conditions in an Expansion Tube

The expansion tube is the only type of ground test facility currently able to simulate high-Mach-number scramjet test flows. These access-to-space flow conditions are characterized by total pressures of the order of gigapascals. The University of Queensland’s X2 expansion tube facility has recently been used to generate scramjet flow conditions between Mach 10 and 14, relevant to a 96 kPa dynamic pressure ascent trajectory, with total pressures up to 10 GPa. These experiments have matched flight enthalpy and Mach number while achieving test flow static pressures considerably higher than the true flight values. This demonstrates that Reynolds number can also be matched, even for subscale models, thereby addressing one of the major challenges for ground testing of high-Mach-number scramjet design concepts. The routine operation of expansion tubes at high flight enthalpies has already been established; but for operation at lower enthalpies (relevant to scramjet testing), the wave processes within these machines become more tightly coupled and difficult to tune. This paper details the combined analytical and numerical processes used to develop new flow conditions in the X2 facility. Experimental results are presented, and axisymmetric computational fluid dynamics analysis is used to fully characterize the test flow properties.

Simulations of Mixing in an Inlet-Fueled Axisymmetric Scramjet

An unsteady simulation of a simple axisymmetric inlet-fueled scramjet engine concept is performed using a hybrid Reynolds-averaged Navier–Stokes and large-eddy simulation approach. The freestream has a Mach number of 7.5 with Mach 8 flight enthalpy. The simulation is of a nonreacting case in which hydrogen is injected into nitrogen. The simulation is used to provide a detailed description of the structure of the flow. The simulation shows that a large-scale pair of counter-rotating vortices forms within the scramjet combustor, with rotation opposite to the rotation of the pair that forms further upstream due to the interaction of the fuel plume with the crossflow. This vortex pair is found to significantly alter the shape of the hydrogen fuel plume and increase the rate at which the hydrogen is mixing by more than a factor of 2 compared to before the vortex pair is formed. The distribution of hydrogen is examined in detail. The time-averaged and fluctuating wall pressures, the mean velocity field, and resolved turbulence quantities are also examined. Additionally, the hybrid Reynolds-averaged Navier–Stokes and large-eddy simulation results are used to evaluate the performance of a steady-state Reynolds-averaged Navier–Stokes simulation of the configuration.

Investigation of Vitiated Species on the Effect of Scramjet Combustion Using Fuel of Hydrogen or Ethylene

In ground test of the scramjet, the air is heated by burning oxygen in oxygen-enriched air to simulate the flight enthalpy. As result, test media are contaminated by species (H 2 O, H, O, OH and CO 2 ), which are not of representative in the actual atmosphere. In the article, the high enthalpy air is simulated by used the resistance heater, meantime vitiated species is introduced to the high enthalpy air. The effect of combustion is investigated by the experiments used the fuel of hydrogen or ethylene, when the test media are contaminated by H 2 O and CO 2 . At the same time, the effect of scramjet combustion is researched by numerical simulated. The result: the effect on the wall pressure and the combustion performance is markedly when the coming flow with contamination of H 2 O and CO 2 .

Experimental Study of Test-Medium Vitiation Effects on Dual-Mode Scramjet Performance

An experimental study was performed to characterize the effects of vitiation due to combustion-air preheating on dual-mode scramjet combustion.Major combustion vitiation species (H2O andCO2)were added to the freestreamof an electrical-resistance-heated, direct-connect facility simulating Mach 5 flight enthalpy. With clean, dry air, the combustor operated in the supersonic mode at fuel equivalence ratios below 0.22, and in the subsonic mode for equivalence ratios above 0.26. Hysteresis was observed in the dual-mode transition region between 0.22 and 0.26, as the mode of combustion was dependent on whether the fuel rate was increasing or decreasing. Adding increasing amounts of water vapor and carbon dioxide to the freestream decreased combustor pressures by 10 to 30% for the same fuel equivalence ratio. Vitiation also caused transition between supersonic and subsonic combustion to occur at a higher fuel equivalence ratio thanwith clean air. This work represents the first direct evaluation of the effect of testmedium vitiation on dual-mode scramjet combustion atMach 5 enthalpy simulation in the same facility. The results indicate the importance of accounting for test-medium vitiation when extrapolating from ground-testing to flight, particularly in the dual-mode transition region between subsonic and supersonic combustion regimes.

Experimental Study of a Dual-Mode Scramjet Isolator

A constant-area isolator was fabricated and tested in conjunction with a Mach 2 hydrogen-air combustor operating at a simulated Mach 5 flight enthalpy. Predicted isolator performance was validated through pressure measurements obtained via low-frequency pressure taps. The maximum pressure ratio measured in the combustor approached the design limit of 4.5. Scramjet operability, the range of equivalence ratios over which combustion was sustained without shock-inlet interaction, was improved to 0.06-0.32, as opposed to 0.32-0.37 without the isolator. For a given change in fuel equivalence ratio, the location of the shock train was easier to control with the isolator modification. Shock-train location repeatability was found to vary somewhat with equivalence ratio. Small fluctuations in the time-resolved pressure history indicated that the shock train was relatively temporally steady for a given equivalence ratio. High-frequency pressure measurements were within a 95% confidence interval of low-frequency pressure measurements. High-frequency results indicated that an increase in pressure and large pressure fluctuations occurred near the leading edge of the shock train. Power spectral analyses also indicated that there is significant variation in the frequency content of the pressure signal upstream and downstream of the shock-train leading edge. These results suggest that methods of shock-train leading-edge detection may be developed using pressure-time history characteristics other than the pressure magnitude.

EFFECTS OF VITIATED AIR ON HYDROGEN IGNITION IN A HIGH-SPEED LAMINAR MIXING LAYER

One associated complication for the use of vitiated air in laboratory supersonic combustion studies to simulate flight enthalpy is that the test media are contaminated by species that are not representative of the actual atmosphere. When burning hydrogen in oxygen-enriched air is employed to produce vitiated air in experimentation, the resulting high-enthalpy airflow contains substantial amounts of, for example, H2O, H, OH, O, and NO. Therefore, the primary objective of the present numerical study is to assess the effects of vitiated air on the ignition characteristics in supersonic experiments. Specifically, the ignition evolution in the high-speed hydrogen/air laminar mixing layer is computationally simulated using detailed chemistry and transport properties. Individual and combined influences of important contaminants on ignition are systematically examined over a range of pressure, air temperature, and freestream velocity variations. Vitiations of the active radicals, like H, O, and OH, are found to enhance ignition, as expected. Results also show that contamination of H2O and NO can inhibit and promote supersonic ignition, respectively. The computed results on the net effect of simultaneous contaminations of H, O, OH, H2O, and NO on the ignition response further provide insight into the interpretation of the experimental data using the vitiated air facilities.

Development of an Experimentally Flexible Facility for Mixing-Combustion Interactions in Supersonic Flow

A new experimental facility has been designed, built and calibrated to support research topics involving supersonic mixing and combustion and will be used to investigate: 1) molecular level mixing; 2) mixing-combustion coupling effects for gaseous and liquid fuels; and 3) atomization, droplet break-up, mixing, and combustion of high-energy/high-density slurry fuels in a supersonic airstream. The facility simulates flight enthalpies corresponding to altitudes between 26 and 36 km and flight Mach numbers up to 4.755. The major features of this facility are the geometric flexibility of the test section configuration, of the fuel injection, and of the experimental conditions, enabling the investigation of a broad range of topics. Parallel and transverse injection, independently controlled in four regions, is available. The construction of the test section is modular and the entrance Mach numbers can be varied between 1.6 and 3.6. An experimental program to define the test flow at the entrance of the facility’s test section is presented with emphasis on the test section entrance ture distribution and calibration. Selected research topics currently being investigated are summarized to highlight the usefulness of the present facility.

Investigation of scramjet injection strategies for high Mach number flows

A method for estimating the axial distribution of thrust performance potential in a supersonic combustor is described. A complementary technique for illustrating the spatial evolution and distribution of thrust potential and loss mechanisms in reacting flows is developed. A wall jet case and swept ramp injector case for Mach 17 and Mach 13.5 flight enthalpy inflow conditions, respectively, are numerically modeled and analyzed using these techniques.