Gordon L. Nelson

Feasibility study of a hybrid MHD/radiatively driven facility for hypersonic ground testing

A new concept is suggested for high Mach number, high dynamic pressure, long run time hypersonic wind tunnel. The concept includes an ultrahigh-pressure (UHP) driver, beamed energy addition to dense supersonic stream, and an MHD accelerator. The hybrid scheme is shown to benefit from the synergistic action of the components. The UHP driver and beamed energy sources add enthalpy and accelerate the flow at high density, relaxing requirements on the MHD accelerator and creating favorable conditions at the MHD channel entrance. On the other hand, MHD accelerator directly increases kinetic energy of the flow with little entropy change, and is capable of significantly extending the performance envelope (Mach number, dynamic pressure) compared with the pure beamed energy addition case. Two major versions of the hybrid facility are considered, corresponding to MHD operation at high and low pressures. In the first scenario, flow exiting the UHP driver will be directly passed into an MHD channel, where a rapid-fire sequence of bright arcs will be initiated by lasers or electron beams. The arcs will be sustained by linear electrodes on each side of the flow, accelerated in the presence of a magnetic field, and used to impart momentum to the bulk flow. The second scenario includes the UHP driver followed by the beamed energy addition and isentropic expansion to low pressure and temperature, at which point the flow will be passed into an MHD duct, where ionization is created by beams of energetic electrons. One-dimensional analysis and analytical estimates show that such a scheme could significantly augment the Mach number and dynamic pressure, or it could be used to substantially relax requirements on the UHP. Key technical issues to be resolved in the development of the hybrid concept are identified and briefly discussed.

OVERVIEW OF THE NASA MARIAH PROJECT AND SUMMARY OF TECHNICAL RESULTS

During the period April 1995 to October 1997, the National Aeronautics and Space Administration (NASA) sponsored the Magnetohydrodynamics Accelerator for Research Into Advanced Hypersonics (MARIAH) Project. The objective of the project was to evaluate MHD as a driver technology for hypersonic ground test facilities. Test requirements developed in consultation with NASA specified that the technology should be capable of supporting near full-scale engine testing at dynamic pressures up to 2,000 lbf/ft2 and free stream Mach numbers up to 16. Test durations of tens of seconds to minutes were also specified. The near full-scale requirement implies large test section areas, large flow rates, and extremely high electric power. A technology evaluation included a review of past United States and Russian magnetohydrodynamics (MHD) technology development, detailed analysis and computational simulation of several configurations of MHD accelerators, and two major experimental efforts directed at measurement of electrical conductivity in seeded and unseeded high pressure air. One of the significant findings of the MARIAH Project is that MHD accelerators, which rely on thermal ionization of an alkali metal to achieve the requisite conductivity, have a restricted pressure-temperature range of operation. Pressures of at least 100 atm and temperatures of 2,500 K or higher are required for this mode of MHD channel operation, irrespective of seed material. This is due in part to constraints on entropy dictated by the targeted test section conditions. The minimum temperature requirement of 2,500 K is determined by electrical conductivity considerations. This temperature-pressure range is difficult to achieve using conventional arc heaters as primary drivers. Consequently, alternative nonequilibrium schemes for creating and sustaining the electrical conductivity were investigated. Unconventional drivers in the form of ultrahigh pressure (UHP) gas piston drivers were also evaluated. This paper discusses the major activities and technical findings, concluding with specific recommendations for future research.

ELECTRON ATTACHMENT IN SEEDED AIR FOR HYPERVELOCITY MHD ACCELERATOR PROPULSION WIND TUNNEL APPLICATIONS

Magnetohydrodynamics (MHD) has been identified as one of only two technologies that is capable of producing hypervelocity, high flight dynamic pressure, clean-air wind tunnel simulations for development of air-breathing propulsion systems. An extensive study of MHD accelerator performance for application to a large-scale testing and evaluation (T&E) facilities was conducted for National Aeronautics and Space Administration (NASA). Several analytical and experimental studies were conducted during this project, including an analytical parametric and optimization study evaluating the performance of MHD accelerators for this application. An alkali metal seed material is generally used in MHD accelerators to enhance ionization. However, when ionization occurs in air, some of the free electrons can be lost through attachment to oxygen species, forming negative ions and reducing the electrical conductivity. Under some circumstances, this can be detrimental to MHD performance since MHD performance depends strongly on the electrical conductivity of the working gas. The results of an analysis to evaluate the effect of electron attachment to oxygen on the electrical conductivity of seeded air and on the performance of MHD accelerators is presented in this paper.

Progress toward a radiative and MHD driven high enthalpy, high pressure, long duration test facility

During the past two years, the U.S. Air Force (USAF) has sponsored the Magnetohydrodynamics Accelerator Research Into Advanced Hypersonics, Radiatively Driven Hypersonic Wind Tunnel (MARIAH II/RDHWT) Program, This program represents a merger of two earlier projects, one sponsored by NASA, which had the objective of evaluating MHD technology; the other sponsored by the USAF for the purpose of evaluating beamed energy addition into supersonic flows. The objective of the current project is to evaluate the feasibility of several novel technologies as drivers for a true enthalpy hypersonic wind tunnel. As presently envisioned, the tunnel would be comprised of three stages. The first stage would consist of an ultra high pressure (UHP) driver in which air is compressed dynamically to pressures up to 2,000 MPa and released through a converging-diverging nozzle. The second stage of energy addition would consist of radiative energy beamed from downstream into the supersonic section of the nozzle. The third stage would be a nonequilibrium magnetohydrodynamic (MHD) accelerator which would rely on electron beam seeding to sustain the required electrical conductivity. The near term goal of the program is to develop credible engineering data to enable the design of a missile scale hypersonic wind tunnel (MSHWT). The ultimate goal is to design and build a Test and Evaluation wind tunnel capable of supporting near full scale engine testing at dynamic pressures up to 2000 lbf/sq. ft. (psf) and free stream Mach numbers up to 15. Test durations of tens of seconds to minutes were also specified. A substantial amount of research has been accomplished to date. Computational simulations of a conceptual facility incorporating reservoir conditions of 2,000 MPa and 900 K indicate that a test section Mach number of 12 at a dynamic pressure of 2,000 psf is attainable using only the first two stages. To achieve the Mach 15 condition at the same dynamic pressure will require MHD augmentation. This paper provides a survey of all significant computational and experimental work being conducted in support of the program. Specific areas of ongoing research which will be described in some detail in the paper include the following. . Laser energy addition tests at a scale of 10 kW conducted at the U.S. Air Force Research Laboratory (AFRL). . Electron beam energy addition tests at the 30 kW level conducted at Sandia National Labor&ories (SNL). . Planned electron beam energy addition tests at the 100 kW level to be conducted at SNL. . Computational simulations of both electron beam and laser energy addition into supersonic air flows. . Computational simulation of nonequilibrium MHD kinetic energy addition sustained by electron beam seeding. . Static cell experiments conducted at SNL for the purpose of validating the concept of electron beam seeding in air. . Design of a UHP driver, called the A-2 facility. It is planned that this facility will operate at reservoir conditions of 2,000 MPa and 900K, and will support future proof of principle tests in the area of beamed energy addition. The paper summarizes all major research activities and concludes with specific recommendations for future research. * Staff Aerospace Engineer, MSE Technology Applications, Inc., Butte, MT, Senior Member AIAA ’ ’ Professor of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, Fellow AIAA Professor of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, Associate Fellow AIAA ’ Research Scientist, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, Member AIAA ‘* Research Scientist, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, Senior Member AIAA ” Staff Scientist Lawrence Livermore National Laboratory, Livermore CA ” Staff Scientist: Sandia National Laboratories, Albuquerque, NM, Member AIAA This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States, 1 American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or published with permission of author(s) and/or author(s)’ sponsoring organization.

PARAMETRIC ANALYSIS OF MHD ACCELERATOR WIND TUNNELS FOR HYPERVELOCITY, AIR-BREATHING PROPULSION DEVELOPMENT

Magnetohydrodynamics (MHD) has been identified as one of only two technologies that is capable of producing hypervelocity, high flight dynamic pressure, clean air wind tunnel simulations for development of air-breathing propulsion systems. An extensive study of MHD accelerator performance for application to large-scale testing and evaluation (T&E) facilities was conducted for NASA. A parametric and optimization study was performed as one of the tasks within this study. The NASA requirements for this study specified that the T&E facility should be capable of producing test conditions equivalent to a post bow shock condition for a flight Mach number of 16 and dynamic pressure of 2,000 lbf/ft2. A one-dimensional performance analysis indicated that cesium seeded MHD accelerators augmenting high pressure arc heaters can not reach this flight condition, but would be able to produce conditions for a flight dynamic pressure of 700 lbf/ft2 at the same flight Mach number. The results of this MHD accelerator performance study are presented in this paper. Nomenclature A - Flow cross-sectional area, (m2) B - Magnetic field strength, (T) E - Electric field, (V/m) Ht - Total enthalpy, (J/kg) j - Current density, (A/m2) K - Faraday loading parameter, (=Ey/uB) m - Mass flow rate, (kg/s) qw - Wall heat transfer, (W/m2) s - Entropy, (J/kg-K) T - Gas temperature, (K) u - Gas velocity, (m/s) x - Axial direction y - Transverse direction 77 - Efficiency a - Electrical conductivity, (mho/m) \l/ - Channel perimeter, (m)