Masters Theses

Date of Award

12-1994

Degree Type

Thesis

Degree Name

Master of Science

Major

Aerospace Engineering

Major Professor

Roger Crawford

Committee Members

Roy J. Schulz, Robert L. Roach

Abstract

The simulation of hypersonic air streams at Mach numbers above 10 in ground test facilities is complicated by the extremely high stagnation temperatures and pressures that the facilities must endure. Operationally, nozzle throat heat transfer limits, air dissociation in the stagnation chambers, and NOx, formation are three facility shortcomings limiting true flight simulation at hypersonic Mach numbers. Many studies and experiments dating from the early 1960's to the present have supported the application of MHD acceleration of a pre-heated conducting gas for hypersonic simulation. The very high development and operating costs of a MHD-augmented continuous flow facility have stopped development of this concept for the last 25 years. Now new high temperature composite materials, improved computational fluid dynamics capability, and a renewed interest in hypersonic flight are making the MHD- augmented hypersonic flow facility essential to the further understanding of hypersonic fluid mechanics and heat transfer.

Due to the large development costs of a continuous flow MHD-augmented hypersonic flow facility, a transient MHD accelerator has been proposed as a low cost research facility for investigating the key technology issues. Studies at the University of Tennessee Space Institute in a railgun research program have demonstrated a significant electromagnetic difference between pulsed or transient electric fields and those formed in steady state devices. Steady state MHD devices have a more uniform magnetic field and current density distributions. Pulsed or transient MHD devices accelerate a gas slug in a time varying magnetic field. Pulsed or transient MHD devices, therefore, lack the flow uniformity associated with steady state devices. research facility needs a uniform freestream flow A to accurately simulate the hypersonic flight conditions. Therefore, the electrode geometry is critical for providing the uniformity in the magnetic field, current density, and accelerating force.

The electrode geometry is a major contributor to reduce the non-uniformities in pulsed or transient MHD devices. The electrode configuration will be the determinant in obtaining a uniform accelerating force in the channel. The desired electrode geometry will produce a more uniform magnetic flux density and a more uniform current density, resulting in a more uniform magnetic force to propel the conducting gas down the channel.

Several electrode shapes were simulated to find an electrode geometry that provides the uniformity needed to accelerate the conducting gas down the channel. The initial accelerator concepts were for a rectangular electrode geometry. UTSI proposed to investigate other geometries which produce a magnetic flux density and current density that reach uniformity at a quicker rate and without the current concentrations in the corners associated with a rectangular electrode geometry. Other criteria such as electrode fabrication costs were not taken into consideration but should be considered before building a device with one of the shape indicated in this thesis.

The electromagnetic code MegaTM has been used to evaluate the moving fields and current density as a slug of potassium-seeded air is being accelerated down a channel. The results of the study are that: (1) of the types of electrode shapes studied, none were able to eliminate the non-uniformity of the magnetic force in the initial time steps, and (2) some shapes can, however, reduce the non- uniformity to tolerable limits as early as 10 microseconds. The magnetic field was simulated in both 2-D and 3-D computer models to find the electrode shape with tolerable flow uniformity and to map the field distribution in the conducting gas.

The simulations show that 1) flat, smooth electrodes help maintain a uniform magnetic force; 2) in the first microseconds, the uniformity of the magnetic force varies little between hollow shapes and solid shapes; 3) a better magnetic force uniformity is provided with an electrode wider than the channel width and with a curved electrode end; and 4) if the current is increased, the magnetic force will increase, but the uniformity of the magnetic force will remain almost the same.

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