Ph.D Student | Ifergan Oshri |
---|---|

Subject | Numerical and Experimental Investigation of Arc Plasma Wind Tunnels |

Department | Department of Mechanical Engineering |

Supervisor | Professor David Greenblatt |

Hot plasma wind tunnels incorporate a tube which includes a constrictor that is characterized by a voltage gradient between its inlet and outlet created by an anode and a cathode. The tube contains high-temperature air (>7000 K) that is electrically conductive due to the ionization processes that begin at these temperatures, resulting in an electric arc.

This research is divided into two distinct, but interrelated, parts: one computational and one experimental. The computational objective was to develop, verify and validate a Simplified Numerical Model for the purpose of providing rapid and accurate solutions of the flow in a constrictor in the presence of an electric arc. The model strikes the middle ground between over-simplified empirically-based correlations and advanced computationally expensive simulations. By assuming a fully developed flowfield, the energy equation is solved in conjunction with a plasma local-equilibrium thermodynamic model and Ohm’s law. Despite its relative simplicity, the model attained excellent correspondence with much more sophisticated and elaborate CFD-based models and captured the essential details of the electro-aerodynamic coupling and the influence of the controlling parameters. Furthermore, voltage, enthalpy and efficiency calculations showed excellent correspondence with published experimental data as well as in-house data generated from the Technion Arc Plasma Tunnel (TAPT).

The experimental
objective of this thesis was to *simultaneously* quantify voltage and heat
losses along the length of the TAPT constrictor, for air and argon gas. Thus,
by acquiring time-locked data along the constrictor, transient processes
associated with the arc during ignition, extinction and nominally steady-state
operation were documented for the first time. The voltage data consistently
showed a significant, sharp jump between the cathode and the first constrictor
segment caused by electrons moving from cathode surface and ions arriving at
arc column. A smaller rise was seen in the near-anode region due to electrons
moving from the arc to the anode. Downstream of the first segment, the voltage
increased in a far less dramatic fashion and in some cases, in particular for
argon gas, it was almost linear. The high spatial resolution voltage
measurements allowed us to identify a wave-like structure in the upstream part
of the constrictor. Although no clear explanation was found, it was postulated
that electrode erosion produced an unstable arc geometry that was further
distorted by the imposed magnetic fields. Alternatively, the wave-like
structure could be related to vortex precession commonly observed in swirl-stabilized
flows. Measurements of heat transfer to the wall, based on differences in
cooling water temperature across each segment, revealed a mainly uniform
heat-transfer distribution along the constrictor, with elevated heat transfer
near the cathode and anode due to the strong electric field strengths in the
near-cathode and near-anode regions. By carefully varying either mass flowrate
or current, these measurements also allowed us, for the first time, to quantify
differences in radiation and convection heat transfer.

A comparison of both voltage and heat loss data along the length of the constrictor with the Simplified Model results, revealed some model deficiencies, even though the integral values’ comparisons were excellent.