Measuring Density Profiles of Electrons and Heavy Particles in a Stable Axially Blown Arc

Abstract

Two-color spatial carrier wave interferometry employing pulsed 532-and 671-nm lasers is used to measure the electron-density and heavy-particle-density profiles in the stagnation point of a stable, axially blown arc in argon for currents of 50 to 200 A and stagnation point pressures of 0.2 to 16 bar. This technique takes advantage of the fact that the free-electron contribution to the refractive index depends strongly on the wavelength, while that of the heavy particles does not. The high spatial resolution achieved allows the hot core of the arc to be readily distinguished from the surrounding boundary layer. A custom-built test device is used to ensure flow conditions that lead to a stable, axisymmetric arc; this permits the reconstruction of the density and temperature profiles using a single projection (interferometric image) of the refractive-index distribution through the arc (at two wavelengths). The arc radius determined from the heavy-particle density decreases with increasing stagnation pressure and increases with the current. These measurements are in good agreement with a simple axially blown arc model taking into account Ohmic heating, radiation losses, and enthalpy flow for core temperatures of approximately 16 500 K. The measured electron density at the center of the arc agrees well with a prediction based on local thermodynamic equilibrium.

Optical Diagnostics of Switching Arcs Near Current-zero: Speckle Imaging and Interferometry

“…Two laser wavelengths allow the measurement of the electron density because the contribution of the electrons to the index of refraction is strongly wavelength dependent, while the contribution of the heavy particles is not. The details of this approach are described in detail in [2,4]. An example of a measurement of the electron density in the stagnation point of the arc zone is given in Figure 6.…”

“…The speckle and interferometry experimental setups are described in detail in [3] and [4], respectively. Here we only briefly present the two techniques; a schematic of the speckle imaging setup is provided in Figure 1; the experimental setup for interferometry is given in Figure 2.…”

“…The resulting interference patterns were analyzed using Fourier techniques to obtain the line-of-sight integrated refractive index (again making use of a reference image). The procedure employed is detailed in [4]. It is important to note that the circuit breaker test devices used to obtain speckle and interferometric images differed considerably.…”

“…The three-dimensional index of refraction and density distribution in this arc and surrounding gas flow can be re-constructed from a single projection. This test device is described in detail in [4].…”

“…Such information can be obtained from computational fluid dynamics (CFD) simulations [1], but such models often lack experimental validation (aside from comparison with the measured current, voltage, and pressure). For this reason, several spatially resolved optical diagnostic methods based on well-established techniques that measure the index of refraction (or its gradient) were recently developed or adapted to image circuit breaker arcs near current zero [2][3][4][5]. The advantages and disadvantages of interferometric techniques compared to other optical methods (such as emission spectroscopy) are discussed in detail in those studies.…”