Temperature measurements of long sparks in air using time-resolved moiré deflectometry

Abstract

This paper presents an original investigation on a time-resolved moiré deflectometry for gas temperature measurements of long sparks in air. In order to perform a rigorous comparative investigation with the widely used quantitative schlieren method, we set up an orthogonal optical measurement system. The impacts of spatial resolution and exposure time on the measurement accuracy are investigated by comparing the measured radial distribution of the gas temperature and its time evolution during the same spark discharge event. It is found that the time-resolved moiré deflectometry with an exposure time of 1.0 μs can accurately measure the gas temperature evolution during the isobaric heating and relaxation process of long sparks. The measured results of the expansion rate of cross-section π·〖R_"g" 〗^2 and average temperature Tg confirms that the energy loss is dominated by thermal conduction during isobaric heating and relaxation process after the extinction of discharge current. The moiré deflectometry can achieve a finer spatial resolution than the quantitative schlieren method, which is beneficial to the reconstruction of steep radial temperature distribution and the suppression of measured temperature fluctuation. Moreover, the moiré deflectometry can obtain a sub-microsecond exposure time by increasing the power of continuous-wave laser source, which shows its potential to capture the rapid changes in gas temperature during the fast-heating process.

Thermal dynamics of leader decay and reactivation in long air gap discharges

“…The grayscale changes in Schlieren images are quantitatively related to the deflection angle of the probing light beam, which can be used to reconstruct the refractive index distribution n(r) of the leader channel cross-section. More details about the fundamental principle of the quantitative Schlieren method can be found in [24,25]. A 6.0 MV/360.0 kJ impulse voltage generator is adopted in this work, which can perform long air gap discharge experiments with a gap length of more than 10 m. When the gap length used in experiments increases, as shown in figure 1, the distance from the collimating and convergence optical units to the high-voltage electrode has to increase accordingly to fulfill the safety insulation requirement.…”

“…These can enhance the vertical component of the electric field and help to control the trajectory of the initial leader segment and avoid the displacement of the leader cross-section. The focal length of L 1 and L 2 is 1.5 m. A secondary imaging lens with a focal length of 20 cm is inserted between the knife edge and the filter instead of the long focal lens in [25], which can help to realize a spatial resolution of 60.0 µm pixel −1 . The high-speed camera is operated at a sampling rate of 200.0 kfps, which means the Schlieren image is recorded every 5.0 µs.…”

“…The high-speed camera is operated at a sampling rate of 200.0 kfps, which means the Schlieren image is recorded every 5.0 µs. The exposure time of a single Schlieren image is set to be 0.37 µs which is shortened by 70% from that of 1.25 µs presented in [25]. The reduction of exposure time is beneficial for avoiding the temporal-averaging effect caused by the time integration of a single Schlieren image under long exposure conditions.…”

“…Although the 1-D numerical model has promoted our physical understanding of the positive leader channel, more experimental observations are needed to obtain the longitudinal property of the leader channel, which is crucial to reveal the formation mechanism of discontinuous positive leaders. Recently, flow visualization techniques, including the quantitative Schlieren method [23][24][25], Moiré reflectometry [25], and Mach-zander interference [26,27] have been introduced to measure the spatiotemporal evolution of gas temperature during the positive leader initiation phase. These measurements provide data support for the validation of the proposed 1D radial thermal hydrodynamic model of leader discharge.…”

Measurement of medium-voltage AC air arc temperature and particle number density based on dual-wavelength Moiré deflection technology

“…In the second zero-off stage shown in figure 8, no electron density drop was detected, which means that t x may be lower than t 2 . Other studies have observed that the initial ionization channel in the air gap can be formed within a few microseconds, well before t 2 [35,36]. • Arcing stage: the arc enters the arcing stage, and its electron density gradually decreases after a rapid decline.…”