Spatially-resolved velocities of thermally-produced spray droplets using a velocity-divided Abel inversion of photographed streaks

Abstract

Droplet velocities of thermal spray are known to have profound effects on important coating qualities, such as adhesive strength, porosity, and hardness, for various applications. For obtaining the droplet velocities, therefore, the TOF (time-of-flight) technique has been widely used, which relies on observations of emitted radiation from the droplets, where all droplets along the line-of-sight contribute to signals. Because droplets at and near the flow axis mostly contribute coating layers, it has been hoped to get spatially resolved velocities. For this purpose, a velocity-divided Abel inversion was devised from CMOS photographic data. From this result, it has turned out that the central velocity is about 25% higher than that obtained from the TOF technique for the case studied (at the position 150 mm downstream of the plasma spray gun, where substrates for spray coatings are usually placed). Further implications of the obtained results are discussed.

Spectroscopic observation of radiation emitted from thermally-produced spray droplets and the resulting temperature determination

“…( 1). It should be noted that this limit was obtained under the general thermal droplet's condition, 16,17) for instance the droplet's flux of about 6 × 10 18 m −2 •s −1 in Ref. 16, and the ambient light condition in a normal laboratory room.…”

“…It should be noted that this limit was obtained under the general thermal droplet's condition, 16,17) for instance the droplet's flux of about 6 × 10 18 m −2 •s −1 in Ref. 16, and the ambient light condition in a normal laboratory room. When the observation area of the spectrometer is continuously filled with radiating bodies, the lower temperature limit is improved much.…”

“…where Ið; TÞ is the intensity of radiation emitted at a wavelength λ from a body surface at a temperature of T, "ð; TÞ is the emissivity which is dependent on the material and its surface conditions, such as surface roughness and the degree of oxidation, along with the wavelength and the temperature of the emitting surface, h is the Planck's constant, c is the speed of light, and κ is the Boltzmann constant. In the simple monitoring system, [12][13][14][15][16] the center wavelengths of 650 and 750 nm with a bandwidth of 50 nm each were chosen in order to measure the temperatures. At the same time, the measurement of the velocity was also facilitated by the time of flight (TOF) technique for the droplets simultaneously using the same optical detection system for two pre-determined spatial positions.…”

Optical monitoring systems for thermal spray processes: droplets behavior and substrate pre-treatments

“…Table 1 summarizes the measured values of temperatures and velocities. It should be noted that there is an uncertainty of up to 20% overestimate in these temperatures [5] and a 25% underestimate in these velocities [7]. The former overestimates come from uncertainty in the emissivity value of the droplets, and the latter underestimates come from the necessity of subtracting the contributions of low velocity droplets existing at the peripheries of the droplet flow which are inevitably observed for this kind of line-of-sight measurements (this subtraction was not carried out in the values in table 1).…”

Forty years (1983–2023) with LAPD and for its future evolution

“…From these photos, one sees a clear axi-symmetry of the discharge between the two electrodes. Therefore, the Abel-inversion [9] of the line-of-sight fringe shifts, shown in figure 11, yields the radial information. As described above, the contributions of electron and neutral densities are different for different wavelengths.…”