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1. A study of the efficacy of flame electrical resistance for standoff measurements during the oxyfuel cutting process

   Martin, Christopher ; Untaroiu, Alexandrina ; Xu, Kemu ; Rahman, S.M. Mahbobur ( June 2021 , Proceedings of Manufacturing Science and Engineering Conference)

   null (Ed.)

   This is a study of the suitability of preheat flame electrical resistance as a potential method for measuring the standoff distance an oxyfuel cutting torch and a work piece. Careful scrutiny of forty seven (47) individual experiments demonstrate that when cut quality is good, there is a linear repeatable relationship between the two with uncertainty about ± .3mm (.015in). As the cut quality degrades, the formation of top-edge dross reduces the electrical path length in the flame, and momentary reduction in the reaction rate in the kerf reduces the free electrons in the flame, causing rises in flame resistance. In these conditions, measurement uncertainty reduces to ± 1mm (.040in) or worse. 

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2. Semiconductor aspects of the oxyfuel cutting torch preheat flame; Part I: Measurements between torch and work

   Martin, Christopher ; Pond, Teresa ; Tomas, Jacob ; Schmit, Jenna ; Miguel, Erikson ; Untaroiu, Alex ; Xu, Kemu ( June 2020 , Proceedings of the ASME Manufacturing Science and Engineering Conference)

   This two-part paper presents precise measurements of the ion currents passing between the torch and work piece of the preheat flame of an oxyfuel cutting torch as a means for replacing contemporary sensing suites. Part I shows that the current-voltage characteristic of the flame exhibits sharp discontinuities common to semi-conductors that we study in various configurations including preheat, pierce, cut, and loss-of-cut. Standoff measurements are made by applying a sinusoidal current signal between the torch and work piece while the resulting voltage amplitude is an indication of flame resistance. Uncertainties are estimated to range from 0.5mm to 1mm (.02in to .04in). Signals for ready-to-pierce and precursors for loss-of-cut are also produced due to the generation of secondary ions from chemical activity at the work piece. 

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3. Burning Characteristics of Small Firebrands in Wildland-Urban-Interface Fires

   Kwon, Byoungchul ; Liao, Ya-Ting ( May 2021 , 12th U. S. National Combustion Meeting)

   Firebrand attack has been shown to be one of the key mechanisms of wildfire spread into Wildland-Urban Interface (WUI) communities. The ignition propensity of materials caused by firebrands depends on not only the attributes (e.g., shape, size, numbers) but also the distribution of firebrands after landing on the substrate materials. To help characterize this process, this study aims to first investigate the effects of gap spacing on the burning behaviors of a group of wooden samples. Experiments were conducted using 9 wooden cubes, 19mm-long on each side. These samples were arranged in a 3 by 3 square pattern on suspension wires. The gap spacing (s) between the cube samples varies from 0 to 30 mm. Burning process was recorded using video cameras. Sample mass loss and temperatures were monitored during the flaming and smoldering processes. The results show that when s ≤ 10 mm, flames from individual samples merged. When the gap spacing reduces, the mass loss rate first increases but starts decreasing at s = 10 mm where flame merging occurs. The flame height has a similar non-monotonic dependency on the gap spacing and the maximum flame height occurs at s = 5 mm. Compared to the case with s = 10 mm, cases with a smaller gap spacing (s = 2.5 and 5 mm) have a larger flame height but a smaller sample mass loss rate. This indicates that a reduced air entrainment leads to an increase in the flame height despite of a decreased flame heat feedback to the solid samples. The heating rates of each sample were also calculated to investigate the local burning behaviors. The analysis showed a weaker flame heat feedback to the sample at the center for cases with under-ventilated combustion. Last, gaseous flame height was corelated to the solid burning rate. The correlation was also compared with previous empirical equations concerning liquid pool fires of different heat release rates. 

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4. Effects of spacing on flaming and smoldering firebrands in wildland–urban interface fires

   https://doi.org/10.1177/07349041221081998  

   Kwon, Byoungchul ; Liao, Ya-Ting T. ( March 2022 , Journal of Fire Sciences)

   Firebrand (ember) attack has been shown to be one of the key mechanisms of wildfire spread into wildland–urban interface communities. After the firebrands land on a substrate material, the ignition propensity of the material depends on not only the attributes (e.g. shape, size, and numbers) but also the distribution of the firebrands. To help characterize this process, this study aims to investigate the effects of gap spacing on the burning behaviors of a group of wooden samples. Experiments are conducted using nine wooden cubes, 19 mm on each side. These samples are arranged in a 3 × 3 square pattern on suspension wires and are ignited by hot coils from the bottom surface. The gap spacing (s) between the samples varies in each test (ranging from 0 to 30 mm). After ignition, the samples are left to burn to completion. The burning process is recorded using video cameras. Sample mass loss and temperatures are monitored during the flaming and smoldering processes. The results show that the flame height and the sample mass loss rate have non-monotonic dependencies on the gap spacing. When the gap spacing reduces, the flame height and the mass loss rate first increase due to enhanced heat input from the adjacent flames to each sample. When s ≤ 10 mm, flames from individual samples are observed to merge into a single large fire. As s further decreases, the air entrainment at the flame bottom decreases and the flame lift-off distance at the flame center increases, resulting in an increased flame height, decreased flame heat feedback to the solid samples, and a decreased mass loss rate. The decreased mass loss rate eventually leads to a decrease in the flame height as well. The gaseous flame height is correlated to the solid burning rate. The correlation generally follows previous empirical equations for continuous fire sources. For the smoldering combustion, compared to a single burning sample, the smoldering temperature and duration significantly increase due to the thermal interactions between adjacent burning samples. To help interpret the results of the burning experiments, thermogravimetric analysis is also performed in air and nitrogen, resulting in heating rates ranging from 10 to 100 K/min.

    

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5. SPATIALLY RESOLVED ION CURRENT DENSITY MEASUREMENTS WITH A TRANSIENT INSERTION LANGMUIR PROBE

   Christopher R. Martin ; Jacob Orr ; S.M. Mahbobur Rahman ; Alexandrina Untaroiu ( October 2023 , Proceedings of the International Mechanical Engineering Congress and Exposition)

   This work presents a novel technique for constructing spatially resolved ion densities from Transient Insertion Langmuir Probe (TIL Probe) measurements in a flame. Similar to a tomographic transformation, this technique is used to deduce the spatial distribution of ions in a flame from many individual measurements that are integrated along a probe’s length. We demonstrate the approach in the oxyfuel cutting torch preheat flame, which presents two severe challenges for electrical measurements: (1) temperatures over 3,000K destroy most probes made from alloys with appropriate chemical stability, and (2) the relevant length scales are on the order 0.15 mm. Presented here are (1) a Fourier series formulation for the current density, (2) a least-square problem for calculating the coefficients, (3) criteria for the highest wavenumber allowed in the expansion, (4) description of an experiment used to measure probe currents in an oxyfuel cutting torch preheat flame, (5) solution for spatially resolved current density in the oxyfuel cutting torch flame. Images of ion current density are produced with a resolution of 0.15 mm (0.0059 in), exhibiting peak current densities around 14 𝜇A/mm. It is found that low-signal regions in the “shadow” of high-signal regions can suffer from signal-to-noise ratio problems due to natural fluctuations in the flame, and improvements are proposed to mitigate the effect. It is found that the numerical cost of setting up the resulting Hermitian-matrix linear problem far exceeds the numerical cost of inversion. High-level packages like Python and MATLAB are far too slow, so a multi-threaded algorithm is implemented in C, and the LAPACKE C library is used for efficient linear algebra support. 

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