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Measurement of the emissivity of quartz glass
Abstract
The technique employed for measuring the total emissivity of a material partially transparent to thermal radiation consists in measuring the temperatures of the sample and of a blackbody at which the radiant fluxes are equal. The apparatus consists of a water-cooled blackened vacuum chamber accommodating a tubular graphite furnace with a partition in the centre. The furnace is heated by an inductor fed from a high-frequency generator. A sample in the form of a disc is suspended inside the upper section of the graphite furnace used as the blackbody; when the temperature stabilizes, the graphite furnace is dropped and the sample screened by a shutter. The measurement time is reduced to 40 ms by using a fast recorder. The total normal emissivity of grade KSG quartz glass was measured from 700 to 1300 K on samples 2 to 10 mm thick. Data are also given for other grades of quartz glass. Measurement errors are estimated.
... This Appendix provides details about an uncertainty analysis conducted on the emissivity of the transparent packed bed with D/d=9. The emissivity increases with an increase in the thickness and decreases with an increase in the temperature [520]. However, for simplification, in this study, the emissivity was considered a constant value. ...
... It is to be expected as resin depletion at the end of the test results in the exposed surface being mostly reinforcement fibres. Furthermore, the decrease in emissivity from approximately 0.9 at room temperature (HDR at 20 • C) to 0.65 at 440 • C (TES with ∆T = 420 • C) is also consistent with the high temperature emissivity measurements of quartz glass at similar temperatures reported in the literature [59,60]. ...
Two-color (2C) pyrometry has long been used for flame temperature and soot concentration studies and is now becoming more widely used to measure surface temperatures of burning materials. With the obvious advantage of being a contact-free method that requires only minimal optical access, 2C pyrometry combined with high-speed acquisition is a promising diagnostic tool to obtain exceptional temporal and spatial resolution of thermally degrading samples. However, its conceptual simplicity relies on a set of basic assumptions that when violated can result in large errors. In this work, we use an experimental configuration representative for fire resistance testing for aerospace and naval applications to analyze the impact of camera parameters and test setup on the accuracy of the surface temperature results obtained. Two types of fibre reinforced polymer composites and a steel plate are used to investigate material specific aspects that effect the measurements. An improved workflow for camera calibration is presented that takes the actual experimental setup into account. The temperature and emissivity mapping obtained trough in-situ IR measurements is compared against data acquired trough thermocouples and post-fire hemispherical directional reflectance measurements at room temperature. This comparison illustrates the necessity for proper post-processing and demonstrates that emissivity values obtained from pristine or burnt samples are not well suited to obtain accurate surface temperatures through conventional (single color) IR thermography. We also present a detailed error budget and suggestions for calibration measurements to keep the overall error well below 50 K in a temperature range from 673 K - 1473 K.
... In the heat transfer calculations developed above, we assumed the following values of emissivity: HASTELLOY, ε = 0.2; 42TE ceramics, ε = 0.9; PROMAFORM ε = 0.9; quartz ε = 0.55; AISI 304, ε = 0.6. For further details, the reader is forwarded to [38][39][40][41]. By setting equal all the emissivities to a unit value, as the surfaces were ideal black bodies unable of reflecting any radiation, by the network of resistance one would obtain a temperature at the heater lower of about one hundred degrees, whilst the outer wall temperature would remain unchanged. ...
In this study, we analyzed the problem of a compact furnace, to be used for in situ experiments in a cone-beam X-ray microtomography commercial system. The design process was accomplished and outlined through its main steps, until the realization of a prototype. The furnace was conceived to carry out wettability experiments at temperatures up to 700 °C and under inert atmosphere on sessile droplets of a molten metal alloy, with a few millimeters diameter, posed on a thin ceramic substrate. X-ray imaging of the molten droplet is expected to permit an accurate three-dimensional reconstruction of the droplet profile and a robust estimation of the related quantities (such as the contact angle and the surface tension) utilized for the assessment of metal-ceramic joints by brazing. The challenges faced during this project, mostly related to the constraints of the setup, and the novel solutions implemented were discussed also with the support of analytical and numerical tools, in terms of interaction of X-rays with matter, geometry and working principle, heat transfer and insulation, material selection.
... Viscosity laws used in the sensitivity analysis have been shown in Fig.9(a). The reference value for emissivity is assumed to be 0.3 but literature data shows emissivity for Quartz in the range between [0.1 − 0.5] see refs [9,16,31]. Thermal conduction and density will influence the diffusivity in the material and therefore can impact the liquid layer thickness. Reference value for density are provided by the sample manufacturer and a 10% variation is considered. ...
View Video Presentation: https://doi.org/10.2514/6.2021-3137.vid We study the ablation and transient thermal response of a quartz sample in air plasma.The experiment is carried out in the Plasmatron facility of the von Karman Institute for Fluid Dynamics. The aero-thermodynamic environment reproducing the atmospheric entry in the boundary layer of a test object is selected with cold-wall heat flux around 2.6 MW/m2 and test chamber pressure of 100 mbar. These conditions leads to surface temperatures higher than 2500 K and overall recession of 2 mm. We propose a numerical comparison based on a traditional transfer coefficient model with thermochemical ablation tables and a sensitivity analysis evaluating material properties and flow conditions influence. The proposed work aims at providing experimental data and improving ablation models to predict the atmospheric entry of space debris silicate materials.
... The emissivity (εw) was 0.6, a value that corresponds to that of 3-mm thick fused quartz at 800-900 K [41]. The Morsi-Alexander drag model [42] for a spherical particle was implemented. ...
This study provides evidence that a helium-neon (He-Ne) laser operating in the Mid-infrared (MIR) at a wavelength of 3.39 μm can detect variations in 1-hexene concentration in the presence of a solid catalyst. The in-situ and online characterization of the concentration of 1-hexene, as an example of a hydrocarbon, is relevant to enhance the current understanding of the interaction between hydrodynamics and chemistry in different heterogeneous catalytic processes. We designed and built a laboratory-scale downer unit that enabled us to analyze heterogeneous catalytic reactions and provided optical access. The lab-scale reactor was 180-cm long, had an internal diameter of 1.3 cm, and was made of fused quartz to allow the passage of the laser beam. 1-hexene was carefully measured, vaporized, and fed into the reactor through two inlets located at an angle of 45 degrees from the vertical descendent flow and 70 cm below the input of a solid catalyst and a purge flow entraining N2. A system of five heaters, which can be moved in the vertical direction to allow the passage of the laser beam, guaranteed temperatures up to 823 K. Computational Fluid Dynamics (CFD) simulations of the hydrodynamics of the system indicated that a uniform temperature profile in the reaction section was reached after the catalyst and the feed mixed. The estimated catalyst to oil ratio and time on stream in the experiments were, respectively, 0.4 to 1.3 and 2 s. After a correction for laser power drift, the experimental results showed a linear response of the fractional transmission to the 1-hexene concentration that was independent of temperature in the 373 K–673 K range. Even in the presence of a catalyst, the absorption of 1-hexene at the MIR frequency of the laser was high enough to enable the detection of 1-hexene since the fractional absorption of the absorbing path length in these experiments was close to zero (0.013 m) and the 1-hexene concentrations were higher than 1.254 × 10-5 mol/cm3. This result demonstrated the ability of the laser system to measure the concentration of 1-hexene in the presence of a catalyst and indicates that it can be used to better decouple hydrodynamics from kinetics in heterogeneous catalytic processes.
The in-flight reduction of iron ore particles using an atmospheric pressure hydrogen plasma is investigated. Iron ore particles with a size less than 75 µm are aerosolized and carried with an argon-hydrogen (90%–10%) gas mixture through an atmospheric pressure microwave plasma. After the treatment, the collected particles are observed to follow three distinct populations: (i) fully reduced nanoparticles, (ii) partially reduced spheres, larger than the feedstock, and (iii) partially melted, partly reduced agglomerates. A model is developed to explain the possible mechanism for the origin of the three populations. The nanoparticles (i) are found to be likely formed from the previously evaporated material whereas the particles (ii) and (iii) result from the partial/complete melting of the particles and agglomerates flowing through the reactor. The gas temperature is estimated to be more than 2000 K, which enables the rapid melting, evaporation, and reduction of these particles within residence times of only a few 10 ms.
Methane pyrolysis is an attractive technology to reduce greenhouse gas emissions, since the carbonaceous part of the hydrocarbon is captured into a solid, easier to store than carbon dioxide. In this framework, computational fluid dynamics (CFD) is an excellent tool to simulate the catalyst deactivation and the complex phenomena arising on the carbon surface. We investigated thermal and catalytical pyrolysis of methane in a tubular quartz reactor with an internal diameter of 3.8 cm. We modeled soot deposition and added a new surface mass balance that parametrize bed porosity as time-dependent variable. Our model predicts occlusion (clogging) time for empty and packed bed reactor: operating at 1373 K, an empty bed clogs in 15 d, which is coherent with industrial operations. The model well predicts methane conversion in the empty reactor even after 24 h time on stream. When a packed bed is used, the experimental conversion follow the predicted activity decay, with deviations due to the size distribution of the carbon during experiments. Radiation is the main heat transfer mechanism (86 % of the total heat absorbed by the system), compared to the others involved in this system, i.e., conduction, which is more than 50 times smaller, natural convection, more than 18 times smaller, and forced convection, more than 300 times smaller.
Two-color (2C) pyrometry has long been used for flame temperature and soot concentration studies and is now becoming more widely used to measure backface temperatures of burning materials. With the obvious advantage of being a contact-free method that requires only minimal optical access, 2C pyrometry combined with high-speed acquisition is a promising diagnostic tool to obtain exceptional temporal and spatial resolution of thermally degrading samples. However, its conceptual simplicity relies on a set of basic assumptions that when violated can result in large errors. In this work, we used an experimental configuration representative for fire resistance testing for aerospace and naval applications to analyze the impact of camera parameters and test setup on the accuracy of the surface temperature results obtained. Two types of fiber reinforced polymer composites and a steel plate are used to investigate material specific aspects that affect the measurements. An improved workflow for camera calibration is presented that takes into account the actual experimental setup. The temperature and emissivity mapping obtained through in-situ IR measurements is compared to data acquired through thermocouples and post-fire hemispherical directional reflectance measurements at room temperature. Excellent agreement is found for post-processed 2C pyrometry measurements and TC probe measurements with an uncertainty of 30 °C at 500 °C. In-situ emissivity mappings of burnt specimens are consistent with data from the literature for degraded composite samples and oxidized steel plates. Our results remind the importance of proper post-processing and demonstrate the potentials of 2C pyrometry for fire testing applications. We also present a detailed error budget and suggestions for calibration measurements to improve measurement accuracy.
Gas-phase solar receivers represent one of the best near-term solutions for providing temperatures in excess of 1000 K. However, as the desired outlet temperature increases, heat losses also inexorably increase. To mitigate this trade-off, this research systematically analyses a semi-transparent packed-bed solar receiver which can enable absorption to occur within the receiver's volume rather than on the surface (where thermal emission losses occur). As such, this study aims to find the limits-and the key limiting factors-for the overall performance of this new class of the receiver. This was done by exploring a wide range of geometrical (e.g., ~0.01 to ~ 1.45 spheres per cm3 or D/dp ranging from 3 to 15) and operational parameters. This parametric study was conducted by coupling a comprehensive heat transfer model with ray tracing and a surface-to-surface radiation model. For a 1000 K outlet temperature, an effective maximum receiver efficiency of just over 70% was achieved with 18 rows of transparent spheres (i.e., D/dp = 9 or ~ 0.3 spheres per cm³). This analysis also revealed a critical dimensionless parameter, Ω, which must be kept near a value of 1 to maximise the effective efficiency. This proposed dimensionless parameter was derived from the ratio of conduction to convective heat transfer multiplied by the ratio of absorbed solar power to input pumping power. Overall, this study's results indicate that semi-transparent packed beds represent a promising near-term design for cost-effective, high-temperature solar receivers, which can be used in advanced power cycles.
Atmospheric pressure microwave plasma reforming was conducted on a methane (CH4) and carbon dioxide (CO2) mixture, and characterized by measuring the temperature of the plasma and gas composition of the reforming product via optical emission spectroscopy (OES) and gas chromatographic measurement. The temperature at the plasma reached as high as 5900 K regardless of the specific energy input, while at microwave power of 2 kW and flow rate of 10 slm, nearly all CH4 and CO2 were converted into hydrogen (H2) and carbon monoxide (CO). The plasma temperature higher than the level achievable from uniform gas heating implied that only a part of the flow will enter the plasma region, and the rest will bypass and mix with the plasma stream downstream. Thus, a reactor network-type simulation was performed by modeling the plasma and surrounding streams, each as a series of perfectly-stirred reactors; the reactors at the same downstream locations interacted for the gas diffusion and heat conduction. The simulation reproduced the measured gas compositions well, revealing that the reforming proceeded as the surrounding gas enters and diffuses out of the plasma stream due to the flow mixing, as well as through the heating of the surrounding stream by the plasma stream.
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