Particle Emissivity Research Articles

Determination of ultrafine particle number emission factors from building materials in standardized conditions.

When comparing the particle emissivity for different materials and/or mechanical activities, a serious methodological issue emerges due to the dynamic nature of solid aerosols. Particle size distribution and concentration depend on initial particle emission that constantly evolves due to aerodynamic collisions. In this context, we propose a methodological approach and an experimental setup that enables to assess the release of fine/ultra-fine particles maintaining a steady-state inhalable mass concentration, here chosen at the Swiss occupational exposure level value for biopersistent granular particles (OEL: 10mg/m3) in a controlled ventilation chamber. As a case study, this methodological protocol was tested in the occupational exposure scenario in which a series of insulating materials based on silica aerogel and conventional mortar and concrete were subjected to handling or sawing. Once the OEL was reached, the particle size distribution and morphology of the aerosols were characterized using direct reading instruments (scanning mobility sizer, aerosol photometer) and electron microscopy (SEM and TEM) analyses. As a main result, the presence of silica aerogel in the mortar did not modify the emission profile for submicronic particles during sawing in comparison to the bulk mortar. Emission factors for ultra-fine particles were found to be 88×106 and 81×106 particles/µg of inhalable dust for the aerogel mortar and bulk mortar, respectively. For concrete sawing, the number concentration of submicronic particles at the OEL is one order of magnitude greater. The aerogel-glass-wool handling generated similar particle number concentration at the OEL with ultra-fine particle emission factors of 647×106 particles/µg of inhalable dust, in comparison to 758×106 particles/µg of inhalable dust during dry concrete sawing. In conclusion, the methodology introduced in this work provides standardized particle emission factors for comparing materials and activities, while establishing a link between particle number emissions and occupational exposure limits.

Characterization of Surface α-Particle Radiation, Internal Traceability and Simulation of Typical Tin Spheres

Surface α-particle emissivity testing and spectral characterization of two leaded tin spheres (Sn10%Pb90%, Sn63%Pb37%) and one lead-free tin sphere (Sn96.5%Ag3.0%Cu0.5%, SAC305) were carried out. The results show that Sn10%Pb90% Sn spheres have the highest α-particle emissivity; Sn63%Pb37% Sn spheres are the next highest, which is an order of magnitude lower than the α-particle emissivity of Sn10%Pb90% Sn spheres; and SAC305 Sn spheres have the lowest emissivity, which is reduced by about 55.6% compared to the emissivity of Sn63%Pb37% Sn spheres. All three types of tin spheres, after purification treatment, achieved the grade of ultra-low alpha particle emissivity (<0.002 α/(cm2·h)). The internal radionuclide traceability of the tin sphere, combined with the energy spectrum, reveals that the emission spectrum of the tin sphere exhibits an obvious “single peak” characteristic, with the peak energy in the interval of 5 MeV~5.5 MeV. Comparative analyses revealed that 210Po is the main nuclide that produces alpha particles, and 210Po originates from the decay of 210Pb. Further Monte Carlo simulations show that α-particles with energies greater than 4.1 MeV in the measured energy spectrum all come from the contribution of radionuclides within 5 μm of the surface layer of the tin sphere, which accounts for 60% of the total radioactivity. Combining the experimental and simulation results, it is found that the internal radionuclides of the tin sphere are characterized by more surface layer and less internal layer. The above results are of great significance for the establishment of α-particle mitigation methods for tin spheres.

Remote measurement and calibration of alpha-emitting radionuclides through the utilization of characteristic ultraviolet fluorescence

During the decommissioning of nuclear facilities and spent fuel reprocessing, conventional alpha particle detectors are insufficient for accurately measuring the emissivity of alpha particles due to their high radiation intensity and complex nuclide types. The air fluorescence method is developed to detect the α particle emissivity through measuring the intensity of ultraviolet fluorescence generated by the interaction between α particles and nitrogen or other inert gas mediums. This method offers advantages such as long detection range and immunity to γ radiation interference. The measurement results are primarily influenced by factors including α particle energy, detection distance, and gas medium composition, with photon noise and detection distance determining the detection limit. The obtained results indicate that: In the condition of measured in darkness, measurement time for 1000s, and distance for 1 m, The minimum detection limit for α sources is 7.6 kBq with a relative standard uncertainty of 10%.

Study on radiation characteristics of coal fired boiler changing with burnout rate

The burnout rate is an important parameter in the combustion of pulverized coal. During the process of pulverized coal burnout, its radiation characteristics and the entire coal-fired boiler vary with the burnout rate, which is a very complex process and current research is not clear enough. This study proposes a new single particle pulverized coal burnout model, which incorporates pore volume correction to fit the density ratio and diameter ratio of multiple coal types with varying burnout rates as the theoretical basis. Using Mie theory, Jiaozuo coal particles are calculated average absorption and scattering efficiency in the band of 0.39 ∼ 16 μm, which is used as the basis for correcting particle emissivity and scattering factor. Then, through user defined function (UDF) in the simulation software, the particle radiation characteristics are modified from the original constants of 0.9 and 0.6 to linear and polynomial models that vary with the burnout rate. Not only do the spatial distribution rules of the coal-fired boilers radiation characteristics with the burnout rate of the furnace be obtained, but also the temperature distribution of the furnace under different particle radiation characteristics models is compared. The results show that the pulverized coal burnout model is consistent with the experimental results in literatures. High burnout rate corresponds to low mass concentration of furnace particles, and low absorption and scattering coefficients. Using the above model to calculate the particle emissivity and scattering factor that vary with the burnout rate, after UDF embedding, the error between the simulation results and the on-site cross-sectional temperature and thermal imaging detection decreases from 4.81 % to 2.59 %, reflecting the high accuracy and rationality of the burnout model and particle radiation characteristic correction method.

Improved oxidation resistance and infrared emissivity of tantalum disilicide particles with sol-gel derived SiO2 coatings

As an important high-temperature structural material, tantalum disilicide (TaSi2) suffers from performance degradation associated with high temperature oxidation due to the poor oxygen diffusion barring effect of the mixed Ta2O5/SiO2 oxidation scale. In this work, silica coating was prepared on the surface of TaSi2 powder by a sol-gel method. The oxidation resistance was studied by thermogravimetric tests in non-isothermal oxidation and isothermal oxidation, as well as by oxidation treatment in a tube furnace at temperatures ranging from 800 °C to 1400 °C. The spectral emissivity of raw TaSi2, coated TaSi2 and the oxidized ones was compared and analyzed. The amorphous SiO2 coating is composed of numerous nanoparticles and covers most of the surface of TaSi2 particles. The oxidation weight gain of coated TaSi2 during heating from RT to 1200 °C and 1400 °C decreases from 29.8% and 30.9–3.0% and 8.6%, respectively. And the oxidation is controlled by diffusion in the overall process. The coated TaSi2 combines the advantageous radiation property of TaSi2 core and SiO2 coating. Compared with that of raw TaSi2, the emissivity of coated TaSi2 at the wavelength range of 200–2500 nm and 2.5–25 µm has increased by 7.5% and 13.2%, respectively, and the property did not deteriorate after oxidation at 1200 °C, suggesting that the coated TaSi2 is an excellent emitting agent during the entire service life.

Spectral emissivity measurement of free-falling particle groups

A solar energy receiver is a device that absorbs solar radiation and transfers the generated heat energy to a heat transfer medium. As a heat transfer medium in solar energy receivers, solid particles can safely absorb strong solar radiation flux and achieve a higher operating temperature. Hence, they have attracted considerable research interest. The emissivity of particles is an important parameter in numerical simulation and efficiency analysis of solid particle receivers. However, it is rarely reported, especially for dispersed particles flowing in receivers. In this study, a measurement system for the normal spectral emissivity of free-falling particles is designed and developed. The system is based on the energy method and uses an FTIR spectrometer as a radiation measurement device, which can achieve measurement in the wavelength range of 2.5–16 µm below 800 °C. The emissivity of the sheet SiC material is measured to verify the reliability of the system. The results are in good agreement with the data published in NIST. Furthermore, the effects of the particle material, temperature, and particle group thickness on the particle group emissivity are studied. It is found that the emissivity of gray ceramic sand particles is not sensitive to the temperature, whereas that of alumina ceramic particles is positively correlated with the temperature. The emissivity of the two particles increases rapidly with the release width of the particle group, which is related to the increase in the mass flow rate of the particle group. Finally, uncertainty analysis of the measurement system is performed. The results measured in this paper can provide the original emissivity data for heat transfer simulation and efficiency evaluation in the receiver.

Universal Formulas for Calculating Emissivity and Integral Radiation Flux Densities of Black Bodies and Subwavelength Particles

Universal Formulas for Calculating Emissivity and Integral Radiation Flux Densities of Black Bodies and Subwavelength Particles

Study on the combustion behavior of single coal particle using a thermal-imaging technique

This study proposes a novel thermal-imaging detection technique that combines thermogravimetric analysis (TGA) with hyperspectral imaging to investigate the combustion behavior of single coal particles. The proposed technique enables the combustion reaction kinetics to be correlated with the observation of combustion regimes, as well as the combustion temperature and radiative parameters. The combustion behavior of two kinds of single coal particles, named Yinni (YN) and Datong (DT), is detected by the thermal-imaging technique. The results show that the single coal particles undergo a brief release of moisture/volatile, simultaneous combustion of volatile and char, and a relative long-term char oxidation during combustion. The combustion temperatures of the two single coal particles are approximately in the range of 750–1250 K, while the band-averaged emissivity values vary between 0.05 and 0.85. Furthermore, the simultaneous reaction rate, average temperature, and average emissivity of the two single coal particles during combustion are also obtained. It is found that the single coal particles follow a first-order reaction in the moisture/volatile release and char oxidation stages, and a complex reaction kinetics in the simultaneous combustion stage. The average temperature of the single coal particles is maintained at about 1000 K during the moisture/volatile release stage, but gradually increases during the simultaneous combustion and char oxidation stages. The average emissivity of the single coal particles keeps increasing in the release of moisture/volatile and simultaneous combustion stages, whereas remains basically unchanged in the long-term and slow char oxidation stage.

Universal formulas for calculating emissivity and integral radiation flux densities of black bodies and subwavelength particles

Universal formulas for calculating emissivity and integral radiation flux densities of black bodies and subwavelength particles

Flame Temperature Measurement Based on Laser-Induced Breakdown Spectroscopy and Element Doping.

In this study, based on the existing high-temperature measurement and calibration equipment, calibration experiments using the spectral emissivity of intrinsic element particles in the field were designed to achieve the accurate measurement of a temperature field. Laser-induced breakdown spectroscopy was used to select the corresponding elements, and the element doping method was used to approximate the real temperature field. After calibrating the camera, the temperature distribution and spectral emissivity distribution of the flame were calculated. The range of calculated values was determined to be well-consistent with data collected using an infrared thermal imager, which verified the accuracy of the experiment.

Effect of Laser Irradiation on Emissivity of Flame-Generated Nanooxides

The application of pyrometry to retrieve particle temperature in particulate-generating flames strictly requires the knowledge of the spectral behavior of emissivity of light-emitting particles. Normally, this spectral behavior is considered time-independent. The current paper challenges this assumption and explains why the emissivity of oxide nanoparticles formed in flame can change with time. The suggested phenomenon is related to transitions of electrons between the valence and conduction energy bands in oxides that are wide-gap dielectrics. The emissivity change is particularly crucial for the interpretation of fast processes occurring during laser-induced experiments. In the present work, we compare the response of titania particles produced by a flame spray to the laser irradiation at two different excitation wavelengths. The difference in the temporal behavior of the corresponding light emission intensities is attributed to the different mechanisms of electron excitation during the laser pulse. Interband transitions that are possible only in the case of the laser photon energy exceeding the titania energy gap led to the increase of the electron density in the conduction band. Relaxation of those electrons back to the valence band is the origin of the observed emissivity drop after the UV laser irradiation.

Thermal radiation ofsubwavelengthparticles

A method is proposed for calculating the dependences of the emissivity of subwavelength parti-cles (SP) from various materials in the form of disks, spheres, cubes and cylinders on their sizes (D) and temperature (T), for cases when external electromagnetic radiation practically does not affect their temperature. For all the listed types of particles, the cutoff wavelengths λcutoffdepending on the size of the SP and the particle shape coefficients ξare determined. With a decrease in the particle size, from the radiation spectrum, which was originally described by Planck's law, wavelengths exceeding λcutoffare gradually excluded. This leads to a decrease in the integral radiation, a decrease in the emissivity and a shift of the radiation spectrum to the blue region. A simple scheme is also proposed for determining ε -the emission coefficients of the midrange according to the calculated graph ε (U), where: U = (ξ × D × T) / B; B is the con-stant of the Wien displacement formula.

Radiative heat transfer in the discrete element method using distance based approximations

A model to estimate radiative heat transfer in particle beds is developed for use in the Discrete Element Method (DEM). Monte Carlo ray tracing simulations are run to find the Radiation Distribution Factor (RDF) between pairs of particles and between particles and a wall, in particle beds with random packing. Curves are found to express the average RDF as a function of distance, and within DEM these curves are used to estimate particle-particle and particle-wall radiative transfer. The resulting Distance Based Approximation model is computationally efficient and simple to implement. RDF-distance curves are given as a set of tables covering two particle emissivities (0.65, 0.86), four wall emissivities (0.4, 0.6, 0.8, 1.0), and five solid fractions (0.25, 0.35, 0.45, 0.55, 0.64). The accuracy of the model is investigated, with accuracy sufficient for many engineering applications shown. An initial implementation is demonstrated for a heat exchanger with a dense granular flow.

Interpretable Forward and Inverse Design of Particle Spectral Emissivity Using Common Machine-Learning Models

Author(s): Elzouka, M; Yang, C; Albert, A; Prasher, RS; Lubner, SD | Abstract: Radiative particles are ubiquitous in nature and in various technologies. Calculating radiative properties from known geometry and designs can be computationally expensive, and trying to invert the problem to come up with designs specific to desired radiative properties is even more challenging. Here, we report a machine-learning (ML)-based method for both the forward and inverse problem for dielectric and metallic particles. Our decision-tree-based model is able to provide explicit design rules for inverse problems. Furthermore, we can use the same trained model for both the forward and the inverse problem, which greatly simplifies the computation. Our methodology shows the promise of augmenting optical design optimizations by providing interpretable and actionable design rules for rapidly finding approximate solutions for the inverse design problem. Inverse design is usually done by expensive, slow, iterative optimization. Elzouka et al. show how a simple machine-learning model (decision trees) can efficiently perform inverse design in one shot, while recovering design rules understandable by humans. Inverse design of the spectral emissivity of particles is used as an example.

Investigation of Pulverized Biomass and Coal Char Emissivity

Current work presents an optical setup, its calibration and reference process and the first results from single particle emissivity measurements of pulverized biomass and coal fuel particles. In contrast to earlier attempts, the setup offers the possibility of emissivity measurements during the whole particle burn-off. A laser ignites a single particle, placed in the center of the setup. Two photomultipliers observe the emitted particle radiation in the visible range (550 nm and 700 nm) for temperature calculation, using two-color pyrometry. An InSb-detector records the emitted particle radiation between 2.4 µm and 5.5 µm, which is later used to calculate particle emissivity in this range. The conclusion of multiple particle measurements lead to decreasing particle emissivity with increasing temperature. For coal particles the emissivity decreases from 0.45 at 2300 K to 0.03 at 3400 K. Biomass char shows a similar trend with a decrease from 0.18 (2100 K) to 0.03 (2900 K).

Emissivity and absorption function measurements of Al2O3 and SiC particles at elevated temperature for the utilization in concentrated solar receivers

Solar thermal receivers collect and can store concentrated solar radiation using solid particles. Solid ceramic particles have shown to be a practical and efficient heat transfer media in solar-particle receivers, however, their emissivity and absorptivity at high temperatures are scarcely reported. This gap has led to large uncertainties in the assessment of solar thermal receivers’ efficiency. In this work, an experimental method was developed to measure the emissivity and absorption function of solar particles at elevated temperatures up to 1200 K. Two types of solar particles, aluminum oxide (Al2O3, ∼95% purity) and silica carbide (SiC, ∼99% purity), were studied, particularly aiming to understand the dependence of emissivity and absorption function on temperature. Using a heat transfer model, the emissivity of particles was evaluated based on the fitting of the cooling rate, while the particle absorption function was obtained by fitting of the heating rate, following a well-controlled heating radiation at 910 nm. It was found that the emissivity values of the two particles are independent of temperature, showing constant values of 0.75 ± 0.015 and 0.92 ± 0.012 for Al2O3 and SiC respectively, in the temperature from 300 to 1200 K. The absorption function was found to be increased nonlinearly with temperature for Al2O3, while that of SiC dropped slightly. These absorption functions are specified for 910 nm. Using the evaluated experimental values of emissivity and absorption function, the maximum temperature and the temperature rise time of micro-sized particles (hundreds of micrometers) under different radiation fluxes were simulated taking into account the effect of particle diameter.

Influence of the Mineral Content on the Emissivity of Hydrothermal Char

AbstractThe influence of minerals on solid fuel conversion is an important topic and has so far been addressed focusing mainly on the catalytic effects. Here, the impact of iron oxide on the emissivity of burning char particles was investigated. Synthetic, hydrothermally carbonized char was doped with SiO2 or with a mixture of SiO2 and Fe2O3. The sample was then exposed to an oxy‐fuel atmosphere in a laboratory reactor. An established pyrometer/infrared (IR) spectrometer setup was used to measure the size, temperature, and IR radiation of single burning particles. Based on these measurements, the spectral and total emissivity in the near‐IR (1.25–2.25 μm) and the total emissivity in the mid‐IR region (2.4–5.5 μm) were calculated. The results show that the iron content directly influences the emissivity of char during combustion.

Numerical investigation on the impact of variable particle radiation properties on the heat transfer in high ash pulverized coal boiler through co-simulation

Co-simulation of the high-temperature heat and mass transfer processes in coal-fired boilers using CFD together with process-level models of the water/steam circuit is a promising approach to investigate off-design conditions. In the present work, a 1D discretized two-phase model of the water flows in the evaporator and radiative superheaters was developed. This was coupled with a detailed furnace combustion and heat transfer CFD model. The coupled models are used to investigate the impact of variable particle emissivity and scattering efficiency on the evaporator and radiant superheater process conditions, including the furnace steam generation rate and final steam temperature. The results are compared with the case where the radiation properties of the particles are assumed to be constant, thereby highlighting the importance of accounting for the variations in the particle properties throughout the process. The case study boiler is that of a real 620 MWe subcritical power plant firing coal with a high ash content (41 %wt). The results show that the constant property model results in a predicted furnace heat load that is 6.5% lower than the experimental value, while the conversion-dependent model is only 0.7% lower than the experimental value.

Analysis of Clumped-Pebble Shape on Thermal Radiation and Conduction in Nuclear Beds by Subcell Radiation Model

Abstract The core of high-temperature gas-cooled reactor is a dense pebble bed of random packing filled with monosized fuel spheres. Subcell radiation model (SCM) is a generic analytical approach to calculate effective thermal conductivity (ETC) of thermal radiation. For the packed bed of monosized spheres operated in various conditions, it is proven that the SCM is still applicable in the particle size ranges of 1.2–60 mm and temperature ranges of 0–1200 °C. Based on the SCM, radiation-to-conduction ratio ξ is presented and radiation becomes an essential part at ξ>0.1 for the accurate evaluation. For the beds of nonoverlapping clumped-sphere particles, the model combining with discrete element method (DEM) and SCM is presented to study the heat transfer behaviors, including effects of particle shape, emissivity distribution and pebble flow with transient heat transfer. For the experimental nuclear pebble beds, the results of SCM are in good agreement with the empirical correlation and accord well with the experimental data under high temperature range.

Method for calculation of radiative heat transfer in beds of spherical particles

A new technique for implementing external (particle-to-wall) and particle-to-particle radiative heat transfer in discrete elements method (DEM) simulations is proposed. It is based on the idea that an expected view factor value depends on relevant local bed parameters (distance between particles, particle radius ratio, and local bed porosity). Calculation of average view factors via the formula requires considerably less computational effort than direct in situ integration, when this happens a reasonable average value and an overall accuracy comparable to direct calculation are provided. Both mono- and polydisperse mixtures of spherical opaque particles were considered. It was shown that using nondimensional parameters, a simple general dependence for an external radiative heat flux may be introduced. Exponential and linear fits were proposed for estimating the particle-particle radiative heat flux. The generalization of the obtained formulas for various bed porosities is proposed. The distribution of cumulative transferred heat flux across the particles up to a certain distance was found, and the recommendations regarding the choice of that parameter to achieve a desired accuracy were formulated. Also, the method to account for the particle emissivity was proposed on the basis of the empirical dependence between emissivity and radiative heat flux in porous materials. The proposed method satisfies all the requirements to become a standard implementation of radiative heat transfer calculation in DEM.