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The Monte Carlo method (MCM) can be used to calculate the radiative heat flux and divergence with high accuracy, and the Distribution of Ratios of Energy Scattered by the medium Or Reflected by the boundary surface (DRESOR) method based on the MCM provides the radiation intensity with high directional resolution. However, the accuracy and efficiency of the DRESOR method for calculating the radiative heat flux and divergence have not been verified. Therefore, in this article, the DRESOR method is reformed to make it more suitable for calculating the radiative heat flux and divergence, and the DRESOR method and MCM are compared quantitatively. In the comparison, the influences of the Mean Optical Thickness per Element (MOTE), the surface and medium parameters, anisotropic scattering, and a nonuniform medium are considered. The results show that the figure of merit (FoM) of the reformed DRESOR method is 81.18 and 6.04 times that of the MCM for the surface element and space element, respectively when the MOTE is 0.17 for calculating the radiative heat flux and divergence. The larger the surface emissivity, and the smaller the scattering albedo, the better the performance of the DRESOR method than the MCM is. Meanwhile, the MOTE (mesh density), anisotropic scattering, and the nonuniform medium have similar influences on the DRESOR method and the MCM. [ABSTRACT FROM AUTHOR]

Highlights • The caculation error is evaluated in an isothermal and equilibrium state. • The Mean Optical Thickness per Element(MOTE) plays a key role. • MOTE less than 0.1 ensures a minimum error for surface elements. • Scaling laws correlate the effect of number of energy bundles(NEB). • Minimum NEBs are given for a desirable error level of 1.0%. Abstract Generally, there is no common method for directly and quantitatively evaluating computational errors in the calculation of radiative transfer. A method for quantitatively evaluating the accuracy in calculation of radiative transfer using the Monte Carlo method (MCM) is proposed, which is based on setting a radiative transfer enclosure in an isothermal and radiative equilibrium state, i.e. a state that equates exactly to a blackbody. For a gray emitting, absorbing, isotropically scattering, cubic enclosure bounded with diffusely emitting and reflecting surfaces, the computational accuracy and efficiency of the MCM are examined under fixed, constant surface emissivity of 0.5 and medium scattering albedo of 0.5. The concept of Mean Optical Thickness per Element (MOTE), that is the product of the absorption coefficient and the average length of three coordinate directions of the discrete space elements, is proposed to serve as a key scaling parameter to evaluate the calculation errors. When the MOTE is less than around 0.1, the errors for the surface elements reach a minimum level, even if the mesh density continues to increase. Scaling laws have been set up to relate the minimum errors for both the surface elements and space elements with the variation in the number of energy bundles (NEB). A desirable calculation error level is set as 1.0% for radiative heat transfer with an acceptable cost, for which the minimum NEBs are 3000 and 750 for the surface and space elements, respectively. For the surface elements, a scaling law relating the NEB and the maximum MOTE has been obtained that will enable the desirable error not to be exceeded; for the space elements, achieving the desirable error is dominated by the NEB, not the MOTE. It is stated that the cases studied in the paper have many limitations, and additional research should be done in the future to address the real cases of interest. [ABSTRACT FROM AUTHOR]

Abstract: Radiative intensity with high directional resolution at boundaries can provide abundant information about the medium properties and temperature, which is very useful for inverse analysis. Monte Carlo method is a widely used method due to its flexibility for complex radiative transfer problems. In this work Distributions of Ratios of Energy Scattered Or Reflected (DRESOR) method, which is based on Monte Carlo method, is developed for radiative intensity calculation in a three-dimensional rectangular enclosure which contains an emitting, absorbing, and isotropically scattering medium. The method is validated by comparing calculated radiative intensities with those of the reverse Monte Carlo method. Then, radiative intensity for different radiative problems is examined by DRESOR method. Comparing with reverse Monte Carlo method, DRESOR method shows much better time-efficiency. DRESOR method also shows an obvious advantage if radiative intensity with high directional resolution at many different boundary positions is required. To calculate radiative intensity with high directional resolution in a system where radiative source is confined to a small volume, both standard and reverse Monte Carlo method become inefficient, the results show that DRESOR method is still an efficient tool. The results given in this study demonstrate that DRESOR method is very advantageous in situations when high directional resolution is required. [Copyright &y& Elsevier]

The distribution of ratios of energy scattered by the medium or reflected by the boundaiy surface (DRESOR) method can provide radiative intensity with high directional resolution, but also suffers the common drawbacks of the Monte Carlo method (MCM), i.e., it is time-consuming and produces unavoidable statistical errors. In order to overcome the draw-backs of the MCM, the so-called equation-solving DRESOR (ES-DRESOR) method, an equation-solving method to calculate the DRESOR values differently from the MCM used before, was proposed previously. In this method, a unit blackbody emission is supposed within a small zone around a specified point, while there is no emission elsewhere in a plane-parallel, emitting, absorbing, and isotropically scattering medium with transparent boundaries. The set of equations for the DRESOR values based on two expressions for the incident radiation was set up and solved successfully. In this paper, the ES-DRESOR method is extended to a one-dimensional system with diffusely reflecting boundaries. The principle and formulas are given. Several examples with different parameters are taken to examine the peiformance of the proposed method. The results showed that all the DRESOR values obtained using the ES-DRESOR method agree well with those got using MCM. The average relative error for the intensity obtained by the ES-DRESOR method is 9.446 x 10-6, lower by over I order of magnitude than the 2.638 x 10-4 obtained by the MCM under the same conditions. More importantly, the CPU time for computing the DRESOR values, which ranges from several hundred seconds to several thousand seconds using the MCM, is reduced to 0.167 s using the ES-DRESOR method. The computation time is shortened by about 3 orders of magnitude. The overall performance of the ES-DRESOR method is excellent. [ABSTRACT FROM AUTHOR]

An acoustic method can provide a noninvasive, efficient and full-field reconstruction of aerodynamic fields in a furnace. A simple yet reasonable model is devised for reconstruction of a velocity field in a cross section of a tangential furnace from acoustic measurements based on typical physical characteristics of the field. The solenoidal component of the velocity field is modeled by a curved surface, derived by rotating a curve of Gaussian distribution, determined by six characteristic parameters, while the nonrotational component is governed by a priori knowledge. Thus the inverse problem is translated into determination of the characteristic parameters using a set of acoustic projection data. First numerical experiments were undertaken to simulate the acoustic measurement, so as to preliminarily validate the effectiveness of the model. Based on this, physical experiments under different operating conditions were performed in a pilot-scale setup to provide a further test. Hot-wire anemometry and strip floating were applied to compare with acoustic measurements. The acoustic measurements provided satisfactory consistency with both of these approaches. Nevertheless, for a field with a relatively large magnitude of air velocities, the acoustic measurement can give more reliable reconstructions. Extension of the model to measurements of hot tangential furnaces is also discussed. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the ES-DRESOR method, an equation-solving method to calculate the DRESOR values differently from the MCM used before, is proposed. In this method, a unit blackbody emission is supposed within a cell, while there is no emission from the remaining part of the system. This leads to the equations for the DRESOR values based on the incident radiation, and all the DRESOR values can be solved directly from the equations without the rays tracing used in the MCM. Numerical results show an excellent agreement of all DRESOR values found by ES-DRESOR method and MCM for 100 cells in 1D case with emitting, absorbing, and isotropic scattering medium and transparent boundaries. The average relative error for the intensity obtained by the ES-DRESOR method is only 3.372×10−5, lower by one order of magnitude than 4.022×10−4 obtained by the MCM under the same conditions. The CPU time for computing the DRESOR values using the MCM varies from a few hundred seconds to several thousand seconds. In contrast, it is less than 1s using the ES-DRESOR method, showing an obvious improvement in the computational efficiency. [Copyright &y& Elsevier]

The inferior mirage from road surfaces is a common phenomenon, which can be easily seen in everyday life. It has been recognized in the literature as a light refraction phenomenon due to the refractive index gradient caused by the temperature gradient in the air strata above the road surfaces. However, it was also suggested that the mirage is just a phenomenon of specular reflection at grazing incidence. Because of the lack of reasonable and quantitative evidence, the generally accepted light refraction theory has not yet been refuted. Here we show some mirror-like reflection images captured from a road surface stretch in Yujiashan North Road, Wuhan, China, when there was no obvious temperature gradient on or above the road, measured on a winter day in December 2009. This provided direct evidence to doubt the temperature induced light refraction mechanism of the inferior mirage. Furthermore, the critical grazing angle of about 0.2° to the road plane where the mirror-like reflection appears could not make the rough surface scatter incident light as a smooth surface according to the Rayleigh criterion. Therefore the phenomenon is a mirror-like observation effect of scattering from the surface, which cannot be entirely explained by light refraction via air strata. The results demonstrate that the image-formation mechanism and the observer-based-analysis method shown here potentially offer a means of understanding a wide range of scattering phenomena from rough surfaces at grazing angle; for example, the superior mirages of unusual brightness occasionally observed over frozen lakes and the off-specular reflection phenomenon. [ABSTRACT FROM AUTHOR]

The Distribution of Ratios of Energy Scattered by the medium Or Reflected by the boundary surface (DRESOR) method based on the Monte Carlo method is an important new approach to solve the radiative transfer equation accurately and efficiently. This method has been applied extensively for the detection and analysis of the furnace flame temperature. However, the discrete method, the number of direct incident radiation directions (DIRDs), and the number of scattered or reflected energy beams (SREBs) lack quantitative references. Therefore, in this paper, the effects of different discrete methods, the number of DIRDs, and the number of SREBs on the radiative heat flux, divergence error, and computational efficiency of three-dimensional homogeneous absorbing, emitting, and scattering media are analyzed. The results show that the random discrete method of the DIRD for the space element substantially reduces the divergence error of the radiation heat flux. The uniform discrete method of the DIRD should be used for the surface element to minimize the error of the radiation heat flux error. In addition, the relative error of the DRESOR method is reduced to 0.004, and the figure of merit reaches the maximum when the number of DIRDs is about 1000, the number of SREB is 500, and the scattering albedo is less than 0.8. The fitted equations describing the relationship between the calculation errors and the number of SREBs for the surface and space elements are obtained for different scattering albedos. An increase in the number of DIRDs and SREBs does not significantly reduce the calculation error. The results provide a quantitative reference for determining the parameters of the DRESOR method in radiant heat analysis. [ABSTRACT FROM AUTHOR]

• An accurate and efficient formula for calculating scattering fraction was delivered. • A simplified tracking strategy of energy beam was proposed. The Distribution of Ratios of Energy Scattered by the medium Or Reflected by the boundary surface (DRESOR) method, one of the most effective methods to solve the radiation intensity problem with high directional resolution in radiation transfer analysis, suffers performance degradation as the medium scattering albedo becomes large. A scheme is put forward to subdivide the discrete energy beam step length, and take its infinitesimal limit to calculate the scattering/reflection fraction (DRESOR value) with high efficiency and precision. The preliminary simulation results show that when the scattering albedo is greater than 0.5, the optimized DRESOR method can greatly reduce the calculation time and improve the calculation accuracy. [ABSTRACT FROM AUTHOR]

• Advantage ranges of the surface and space parameters for pathlength and collision-based methods are obtained, respectively. • The pathlength method is more sensitive to non-uniformity of medium parameters than the collision-based method. • The pathlength method is more sensitive to uniformity of the scattering coefficient than to the absorption coefficient. • The performance index of the pathlength method is the best when the cutoff level of energy beam tracking is 10−7 and 10−6 for surface and space elements respectively. • The overall advantage of path-length method is greater than that of collision method. Monte Carlo ray tracing (MCRT) methods are important for solving the radiative transfer equation. However, the particular ray-tracing procedure used has a significant impact on the calculational performance. In this paper, we analyze and compare the performances of MCRT pathlength (PL) and collision-based (CB) methods for different surface and medium parameters and different degrees of uniformity. The results show that in a gray medium and with a radiation heat balance system, the PL method is superior when the surface emissivity is less than 0.15 for surface elements, and when the mean optical thickness per element (MOTE) is small and the emissivity is large for space elements; otherwise, the CB method is superior. Also the overall performance of the PL method is better than that of the CB method. However, PL method is more sensitive to non-uniformity of medium parameters than the CB method, and that the PL method is more sensitive to uniformity of the scattering coefficient than to the absorption coefficient. In addition, for a gray body and uniform optical parameters, reducing the number of grids improves the performance of the MCRT. And the performance index of the PL method is the best when the cutoff level of energy beam tracking is 10–7 and 10–6 for surface and space elements respectively. These quantitative conclusions will help in selecting suitable MCRT methods for engineering calculations, based on the physical properties of materials and conditions. [ABSTRACT FROM AUTHOR]

A novel channel-selection convolutional neural network (CS-CNN) is proposed to predict NOx emission from coal-fired boilers under steady-state and transient load conditions. First, a new channel-selection convolutional layer (CS-CL) is presented to replace regular convolutional layer (RCL). The CS-CL evaluates the channel importance of the input variables, selects the Top-C important channels and releases the hyperparameters of the remaining low-importance channels, thus contributing to maximize the utilization of the parameter resources of the model. The advantages of using CS-CLs are the preservation of the great majority of manipulated variables involved in combustion control among the input variables and the prevention of the model overfitting problem due to the redundancy of input variables. Second, a sliding window-based preprocessing method is applied to the historical data of the boiler which is divided into four-dimensional (4D) tensors. Then, comparative tests are performed on long short-term memory (LSTM) model, baseline CNN and CS-CNN using the historical data of a 670 MW boiler. The results of tests showed that CS-CNN has higher prediction performance. Finally, in order to increase the interpretability of the deep learning black box model, this study analyzes the working mechanism of the CS-CNN through ablation analysis and visualization of model parameters. • Channel importance assessment of input variables by channel selection convolutional layer. • Data-driven model for complete burner manipulation variables. • Novel convolutional neural network for predicting NOx emission from coal-fired boilers. • Model visualization analysis of convolutional neural network for industrial time series data. [ABSTRACT FROM AUTHOR]

Highlights • Full temperature fields are visualized by interferometry with simulated local compositions. • Effect of compositions on interferometry is studied on oxygen-enriched flames. • Larger discrepancies are found in oxygen-enriched flames with hot air assumption. Abstract The gas temperature fields of axisymmetric co-annular laminar diffusion ethylene flame jets in air and oxygen-enriched atmospheres were measured and visualized using large lateral shearing interferometry. The relationship between the refractive index and the temperature is obtained using a state equation and the Gladstone–Dale relation. Considering the unknown local gas composition can be a potential source of error, local gas compositions provided by corresponding numerical simulations were employed. The spatial resolution of the temperature fields was 0.1 mm/pixel. The temperature results obtained using hot air assumption and simulated local composition indicate that deviations of up to 400 K occur at the bottom part on the centerline, whereas at larger axial or radial positions, the deviations could be less than 50 K. The results are also validated by comparison with CARS. In addition, compared with flames under air atmosphere, significant deviations occurred in a greater region of oxygen-enriched flames. [ABSTRACT FROM AUTHOR]

Highlights • The LBL, SNB and SNBCK models were used to calculate the heat flux and source. • A one-dimension enclosure between two parallel plates was considered. • The results of HITRAN2016 database is more accurate than previous versions. • The hot lines would affect the accuracy of the results of HITRAN database. Abstract The statistical narrow band (SNB) and the statistical narrow band correlated-k (SNBCK) models were used with HITRAN2008, HITRAN2012, and HITRAN2016 databases, in order to demonstrate the effect that the different HITRAN database versions had on the accuracy of the SNB and SNBCK calculations. The line-by-line (LBL) calculation was based on the HITEMP data. Eight cases were calculated using the LBL, the SNB, and the SNBCK models in a one-dimensional enclosure between two parallel plates filled with CO 2 , H 2 O, or a mixture of both. The results demonstrated that the radiant flux and the source of the SNB and SNBCK models based on the HITRAN2012 and HITRAN2016 databases were in good agreement. This was not true in the 0.1 m short path. The improvement in accuracy from the HITRAN2016 to the HITRAN2012 database was not as accurate as the improvement from the HITRAN2012 database to the HITRAN2008 database. The results calculated using the same HITRAN database by the SNB model were more accurate than those obtained with the SNBCK model. And the lack of contribution of the hot lines in the three HITRAN databases leads to the relative error values of more than 20% on the radiant flux. This indicated that the HITRAN databases are not suitable for engineering calculations at a higher temperature. [ABSTRACT FROM AUTHOR]

Abstract This paper proposed a novel two layer hierarchical Takagi-Sugeno fuzzy hyperbolic tangent static model control scheme for a circulating fluidized bed (CFB) boiler thermal power unit to cope with the complex nonlinearity and large inertia performance which make its existing PI type load-fuel control loop not functioning normally. In the upper layer, a reference governor with Takagi-Sugeno fuzzy hyperbolic tangent static model control algorithm was developed to yield a compensation for the external load command of the existing PI type load-fuel control loop in the lower layer. By constructing a new quadratic integration Lyapunov function, the stability condition of proposed control system was derived. While implemented in the decentralized control system (DCS) of CFB boiler, the reference governor was constructed and adopted the cubic variables hyperbolic tangent function algorithm which can oppose effectively the nonlinearity and large inertia performance. The experimental results shown that the proposed hierarchical fuzzy hyperbolic tangent static model control scheme can improve the control performance of the PI type load-fuel control system obviously, weaken the fluctuation in the main steam pressure and bed temperature apparently and make the controlled system more stable. [ABSTRACT FROM AUTHOR]

Generally, the involvement of hydrocarbons such as C 2 H 4 and its derivative C 2 H 2 in thermal radiation has not been accounted in the numerical simulation of their flames, which may cause serious error for estimation of temperature in the early stage of combustion. At the first, the Statistical Narrow-Band (SNB) model parameters for C 2 H 2 and C 2 H 4 are generated from line by line (LBL) calculations. The distributions of the concentrations of radiating gases such as H 2 O, CO 2 , CO, C 2 H 2 and C 2 H 4 , and the temperature along the centerline of a laminar ethylene/air diffusion flame were chosen to form a one-dimensional, planar enclosure to be tested in this study. Thermal radiation transfer in such an enclosure was calculated using the LBL approach and the SNB model, most of the relative errors are less than 8% and the results of these two models shows an excellent agreement. Below the height of 20 mm, which is the early stage of the flame, the average fraction contributed by C 2 H 2 and C 2 H 4 in the radiative heat source is 33.8%, while that by CO is only 5.8%. This result indicates that the involvement of C 2 H 2 and C 2 H 4 in radiation heat transfer needs to be taken into account in the numerical modeling of the ethylene/air diffusion flame, especially in the early stage of combustion. [ABSTRACT FROM AUTHOR]

The gas temperature fields of axisymmetric co-annular laminar diffusion ethylene flame jets in air and oxygen-enriched atmospheres were measured and visualized using large lateral shearing interferometry technique. Seven cases of reconstructed temperature fields were investigated using various compositions of fuel/oxidant with an average correction factor. The phase difference field were obtained using a fringe trace algorithm. A discrete Abel inversion process was adopted to resolve the refractive index distribution from the line-of-sight integrated data. The peak temperatures of the flame varied between 1980 K and 2050 K using air as the oxidant and increased to more than 2150 K when oxygen was added. Two separate high temperature zones, one located at the side edge of the flame, the other in the upper part of the flame around the centerline of the burner were observed in four of the seven instances of high mixture stoichiometry. The proposed methodology was verified to be suitable for the reconstruction of temperature distributions from the experimental measurement results. [ABSTRACT FROM AUTHOR]

Three kinds of atmospheres, namely, air, oxygen-enriched air, and oxygen/carbon dioxide atmospheres, and a total of ten axisymmetric coflow ethylene laminar flames are experimentally studied using a hyperspectral imaging device. 2-D distributions of the local emission source term in the visible and near-infrared ranges are reconstructed using the least-squares method. Then the distributions of temperature and absorption coefficient are calculated using the Newton-type iterative method, while the wavelength dependence of the absorption coefficient is fitted by polynomials. For the nonoptically thin flame, an iterative algorithm is adopted by substituting the absorption coefficients back into the reconstruction. The experimental results show that there exists a local temperature peak near the flame edge and the flame root. Additionally, for an oxygen-enriched atmosphere, there also exists another local temperature peak in upper center part of the flame in an air/oxygen atmosphere or in the upper edge part in an oxygen/carbon dioxide atmosphere. The results also reveal that the reduction in the flow of ethylene will result in the decrease in the highest soot volume fraction, and a higher oxygen content will generate a higher temperature peak and a lower soot volume fraction peak in an oxygen-enriched air atmosphere, but a higher soot volume fraction peak in an oxygen-enriched oxygen/carbon dioxide atmosphere. In addition, in an air atmosphere or an atmosphere with a large quantity of carbon dioxide, there will be another local peak of the soot volume fraction in the upper center part of the flame, which may be aggravated by the reduction in the amount of ethylene. [ABSTRACT FROM AUTHOR]

[Display omitted] • EffNet and ResNet excel in microalgae biomass quantitative monitoring. • Rhodophyta RGB trends are more suited for quantitative forecasts than Spirulina. • ResNet demonstrates greater generalization and robustness than EffNet. The effective monitoring of microalgae cultivation is crucial for optimizing their energy utilization efficiency. In this paper, a quantitative analysis method, using microalgae images based on two convolutional neural networks, EfficientNet (EFF) and residual network (RES), is proposed. Suspension samples prepared from two types of dried microalgae powders, Rhodophyta (RH) and Spirulina (SP), were used to mimic real microalgae cultivation settings. The method's prediction accuracy of the algae concentration ranges from 0.94 to 0.99. RH, with a distinctively pronounced red-green-blue value shift, achieves a higher prediction accuracy than SP. The prediction results of the two algorithms were significantly superior to those of a linear regression. Additionally, RES outperforms EFF in terms of its generalization ability and robustness, which is attributable to its distinct residual block architecture. The RES provides a viable approach for the image-based quantitative analysis. [ABSTRACT FROM AUTHOR]

This paper presents a method to invert the two-dimensional distribution of a temperature and volume concentration of soot particles from color images. By using numerical simulation, the temperature field and particle volume-concentration field of a non-uniform soot flame are simultaneously reconstructed using the wide-response spectrum of a color CCD camera without adding monochromatic filters. The influence of number of cameras, error of camera position angle, measurement noise and different reconstruction algorithms on measurement accuracy are analyzed. The numerical-simulation results demonstrate that camera-position angle errors play a crucial role in the reconstruction accuracy. In addition, increasing the number of cameras can improve the reconstruction result accuracy. Compared with the least squares algorithm, the Tikhonov-regularization algorithm has a stronger anti-noise ability and can resist 39 dB of noise. The conclusions obtained in this paper are helpful to guide following experimental studies. [ABSTRACT FROM AUTHOR]

[Display omitted] • An initial reaction mechanism for plasma-assisted steam reforming of toluene. • Over 60 % toluene was converted into small molecules (H 2 , CO, C 2 H 2) at 300 °C. • Combining kinetic modeling of plasma reactions and quantitative analysis of OES. • Interaction mechanism of plasma species and radicals was unveiled. • Synergy rules for plasma and thermal processes have been preliminarily obtained. Plasma-enhanced reforming has been widely studied as an efficient technology for online tar removal. By combining reforming experiments with kinetic modeling and in-situ optical emission spectrometry (OES) analysis, interaction mechanism and coupling rules of plasma/thermal reactions during plasma-assisted steam reforming of toluene were well explored in this study. A chemistry model was innovatively developed for plasma-assisted steam reforming of toluene, and predicted experimental results under different operations conditions with superb agreements. The rate of production and sensitivity analysis implied that OH, H and N 2 * mainly contributed to primary toluene conversion, while the electronic attachment dissociation of H 2 O, H addition onto benzene ring and benzyl oxidation by OH were the most enhancing-sensitive reactions. Resultantly, toluene conversion was greatly increased with steam addition before Vf H2O = 10 %, but after which Vf H2O increase deteriorated the reforming performance due to the inhibition of E-N 2 collision by high Vf H2O. As temperature increased, plasma and thermal synergistically promoted reforming performance before 300 °C, at which > 60 % toluene was converted into small molecules such as CO, H 2 and C 2 H 2, with yields of 23.6 %, 9.4 % and 16.3 % correspondingly at 12 kJ/L, but further temperature increase would not benefit due to the reduced discharge field strength at higher temperatures, being confirmed by the decreased relative intensity of OH, CN feature bands on OES. It was initially proposed that non-thermal plasma governed the reforming process at T < 300 °C, and thermal reactions predominated at T > 500 °C after a transition region (300 ∼ 500 °C). Increase of input energy caused higher discharge field strength, promoted the electronic collision ionization or dissociation of N 2 and H 2 O, thus enhanced toluene conversion and gas yields. A detailed reaction pathway has also been clarified. [ABSTRACT FROM AUTHOR]

Abstract: Within a graded index medium, the radiative energy rays propagate in curved paths and with a varied speed of propagation, making the solution of the transient radiative transfer complex and difficult. The DRESOR method is extended to calculate the time-resolved reflectance and transmittance from a one-dimensional, non-emitting, absorbing, anisotropically scattering medium with a linear increasing graded index exposed to a collimated truncated Gaussian pulse. Comparisons are made with the results obtained by DOM and DFEM, which indicate that the DRESOR method is accurate and effective. Compared with uniform graded index, the increase in the gradient of the refractive index significantly reduces the peak value of the reflectance and transmittance, delays the occurrence time of the transmittance and increases the duration time. The double-peak phenomenon of the time-resolved transmittance is caused by the fact that the maximum values of intensity in different directions appear at different times, which occur much later as the polar angle increases. The double-peak phenomenon of the time-resolved transmittance becomes more obvious as the increase in the graded index, the scattering albedo and the optical thickness; and the decrease in the anisotropically scattering phase function coefficient. [Copyright &y& Elsevier]

Abstract: Radiation heat transfer in a two-dimensional rectangular enclosure containing CO2/H2O/N2 mixtures was calculated using the line-by-line and the statistical narrow-band correlated-k models. The radiative transfer equation was solved using the discrete ordinates method along with the T7 quadrature scheme. The updated HITEMP spectroscopic database, HITEMP2010, was employed in line-by-line calculations to obtain the most accurate line-by-line results. The statistical narrow-band model parameters of Sourfiani and Taine [IJHMT 40, 987–991, 1997] were used to obtain the statistical narrow-band correlated-k model results. Calculations were carried out for five test cases involving isothermal/non-isothermal and homogeneous/inhomogeneous CO2/N2, H2O/N2, and CO2/H2O/N2 mixtures. The present line-by-line results can serve as benchmark results for the purpose of validating other approximate models in gas radiation heat transfer calculations in two-dimensional enclosures. Results of the statistical narrow-band correlated-k model are in good agreement with those of the line-by-line method in all the test cases. The statistical narrow-band correlated-k model was found sufficiently accurate to generate benchmark solutions in multi-dimensional gas radiation transfer problems where line-by-line calculations are infeasible due to excessively long computing time. [Copyright &y& Elsevier]

Abstract: Thermal radiation transfer in one-dimensional enclosure between two parallel plates filled with real gases, namely CO2, H2O, or their mixtures, was calculated using the line-by-line approach and the statistical narrow-band model. Line-by-line calculations were carried out using the HITEMP1995, HITRAN2004, HITRAN2008, HITEMP2010, and updated CDSD-1000 databases. This study demonstrates the importance of spectral database to the accuracy of line-by-line calculations through a systematic comparison of line-by-line results using different databases. Calculations of the statistical narrow-band model were conducted using the EM2C narrow-band database. The strong dependence of line-by-line results on the spectral database was demonstrated through several gas radiation transfer problems in planar-plate enclosure containing real gases of both isothermal or non-isothermal and uniform or non-uniform concentrations at 1atm. Fairly significant differences were found between the line-by-line results using the HITEMP2010 database and those using older databases. Very good agreement in both the wall heat flux and the radiative source term was observed between the line-by-line results using the HITEMP2010 database and the results of the statistical narrow-band model in all the cases tested, confirming the EM2C narrow-band parameters for both H2O and CO2 are accurate. For cases involving CO2 the line-by-line results using the HITEMP2010 database are in excellent agreement with those using the updated CDSD-1000 databases. The line-by-line results based on the HITEMP2010 database should be used as benchmark solutions to evaluate the accuracy of other approximate models. [Copyright &y& Elsevier]

Abstract: This work presents a new conception of in-situ combustion control schemes for pulverized coal-fired boilers to improve the installed conventional control equipment of power plant units. In this paper, radiant energy signal (RES), based on a flame image processing technique, is put forward and an optimal on-line combustion supervisory strategy is developed for introducing RES into the combustion control circuit. The results show that better combustion efficiency can be obtained in association with the control property. Also a decrease in fuel consumption with a corresponding decrease in the consumption of energy, as well as a reduction of emissions, has been studied and tested for the optimization. In this paper, the optimal scheme is proved to be practical and economical, and it can be used to optimize the combustion control design and operation of pulverized coal-burning boilers, when considering both the stability of steam pressure as a traditional calorific value signal and the rapidity of response by using RES. [Copyright &y& Elsevier]

The transport process of ultra-short light pulse propagation inside the nonemitting, absorbing, and scattering two-layer media is studied. Under the appropriate combination of the pulse width and scattering property of the probed medium, the temporal signal of the reflectance exhibits a direct correlation between the reflectance signal rise time and the location of the interface. The geometry under study is a two-layer structure. This study extends a prior work by authors, giving more extensive numerical simulations to rigorously test and revise the conditions of the optimal pulse width, which would reveal the dual-peak in the temporal reflectance signals. [ABSTRACT FROM AUTHOR]

In the field of micro and mesoscale combustion, the feature of flame-wall thermal coupling is of great significance because of its small scale nature. Thus, this work provides a comprehensive heat transfer analysis in cylindrical combustors from the perspective of numerical simulation. The combustor has a fixed length-to-diameter aspect ratio of 10, and the channel diameter is scaling up from 1 mm to 11 mm to explore the influence of chamber dimension on heat transfer and flame structure. The distribution of convective and radiative heat flux on inner surface, contribution of thermal radiation are given. Moreover, the role of radiation in flame structure is analyzed, and the convective and radiative heat losses are quantitatively analyzed. We find that radiative heat flux is smaller compared to convective heat flux, and the proportion of radiative heat flux becomes larger with an increasing diameter. Thermal radiation does not change the flame structure when the diameter is less than 3 mm. When the diameter is greater than 5 mm, thermal radiation changes the location of flame front. The heat loss becomes larger at a smaller diameter, and heat loss ratio can reach approximately 73.6% in the combustor with diameter of 1 mm. [Display omitted] • Comprehensive heat transfer analyses in micro combustors are revealed. • The contributions of thermal radiation in combustors with different scales are investigated. • The role of radiation in the flame structure is analyzed. • Heat losses in cylindrical combustors from microscale to mesoscale are quantified. [ABSTRACT FROM AUTHOR]

• A mesh in downstream region can suppress oscillation by 50% and raise frequency. • Incorporating a mesh leads to more system state variations from recurrence analysis. • Mesh number can affect system behaviours to a larger extent than mesh position. • Degree of mesh temperature fluctuation is related to both mesh number and position. • The mesh acts as a heat and flow disturbance source in the downstream region. Self-excited thermoacoustic oscillations are undesirable in most combustion systems due to their negative influence on combustion efficiency and structural vibration. Nonlinear thermoacoustic oscillations driven by combustion can be sensitive to changes in the heat and flow conditions. Therefore, a comprehensive study on self-excited thermoacoustic oscillations was conducted to investigate the effect of incorporating a woven mesh with varying mesh numbers and positions into the downstream region of a Rijke tube. This study determined system frequency and amplitude response and revealed dynamic properties of the system by applying recurrence analysis. To obtain temperature distributions, a thermal imaging system was employed, comprising a short wavelength infrared (SWIR) camera for the mesh and a long wavelength infrared (LWIR) camera for the tube wall. A high-speed schlieren imaging system was utilised for visualising heat and flow conditions at the tube end. The study demonstrated the effect of including a mesh in changing the oscillation eigenfrequency, suppressing the oscillation amplitude by up to 50 % and influencing the system dynamics. Higher mesh number demonstrated greater effectiveness. The captured thermal and schlieren images clearly evidenced the mesh's significant influence on the downstream heat and flow conditions. The insights gained from this work provide a potential control method of self-excited, nonlinear thermoacoustic oscillations. [ABSTRACT FROM AUTHOR]

Effects of radiation reabsorption on the burning flux of freely propagating laminar premixed flames at atmospheric and elevated pressures were numerically investigated for CH 4 /O 2 /N 2 /CO 2 mixtures. Models with SNBCK 9 bands and 7 bands based on the spectral band lumping for H 2 O, CO 2 , CO and CH 4 were developed to improve the computational efficiency. It is found that the burning flux is promoted by the upstream radiation reabsorption and with increasing CO 2 concentration. Furthermore, with increasing pressure, radiation reabsorption first increases and then reduces the burning flux because of the corresponding increases of the reabsorption efficiency and the optical thickness, respectively. The blockage of radiation emission from the burnt mixture due to the increased optical thickness is dominant with the addition of the stronger radiative species CO 2 at higher pressures. Extensive computation further demonstrates that, compared with the benchmark case of 367 bands, the SNBCK 9 bands lumping retains good accuracy while substantially facilitates the computational efficiency. [ABSTRACT FROM AUTHOR]

A candle flame is experimentally studied using a hyperspectral imaging device in this paper. The distributions of temperature and wavelength-dependent emissivity are calculated using a Newton-type iterative method. The results show that the spectral radiation intensity increases from the outside to the inside in the candle flame, and at the same spatial point, the intensity increases as the wavelength increases. The highest temperature of the candle flame is about 1980 K, occurs close to the flame border, indicating the most vigorous reaction. Additionally, the radiation characteristics of the candle flame differ from a gray-body, and the emissivity of the flame decreases as the wavelength increases. [ABSTRACT FROM AUTHOR]

A SNBCK/DRESOR combined method is presented in order to predict the radiative heat transfer of gas and soot mixtures at different temperature configurations. The mixtures of the combustion gases (H 2 O, CO 2 , CO) and the different soot loadings were analyzed to explore the influence of the soot on the non-gray radiation. The radiative source and the intensities with high directional resolution calculated by the DRESOR method were investigated at different temperature configurations. The accuracy of the SNBCK/DRESOR model was also analyzed and compared to the LBL/DOM approach, where the relative errors were less than 4%. The results indicate that the SNBCK/DRESOR method has a better efficiency than the SNBCK/DOM method. [ABSTRACT FROM AUTHOR]

A measurement system consisting of a large lateral shearing interferometry and an experimental setup which can be used under high pressures (pressure ≤ 10 MPa) has been built up to measure the binary gas diffusion coefficients. Fringe pattern in the large lateral shearing interferometry can be as simple as the holographic interferometry. The preliminary experimental results of the diffusion coefficients of CO 2 and CH 4 in O 2 near room temperature and atmospheric pressure have been obtained. Comparison of the experimental results with the data calculated by the Fuller correction and the formulas of Landolt-Börnstein database shows that our measured results are accurate and credible. The experimental system used in this study is suitable for the measurement of the gas diffusion coefficients under high pressures which have not been thoroughly experimental studied. [ABSTRACT FROM AUTHOR]

The selective catalytic reduction (SCR) denitrification efficiency of coal-fired boilers is used to assess the clean production of power plants. Accurate predictions of NOx emissions at the SCR inlet can help improve the denitrification efficiency. However, the generation and distribution of NOx in a furnace involves complex combustion, thermodynamics, and fluid mechanics processes. The physical-based method for NOx emission models is complex and requires expensive computational resources. To address these issues, deep learning was employed to develop a prediction approach based on the random forest (RF) algorithm and lightweight convolutional neural network (CNN). To avoid curse of dimensionality, the random forest algorithm was utilized to select the importance of the candidate variables. The selected variables were in line with the boiler NOx generation and combustion control trends. The lightweight CNN was developed to build the model that predicts NOx emissions. The key was that a Cross-Channel Communication (C3)-Block was introduced into the lightweight CNN to resolve the issue in which there was no communication between feature maps in the same convolutional layer. This model was named C3–CNN. The predictive effectiveness of the C3–CNN-based model was assessed on the real historical dataset of a 600 MW down-fired boiler. The experimental results indicated that the proposed method reduced the model depth of the CNN (saved 3 convolutional layers, 1.5 times parameters, and 1 times FLOPs than 6-layer CNN) while also ensuring the model prediction accuracy (RMSE = 13.52704 ± 0.21036 mg/m3, MAE = 9.89313 ± 0.18288 mg/m3, R 2 = 0.93337 ± 0.00222), which was suitable for the online optimization of industrial pollutant control and may assist cleaner production. [Display omitted] • A novel regression approach based on convolutional neural networks (C3–CNN) is proposed. • C3–CNN can make NOx emission predicts for coal-fired boilers. • C3–CNN improves efficiency and accuracy of NOx emission predicts under steady-state and transient conditions. • C3–CNN's lightweight framework encourages cross-channel communication between feature maps of the same convolutional layer. • Random forest algorithm is utilized to prioritize the model's manipulated variables for high-dimensional industrial variables. [ABSTRACT FROM AUTHOR]

Driven by the demand for accurate intrinsic kinetics for the prediction of the combustion rate of purely fired Zhundong coal (ZDC) in a slag tapping boiler, the surface activation function (SAF) and intrinsic kinetic parameters of two types of rapid-pyrolysis ZDC chars were revealed via the isothermal and nonisothermal thermogravimetric analysis methods. The isothermal kinetic results show that the activation energy (E), reaction order and SAF of these two chars were the same, at 146 kJ/mol, 0.78 and X 0.44 (1 – X)−0.48, respectively. Therefore, the difference in the intrinsic reaction rate between these two chars was caused only by the different pre-exponential factors, which are mainly influenced by the specific surface area and catalytic elements in char ash. Nonisothermal kinetic parameters were obtained via the isothermal model-fitting method (ISOM), thereby using the isothermal SAF to fit the nonisothermal data at different heating rates (β s). The results indicate that the E values of the two chars were 148.95 ± 3.12 kJ/mol and 146.16 ± 1.66 kJ/mol, indicating that β has little influence on E. These measured values are consistent with the isothermal E values, so the ISOM is suitable for the determination of the nonisothermal kinetics of rapid-pyrolysis char combustion. • Intrinsic combustion kinetics of rapid-pyrolysis Zhundong coal chars were obtained. • Different chars have the same E , reaction order and surface activation function. • Catalytic elements have a positive effect on A but have little influence on E. • The heating rate has little influence on E of rapid-pyrolysis char combustion. • ISOM is suitable to describe the combustion kinetics of rapid-pyrolysis char. [ABSTRACT FROM AUTHOR]

In this paper, a mathematical model of the fly ash deposition in a 660 MW supercritical once-through boiler was developed. The mechanism of particle-particle and particle-wall collisions has been considered. Particle collision is modeled by a direct simulation Monte Carlo (DSMC) method. Three submodels (impaction, thermophoresis, and sticking) were included in the mathematical model. In the ash deposition model, the critical velocity model was used to calculate the ash sticking rate during initial deposition, and the viscosity model was used for the outer layer deposition. An ash deposition probe was inserted into the peephole and ash deposits were collected. The morphology of these samples was analyzed using SEM. The deposit thickness and the distributions of adhesion particle size were obtained by mathematical model. The mean deposit thickness calculated by the DSMC method agreed with the value measured by the ash deposition probe with the relative error within 3.79%. The simulation results show that the deposition inner layer consists of fine particles less than 10 μm in size and sticky fine particles were selectively captured during the initial deposition stage. The DSMC method may be a practical alternative for studies of ash deposit formation. [ABSTRACT FROM AUTHOR]

There has been a controversy about the reactivity of coal/char combustion under oxyfuel and air conditions. The conversions of NCP anthracite and DT bituminous coal chars were studied under different atmospheres, using simultaneous thermogravimetry and differential scanning calorimetry (TG–DSC). A model of the intrinsic reaction rate was developed to obtain the variable activation energy at different temperatures and char conversion ratios in terms of a TG–DSC curve only. In the endothermic stage, the chemisorption obeys the same mechanism in O 2 /CO 2 , O 2 /N 2 and O 2 atmospheres, in which the reaction order is about 0.48 in the n th-order model, while in CO 2 is a zeroth-order reaction; both of them are not affected by coal rank. The chemisorption activation energy of CO 2 is less than that of O 2 . The chemisorption activation energies in O 2 /CO 2 are more than that in either 100% CO 2 or 100% O 2 , showing that both O 2 and CO 2 compete for the same active sites. The char-O 2 chemisorption is inhibited by the char-CO 2 chemisorption, resulting in a lower reactivity in O 2 /CO 2 . Subsequently, in the exothermic stage, the NCP and DT char combustions obey different mechanisms, in which the reaction orders are 1.65 and 1, respectively. According to the significant differences in variable activation energies obtained from the TG and the DSC data, the char reaction can be divided into a multi-step chemisorption oxidation stage and a one-step burning stage. Compared with TG method, DSC method is more suitable for describing the reactive characteristics of the multi-step stage. The results show that the char-O 2 /CO 2 chemisorption partially shares common active sites with a complicated multi-step mechanism. The trends of variable activation energies with char conversion ratio at different O 2 concentration in O 2 /CO 2 are similar. The activation energy is not significantly affected by CO 2 , and is about 178 ± 11 kJ/mol. The intrinsic reaction mechanisms of DT char combustion in O 2 /CO 2 and O 2 /N 2 have no fundamental difference; however, the NCP char combustion is affected by both CO 2 and oxygen partial pressure. [ABSTRACT FROM AUTHOR]

• An experimental design was used to investigate the effects of different torrefaction conditions on the properties of biomass. • Two spectrometers were simultaneously used to capture high SNR signals of continuous spectrum and characteristic spectrum. • In-situ detection of the temperature and K release concentration of torrefied biomass pellet combustion was achieved. • Changes in combustion characteristics and K release characteristics of torrefied biomass were evaluated. The orthogonal experiment designed in this paper investigated the effect of changes in experimental parameters such as biomass species, torrefaction temperature, torrefaction time, torrefaction oxygen concentration on the physicochemical properties and combustion characteristics of the torrefaction products. Two optical fiber spectrometers were used to capture the spontaneous emission spectra of the combustion flame of compressed pellets of torrefied biomass, and the in-situ measurement of pellet combustion temperature and K release concentration at high heating rate was realized by multi-spectral analysis algorithm. The experimental results indicate that for biomass, torrefaction will lead to a decrease of volatile and moisture content and an increase of fixed carbon and ash content in components, a decrease of O and H and an increase of C content in elements, a certain loss of biomass quality, an increase of high heat value, and each indicator move closer to coal. The effects of various experimental parameters on the volatile content, mass yield, high heat value increase rate and energy yield in biomass torrefaction products have similar laws, i.e., biomass species / torrefaction oxygen concentration > torrefaction temperature > torrefaction time. The mild torrefaction product pellets have a shorter ignition delay, increased volatile combustion flame height and peak temperature, longer volatile and char combustion duration, but lower peak char combustion temperature, among which the selected torrefaction products of rice straw and pine wood showed an increase in peak temperature of 0.56 % (11 K) and 0.61 % (12 K) for the volatile stage respectively, and a decrease of and 1.29 % (20 K) and 1.29 % (21 K) for the char stage respectively. In addition, the peak K release concentration during combustion of the mild torrefaction product pellets was reduced, among which the peak K release concentrations during the volatile and char stage were reduced by 43.22 % (3.89 ppm) and 52.93 % (51.55 ppm) of the rice straw torrefaction product respectively, and by 73.14 % (1.28 ppm) and 62.08 % (15.01 ppm) for the pine wood torrefaction product respectively. Biomass torrefaction products have fuel properties closer to coal, less K released from the combustion process, which have the potential to become an alternative to coal. [ABSTRACT FROM AUTHOR]

The effects of total pressure on gas radiation heat transfer are investigated in 1D parallel plate geometry containing isothermal and homogeneous media and an inhomogeneous and non-isothermal CO 2 –H 2 O mixture under conditions relevant to oxy-fuel combustion using the line-by-line (LBL), statistical narrow-band (SNB), statistical narrow-band correlated-k (SNBCK), weighted-sum-of-grey-gases (WSGG), and full-spectrum correlated- k (FSCK) models. The LBL calculations were conducted using the HITEMP2010 and CDSD-1000 databases and the LBL results serve as the benchmark solution to evaluate the accuracy of the other models. Calculations of the SNB, SNBCK, and FSCK were conducted using both the 1997 EM2C SNB parameters and their recently updated 2012 parameters to investigate how the SNB model parameters affect the results under oxy-fuel combustion conditions at high pressures. The WSGG model considered is the recently developed one by Bordbar et al. [19] for oxy-fuel combustion based on LBL calculations using HITEMP2010. The total pressure considered ranges from 1 up to 30 atm. The total pressure significantly affects gas radiation transfer primarily through the increase in molecule number density and only slightly through spectral line broadening. Using the 1997 EM2C SNB model parameters the accuracy of SNB and SNBCK is very good and remains essentially independent of the total pressure. When using the 2012 EM2C SNB model parameters the SNB and SNBCK results are less accurate and their error increases with increasing the total pressure. The WSGG model has the lowest accuracy and the best computational efficiency among the models investigated. The errors of both WSGG and FSCK using the 2012 EM2C SNB model parameters increase when the total pressure is increased from 1 to 10 atm, but remain nearly independent of the total pressure beyond 10 atm. When using the 1997 EM2C SNB model parameters the accuracy of FSCK only slightly decreases with increasing the total pressure. [ABSTRACT FROM AUTHOR]

In this paper, a distributed parameter model for tubular reactor in the ethylene cracking furnace based on 3-D temperature reconstruction is developed. The mathematical model was formulated to predict the distributions of the heat flux and the tube temperature while considering a non-uniform distribution of the surface heat transfer and the ethylene pyrolysis process. The results show that the heat flux distribution on the tubular reactor has four high heat flux zones, and the tube temperature reaches a maximum of 1088 °C. The value of the maximum tube temperature decreases compared with the value of the maximum heat flux due to the heat exchange coefficient of pyrolysis gas increasing slowly in the bend region. The tube temperature distribution was validated by using a spectrometer system and the errors between three major product yields at the tube outlet obtained by the proposed model and in-situ measured values were within 5%. This distributed parameter model can be used as a guide for ethylene cracking furnace operators and as a tool for design. [ABSTRACT FROM AUTHOR]

Abstract: During a numerical analysis of radiative transfer in some cylindrical optical thermal analysis and thermal design, applying a cylindrical coordinate system would be much more convenient and precise than that using a Cartesian coordinate system. In this paper, the DRESOR method under a cylindrical coordinate system is proposed to address radiative transfer in a semitransparent graded index cylindrical medium. The dimensionless incident radiation and net radiative heat flux are obtained using the DRESOR method. The accuracy and validity of the proposed method is verified by comparison with other techniques. The effects of isotropic scattering albedo and graded index on radiative transfer are also considered. Additionally, the high directional radiative intensity information is obtained to show the performance of the DRESOR method. It shows that the DRESOR method is an effective technique to address the radiative transfer problem in the graded index cylindrical medium with complex surface temperature characteristics. [Copyright &y& Elsevier]

An on-line three-dimensional temperature measurement experiment was carried out in a gas-fired pilot tubular furnace. Four flame image detectors were utilized to obtain two (red and green) monochromatic radiation intensity distributions, which can be calculated by the DRESOR method based on the radiation image processing technology. Then a revised Tikhonov regularization method was developed to reconstruct three-dimensional temperature distributions from the green monochromatic radiative intensity. Meanwhile, a Newton method combined with a least-squares method was used to simultaneously reconstruct radiative properties from the red one. The two calculation procedures were performed alternately, forming an iterative algorithm to a simultaneous reconstruction of temperature and radiative properties. The reconstructed temperatures agreed well with those measured by thermocouples for different cases with different calorific values and components of gas. The largest relative error was less than 3%, which validated the effectiveness and accuracy of this reconstruction algorithm. Moreover, the nonuniform radiative properties for the flame and nonflame regions were determined to improve the accuracy of temperature measurement by a rigorous comparison test. Finally a set of reasonable fixed radiative properties for the media and walls was chosen for the on-line detection of temperature. The visualized temperatures obtained by the present method agreed reasonably with those measured by thermocouples for all cases, with the largest relative error less than 5%. The present method based on radiation image processing technology is reliable for on-line temperature measurement and shows a good accuracy for its application in the combustion industry. [ABSTRACT FROM AUTHOR]

Highlights: [•] Non-imaging concentrating reflectors designed for providing uniform irradiance on the receiver are proposed. [•] Ray tracing technique is employed in the estimation of flux distributions on the receiver. [•] Effects of parameters on the performance are discussed. [Copyright &y& Elsevier]

Abstract: A kind of concave silicon complex grating based on the superposition of multiple simple silicon gratings with different groove depths (ridge heights) is proposed for a potential application as an absorber for solar cells in the visible and near-infrared wavelength regions. For simple binary gratings, the associated absorptance peak is narrowband and direction sensitive. This study demonstrates that these drawbacks can be remedied by using this kind of concave complex grating with different groove depths, which can tailor radiative properties in the whole of the interested spectral range by making full use of the microcavity resonance effect to enhance absorption. A parametric study is carried out to evaluate the influences of filling ratio, incident angle and groove depth on the spectral absorptance of a silicon grating. And an efficient way of searching for the optimal profiles for the complex gratings is employed. The orthogonal array from the Taguchi method benefits the optimization of structure depths, periods, and filling ratios among thousands of combinations. The average spectral absorptance of the optimized structure is above 0.94 within wavelength region from 0.3 to 1.1μm for the silicon complex grating with two groove depths, which suggests that the proposed structure can be well suitable for solar absorber applications. All numerical results are obtained from validated programs based on the rigorous coupled-wave analysis (RCWA) method. [Copyright &y& Elsevier]

A novel planar micro-combustor with four-corner entrances is proposed by considering high and uniform outer wall temperature distribution used in micro thermo-photovoltaic generator. For this, a three-dimensional numerical model with detailed chemical mechanism, transport, and heat transfer and a thermo-photovoltaic energy conversion model are developed to examine its performance. Results show that the proposed micro-combustor has excellent wall temperature uniformity because a uniform distribution of high-temperature zones of four individual flames is formed in the combustion chamber. Thermal performance and energy efficiency of the proposed micro-combustor are better than conventional micro-combustor at a low flow rate (m) and large equivalence ratio (φ). Additionally, the implemented plate-insertion strategy effectively overcome the deterioration of combustor performance under higher m or lower φ. Proposed micro-combustor with plates insertion has the largest wall temperature and energy efficiency, no matter what φ and m are adopted. Plates extend the fluid ravel path and produce some recirculation zones, which promotes heat transfer, preferential diffusion, and flame anchoring, while its pressure loss is larger. Of all the tested cases, the system efficiency reaches a maximum of 6.39%, corresponding to 12.1 W electricity output. This work provides effective combustor design and research insights to develop high-efficiency micro-power system. • A novel micro-combustor with four entrances is proposed for micro-TPV system. • Excellent performance of wall temperature uniformity is obtained. • Inserting plates boosts thermal performance and energy conversion efficiency. • A maximum system efficiency of 6.39% is achieved with 12.1 W electric output. [ABSTRACT FROM AUTHOR]

This paper proposes a hyperspectral imaging technique for capturing radiative images at a high temporal–spatial resolution, for the purpose of spectrally visualizing the instantaneous combustion behaviors of single coal particles. We initially use a hyperspectral imaging device to record the combustion processes of two types of single pulverized coal particles on a McKenna flat-flame burner. From the snapshot images captured by the device, we observe the combustion processes, ignition characteristics, flame morphologies, and sizes of the two single coal particles. The observations indicate that the two single coal particles undergo volatile release/combustion and char ignition/oxidation during the luminous combustion process; moreover, both particles exhibit homogeneous ignition characteristics under the experimental conditions. Furthermore, based on the spectral radiative intensity images obtained via the device, the soot temperature and volume fraction images of the two single coal particles in the envelope flame are obtained at different burning times. Thereafter, the influences of the combustion behaviors and soot formation mechanisms on the distribution rules of the soot temperature and concentration at different combustion periods are discussed. Finally, the temperature and apparent spectral emissivity values of the two single char particles during the char oxidation process are also calculated to analyze their combustion behaviors. The results show that the temperature changes within a relatively narrow range with respect to the burning time during the oxidation stage, whereas the spectral emissivity continues to increase with the oxidation of chars and subsequently decreases. [ABSTRACT FROM AUTHOR]

Abstract: Thermal effects greatly influence the optical properties of the acousto-optic deflectors (AODs). Thermal analysis plays an important role in modern AOD design. However, the lack of an effective method of analysis limits the prediction in the thermal performance. In this paper, we propose a finite element analysis model to analyze the thermal effects of a TeO2-based AOD. Both transducer heating and acoustic absorption are considered as thermal sources. The anisotropy of sound propagation is taken into account for determining the acoustic absorption. Based on this model, a transient thermal analysis is employed using ANSYS software. The spatial temperature distributions in the crystal and the temperature changes over time are acquired. The simulation results are validated by experimental results. The effect of heat source and heat convection on temperature distribution is discussed. This numerical model and analytical method of thermal analysis would be helpful in the thermal design and practical applications of AODs. [Copyright &y& Elsevier]

Abstract: This paper extends the DRESOR (Distribution of Ratios of Energy Scattered by the medium Or Reflected by the boundary surface) method to radiative transfer in a variable refractive index medium. In this method, the intensity is obtained from the source term along the curved integration paths determined only by the variable refractive index, and the DRESOR values are calculated by the Monte Carlo method in which the propagation of the energy bundles are affected by Snell''s law. With given temperatures on the black boundaries of a one-dimensional medium, the temperature distribution inside the medium with a variable scattering property is calculated under the condition of radiative equilibrium. It is shown that the DRESOR method has a good accuracy in the cases studied. For an isotropic-scattering medium with the same optical thickness, the scattering albedo has no effect on the temperature distribution, which can be obtained from the general equations and can be seen as an extension of what exists for a constant refractive index; however, the different refractive index causes obvious changes in the temperatures inside the medium. The effect of anisotropic scattering on the temperature distribution cannot be ignored, although it is still weaker than the effect caused by variation in the refractive index. [Copyright &y& Elsevier]

Abstract: In this paper, a distributed parameter model for the evaporation system of a subcritical once-through, coal-fired, twin-furnace boiler based on 3-D temperature field reconstruction is developed. The imaginary wall surface was put forward to simplify the twin-furnace problem. The mathematical model was formulated for predicting the transient distributions of parameters, such as the heat flux, the metal-surface temperature and the steam quality; while considering the non-uniform distributions of the surface heat transfer coefficient and frictional resistance coefficient. The model was based on the 3-D temperature distribution got by a flame image processing technique. The model was validated by some measurable parameters of the evaporation system at three typical loads. The results show that the heat flux and the temperature which are located at the overlapping region of the two tangential flames are higher than those in other corners. This distributed parameter model on evaporation system reflects the in-situ operating status of the power plant boiler, which may lead to the subsequent research on a supercritical boiler. [Copyright &y& Elsevier]

Abstract: This paper presented an experimental investigation on the estimation of radiative properties and temperature distributions in a 670t/h coal-fired boiler furnace by a portable imaging processing system. The portable system has been calibrated by a blackbody furnace. Flame temperatures and emissivities were measured by the portable system and equivalent blackbody temperatures were deduced. Comparing the equivalent blackbody temperatures measured by the portable system and the infrared pyrometer, the relative difference is less than 4%. The reconstructed pseudo-instantaneous 2-D temperature distributions in two cross-sections can disclose the combustion status inside the furnace. The measured radiative properties of particles in the furnace proved there is significant scattering in coal-fired boiler furnaces and it can provide useful information for the calculation of radiative heat transfer and numerical simulation of combustion in coal-fired boiler furnaces. The preliminary experimental results show this technology will be helpful for the combustion diagnosis in coal-fired boiler furnaces. [Copyright &y& Elsevier]

Abstract: Numerical studies of the slagging characteristics under different operational conditions in a 300MW down-fired boiler were carried out using slagging models coupled with gas–solid two phase flow and combustion models. Combined with the real operating conditions; comparative and detailed analysis on the slagging position, extent, and causes is presented. The results show that the serious slagging is mainly on the side walls of the lower furnace. Because of the more rapid expansion of the flue gas under the higher temperature, the flue gas in the furnace center makes the flue gas on both sides deflect and flow to the side walls; and the pulverized-coal flame impinges on the side walls. This results in the slagging on the side walls. Under off-design operating conditions, such as stopping some burners, the local flow field is asymmetric and impinges on the local arch burner, front and rear wall regions where the stopped burners are located. It leads to slight slagging on the arch burner regions and the front and rear wall regions of the lower furnace. Based on the investigation, it has been found that the serious slagging on the side walls can be effectively alleviated by cutting off the burners close to the side walls, reducing boiler load and burning low slagging-tendency coals. [Copyright &y& Elsevier]

To improve the efficiency of full-spectrum correlated- k -distribution (FSCK), a new method FSCK-RSM has been proposed based on response surface methodology (RSM) in this paper. The k -distributions of FSCK was used to fit the response surface model based on radial basis function and the radiative calculation efficiency was improved by avoiding multiple computations and interpolation in the FSCK-RSM. The thermal radiation heat transfer of five combustion gases (H 2 O, CO 2 , CO, C 2 H 2 and C 2 H 4) in a one-dimensional layer was investigated and the radiative sources calculated by the LBL, SNB, FSCK and FSCK-RSM methods were given at different distributions of temperature and gas concentration. The results showed that the needed amount of input data was reduced by 677 times using FSCK-RSM comparing to the FSCK and the maximum of the average normalized deviation for FSCK-RSM was 2.46% in the non-isothermal homogeneous medium. The model was finally used for radiation reabsorption calculations in planar C 2 H 4 /O 2 /N 2 /CO 2 flames with full coupling to heat transfer and multi-species chemistry. The computational time using the FSCK-RSM was found to be at most half of that using the FSCK method. This FSCK-RSM model was an effective method for addressing the radiation problems that occur in combustion systems. [ABSTRACT FROM AUTHOR]