On the treatment of non-optimal regularization parameter influence on temperature distribution reconstruction accuracy in participating medium

The choice of the regularization parameter plays a very important role in the inverse radiation problem of temperature distribution in participating medium and in practice the regularization parameter is not easy to determine accurately, which can directly affect the reconstruction accuracy and introduce errors into reconstruction results. This paper presents the alleviation of non-optimal regularization parameter influence on the temperature distribution reconstruction accuracy in participating medium using coupled methods, i.e., two kinds of regularization method (least square QR decomposition (LSQR) method and truncated singular value decomposition (TSVD) method) coupled with genetic algorithm (GA). The radiative heat transfer was described by the backward Monte Carlo method for its efficiency. Two kinds of temperature distributions with one peak and two peaks are considered. The results show that GA can still improve the accuracy of solutions even though the optimal regularization parameters are used in the coupled methods (LSQR-GA and TSVD-GA). GA can also reduce the temperature reconstruction errors due to the non-optimal choice of the regularization parameter and improve the accuracy of the reconstruction results in the coupled methods. Moreover, the coupled methods can even reach the same or better solutions accuracy for some samples with non-optimal regularization parameter, compared with the accuracy of solutions obtained by the single LSQR method or TSVD method with the optimal regularization parameter. This study demonstrates that the coupled method can alleviate non-optimal regularization parameter influence and obtain more accurate results for the inverse radiation problem of temperature distribution in participating medium.     

Evaluation of Turbulent Models for Natural Convection of Compressible Air in a Tall Cavity

Numerical simulation of turbulent natural convection of compressible air in a tall cavity is carried out. In order to evaluate the accuracy of turbulent models, various turbulent models are applied to solve the natural convection in a tall cavity that has different temperatures imposed on two opposing vertical walls. For the large-eddy simulation (LES) model, Smagorinsky subgrid scale (SGS) and dynamic Smagorinsky SGS are also applied to the same cases in order to investigate the differences in temperature and velocity caused by different turbulent models. It is found that the k–? model has a high accuracy of predicting velocity distribution at various sampled lines by comparing with experimental data at Rayleigh number of 2.03 × 10¹⁰ and 3.37 × 10¹⁰, while the LES model has good performance in predicting temperature distributions.     

SLOWLY DIVERGENT SPACE MARCHING SCHEMES IN THE INVERSE HEAT CONDUCTION PROBLEM

Abstract

Recently developed “slowly divergent” space marching difference schemes, coupled with Tikhonov regularization, can solve the one-dimensional inverse heal conduction problem at values of the nondimensional lime step δt⁺ as low as δt⁺ = 0.0003. A Lax-Richtmyer analysis is used to demonstrate dramatic differences in error amplification behavior among various space marching algorithms, for the same problem, on the same mesh; maximum error amplification factors may differ by more than 10 orders of magnitude at parameter values that are of interest in rocket nozzle applications. Slowly divergent schemes are characterized by their damping behavior at high frequencies. A widely used benchmark problem, where the surface temperature gradient is a step function, provides a basis for evaluating Tikhonov-regularized marching computations. With standard marching procedures, relatively high values of the regularization parameter r are found to be necessary; the resulting loss of resolution leads to erroneous solutions. When slowly divergent schemes are used, much lower values of r are possible, leading to reasonably accurate reconstruction of thermal histories at the active surface.     

Kinetic parameter estimation and simulation of trickle-bed reactor for hydrodenitrogenation of whole-fraction low-temperature coal tar

A kinetic model of whole-fraction low-temperature coal tar (LTCT) hydrodenitrogenation (HDN) was established and applied to a bench-scale trickle-bed reactor (TBR). The gPROMS software was used for parameter estimation, and nonlinear regression method was used to get the kinetic parameters: Ea = 98,173; k₀ = 1.7 × 10⁶; n = 1.56; α = -0.16; m = 0.5; t_c = 12,196; β = 1.06. The concentration of nitrogen compounds in the liquid phase at different axial positions of TBR was obtained. At the same time, the order of influence of reaction conditions on HDN was temperature, LHSV, and pressure. The effectiveness factors at different positions of the catalyst bed were analyzed, and the influence of different reaction conditions on the effectiveness factor was studied.     

Comparison of the explosion characteristics of hydrogen,propane, and methane clouds at the stoichiometric concentrations

Numerical simulations were performed to study explosion characteristics of the unconfined clouds. The examined cloud volume was 4 m × 4 m × 2 m. The build-in obstruction inside the cloud was the 8 × 8 × 4 perpendicular rod array. The obstacle volume blockage ratio was 0.74. Three gases were considered: hydrogen/air at the stoichiometric concentrations, propane/air at the stoichiometric concentrations, and methane/air at the stoichiometric concentrations. The hydrogen/air cloud explosion has higher peak overpressure and the overpressure rises locally at the nearby region of the cloud boundary. The explosion overpressures of both methane/air and propane/air are lower, compared with the hydrogen/air, and decreases with distance. The maximum peak dynamic pressure is reached beyond the original cloud, which is clearly different from the explosion peak overpressure tends. Furthermore, dynamic pressure of a cloud explosion is of the same order as overpressure. The explosion flame region for the hydrogen/air cloud is approximately 1.25 times of the original width of the cloud. The explosion flame regions for propane/air or methane/air clouds are approximately 1.4 times of the original width of the cloud. Unlike the explosion overpressures, the explosion temperatures have little difference between the three mixture examined in this study. The higher energy of explosive mixture generates a high temperature hazard effect, but the higher energy of explosive mixture may not generate a larger overpressure hazard effect in a gas explosion accident.     

Numerical Study of Transient Natural Convection in Air in Vertical Divergent Channels

This article carries out a numerical, transient, two-dimensional analysis of natural convection in air in a divergent channel, characterized by two inclined flat plates heated at a uniform heat flux. The flow is assumed to be laminar and incompressible. Simulations allow to detect the complex structures of the flow inside and outside the channel. Results, in terms of temperature distributions, average Nusselt and Reynolds profiles, depending on time as a function of the divergence angle and channel spacing, are presented. Flow visualizations and stream function contours confirmed the disturbances inside the channel for the highest divergence angles (>5°). Correlations in terms of Nusselt numbers as a function of Rayleigh and Rayleigh modified numbers, ranging from 7.6 × 10² to 1.3 × 10⁹ and from 30 to 8.2 × 10⁸, respectively, were proposed. They were in very good agreement with the experimental relations.     

Particulate matter emission and K/S/Cl transformation during biomass combustion in an entrained flow reactor

This study aims to demonstrate the effect of ash chemistry, especially, the transformation of potassium (K), chlorine (Cl), and sulfur (S) species, on the fine particle emission during biomass combustion. Biomass was burned in an entrained flow reactor at varied temperature from 1000 to 1300 °C, where fine particles were sampled using a 13-stage low pressure impactor, and the morphology and composition of the fine particles were analyzed. The fates of K, Cl, and S during biomass combustion were compared between the entrained flow reactor and the muffle furnace. Results show that the particle size distributions of PM₁₀ are bimodal for all studied cases. A higher concentration of fine-mode particle is observed at 1000 °C, with the peak position at 0.274 μm. When the temperature is increased from 1000 to 1100 °C or higher, the concentration of fine-mode particle is reduced by about 50%, and its size becomes smaller with a peak position at 0.097 μm. K, Cl and S are enriched as potassium chloride and sulfate, dominantly in PM_1.0; while Mg, Ca and Si are enriched in PM_1.0–10. A certain amount of sulfur in PM_1.0 at 1000 °C is observed, while the sulfur disappears above 1100 °C. This indicates that the process of potassium sulfation tends to occur at a moderate temperature, and affects the emission amount and the particle size distribution of particulate matters. Analyzing results of the fates of K, Cl and S in the particle phase indicate a completed sulfur-release from biomass ash above 1200 °C, as well as a maximum capture efficiency for potassium-containing vapors at 1100 °C, which results in a minimum PM_1.0 emission at 1100 °C.     

Kinetics of CO2 gasification of biomass char in granulated blast furnace slag

The CO₂ gasification reactions of biomass char in granulated BFS (blast furnace slag) were isothermally investigated using a thermogravimetric analyzer with the temperature ranging from 1173 K to 1323 K. The effects of temperature, biomass type and granulated BFS on the kinetic characterizations of CO₂ gasification of biomass char were illuminated. The kinetic mechanism models and parameters were obtained through a novel two-step calculation method. The results indicated that the CO₂ gasification reactivity of biomass char as conversion and gasification index increased with the increase of temperature and it could be promoted through granulated BFS. The CO₂ gasification reactivity of CS (cornstalk) char with lower alkali index was lower than that of PS (peanut shell) char. The A₄ model (Avrami-Erofeev (m = 4) model) and A₃ model (Avrami-Erofeev (m = 3) model) were demonstrated as the best appropriate models for CO₂ gasification of CS char and PS char, respectively. The gasification activation energy of CS char ranging from 155.08 to 160.48 kJ/mol was higher than that of PS char whether with or without granulated BFS. Granulated BFS could decrease the activation energy of CO₂ gasification of char at any biomass type.     

Electrochemical and thermal properties of SmBa0.5Sr0.5Co2O5+δ cathode impregnated with Ce0.8Sm0.2O1.9 nanoparticles for intermediate-temperature solid oxide fuel cells

SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC55) impregnated with nano-sized Ce_0.8Sm_0.2O_1.9 (SDC) powder has been investigated as a candidate cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The cathode chemical compatibility with electrolyte, thermal expansion behavior, and electrochemical performance are investigated. For compatibility, a good chemical compatibility between SBSC55 and SDC electrolyte is still kept at 1100 °C in air. For thermal dilation curve, it could be divided into two regions, one is the low temperature region (100–265 °C); the other is the high temperature region (265–850 °C). In the low temperature region (100–265 °C), a TEC value is about 17.0 × 10^?6 K^?1 and an increase in slope in the higher temperatures region (265–800 °C), in which a TEC value is around 21.1 × 10^?6 K^?1. There is an inflection region ranged from 225 to 330 °C in the curve of d(δL/L)/dT vs. temperature. The peak inflection point located about 265 °C is associated to the initial temperature for the loss of lattice oxygen and the formation of oxygen vacancies. For electrochemical properties, the polarization resistances (R_p) significantly reduced from 4.17 Ω cm² of pure SBSC55 to 1.28 Ω cm² of 0.65 mg cm^?2 of SDC-impregnated SBSC55 at 600 °C. The single cell performance of SBSC55∣SDC∣Ni-SDC loaded with 0.65 mg cm^?2 SDC exhibited the optimum power density of 823 mW cm^?2 at operating temperature of 800 °C. Based on above-mentioned properties, SBSC55 impregnated with an appropriate SDC is a potential cathode for IT-SOFCs.     

NATURAL-CONVECTIVE HEAT TRANSFER IN A SQUARE CAVITY WITH TIME-VARYING SIDE-WALL TEMPERATURE

ABSTRACT

The natural-convective heat transfer in an inclined square enclosure is studied numerically. The top and bottom horizontal walls are adiabatic, and the right side wall is maintained at a constant temperature T ₀. The temperature of the opposing vertical wall varies by sine law with time about a mean value T ₀. The system of Navier–Stokes Equations in Boussinesq approximation is solved numerically by the control-volume method with SIMPLER algorithm. The enclosure is filled with air (Pr = 1) and results are obtained in the range of inclination angle 0° ≤ α ≤ 90° for two values of Grashof number (2 × 10⁵ and 3 × 10⁵). It can be noted that there is a nonzero time-averaged heat flux through the enclosure at α ≠ 0°. The dependencies of time-averaged heat flux on oscillation frequency and inclination angle are depicted. It is found that the maximal heat transfer corresponds to the values of inclination angle α = 54^○ and dimensionless frequency f = 20π for both Grashof numbers studied (2 × 10⁵ and 3 × 10⁵).     

基于CFD数据库的炉膛压力管道泄漏定位研究

对非均匀温度场内定位方法进行研究,以解决炉膛热态情况下压力管道泄漏定位问题。使用广义互相关方法进行远距离时延值测量实验并进行低信噪比仿真;使用球型插值法进行三维空间定位仿真;在CFD数据库的基础上,假设声波直线传播,使用初值迭代寻优模型对泄漏定位。实验中,传感器相距10 m,仿真环境为-6 db,三维空间深9 m、宽9 m、高5 m。单峰温度场下,将温度场平均与假设声波直线传播的时延值误差对比。结果表明,广义互相关测量时延值在大空间、低信噪比环境下可行;单峰温度场情况下,假设声波直线传播、温度场平均的情况下所得时延值误差分别在0.2和2.2 ms范围内,由此导致的定位误差分别为0.4和8 m范围内,初值迭代寻优模型能够有效解决非均匀温度场中声速不固定而导致被动定位算法时延值无法反推距离差的问题;假设声波直线传播的时延值误差及由此导致的定位误差满足工程精度要求。     

Effectiveness factor correlations from simulations of washcoat nickel catalyst layers for small-scale steam methane reforming applications

In this paper, the effectiveness factors of Ni/MgAl₂O₄ washcoat catalyst layers under steam methane reforming (SMR) conditions relevant to small-scale hydrogen production systems (1–3 bar pressure, 600–800 °C temperature, and 2–4 steam-to-carbon ratio) are numerically investigated. The effects of the washcoat properties, including the layer thickness (20–80 μm), the mean pore diameter (10–40 nm), and the volume-specific catalyst surface area (1.1–3.3 × 10⁷ m²/m³), are also considered. The simulation is conducted by fully considering the intrinsic reaction kinetics (Xu and Froment model) and multicomponent mass diffusion (Maxwell-Stefan equation). The numerically obtained effectiveness factors are presented as a function of the methane conversion ratio and effective Thiele moduli, and simple correlation equations are proposed for easy evaluation of the effectiveness factors.     

Pore structure and effective diffusion coefficient of catalyzed electrodes in polymer electrolyte membrane fuel cells

For polymer electrolyte membrane (PEM) fuel cells, the pore structure and small effective diffusion coefficient (EDC) of the catalyst layers have significant impact on the cell performance. In this study, both the pore structure and EDC of the catalyst layers are investigated experimentally; the pore structure of the catalyst layer is characterized by the method of standard porosimetry, and the EDC is measured by a modified Loschmidt cell for oxygen-nitrogen mixture through the catalyzed electrodes. It is found that Pt loading has a direct impact on the pore structure and consequently the EDC of the catalyzed electrodes. As the Pt loading is increased, the porosity and mean pore size of the catalyzed electrode decrease, and the EDC decreases accordingly, however, it is increased by 15–25% by increasing the temperature from 25 °C to 75 °C. The EDC of the catalyst layer is about 4.6 × 10^?7 m² s^?1 at 75 °C, compared with 25.0 × 10^?7 m² s^?1 for the uncatalyzed electrode at the same temperature.     

Experimental investigation on simultaneous measurement of temperature distributions and radiative properties in an oil-fired tunnel furnace by radiation analysis

The paper reports experimental investigations on simultaneous measurement of temperature distribution and radiative properties in an oil-fired tunnel furnace by radiation analysis. Two color CCD cameras were used to obtain visible thermal radiation in the furnace. A radiation imaging model was established by the calculation of radiative transfer equation in the furnace. The temperature distribution and radiative properties can be obtained from the inversion of the radiative imaging model. The validity of radiative imaging model was verified by the numerical analysis of cavity radiation and isothermal system radiation, and the accuracy of reconstruction method was validated by simulation reconstruction. The experimental analysis was divided into two parts. Firstly, the temperatures of wall surface were calculated from the radiative image of refractory wall and compared with the measured temperature of a thermocouple. The difference between the two methods was only about 20 K. Secondly, the temperature distributions in the furnace, absorption coefficients of combustion medium, and emissivities of refractory wall were reconstructed. Because of a single burner in the tunnel furnace, the temperature distributions in the XY vertical sections in the furnace were with temperature higher in the center and lower near the refractory wall surface, and the temperatures decreased along the length of the tunnel furnace. The measured emissivity of refractory wall showed that the refractory material of RPA-MC30 is with high reflectivity in visible spectrum.     

Simultaneous reconstruction of temperature distribution, absorptivity of wall surface and absorption coefficient of medium in a 2-D furnace system

For a 2-D furnace system filled with a gray medium, surrounded by gray emitting/absorpting and diffusely reflecting wall surfaces, the temperature distribution is reconstructed using an improved Tikhonov regularization method with radiative energy images detected from the boundary of the furnace, uniform absorptivity of both the wall surfaces and the medium being updated from the temperature images grasped from the boundary too. These steps are taken alternately till a convergence is reached. The measurement errors with normal distribution of standard square deviation of 0.01 are taken into consideration for the radiative energy image and temperature image data. The reconstruction errors for radiative properties vary from 1.45% to 10.75%, and for the highest temperature are within 2%. Comparatively, the reconstruction result for the sharper temperature distribution is not as good as that for the smoother temperature distribution. The applicability of the proposed method may be practically valuable.     

基于少量声学数据的炉内温度场重建

针对炉内温度场声学测量中只有少量声学数据问题,提出了基于平面像素分割和正则化的温度场重建方法.利用二维温度场像素之间的邻近关系,给出了正则化矩阵的一般求取方法,并对正则化参数进行了研究.分别对单峰对称和非对称、双峰对称和非对称的温度场模型进行了仿真实验,得出了三维显示图和等温线图,并给出了温度场的绝对误差和均方根误差.仿真结果表明:该模型的准确度较高,时各种温度场计算有一定的通用性.     

Simulations of Three-Dimensional Buoyant Laminar Plumes in a Martian Environment

Natural convection in laminar martian plumes is investigated numerically for the cases of two interacting area source plumes in a quiescent environment and of a single plume in a crossflow. This article deals specifically with environmental conditions typically seen on Mars (Ga = 2.02 × 10⁵, Ec = 4.35 × 10^?5, and Pr = 0.874). Resulting parametric studies provide detailed information on the importance that certain readily measurable variables have on the dimensionless temperature difference, Θ, and provide correlations for the magnitude and location of the maximum local temperature in the plume for two interacting plumes and a single plume in a crossflow.     

Utilization of Pistia stratiotes (aquatic weed) for fermentative biohydrogen: Electron-equivalent balance,stoichiometry, and cost estimation

This study investigated the utilization of Pistia stratiotes for biohydrogen production via a dark-fermentation process. The aquatic plant was subjected to acid-hydrolysis using H₂SO₄: 3.0% (v/v) for 40 min, resulting in sugar yield: 122.2 ± 5.2 mg/g. The optimum culture pH was 5.5, achieving hydrogen yield (HY): 2.46 ± 0.14 mol-H₂/mol-glucose (3.51 ± 0.20 mg-H₂/g-dry weight) at fermentation time 8 h, temperature 25 °C, and substrate-to-biomass (S/X) ratio 1.0 g-COD/g-VSS. The organic mass balance (92–96%) and electron-equivalent balance (92–98%) indicated the reliability of fermentation data. The dominant species included Planctomycetales, Verrucomicrobiales, Clostridiaceae, and Gammaproteobacteria. The phylogenetic analysis confirmed the abundance of hydrogen-producing bacteria such as Bacillus, Clostridium, and Enterobacter. The hydrogenase gene expression provided the highest activity at pH: 5.5 with a cell number 2.53 × 10⁴ copies/ng-DNA compared to pH: 4.5 (6.95 × 10³ copies/ng-DNA) and pH: 8.5 (7.77 × 10³ copies/ng-DNA). The total cost of the fermentation system including the amortization cost of investment and operating cost was 0.08 $/kg-dry weight (22.8 $/kg-H₂ produced).     

Effects of congestion and confining walls on turbulent deflagrations in a hydrogen storage facility-part 1: Experimental study

If the general public is to use hydrogen as a vehicle fuel, customers must be able to handle hydrogen with the same degree of confidence, and with comparable risk, as conventional liquid and gaseous fuels. Since hydrogen is stored and used as a high-pressure gas, a jet release in a confined or congested area can create an explosion hazard. Therefore, hazards associated with jet releases from leaks in a vehicle-refuelling environment must be considered. As there was insufficient knowledge of the explosion hazards, a study was initiated to gain a better understanding of the potential explosion hazard consequences associated with high-pressure leaks from hydrogen vehicle refuelling systems. Our first paper [1] describes the release and subsequent ignition of a high-pressure hydrogen jet in a simulated dispensing area of a hydrogen vehicle refuelling station. In the present paper, an array of dummy storage cylinders with confining walls (to represent isolation from the forecourt area) was used to represent high-pressure hydrogen cylinder storage congestion. Experiments with ignition of premixed 5.4 m × 6.0 m × 2.5 m hydrogen-air clouds and hydrogen jet releases up to 40 MPa pressures were performed. The results are presented and discussed in relation to the conditions giving the highest overpressures. We concluded from the study that the ignition of a jet release gives much higher local overpressure than in the case of ignition of a homogeneous mixture inside the cylinder storage congestion area. The modelling of these results will be presented in Part 2 of this paper.     

Simultaneous Measurement of Three-Dimensional Temperature Distributions and Radiative Properties Based on Radiation Image Processing Technology in a Gas-Fired Pilot Tubular Furnace

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.