High Temperature Particle Research Articles

An approximation function model for solving effective radiative heat transfer in packed bed

Radiative heat transfer is significant but rather complicated in the bed packed with lots of high temperature particles. Unfortunately, it cannot use traditional radiative transfer equation (RTE) to efficiently predict thermal radiation between particles in packed beds. With continuum assumption, an approximation function model (AFM) is proposed here. A generic physical equation of radiation exchange factor of packed bed is given, which is determined by surface emissivity and particle-scale packing structure. Compared with discrete element method (DEM) simulation, current radiation interaction function (RIF) gives a good estimation of the obstructed view factor of packed bed. The radiation in AFM is equivalent to that of heat conduction at size parameter ξ ≪ 1, and effective thermal conductivity (ETC) in central region of packed bed is higher than that of the whole bed. The AFM is also applicable for pebble bed with both radiative and conductive heat transfer, and it can predict consistent radial temperature distributions with that in experiments. The AFM works as a good replacement of traditional radiative transfer equation (RTE), and is also feasible to apply current equation to analyze other radiation models.

Lagrangian simulation and exergy analysis for waste heat recovery from high-temperature particles using countercurrent moving beds

The gas–solid countercurrent moving bed provides an effective method of waste heat recovery from high-temperature particles. The Lagrangian descriptions of particle and gas energy equation in a countercurrent moving bed are established. Two typical scenarios for waste heat recovery from high-temperature particles are studied. The first scenario is the calculation of the necessary particle cooling time and bed height for different cases, where the value of outlet particle temperature is first set as a required design condition. The second is the investigation of the effects of particle size and gas velocity on the heat recovery process and recovery gas exergy in a moving bed with a fixed bed height. Detailed results regarding the particle cooling behavior, necessary cooling time, particle, and gas temperature distribution in the bed, and the pressure drop of each case are obtained. The corresponding thermal exergy, pressure exergy loss, and net exergy of the recovered gas are analyzed. Suggestions on the design and operation for waste heat recovery from high-temperature particles using countercurrent moving beds are proposed.

A numerical study on the pyrolysis of large coal particles: Heat transfer and volatile evolution

A pyrolysis model coupled with heat transfer and volatile evolution has been presented to characterize the internal heat transfer and pyrolysis behavior of a large coal particle. The applicability of kinetic model (DAEM) was first verified, then the ability of the large particle pyrolysis model to estimate the particle temperature variation was validated. In order to investigate the effects of pyrolysis temperature and particle size on pyrolysis characteristics of large particles, temperature and volatile yield profiles inside the particle were numerically studied. Results showed that the influence of pyrolysis temperature and particle size were quite significant. For the low pyrolysis temperature (600 °C) and small particle size (6 mm), the maximum temperature difference inside the particle was less than 100 °C, an assumption of homogeneous pyrolysis may be roughly adopted. While for the high pyrolysis temperature (1000 °C) and large particle size (22 mm), more than 200 °C temperature difference was found in the particle, notable gradients could be observed in the transient profiles of volatile yield. The effects of pyrolysis temperature and particle size on the residual density inside the coal particle have also been investigated numerically using the pyrolysis model.

The Relationship of Fracture Mechanism between High Temperature Tensile Mechanical Properties and Particle Erosion Resistance of Selective Laser Melting Ti-6Al-4V Alloy

In this study, selective laser melting (SLM) Ti-6Al-4V is subjected to heat treatment for 4 h at 400 °C, 600 °C, and 800 °C, followed by air cooling. After heat treatment at 400 °C and 600 °C, the ductility was lower (strength increased). This was could be for two reasons: (1) high temperature tensile properties, and (2) particle erosion wear induced phase transformation. Finally, the particle erosion rates of as-SLM Ti-6Al-4V and heat treatment for 4 h at 800 °C (labeled 800-AC) were investigated and compared; the lamellar α + β phases in 800-AC are difficult to destroy with erosion particles, resulting in the erosion resistance of 800-AC being higher than that of the martensitic α’ needles in the as-SLM Ti-6Al-4V at all impact angles (even the hardness of the 800-AC specimen was lower). The as-SLM Ti-6Al-4V alloy needs heat treatment to have better wear resistance.

Efficient waste heat recovery system for high-temperature solid particles based on heat transfer enhancement

A series of efficient heat recovery methods including PCHEE-A, PCHEE-K and a double-medium system is proposed in this paper, which is to provide comprehensive solutions for high temperature particle in different situations. The PCHEE-A and PCHEE-K can increase the heat transfer area and heat transfer coefficient, through extending specific area to 1000–2000 m2/m3 and inducing turbulence in the heat transfer boundary, respectively. For the double-medium system which can achieve deep heat recovery and utilization, a multi-dimensional thermal resistance model is established to conduct heat analysis. Based on the model, Pareto frontier optimization of the double-medium system is carried out, and the optimal medium arrangement as well as the heat load distribution is derived. By combing the above methods, three typical systems (single-medium vertical plate-type with PCHEE-A, double-medium vertical tube-type and horizontal-type with PCHEE-K) are established and can get heat recovery efficiency at around 80%.

Preparation and photoelectric properties of cadmium sulfide quantum dots**Project supported by the Equipment Pre-research Fund under the Equipment Development Department (EDD) of Chinaʼs Central Military Commission (CMC) (Grant No. 1422030209), the Innovation Team Program of NORINCO Group (Grant No. 2017CX024), and the National Natural Science Foundation of China (Grant Nos. 61106098 and 11864044).

Cadmium sulfide quantum dots (CdS QDs) are widely used in solar cells, light emitting diodes, photocatalysis, and biological imaging because of their unique optical and electrical properties. However, there are some drawbacks in existing preparation techniques for CdS QDs, such as protection of inert gas, lengthy reaction time, high reaction temperature, poor crystallinity, and non-uniform particle size distribution. In this study, we prepared CdS QDs by liquid phase synthesis under ambient room temperature and atmospheric pressure using sodium alkyl sulfonate, CdCl2, and Na2S as capping agent, cadmium, and sulfur sources respectively. This technique offers facile preparation, efficient reaction, low-cost, and controllable particle size. The as-prepared CdS QDs exhibited good crystallinity, excellent monodispersity, and uniform particle size. The responsivity of CdS QDs-based photodetector is greater than , which makes them suitable for use as ultra-violet (UV) detectors.

Experimental Analysis on Thermal Characteristics of Argan Nut Shell (ANS) Biomass as a Green Energy Resource

Argan nut shell (ANS) is one of the most abundant agricultural by-product in the southwestern of Morocco. This study aims to focus on thermal characteristics of ANS biomass for alternate energy production. Physicochemical studies were done to characterize biomass-based material. Moreover, thermal degradation characteristics and kinetic parameters were assessed under pyrolysis and oxidizing conditions by means of thermogravimetric methods. Fundamental tests involving particle combustion of ANS and coal were carried out, and a comparison was performed. The results exhibit a heating value of 20.63 MJ kg -1 and negligible amount of nitrogen and sulfur. Furthermore, ANS follows the usual structural decomposition of lignocellulosique biomass, while coal reveal a discrete devolatilization zone. In addition, combustion tests show a significant burning period and high particle temperature. This experimental research is intended to provide a reference data of ANS biomass in engineering applications.

A Guided Vehicle under Fire Conditions Based on a Modified Ultrasonic Obstacle Avoidance Technology.

Low visibility and hot smoke environment under fire conditions can largely hamper the related fire rescue processes. Ultrasound obstacle avoidance technology is then useful for guidance. However, the biggest challenge of adopting ultrasound technology comes from accurate distance measurements under the disturbances of high temperature and soot particle concentration. It is critical to measure the propagation speed under the complicated fire conditions. Therefore, in this study, a baffle calibration method was proposed to improve the accuracy of distance measurement of an obstacle. The method is based on two ultrasound measurement systems, while one is used to calibrate the propagation speed of ultrasound based on the fixed distanced baffle and the other is for the dynamic measurement of obstacle distance based on the calibrated speed. The viability of this method on the guided vehicle was confirmed based on the experiments. From its comparison to those existing methods, such as constant speed and temperature compensation methods, it was known from that the proposed baffle calibration method provides the best prediction. It was obtained that the maximum errors based on the baffle calibration method are 2.75% and 2.62% under the two representative fire scenarios, respectively, which are much lower than those of constant speed (7.81% and 8.4%) and temperature compensation methods (10.4% and 5.12%).

Heat Transfer Models of Moving Packed-Bed Particle-to-sCO2 Heat Exchangers

Particle-based concentrating solar power (CSP) plants have been proposed to increase operating temperature for integration with higher efficiency power cycles using supercritical carbon dioxide (sCO2). The majority of research to date has focused on the development of high-efficiency and high-temperature particle solar thermal receivers. However, system realization will require the design of a particle/sCO2 heat exchanger as well for delivering thermal energy to the power-cycle working fluid. Recent work has identified moving packed-bed heat exchangers as low-cost alternatives to fluidized-bed heat exchangers, which require additional pumps to fluidize the particles and recuperators to capture the lost heat. However, the reduced heat transfer between the particles and the walls of moving packed-bed heat exchangers, compared to fluidized beds, causes concern with adequately sizing components to meet the thermal duty. Models of moving packed-bed heat exchangers are not currently capable of exploring the design trade-offs in particle size, operating temperature, and residence time. The present work provides a predictive numerical model based on literature correlations capable of designing moving packed-bed heat exchangers as well as investigating the effects of particle size, operating temperature, and particle velocity (residence time). Furthermore, the development of a reliable design tool for moving packed-bed heat exchangers must be validated by predicting experimental results in the operating regime of interest. An experimental system is designed to provide the data necessary for model validation and/or to identify where deficiencies or new constitutive relations are needed.

High-Temperature and High-Pressure Particle Attrition Characteristics of Dry Sorbents for Pre-Combustion CO<sub>2</sub> Capture by a Gas Jet in a Bubbling Fluidized Bed

High-Temperature and High-Pressure Particle Attrition Characteristics of Dry Sorbents for Pre-Combustion CO<sub>2</sub> Capture by a Gas Jet in a Bubbling Fluidized Bed

Experimental and theoretical study of ultraviolet-induced structural/optical instability in nano silicon-based luminescence

Nano silicon is emerging as an active element for UV applications due to quantum confinement-induced widening of the Si bandgap, amenability to integration on Si, and less sensitivity to temperature. NanoSi-based UV applications include deep space exploration, high temperature propulsion, solar photovoltaics, and particle detection in high energy accelerators. However, the viability of the technology is limited by a complex nanoSi optical quenching instability. Here, we examined the time dynamics of UV-induced luminescence of sub 3-nm nanoSi. The results show that luminescence initially quenches, but it develops a stability at ∼50% level with a time characteristic of minutes. Upon isolation, partial luminescence recovery/reversibility occurs with a time characteristics of hours. To discern the origin of the instability, we perform first principles atomistic calculations of the molecular/electronic structure in 1-nm Si particles as a function of Si structural bond expansion, using time dependent density functional theory, with structural relaxation applied in both ground and excited states. For certain bond expansion/relaxation, the results show that the low-lying triplet state dips below the singlet ground state, providing a plausible long-lasting optical trap that may account for luminescence quenching as well as bond cleavage and irreversibility. Time dynamics of device-operation that accommodates the quenching/recovery time dynamics is suggested as a means to alleviate the instability and allow control of recovery, which promises to make it an effective alternative to UV-enhanced Si or metal-based wide-bandgap sensing technology.

A tungsten guard limiter for EAST 4.6 GHz lower hybrid waveguide antennas

A new guard limiter (GL) with tungsten tiles is developed for the 4.6 GHz lower hybrid waveguide antennas (LHWA) on experimental advanced superconducting tokamak (EAST). The tungsten blocks assembly with copper chromium zirconium (CuCrZr) pipes employed for the LHWA lateral protections, which are accommodated to the LHWA profiles. The tungsten block with a wedge-shape in the toroidal cross section is designed to reduce the high temperature particle flux from LHWA side. The explosive welding technology is applied for fabricating the connecting water tanks between the main pipes and the cooling branches. The eddy current analysis under the vertical displacement event and the disruption of plasma indicates the maximum current is 1100A. The GL temperature field and structural stress are estimated using finite element method (FEM) based on ANSYS code. It is found that there is the peak temperature zone located on each center of the 12 sub waveguides zones. It indicates that the GL can be operated at peak power density with 12.92MW/m2 coming from the 4.6 GHz LHWA. It is also confirmed that the frame supports are safety for the GL structure based on the mechanics analysis.

Orientation effect of asymmetric diatomic molecules in transport diffusion

Non-equilibrium transport is an important research area in statistical physics. The influences of the structures of polyatomic molecules on their transport have attracted the attention of researchers. Up to now, most of researchers deemed that temperature gradient is the main factor for molecular orientation and neglected the effect of the chemical potential gradient on the molecular orientation. To make up the deficiency in the study of chemical potential gradients, we build a non-equilibrium system with both chemical potential gradient and temperature gradient, and study the transport diffusion behavior of asymmetric diatomic molecules by using molecular dynamics and Monte Carlo methods. It is found that the diatomic molecules implement the orientation effect during non-equilibrium transport. Under the chemical potential gradient, the molecular orientation effect leads to the fact that the large atom tends to be in the direction of low concentration particle bath, while the small atom tends to be in the direction of high concentration particle bath. The molecular orientation is opposite to the direction of the flow. Under the temperature gradient, the molecular orientation effect leads to the fact that the large atom tends to be in the direction of high temperature particle bath, while the small atom tends to be in the direction of low temperature particle bath. The molecular orientation is the same as the direction of the flow. The orientation direction caused by concentration gradients is opposite to that caused by temperature gradients and it appears as a competitive relationship. At the same time, the influence of the asymmetry of the molecule itself on the molecular orientation is also studied. The larger the asymmetry of the molecule itself (σB/σA), the more obvious the molecular orientation effect is. When σB/σA>1.6, the influence of the asymmetry of the molecule itself on the orientation effect is gradually saturated. When σB/σA=1, which is also for a symmetric molecule, even if neither the temperature gradient nor the chemical potential gradient is zero, no molecular orientation occurs. We explain the physical mechanism of orientation through the principle of minimum entropy production. This work is of theoretical significance for in depth understanding the relationship between mass transport and molecular structure under non-equilibrium conditions.

O2/CO2 and O2/N2 combustion of bituminous char particles in a bubbling fluidized bed under simulated combustor conditions

Most of the time char particles in commercial FB units encounter considerably lower oxygen concentrations than that in the entrance, say between 2% and 10%, and the effects of the oxy-fuel atmosphere on the conversion history of char particles need to be clearly treated under these conditions. In the present work, an experimental study of combustion of bituminous coal char was carried out in a laboratory FB in O2/CO2 and O2/N2 atmospheres under simulated FB combustor conditions at O2 concentrations of 4–10% v/v, bed temperatures of 800–900 °C and char particle sizes of 2–8 mm by continuously measuring the concentrations of O2 and CO in the flue gas. The results indicate that the conversion of char is controlled by diffusion of O2 in the boundary layer of the particle in O2/CO2 and O2/N2 environments and that the gasification of char in O2/CO2 is limited by chemical kinetics. A char conversion model, taking into account the mass transfer from the bed to the particle and the gasification kinetics of char, was built, based on the experimental results and the intrinsic reactivity of char obtained in TGA tests. Simulations, carried out under the same conditions as in the FB experiments, give good agreement in terms of burnout time and instantaneous reactivity during the conversion of single particles. Simulated results prove that the low diffusivity of O2 in CO2 is the main reason for the decreased reaction rate of the char particle in O2/CO2 compared with O2/N2. The contribution of gasification to the consumption of char is more notable at high bed temperature (900 °C) and coarse particles (8 mm), particularly at lower oxygen concentration (4–6% v/v).

Modeling dispersion mode of high-temperature particles transiently produced from industrial processes

High-temperature particles that are transiently produced at large concentrations become common contaminants in industrial buildings, and cause significant harm to worker health and indoor air quality. Along with high-temperature particle movement, ambient air will be heated and the airflow velocity distribution will change accordingly. Some particles will separate with the airflow, depending on the air velocity around particles and particle diameters. Therefore, different dispersion modes will occur. This study developed a numerical model to investigate particle dispersion in industrial buildings. Three dispersion modes were studied, with a consideration of the influence of initial temperature (T0) and particle diameter (dp). Particles with higher T0 and smaller dp (T0 = 673 K; and 1 μm ≤ dp ≤ 10 μm, respectively) were associated with significant horizontal diffusion. This included the movement of most particles within the upper space. Particles with either a larger dp and higher T0, or smaller dp and lower T0 (T0 = 673 K, dp = 30 μm and 293 K ≤ T0 ≤ 373 K, dp = 10 μm), experienced the least horizontal diffusion. Upward transport was limited, with the majority of the particles staying at lower levels. Particles for which T0 and dp ranged from T0 = 673 K, 10 μm ≤ dp ≤ 20 μm and 473 K ≤ T0 ≤ 573 K, dp = 10 μm, respectively, experienced moderate horizontal diffusion. Most particles moved in the upper-middle space and spent the longest time in the vertical direction. Our conclusions inform future studies focusing on human protection and ventilation system design.

Vision-based investigation on the ash/slag particle deposition characteristics in an impinging entrained-flow gasifier

Coal gasification is widely used for chemical production as well as energy generation, and slag tapping process is crucial to entrained-flow coal gasification, before which particle deposition play an important role in slag formation. In order to investigate the slag tapping under special or abnormal operating conditions, the ash/slag particle deposition and ash/slag layer behavior characteristics have been studied based on a bench-scale impinging entrained-flow gasifier. High temperature endoscopy combined with high speed photography is applied to obtain images inside the gasifier and image processing techniques are used to distinguish the object from the high brightness background. The results show that an ash/slag deposition layer is formed under low operating temperature. In the case of particle deposition: High temperature particle (HTP) with medium size would slide on ash/slag layer after impacting, low temperature ash/slag on the deposition layer adheres on the particles and detach from the deposition layer together. HTPs with big size would attract low temperature ashes while travelling in gasifier space and ash/slag surface after impact, then form a low temperature particle (LTP) group with high temperature core and detach from the basic ash/slag layer. LTPs with big size and high space speed would impact and embed into the ash/slag deposition layer, then particle temperature rises while particle volume decreases. The embedded particles hardly detach from the basic layer. The low temperature ash/slag deposition layer would detach from the old slag layer on refractory wall, and the detachment of ash/slag layer is mainly classified into three typical patterns according to the behaviors of the detach process: detach after sliding pattern, detach after tilting and shaking pattern, detach after warping and fracturing pattern. Detachment after warping and fracturing is the commonest pattern of ash/slag deposits behavior that has been observed. Due to the huge volume of detached pieces, it could be the major source of slagging under abnormal operating condition. Under certain extreme conditions, slag tap hole would be blocked by the bridging of huge ash/slag deposits pieces and leads to overpressure of gasification chamber and slagging failure. In industrial applications, the results of this study would be the theoretical support for the suggested minimum load and suggested feeding period of the gasifier during the unique online pressurized continuous coal-water slurry feeding technique of the opposed multi-burner (OMB) gasification.

Morphology development during sessile single droplet drying of mixed maltodextrin and whey protein solutions

A sessile single droplet drying method is employed to gain understanding on spray drying behaviour of mixed maltodextrin and whey protein solutions. Skin formation and subsequent locking of the skin and morphology development were the main aspects studied. Droplets of initial solids content of 30% (w/w), containing maltodextrin DE12 and whey protein in varying ratios were dried at temperatures ranging between 40 °C and 90 °C at a sessile droplet drying platform. The droplet composition did not influence the initial drying rate or locking point, but had major influence on the morphology of the final particle. Particles with high amount of whey protein and/or dried at low temperatures were smooth and spherical with one large vacuole, whereas high maltodextrin concentrations and/or high temperature particles gave a wrinkled surface and multiple vacuoles. Results suggest that composition and drying temperature have a strong influence on particle morphology, and can be altered to steer morphology. Skin formation being a determining factor in morphology development in whey protein-rich systems is explained by sol-gel transition and in maltodextrin-rich systems by reaching a viscosity value that leads to critical skin rigidity.

Pyrolysis of low-rank coal with heat-carrying particles in a downer reactor

A novel co-current downer pyrolyzer was developed that can feed heat-carrying particles (silica sand, particle size, dp,sand=0.116mm) preheated in a fluidized bed separately with coal from the top of the reactor. This allows the high-temperature particles to directly contact coal particles. The influence of the pre-heated heat-carrying particles on the pyrolysis of dried Loy Yang coal particles (particle size, dp,coal=0.350–0.500mm) in a quartz glass downer reactor (1.0m in length and 20mm in inner diameter) was investigated at 1173K in the absence and presence of the steam (up to 30vol%) under different silica/coal ratios. Increase in sand feed rate increased the amount of carbon surface deposits and the yields of CO2 and CO. TOF-MS and FT-IR spectra of the produced heavy tar and deposited carbon on silica sand indicated that the heavy tar very rapidly deposited on the surfaces of sand and forms cokes by polymerization. No significant influence of the steam on pyrolysis reaction was observed within the estimated residence time of coal particles (~0.54–0.61s) in this pyrolyzer.

Study of Nonequilibrium Size and Concentration Effects on the Heat and Mass Diffusion of Indistinguishable Particles Using Steepest-Entropy-Ascent Quantum Thermodynamics

Conventional first-principle approaches for studying nonequilibrium processes depend on the mechanics of individual particles or quantum states and as a result require many details of the mechanical features of the system to arrive at a macroscopic property. In contrast, thermodynamics, which has been successful in the stable equilibrium realm, provides an approach for determining macroscopic properties without the mechanical details. Nonetheless, this phenomenological approach is not generally applicable to a nonequilibrium process except in the near-equilibrium realm and under the local equilibrium and continuum assumptions, both of which limit its ability to describe nonequilibrium phenomena. Furthermore, predicting the thermodynamic features of a nonequilibrium process (of entropy generation) across all scales is difficult. To address these drawbacks, steepest-entropy-ascent quantum thermodynamics (SEAQT) can be used. It provides a first-principle thermodynamic-ensemble based approach applicable to the entire nonequilibrium realm even that far-from-equilibrium and does so with a single kinematics and dynamics, which crosses all temporal and spatial scales. Based on prior developments by the authors, SEAQT is used here to study the heat and mass diffusion of indistinguishable particles. The study focuses on the thermodynamic features of far-from-equilibrium state evolution, which is separated from the specific mechanics of individual particle interactions. Results for nonequilibrium size (volume) and concentration effects on the evolutionary state trajectory are presented for the case of high temperature and low particle concentration, which, however, do not impact the generality of the theory and will in future studies be relaxed.

A novel configuration of natural gas diffusion burners to enhance optical, thermal and radiative characteristics of flame and reduce NOx emission

A novel, simple and feasible method was proposed for increasing intermediate soot concentration and enhancing thermal and radiative characteristics of natural gas flame. In the method, based on pyrolysis technique, a portion of natural gas, as the secondary gas injection rate, was directly injected into the flame to create high temperature and emissive intermediate soot particles. The portion of the fuel dedicated to secondary injection rate varied in the range of 20–55%. The flame structure, axial temperature, soot concentration and radiation flux were examined and compared with those resulted from a prevalent burner. Also, the optical properties of intermediate soot particles were studied by BOMEM FTIR, IR flame photography and TES-1332A digital luminance meter. The results indicated that natural gas molecules injected into the flame were dominantly pyrolyzed into soot particles. These highly luminous emissive particles enhance the total flame emissivity and radiation. Moreover, considering exhaust emission limits, there was an optimum secondary injection rate (40%), which, besides enhancing the radiation of flame, maintained the emission of CO pollutant and UHC lower than the standard values. Furthermore, in the pyrolysis process absorption of heat from flame reduces maximum flame temperature and consequently NOx emission by 35% in the optimum case.