•Particle behaviors in biomass gasification are studied via reactive CFD-DEM.•Higher temperature and S/B promote the solid vertical dispersion coefficient.•The contribution of different heat transfer ...modes during gasification is revealed.•Increasing temperature and S/B promotes all heat transfer modes.•The middle region of the reactor has the highest pyrolysis rate.
Biomass gasification in a bubbling fluidized bed (BFB) reactor is numerically studied based on a particle-scale computational fluid dynamics-discrete element method (CFD-DEM), with thermochemical and polydispersity effects featuring. After model validation, the particle-scale information (e.g., particle motions, particle mixing, solid dispersion, and heat transfer contribution) are thoroughly explored with the discussion of the effects of several critical operating parameters on particle behaviours. The results show that the middle dense region has the highest biomass pyrolysis reaction rate due to the vigorous particle motion. Sand and biomass particles show synchronous horizontal motions, and the solid vertical dispersion coefficients are much higher than the solid horizontal ones, denoting that the vertically introduced gas flow dominates bed hydrodynamics. A higher operating temperature causes a higher solid dispersion coefficient. Elevating temperature and steam to biomass ratio (S/B) first increases and then decreases the particle mixing index. Convection plays a dominant role during the biomass gasification process, followed by the radiation and heat of reaction. The conduction accounts for the smallest proportion and can be neglected. Increasing operating temperature promotes chemical reactions, biomass temperature, and all heat transfer modes. Increasing S/B promotes biomass motions and gasification reactions, leading to more heat consumed and biomass temperature decrease. Decreasing biomass temperature results in a larger temperature difference between biomass particles and bed material, which enhances the conduction, convection, and radiation.
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•An Al-based MIC (EF@EMOF) was prepared via electrospinning combining with in situ growth technique.•Obtained MIC has significantly increased heat release and burning rate.•The ...etching reaction avoids the sintering and improves combustion efficiency.
Combustion is a kind of reacting process involves fluid mechanics and chemical reactions at the same time. In the past decades, little attention has been paid to the improvement on the heat and mass transfer rate of EMs, especially for metastable intermixed composites (MICs). In this paper, an Al-based MIC (EF@EMOF) with modified chemical kinetics as well as improved heat and mass transfer rate was prepared by precisely designing the reaction process and introducing energetic metal organic frameworks (EMOF) with high specific surface area as the reactants. The overall reaction process includes the activation of n-Al by eliminating Al2O3, decomposition of EMOF producing metal oxide, followed by exothermic reactions between the activated n-Al with metal oxide and PVDF. Results show that obtained MIC has significantly increased heat release (3464 J g−1), burning rate (more than 5 times faster than that of mechanically mixed one), and improved combustion efficiency. Furthermore, it is found that the decomposition of EMOF as well as the etching reaction generates massive gas products on the interface layer which avoid the sintering and form lots of holes. Those holes, in return, provide new channels for the further reaction, thus significantly improving the energy output and chemical reaction kinetics.
In this work, a numerical simulation model of an industrial-scale magnesite flash calciner (MFC) was established based on computational fluid dynamics (CFD) method. The discrete phase model (DPM) was ...employed, and the results of kinetic analysis for magnesite decomposition were taken into consideration. Then, the influence of swirling gas inlet design on particle motion and decomposition in MFC was analyzed, thereby providing suggestions for production. The results indicate that incorporating a swirling design can attenuate the particle deposition at the feeding port and increase particle residence time in furnace. However, the conversion degree of magnesite is decreased. This phenomenon can be attributed to the particle accumulation near the wall, resulting in a localized lower gas temperature and subsequently leading to a gas-solid heat transfer degradation. By elevating the gas temperature and reducing its flow rate, the enhancement of magnesite decomposition and reduction in energy consumption are achieved. The gas-solid water equivalent ratio, determined by heat of reaction and sensible heat of flue gas, needs to be higher than 1.8 to maintain a mass fraction of MgO above 0.9.
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•Effect of swirling gas inlet on magnesite flash calciner performance is studied.•The particle motion and reaction behavior in furnace are analyzed using CFD method.•The particle motion is improved while the mass fraction of MgO is reduced.•The gas-solid equivalent ratio is introduced and a practical range is obtained.
•Discussion on metal foam heat exchangers for various energy-intensive industries.•Comprehensive review of particulate fouling through porous metal foams.•Proposition of oscillatory fluids as a heat ...exchanger fouling mitigation technique.•Numerical and experimental methods to investigate heat exchanger fouling.
Curtailing the ever-increasing global energy demand remains an arduous challenge. The U.S. Energy Information Administration emphasized that the deployment of new heat exchanger technologies could revolutionize how industry uses energy. In this review, we highlight the significance of metal foam heat exchangers as an attractive alternative to traditional heat exchanger technologies. Research on metal foam heat exchangers is steadily gaining momentum. However, metal foam heat exchangers are highly susceptible to fouling which results in a myriad of issues such as low heat exchanger performance and high energy consumption. These issues are further compounded by the fact that the fundamental mechanisms governing particulate fouling in metal foam heat exchangers are poorly understood. As such, the overarching goal of reducing energy consumption and greenhouse gas emissions could be met by deploying energy-efficient heat exchanger technologies and also by gaining a solid comprehension of multiphase flows, heat transfer, and heat exchanger fouling mechanisms. The development of advanced numerical methods to unravel heat exchanger fouling mechanisms could pave the way for an optimized heat exchanger design with minimum energy consumption and greenhouse gas emissions. This paper provides researchers a review of the performance of metal foam heat exchangers for various industrial applications and the implications of particulate fouling on the thermal performance of metal foam heat exchangers.
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•Heat and mass transfer behaviours of char combustion are studied at a particle scale.•Polydisperse drag model has to be used to accurately reproduce bed hydrodynamics.•Axial ...dispersion coefficient is one order of magnitude larger than horizontal one.•For reactive particles, the radiation and heat of reaction are dominated.•For inert particles, the radiation and convection are dominated simultaneously.•Particle concentration shows a weak influence on Nup and Shp.
In this study, the multiphase flow and thermochemical behaviours of char combustion in a bubbling fluidised bed (BFB) are simulated using CFD-DEM approach featuring particle size polydispersity and thermochemical sub-models. The model is first validated in terms of mixing index, particle temperature, and particle diameter. Then, it is applied to examine the contribution of each heat transfer mode and study particle-scale behaviours of char and sand comprehensively. The results show that the polydisperse drag model should be used to accurately reproduce bed hydrodynamics when simulating a BFB system with polydisperse particles. Under the simulation conditions, the particle-averaged heat fluxes to char particles through convection, conduction, radiation, and char reaction take 9.79%, 0.82%, 40.44%, and 48.95%, respectively; the particle-averaged heat fluxes to sand particles through convection, conduction, and radiation take 30.28%, 1.0%, and 68.72%, respectively. For reactive char particles, the radiation and heat of reaction are dominated, while for inert sand particles, the radiation and convection are dominated; and for both particle species, the conduction is negligible. Axial dispersion coefficient is one order of magnitude larger than the horizontal one, demonstrating the dominant role of the introduced gas flow in determining bed hydrodynamics.
•A combined micropolar and nanofluid effects of thermal system has been investigated.•Energy conversion is application for thermal system.•Thermal energy enhancement are observed with various values ...of pertinent parameters.•The results are comparison with other studies have been examined.
An applied thermal system for heat and mass transfer and energy management problem of hydromagnetic flow with magnetic and viscous dissipation effects micropolar nanofluids towards a stretching sheet has been studied. A system of partial differential equations for micropolar and nanofluid has been analyzed by a combination of the similarity transformation and accurate finite-difference method. Those solutions are used to obtain distributions of the local convective heat transfer coefficient and the stretching sheet temperature. The related importance dimensionless parameters are Prandtl number Pr, magnetic parameter M, material parameter K, Eckert number Ec, Brownian motion number Nb, thermophoresis parameter Nt and the Schmidt number Sc, respectively. The numerical results are indicated that an increasing in the magnetic parameter is given a decreasing in the values of the velocities and Nusselt number, or an increasing in the values of the shear stress, couple stress at the surface, temperature and concentration. The material parameter K has the opposite effect of magnetic field parameter on the values of the velocities, temperature, concentration, shear stress, Nusselt number and Schmidt number. The temperature is decreased as the Prandtl number increasing, or is increased as the Eckert number increasing. The concentration is decreased as Schmidt number increasing. At last, the study has been presented one multimedia video to show its main contain, it will be appeared at Elsevier AudioSlides website.
Machine learning techniques have received a lot of interest in the exploration to minimize the computational cost of computational fluid dynamics simulation. The present article investigates ...application of heat and mass transfer in magnetohydrodynamic flow over a stretching sheet in metallurgy process by employing the learning methodology based on gradient descent. It is anticipated that the consequences of the current work will show the benefits of future research to enhance the development in the domains of science and engineering. A tabular and graphical evaluation greatly demonstrates the similarity between current and previous outcomes in the prescribed fluid flow model.
Recently, the development of modern vehicles has brought about aggressive integration and miniaturization of on-board electrical and electronic devices. It will lead to exponential growth in both the ...overall waste heat and heat flux to be dissipated to maintain the devices within a safe temperature range. However, both the total heat sinks aboard and the cooling capacity of currently utilized thermal control strategy are severely limited, which threatens the lifetime of the on-board equipment and even the entire flight system and shrink the vehicle’s flight time and range. Facing these thermal challenges, the USA proposed the program of “INVENT” to maximize utilities of the available heat sinks and enhance the cooling ability of thermal control strategies. Following the efforts done by the USA researchers, scientists in China fought their ways to develop thermal management technologies for Chinese advanced energy-optimized airplanes and spacecraft. This paper elaborates the available on-board heat sinks and aerospace thermal management systems using both active and passive technologies not confined to the technology in China. Subsequently, active thermal management technologies in China including fuel thermal management system, environment control system, non-fuel liquid cooling strategy are reviewed. At last, space thermal control technologies used in Chinese Space Station and Chang’e-3 and to be used in Chang’e-5 are introduced. Key issues to be solved are also identified, which could facilitate the development of aerospace thermal control techniques across the world.
•A numerical model is developed and experimentally validated.•Fiber microstructure parameters are obtained by experimental and theoretical methods.•The heat and mass transfer performance of ...moisture-conducting fibers is evaluated.•The Coolmax fiber has superior heat and moisture transfer characteristics.•Ambient air parameters affect the heat and moisture transfer performance of fibers.
Moisture-conductive fibers exhibit potential to serve as evaporative medium in evaporative cooling systems. Their heat and mass transfer performance significantly influences the energy efficiency of evaporative cooling systems. This study evaluated the heat and mass transfer characteristics of moisture-conducting fibers through numerical simulation based on experimental validation. The experimental systems and numerical model with moisture-conducting fibers were developed. The microstructural parameters (porosity and permeability) of fibers were determined according to experimental data and theoretical analysis. The accuracy of parameters and model was verified by experiment. The differences in heat and mass transfer performance of various types of fibers and the effects of ambient air parameters (temperature, relative humidity and flow rate) on the heat and moisture transfer characteristics of fibers were investigated through simulation. The results indicated that the Coolmax fiber had superior heat and mass transfer performance compared to Coolpass, cellulose/PET and polyester fibers. The evaporative cooling efficiency between air and fibers was facilitated by increasing air temperature, decreasing air relative humidity and reducing air flow rate under other conditions unchanged. The results are beneficial to the optimization of the evaporative cooling medium and provide theoretical guidance for the selection of fibers for evaporative cooling.
•FR-TGA can provide excellent heat-mass transfer conditions for sludge pyrolysis.•The influence of heat-mass transfer rates on the sludge pyrolysis was studied.•A multi-step reaction kinetics model ...for sludge pyrolysis was established.•Moisture can improve the reactivity of sludge pyrolysis at high heating rate.
The pyrolysis process of sewage sludge (SS) in fluidized bed has the characteristics of high speed heat-mass transfer rate, which challenges the reliability of thermal analysis methods. In this study, TGA and fast-reaction thermogravimetric analysis (FR-TGA) were used to investigate the pyrolysis characteristics of SS. The apparent kinetic parameters of pyrolysis at constant heating rate (β) and excellent heat-mass transfer rate were obtained, respectively. Resuls shows that, the pyrolysis of SS under both conditions showed multi-step reaction characteristics. The apparent activation energy (Eα) of pyrolysis of SS determined by FR-TGA was significantly lower than that determined by TGA. With the increase in temperature of the FR-TGA reactor, E decreased further. The excellent heat-mass transfer rate could improve the reactivity during SS pyrolysis. In addition, FR-TGA experiments showed that moisture could promote the pyrolysis of organic components in SS and further reduce Eα. Based on these conclusions, a kinetic model of SS pyrolysis under excellent heat-mass transfer rate was established. This study can serve as a reference for predicting the pyrolysis behavior of SS in a fluidized bed reactor.
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