Particle Heat Transfer Coefficient Research Articles

Insights into the gas-solid hydrodynamics and thermochemical characteristics in a pilot-scale chemical looping combustion unit using MP-PIC

Chemical looping combustion (CLC) emerges as an efficient pathway for CO2 capture and storage, yet complex in-furnace phenomena are still far from being understood. Accordingly, the multiphase reactive flow during the CLC process in a pilot-scale dual circulation fluidized bed (DCFB) is numerically investigated by the multi-phase particle-in-cell method considering thermochemical sub-models. After model validations, the thermochemical behaviors and the impact of operating parameters on CLC performance are explored, followed by unveiling the underlying mechanisms of heat and mass transfer characteristics. The pressure gradient is explicitly correlated with the solid holdup in two reactors. Gas-solid flow in the fuel reactor (FR) is more heterogeneous than that in the air reactor (AR) due to complex reactions and large amounts of gas/particle exchanges with other parts of the DCFB, and the whole CLC unit shows good circulating and heat transfer performance. Higher temperatures improve fuel conversion and increase CO2 yield. Elevating the air/fuel ratio (ψ) significantly enhances CLC performance at ψ ≤ 1.0 but it insignificantly affects the CLC performance at ψ > 1.0. A higher total inlet flow rate leads to suppressed fuel conversion and a subsequent decline in CO2 yield. The particle heat transfer coefficient (HTC) in the AR is approximately 450 W/(m2·K), significantly higher than that in the FR of about 300 W/(m2·K).

Numerical simulation study of impact of polydispersity on biomass pyrolysis in draft-tube spouted reactor with fountain confiner

During biomass pyrolysis within spouted reactor, particles exhibit behavior characterized by polydispersity. To explore the impact of polydispersity of bed materials on the biomass pyrolysis, the multiphase particle-in-cell model is constructed to simulate such process. The focus of the study is examining the impact of size distribution of solid materials and the presence of a draft tube on the particle-level properties. The results show that the yields of Gas1 and tar during the first pyrolysis increase about 13.1 % and 14.8 %, respectively, as the PSD width of sand increase from 0.1 to 0.7. In addition, the influence of the draft tube is primarily centered on enhancing interphase interactions rather than pyrolysis yields when a fountain confiner is applied. Fine particles mainly distribute along the interphase of annulus and its vicinity while coarse particles mainly accumulate along the interphase of annulus and spout. Expanding the PSD widths of bed materials from 0.1 to 0.7 results in an approximate temperature increase of 10 K in the floating biomass. The variation of particle slip velocity and heat transfer coefficient exhibits similar trend, mainly due to their interrelation in convective heat transfer. These findings hold valuable insights that can guide optimization design and parameter selection in the conical fountain confined spouted bed reactors.

Effects of ellipsoidal and regular hexahedral particles on the performance of the waste heat recovery equipment in a methanol reforming hydrogen production system

Hydrogen production from methanol has attracted attention due to its wide range of raw material sources and mature technology. Using waste heat of industrial high temperature solid particles like blast slag and steel slag etc. To provide vaporization heat and reaction heat for the reaction between methanol and water is an emerging technology for hydrogen production from methanol, which can save additional thermal energy resources. Herein, the performances of equipment that uses the waste heat of ellipsoidal and regular hexahedral particles to provide a heat source for methanol to hydrogen were explored by the DEM-CFD method. Compared with spherical particles of the same equivalent diameter, ellipsoidal and regular hexahedral particles have poor fluidity in the stagnant area, and the empty area is enlarged and irregular in shape. The average velocity peaks of the ellipsoidal and regular hexahedron particles are larger than those of spherical particles, and the overall mean velocity fluctuation of ellipsoidal particles is similar to that of spherical particles while the regular hexahedron particles' is larger. The average temperature drop rate of the ellipsoidal and regular hexahedral particles is slower than that of spherical particles, the uniformity of temperature distribution is worse than that of spherical particles. The ellipsoidal and regular hexahedral particles’ average effective heat transfer coefficient is smaller than that of spherical particles, and the heat transfer effect is weaker than that of spherical particles. The effective heat transfer coefficient of ellipsoidal particles is 2.95 W/(m−2∙K−1) lower than that of spherical particles and the effective heat transfer coefficient of hexahedral particles is 6.09 W/(m−2∙K−1) lower than that of spherical particles. Therefore, compared with the spherical particles of the same equivalent diameter, ellipsoidal and regular hexahedral particles produce less hydrogen.

Computational fluid dynamics study of hydrogen production by sorption enhanced steam ethanol reforming process in fluidized bed

As a renewable resource, hydrogen plays a pivotal role in the energy transition of the current world. A promising route for hydrogen production is the sorption enhanced steam ethanol reforming (SE-SER), which is characterized by the complex multi-scale characteristics. This paper aims to explore the hydrodynamic (e.g., gas and solid flux, solid dispersion and velocity) and thermodynamic (e.g., particle temperature and heat transfer coefficient) characteristics on the micro-level together with the effect of operating parameters on the SE-SER performance. Accordingly, the reactive multiphase particle-in-cell (MP-PIC) model has been developed under the Lagrangian manner for studying the SE-SER process in the bubbling fluidized bed. In this model, it is assumed that uniform temperature appears inside the particle, and particles with similar properties are packed into a parcel to reduce computational cost. This model has been well validated with experimental data. The results indicate that along the bed height, the solid temperature decreases while the heat transfer coefficient increases. H2 and CO2 have similar distribution in the bed, mainly accumulating in the upper part of the reactor. Enlarging the ratio of H2O to C2H6O in the gas mixture promotes the yields of H2 and CO2. The relationships between key particle variables are explored. It is found that increasing the slip velocity enlarges the particle heat transfer coefficient, and large particle Reynolds number mainly appears with small solid volume fraction. The explored results provide valuable insights into deeply understanding complex multiphase flow and optimizing actual operation of the SE-SER field.

Study of biomass gasification in an industrial-scale dual circulating fluidized bed (DCFB) using the Eulerian-Lagrangian method

Dual circulating fluidized bed (DCFB) has emerged as an efficient reactor for biomass gasification due to its unique feature of high gas-solid contact efficiency and separated reactions in two reactors, yet the understanding of complex in-furnace phenomena is still lacking. In this study, biomass gasification in an industrial-scale DCFB system was numerically studied using a multiphase particle-in-cell (MP-PIC) method featuring thermochemical sub-models (e.g., heat transfer, heterogeneous reactions, and homogeneous reactions) under the Eulerian-Lagrangian framework. After model validation, the hydrodynamics and thermochemical characteristics (i.e., pressure, temperature, and species) in the DCFB are comprehensively investigated. The results show that size-/density-induced segregation makes solid fuels concentrate on the bed surface. Interphase momentum exchange leads to the continuous decrease of the gas pressure axially. In the gasifier and combustor, the lower heating value (LHV) of the gas products is 5.56 MJ/Nm3 and 0.2 MJ/Nm3 and the combustible gas concentration (CGC) is 65.5% and 1.86%, respectively. The temperature in the combustor is about 100 K higher than that in the gasifier. A higher solid concentration results in a smaller value of particle heat transfer coefficient (HTC). The HTCs range from 50 to 150 W/(m2 K) for a solid concentration larger than 0.3 in the combustor while the HTCs range from 100 to 200 W/(m2 K) in the gasifier. The Reynolds number of biomass particles is two orders of magnitude larger than that of the sand particle. The numerical results shed light on the reactor design and process optimization of biomass gasification in DCFBs.

CFD investigation of the complex multiphase flow of biomass gasification in industrial-scale dual fluidized bed reactor

In this work, the process of biomass steam-gasification via a dual fluidized bed with a thermal input of 8MWth is numerically modeled by applying the multiphase particle-in-cell method. This model is successfully validated with the experimental data. Based on the simulation, the results can be epitomized as: distinct fluidization regimes in gasifier and combustor reactors give rise to different flow pattern and dispersion coefficients of solid phase in both reactors. Large particle heat transfer coefficients mainly appear in the upper regions of both reactors due to violent reactions. In this industrial-scale apparatus, the time-averaged heat transfer coefficient of sand in the combustor is larger than that of gasifier sand for about 40 W/(m2·K). Relationships between key variables such as volume fraction, heat transfer coefficient, slip velocity, Reynolds number and temperature of solid phase are discussed. Particles with small volume fraction tend to possess large heat transfer coefficient and slip velocity. The results explored at the particle-level might be useful in the industrial operating regarding the biomass steam-gasification processes.

MP-PIC investigation of glycerol steam reforming in bubbling fluidized bed for high-quality hydrogen production

A reactive multiphase particle-in-cell approach is adopted to numerically investigate the physical and thermochemical characteristics of glycerol steam reforming process in a three-dimensional bubbling fluidized bed. Via the model verification, the simulated results are in acceptable agreement with the experimental measurements. The gas-solid flow hydrodynamics and the particle thermochemical properties are explored. In addition, the effects of different operating parameters (reaction temperature, particle diameter, steam/glycerol ratio, operating pressure) on the performance of the glycerol steam reforming system are discussed. The results reveal the significantly non-uniform spatial distribution of particle temperature and heat transfer coefficient (HTC) due to the intense endothermic reactions and nonlinear hydrodynamics. Comparatively large HTC of solid particles appears in the lower part of the bed. Enlarging the reaction temperature and operating pressure give rise to a larger particle HTC. The back-mixing and internal circulation behavior of particles highly influence the spatial distribution of particle velocity inside the bed. The slip velocity of particle accumulated around the gas feeding port is obviously large. The slip velocity and HTC of particle are almost positively correlated. Decreasing the steam/glycerol ratio enhances the particle thermochemical properties. Moreover, elevating the reaction temperature significantly promotes the yields of H2 and CO while the formation of CH4 is suppressed.

Numerical investigation of the Ni-based catalytic methanation process in a bubbling fluidized bed reactor

In this study, the CO methanation process in a three-dimensional bubbling fluidized bed with Ni-based catalyst is simulated with the reactive multiphase-particle-in-cell method, considering the complex gas-catalyst hydrodynamics, chemical reactions, and heat and mass transfer between phases. The consistency between numerical results and experimental data verifies the feasibility and accuracy of the model. The effects of operating parameters on the purity of CH4, the yield of CH4, the CO methanation performance, and gas-catalyst thermophysical properties inside the reactor are explored. The results show that the CO methanation process mainly occurs in the dense phase region. The fresh gas injected has a significant effect on gas and solid flux, and the heat transfer coefficient (HTC) of catalyst particles. A higher inlet gas velocity and operating temperature lead to a larger HTC of catalyst particles. Furthermore, the HTC of catalyst particles close to gas distributor is the largest, and then the dense region. The gas production of CH4 has an inverse relationship with the ratio of CO/H2. Increasing the CO/H2 ratio reduces the yield of CH4. Also, increasing the inlet velocity of gas decreases the reactant gases residence time, which results in the reduction of CH4 production.

Effect of powder loading on plasma spheroidization of hydride-dehydride titanium powders

Spherical titanium particles have been produced from hydride-dehydride titanium raw powders using an inductively coupled plasma spray system. Taking into account the variously modified particle morphologies with different plasma conditions, the effect of powder loading on spheroidization was quantified and evaluated in terms of the altered particle heating capabilities. The averaged particle heat transfer coefficient was found to vary with powder feeding rate and associated strongly with Ar–H2 gas thermal conductivity. Furthermore, there observed a most efficient particle heating condition for each powder feeding rate, but the efficient heating condition tends to vaporize small raw powders and increase the overall oxygen content unintentionally as a result of the attachment of satellites nanoparticles on spherical particles. It is therefore important that the plasma input power is optimally controlled with particular attention to the altered degree of heat transfer especially for the case of small powder injection.

Numerical study of fluid dynamics and heat transfer property of dual fluidized bed gasifier

In this work, the Eulerian-Lagrangian multiphase particle-in-cell approach is adopted to track the reactive gas-solid motion in the biomass gasification of dual fluidized bed gasifier. Firstly, the mole fraction of dry gaseous products obtained at the gasifier outlet is evaluated and compared with the experimental measurement to prove the accuracy and adaptability of numerical model. Then, the gasification properties of biomass together with the influence of operating parameters in the full-loop system are studied. The results demonstrate that the gas-solid flow with a large particle Reynolds number and vertical flux is relatively fast in the combustor. The combustible gaseous species generated in the biomass gasification mainly locate in the upper part of bubbling fluidized gasifier. Particle temperature is the largest in the standpipe of the full-loop apparatus. Vigorous heat exchange between gas and solid phase occurs in the combustor. Moreover, the heat transfer coefficient of solid phase in the combustor is apparently higher than that in the gasifier. Increasing the steam/biomass ratio and biomass flow rate enlarges the heat transfer coefficient of particles in the bubbling fluidized gasifier but slightly impacts that in the combustor. The results obtained provide meaningful insights for further study of the complex dual fluidized reactor.

A Computational Study on Inter-Phase Heat Transfer in a Conical Fluidized Bed Reactor Using Hot Air

This paper presents a computational study on the inter-phase heat transfer inside a conical fluidized bed reactor when hot air is introduced through the bottom inlet. Two different diameters, 2.0[Formula: see text]mm and 4.0[Formula: see text]mm glass particles are used as the first and second solid phase and hot air is used as the third phase. A gas–particle heat transfer and particle–particle heat transfer are investigated by using computational fluid dynamics. Euler–Euler two-fluid model is used to describe dynamics of particles and fluid flow in the reactor. We observe that gas–particle heat transfer coefficient is large when solid particle is small. This is the same tendency as the gas–particle heat transfer coefficient when cold air is introduced among hot particles. Particle-to-particle heat transfer depends much on the superficial velocity at the inlet.

Drag and heat transfer coefficients for axisymmetric nonspherical particles: A LBM study

In this work, the thermal lattice Boltzmann method with immersed moving boundary conditions has been employed to calculate the drag coefficients and Nusselt numbers for isolated axisymmetric nonspherical particles in uniform flow for a wide range of Reynolds numbers (Re) and aspect ratios (Ar). The simulation results demonstrate that the drag coefficient for ellipsoids and a spherocylinder evolves as sin2θ, viz. θ being the incident angle of the particle. However, the Nusselt number for prolate ellipsoids evolves as sinmθ, viz. m being a function of particle aspect ratio. A correlation for m has been developed for 1<Ar≤4 based on the simulation data. The Nusselt number for a spherocylinder of aspect ratio 4 evolves as sin1.278θ. Moreover, we observe that the drag coefficients and Nusselt numbers for a prolate ellipsoid of aspect ratio 4 and a spherocylinder of aspect ratio 4 are comparable with a maximal relative deviation 5%. Based on the simulation data and the data from literature, a drag correlation for oblate (0.2≤Ar<1 and 1≤Re≤100) and prolate (1≤Ar≤5 and 1≤Re≤2000) ellipsoids has been developed to broaden the range of applicability of existing drag correlations. The average relative deviation between the drag correlation and the fitting data is 9.4%. The Nusselt number correlation for prolate ellipsoids is valid for 1≤Ar≤4 and Re≤500. On the other hand, the Nusselt number correlation proposed for a spherocylinder is valid for Ar=4 and Re≤500. The new drag and Nusselt number correlations are expected to improve the accuracy of Euler-Lagrangian simulations of non-isothermal particulate flows comprising ellipsoids or spherocylinders.

Bubblig Fluidized Bed Combustor (Dept.M)

A Bubbling Fluidized Bed Combustor (BFBC) is designed and set-up to investigate the combustion processes of sub-bituminous Egyptian coal particles and wood. The effect of changing the coal particles size, with keeping the fluidized air velocity constant, on: gas temperatures, heat flux to the combustor wall, radiant-convective overall heat transfer coefficient and combustion level of coal particles is investigated. The highest combustion level is found at coal particles size of 17 mm. The heat transfer coefficient in the case of coal combustion is extremely greater by 500% than that of wood combustion.

Particle-scale evaluation of the pyrolysis process of biomass material in a reactive gas-solid spouted reactor

Biomass pyrolysis is numerically studied in a spouted bed reactor. Thermophysical processes with complex reactions are considered for gas and solid phases under Eulerian and Lagrangian frameworks. After thorough model validations with experimental measurements, particle-scale evaluation of sand and biomass species combined with the segregation phenomenon is presented. The results show that the axial segregation significantly affects the spatial distribution of particle-scale information in three distinct regions. Particle heat transfer coefficient (HTC) has the largest value in the spout region, and then fountain region. A continuous decrease of particle HTC along the bed height appears. Particles close to the spout-annulus interface have a comparatively smaller temperature. Different distribution patterns of the HTC for sand and biomass can be observed. The increase of pyrolysis temperature raises the biomass temperature and HTC, while the effect of gas velocity is negligible. The dispersion intensity of biomass species is obviously larger than that of sand particles. A smaller sand size results in a smaller temperature, but a larger HTC, and a reduced dispersion coefficient.

Experimental evaluation of hydrodynamics and tube-to-bed heat transfer of fluidized Ilmenite ore particles at elevated pressures

The minimum fluidization velocity (Umf), bubble size characteristics, and tube-to-bed heat transfer coefficient (hc) of ilmenite particles are investigated at pressures up to 2000 kPa. The Umf was found to decrease with pressure for dsv ≥ 236 μm and remain constant for dsv ≤ 109 μm. Umf was compared to various Wen and Yu type correlations (Remf=C12+C2Ar−C1) with the Saxena & Vogel (1977) constants resulting in the best fit (AARD = 10.5%) for dsv ≤ 109 μm and the Chitester et al. (1984) constants resulting in the best fit (AARD = 6.4%) for dsv ≥ 236 μm. Pressure marginally effected bubble size relative to gas velocity, with the data fitting the Mori & Wen (1975) correlation best (AARD = 26.4%). The tube-to-bed heat transfer coefficient experienced a maximum with increasing gas velocity and marginally increased with pressure. The Molerus et al. (1995) correlation matched the atmospheric heat transfer data.

A numerical study on gas-to-particle and particle-to-particle heat transfer in a conical fluidized bed reactor

In this present study, the particle behavior and heat transfer coefficients for particle-gas and particle-particle re found by using a computational model in a fluidization process. A conical shape is selected as a reactor due to its wide ange of applications. The equations describing gas and particle motions and heat transfer are solved by using the Eulerian two-fluid approach. Glass bead particles of two different sizes (2 mm and 4 mm) are used as bed materials, and the air is used as a fluidized gas. The velocity of the gas inlet is varied from 1.3 m/s to 2.6 m/s. The results demonstrate that the particle-gas eat transfer coefficient according to the velocity of the air inlet reaches its maximum value around 2.1 m/s, then decreases thereafter. Besides, at the same velocity, the particle-particle heat transfer coefficient reaches its maximum value, then decrease thereafter with increasing the velocity of the inlet air.

Numerical Study of Heat Transfer in Gravity-Driven Particle Flow around Tubes with Different Shapes

In the present paper, the heat transfer of gravity-driven dense particle flow around five different shapes of tubes is numerically studied using discrete element method (DEM). The velocity vector, particle contact number, particle contact time and heat transfer coefficient of particle flow at different particle zones around the tube are carefully analyzed. The results show that the effect of tube shape on the particle flow at both upstream and downstream regions of different tubes are remarkable. A particle stagnation zone and particle cavity zone are formed at the upstream and downstream regions of all the tubes. Both the stagnation and cavity zones for the circular tube are the largest, and they are the smallest for the elliptical tube. As the particle outlet velocity (vout) changes from 0.5 mm/s to 8 mm/s at dp = 1.72 mm/s, when compared with the circular tube, the heat transfer coefficient of particle flow for the elliptical tube and flat elliptical tube can increase by 20.3% and 15.0% on average, respectively. The proper design of the downstream shape of the tube can improve the overall heat transfer performance more efficiently. The heat transfer coefficient will increase as particle diameter decreases.

Experimental evaluation of the convection heat transfer coefficient of large particles moving freely in a fluidized bed reactor

Heat transfer is a key aspect in the performance of fluidized bed reactors. In particular, the dense phase heat transfer coefficient, which determines the heat exchanged between the bed and fuel particles, is of special relevance. This work presents a study on the heat transfer that occur when introducing pelletized particles of dry ice in a fluidized bed. The convection heat transfer coefficient is experimentally evaluated by means of the sublimation of dry ice particles using a novel methodology, which consists in a measurement system based on a macro-TGA fluidized bed and the corresponding evaluation of the acquired data. The system is capable of measuring the real time mass of the fluidized bed, which is a direct measure of the mass loss of the dry ice particles. The evaluation of such measurements allows to determine the convection heat transfer coefficient between the dry ice particles and the bed. Fixed and fluidized bed regimes, varying the excess gas velocity from -0.03 m/s up to 0.35 m/s, were tested for two different bed materials, namely Ballotini glass beads and alumina. The results of the sublimation and heat transfer coefficients are in good agreement with values reported in the literature, confirming the reliability of the experimental and postprocessing methodologies. In general, higher gas velocities enhance the dry ice particles mixing in the bed up to a limit in which the sublimation and convection heat transfer coefficients do not further increase. Additionally, the influence of the buoyancy behaviour of the dry ice particles in the bed on the convection heat transfer coefficient is discussed. The convection heat transfer coefficient decreases substantially if the dry ice particles present a flotsam behaviour and the gas velocity is not sufficiently high to ensure a proper circulation of the particles in the whole bed.

Heat transfer to a stationary cubic particle in a laminar tube flow: Computational fluid dynamics simulations and experiments

In continuous aseptic processing involving liquid food suspensions, the suspended particles must receive sufficient heat treatment without affecting their nutritional or sensory qualities. Optimal process design for the sterilization of particulates in liquid food therefore requires good estimates of convective heat transfer coefficient at the fluid to particle interface. Relatively few studies have been reported involving coarse non-spherical particles in a confined flow. In this work, we have used a combination of computational fluid dynamics (CFD) simulations and experiments to investigate the heat transfer rate to a coarse cubic particle suspended in a laminar fluid flow within a circular tube. We carried out the simulations using ANSYS Fluent software, whereas for the experimental investigations, we used the stationary particle method, employing a thermocouple to measure the particle temperature at the centre. We conducted all tests using a sugar solution as the heat-carrying fluid, and we employed two types of cubic particles, one fabricated from peach fruit and one from polycarbonate plastic. We varied the particle orientation, the fluid flow rate, and the radial position of the particle in the tube. For all the above parameters, we validated the time-temperature profiles of the particle obtained from the simulations against the values measured in the experiments. The temperature data from the simulations for the polycarbonate particle showed fairly good agreement with the experimental data (within 5%) for all flow rates and particle orientations. However, for the peach fruit particle, the agreement between the simulated and experimental data was unacceptable, especially for non-central positions in the tube, with obvious deviations between the simulated and experimental data, where the maximum deviations were between 15% and 25%. The evaluated average fluid-particle heat transfer coefficients for polycarbonate particles were compared with the values obtained from correlations in the literature. The comparison resulted in either overestimation (up to 50%) or underestimation (up to 35%) of the average fluid-particle heat transfer coefficients.

Thermal analysis of fluidized particle flows in a finned tube solar receiver

This paper addresses experimental results on fluidized particle-in-tube solar receiver using a finned tube in order to increase wall-to-particle heat transfer. On-sun tests of a single finned tube solar receiver were performed at the focus of the 1 MW solar furnace of Odeillo. Several solar flux densities and distributions (mean values 236–485 kW/m2) and particle mass flux densities (G = 20–110 kg/m2·s) were tested. A detailed analysis of tube wall and particle temperature distributions and temperature measurement accuracy is proposed. The power extracted by the particle suspension ranges between 17.8 kW and 32 kW and the typical thermal efficiency of this lab-scale solar receiver is about 75%. The mean global wall-to-fluidized particle heat transfer coefficient is calculated as 1200 ± 400 W/m2·K for G in the range 30–110 kg/m2·s. The main uncertainty on the heat transfer coefficient is due to uncertainty on wall temperature measurement during on-sun experiments. The range of this uncertainty is estimated by comparing infra-red camera measurements and wall-welded thermocouple data.