Bed Temperature Research Articles

Effect of scanning strategy on the thermo-structural coupling field and cracking behavior during laser powder bed fusion of Ti48Al2Cr2Nb alloys

Laser powder bed fusion (LPBF) is characterized by complicated non-equilibrium processing characteristics, involving extremely high temperature gradient and cooling rate within the mesoscopic dynamic molten pool, which brings great challenges to the forming of metal materials with high crack sensitivity (such as γ-TiAl alloy). In this paper, based on the LPBF forming process of Ti48Al2Cr2Nb alloy, a corresponding multi-track and multi-layer thermal-structure sequential coupled finite element model was constructed, and the influence of four different scanning strategies on the temperature and stress evolution behavior in the powder bed forming region was quantitatively studied, and the crack initiation criterion was also established according to the relationship between nodal flow stress and equivalent stress. It was found that island scanning strategy could induce the higher molten pool temperature and attendant larger molten pool size compared with the non-island linear scanning strategy. Besides, island scanning strategy was conducive to relieving the thermal stress inside the division region but also inducing the stress concentration in the sub-island overlap region. Specially, the strip island scanning strategy exhibited the lowest residual stress and most homogeneous stress distribution. By the experimental measurements, the strip island scanning strategy contributed to the lowest crack density of 0.33mm/mm2.

Influence of bed temperature on mechanical resistance, surface and dimensional quality of 3D printed parts by robotized fused pellet modeling

Influence of bed temperature on mechanical resistance, surface and dimensional quality of 3D printed parts by robotized fused pellet modeling

Performance improvement of magnesium-based hydrogen storage tanks by using carbon nanotubes addition and finned heat exchanger: Numerical simulation and experimental verification

One of the important tasks of improving the properties of metal hydride reactors is the design of system with optimized heat and mass transfer. Many works are devoted to the selection of the optimal heat exchanger for metal hydride hydrogen storage systems. At the same time, it is necessary to consider the composition of the metal hydride bed, which affects thermal conductivity as well. Little attention has been paid to the study of heat transfer properties in hydrogen storage systems with a heat exchanger and a metal hydride bed based on magnesium hydride and carbon nanotubes. In this work we used numerical simulation to determine the effectiveness of carbon nanotubes addition and heat exchangers with different fin number and geometries on high-temperature magnesium-based hydrogen storage tanks performance. It was found, that increasing the number of fins from three to five makes a smaller contribution in the temperature increasing as compared to the addition of three fins to the heater. The three solid, radial and complex fins have a similar effect on the average Mg/MgH2 bed temperature at 60 min. The most optimal fin number and geometry is three radial fins in terms of the efficiency and the volume it occupies. The addition of carbon nanotubes increases the average temperature of the Mg/MgH2 bed by 67 K for a reactor equipped with a heater without fins. Hydrogen storage system with three radial fins and Mg/MgH2 bed enhanced with carbon nanotubes provides an increase in the average bed temperature of about 180 K as compared to system without fins and nanotubes. This leads to a reduction in the hydrogen absorption time to reach 90% saturation by almost 63% for the three radial fins reactor with carbon nanotubes addition to the metal hydride bed. It has been confirmed that the addition of carbon nanotubes to Mg/MgH2 makes a significant contribution to the heat transfer in the metal hydride bed. In addition, it is shown that both an increase in the heat transfer area and the thermal conductivity coefficient leads to a significant improvement in the efficiency of the metal hydride reactor. In the future, the obtained results can be taken into account when designing hydrogen storage systems based on these materials.

Regeneration of bulk and silica-supported gallium oxide catalysts applied in the dehydrogenation of methanol to formaldehyde and hydrogen by reaction-oxygen regeneration cycles and oxygen pulses in the feed

The oxygen-free dehydrogenation of methanol to formaldehyde and hydrogen is a potential alternative to the established industrial oxidative dehydrogenation yielding aqueous formaldehyde solutions. Advantages are the production of anhydrous formaldehyde and of hydrogen as a valuable coupled product. However, fast deactivation of the catalyst occurs under the high-temperature reaction conditions mainly due to coking, which can be overcome by applying oxygen at a temperature high enough to allow fast oxidation of the carbon deposits but low enough to prevent the loss of Ga. From an engineering point of view, a stable reaction temperature in the catalyst bed and a high H2 yield have to be maintained, requiring that the catalyst enables methanol dehydrogenation, but not the oxidation of hydrogen to water. Thus, two modes of regeneration of GaOx-based catalysts were investigated using reaction − oxygen regeneration cycles and oxygen pulses in the feed. When using bulk Ga2O3 as catalyst and applying optimized reaction-regeneration cycles, a formaldehyde yield of 51 % and a hydrogen yield of 45 % at a stable methanol conversion of 65 % were achieved, whereas undiluted O2 pulsed into the feed every 30 s resulted in a continuous formaldehyde yield of 40 % and a hydrogen yield of 35 % at a stable methanol conversion of 59 % with water yields below 2 % and 4 %, respectively. The effects of the two operation modes on stability and product distribution are discussed in terms of the carbon removal rate from the surface, refilling of oxygen vacancies and parasitic reactions in the presence of oxygen. Specifically, the consumption of O2 did not lead to excessive water formation but preferentially effected the oxidation of the carbon deposits to CO and CO2.

Adhesive solid-state fermentation producing Aspergillus niger conidia on stainless-steel dixon ring supports.

An adhesive solid-state fermentation (adSSF) mode was developed to produce Aspergillus niger conidia, which used a stainless-steel Dixon ring as the support and water-retaining adhesive to load nutritional media on its surface. To obtain high conidia yields, the components of the water-retaining adhesive were screened, optimized by single-factor optimization and response surface methodology, and the optimal dosages of the main components were: wheat bran powder 0.023g·cm-3bed, cassava starch 0.0022g·cm-3bed, and xanthan gum 0.0083g·cm-3bed. The experimentally tested conidia yield was 4.2-fold that without water-retaining adhesive but was 3.7% lower than the maximum yield predicted by the model. The observed double-side growth of A. niger on the Dixon ring supports improved space utilization of the fermentation bed, and the void fraction can increase with the shrinkage of the gel layer. In 1.6L tray reactors with three-point online temperature monitoring, the inner-bed temperature of adSSF was at most 4°C lower than the adsorbed carrier solid-state fermentation (ACSSF) mode, and the conidia yield was 1.68 × 108 conidia.cm-3bed, 61.5% higher than that of the ACSSF bed at the same time, but when the fermentation time was extended to 168h, the conidia yield of the adSSF bed and ACSSF bed were close to each other. The results revealed that the high voidage of the adSSF bed was the main reason for low bed temperature, which can benefit the inner-bed natural convection and water evaporation.

CORRELATION FOR MAXIMUM HEAT TRANSFER BETWEEN FLUIDIZED BED AND ITS WALL AND APPLICATION TO SOLAR POWER PLANTS

Abstract Heat transfer from and to fluidized beds is involved in many applications including chemical processes, power generation, solar collectors etc. It is generally desired to operate in the range in which heat transfer coefficient is maximum. Several good correlations have been developed for maximum heat transfer to objects immersed in fluidized beds. However, there is no correlation for maximum heat transfer between the fluidized bed and its enclosing wall. The present research was done to develop such a correlation. A correlation for maximum heat transfer to bed wall has been developed which is in satisfactory agreement with available test data. The data include a wide variety of particles with diameters 70 to 900 μm, bed diameters 25 to 101 mm, particle density from 1800 to 3200 kg/m3, particle thermal conductivity 0.5 to 237 W/mK, and bed temperature from 18 to 316 °C. A correlation for the optimum velocity was also developed based on the same data. The new heat transfer correlation was compared to test data for fluidized beds made to flow into solar collector tubes by the application of pressure difference. A modification of it was developed which agrees well with available data.

Design and experimental study of solar-driven biomass gasification based on direct irradiation solar thermochemical reactor

Solar-driven biomass steam gasification is a promising technology to produce H2-rich syngas, meanwhile achieve flexible storage of renewable energy. However, the solar gasification application also faces numerous challenges, due to its involved complex chemical reaction characteristics and the inherent the multi-physics energy conversion processes. This work designs a novel direct irradiation solar gasification reactor, with the improved optical acceptance structure and reasonable test module, and the wheat straw particle is selected as experimental sample. Employing a 9 kWe high flux solar simulator, the maximum temperature of reaction bed in this prototype reactor reaches to 1260 °C, with the average solar flux of 1171.3 kW/m2. With adjustable radiation flux as experiment factor, under the highest total radiation of 3.35 kW, the molar fraction of H2 in the produced syngas is 47.1 % with the energy upgrade factor of 1.15, and the cellulose component completes decomposition. Increasing the mass flow rate of steam reduces the syngas output while raising the H2/CO ratio of the syngas. In addition, smaller biomass particle size facilitates the reaction, thereby improving the conversion efficiency of direct irradiation gasification. The investigation results provide a meaningful reference for the efficient utilization of abundant solar and biomass energy.

Coral growth along a natural gradient of seawater temperature, pH, and oxygen in a nearshore seagrass bed on Dongsha Atoll, Taiwan.

Coral reefs are facing threats from a variety of global change stressors, including ocean warming, acidification, and deoxygenation. It has been hypothesized that growing corals near primary producers such as macroalgae or seagrass may help to ameliorate acidification and deoxygenation stress, however few studies have explored this effect in situ. Here, we investigated differences in coral growth rates across a natural gradient in seawater temperature, pH, and dissolved oxygen (DO) variability in a nearshore seagrass bed on Dongsha Atoll, Taiwan, South China Sea. We observed strong spatial gradients in temperature (5°C), pH (0.29 pH units), and DO (129 μmol O2 kg-1) across the 1-kilometer wide seagrass bed. Similarly, diel variability recorded by an autonomous sensor in the shallow seagrass measured diel ranges in temperature, pH, and DO of up to 2.6°C, 0.55, and 204 μmol O2 kg-1, respectively. Skeletal cores collected from 15 massive Porites corals growing in the seagrass bed at 4 sites revealed no significant differences in coral calcification rates between sites along the gradients. However, significant differences in skeletal extension rate and density suggest that the dynamic temperature, pH, and/or DO variability may have influenced these properties. The lack of differences in coral growth between sites may be because favorable calcification conditions during the day (high temperature, pH, and DO) were proportionally balanced by unfavorable conditions during the night (low temperature, pH, and DO). Alternatively, other factors were simply more important in controlling coral calcification and/or corals were acclimated to the prevailing conditions at each site.

Influence of bed temperature on the final properties of PLA parts manufactured by material extrusion

Purpose This paper aims to study the influence of bed temperature on the properties of printed parts and their structural stability. Design/methodology/approach Material extrusion is a manufacturing technique in which a part is completed layer by layer with molten filament. The first layer is deposited on a build platform called bed, which is usually at a controlled temperature above room temperature. The density, coefficient of thermal expansion and Young’s modulus were determined as a function of the bed temperature. The complex permittivity was determined at different temperatures, with the aim of analyzing the influence of the bed temperature and isothermal treatments on the characteristics of the amorphous phase. Findings The Young’s modulus presented a non-monotonic behavior, while the coefficient of thermal expansion did not present a clear dependence on the bed temperature. However, a contraction of the dimensions of the parts was observed after heating at temperatures above the glass transition. With treatments at temperatures lower than the glass transition temperature, no changes were observed. However, with treatments at temperatures higher than this, changes in the mobile amorphous region were inferred. Originality/value Issues related to the use of parts manufactured by 3D printing after a posterior heating were analyzed: an improvement in the Young’s modulus and a slight variation of the coefficient of thermal expansion were observed. However, a significant variation in dimensions was detected, mainly for the lowest bed temperatures. This is important for possible applications at temperatures above 60°C.

Low temperature powder bed fusion of polyamide 6: transient process characteristics and process-dependent part properties

AbstractThe powder-based additive manufacturing of high-melting polyamides is impeded by unfavorable fracture properties and oxidation tendencies, limiting the technical applicability and the reuse of unfused powder. Overcoming these limitations through new processing strategies provides the opportunity for significantly extending the range of materials applicable for powder bed fusion processes. Based on fractal, locally quasi-simultaneous exposure strategies, a mesoscale compensation of thermal and crystallization-induced shrinkage is obtained, enabling the non-isothermal processing and stable layer formation of polyamide 6 at powder bed temperatures of 25 °C, 50 °C, and 75 °C. Thermographic in situ investigations reveal the layerwise quenching of discrete layers, leading to cooling rates exceeding − 250 K s−1 at 130 °C. Based on microscopic, mechanical, thermal, and spectroscopic investigations, insights into the structure formation and corresponding mechanical properties can be obtained, yielding reduced fractions of α-PA6 while increasing the elongation at break. The inherent layerwise quenching promotes the formation of microspherulitic morphologies while minimizing oxidation-induced material degradation, reflected in infrared spectroscopic material characteristics. Relying on obtained findings, process optimization strategies can be derived that enable the processability of high-melting polyamides at reduced temperatures, reducing the energy consumption of the build process while adapting underlying material characteristics based on accelerated intermediate cooling conditions.

Robust design optimization of Critical Quality Indicators (CQIs) of medical-graded polycaprolactone (PCL) in bioplotting

Polycaprolactone (PCL), either in its pure grade or as a polymeric matrix for bio-composites, plays a key role in the biomedical and bioengineering industries. It is also considered a multifunctional and versatile polymer for bioprinting and bioplotting purposes, especially in tissue engineering. Herein, an undiscovered yet valuable aspect of PCL extrusion-based bioprinting, such as the predictability of Critical Quality Indicators (CQIs), is investigated in depth. With the aid of the robust L25 orthogonal matrix design, the six most generic and device-independent control factors proved their impact on quality metrics such as global porosity, dimensional conformity, and surface roughness, determined with the aid of highly evolved Nondestructive Testing (NDT) and algorithms. To this end, 25 experimental runs were set, and 125 specimens were fabricated using an industrial-scale bio-plotter and medical-graded polycaprolactone. Various infill densities (ID), layer thicknesses (LT), raster deposition angles (RDA), printing speeds (PS), nozzle temperatures (NT), and bed temperatures (BT) were applied. CQIs were determined using optical profilometry and microscopy, and micro-computed tomography. Quadratic predictive equations were compiled and verified using two additional, well-chosen experimental runs. These generally applicable predictive models carry a massive amount of research and industrial merit, as they ensure visibility in bioprinting with PCL.

Numerical simulation of fluidized bed reactor for calcium looping energy release process in thermochemical storage: Influence of key conditions

The thermochemical energy storage technology applied to concentrating solar power is expected to realize the large-scale deployment of solar power. Reactor design is recognized as a key challenge. In this study, the calcium looping energy release process in a bubbling fluidized bed reactor was numerically simulated using an Eulerian-Eulerian Two-fluid Model. The effects of immersed tubes, particle size, CO2 concentration, and flow velocity on the multi-physical coupling characteristics during the carbonation process were explored. The result indicated that the presence of immersed tubes improves flow characteristics and effectively controls the bed temperature. Particle size had no significant effect on the carbonation process during the kinetically controlled stage. Increasing CO2 concentration significantly enhanced the conversion rate; however, it could lead to a reduction in particle renewal frequency, thereby deteriorating the heat transfer coefficient (HTC). The study on flow velocity indicated that a balance should be struck between bed stability and reaction conversion rate, with the model identifying 0.48 m/s to 0.74 m/s as the suitable operating gas velocity. Additionally, the study compared two methods for calculating HTC and analyzed the energy utilization efficiency during the carbonation process, providing recommendations for industrial applications.

Analysis of Biomass Co-firing Combustion Test using Corn Cobs at Punagaya CFPP as A Green Energy Alternative

Abstract Indonesia is determined to reduce fossil fuels by 2060 set a national renewable energy mix target of 23% by 2025 and be free of fossil fuels by 2060. One of the PLN’s (a national electrical company in Indonesia) efforts to increase carbon emissions reduction and strengthen local new renewable energy is co-firing biomass on CFPP. Indonesia, which is located in a tropical area, has a large potential for biomass resources. Corn cob, one of the biomass types, has a big potential to replace coal as fuel in CFPP especially in Jeneponto Region, South Sulawesi. By utilizing corn cobs as a fuel with a co-firing mechanism, it can generate green energy. The methodology for assessing the corn cob co-firing combustion test’s performance at Punagaya CFPP is presented in this paper. It is based on a multi-criteria assessment designed to examine the technical, financial, and environmental effects of operating parameters with a mixture composition of 95% coal and 5% corn cobs at 70 MW load for 10 hours. From the test results in terms of operational aspects, the parameters have changed but it still in the normal operating range; FEGT increased by 5.3°C, bed temperature increased by 8.9°C, air chamber pressure decreased by 0.05 kPa, and total airflow decreased by 4.5 T/h. From the financial aspect, it has benefited from a decrease in the value of Specific Fuel Consumption (SFC), which is 0.014 kg/kWh compared to 100% coal. From the environmental aspect, the overall results of emission parameters are still by environmental quality standards and have decreased for NOx and SO2 parameters.

Evaluation of Fused Deposition Modelling process parameters influence on the porosity of printed parts

Fused Deposition Modelling (FDM) is an additive manufacturing technology that is rapidly gaining popularity due to its ability to produce complex shaped parts in a short time. However, parts produced by means of FDM have porosity that results from the printing process. The mechanical properties of printed parts depend on the FDM process parameters and porosity. This study investigates the effect of FDM process parameters on the porosity of FDM parts. The results of the study show that properly selected FDM process parameters can significantly minimize the percentage of part porosity. The effect of FDM process parameters on porosity was determined by analyzing nine process variables: layer height, extrusion temperature, printing speed, extrusion multiplier, bed temperature, infill pattern, deposited strand width, number of shells, number of solid top and bottom layers. The percentage of samples porosity was measured by means of the hydrostatic weighing method. Using the analysis of variance, statistically significant factors and their combinations affecting the percentage of parts porosity were determined. It was shown that the extrusion multiplier had the greatest influence on the percentage of parts porosity. A significant contribution to the formation of porosity was made by the infill pattern, layer heights, printing speed, number of shells, number of solid top and bottom layers and their interaction. A regression model has been developed that allows predicting the porosity of parts achieved by different combinations of FDM process parameters.

An experimental setup for studying conduction and radiation heat transfer in a rotary drum

This study presents the development of a novel heating system that aims to bridge the experimental gap for analyzing both radiation and conduction heat transfer in a rotary drum. The experiments were conducted by loading the drum with 4 mm silica glass beads at varying fill levels (10 %, 17.5 %, and 25 %) and rotation rates (2, 5, and 10 RPM), and monitoring the temperature evolution with an infrared camera. Higher fill levels resulted in lower average particle bed temperatures, while rotation rate influenced the contact time between particles and the drum wall and the exposure to radiative heat. A rotation rate of 5 RPM resulted in the highest average bed temperature due to optimal contact and exposure time for conduction and absorption of radiative heat, respectively. Thus, a balance between adequate particle mixing and optimal contact/exposure time is crucial for maximizing heat transfer efficiency in rotary drums exhibiting conduction and radiation.

A Numerical and Experimental Performance Assessments of an Improved Natural Draft Biomass Cook Stove

ABSTRACT This article analyses the performance of an efficiently designed biomass cook stove using standard performance parameters. The investigation was first conducted using mathematical modeling developed in the ANSYS Fluent software. The results of primary air and the temperature profiles inside the combustion chamber were found through simulation. It revealed that the flue gas left the combustion chamber with an average velocity of 1.7 m/s. The average pot bottom and fuel bed temperatures were 650 K and 1100 K, respectively. A copper coil was wrapped around the combustion chamber wall to recover its waste heat. The recovered waste heat was utilized to warm the water flowing through the copper coil. The heated water could be used for other domestic applications. The fabricated design of the improved stove was experimented with water boiling test. The maximum thermal efficiency achieved was 42% with a firepower of 8.25 kW, after incorporating the waste heat recovery system, corresponding to 7.5 L test. The overall efficiency, which is the product of the modified combustion efficiency and the maximum thermal efficiency, was found to be 41.50%. The performance was found to be significantly better than that of a commercial biomass cook stove. The developed model was also evaluated for indoor CO, CO2 and PM emissions. The highest levels of CO2 and CO were 940 ppm and 23 ppm. The average amount of PM1, PM2.5 and PM10 found out to be 18 µg/m3, 20 µg/m3 and 35 µg/m3 respectively.

Superhydrophobic Surface Modification of a Co-Ru/SiO2 Catalyst for Enhanced Fischer-Tropsch Synthesis

Commercial silica support pellets were impregnated and calcined to contain cobalt oxide and ruthenium oxide for Fischer-Tropsch synthesis (FTS). The precatalyst pellets were split evenly into two groups, the control precatalyst (c-precat) and silylated precatalyst (s-precat), which were treated with 1H,1H, 2H, 2H-perfluorooctyltriethoxysilane (PFOS) in toluene. The samples of powderized s-precat were superhydrophobic, as determined by the water droplet contact angle (>150°) and sliding angle (<1°). Thermal analysis revealed the PFOS groups to be thermally stable up to 400 °C and temperature programmed reduction (TPR) studies showed that H2 reduction of the cobalt oxide to cobalt was enhanced at lower temperatures relative to the untreated c-precat. The two active catalysts were examined for their FTS performance in a tubular fixed-bed reactor after in situ reduction at 400 °C for 16 h in flowing H2 to give the active catalysts c-cat and s-cat. The FTS runs were performed under identical conditions (255 °C, 2.1 MPa, H2/CO = 2.0, gas hourly space velocity (GHSV) 510 h–1) for 5 days. Each catalyst was examined in three runs (n = 3) and the mean values with error data are reported. S-cat showed a higher selectivity for C5+ products (64 vs. 54%) and lower selectivity for CH4 (11 vs. 17%), CO2 (2 % vs. 4 %), and olefins (8% vs. 15%) than c-cat. S-cat also showed higher CO conversion, at 37% compared to 26%, leading to a 64% increase in the C5+ productivity measured as g C5+ products per g catalyst per hour. An analysis of the temperature differential between the catalyst bed and external furnace temperature showed that s-cat was substantially more active (DTinitial = 29 °C) and stable over the 5-day run (DTfinal = 22 °C), whereas the attenuated activity of c-cat (DTinitial = 16 °C) decayed steadily over 3 days until it was barely active (DTfinal < 5 °C). A post-run surface analysis of s-cat revealed no change in the water contact angle or sliding angle, indicating that the FTS operation did not degrade the PFOS surface treatment.

Atmospheric Bubbling Fluidized Bed Risers: Effect of Cone Angle on Fluid Dynamics and Heat Transfer

Abstract In this paper, a comparative study of fluid flow behavior and thermal characteristics of sand particles has been carried out numerically and experimentally in bubbling fluidized beds for five-cone angles of the riser wall having 0 deg, 5 deg, 10 deg, 15 deg, and 20 deg. An Eulerian model with a k–ε turbulence model is used to explore the numerical analysis, and the findings are compared to those of the experiments. For the study, the inlet air velocity is fixed at 1.5 m/s with sand particles filled up to 30 cm to maintain bubbling conditions in the risers. The results indicate that increasing the cone angle up to 10 deg while maintaining the amount of bed materials constant leads to a reduction in pressure drop. The expansion of particles along the riser is observed to decrease with the increase in the cone angle up to 10 deg. The radial solid volume fraction profile transforms to a U shape from the W-type profile as the cone angle increases up to 10 deg. Correspondingly, the solid velocity is found to have an inverted U-type and W-shaped profile for the risers. The granular temperature is also found to increase with a decrease in the solid percentage at any location. The average bed temperature, interphase, and bed-to-wall heat transfer coefficient at a location of 10 cm axial height also increase with the cone angle increase up to 10 deg. As a result, the conical riser, when designed with a greater cone angle up to 10 deg, exhibits more efficiency in terms of heat transfer characteristics. The 3D simulation results are in strong concurrence with the experimental results in all investigations.

Heat transfer efficiency in gas–solid fluidized beds with flat and corrugated walls

Abstract Gas–solid fluidized bed reactors exhibit improved heat and mass transfer performance as compared to packed beds. Corrugated walls installed in narrow gas–solid bubbling fluidized bed (CWBFB) enclosures have been observed to decrease minimum bubbling velocity, reduce bubble size, improve gas distribution, provide stable operation, and minimize particle carryover or loss. Thorough analyses of the wall-to-bed heat transfer coefficient in flat- (FWBFB) and corrugated- (CWBFB) wall bubbling fluidized beds have been performed for a variety of operating conditions and geometric parameters. Fast-response self-adhesive heat flux probes and thermocouples were used to simultaneously measure the wall-to-bed heat flux, surface and bed temperatures, and were used to determine the heat transfer coefficient (HTC) at various axial and lateral locations. For a given set of parameters, a significant increase in HTC was observed at lower gas flow rates in CWBFB as compared to FWBFB. It was shown that CWBFB inventory required lower U mb (gas flow rate) as compared to FWBFB. Full 3-D transient Euler–Euler CFD simulations using the kinetic theory of granular flow were also performed, which confirmed the experimental results.

Novel packed-bed CFD model for pellet combustion: Validation and application to a stove

An advanced CFD (computational fluid dynamics) model has been developed to describe the combustion of wood pellets. It is based on a packed-bed approach and provides a realistic description of the combustion of pellets, resolved in 3D over the whole fuel bed. It considers the properties of the packed bed rather than the properties of the single fuel particles. Compared to a particle-based description, simulation times are shorter due to the reduced complexity. In the model, the local conditions in different parts of the fuel bed (temperatures, flow conditions, oxygen concentrations …) influence the various stages of the combustion process (drying, pyrolysis, charcoal gasification and combustion). The compaction of the fuel bed due to shrinkage of the fuel particles during conversion and the loss of the particle structure shortly before the conversion is complete are also included in the description. The formulation of the bed compaction, the radiation absorption model and the charcoal conversion reactions have been specifically tuned to the combustion of wood pellets. The detailed pyrolysis model includes not only the main components, but also higher hydrocarbon species and tars in a manner specific to the fuel considered. It has been combined with a chemical gas phase mechanism which is a reduction of a detailed mechanism developed for thermo-chemical biomass conversion. The model has been validated against measurement data from a lab-scale batch reactor, and then applied to simulate the fuel bed of a commercial pellet stove. The simulation results are in good agreement with the corresponding measurements.