Inlet Gas Velocity Research Articles

Investigation of convective heat transfer in a cold flow pebble bed nuclear reactor using advanced pebble probe heat transfer sensor

The aim of this study was to investigate the local heat transfer coefficients between the pebbles and the coolant gas in a PBR using an advanced measurement technique that consists of a heated pebble probe, a micro-foil heat flux sensor mounted flush on the surface of the heated pebble probe, and a thermocouple in the center of the bed void in front the sensor. In this work, the local heat transfer coefficients were derived at various axial levels, radial and angular locations and at different superficial inlet gas velocities that cover both transitional and turbulent flow conditions. The local heat transfer coefficients were found to be 22-32%, 34-39%, and 18-20% higher near the wall at superficial inlet gas velocities of 0.3,1.2, and 2m/s, respectively. This is due to higher volumetric flow at the wall where larger void in the pebble bed exists compared to the center region of the bed where the flow of the gas in the packed bed follows the least resistance path. Large deviations were obtained between the experimental overall heat transfer coefficients in the bed and the predictions of seven correlations that were selected from the literature. Furthermore, these correlations cannot predict the local heat transfer coefficients inside the PBR. This necessitates the development of new correlations for the prediction of the local heat transfer coefficients using the data obtained in this work. A pseudo-3D correlation was developed and was found to provide predictions that are in good agreement with the experimental values for the conditions used, with an averaged absolute relative error (AARE) of 3.33% at high Reynolds numbers for our operating and design conditions.

Parametric study on hematite pellet direct reduction by hydrogen

This study investigates the intrinsic kinetics of hematite reduction to iron using hydrogen through the utilization of a computational fluid dynamics (CFD) coupled with a modified unreacted shrinking core model. The new mathematical model is developed to analyze the reduction process of a real pellet, taking into account the variation in porosity at different reaction rates. The impact of temperature, particle size, gas inlet velocity, and porosity on hydrogen-based direct reduction were investigated. The study reveals that size affects reduction degree at temperatures below 973 K, while it is independent of size at 1273 K. The optimal inlet velocity is 0.28 m∙s−1, and the time for complete reduction is over 3500 s and 800 s at 973 K and 1273 K, respectively. The numerical results align with experimental results. The study provides a solid foundation for dynamic modeling of reduction process but further work is needed for a realistic model.

Analysis of Influencing Factors on Silicon Epitaxial Growth in Horizontal Single‐Wafer Reactor through Orthogonal Test

AbstractSilicon epitaxy is a crucial process used in semiconductor manufacturing to deposit high‐quality films of silicon. This technique is widely used in the production of integrated circuits, as it enables the fabrication of intricate electronic structures with enhanced performance characteristics. This study conducts numerical simulations on a chemical vapor deposition (CVD) reactor to explore the impact of process parameters on the growth rate and non‐uniformity of silicon. The investigation encompasses both the gas and surface reactions of the trichlorosilane‐hydrogen (TCS‐H2) system. The distributions of gas flow velocity, temperature, and main components are systematically studied by varying the process parameters, including susceptor temperature, inlet gas velocity, susceptor rotating speed, inlet gas temperature, upper wall temperature, and TCS mole fraction. Furthermore, the orthogonal test method is introduced to assess the effect of all parameters on the growth non‐uniformity. The results reveal that the inlet gas velocity and susceptor temperature have a significant influence on the growth rate and non‐uniformity. The silicon growth rate is primarily influenced by the TCS mole fraction, whereas the rotation speed of the substrate primarily influences the growth non‐uniformity of growth. Finally, the optimal scheme is proposed as valuable guidance for enhancing silicon chemical vapor deposition processes in industrial applications.

Analysis of the Multiphase Flow With Condensation in the Two-Phase Ejector Condenser Using Computational Fluid Dynamics Modeling

Abstract The liquid-driven two-phase ejector condenser is the object of the numerical investigation. The spray-ejector condenser is one of the critical components of the developed gas power plant with negative CO2 emission. The task of the ejector is to entrain exhaust gas and condense steam contained in it. Computational fluid dynamics (CFD) modeling allows analyzing complex phenomena and predicting the influence of a wide range of operating parameters on the local structure of the multiphase flow with condensation. The geometrical model of the ejector was designed to provide efficient steam condensation and generate sub-pressure region at the gas inlet. The 2D, axisymmetric CFD model was created using simcenter star ccm+ software. The multiphase mixture model was used to take into account two-phase flow. Turbulent flow was computed using k–ω SST model. Direct contact condensation of steam was calculated using two different approaches: the Spalding/evaporation model and the thermally-driven boiling/condensation model. The influence of various gas inlet velocities and the presence of CO2 on the operation of the ejector condenser were investigated based on scalar fields and charts representing changes in the most important variables along the flow path. The condensation is the most intense in the suction chamber. The boiling/condensation model predicts lower suction pressure and higher condensation effectivity than Spalding/evaporation model. The CO2 considerably affects the pressure and temperature distributions and reduces the condensation rate.

Simulation study on effect of nozzle geometry on flameless combustion of non-preheated methane gas

In this paper, the effects of different burner configurations on the characteristics of flameless combustion were evaluated by comparing the temperature field, NOx, OH, and H2CO at different burner inlet velocity and angles through a combination of experimental and numerical simulations. The results show that increasing the burner inlet angle and gas velocity is imperative in achieving the flameless combustion, increasing the re-circulation rate in the furnace, making the temperature distribution in the furnace uniform, and reducing the emission of NOx at the end of the furnace. During the simulation of flameless combustion, it was found that OH radicals and H2CO radicals were well correlated with the reaction exothermic zone, and the Reynolds number was positively correlated with the re-circulation rate in the furnace. With the increase of Reynolds number, the entrainment rate of flue gas increases, and the combustion state is closer to flameless combustion. When re-circulation rate Kv >2, combustion becomes flameless. Through the summary analysis of the data, it can be found that there is a critical Reynolds number for the burner to achieve flameless combustion, and flameless combustion occurs only when the Reynolds number is greater than 1.0?104.

Effect of inlet conditions on the thermal insulation performance of marine gas turbine exhaust systems

A marine gas turbine insulation solution was designed to reduce exhaust system temperatures. The effects of cooling gas temperature (45?C, 55?C, 65?C), cooling air velocity (2 m/s - 10 m/s), and a range of classic aerodynamic conditions ranging on the thermal insulation performance of the gas turbine exhaust system were investigated using numerical simulations. The results indicate that the use of aerogel insulation material effectively reduces the average temperature of the exterior volute casing to 71?C from 315?C under rated turbine conditions. The exterior volute casing temperature increases with higher cooling gas inlet temperatures but decreases with increasing cooling gas inlet velocities. Additionally, alterations in the aerodynamic conditions at the gas inlet will induce changes in the thermal insulation performance of the exhaust system, and excessive circumferential flow velocities can cause localized overheating in the exhaust volute casing.

Numerical simulation study on the drag reduction characteristics of grooves-microbubbles coupling surfaces

Surface drag reduction technologies can significantly reduce the resistance during ship navigation, enhancing speed, efficiency and adaptability under various operating conditions. This paper uses numerical simulation technology to analyze the drag reduction characteristics of grooved and grooves-microbubbles coupling surface, focusing on the effects of groove width, gas flow rate, and liquid flow velocity on the drag reduction performance. The research results indicate that the grooved surface is suitable for full surface drag reduction at velocity below 3 m/s with a maximum drag reduction rate of 4.02%. Microbubbles can greatly improve the drag reduction effect of the grooved surface, and the drag reduction effect of the coupling surface gradually increases with the gas flow rate increases. The maximum drag reduction rate can reach 89.86% at the gas inlet velocity of 1 m/s. The liquid flow velocity has a significant impact on the drag reduction. In both the groove model and the coupling model, the drag reduction rate initially rises and then declines with the liquid flow velocity increases.

Investigation on gas-liquid two-phase frictional pressure drop in pipeline riser

The pressure drop characteristics of gas-liquid two-phase flow in pipeline-risers are important to the design optimization of marine risers lifting oil and gas. In this study, experimental investigation on single-phase and air-water two-phase flow pressure drop characteristics in vertical and S-shaped risers have been carried out in a long-distance pipeline with the gas superficial velocities ranging from 0.03 to 6.2 m/s and the liquid superficial velocities from 0.02 to 1.3 m/s. The test loop with 50 mm i. d. Consists of a horizontal pipeline with 114 m in length, followed by a 16 m downward inclined section, and ended at a riser. The downward section is adjustable in inclination with various angle of −2°, −3°, −4° and −5° with respect to horizontal. A vertical riser and an S-shaped flexible riser were used in the experiment. The effects of flow and geometry parameters on two-phase frictional pressure drop multiplier are studied. For flow parameters, the two-phase frictional pressure drop multiplier increases with increasing the inlet gas and liquid superficial velocity. For geometric parameters, the two-phase frictional pressure drop multiplier in S-shaped riser is larger compared to that in straight vertical riser. The existing correlations reported in the literature are evaluated. A single-phase friction factor correlation and a two-phase frictional pressure drop multiplier correlation for pipeline-riser system are developed. Comparison of the predicted value and the measured results shows good agreements.

Study of fluidized bed freeboard for effect of Geldart A particles shape using particle image velocimetry (PIV)

A study was carried out using Particle Image Velocimetry (PIV) to reveal velocity vector flow fields in the freeboard region of a Geldart A fluidized bed. For the first time the effect of particle shape, was studied for three powder types, namely China clay (platelet-like), Diatomite (spherical) and Quartz (irregular). A bench-scale fluidized bed set-up of perspex glass was used for direct PIV imagining over a substantial inlet gas velocity range. Interestingly the freeboard was not found to be highly turbulent as has been reported in the case of Geldart B particles. Rather a transition regime was observed which was dominated by laminar streamlines and punctuated by short-lived eddies with life span even less than 400 μs. Bed expansion was significantly affected by particle shape. This was explained by an ‘effective cluster size’ hypothesis for the three powder types. The ‘uniformity’ of the velocity field was also quantified for each powder.

Study on motion characteristics of filamentous biomass particles in a cyclone separator

Cyclone separators are important equipment for biomass transportation and separation. At present, the separation mechanism of particles with special shapes is rarely explored. A mutable filamentous particle was established to simulate the separation process of biomass materials. Firstly, comparing the simulateded results with the experimental images. Further comparison of the simulation results with the experimental airflow tangential velocity to verify the calculation model. This study explores the separation characteristics and forces acting on filamentous biomass particles through computational fluid dynamics (CFD) and discrete element method (DEM). The distribution characteristics of airflow are also described in detail. The research results indicate that after 0.8 s, the filamentous biomass particles are separated stably in the cyclone separator and form a particle vortex line on the conical wall. The higher the inlet gas velocity, the lower the separation efficiency of filamentous biomass particles. The inlet gas velocity is 4.9 m/s, and the separation efficiency of filamentous biomass particles reaches 98.7%. The total force acting on the filamentous biomass particles gradually decreases after a sharply increase at 0.2 s, and it no longer changes after 0.5 s. The airflow vortex in a cyclone separator is composed of an internal vortex and an external vortex. The tangential velocity of the internal vortex varies approximately linearly. The internal vortex of the cyclone separator has a negative pressure area, which can lead to a decrease in the separation efficiency of filamentous biomass particles. These conclusions provide valuable theoretical guidance for the process and separation of biomass materials.

CFD-DEM study on the synergistic effect of coke consumption and particle dynamics in the blast furnace raceway

In this study, the gas-solid flow process in the blast furnace raceway is numerically simulated using coupled computational fluid dynamics and the discrete element method (CFD-DEM). The coke reaction kinetics data are imported into the DEM model to reproduce the consumption process of each coke particle. The effects of inlet gas velocity and angle on the morphology of the raceway, coke consumption rate, coke bed temperature, and particle size distribution in the blast process are systematically investigated and analyzed. The results show that the consumption of coke particles promotes the formation of raceways during the blast process. At the same time, a coke mixture layer is produced at the edge of the raceway. The higher the inlet gas velocity, the thicker the coke mixture layer in the middle and upper parts of the raceway region, and the larger the proportion of small particles in the coke mixture layer. The effect of the inlet gas angle on the raceway region is less than the inlet gas velocity. However, with the increase in the inlet gas angle, the high-temperature region of the coke bed extends downward gradually, which is conducive to activating the hearth.

Multiscale computational fluid dynamics modelling of spatial ALD on porous li-ion battery electrodes

The self-limiting surface reaction characteristic of atomic layer deposition (ALD) makes it ideal for the surface modification of electrode materials for lithium-ion batteries (LIBs). Spatial ALD shows promise as a scalable method for the coating on pre-fabricated electrode sheets. As a strong-coupled multiscale process, various process conditions and microstructure parameters have great influences on the macroscale fluid dynamics and the pore-scale diffusion–reaction process, thus affecting the coating efficiency. This study presents a multiscale numerical model that combines computational fluid dynamics (CFD) with multilevel pore-scale diffusion–reaction kinetics to explore the spatial ALD process on porous LIB electrodes. The dynamic mesh method is utilized to simulate electrode movement. The considerable active surface-to-volume ratio of the porous structure limits the precursor infiltration depth due to the low diffusion rate and inadequate precursor supply. As the electrode velocity increases, an asymmetric distribution of precursor concentration under the injector is observed with a rapid decrease. Elevating both the precursor concentration and inlet gas velocity augments the coating depth by enhancing the supply of the precursor. The experimental data aligns well with our numerical results, verifying the accuracy of the multiscale CFD model. Our observations reveal that a relatively lower operating pressure, around 0.1 atm, compared to 0.01 atm and 1 atm, optimizes the deposition rate along the electrode depth during the half-ALD cycle, especially when the pore size is larger. Electrode porosity of about 0.4 notably improves coating uniformity by elevating the precursor diffusion rate. Predictions show that with a substrate velocity of 0.2 m/s, the coating depth on an electrode having higher porosity at the top compared to the bottom via atmospheric spatial ALD could reach a depth of 38 μm with a precursor utilization of 78 %.

Experimental investigation of three-dimensional gas-solid flow in CFB cyclone separator using electrical capacitance volume tomography

The anisotropic three-dimensional flow field inside cyclone separators makes it challenging to capture the gas-solid flow characteristics, and the internal behavior of the cyclone separator remains not fully understood. The gas-solid flow characteristics inside a cyclone separator of the circulating fluidized bed (CFB) are investigated using electrical capacitance volume tomography (ECVT). The solid fraction, average velocity, and frequency characteristics of particles were extracted from ECVT images. The experimental results show that the number of turns for the dust strands spirals gradually increases as the inlet gas velocity Uf increases. The axial velocity of the particles in the cylindrical part of the cyclone separator is always greater than in the conical part. In addition, the difference in inlet gas velocity and particle size results in different particle flow characteristics in the CFB riser. It leads to the differences in the particle circulation rate and flow pattern in the cyclone separator.

Study on hydro-thermal coupling heat transfer performance of flue gas-water heat exchanger

The finned tube heat exchanger is the core component of the flue gas waste heat utilization system. In order to optimize and control the heat transfer efficiency of the heat exchanger in real time under smart heating conditions, we use the entransy dissipation thermal resistance as a kernel index to assess its heat exchange performance, and Trnsys simulation software is employed to figure out the influence of heat exchanger operating parameters (flue gas inlet velocity) and fin structure parameters (fin height, fin pitch) on heat exchanger heat transfer characteristics. The results show that in a reasonable range, increasing the flue gas inlet velocity can decrease the irreversible loss of heat transfer, obviously improving the heat transfer effect. The pitch and height of the fin also have a large impact on the performance of heat transfer. To significantly promote the performance of the heat exchanger, it is useful to reduce the pitch of the fin and increase the fin height within a reasonable range.

CFD-DEM simulation of high density particles fluidization behaviors in 3D conical spouted beds

The spouted bed has been used in the coating process of high-density nuclear fuel particle. The particle fluidization behaviors in pseudo-2D and 3D spouted beds were simulated and validated. The effects of four independent variables (cone angle, particle density, inlet gas velocity, and particle loading) on particle fluidization behaviors in the 3D spouted bed were investigated systematically. The cone angle effect on fluidization mechanism was studied quantitatively first time. A new fluidization quality index was proposed based on the particle entrainment principle, and an extreme value was obtained when the cone angle was 60°, considered to be the optimum value for the 3D conical spouted bed. It was indicated the gas–solid contact efficiency can be kept up if the gas velocity was proportional to ρp0.65 and Np0.78 when the particle density or loading was increased. These results will be useful for geometry and operation parameters design of the 3D conical spouted bed and helpful for developing the fluidization mechanism of high-density particles.

Parametric investigation of premixed gas inlet conditions effects on flow and combustion characteristics

The need to examine combustion devices' combustion characteristics and reduce emissions has gained importance with the implementation of the Energy Related Products Directive in Europe. Therefore, this study aims to investigate flow and combustion characteristics, including emissions in the combustion domain of a condensing boiler. Different premixed gas input velocities and temperatures in the premixed gas burner are considered in the numerical study validated with literature. Effects of inlet velocity and temperature to flow and combustion characteristics are examined parametrically. Variations of velocity and temperature along the combustion domain and CO and NO emissions are obtained at the outlet region.Variations in premixed gas inlet velocity directly influence the residence time and average temperature of the flue gases, therefore, CO and NO emissions. The velocity profiles are effective for flame structure and emission production. In terms of emissions, a substantial escalation occurs in outlet CO emissions with the increase in gas velocity. An increase in NO emissions is observed with the rise in average temperatures and a decrease in NO emissions is observed by reducing residence time. It is concluded that crucial flow and combustion characteristics can be estimated with the use of the current approach to contribute to design improvements and pollutant emission reduction.

Evaluation of frictional pressure drop correlations for air-water and air-oil two-phase flow in pipeline-riser system

Accurate prediction of the frictional pressure drop is important for the design and operation of subsea oil and gas transporting system considering the length of the pipeline. The applicability of the correlations to pipeline-riser flow needs evaluation since the flow condition in pipeline-riser is quite different from the original data where they were derived from. In the present study, a comprehensive evaluation of 24 prevailing correlation in predicting frictional pressure drop is carried out based on experimentally measured data of air-water and air-oil two-phase flows in pipeline-riser. Experiments are performed in a system having different configuration of pipeline-riser with the inclination of the downcomer varied from −2° to −5° to investigated the effect of the elbow on the frictional pressure drop in the riser. The inlet gas velocity ranges from 0.03 to 6.2 m/s, and liquid velocity varies from 0.02 to 1.3 m/s. A total of 885 experimental data points including 782 on air-water flows and 103 on air-oil flows are obtained and used to access the prediction ability of the correlations. Comparison of the predicted results with the measured data indicate that a majority of the investigated correlations under-predict the pressure drop on severe slugging. The result of this study highlights the requirement of new method considering the effect of pipe layout on the frictional pressure drop.

Modeling of water film generation in up-inclined low liquid loading natural gas pipeline elbows and its implication on internal corrosion occurrence

Condensed water often accompanies natural gas in the transmission process. The water forms thin film when parameters such as temperature and pressure change. This accelerates pipeline corrosion and makes the pipeline wall become thinner. Severe corrosion can lead to pipeline rupture, endangering the pipeline safety and leading to economic losses. The distribution of a water film plays a significant role in the corrosion process, So the work of predicting the distribution of a water film and flow characteristics becomes a key priority when analyzing corrosion in a pipe elbow. In this study, Eulerian, RNG (re-normalization group) k-ε and Eulerian wall film models are employed to establish a pipe elbow model that predicts the locations of water phase accumulation. Combining with a limiting diffusion current density, this model can be used to predict the potential corrosion locations and extent. The proposed model is validated by using velocity and water film distributions and it shows substantial agreement with previous models and results in the literature. From the simulation outcomes, a water film is mainly distributed at bottom while the thickness of the water film increases significantly at the pipe elbow. At the elbow bottom the inlet gas velocity increases, this shifts the high corrosion rate zone from elbow rear to front. Corrosion at the elbow front is more serious regardless of the changes in the inlet liquid velocity, elbow angle or water holdup. Meanwhile, when the gas velocity or elbow angle is large, the water film gradually spreads from the elbow bottom to the side and top parts which are in a high corrosion rate zone and more dangerous. Therefore, weak positions where pipeline elbow corrosion failures may occur can be quickly and conveniently computed and determined by the proposed model.

Study of the gas-solid flow characteristics of ice particles in helically coiled tubes

Efficient drilling operations in polar regions are crucial given their unique climate and surface conditions. To improve drilling efficiency, a novel technology utilizing helically coiled tubes (HCT) for transporting ice particles has been proposed. This study focuses on the gas-solid flow characteristics of ice particles within the HCT employing the Eulerian-Eulerian models of Ansys-Fluent 2022 R2. Numerical simulations were validated against experimental data, showing good agreement with a maximum error of merely 11.27%. Three distinct flow regions perpendicular to the flow direction were observed, influenced by centrifugal force and secondary flow. The inlet gas velocity, tube inner diameter, coil diameter, particle diameter, and penetration rate all affected the ice particle transport ratio (ITR) and the pressure drop in the HCT. Notably, increasing the tube inner diameter from 30 mm to 75 mm led to a remarkable 398.7% rise in the ITR, while particle diameter changes minimally affected ITR.

Investigating the impact of modeling assumptions and closure models on the steady-state prediction of solar-driven methane steam reforming in a porous reactor

In this work, the effect of different modeling assumptions and closure models — frequently considered in the literature of solar thermochemical reactors and closely-related areas — is investigated. The application of different modeling assumptions and closure models may strongly affect the results accuracy and compromise a meaningful comparison across literature works. The following is herein investigated: (i) the role of upstream and downstream fluid regions to the two-phase reactor region; (ii) relevance of species and gas-phase thermal diffusion mechanisms; (iii) reaction heat accounted for in gas- or solid-phase energy balance equations; (iv) local thermal equilibrium (LTE) vs. local thermal non-equilibrium (LTNE) models; (v) model dimension (one-dimensional vs. two-dimensional axisymmetric models); (vi) local volumetric convection heat transfer correlations; and (vii) effective solid thermal conductivity correlations. The relevance of this work extends well beyond the current application (methane steam reforming in a volumetric solar reactor), since similar models and assumptions have also been widely applied for predicting the performance of volumetric solar absorbers and dry reforming solar reactors. The results show that an upstream fluid region should be considered while applying inlet first-type boundary conditions and diffusion transport in the corresponding governing equations to ensure full-conservation and simultaneously to account for developing profiles upstream the reactor inlet section. For the operating conditions considered, species diffusion and gas-phase heat conduction are particularly relevant at the reactor centerline but with a negligible integral effect. A significant difference in the reactor performance is observed while accounting for the reaction heat from surface reactions in the solid- or gas-phase energy balances — for the lowest inlet gas velocity herein considered, the thermochemical efficiency (methane conversion) is approximately equal to 70.8% (76.3%) and 77.3% (84.7%) assigning the reaction heat to the gas- and solid-phase energy balances, respectively. LTE model results are strikingly different from the results obtained with the LTNE model considering the reaction heat accounted for in the gas-phase energy balance but not significantly different from the results computed with the LTNE model with the reaction heat assigned to the solid-phase energy balance. One-dimensional reactor modeling provided with an average concentrated solar heat flux value results in a similar average performance as that given by a two-dimensional model but fails to predict the high temperatures observed at reactor centerline — for the lowest inlet gas velocity, the difference between the maximum solid temperatures predicted by one- and two-dimensional models is about 340K.