Uniform Velocity Distribution Research Articles

Hydraulic and heat transfer characteristics in structured packed beds with methane steam reforming reaction for energy storage

Packed beds with methane steam reforming reaction (MSR) can be used as thermochemical storage devices. The packing configuration can have a significant effect on the flow distribution which will further influence the heat/mass transfer, dispersion and reaction characteristics. In the present paper, the hydraulic, dispersion, heat/mass transfer and the MSR reaction properties are investigated in packed beds with the simple-cubic (SC) packing, the body-centered cubic (BCC) packing and face-centered cubic (FCC) packing forms. ANSYS FLUENT integrated with user defined scalars (UDS) and user defined functions (UDF) are used to perform the simulations. Results show that firstly, the residence time distribution (RTD) can be used to evaluate the dispersion property in packed beds where a narrower RTD curve reflects a more uniform velocity distribution with less dispersion. Secondly it is found that, regions with backflow are not beneficial for heat/mass transfer and chemical reactions from two aspects. One is that the heat transfer between the main fluid and the particle surfaces is weak and low temperature of the particles will reduce the reaction rates. The other is that the poor species transport will prevent the reactant to flow into the reaction region and the products cannot be taken away by the main flow timely, which will have a negative effect on the reaction. Thirdly, it is revealed that, the dimensionless variation of the residence time distribution (DVRTD) can reflect the heat/mass transfer and reaction performances, where a smaller DVRTD corresponds to better heat/mass transfer and higher reaction rates.

Numerical simulation of detailed airflow distribution in newly developed photosynthesis chamber

Predictive numerical simulation of airflow uniformity in canopy plants could provide a suitable environment for plant growth. A numerical investigation of airflow in a photosynthesis chamber was conducted using the Computational Fluid Dynamics (CFD) model. This research-validated the numerical model with measurements performed in a bare bottom open chamber. The chamber has bottom openings with three exhaust fans on the roof. After model validation, airflow patterns and their uniformity were evaluated in different fan arrangements and doubled air volume rates. The obtained results showed that a more uniform airflow distribution was observed with increasing the fan’s air volume rate (0.0187, 0.0172, and 0.0177 m3s−1), particularly fan in the middle position and diagonally position inside the plant with coefficients of variation of 14.36%, 9.3% and 10%, respectively. Moreover, increasing the fan’s air volume rate and moving the fan positions to the middle and diagonally can significantly help produce uniform air velocity distribution inside the plant.

PENGARUH JUMLAH SALURAN MASUK UDARA TERHADAP DISTRIBUSI KECEPATAN UDARA PENGERING PADA ALAT PENGERING TIPE SWIRLING FLUIDIZED BED

Numerical and experimental studies have been carried out on the effect of inlet air number to the air velocity distribution in the drying chamber of the Swirling Fluidized Bed Dryer (SFBD). The main components of SFBD are a drying chamber, plenum chamber, center body, distributor, inlet air, heater, and blower. Numerical analysis was performed using the finite volume method on a three-dimensional model using computational fluid dynamic (CFD) software. An experimental study was carried out by measuring the air velocity at eight measurement points in the drying chamber. The results showed that using two inlet air produced a more uniform distribution of air velocity compared to one inlet air.

The Effect of External Geometry Factors on the Characteristics of Flow Profiles and Segregation in a Vertical Sinter Cooling Bed

Good particle flow patterns and uniform particle velocity distributions enhance the performance of heat transfer and smooth flow processes in vertical sinter cooling beds (VSCBs). The effect of three typical geometries, conical, curved and rectangular, on the performance of flow profiles and segregation in a VSCB is investigated comparatively and quantitatively based on the discrete element method (DEM). The evolution of flow profiles and particle segregation directly influence the evenly distributed sinter layers and the efficiency of heat exchange in VSCBs. In this research, a 3D packed bed model is established for the three geometry types to quantitatively and qualitatively investigate the influence of structural parameters on the evolution of flow patterns and segregation. The comparison of the effect of the three geometry types on the particle flow process showed that the curved geometry types greatly improve the performance of the flow pattern and size segregation. The height of the mass flow pattern for the curved geometry varies with the structural parameters by 1.5-fold that of the flow pattern for the other two geometry types. The curved geometry dramatically reduces the magnitude of the segregation index (SI) near the sidewall, while this magnitude fluctuates near 1.0 in the central flow passage of the VSCB.

Simulation of forced convective heat transfer in Kelvin cells with optimized skeletons

The optimization of pore structure for metal foam is a feasible method for reducing the pressure drop and improving the overall heat transfer performance, especially at high velocity. In this regard, the optimized Kelvin cell with elliptical skeletons with a cross-section ratio of the long axis to the short axis (b/a) of 1.0, 1.4, and 2.0 are considered to evaluate the effects of b/a on the pressure drop and the heat transfer coefficient (HTC). The results indicate that both the pressure drop and HTC decrease with an increase in b/a. However, the pressure drop reduces more significantly. For instance, as b/a increases from 1.0 to 2.0, the pressure drop and the volumetric HTC at 90 m/s decrease by 98.5% and 6.3%, respectively. Therefore, the value of the volumetric area goodness factor (which considers both the effect of heat transfer coefficient and pressure drop on the overall heat transfer performance) jv/fof the sample with b/a = 2.0 is 86.7% higher than the sample with b/a = 1.0 at 90 m/s. As the velocity increases, the effect of b/a on the overall heat transfer performance increases. Compared with the cylindrical skeleton velocity distribution and pressure drop, the elliptical skeletons lead to a more uniform velocity distribution and reduce pressure drop significantly; nevertheless, the temperature distributions barely change. Moreover, the elliptical skeleton increases the specific surface area. Therefore, the jv/fof the elliptical skeleton significantly improves. This study provides a new direction for the design of novel metal foams with excellent overall heat transfer performance for heat transfer devices.

Design optimization of a high-temperature fin-and-tube heat exchanger manifold – A case study

High-temperature fin-and-tube heat exchangers are widely used in many industries such as petrochemical industry, automotive, energy, and many others. The major advantage of those heat exchanger is a large heat transfer area within a compact shape. However, it is necessary to ensure uniform distribution of velocity in all the tubes. Failing that, it leads to large differences in mean temperature in the tubes. Consequently, excessive thermal stress occurs, that may cause the heat exchanger to break down. The small volume of collectors of the heat exchangers implicates possibility of improper flow condition inside the tubes, causing an unsuitable inner distribution of thermal and mechanical loads. This case study presents a design optimization of high temperature fin and tube heat exchanger manifold. The modified design of heat exchanger manifold is proposed. To optimize the manifold shape, the Particle Swarm Optimization and Continuous Genetic Algorithms are used. The design consists of tube used for a typical manifold, welded to the wedge, that enlarges the volume of the fluid and improves the flow distribution to tubular space of heat exchanger. The ANSYS CFX based CFD simulations are performed in order to determine the flow distribution in the tubes of fin-and-tube heat exchanger. The structural analysis is performed with ANSYS to determine the occurring thermal stresses. The new design of heat exchanger manifolds allowed one to reduce the tube wall temperature from 185 °C to 134 °C and compressible stresses in the construction of heat exchanger by nearly five times (from 105 MPa to 23 MPa), compared to the traditional fin-and-tube heat exchanger manifold.

Interfacial solitons propagating through a background shear current

Nonlinear internal solitary waves (ISWs) propagating through a two-layer stratified system, in the presence of a shear background current, are theoretically investigated. We implement a new version of the Miyata–Choi–Camassa model with mobile layers (MCC-ML), by considering an asymptotic, uniform velocity distribution for each layer. To investigate the typical geophysical flow conditions observed in the coastal oceans, we focused on theoretical predictions for a density ratio between the two layers set to 0.99. A rigid-lid at the top of the theoretical domain is considered since it represents a good approximation under the Boussinesq condition. By varying the ratio of the undisturbed layer thickness from 0.1 to 10, we considered ISWs with both positive and negative polarities, when the background fluid is at rest. For increasing velocity differences between the two layers, ISWs tend to broaden (steepen) when the background velocities assume the same (opposite) direction of those induced by the wave. We show that the polarity conversion can be easily predicted since it directly depends on both stratification features and ambient velocities. The shear current affects also the wave celerity: for increasing background shear, upstream-propagating solitons reach a critical condition for which the wave celerity is equal to zero. We found that this occurrence is associated with a well-defined value of the wave amplitude. For even larger background shears, the waves are observed to change their direction of propagation. By linear analysis, we finally obtained the limiting background shear current for which the MCC-ML model does not provide any solution.

Improvement of ammonia mixing in an industrial scale selective catalytic reduction De-NOx system of a coal-fired power plant: A numerical analysis

In this study, several methods are proposed to improve the denitrification (De-NOx) performance of the selective catalytic reduction (SCR) system. To illustrate how the proposed methods can be applied, the numerical analysis for the SCR system of an industrial-scale thermal power plant is conducted. There are two important indices affecting the performance of SCR system: one is the degree of mixing of the injected NH3 and the other is the uniformity of flow velocity before entering the catalyst layer. To improve the NH3 mixing, a static mixer in a location downstream of the ammonia injection grid (AIG) is generally installed. However, the uniformity of flow velocity distribution is severely aggravated as the mixer induces vortex. Therefore, it is suggested to place the mixer away from the catalyst layer to enhance the NH3 mixing and the uniformity of flow velocity together. The effect of the number of injection nozzles of the AIG on the SCR performance is also investigated. The number of nozzles of the ammonia injection grid can be reduced without affecting the uniformity in flow velocity. However, too much reducing the number of nozzles would result in improper ammonia mixing. Therefore, it is recommended to reduce the number of nozzles within a tolerable range of the NH3 mixing.

Analysis of inlet flow passage conditions and their influence on the performance of an axial-flow pump

The inlet flow conditions will directly affect impeller performance, which is of great concern to pump designers. In this study, based on two axial-flow pump devices, the influence of the evaluation criteria of inlet flow conditions and numerical grid scales on the accuracy of the simulation are investigated, the correctness of the numerical simulation are verified by experiments. The axial velocity distribution uniformity, axial velocity weighted average angle and hydraulic loss are calculated with three grid scales commonly used in engineering. The applicability of three turbulence models in engineering is verified. The influence of the uniformity of the axial velocity distribution on the impeller is quantitatively explored by installing a group of vortex generators. The results show that the simulation errors of the common formula of the axial velocity distribution uniformity for the elbow inlet passage and front-shaft tubular inlet passage are 16.3% and 14.6%, respectively; the modified formula limited the computational error to 0.2%, which reduced the axial velocity distribution uniformity dependence on the grid. The quantitative relationship between inlet flow conditions and pump performance was established, as the impeller efficiency decreased linearly with decreasing axial velocity distribution uniformity.

Design of non-uniformly distributed annular fins for a shell-and-tube thermal energy storage unit

The shell and tube latent heat thermal energy storage systems are widely recognized as one of the most effective ways to store and utilize solar energy due to their high energy density, constant storage/releasing temperature, structural feasibility and rational price. The application of fins is an effective method to enhance heat exchange through extend the heat transfer surface area. However, there existed strong uneven melting behavior including melting fraction, temperature uniformity in a vertical thermal energy storage unit. To improve its uniformity of melting fraction and temperature, this study designed various conditions for fin pitch and positions in a non-uniform pattern to reduce the inhomogeneity for the melting process. Numerical models were established and validated by the experimental measurement conducted in this study. The effects of fin pitch and position on the thermal performance of the melting process were quantified via analyzing the melting front evolution, temperature and velocity distribution, melting rate and temperature uniformity. Results demonstrated that a 62.8% and 34.4% reduction in full melting time and average temperature difference in the phase change material region was separately obtained for the thermal energy storage unit with non-uniformly distributed annular fins, compared to the original case with uniform fins. Besides, the melting rate uniformity was also improved by 84.7%. The non-uniform design on fin position and pitch give a perspective to enriching the design methods for practical application in engineering.

Precise Forming of Complex Magnesium Alloy Components Based on Finite Element Method and Quantitative Preforming Design

To solve the difficulties associated with integral precise extrusion forming for complex components of magnesium alloy, Deform-3D finite element simulation software was used to conduct numerical analyses for the forming process. Different extrusion schemes were used for determining the overall extrusion difficulties associated with special-shaped thin-walled complex components of magnesium alloy. The effects of a preformed blank on the filling and formation of folding defects in the forming process were studied. Based on the principle of minimum energy in the process of plastic deformation and the law of least resistance, volume predistribution and equal distance flow quantitative compensation methods were put forward. These methods were used to optimize the size and structure of the preform, forming a uniform material flow distribution. They also enabled one to manifest a uniform material flow velocity distribution, reduce the forming load, and avoid the creation of folding and filling defects during the final forming, for the realization of an efficient and reliable preform optimization design for magnesium alloy profiled complex components. The results obtained from a forming test, microstructure and mechanical properties test showed that the mechanical properties of the extruded component all met the service indexes. This design method can provide a theoretical reference for preforming design of profiled complex components.

Numerical Simulation of Internal Flow Field and Structural Optimum Design for the diesel engine SCR Catalytic Converter

A three-dimensional model and finite element model of the diesel engine SCR catalytic converter were established by using ANSYS software. The numerical simulation of the internal flow field for the diesel engine SCR catalytic converter was performed. The results show that the pressure loss of the catalytic converter is larger and the velocity distribution in the front of substrate is nonuniform. On this base, the effects of the diameters and lengths of the substrate on the catalytic converter’s internal flow field characteristic were studied. The results show that when the substrate’s length increases, the pressure loss increases and the velocity distribution uniformity in the front of substrate becomes worse. When the diameter of substrate increases, the pressure loss decreases and the velocity distribution uniformity in the front of substrate becomes better. Therefore, the structure of catalytic converter was optimized. Through the optimization of the structure, the velocity distribution in the front of substrate is more uniform. The passing capacity of substrate is increased. The conversion efficiency of catalytic converter is improved. The back pressure is reduced.

Computational fluid dynamics analysis of char conversion in Sandia's pressurized entrained flow reactor.

Design and analysis of practical reactors utilizing solid feedstocks rely on reaction rate parameters that are typically generated in lab-scale reactors. Evaluation of the reaction rate information often relies on assumptions of uniform temperature, velocity, and species distributions in the reactor, in lieu of detailed measurements that provide local information. This assumption might be a source of substantial error, since reactor designs can impose significant inhomogeneities, leading to data misinterpretation. Spatially resolved reactor simulations help understand the key processes within the reactor and support the identification of severe variations of temperature, velocity, and species distributions. In this work, Sandia's pressurized entrained flow reactor is modeled to identify inhomogeneities in the reaction zone. Tracer particles are tracked through the reactor to estimate the residence times and burnout ratio of introduced coal char particles in gasifying environments. The results reveal a complex mixing environment for the cool gas and particles entering the reactor along the centerline and the main high-speed hot gas reactor flow. Furthermore, the computational fluid dynamics (CFD) results show that flow asymmetries are introduced through the use of a horizontal gas pre-heating section that connects to the vertical reactor tube. Computed particle temperatures and residence times in the reactor differ substantially from the idealized plug flow conditions typically evoked in interpreting experimental measurements. Furthermore, experimental measurements and CFD analysis of heat flow through porous refractory insulation suggest that for the investigated conditions (1350 °C, <20 atm), the thermal conductivity of the insulation does not increase substantially with increasing pressure.

Effective velocity range of theoretical hypervelocity penetration models

This study attempted to improve the applicability of existing penetration models. First, the correlation of three existing hypervelocity penetration models of Birkhoff et al. (Birkhoff), Abrahamson and Goodier (A-G), and DiPersio, Simon and Merendion (DSM) was analyzed. The A-G model predicted the same penetration depths as Birkhoff and DSM models for jets of uniform and linear velocity distributions. Second, the accuracy of each penetration model was verified to check the differences in the experimental results. Finally, the penetration of jet elements with various velocity distributions was analyzed with finite element analysis. The effective velocity ranges of the analytical models were identified to be approximately 4 km/s for uniform velocity jets, and above 3 km/s for linear velocity profiled jets. This study is expected to provide efficiency to the study of hypervelocity penetration.

Effect of the Gas-Vortex Stabilization Method on the Plasma Cutting Quality

Various methods of gas-vortex stabilization in plasma torches for metal cutting are investigated. The influence of an angle of plasma-forming gas injection into the nozzle on kinematic characteristics of the plasma jet is shown. The increase in the radial component of the velocity at the output from the swirler makes it possible to increase the uniformity of velocity distribution and the kinetic properties of jet in the zone of influence on the cut metal. For the cutting of the thin sheet metals, it is advisable to use the technology of "narrow-jet plasma".

Three-dimensional simulation of wind tunnel diffuser to study the effects of different divergence angles on speed uniform distribution, pressure in outlet, and eddy flows formation in the corners

Despite the widespread use of diffusers in various industries, there is no comprehensive research so far. This is expressive on how the flow diffuses throughout the diffuser geometry, the amount of non-uniformity of speed distribution at the outlet, and the rate of eddy flows at the corners. The present study simulated a three-dimensional diffuser with a square geometry at different divergence angles in order to obtain a better understanding of the flow diffusion across the geometry, velocity distribution at the outlet, and reverse flow. Moreover, the aspect ratio and the Reynolds number were considered constant in all cases. The turbulence model was used along with a high-resolution discretization and a root-mean-square convergence criterion to solve this problem. The speed was substantially reduced to nearly zero at corners of the diffuser with a square cross section. Reverse flow and eddy currents were also observed in the same regions. By increasing the divergence angle, this effect was further intensified, and in addition to the corners, flow separation and eddy currents were formed in the regions close to the wall due to the adverse pressure gradient. The maximum velocity and flow distribution at the outlet cross section was located in the central region, which was intensified by increasing the divergence angle. The deviation of the average velocity at the diffuser outlet with a divergence angle of 5°, with a completely uniform velocity distribution at the outlet, was observed to be 15.3%. This deviation grew with an increase in divergence angle and reached 128.9% at a divergence angle of 30°.

Reverse optimization algorithm of velocity uniformity in microchannels based on a simplified resistance network model

Velocity distribution plays an important role on the performances of microchannel reactors. In order to obtain an ideal velocity distribution among microchannels, a reverse optimization algorithm is proposed based on a simplified resistance network model, in which the relationship between the pressure drop and resistance is established, and the relationship between the flow rate and the channel velocity is also provided. The reverse optimization algorithm was developed based on the assumption of equal velocity values in all microchannels. Some structural parameters are set as unknown quantities and then optimized. In this work, microchannel width and manifold shapes are selected for optimization by using the reverse optimization algorithm. The results indicate that the non-equal microchannel width (NEMW) model can achieve more uniform velocity distribution than the corresponding equal microchannel width (EMW) model. The non-linear manifold structure (NLMS) can also present relatively uniform velocity distribution, but relatively poor velocity uniformity of microchannels compared to the NEMW model. Considering the requirement of practical application, a mass calculation algorithm is proposed to optimize the structural parameters in a certain range. However, the computing time of this algorithm is too large.

Three-dimensional simulation of solid oxide fuel cell with metal foam as cathode flow distributor

In this study, the use of metal foam as a flow distributor at cathode is evaluated numerically by a comprehensive three-dimensional solid oxide fuel cell (SOFC) model. The results show that the adoption of metal foam improves the power density by 13.74% at current density of 5000 A m−2 in comparison with conventional straight channel design. It is found that electronic overpotential, oxygen concentration and reaction rates distribute more uniformly without the restriction of ribs. The effects of cathode thickness on the two different flow distributors are compared. Compared with conventional straight channel, the metal foam is found to be more suitable as a distributor for anode supported SOFC with thin cathode gas diffusion layer. Moreover, when metal foam is applied to the fuel cell with a larger reaction area, a more uniform velocity distribution and a lower temperature distribution can be achieved. It is also found that an appropriate permeability coefficient should offer a reasonable pressure drop, which is beneficial for the fuel cell system performance improvement.

Simulation Research on Double-Side Grinding Method with Variable Position Rotation

In this paper, a double-side grinding method with variable position rotation is proposed, which changes the motion mode of the workpiece in the fixed distance rotation grinding from the grinding principle, and realizes the irregular planar motion of the workpiece in the isolation dics at the same time as the rotation. That is, the movement trajectory of the workpiece rotation center relative to the grinding tool is the curve of the change. Different grinding principles fundamentally overcome some defects of traditional planar grinding methods. A prototype model of variable position rotation grinding is established, and the feasibility of the new method is verified by simulation experiments. The simulation data also further proves the time-varying nature of this grinding method in improving the grinding trajectory, the uniformity of relative velocity distribution of grinding and the superiority and diversity of the mechanical wear of the grinding mechanism and the ability to adjust the grinding trajectory.

3D CPFD Simulation of Circulating Fluidized Bed Downer and Riser: Comparisons of Flow Structure and Solids Back-Mixing Behavior

The difference of gas-solids flow between a circulating fluidized bed (CFB) downer and riser was compared by computational particle fluid dynamics (CPFD) approach. The comparison was conducted under the same operating conditions. Simulation results demonstrated that the downer showed much more uniform solids holdup and solids velocity distribution compared with the riser. The radial non-uniformity index of the solids holdup in the riser was over 10 times than that in the downer. In addition, small clusters tended to be present in the whole downer, large clusters tended to be present near the wall in riser. It was found that the different cluster behavior is important in determining the different flow behaviors of solids in the downer and riser. While the particle residence time increased evenly along the downward direction in the downer, particles with both shorter and longer residence time were predicted in the whole riser. The nearly vertical cumulative residence time distribution (RTD) curve in the downer further demonstrated that the solids back-mixing in the downer is limited while that in the riser is severe. Solids turbulence in the downer was much weaker compared with the riser, while the large clusters formation near the wall in the riser would hinder solids transportation ability.