Flow Field Model Research Articles

A Streamline Dot Flow Field Design for Proton Exchange Membrane Fuel Cell

Flow field optimization plays a vital role in improving the output performance of proton exchange membrane fuel cells (PEMFCs), especially at high current density. A streamline dot flow field is proposed in this study to enhance the mass transfer of reactant gas and compared it with parallel, rectangular dot, and rhombic dot flow fields. The influence of the tail length of streamline dot block on the performance of fuel cells is further analyzed. The results show that the uniform distributions of pressure and oxygen molar fraction are significantly increased in dot flow fields compared with parallel flow field. In dot flow fields, vortices produced behind the dot blocks could increase the flow resistance and pressure drop. Among six models, the dot flow field with 2 mm tail has the most uniform current density distribution at PEM-CL cross section, and its index of uniformity current density (IUCD) is the smallest with the value of 0.012991. And the parallel flow field model has the largest value of IUCD. Compared with parallel flow field, the current density of streamline dot flow field model with 2 mm tail increased by 4.9%.

PIGNN-CFD: A physics-informed graph neural network for rapid predicting urban wind field defined on unstructured mesh

Urban wind field plays an important role in quantitative assessment of urban environment. Compared to field measurement and wind tunnel experiment, Computational Fluid Dynamics (CFD) simulation having the advantages of low cost, repeatability and reliable precision is becoming a common scheme to model flow field of one fixed urban scenario, but still faces the problems of time-consuming computation and lack of scalability for practical engineering application. This paper proposes PIGNN-CFD, a novel physics-informed graph neural network for rapid predicting urban wind field based on irregular unstructured mesh data of CFD simulation. Specifically, a CFD model employing the unsteady Reynolds-Averaged Navier-Stokes (RANS) equations with the standard k-ε turbulence model, is constructed and then numerically solved by OpenFOAM to simulate urban wind field defined on unstructured mesh. After being validated by publicly available wind tunnel test data provided by the Architectural Institute of Japan (AIJ), the proposed CFD model is employed to build the training and test sample sets of urban wind fields by simulating the wind blowing through various randomly generated small-scale urban scenes. A novel physics-informed graph neural network, both approximating the training data and automatically satisfying the RANS equations, is designed and trained to perform wind field inference on unstructured mesh graph, and then scaled up to predict wind fields of arbitrary large-scale urban scenes. The predicted wind field results of two urban environments at different scales show that the well-generalized PIGNN-CFD model runs 1–2 orders of magnitude faster than the CFD model on which it is trained, while obtaining the consistent computational accuracy.

Numerical simulation of the spatial and temporal distributions of the downwash airflow and spray field of a co-axial eight-rotor plant protection UAV in hover

The downwash airflow of a multi-rotor unmanned aerial vehicle (UAV) in hover can improve the penetration rate of droplets in the canopy of fruit trees, but the uncertain distribution of the downwash airflow can also lead to the generation of uneven distribution of droplets. With a co-axial eight-rotor plant protection UAV as the research object, the computational fluid dynamics (CFD) method is adopted in this study to establish the coupled flow field model of the downwash airflow and the two-phase flow model of airflow-droplets in hover. The spatial and temporal distribution law of the downwash airflow is analyzed, and the impact of the motion law of the droplets in the spray field is investigated. The results show that the downwash airflow shifts from an unsteady state to a steady state over time. Various wind vortices such as wake vortex, ground diffusion airflow, and vortex ring are generated during the evolution process, with a “negative velocity channel” of downwash airflow velocity identified in the central region under the rotors. The farther the airflow is from the rotor, the larger the area field of wake vortex airflow. Consequently, there is a gradual coupling of the left and right adjacent wake vortices from mutual independence, with the “spiral effect” of the downwash wind field offset by the mutual coupling of the upper and lower rotors of the UAV. When the installation position of the nozzle falls within the below-rotor region, the distribution of droplet deposition becomes more penetrative than the central axis installation position of the nozzle. It indicates that the below-rotor installation position of the nozzle can prevent droplets from entering into the coupling region of the trailing vortex, which reduces droplet drift. According to the field test, the measurement and simulation results of the downwash air velocity are consistent, with the minimum and maximum relative error reaching 4% and 29%.

Analysis and Optimization of Heat Dissipation Module in the Motor Grader Engine Compartment

Abstract To solve the problem of heat reflow in the fan rotation center of the forced convection cooling system in the engine compartment, this study established a calculation model of the external flow field of the engine compartment’s cooling module. It then used the CFD numerical simulation method to calculate and analyze the heat flow characteristics of the existing radiator and compared these with the experimental results. Accordingly, the region where the heat reflux occurred and the reason for the heat reflow were found. The existing heat dissipation scheme was recalculated by using the secondary heat dissipation model, and an optimized and improved scheme was proposed to introduce a deflector cone structure to eliminate heat reflow. The research results showed that the secondary heat dissipation model could more accurately describe the heat reflow problem of the engine compartment, the heat flow organization of the improved structure was more reasonable, and the temperature distribution was more uniform. Moreover, the theoretical heat dissipation effect of the improved structure was more than 10% higher than that of the existing structure.

Large Eddy Simulation of Gasoline Sprays in a Lagrangian–Eulerian Framework Using the High-Order Spectral Element Method

Abstract Predicting the spray evolution using simulations requires accurate modeling of the turbulent gas-phase flow field. In this study, the high-order spectral-element method (SEM), implemented in the code Nek5000, was used to provide highly resolved solutions to the turbulent flow field. Spray modeling capabilities were implemented into the Nek5000 code. The spray is modeled in a Lagrangian–Eulerian (LE) framework, where the liquid is represented by discrete parcels of droplets. The method for coupling liquid and gas in the context of SEM is described, which allows for very fine meshes to be used without affecting the stability of the solution. Large-eddy simulations (LES) of the eight-hole ECN Spray G gasoline injector were conducted. Numerical results are compared against experimental data for liquid penetration, droplet size and gas velocity. The morphology of the multiplume spray is compared against experimental data. The effect of different spray injection inputs is analyzed. It was found that using a plume direction of 33 deg and an injection cone angle of 30 deg produced the best results overall. This work shows the applicability of SEM for spray modeling applications, where use of a high-order flow solver can help us understand the multiplume spray aerodynamics and how it leads to plume collapse under certain conditions. Results also highlight the need for tuning spray input parameters in the LE framework, even when high-fidelity gas flow solutions are possible.

Intelligent computing architecture for the analysis of Williamson Magneto hydrodynamic fluid model and heat transfer through a permeable stretching surface

The Williamson fluid model's flow field properties have drawn researchers' attention in recent years. The objective of this research is to analyze the Williamson Magneto Hydrodynamic Fluid Model's unsteady flow field properties by applying the algorithm of Bayesian regularization with an artificial neural network of backpropagation. A magnetic field interacts with the nanosized particles, and the stretched porous surface causes the fluid to flow. Under similarity transformation, the non-linear partial differential equations of the Williamson fluid model are turned into ordinary differential equations by varying the thermophoresis parameter, the mass transfer parameter, the unsteadiness parameter and the Lewis number. A set of data is generated in Matlab for various Williamson fluid model scenarios using the Adam numerical technique for the proposed Bayesian regularization algorithm. The solution and error analysis plots are evaluated using the reference dataset. Furthermore, the performance of the algorithm of Bayesian regularization is clarified by mean square error results, error analysis plots, regression, and error histograms. The solution of the Williamson fluid model is exploited by the validation, training, and testing procedures. The validity of the obtained results is confirmed by the mean square error up to the level of E-13.

Geothermal dynamic constraints of groundwater source heat pump system in shallow aquifers

Due to the temperature of shallow aquifers being affected by atmospheric temperature, groundwater source heat pumps (GWSHPs) become unstable and the operation efficiency of GWSHP is constrained. In the study, the coupling numerical simulation model of the groundwater flow field and temperature field is established based on the continuous monitoring results in an actual experimental site, and the water and thermal migration of shallow aquifer is simulated under the influence of the atmospheric environment. The influence of the dynamic change in ground temperature is analyzed on a GWSHP. The results indicated that the temperature of the shallow aquifer is affected by the external temperature, and the recharge temperature in the summer cooling period was 33°C, and that in the winter heating period was 6°C in the actual site, to avoid the occurrence of thermal penetration when there is a gap between the actual situation and the design situation, the single cooler can balance the insufficient cooling capacity in summer under the most unfavorable situation. The research results can also provide a reference for the development and utilization of geothermal energy resources in shallow aquifers.

Flow Field Modeling and Simulation of High-Speed Gear Pump Considering Optimal Radial and End Clearance

Flow Field Modeling and Simulation of High-Speed Gear Pump Considering Optimal Radial and End Clearance

A Study of the coarse droplet formation from the liquid film in steam turbines

For all the deployment of the non-fossil energy sources the steam turbine remains the most important energy transforming device. In the current situation when the daily operation of the turbines is not predictable due to electricity supply from the photovoltaic and the wind power plants, the turbines work in the non-design conditions, the danger of the blade’s erosion caused by the coarse droplets is assumed. The purpose of this paper is to suggest the possible formation process of the coarse droplets in the steam turbine. The current studies were focused mostly on the measurement or identification of the coarse droplets in the steam turbine, but the explanation of the archived results was not sufficient. An analysis of the droplets formed in the wind tunnel indicates, for the majority of the measurements, the part of the formed droplets is bigger than the critical Weber number limit. Similar, but less detail, experience was achieved for measurement in the steam turbine. This paper introduces simple analysis of the optically measured coarse droplets size distribution with the modelled flow field behind the blade fitted by measured velocity field. A detailed hot wire measurement was performed in the vicinity of the trailing edge where the liquid film is disrupted to the coarse droplets.

Modelling of Flow Characteristics of Main Rotor with Elliptical Root Section in Hover Mode

The modelling of a helicopter main rotor flow field in hover regime is being considered. The aerodynamic and aeroacoustics characteristics of two rotor models with different blade root geometries are compared. The first rotor has the shape of NACA0012 air foil in the root section of the blades. The second rotor model has an elliptic air foil in the root of the rotor blade. The amplitude and temporal characteristics of the aeroacoustics emission in near-field are investigated using Computational Fluid Dynamics (CFD).

Study on Unblocking and Permeability Enhancement Technology with Rotary Water Jet for Low Recharge Efficiency Wells in Sandstone Geothermal Reservoirs

In China, sandstone geothermal reservoirs are large in scale and widely distributed, but their exploitation is hindered by low recharge efficiency. In this paper, an unblocking and permeability enhancement technology using a rotary water jet for low recharge efficiency wells in sandstone geothermal reservoirs is proposed to solve this problem. This paper presents a series of studies about the proposed technology, including experiments, simulation and field application. Firstly, an experiment was carried out to verify the scale removal effect of a high-pressure water jet on the inner wall of the screen tube and its impact on sandstone. Secondly, the numerical models of the rotary jet flow field in the wellbore were established by ANSYS Fluent to study the influence of parameters. Finally, based on the simulation and experiment results, a rotary jet tool applicable to unblocking and descaling low-efficiency wells was designed, and a field application for low-efficiency wells in sandstone thermal reservoirs was conducted. The study results show that the unblocking and permeability enhancement technology using a rotary water jet is effective in removing the blockages and improving the permeability near the well. In conclusion, the presented technology can solve the problem of low efficiency during the reinjection of cooled thermal waters back into sandstone geothermal reservoirs and has great effectiveness in field application.

Analysis of pressure oscillation and structural parameters on the performance of deflector jet servo valve

The performance of deflector jet servo-valve is highly affected by the flow field characteristics in the pilot stage of the deflector jet servo-valve. This research used the large eddy simulation method to study the pressure pulsation characteristics of the front stage flow field of DJSV, which has a high significance for the reasonable design of DJSV and the suppression of self-excited oscillation. The geometric model of the three-dimensional flow field of the jet pilot stage of the servo valve deflection plate was established. LES method was used to analyze the influence of the geometric parameters of the jet outlet, amplifier and receiving port on the flow field distribution characteristics of the pre-stage. The results show that pressure oscillation is highly affected by the inlet pressure. Structure optimization is a good solution for decreasing pressure oscillation in deflector jet servo valve.

Heat transfer and entropy analysis in squeezing flow of hybrid nanofluid (Au-CuO/NaAlg) with D-F (Darcy-Forchheimer) and C-C (Cattaneo-Christov) heat flux

Hybrid nanofluid bounded between two infinite parallel surfaces in presence of D-F (Darcy-Forchheimer) porous medium is explored in this analysis. Heat transport is explored by C-C (Cattaneo-Christov) heat flux and thermal radiation. Flow is originated due to squeezing of upper and stretching infinite surfaces. Entropy and Bejan number are part of this study. Lower surface is subject to convective boundary condition. Mathematical modeling of flow field is done under aforementioned assumptions. By transformation technique, PDEs (flow and heat transport expressions) are changed into ODEs. Solutions are obtained via ND-solve (shooting technique). Quantities of our interests (flow, entropy, velocity, Bejan number, skin friction) are evaluated under involved variables. Velocity of both hybrid nanomaterial and nanofluid boosts against higher squeezing and porosity parameters and Forchheimer number. Dominated behavior of velocity in case of hybrid nanomaterial is observed when compared with nanofluid. Fluid temperature intensifies by squeezing parameter and Eckert number while reduction occurs for radiation parameter. Variation in temperature is more for hybrid nanomaterial. Entropy production rate reduces for higher squeezing parameter while it increases through porosity parameter. Bejan number is an increasing function of squeezing parameter while it reduced for porosity parameter. Skin friction is controlled by squeezing parameter and Forchheimer number.

Free vibration and stability of a fully submerged pipe aspirating water: An experiment and new physical insights

Dynamic stability due to internal axial flow is a considerable problem for a pipe conveying fluid such as deep seawater intaking for an Ocean Thermal Energy Conversion (OTEC) plant. However, there has been much ambiguity about its dynamics, and this raises a question about whether such an aspirating pipe submerged in water flutters or not. Therefore, the objective of this paper is to provide an experiment to take a new look at the dynamics of pipe aspirating fluid (water). The experimental apparatus is constructed to eliminate expected disturbances, and we measure free damped vibrations of a submerged 4 m length pipe with internal flow. As a result, we observe the nonlinear and non-planar behavior, however, the pipe converges to the zero point and remains stable at a maximum velocity of 1.66 m/s. Subsequently, we review existing theoretical models, and present a comparison with the results from the tank experiment. In addition, we provide a new model of the inlet flow field, which plays an important role on stability, considering the flow separation and jet formed inside of the pipe entrance. This equation is solved by FEM for time integration and eigenvalue analysis, and the results seem to reproduce the experimental natural period and amplitude of the free vibration with internal flow. The model also suggests that an aspirating pipe submerged in water does not flutter up to the maximum flow velocity attainable in the experiment.

Study on Interior Aerodynamic Noise Characteristics of the High-Speed Maglev Train in the Low Vacuum Tube

As the next-generation high-speed transportation system, the low vacuum tube high-speed maglev system combines the tube with a certain degree of vacuum with the high-speed maglev train, which can realize high-speed operation under low aerodynamic resistance and noise mode. In order to study the interior aerodynamic noise characteristics of the high-speed maglev train in the low vacuum tube, a computational model of the external flow field of the high-speed maglev train in a low vacuum tube was established, and the computational model of the interior aerodynamic noise of the high-speed maglev train was established using the statistical energy analysis method; then the interior aerodynamic noise characteristics of the high-speed maglev train in the low vacuum tube were studied. The research results show that in the low vacuum tube, the distribution of the interior aerodynamic noise of the high-speed maglev train shows the characteristics of large head car and tail car and small middle car, and the aerodynamic noise on the top of the car is smaller than that on the floor. With the increase in frequency, the sound pressure level of the interior aerodynamic noise of the high-speed maglev train has the tendency of increasing first and then decreasing, and the main energy of the interior aerodynamic noise is distributed in the range of 200–1000 Hz. From the perspective of the total sound pressure level of the interior aerodynamic noise, the interior aerodynamic noise of the tail car is the greatest, followed by the head car, and the interior aerodynamic noise of the middle car is the smallest. As the direction of the travel of the maglev train will change, the optimization design of the interior aerodynamic noise of the head and tail cars should be emphasized.

Numerical Study on Transverse Jet Mixing Enhanced by High Frequency Energy Deposition

Supersonic incoming flow has a large momentum, which makes it difficult for transverse jets to have a large penetration depth due to the strong compression of the incoming flow. This impacts the mixing efficiency of the jet in the supersonic combustor. This paper proposes a method to improve the mixing efficiency of a rectangular flow field model using pulsed energy deposition, which is verified numerically. In the simulations, the Navier–Stokes equations with an energy source are solved to simulate the effects of energy deposition with various distributions on the fuel mixture. The results show that the energy deposition increases the turbulent kinetic energy, which enlarges the scale of the flow vortex and improves the fuel mixing performance. The energy deposition is distributed upstream and significantly improves the mixing performance. Energy deposition can improve the penetration depth of fuel, which is more significant when the energy deposition is distributed downstream of the jet orifice. The energy deposition also slightly reduces the total pressure recovery coefficient. In general, an energy deposition that is distributed upstream of the jet has the best effect on the mixing efficiency.

Learning time-aware multi-phase flow fields in coal-supercritical water fluidized bed reactor with deep learning

The supercritical water fluidized bed (SCWFB) reactor has been utilized to achieve clean and efficient conversion of coal as well as the generation of hydrogen. The complicated time-aware multi-phase flow fields inside the reactor however are usually obtained via time-consuming experiments or transient numerical simulation. To this end, we propose a data-driven deep spatio-temporal sequence model, named ReactorNet, for intelligent modeling and efficient prediction of the complex time-aware multi-phase flow fields inside the SCWFB reactor. Particularly, the model employs the U-Net architecture wherein the coupled convolution module and BiConvLSTM module in a single block are stacked to simultaneously extract the multi-scale spatial and temporal features of multi-phase flow fields in the reactor. The numerical results showcase that the ReactorNet could successfully and simultaneously learn the evolution of multi-phase flow fields in the reactor under different temperature conditions, thus performing accurate multi-step-ahead prediction. The flow fields predicted by ReactorNet are not only highly consistent with the simulated results, but also hundreds of times faster than the traditional numerical simulation. In addition, the ReactorNet can carry out both reasonable spatial and temporal extrapolation and even long-term rollout prediction according to the learned evolution principle of flow fields, thus showcasing remarkable generalization ability. It is the first attempt to apply deep learning to the modeling and predicting of time-aware multi-phase flow fields inside the challenging SCWFB reactor.

Hydrodynamics Simulations and Analyses of a Fluid Lubricated Screw-Nut Pair

A new method is proposed to solve the hydrodynamics performances of a fluid lubricated screw-nut pair using FLUENT. Before the simulations, the Computational Fluid Dynamics (CFD) model of the gap flow field is built, based on some approximation rules. During the simulations, the dynamic mesh technology is employed to realize the real-time update of the grids of the computational domain. For a given velocity perturbation, the stiffness and damping coefficients of the system are solved using the finite difference method, and the influences of the perturbations on the system are compared among different ranges. With the fluid–solid interaction and the real-time restriction of the restrictors considered, the system is solved under different loading conditions. A more accurate solution method for the dynamic stiffness and damping coefficients is provided, and the dynamics characteristics of the system after loading are analyzed. On this basis, a qualitative and quantitative comparison is carried out between the method based on the simplified Reynolds equation and the proposed method in this paper, showing the latter superiorities in illustrating the field. A general understanding of the dynamics properties under different loading conditions of the system is obtained through this research, providing a basis for the precision control of the system in the future research.

Modeling of CSTR flow field for Agaricus bisporus residue fermentation based on CFD numerical simulation.

Agaricus bisporus production gets a lot of residues, which could be fermented by a continuous stirred tank reactor (CSTR). This research was conducted to study the characteristics of the multiphase flow field in the reactor and its influence on the efficiency of biogas production in the CSTR fermentation process of Agaricus bisporus residue by using CFD numerical simulation technique. The aim is to reveal the relationship between the reactor operating conditions, flow field characteristics, and biogas production efficiency at the micro-level. We compared the results of different turbulence models by evaluating the power quotients and flow quotients with the experimental results to derive the most suitable flow field model inside the reactor for the Agaricus bisporus residues. The results showed that, under the condition that the number of grids does not affect the simulation results, and considering the model accuracy and efficiency, the numerical method can be chosen as the multiple reference frame (MRF) method of the second-order upwind discrete scheme with the realizable k - ε model. In this way, we can make use of edible mushroom residue as a substrate for resource utilization and provide basic data and theoretical basis for the design and scale-up with anaerobic fermentation to biogas reactor.

Numerical investigation into characteristics of far-field aerodynamic noise radiated by inter-coach space of high-speed maglev trains

Self-sustain oscillation leads to aerodynamic noise at inter-coach space. It can be seen from existing literature that the aerodynamic noise at inter-coach space has already become an important branch in the domain of vibration and noise reduction of high-speed wheel-rail trains. However, to the best of our knowledge, approaches focusing on the identical problem that does exist in the windshield area of high-speed maglev train have not yet been published. In this paper, a computational model is established for the inter-coach space of high-speed maglev train running at the speed of 600 km·h−1. The model includes two half-mid-train with one outer full windshield connected. On the basis of Realizable k-ε model and Spalart-Allmaras DDES model, respectively, the steady and unsteady flow field are initially solved, and the pressure fluctuation on rigid surfaces, the vortex shedding process and the distribution of sound source power are then visualized to make clear the genesis mechanism of aerodynamic noise in the windshield area. With penetrable integration surfaces constructed, the far-field sound radiation is obtained from the Ffowc Williams-Hawkings (FW-H) equation, and the contribution rate of the quadrupole noise sources to the total aerodynamic noise is then calculated as a comparison to the previous conclusions that aerodynamic noise at inter-coach space of high-speed trains was mainly dipole. Of great significance, it is concluded in this paper that at the speed of 600 km·h−1, the quadrupole sound source contributes about 35.6% to the total sound energy in the far field, which is lower than the percentage using the entire train model at the same speed level but still cannot be ignored. The results in this paper are able to provide guidance for future researches on vibration and noise reduction of high-speed maglev train.