Changes In Velocity Field Research Articles

Mixed convection heat transfer characteristics of nanofluid under magnetic field based on discrete unified gas kinetics scheme

In this paper, we proposed a discrete unified gas kinetic scheme (DUGKS) into the mixed convection issue coupled in a magnetic field and verified the accuracy by comparing with a exist literature. The flow and heat transfer characteristics of Fe3O4-water nanofluid in the mixed convection are investigated using the DUGKS model. The investigation explores the influence of Hartmann number (0.01 ≤ Ha ≤ 100), Rayleigh number (2 × 103 ≤ Ra ≤ 6 × 105), Richardson number (0.5 ≤ Ri ≤ 10), and nanoparticle fraction (0.01≤ϕ ≤ 0.05) on mixed convection heat transfer and flow characteristics. The results reveal that the critical point for the influence of the Hartmann number on the average Nusselt number is 1. When Ha < 1, the variation of Ha has little effect on Nu, but when Ha > 1, the influence of Ha becomes significant. When Ri = 1, the heat convection between the cold wall and the hot wall is strongest, and the Nusselt number at the cold wall reaches its maximum. The changes in velocity and temperature fields under all conditions are effectively captured by the DUGKS method. Therefore, the proposed method exhibits good mesh adaptability and accurately simulates the mixed convection heat transfer problem.

The polymer melt swelling behavior in micro-extrusion process for preparation of continuous carbon fiber reinforced PP prepreg filament

The micro-extrusion process for polymer melt has been increasingly used for the production of high value-added prepreg filaments with applications in the fields of fused deposition modeling (FDM). In this study, the effect of characteristic scale, defined as the gap of die land in an annular micro-extrusion die, on the extrudate swelling behaviour of viscoelastic melt is analyzed through numerical simulations and micro-extrusion experiments. The results show that the swelling behaviour displays an obvious dependence on the characteristic scale. An increase in the characteristic scale reduces the swell ratio and retards the process to reach the equilibrium state. In contrast, a decrease in the characteristic scale results in a larger magnitude of change in velocity field and faster relaxation development of stress field. The location of the maximum velocity layer for the laminar flow gradually deviates from the geometric center of channel toward the wall of mandrel with the increase in the characteristic scale. It is imperative to consider the complicated swelling behavior and remarkable viscoelastic effect simultaneously in micro-extrusion process.

Statistical analysis of equatorial electrojet responses to the transient changes of solar wind conditions

Introduction: Prior case studies have indicated that changes in solar wind conditions have a significant impact on equatorial ionospheric electrodynamics. However, there have been limited statistical studies on this topic, impairing our understanding of the coupling between solar wind, magnetosphere, and equatorial ionosphere electrodynamics.Methods: In this study, we conducted a superposed epoch analysis of long-term data from the South America equatorial electrojet (EEJ) spanning from 2001 to 2021 examining the responses of the equatorial ionospheric electric field to step-like changes in solar wind velocity, density, dynamic pressure, and interplanetary magnetic field (IMF) Bz.Result: Our study shows that step-like changes in solar wind velocity, density, and dynamic pressure can trigger changes in EEJ within ∼20–40 min. EEJ exhibits the highest sensitivity to variations in solar wind velocity while being relatively less sensitive to changes in dynamic pressure. Furthermore, the response of EEJ shows greater responsiveness to northward IMF Bz compared to southward IMF Bz.Discussion: Our work provides statistical evidence of how changes in solar wind can lead to changes in low-latitude ionospheric EEJ. We inferred that the changes in solar wind conditions cause magnetospheric deformation and changes in magnetic reconnection rates, leading to the fluctuations of the ionospheric electric field and the resultant EEJ variations.

Magnetic Discontinuities in the Inner Heliosphere: Do Intermediate Shocks Exist?

Magnetic discontinuities are fundamental structures in space and laboratory plasmas where the changes in magnetic and velocity fields are constrained by Rankine–Hugoniot relations. Due to the absence of precise measurements for particles, some issues therein are hardly investigated. The nature of discontinuities driven by the magnetohydrodynamics (MHD) turbulence, and the intermediate shock are two puzzles to be solved. The MHD turbulence generates numerous discontinuities with both small normal magnetic fields and nearly constant magnetic field magnitudes in statistics. By utilizing the data from the Parker Solar Probe, we identify among the turbulence-driven discontinuities two components that exhibit diverse statistical characteristics of the plasma density, and reveal that these discontinuities comprise 80.2% rotational and 19.8% tangential discontinuities. Then, we note a special class of discontinuities within 0.35 au that have jump conditions similar to that of the rotational discontinuity and the shock simultaneously, including (1) positively correlated jumps in the plasma density and temperature, (2) a small change in the magnetic field magnitude, and (3) opposite tangential magnetic fields on two sides. These features conform to the theoretical intermediate shock, which previous studies have found to not practically exist due to the breakdown of the evolutionary condition. By the conservation law of the mass flux across a boundary, we calculate their propagation speeds and find three intermediate shock candidates with super-Alfvénic upstream and sub-Alfvénic downstream flows. This work can improve our understanding of plasma intermittencies and suggests reassessing conclusions based on ideal MHD Rankine–Hugoniot relations.

Comparative thermal analysis of Nickel and Tantalum based hybrid nanofluid using constant proportional Caputo and Atangana–Baleanu operators with time-controlled condition

The main purpose of this research work is to examine how the immersion of nickel and tantalum nanoparticles in water affects the flow and thermal properties of water, with the aim of anticipating potential enhancements. The investigation pertains to the flow behavior of a hybrid nanofluid over an extensively elongated vertical surface under the influence of a condition that varies with time. The phenomenon of heat transfer transpires due to the combined effects of natural convection and thermal radiation. An in-depth comprehension of flow patterns and heat transfer over a vertical surface is essential for optimizing heat exchange processes, designing efficient cooling systems, and enhancing overall energy efficiency. It also helps in climate modeling, weather prediction, and the improvement of the performance of solar thermal systems. The basic model is generalized using Atangana–Baleanu (AB) and constant proportional Caputo (CPC) operators. This generalization leads to the development of two different fractional settings for which separate mathematical analyses are performed. With the help of some new unit-independent quantities, exact solutions for both fractional frameworks are computed using the Laplace transform. For the elucidation of changes in velocity and thermal fields due to different physical effects, several graphical and tabular representations are produced. The outcomes illustrate that the incorporation of uniform concentrations of nickel and tantalum nanoparticles in water leads to raising its thermal efficacy up to 40.39%. The nanoparticles consisting of lamina shapes are observed to produce the maximum amelioration in thermal features. The CPC model based profiles of flow and temperature fields are lower than those procured via the AB model. As a result of nanoparticles’ intrinsic characteristics, the insignificant heat transfer capability of water is greatly amplified, leading to a notable boost in its thermal performance. Thus, the examined hybrid nanofluid can be used as a viable alternative to water in various engineering applications.

Simulation and Experimental Study on the Low NOx Combustion of 35 Kw Biomass Boiler with Air Staging

In the present research, numerical simulations and combustion experiments were carried out using a 35 kW biomass low nitrogen combustion furnace. For this purpose, the boiler structure and fluid area in the furnace were modeled. Subsequently, the grid was divided and the parameters such as speed and pressure were set using Gambit. Then, the model was introduced into Fluent in order to simulate the combustion process and for flow field output and analysis. By analyzing the distribution and changes in temperature field, velocity field, and NOx concentration field under different air supply schemes, and performing low nitrogen combustion experiments, the influence of different air supply schemes on fuel combustion and NOx emission was determined. Through the mutual verification of simulation and experimental research, the optimal air distribution scheme was obtained. The results showed that: without the air staged combustion, combustible gas and air were not properly mixed. As a result, the loss of mechanical incomplete combustion occurred. Under air-staged combustion, the addition of the secondary air reduced the “chimney flow” phenomenon in the furnace to a certain extent. The combustible gas and air were mixed more adequately and the reaction rate and efficiency significantly increased. In addition, with the introduction of the secondary air, the air and the flue gas in the furnace were more efficiently mixed. Moreover, the temperature in the upper combustion area of the furnace decreased by 300–500 K, which slowed down the rate of nitrogen oxides production. This effect was intensified when the proportion of the secondary air increased. The status of the combustion process and changes in the concentration of NOx in the flue gas were compared and analyzed under various working conditions. Data indicated that the optimal air distribution scheme was that with 70% of the primary air and 30% of the secondary air. In this case, the NOx emissions were 27% lower than those without the air staged combustion.

The velocity field characteristics of high‐viscosity fluids falling film flow down clamped channels

AbstractThe complexity of falling film flow has been studied in many industrial applications. In this work, the velocity field of high‐viscosity fluids falling film flow down clamped channels was investigated numerically and experimentally. The results show that the numerical simulation results are consistent with the experimental results, and the characteristics of the velocity field are related to the fluid properties, operating conditions, and structure of the clamped channels. When the fluid viscosity is greater than or equal to 10 Pa ⋅ s, the type of velocity field changes into I shape, U shape, and V shape. While the fluid viscosity drops to 0.89 Pa ⋅ s, the viscous force cannot resist the inertial force and gravity, resulting in a cardioid velocity field. By adjusting the structure of the clamped channels and operating conditions, the tension of the liquid film can be changed, and the velocity distribution of the liquid film can be manipulated. Significantly, under the fluctuating curtain flow, the liquid film coalesces and breaks frequently, which enlarges the surface area of the liquid film and strengthens the surface renewal frequency. Hence, this form of falling film flow can be applied to process intensification of high‐viscosity materials.

Modulation effect on rotor-stator interaction subjected to fluctuating rotation speed in a centrifugal pump

In the paper, the modulation effect of rotor-stator interaction subjected to fluctuating rotation speed is investigated numerically. A quasi-bivariate variable mode decomposition method for analyzing non-stationary flow fields is proposed for the first time. Different from the previous studies, this paper emphasizes the local features of the transient flow field at fluctuating rotation speed. The results show that the impact of fluctuating rotation speed on the impeller force is limited, but it significantly impacts the pressure pulsation at fluctuating frequency. The correlation between amplitude and fluctuation ratio is positive. Meanwhile, the fluctuating rotation speed also induces sidebands centered on the blade passing frequency and its harmonics. The periodic changes in rotational speed lead to frequency and amplitude modulation of pressure pulsations. The magnitude of the instantaneous frequency and amplitude increases as the fluctuation ratio grows. The change in pressure field is well synchronized with the rotational speed, while the change in velocity field lags behind the rotational speed. The variation of pressure field with rotational speed is mainly attributed to two aspects. One is the flow structure at the fluctuating frequency, and the other is the instantaneous component of the flow structure at blade passing frequency.

INVESTIGATION ON IMPACTS OF ELLIPTICAL DUCTS ON MACROSCOPIC SPRAY CHARACTERISTICS OF DUCTED FUEL INJECTION

To study the influence of ducts other than circular shapes on spray characteristics of ducted fuel injection (DFI), experimental and simulation methods were used to study the impacts of elliptical ducts on DFI macroscopic spray characteristics. Two elliptical ducts, small and large, were used, with a circular duct for comparison. The small elliptical duct had the same hole cross-sectional area as the circular duct, and the large elliptical duct had a larger cross-sectional area. The DFI spray configuration with the elliptical duct promoted the axial dispersion and weakened the radial dispersion of spray with respect to free spray. Different spray-duct interactions caused differences in spray characteristics, manifested in changes in velocity field and pressure field. The spray velocity field of the circular duct had the best effect on promoting spray dispersion, followed by the small elliptical duct, and finally by the large elliptical duct compared to that of free spray. Furthermore, the pumping effect caused by the pressure differences between inside and outside the duct promoted the thorough mixing of spray and ambient gas inside the duct. From the radial velocity of ambient gas flowing to spray around the duct inlet, the pumping effect of the circular duct was the strongest, followed by the small elliptical duct and finally by the large elliptical duct.

Gas Temperature Distribution and Fluctuation in a Lab-Scale Fire Whirl

Fire whirls cause an increase in fire damage. This study clarified the unsteady behavior of fire whirls, considering that instantaneous changes in the temperature and flame shape of fire whirls can affect the damage to the surrounding area. Numerical simulations of a lab-scale flame that simulates a fire whirl were performed to investigate the changes in gas temperature and velocity fields under various fuel inflow velocities. The flow field was obtained by solving a continuity equation and a three-dimensional Navier-Stokes equation, and the turbulence was resolved using a large eddy simulation. A chemical equilibrium partially premixed combustion model was used, and radiation effects were considered. The time-averaged gas temperature distribution along the burner central axis revealed that the gas temperature decreased monotonically from upstream to downstream. The time-averaged velocity distribution along the burner central axis showed that the velocity decreased as one moved downstream, but the decrease was uneven. The time variation of the gas temperature demonstrated that the higher the fuel inflow velocity, especially near the burner, the greater the gas temperature flutter. Furthermore, the larger the fuel inflow velocity, the larger the flame swell and wobble. The results showed that the fuel inflow velocity affected temperature fluctuation and flame undulating movement.

Onset of nonlinearity in oscillatory flow through a hexagonal sphere pack

We simulated laminar flow through a hexagonal sphere pack driven by a sinusoidal volume force using direct numerical simulation. We vary two independent parameters, the Hagen and Womersley numbers, representing the amplitude and frequency of the forcing. First, we determine for which regions in the parameter space nonlinear effects have to be considered. We judge the presence of nonlinear effects from the departure of the superficial velocity and kinetic energy from a linear behaviour as well as from the presence of higher harmonics in the discrete Fourier transform of the velocity field. We discuss the asymptotic behaviour of the onset of nonlinearity in the limits of low and high Womersley number, and we delineate approximately the parameter region that can be described using the linear theory. Second, we document the changes of instantaneous velocity fields with Hagen and Womersley numbers. We show that the onset of nonlinearity is accompanied by a loss of fore–aft symmetry of the flow, and subsequently, we employ the deviation from this symmetry to quantify the strength of nonlinear effects in the instantaneous velocity fields. Based on this analysis, we demonstrate that for higher Womersley numbers, the strongest nonlinear effects occur during the deceleration of the superficial velocity; consequently, the development of the nonlinearity is not in phase with the superficial velocity. Finally, we describe the leading-order nonlinear effects in the frequency domain and the interaction among the nonlinear Fourier modes that leads to a temporal variation in the strength of nonlinear effects.

Progress in numerical simulation of casting process

The numerical simulation of casting process can calculate casting filling process, solidification process, and obtain change and coupling information of temperature field, velocity field, pressure field, stress field and microstructure. At present, numerical simulation of casting process has been quite mature at the macro level, and is developing towards the direction of micro level. The coupling and integration of different fields between temperature, flow, stress, and microstructure, is the shape of things for numerical simulation research of casting process. This paper reviews and summarizes the research history and current situation of numerical simulation of casting process. The progress in numerical simulation from five aspects of casting solidification, casting filling, stress field, microstructure, commercial software, is presented.

Numerical investigation of a novel smoldering-driven reactor for plastic waste pyrolysis

This study proposed a novel smoldering-driven reactor for plastic waste (PW) pyrolysis. The heat from self-sustaining smoldering was used to pyrolyze PW to produce oil and gas, a sustainable waste-to-energy approach. A multidimensional numerical model was developed to verify the feasibility and evaluate the performance of the proposed reactor. It was the first attempt to study the smoldering-driven pyrolysis of PW, including the melting process. The findings revealed that char smoldering could provide a stable propagating heat source for PW pyrolysis. The char concentration and air inlet velocity in the smoldering chamber could regulate the PW pyrolysis duration and product yields by controlling the peak temperature and heat propagation velocity. The pyrolysis product yields were controlled by PW content due to the change of velocity field in the pyrolysis chamber. The energy efficiency of PW pyrolysis was also evaluated under critical parameters.

Flow Deflection between Guide Vanes in a Pump Turbine Operating in Pump Mode with a Slight Opening

During the startup and shutdown processes of a reversible-pump turbine (RPT) working in pump mode, abnormal sounds and vibrations usually occur in the distributor when the guide vanes (GVs) are at a slight opening (max opening of about 6%). The objective of this paper is to apply a three-dimensional numerical CFD method to study the unsteady flow behavior in the guide vane region of a pump turbine operating in pump mode. The dynamic meshing technique is introduced to simulate the startup and shutdown processes, and it is shown to be critical in accurately capturing the details of the flow pattern variations. In addition, the RNG k-epsilon two-equation turbulence model is applied and the governing equations are discretized with the finite volume method. Moreover, the boundary conditions are set through the calculation of the transient process of the power station. The results show that the main flow between the GVs is deflected during the startup and shutdown processes. In the shutdown process, the deflection occurs when the guide vane opening (GVO) is between 1.99 and 5.32 degrees, on average. In the startup process, the deflection occurs when the GVO is between 2.83 and 4.11 degrees, on average. In these processes, the velocity field and pressure field change dramatically. Simultaneously, the hydraulic torque (HT) on the GVs has a sharp change. The abrupt change in the HT leads to vibrations and abnormal sounds.

Bolus formation from fission of nonlinear internal waves over a mild slope

AbstractBreaking nonlinear internal waves (NLIWs) of depression on boundary slopes drives mixing in the coastal ocean. Of the different breaker types, fission is most commonly observed on mild slopes of continental margins. However, fission on mild slopes has rarely been investigated in the laboratory owing to limitations on flume length. In the present work, a train of NLIWs of depression is generated in an 18.2 m wave flume and shoaled upon a mild uniform slope. During fission, each NLIW of depression scatters into one or two NLIWs of elevation, which transforms into a bolus at the bolus birth point, where shear instability occurs through the pycnocline. The bolus propagates upslope, decreasing in size until it degenerates by shear and lobe-cleft instability, while losing volume to a return flow along the bed. The location of the bolus birth point, bolus propagation length scale, initial size and the number of boluses from each incident wave are parameterized from the wave half-width and the wave Froude number associated with the incident NLIW. These are compared with the characteristics of boluses generated by other breaking mechanisms on steeper slopes. Some bolus characteristics (height to length ratio, change in size and velocity field) are similar for boluses generated by fission, collapsing sinusoidal waves and internal solitary waves of elevation; however, the number of boluses, their birth point and initial height differ. The boluses formed by fission have more initial energy and no reflection. Further research is required to better quantify bolus-driven mixing on continental margins.

Experimental study of flow dynamics of a sweeping jet in a slot channel

In recent years, fluidic oscillators have been actively applied as devices for flow control in the field of aero and hydrodynamics. This study aims to investigate the structure of a flow of sweeping jet ejected from a fluidic oscillator into a confined area – slot channel. Dynamics of sweeping jet flow are investigated using the PIV method with high temporal resolution. The effect of the Re number on the sweeping jet oscillation frequency was studied in the range from 1 500 to 8 000. Linear frequency dependence on Re number was obtained. Bounding walls affect the dynamics of sweeping jet flow that leads to a change of average velocity field. For low Re numbers, obtained results are in good agreement with the results of other studies.

Study on Calculation Method of Courant Limit in Thermal Hydraulic System Analysis Code

In order to analyze the influence of different Courant limit calculation methods on the calculation speed and accuracy of reactor thermal hydraulic system analysis code, two kinds of Courant limit calculation methods were studied: synthesis method and grouping method. The calculation principles of the two methods were analyzed, and the two methods were adopted to calculate the steady-state condition and large break loss of coolant accident condition of a PWR respectively. The results show that there is no significant difference between the two methods in steady-state condition; in the case of large break loss of coolant accident with drastic change of velocity field, the synthesis method can obtain more accurate calculation results, but it takes more time, and the grouping method can obtain faster calculation speed, however its calculation accuracy is lower.

Numerical Simulation Study on Turbulent Heat Transfer Characteristics of Supercritical Fluid in Micro-Fin Tubes

In consideration of the technical requirements for compact and efficient heat-exchange equipment in industrial fields, such as sludge incineration heat utilization, solar thermal power generation and aerospace, this paper analysed the supercritical nitrogen flow and heat transfer processes in square straight micro-fin tube, triangle straight micro-fin tube and smooth circular tube under different Re numbers, and obtained the changes in velocity field, local turbulence intensity, average Nusselt number and resistance coefficient of supercritical fluid in micro-fin tubes. Moreover, it was found that the introduction of micro fin was able to improve the turbulent heat transfer performance of supercritical fluid in tubes within certain Re number range.

Passive control of the flow-induced noise from a rectangular cylinder using porous walls

A noise reducing technique for the flow-induced noise using a porous material was studied experimentally and numerically. In the experiment, flow-induced noises emitted from three types of rectangular cylinders were measured in a low-noise wind tunnel. One cylinder was made of four aluminum plates and others were made of two or three aluminum plates. Measurement results show that the frequency of the distinct tonal noise was different among three cylinders, that frequency was higher for using porous material. It was also found that the sound pressure lelvel of the noise was also different and that of the cylinder using two porous material plates was 25 dB smaller at maximum. Velocity field of the wake of cylinders were examined by the PIV measurement and that showed that time and space scale of separated vortices around cylinder were smaller for using two porous material plates. It is assumed that the change of aerodynamic sound was caused by that change in velocity field. In the numerical simulation, we could simulate changes of the emitted noise and the wake of the cylinder by applying the slip boundary condition of the velocity to the wall of the cylinder.

Influence of propeller diameter mounted at wingtip of high aspect ratio wing on aerodynamic performance

In this paper, the propeller diameter effect on cruising characteristics of a solar-powered aircraft with a high aspect ratio wing is considered. Numerical studies were performed using a program based on the Reynolds-averaged Navier-Stokes equations for Mach numbers M=0.145 and Reynolds numbers Re = 0·3-106. Three aircraft configurations were considered: without propellers, with running two-bladed propellers with diameters of 0.22 m and 0.33 m. Numerical studies showed that the installed propeller introduces significant non-linearity into the lift curves and propeller thrust coefficient versus angle of attack curves. The propeller slipstream velocity field changes depending on the propeller advance ratio and the flow velocity, thus influencing the pressure distribution on the wing and the aircraft aerodynamic performance.