Convective Flow Field Research Articles

Numerical investigation on designs and performances of multi-dimensional forced convection flow field design of proton exchange membrane fuel cell

The geometric modification of the flow field is validated as an effective measure for improving proton exchange membrane fuel cell (PEMFC) performance and reactant mass transfer ability. In this paper, a parallel flow channel with X-structure connecting two adjacent channels is established. The multi-dimensional forced convection is formed by adding the stepped shape with three connecting flow fields to increase the performance and mass transfer capacity. Six cases based on the flow field of the X shaped channels are proposed to assess the impact of different designs. The results indicate that the X-structure design improves the uniformity of oxygen distribution and drainage capacity. A stepped design with gradually increasing height improves power density and generates a large area of localized gas vertical velocity. The addition of connecting channels results in different flow behaviors at the nodes, such as slight backflow, increased flow velocity, decreased water content, and temperature rise. With a power density improvement of up to 6.9 % compared to the initial flow field, cell performance improves as the order of step height rises and the diamond structure connection channels set. The innovative flow fields are supplied as a reference for future flow field design.

Modeling of magnetohydrodynamic free convection in an enclosure having elliptical sinks using lattice Boltzmann method

AbstractThis paper adopted the Lattice Boltzmann Method (LBM) to compute a two‐dimensional incompressible convective flow field with two isothermal elliptical sinks attached vertically to the horizontal bottom north side of the enclosure, which is subjected to a horizontal magnetic field. The attained results show that LBM can effectively capture vortex shedding structures and other features inside such complex flow. Also, the Hartmann numbers with a panoply of heated sinks location were studied, and the results showed that fluid flow and heat transfer rate depend on the Hartmann number and are greatly influenced by the heated sinks positions. The effect of the Prandtl number on the average Nusselt number is also highlighted.

Corner melting in low Pr metals: A study using lattice Boltzmann method

Evolution of melting boundary in the solid–liquid phase-change problem involves nonplanar geometry due to the presence of Rayleigh–Benard convective cells in the melted region. The transition of flow behavior from steady to oscillatory in the melt zone is particularly an interesting phenomenon to study. In this work, numerical investigation of melting of low Prandtl number materials in a square cavity with two adjacent heated walls has been carried out using total enthalpy-based lattice Boltzmann method. The local thermal and fluid flow behavior within a moving melting front has been investigated. The influence of natural convection in the melt zone has been observed for two different cases: (1) heating from a corner formed by the left and the bottom walls and (2) heating from a corner formed by the top and the right walls. The effect of Rayleigh number in the range of Ra = 102–5 × 107 on the convective flow field is evaluated for a typical parametric value of Stefan number of 0.01 and Prandtl number of 0.025. Results show distinct convection rolls at the melt zone for the cases under investigations. Evolution of flow fields in the melt zone has been described by a set of isotherms and streamlines for both the cases. The interface fronts for both the cases are initially of convex shape, and around Fourier number of 0.5 turn into concave shape, as viewed from the direction of the movement of melting front. At higher Ra, the rates of melting for top-right side and left-bottom side heated cavities are nearly the same above Fourier number of about 1.2. The average heat flux from the walls scales with Ran , where n is a constant which varies between 0.98 and 1.11. In the melt region, the instability pattern of flow and thermal field changes with the change of effective melt area. Transition to periodic flow is observed at Ra of 7.5 × 106 for case 1 and 2.2 × 107 for case 2, respectively.

A Numerical Case Study of Particle Flow and Solar Radiation Transfer in a Compound Parabolic Concentrator (CPC) Photocatalytic Reactor for Hydrogen Production

Compound parabolic concentrator (CPC) photocatalytic reactors are commonly used for photocatalytic water splitting in hydrogen production. This study aimed to gain a better understanding of the physical processes in CPC photocatalytic reactors and provide theoretical support for their design, optimization, and operation. The analysis involved the ray tracing approach, Euler–Euler two-fluid model, and discrete ordinates method (DOM) to study solar radiation transfer and particle flow in the reactor. The distribution of solar radiation on the receiving tube’s surface after CPC concentration was obtained by conducting the ray tracing approach. This solar radiation distribution was then coupled into the Euler–Euler two-fluid model to solve for the natural convection flow field, the temperature field, and particle phase volume fraction distribution inside the receiving tube over a period of 120 s. Lastly, the discrete ordinates method (DOM) was used to analyze the transfer of radiation inside the receiving tube at different times, obtaining the distribution of local volume radiative power absorption (LVRPA) and the total radiative power absorption (TRPA) inside the tube. The results showed that the TRPA reached its maximum at 120 s, accounting for 66.61% of the incident solar UV radiation. According to the above results, it could be suggested that adopting an intermittent operation mode in CPC photocatalytic reactors is reasonable and efficient.

Typhoon eye-shaped global convective flow field-induced colloidal motor swarm

In nature, living individuals such as fish, gather as much as possible in the wild to carry out life activities to improve their survival rate and environmental adaptability. Colloidal motors are often used as models to form various swarms and reveal their intrinsic mechanisms. Here, we rely on a global convective flow field to prompt the formation of a colloidal motor swarm that can absorb all motors in the range of light radiation without assembly limits. The peanut-shaped colloidal motor photocatalytically decomposes hydrogen peroxide and generates the chemical concentration gradient, which triggers convective flows at its waist in the x-z plane and attracts nearby motors. When motors converge towards the center of the light gradient field due to their positive phototaxis, the chemical gradient fluid resulting from motors’ non-uniform distribution in the entire domain induces a typhoon eye-shaped global convective flow field that flows from the edge of the light to the center. Compared with individuals, the global convective flow has a wider attraction range and gathers motors into a swarm. Moreover, with the number of collected motors increasing, the convective flow integrated in the x-z plane is stronger and ceaselessly accumulates more motors toward the z-axis. The formed swarm can also move to other areas under light control to collect more motors. Such collective phenomena provide a chance for understanding the emergence mechanism of biological swarms and applying artificial swarms to environmental governance, information collection, and drug release.

Observing atmospheric convection with dual-scanning lidars

Abstract. While convection is a key process in the development of the atmospheric boundary layer, conventional meteorological measurement approaches fall short in capturing the evolution of the complex dynamics of convection. To obtain deeper observational insight into convection, we assess the potential of a dual-lidar approach. We present the capability of two pre-processing procedures, an advanced clustering filter instead of a simple threshold filter and a temporal interpolation, to increase data availability and reduce errors in the individual lidar observations that would be amplified in the dual-lidar retrieval. To evaluate the optimal balance between spatial and temporal resolution to sufficiently resolve convective properties, we test a set of scan configurations. We deployed the dual-lidar setup at two Norwegian airfields in a different geographic setting and demonstrate its capabilities as a proof of concept. We present a retrieval of the convective flow field in a vertical plane above the airfield for each of these setups. The advanced data filtering and temporal interpolation approaches show an improving effect on the data availability and quality and are applied to the observations used in the dual-lidar retrieval. All tested angular resolutions captured the relevant spatial features of the convective flow field, and balance between resolutions can be shifted towards a higher temporal resolution. Based on the evaluated cases, we show that the dual-lidar approach sufficiently resolves and provides valuable insight into the dynamic properties of atmospheric convection.

Influence of heat generation/absorption on mixed convection flow field with porous matrix in a vertical channel

The effect of heat and mass transfer on mixed convection flow field with porous matrix in a vertical channel has investigated analytically. The analytical solutions of the coupled nonlinear differential equations are obtained for velocity, temperatures and concentrations are showed graphically. The results obtained analytically by using regular perturbation method are considered to be favorable as no such investigations are performed previously. The governing equations have been made smaller than Non-linear ordinary differential equations by means of flow parameters. Investigations are carried out on the flow characteristics for different values of Grashof number, Reynolds number, Brinkman number, heat generation/absorption parameter and porous parameter. It is found that an increases in the Grashof number cause the buoyancy to increase, which in turn causes the fluid flow for heat generation and absorption to increase.

Theoretical and numerical studies on dissolution of horizontal salt body and pattern formation incorporated with a dynamic moving interface

The dissolution of a horizontal salt body is theoretically and numerically investigated by considering the coupled dynamics of the dissolution reaction and the diffusive and convective mass transfer at the salt surface. To take into account the nature of the recessing salt surface and the dissolution-driven convection, a dynamic moving boundary condition coupled with a dissolution reaction are employed By employing a newly derived moving interface condition and parameters, a theoretical analysis predicts the onset of gravitational instability, suggesting scaling relationships between parameters to describe the onset of instability (in transport-dominant (Damkholer number $Da \to \infty $ and Rayleigh number $Ra > {10^5}$ ), onset time ${\tau _d}\sim R{a^{ - 2/3}}$ and ${\tau _d}\sim (RaDa)^{1/4}$ for reaction-dominant regimes $(Da \to 0)$ ). The scaling relationships on dissolution (the average height change, $\Delta {h_{avg}}\sim \sqrt \tau $ for the diffusion-dominant regime and $\Delta {h_{avg}}\sim R{a^{0.79/3}}{\tau ^{0.86}}$ for the convection-dominant regime) are also suggested to explain the pattern formation. Under a convective mass transfer condition, the concentration gradient of the solute developed during the recession of the salt surface results in an adverse density gradient, which causes gravitational instability motion and finally determines the formation of a dissolution pattern. In addition, two-dimensional and three-dimensional numerical simulations visualize dissolution-driven convective flow fields, the recession of the liquid-solid interface and pattern formation on the dissolving interface. Theoretical and numerical analyses of the dissolution process suggest that the control of gravitational instability is important to prevent or enhance dissolution pattern formation. This systematic parameter study provides a deep understanding of the effect of gravitational instability on convection-driven dissolution in a horizontal geometry.

Bejan’s heatline and mulitphysics analysis of double diffusive free convection in a doubly stratified non-Darcian porous enclosure with heat generation

The current article illustrates the characteristics of the MHD free convective transport phenomenon in the thermal and mass stratified fluid-saturated Darcy-Forchheimer porous square cavity under the action of the cross-diffusion (Soret and Dufour effects) heat generation and chemical reaction effects. To acquire the spatial distribution of thermal fluxes along with directional propagation in chemically reactive and heat generative non-Darcy fluid flow, a newly developed heatline model for multi-force effect following the heat function theory is proposed in this study. In addition to the thermal profile visualization tools, the solutal free convective fluid flow fields are visually demonstrated by the massline contours curves using a novel mass-function model analogous to the heat function methodology. On the other hand, for the numerical solution of the non-dimensional form of the partial differential equations governing the study, the finite element method is applied. Moreover, the non-Darcy fluid flow attributes subjected to the multiple forces are graphically portrayed by the streamlines, isotherms, iso-concentrations, cumulative Nusselt/Sherwood number plots, heatlines, and masslines for the related dimensionless parameter. Here, these visuals delineate the contra-rotating fluid flow transition under both the thermal and concentration stratification forces. Especially, forceful retardation in both the thermal and concentration flux’s movements is noticed when magnetic forces are intensified. An upward geometrical shift of the higher concentration region is seen with increasing chemical reaction parameter.

Natural convection in nanofluid flow with chemotaxis process over a vertically inclined heated surface

The thermal energy transport analysis with chemotaxis in the free convective flow of viscous nanofluid over stretchable vertically inclined heated sheet is addressed in this article. The fluid forced and free convection motion is investigated and discussed with physical reasoning. The fluid also contains microorganism heavy-bottom species, and their chemotactic motion is studied. In the light of Buongiorno model, the impact of Brownian motion and thermophoresis slip mechanism on thermal conduction in the nanofluid is analyzed. The work is based on the similarity analysis of governing partial differential equations (PDEs) which lead to non-dimensional ordinary differential equations (ODEs). The solution of resulting flow and heat equations is computed via bvp4c technique. The outcomes are represented in graphical abstract. It is noted that free convective flow field increases near to the surface of sheet then it decays to free stream exponentially. Higher magnitude of thermophoretic force boost up the thermal energy transport in nanofluid flow. The Brownian motion enhances temperature profile and lower down the convection velocity. Chemotaxis motion of species in nanofluid is increasing function of bioconvective Peclet number.

Numerical Analysis of Heating Technique in Corium Melt Pool Convection Flow Field & Thermal Interaction in a Volumetrically Heated Molten Pool

In-Vessel Retention (IVR) is one of the existing strategies of severe accident management of LWR, which intends to stabilize and isolate corium & fission products inside the reactor pressure vessel (RPV) and primary containment structure. Since it has become an important safety objective for nuclear reactors, it is therefore needed to model and evaluate relevant phenomena of IVR strategy in assessing safety of nuclear power reactors. One of the relevant phenomena during accident progression in the oxidic pool is non-uniform high heat generation occurring at large scale. Consequently, direct experimental studies at these scales are not possible. The role computer codes and models are therefore important in order to transpose experimental results to reactor safety applications. In this paper, the state-of-the-art ANSYS FLUENT CFD code is used to simulate Non-uniform heat generation in the lower plenum by the application of Cartridge heating under severe accident conditions to derive the basic accident scenario. However, very few studies have been performed to simulate non-uniform decay heat generation by Cartridge heaters in a pool corresponding lower plenum of power reactor. The current investigation focuses on non-uniform heating in the fluid domain by Cartridge heaters, which has been done using ANSYS FLUENT CFD code by K-epsilon model. The computed results are based on qualitative assessment in the form of temperature and velocity contour and quantitative assessment in terms of temperature and heat flux distribution to assess the impact of heating method on natural convective fluid flow and heat transfer.

Study on Intermittent Microwave Convective Drying Characteristics and Flow Field of Porous Media Food

Numerical simulations were carried out for moist, porous media, intermittent microwave convective drying (IMCD) using a multiphase flow model in porous media subdomains coupled with a forced-convection heat-transfer model in an external hot air subdomain. The models were solved by using COMSOL Multiphysics was applied at the pulse ratio (PR) of 3. Based on drying characteristics of porous media and the distribution of the evaporation interface, IMCD was compared with convection drying (CD). Drying uniformity K, velocity difference, temperature difference, and humidity difference were introduced to evaluate the performance of three models with different inlets and outlet wall curvature. The numerical results show that as the moisture content of slices was reduced to 3 kg/kg, the drying rate in IMCD was 0.0166–0.02 m/s higher than that in CD, and the total drying time of the former was 81.35% shorter than that of the latter. In the late drying stage of IMCD, the core of the sample still had a high vapor concentration and temperature, which led to the evaporation interface remaining on the surface. The vapor evaporated from the slices can diffuse rapidly to the outside, which is why IMCD is superior to traditional convection drying. Through the comprehensive analysis of the models with different inlet and outlet wall curvatures, the drying uniformity K of the type III was the highest, reaching 89.28%. Optimizing flow-field distribution can improve uniform of airflow distribution.

Control of quasi-equilibrium state of annular flow through reinforcement learning

Stability control of the convection flow field has always been a focal issue. The annular flow discussed in this work is a typical research model of microgravity fluid physics, which is extracted from the industrial crystal growth by the Czochralski method. It is believed that the instability of thermal convection is the key factor affecting the quality of crystal growth. Combining the reinforcement learning algorithm with the neural network, this paper proposes a control policy that makes forced convection compete with thermocapillary convection by changing the dynamic boundary conditions of the system. This control policy is successfully applied to the control of the quasi-equilibrium state of annular flow, and the global stability of the flow field is well maintained. It first experimentally makes the annular flow field under low and medium Ma numbers achieve a quasi-equilibrium state, which is different from that before the onset of flow oscillations. Then, a simulation environment is created to imitate the experimental conditions. After training in the simulation environment, with the self-optimized algorithm, the machine learning approach can successfully maintain the simulation environment in a quasi-equilibrium state for a long period of time. Finally, the learning method is validated in the experimental environment, and a quasi-equilibrium state control policy is completely optimized by using the same optimization policy and similar neural network structure. This work demonstrates that the model can understand the physical environment and the author's control objectives through reinforcement learning. It is an important application of reinforcement learning in the real world and a clear demonstration of the research value of microgravity fluid physics.

On the Cattaneo–Christov heat flux theory for mixed convection flow due to the rotating disk with slip effects

The maximum heat conduction difficulty has been recently termed and scrutinized using Fourier’s law. It is acknowledged that this established diffusion theory may collapse when one is absorbed in the transient problems in a minimal period, extraordinary flux, or for precise low temperatures. In this article, a mechanism of heat flux model with a mixed convective flow field is considered. Here fluid motion under the applied magnetic field and velocity slip condition take place due to the rotating disk. After using the Cattaneo–Christov approach, the heat equation is derived for the heat flux model. The thermal relaxation features are examined in detail. The modeled partial differential framework is reduced to an ordinary differential framework through an appropriate transformation, which is solved numerically by implementing the shooting technique with Runge–Kutta–Fehlberg method. The velocity and fluid temperature are simulated for involved physical parameters. The heat generation/absorption factor has a reverse impact on the temperature field. Furthermore, the fluid temperature decays for the thermo-relaxation factor. The results for skin friction and Nusselt number are also computed.

Heat Transfer Enhancement of Water Based Al<sub>2</sub>O<sub>3</sub>- Cu Hybrid Nanofluid Trough Square Cavity

The present numerical work, based on the finite volume method, deals with the characterization of natural convective flow and thermal fields inside differentially vertical heated square cavities filled with a nanofluid as well as the quantification of the convective exchanges. The investigation is devoted to study the influence of the hybrid nanofluid (Al2O3-Cu / water) on the flow’s general structure with a particular attention to the Nusselt number. An exhaustive parametric study is conducted considering different combinations of Al2O3 and Cu nanoparticles (NPs) dispersed in water for a range of Rayleigh numbers (Ra) and total volume fractions An appropriate agreement with experimental data was observed for the estimation of the hybrid nanofluid thermal conductivity. From the results, it is observed that the heat transfer intensifies by increasing the Ra number and the nanoparticles volume fraction. The hybrid nanofluid seems to be the most efficient nanofluid in comparison with a base fluid and a single nanofluid. This heat transfer enhancement becomes more convincing with the increase of the Cu NPs content (% in volume).

Feedback and reactive flow effects on living crystal formation

Active spherical particles in a water film form aggregate on two neighboring water bubbles, where one of the bubbles’ half surface is heated by a laser source, creating a convective flow field. The specific geometry is similar to the experimental setup we used for colloidal particles (Ilday et al., 2017). Water flow is considered reactive during the simulation due to changing flow field, as well as temperature field. A feedback interaction is introduced, which may be due to the concentration of particles in some region of fluid, effectively a time-dependent accumulation. We show the effect of attractive and repulsive feedback forces, and observe, under attractive feedback, the formation of patches of aggregate distributed all over the space, and eventually a greater aggregate formation on the bubble surface. Reactive flow effects, with and without feedback effects, are analyzed, by examining the pair correlation and mean square displacement indicators, as well as the single-particle trajectories.

Flow instabilities and heat transfer in a differentially heated cavity placed at varying inclination angles: Non-intrusive measurements

We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities on the orientation angle of a differentially heated cavity of aspect ratio three. The cavity orientation angles considered are 60° and 30°. While moving from 60° to 30°, the cavity is inclined toward its stable configuration, wherein convection reduces. Flow instabilities have been captured through the spectral analysis of the transient history of temperature distribution recorded in a completely non-intrusive manner using a Mach–Zehnder interferometer. By virtue of the fact that in such configurations, corners of the cavity are the most active regions with regard to the interaction of buoyancy-driven fluid with the cavity walls, and the flow behavior is centrosymmetric (diagonal symmetry), the flow field in the top two corners of the cavity has been mapped. The spatio-temporally resolved interferometric measurements identified two distinct frequencies for cavity inclination angle (θ) of 60°. These two frequencies correspond to two different flow instabilities, namely, the Tollmien–Schlichting (TS) and gravity wave-induced instabilities. As the cavity is further inclined toward 30°, the instability in the boundary layer, i.e., the TS instability, ceases to exist, and only the gravity wave-induced instability is observed. The dependence of flow instabilities on cavity orientation angle is explained on the basis of interferometry-based measurements made in the form of interferograms and the corresponding whole field maps of temperature contours. The convective flow field in the differentially heated cavity has also been qualitatively captured using smoke visualization to provide direct support to interferometric measurements.

The use of passive baffles to increase the yield of a single slope solar still

Despite their relative simplicity, single slope solar stills remain one of the most viable solutions to the production of freshwater from saline and non-potable water sources. Over the years, numerous researchers have attempted to improve the yield of single slope solar stills through changes to their geometry and the use of various design modifications. Despite this long history of research, one modification that has not been fully explored is the use of passive baffles to alter the natural convection inside them.Though baffles have frequently been used to suppress natural convection in enclosures, there may also be the opportunity to use them to enhance natural convection. This judicious placement of baffles to enhance the natural convection could potentially improve the yield from single slope solar stills, however it has not been well explored in the literature.To address this issue, this work examined the effect of vertically mounted passive baffles on the natural convection inside a single slope solar still geometry. In this vein, a computational fluid dynamics simulation was conducted for a still containing a baffle, with the results validated experimentally using particle image velocimetry. On this basis, subsequent simulations explored the effect of baffle length and position on stills with cover angles from 10–60°. The results showed that baffles did have a marked influence on the natural convection flow field and could increase the natural convection heat transfer coefficient, which is directly proportional to a solar still’s yield, by up to 20%.It was also found that short baffles enhanced natural convection at low cover angles, while baffles mounted close to the front of a still provided an improvement to the transfer processes over the widest range of conditions. Finally, the work led to the development of a relationship to describe the effect of baffles on natural convection, as shown in Equation (1), that it is hoped will aid designers of single slope solar stills in the future.(1)Nu'=Ra'0.188∗AR-0.29∗cosθ-0.5∗bB0.08∗dD0.09

Transverse buoyant jet-induced mixed convection inside a large thermal cycling test chamber with perforated plates

Transverse buoyant jet-induced mixed convection inside a large thermal cycling chamber with perforated plates is investigated experimentally and numerically. The mean temperature distribution and temperature fluctuation are obtained experimentally under cooling and heating conditions. The numerical simulation is conducted by solving the Reynolds-averaged Navier-Stokes (RANS) equations using the low-Re k-epsilon model. The modified body force based on the pressure loss analogy is adopted to describe the flow characteristics through the perforated plate. An additional transport equation is solved to determine the root mean square (RMS) of the temperature fluctuation. The proposed numerical method is validated with the experimental data. The flow field, mean temperature distributions, and RMS of the temperature fluctuation are studied for various values of the pressure loss coefficient of the perforated plate Eu and the dimensionless heights of the plenum h. The results indicate that the mixed convection flow field is characterized by the inertial force from the inlet, the buoyancy force due to the temperature difference, and the dissipation effect of the perforated plate. For a larger Eu, the temperature uniformity of the test zone is greatly improved, and the average RMS temperature fluctuation also decreases. The dimensionless height of the plenum h has little effect on the development of the buoyant jet. The average temperature deviation and RMS temperature fluctuation exhibits a slight decrease with increasing h.

Helioseismological determination of the subsurface spatial spectrum of solar convection: Demonstration using numerical simulations

Context. Understanding convection is important in stellar physics, for example, when it is an input in stellar evolution models. Helioseismic estimates of convective flow amplitudes in deeper regions of the solar interior disagree by orders of magnitude among themselves and with simulations. Aims. We aim to assess the validity of an existing upper limit of solar convective flow amplitudes at a depth of 0.96 solar radii obtained using time-distance helioseismology and several simplifying assumptions. Methods. We generated synthetic observations for convective flow fields from a magnetohydrodynamic simulation (MURaM) using travel-time sensitivity functions and a noise model. We compared the estimates of the flow amplitude with the actual value of the flow. Results. For the scales of interest (ℓ < 100), we find that the current procedure for obtaining an upper limit gives the correct order of magnitude of the flow for the given flow fields. We also show that this estimate is not an upper limit in a strict sense because it underestimates the flow amplitude at the largest scales by a factor of about two because the scale dependence of the signal-to-noise ratio has to be taken into account. After correcting for this and after taking the dependence of the measurements on direction in Fourier space into account, we show that the obtained estimate is indeed an upper limit. Conclusions. We conclude that time-distance helioseismology is able to correctly estimate the order of magnitude (or an upper limit) of solar convective flows in the deeper interior when the vertical correlation function of the different flow components is known and the scale dependence of the signal-to-noise ratio is taken into account. We suggest that future work should include information from different target depths to better separate the effect of near-surface flows from those at greater depths. In addition, the measurements are sensitive to all three flow directions, which should be taken into account.