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In this work, supercritical n-decane flowing in ducts with different orientations is thoroughly investigated in terms of secondary flow, wall shear stress, and thermal acceleration. It is known tha...

Enhanced thermal transportation for bio convection and mixed convection of Casson nano fluid flow in stagnation region of rotating sphere is scrutinized in the existence of magnetic field. The bioconvection of nano fluid transportation across spherical surface is considered important for heat storage and thermal balancing in technological processes. To enhanced heat transportation, the gyrotactic microorganisms are used with nano sized particles to improve stability. In this communication, time dependent partial differential formulation is transmuted to a set of dimensionless partial differential equation via similarity transformation. A numerical solution is sought through Glarikan based finite element discretization while it is coded and run on Matlab platform. The role of variables for the dependent quantities is monitored through computational procedure. The parameters of rotating sphere and Casson fluid speeded up the fluid flow in x-direction, but it recedes the flow in y-direction. The larger inputs of Brownian motion and thermo pharoses parameters raised the nano fluid temperature. The absolute value of Nusselt number is up-surged with enhanced buoyancy and rotation but is diminishes against parameters of Brownian motion, bio convection, and Rayleigh number.

An experimental study has been conducted to investigate the heat transfer characteristics of supercritical carbon dioxide (sCO2) uniformly heated in the horizontal circular smooth tube. The results illustrated that there was a significant difference in heat transfer between the top wall and bottom wall due to the buoyancy. Bulk flow acceleration cannot be negligible in the high heat flux region, which leads to heat transfer deterioration. A new heat transfer correlation is proposed, in which the buoyancy parameter and bulk flow acceleration have been taken into account. The new correlation and six classic correlations for sCO2 are examined in horizontal tubes. The comparison indicates that the new correlation has a better performance for sCO2 flowing through a horizontal heating tube under natural circulation conditions. For example, 94.9% of the calculated results using the new heat transfer correlation were within ±30% of the experimental results while only 87.9% of that using the Jackson correlation (the best of the six) were within the same error bands.

A momentum-balance theory for the orientation of updrafts in density currents is developed for understanding squall-line–shear interactions. The motivation arises from studies showing the diagnostic limitations of the vorticity-balance theory by Rotunno et al. (1988) (RKW theory) under varying shear profiles, together with the flow-force balance constraint (FFB) which determines the shear-layer depth in optimal density currents. Considering that the FFB is derived from the horizontal momentum equation, momentum-balance concepts are explored as an alternative to RKW theory, by assuming that the updraft’s slope is determined by the balance between advective tendencies of inflowing air and the work done by pressure perturbations within the denser fluid. Density currents were simulated under diverse shear and buoyancy profiles. Results show that momentum-balance effectively diagnoses the updraft’s slope at low and mid-levels in experiments contemplating both “classic” c-ΔU variations, as well as changes to the shear and buoyancy vertical profiles. It is also found that cases with stronger system-relative inflow tend to produce deeper lifting of near-surface environmental air, notwithstanding the updraft’s slope. RKW theory’s quantitative criterion (c/ΔU) is not as effective at diagnosing the updraft’s slope nor the depth reached by near-surface parcels, although c/ΔU provides guidance for the updraft’s slope at upper levels. This result justifies a reinterpretation of c/ΔU as a measure of the impacts of wind velocities aloft on the updraft

Monodispersed microbubbles and microdroplets are widely used as reaction carriers in microfluidics. In this study, the generation processes of bubbles and droplets in a step-emulsification microfluidic device are compared to show the similarities and differences in the emulsification process. By changing the placement of the microdevice, the effects of buoyancy and gravity on the generation of bubbles and droplets are introduced, and the feedback mechanism of the bubble layer and the effect of droplet accumulation on the emulsification process are clarified. Finally, based on the analysis of the difference of the pinch-off of the dispersed phase between the bubble and the droplet in this configuration, the Plateau-Rayleigh instability processes for the formation of bubble and droplet are revealed by using a high-speed camera system, and the reasons for the difference of the operating ranges of the gas flow rate and liquid flow rate in the dripping flow regime are explained.

The current non-homogeneous nanofluid flow model is carried out to scrutinize the performance of the generalized Fourier's and Fick's laws on the MHD bioconvective aspects of couple-stress nanofluid flows through a convectively heated stretching sheet in the presence of activation energy and multiple stratified boundary conditions. Herein, both the concentrations of solid nanoparticles and motile microorganisms are incorporated explicitly into the nonlinear differential expressions describing the present non-Newtonian nanofluid flow model. Besides, the combined thermal influence of the Cattaneo-Christov heat flux and thermal radiation are also discussed. From a practical point of view, the couple-stress nanofluids are useful for examining different types of thermophysical and rheological features, since this kind of enhanced fluids can clarify realistically the dynamical behavior of various liquids, like the human blood and some polymeric suspensions. For reducing the mathematical complexity of the present physical problem, several effective similarity transformations are introduced formally to simplify the resulting partial differential equations (PDEs) into a nonlinear coupled structure of ordinary differential equations (ODEs). Moreover, the transformed dimensionless self-similarity equations are then numerically solved using the built-in shooting technique with the aid of the bvp4c solver MATLAB package. Furthermore, The obtained results are authenticated with an outstanding agreement. In this respect, the engineering quantities of interest are computed extensively with a higher level of accuracy and then summarized tabularly. To illustrate the impacts of the embedded physical parameters on the profiles of velocity, temperature, nanoparticles concentration, and microorganisms concentration, various illustrations are done successfully along with detailed elucidations. As the main findings, it is found that the temperature distribution and the microorganisms concentration profile can be enhanced with the higher values of the bioconvection Rayleigh number. Similarly, it is revealed that the nanoparticles concentration sketch and the microorganisms concentration profile can be boosted up for the higher magnitudes of the buoyancy ratio parameter.

Owing to the unique advantages in flight altitude, dwelling time and wide coverage area, stratospheric airships provide permanent monitoring and surveillance for both civil and military applications. Here we propose a semi-rigid stratosphere airship design with circumferential high-pressure inflatable rings and a longitudinal carbon fiber skeleton supported inside. We perform numerical simulations to analyze the deformation characteristics during the whole ascending and descending process. An equivalent internal gradient pressure model of helium is established based on the capsule shape and buoyancy-weight equilibrium conditions. The implicit dynamic method is used to deal with the large deformation of the airship capsule under a low negative pressure condition. Deformation and load-bearing performance of the airship capsule, inflatable ring, skeleton, and suspension line are obtained under different working conditions. The results show that the airship, supported with the inflatable rings and the suspension lines, effectively maintains the shape and ensures the stiffness during the ascending, dwelling, and descending stages, especially suffering from negative pressure.

The management of fluids in space is complicated by the absence of relevant buoyancy forces. This raises significant technical issues for two-phase flow applications. Different approaches have been...

This paper reports a numerical study of double-diffusive natural convection through an annular space delimited by a square cylinder on the outside and a cylindrical cylinder on the inside covered by a porous layer. The Darcy-Brinkmann-Forchheimer is used for modeling flow in both fluid and porous areas. The annular space is partially or completely filled with an isotropic porous medium. A finite volume method, using the Patankar-Spalding technique is used for solving the discretization of the dimensionless equations governing the problem. The effects of simultaneously applied thermal and solutal buoyancy forces on heat and mass transfer are shown in the results for a large range of buoyancy ratios N, Rayleigh number, and thermal conductivity. Streamlines, isotherms, and iso-concentrations are presented to analyze the flow structure transition from mass species dominated to thermal dominated flow. Results show that the buoyancy ratio can change the flow pattern and the increased thermal conductivity ratio can improve heat and mass transfer. A good agreement was obtained between the present results and those published were found.

This chapter deals with the mechanics of inelastic bending in the presence of tension. Bending a circular tube ovalizes its cross sections and leads to a limit-load instability. Bending in the presence of tension can aggravate the growth of ovalization. Two types of combined bending-tension loading have been identified. The first is encountered in a suspended line, where the tension is used to control the shape of the suspended section. In this case, the tension is reacted by a distributed force normal to the axis of the tube that occurs because of the weight of the pipe and the buoyancy force. The transverse force acts at the axis of the tube and thus does not influence ovality. In this type of loading, the tension interacts with the bending (and possibly pressure) through an inelastic action by aggravating the state of stress. In the second type of combined bending-tension loading, the pipe is bent over a rigid surface, in which case the tension is reacted by a distributed force acting on the surface of the bent tube. This type of loading is encountered, for example, by a pipeline passing over a curved stinger and when winding or unwinding a line on a reel.

Multi-fluid models have recently been proposed as an approach to improving the representation of convection in weather and climate models. This is an attractive framework as it is fundamentally dynamical, removing some of the assumptions of mass-flux convection schemes which are invalid at current model resolutions. However, it is still not understood how best to close the multi-fluid equations for atmospheric convection. In this paper we develop a simple two-fluid, single-column model with one rising and one falling fluid. No further modelling of sub-filter variability is included. We then apply this model to Rayleigh-Benard convection, showing that, with minimal closures, the correct scaling of the heat flux (Nu) is predicted over six orders of magnitude of buoyancy forcing (Ra). This suggests that even a very simple two-fluid model can accurately capture the dominant coherent overturning structures of convection.

Particle-laden turbulent flow under saturated electrostatic field in 90-degree bend pipe at Reb = 58000 is investigated using one-way coupled Large Eddy Simulation (LES) with a Lagrangian particle tracking technique. Drag, lift, buoyancy, gravity, and electrostatic force are considered. Three particle sizes (dp = 5, 10, 50 μm, St = 5.47, 21.91, 547) are considered. The results show that particle concentration is higher in outer bend than that in inner bend and the concentration increases with particle size. At the stagnation point of Dean vortices near the inner arc of bend, particle concentration is significantly increased under the synergy effect of electrostatics and secondary flow. In addition, particles trapped in the viscous layer are difficult to resuspend under electrostatic effect. Particle concentration near the wall under the presence of electrostatics is larger than that under the absence of electrostatics. However, at the bend, particle concentration in the core region under the presence of electrostatics is lower than that in the absence of electrostatics. For particle behavior, drag force plays the most important role in the core region while electrostatic force dominates the near wall region and causes particles to accumulate there.

In this new-fangled research, the mixed convective stagnation point flow of a hybrid nanofluid over a yawed cylinder with radiation impact is considered. In reality, the perception of mixed convection in the study of yawed cylinder rises in heat exchangers. To explore the heat diffusion through the system of mixed convection, the mathematical problem is formed in the form of partial differential equations which are coupled and nonlinear. A solution technique is applied and described for indulgencing non-similar thermal boundary layers. The non-similar terms involving in the transformed equations are held without approximations. The non-similar equations are solved numerically using the bvp4c technique. The exploration divulges numerous significant results involving yaw angle, hybrid nanoparticle volume fraction, mixed convection, radiation, and non-similarity phenomena. The impact of the yaw angle in enhancing the velocity of the hybrid nanofluid in the span-wise and chord-wise directions in the aiding buoyancy flow and decelerating in the opposing flow, while the contrary behavior is noticed for the dimensionless temperature profile. Also, the hybrid nanoparticles reduce the velocity in both directions in case of assisting as well as opposing flow, whereas the dimensionless temperature augments.

Solar chimney coupled with earth-to-air heat exchanger (SCEAHE) can provides fresh air and cooling capacity simultaneously without any electricity consumption. To understand the complex working mechanism of the coupled system, a numerical model has been established and verified to investigate various geometric and climatic parameters related to system performance. It is found that the system has the optimum performance when the pipe length is 60 m and pipe diameter is 0.6 m. Increasing the solar collector length or the chimney height can both increase the system performance. However, the effect of chimney height is not as significant as that of solar collector length. For the same increase in chimney height and solar collector length, the cooling capacity is increased by 51.6% and 77.8%, respectively. The higher the solar intensity, the higher the buoyancy force, airflow rate, outlet air temperature, and cooling capacity. The cooling capacity is increased by 101.4% by increasing solar intensity from 100 W/m2 to 600 W/m2. The higher the outdoor air temperature, the lower the buoyancy force and airflow rate, but the higher the outlet air temperature and cooling capacity. Moreover, the effect of outdoor air temperature on outlet air temperature is more significant than that on airflow rate. When the outdoor air temperature increases from 36 °C to 46 °C, the temperature reduction is increased by 75.8%, while the airflow rate is only decreased by 15.6%. The model developed in this study can be used for design and performance prediction of SCEAHE system.

Electromagnetic expulsion acts on a body suspended in a conducting fluid or plasma, which is subject to the influence of electric and magnetic fields. Physically, the effect is a magnetohydrodynamic analogue of the buoyancy (Archimedean) force, which is caused by the nonequal electric conductivities inside and outside the body. It is suggested that electromagnetic expulsion can drive the observed plasma counter-streaming flows in solar filaments. Exact analytical solutions and scaling arguments for a characteristic plasma flow speed are reviewed, and their applicability in the limit of large magnetic Reynolds numbers, relevant in the solar corona, is discussed.

The mixing process and the residence time distribution (RTD) of molecules inside reactors are well-known topics in chemical engineering; good radial mixing and poor axial mixing of chemical species...

This numerical study is intended for the analysis of thermal convection induced by buoyancy forces generated within a conical annular porous gap. The annulus was vertically positioned, it contains a discrete heat source and is filled with a Single-Walled Carbon Nanotubes-Water (SWCNT- H 2 O) nanoliquid exposed to a Lorentz force. To describe the porous medium in question, we have adopted the Darcy–Forchheimer model. Galerkin Finite Element Method (GFEM) has been used in this study to predict both thermal and hydrodynamic fields in the physical model. An extensive range of parameters are explored, i.e., the Rayleigh number (103 to 106), Hartman number Ha (1 to 100), and the volume fraction of nanoparticles (0 ≤ ϕ ≤ 0.08). For the purpose of exterminating the effects of heat source location on thermal and hydrodynamic fields, three locations (bottom, middle and upper) have been considered. Findings include current lines, isotherms, and Nusselt number evolution according to the previously stated variables. Predictably, our findings prove that heat transfer rate exhibits a decreasing function of Ha and an increasing function of Da and ϕ . Also, it was revealed that the convective regime is preponderant when the heat source was located in the bottom wall.

The Kelvin-Helmholtz instability (KHI) occurs at the interface amongst two fluids, which are in relative motion with a common boundary. The growth rate of waves occurs whenever the relative velocity is greater as compared with the critical relative velocity. In the present paper, the influence of boundary roughness on KHI under the impact magnetic field in a couple-stress fluid layer bounded by a rigid surface at the lower side and upper side by a fluid saturated porous layer. Using suitable surface and boundary conditions, we have derived the dispersion relation and results are depicted graphically. As observed in presence of sharp interface, magnetic field exhibits stabilizing effect however, destabilizing effect is shown by the buoyancy force on KHI. Also, noted that the growth rate of interface reduces, as there is a rise in roughness parameter value.

Dispersion and natural ventilation of hydrogen leaked at the floor center of a confined space are analytically and numerically studied in this contribution. Two symmetrical openings located atop two opposite walls and several sets of partition boards with varying heights, 0–1.0 m with 0.2 m increment, mounted under the ceiling were designed to examine their effects on ventilation efficiency. Without partition boards, a semi-analytical model combining a modified buoyancy ceiling jet theory and zone model was established to predict the key parameters at steady stage. The analytical predictions were compared with numerical results. 15 monitors were predesigned at an opening and the channels separated by the partition boards to collect the velocity and hydrogen concentration evolutions. The results show that the existence of the partition boards significantly promotes the ventilation efficiency by decreasing the hydrogen concentration in the confined space, constraining the flammable regime into a smaller volume, and increasing both the inflow and outflow velocities. Better ventilation efficiency is achieved by higher partition boards until a critical height after which no appreciable improvement can be gained. Applying the partition boards considerably shortens the developing stage of the dispersion process and intensifies the turbulence of outflow. Conclusions obtained in current work may benefit the natural ventilation system and building structure design for hydrogen safety in practical applications.

Summary Torque and drag models have been used for several decades to calculate tension and torque profiles along drillstrings, casing strings, and liner strings. Buoyancy forces contribute to the loads acting on the pipe and affect its interaction with the borehole wall. Torque and drag calculations account for these localized effects, as well as the material internal forces, torques, and moments on each side of the contact. When the analysis is applied to a discrete length of pipe, the cross sections at each end do not contribute to the buoyancy forces because they are not in contact with the fluid, except where there is a change in diameter or at the end of the string of pipe. We argue that it is important to check that the models used for solid pipe torque and drag calculations remain valid for sand screens, in particular, the extent to which the buoyancy forces acting on a perforated tube might differ from those on a solid pipe. Because the buoyancy force is the result of the pressure gradient acting on the surface of the pipe, the presence of holes may also influence the buoyancy force. We propose that there are theoretical differences between local buoyancy forces acting on plain or perforated tubes. This paper describes how to calculate the local buoyancy force on a portion of a drillstem by the application of Gauss’ theorem and accounting for the necessary corrections arising from the cross sections not being exposed to the fluid. We built an experimental setup to verify that the tension inside a pipe subject to buoyancy behaves in accordance with the derived mathematical analysis. With complex well construction operations, for instance during extended-reach drilling or when drilling very shallow wells with high buildup rates, the slightest error in torques and drag calculations may end up jeopardizing the chances of success of the drilling operation. It is therefore important to check that the basis of design calculations remain valid in those contexts and that, for instance, sand screens or slotted liners may be run in hole safely after a successful drilling operation.

In this paper, the air buoyancy effect on Micro-Electro-MechanicalSystem (MEMS)-based gravity sensors for high-resolution gravity measurements is investigated. The MEMS gravimeter is operated in an atmospheric environment without any vacuum chamber; thus significantly simplifying the design, implementation and maintenance, and reducing the cost of the instrument. It is experimentally observed that the measured acceleration signal shows a clear correlation with the air buoyancy, and consequently the air pressure. A detailed theoretical model of the air buoyant force acting on the MEMS gravity sensor is proposed, giving a gravity-air pressure coefficient of 501.5 $\mu$ Gal/hPa for the silicon springmass system. After removing the error introduced by the air buoyant force, the MEMS gravity sensor exhibits an ultra-low self-noise floor of 1 $\mu {\mathrm {Gal}}/\sqrt{\rm Hz}$ @1 Hz, as well as an excellent stability, with an Allan deviation of 3 $\mu$ Gal (40 s integration time). The sensor is capable of measuring the Earth tides in a 16-day span. This discovery identified one major error source in high-resolution MEMS gravity sensors operating in atmosphere, which could potentially be useful for the development of future MEMS-based gravimeters.

Several industrial activities require the introduction of mechanical agitation and mixing process in vessels with heated walls. This operation is performed in order to accelerate a certain physical or chemical desired homogeneity. Most of the numerical studies presented for the mixing and mechanical agitation process were based on the enhancement of the hydrodynamic performance and energy consumption, while comparatively less research is carried out on thermal phenomena in the literature. Based on this deficiency, the adoption of computational fluid dynamics (CFD) to study mixed convection in a mechanically stirred tank is the subject of the present paper. The numerical investigation focuses on a 3D unsteady laminar flow in a cylindrical tank equipped with an anchor and filled with an alumina-water nanofluid. The wall of this tank is exposed to a constant hot temperature. However, the anchor, the bottom of the tank and the free surface are assumed adiabatic and the nanofluid has an initial cold temperature. The rotary flow is governed by the equations that describe mixed convection, introducing the buoyancy force in the vertical direction into momentum equations. The Richardson number and the volume fraction of the alumina nanoparticles have been considered as control parameters in order to demonstrate their effect on the thermo-hydrodynamic behavior during the temporal evolution of the thermal state. The numerical results show that the predominant nature of the flow generated by the anchor is no longer tangential in the stirred tank by increasing the Richardson number in the unsteady state. In addition, a relative increase in the average Nusselt number is directly attributed to the increase in buoyancy effects on the one hand, and to the volume fraction of alumina nanoparticles on the other hand.

The lack of buoyancy forces in microgravity stagnates vapor bubbles on heated electronic surfaces, raising local surface temperatures above device operating limits. Though flow boiling can help dissipate higher heat fluxes, the addition of pumps and regulated flow loops increases design complexity for electronic systems. Passive directional motion of the fluid using an asymmetric sawtooth microstructure has demonstrated motion along the surface due to viscous forces. The current study explores the sawtooth microstructure in the nucleate boiling regime for a horizontal upward- and downward-facing orientation with varying sawtooth profiles (60°–30° and 75°–15°) in terrestrial gravity conditions as a precursor to the International Space Station (ISS) experiments. A laser powder bed fusion metal additive manufacturing technique was used to fabricate the test surfaces with 250- $\mu \text{m}$ nucleation sites located on the long slopes. For the test surface with cavities spaced 2-mm apart, the wall superheat was ~4 K lower compared to cavities spaced 1 mm apart in the upward-facing orientation. The downward-facing surface induced passive vapor mobility in the direction of the long slope, and a thin liquid film was observed between the vapor mass and microstructure. This liquid film thickness was predicted using a simplified force balance model modified from a prior parabolic flight experiment. The model predicted a uniform liquid film thickness of $49~\mu \text{m}$ for a vapor slug moving at 13.7 mm/s with the input heat flux at 1.25 W/cm2. These terrestrial tests in the adverse orientation suggest that the sawtooth microstructure will successfully induce vapor mobility in upcoming ISS microgravity experiments. Elements of the final flight ampoule, including the cooling zone, the two-stage pressure relief system, and the high-speed imagery setup, are discussed.

Experiments on measuring the rate of evaporation of liquid sessile droplets into air show that the rate of evaporation increases in the presence of forced convection flows. However, data on the effect of convection on evaporation are often contradictory and should be clarified. The paper presents a numerical analysis of evaporation from the surface of a water droplet subjected to forced convection in the gas phase. The drop is located on a smooth horizontal isothermal substrate; the mode with constant contact angle is considered. The shape of the drop has axial symmetry, the same for the velocities and pressure. Forced convection compatible with the symmetry conditions are represented by flows directed downward along the axis of the system and diverging along the sides near the drop and the substrate. The mathematical model is constructed for evaporation controlled by diffusion in the gas phase and takes into account surface tension, gravity, and viscosity in both media, buoyancy and Marangoni convection. The results indicate the existence of the mutual influence of liquid and gaseous media. Thus, a drop vibrates under the influence of movements in the atmosphere, which generates a density wave in the gas: the drop «sounds». The magnitude of the velocity in a liquid is 50 times less than the characteristic velocity in air. It is found that the evaporation rate does not change in the presence of forced convection flows, which contradicts most of the experimental works. The reason for the discrepancies is supposed to be the appearance of nonequilibrium conditions at the boundary of the condensed phase: under these conditions, the evaporation regime ceases to be diffusional.

This research communicates the thermal assessment of Sutterby nanofluid containing the gyrotactic microorganisms with solutal and Marangoni boundaries. The applications of melting phenomenon and thermal conductivity are also considered. The flow is confined by a stretched cylinder. The prospective of Brownian motion and thermophoresis diffusions are also taken account via Buongiorno nanofluid model. The problem is formulated with help of governing relations and equations which are altered into dimensionless form via appropriate variables. The numerical scheme based on shooting scheme is employed to access the solution. A comparative analysis is performed to verify the approximated solution. The observations reveal that the velocity profile enhanced with the Marangoni number while a declining velocity profile has been observed with Sutterby nanofluid parameter and Darcy resistance parameter. The nanofluid temperature get rise with thermal conductivity parameter and thermal Biot number. An arising profile of nanofluid concentration is observed for concentration conductivity parameter and buoyancy ratio parameter.

The structure of Jiangmen Underground Neutrino Observatory (JUNO) central detector is a complex structure subjected to a complex environment. The inner continuous acrylic spherical shell is connected to the outer reticulated stainless-steel (SS) shell through SS rods, and all these structural components are supported by an SS supporting frame. The inner shell is subjected to considerable buoyancy since the detection liquid inside the inner shell has less density than the outside water. Optimization of load path of such an underground structure subjected to buoyancy and pressure loads is difficult due to these complex situations, and in this paper, we tackle the problem of finding the optimal load path to minimize the rod forces. We adopt three types of design variables relevant to the load path, i.e., rod locations, chimney liquid level, and disc spring stiffness. Specifically, B-spline function is adopted to map the rod locations so as to significantly reduce the number of variables, and the chimney liquid level that influences the pressure distribution is taken as a new type of design variable. Subsequently, a numerical method combining the finite element analysis and a derivative-free optimization algorithm is proposed. Based on the optimization results, influences of the design variables, including their types, number, and initial values, on the optimal solutions are discussed in depth. The results suggest that the chimney liquid level and disc spring stiffness have a global effect of shifting rod force distribution, while distribution of rod forces strongly depends on the rod locations. The proposed method is shown to be effective to find an optimal load path of a complex spherical shell structure under various nonlinear constraints.

The modeling of the fluidization or sedimentation velocity of a suspension of solid particles is revisited by examining experiments conducted in either a liquid or a gas. A general expression is found in the case of negligible fluid inertia, i.e. at low Reynolds or Archimedes number. It is built as the product of the velocity of an isolated particle by three non-dimensional corrections that each takes into account a specific physical mechanism. The first correction reflects the variation of the buoyancy with the particle concentration. The second correction describes how the drag force increases with the concentration in case of negligible particle inertia. The third one accounts for the further increase of the drag when the particle inertia is increased. Remarkably, each correction only relies on a single of the three independent non-dimensional groups that control the problem: (1) the particle volume fraction Φs; (2) the ratio Φs/Φpack where Φpack is the bed packing concentration; (3) the Stokes number S t 0 , which characterizes the inertia of the particles and controls their agitation. Moreover, the onset of the instability that separates the homogeneous regime from the heterogeneous one is found to be controlled similarly by the Stokes number. Empirical expressions of the corrections are given, which provide a reliable tool to predict fluidization and sedimentation velocities for all values of the three non-dimensional numbers. The present results emphasize the crucial role of particle inertia, which is often disregarded in previous modeling approaches, such as that of Richardson and Zaki.

The combined effects of thermal radiation, buoyancy force and variable heat source on an unsteady MHD flow of a conducting fluid through a porous walled channel is theoretically investigated. Base on some simplified assumptions, the model partial differential equations are obtained and tackled analytically using variable separable technique. Numerical solutions depicting the impact of various embedded thermophysical parameters on the fluid velocity and temperature profiles, skin friction and Nusselt number are displayed graphically and quantitatively discussed. An escalation in both skin friction and heat transfer rate is observed with a rise in fluid injection–suction at the channel walls.

This paper examines the aggregate impacts of magnetic field, thermophoresis, Brownian motion, variable viscosity, chemical reaction and radiative heat flux on the thermal putrefaction and immanent irreversibility of a channelling nanoliquid film flowing along with a slanted heated permeable plate. Following Buongiorno approach, the two-phase nanoliquid nonlinear model is obtained and addressed numerically using shooting technique as well as the Runge-Kutta- Fehlberg integration scheme. Effects of various emerging parameters on the overall flow structure with heat and mass transfer characteristics including entropy generation rate and Bejan number are displayed using diagrams and discussed. It is found that the entropy generation rate lessened with an upsurge in a magnetic field but heightened with an elevation in the buoyancy forces.

In many industrial applications, heat transfer and tangent hyperbolic fluid flow processes have been garnering increasing attention, owing to their immense importance in technology, engineering, and science. These processes are relevant for polymer solutions, porous industrial materials, ceramic processing, oil recovery, and fluid beds. The present tangent hyperbolic fluid flow and heat transfer model accurately predicts the shear-thinning phenomenon and describes the blood flow characteristics. Therefore, the entropy production analysis of a non-Newtonian tangent hyperbolic material flow through a vertical microchannel with a quadratic density temperature fluctuation (quadratic/nonlinear Boussinesq approximation) is performed in the present study. The impacts of the hydrodynamic flow and Newton’s thermal conditions on the flow, heat transfer, and entropy generation are analyzed. The governing nonlinear equations are solved with the spectral quasi-linearization method (SQLM). The obtained results are compared with those calculated with a finite element method and the bvp4c routine. In addition, the effects of key parameters on the velocity of the hyperbolic tangent material, the entropy generation, the temperature, and the Nusselt number are discussed. The entropy generation increases with the buoyancy force, the pressure gradient factor, the non-linear convection, and the Eckert number. The non-Newtonian fluid factor improves the magnitude of the velocity field. The power-law index of the hyperbolic fluid and the Weissenberg number are found to be favorable for increasing the temperature field. The buoyancy force caused by the nonlinear change in the fluid density versus temperature improves the thermal energy of the system.

An analysis has been carried out to understand the consequences of side-by-side gap-ratio on thermal buoyancy-assisted two-dimensional flow past a pair of heated circular cylinders for a dominant viscous flow field. This is implemented through studies at Reynolds number ( Re) ranging from 5 to 40, Prandtl number ( Pr) 0.7, gap-ratio ( T/D) 1.5 to 4 and Richardson number ( Ri) 0 to 1. An ANSYS-based incompressible flow solver is used with Boussinesq approximation to account for density variations in the momentum equation. One can realize features like the steady-separated and steady-unseparated flow on varying flow and thermal parameters. Unlike streamlines, non-interacting isotherms are non-existent in the current numerical framework. The influence of gap-ratio on enhancement in Nusselt number ( Nu) is the best realized at T/D = 1.5 and buoyancy-aided effects play a dominant role for enhancement in Nu at diffusion and/or viscous-dominant conditions occurring at Re = 5. Correlations are developed to quantify the impact of T/D, Re, and Richardson number Ri on Nu. For the first time, Nu’s correlation based on varying side-by-side gap-ratio has been stated in a single expression. Finally, a comparison for the heat transfer enhancement/reduction in Nu under a similar numerical framework is provided with cases of high-Pr flow and/or different relatable flow arrangements for circular and square cylinders.

Purpose The purpose of this article is to study the steady laminar magnetohydrodynamics mixed convection stagnation-point flow of an alumina-graphene/water hybrid nanofluid with spherical nanoparticles over a vertical permeable plate with focus on dual similarity solutions. Design/methodology/approach The single-phase hybrid nanofluid modeling is based on nanoparticles and base fluid masses instead of volume fraction of first and second nanoparticles as inputs. After substituting pertinent similarity variables into the basic partial differential equations governing on the problem, the authors obtain a complicated system of nondimensional ordinary differential equations, which has non-unique solution in a certain range of the buoyancy parameter. It is worth mentioning that, the stability analysis of the solutions is also presented and it is shown that always the first solutions are stable and physically realizable. Findings It is proved that the magnetic parameter and the wall permeability parameter widen the range of the buoyancy parameter for which the solution exists; however, the opposite trend is valid for second nanoparticle mass. Besides, mass suction at the surface of the plate as well as magnetic parameter leads to reduce both hydrodynamic and thermal boundary layer thicknesses. Moreover, the assisting flow regime always has higher values of similarity skin friction and Nusselt number relative to opposing flow regime. Originality/value A novel mass-based model of the hybridity in nanofluids has been used to study the foregoing problem with focus on dual similarity solutions. The results of this paper are completely original and, to the best of the authors’ knowledge, the numerical results of the present paper were never published by any researcher.

Over a decade, coal-based thermal power plants are upgraded to operate at supercritical pressure conditions due to its high efficiency and low emissions. Water wall panels of a typical supercritical boiler are structured spirally in the lower furnace and vertically placed in the upper furnace. The spiral tubes are inclined at 19 to 22 degrees in which fluid behaves as in horizontal tubes. The design of water wall panels plays the key role in designing a supercritical boiler. The present work aims to numerically investigate the heat transfer behavior of both vertical and horizontal tubes at the supercritical conditions. Since the temperature distribution across the cross-section of vertical tube is uniform, a 2D axis symmetry tube has been considered for analyzing the vertical tube. Unlike vertical tube, the heat transfer characteristics is different for horizontal tubes. Therefore, a 3D tube has been modelled for the computation of horizontal tubes. In order to gain confidence, the present simulations are validated with experiments results available in the literature. Ansys-Fluent has been used in the present simulation. SST k-ω turbulence model is used in this analysis. In the present work, 10 mm diameter of 4m length of vertical tube has been chosen and simulated at low heat flux to mass flux ratio 0.27 and high heat flux to mass flux ratio 0.67 with pressure 241 bar. The effect of heat flux (q) to mass flux (G) ratio which is responsible for heat transfer enhancement and heat transfer deterioration has been studied for both vertical and horizontal tubes. The wall temperature has been plotted along the length of the tube for both top and bottom portion of horizontal tube and compared with wall temperature of vertical tube. The effect of buoyancy plays a vital role in the heat transfer behavior of horizontal tube compared to vertical tube. Heat transfer deterioration occurs due to buoyancy which has a direct linkage with gravity. Three cases were studied, one with full gravity (factor 1), half gravity (factor 0.5) and zero gravity (factor 0). It has been observed that, sudden rise in wall temperature occurs for the case gravity factor 1.0, i.e, considering the gravity effect. For the case of zero gravity, no sudden peak of local wall temperature is observed due to the absence of buoyancy term in the Navier-Stokes equations. Some of the thermo-physical properties like velocity, turbulent kinetic energy, density, wall temperature and turbulent viscosity are analyzed for three cases.

In this numerical study, sediment transport and bed evolution in the surf zone under plunging breakers are investigated; this is a challenging topic that involves complex hydrodynamics coupled with sediment dynamics. Here, a Eulerian–Eulerian multi-phase model for sediment transport, with the capability to capture the free surface, is used. Waves are generated using an open-source toolbox ‘waves2foam’, based on a relaxation method. The model is validated with published experimental results in three cases: (i) surf-zone hydrodynamics over a sloped-rigid bed, (ii) suspended sediment concentration over a sloped-sediment pit, and (iii) bed profile change of a sloped-mobile bed. The profiles of time-averaged velocity, concentration, sediment fluxes, and the time series of concentration are presented. The numerical results suggest that the undertow plays a key role in offshore sediment transport. The instantaneous flow fields show that the vortices under plunging breakers roll up the sediment layers originally near the bed; in such an event, the buoyancy provides a centripetal force to sediment particles. The vortices further transport the suspended sediment onshore and cause strong mixing in water column. This is marked as the underlying mechanism for significant wave-driven flux in the surf zone.

To investigate the effect of the tracer on the fluid flow in the water model, a mathematical model which involves in solute buoyancy is developed to describe the flow field in a single-strand tundish water model. The numerical results show that the tracer solute buoyancy has a great effect on the flow state in the water model, and the RTD curve by saturated KCl solution tracer is closer to the ideal RTD curve than that by saturated NaCl solution tracer. With the decrease of the saturated KCl solution volume or mass concentration, the RTD curves change from the double-peak RTD curve to the multi-peak RTD curve and then to the single-peak RTD curve. The dimensionless tracer number Tr is proposed to analyze the effect of solute buoyancy on the flow field, and its distribution is similar to the tracer mass concentration distribution. The dimensionless tracer parameter number Dtp is proposed to regulate the parameters of the tracer. For the water model, KCl tracer is better than NaCl tracer, and Dtp < 4.44 $$\times$$ 10-5 can be applied to determine the KCl tracer parameter. In the industry production case, when Dtp < 1.18 × 10-5, the effect of Ni on flow field can be ignored.

Hot-point drills have been widely used for drilling boreholes in glaciers, ice caps and ice sheets. A hot-point drill melts ice through the thermal head at its bottom end. Penetration occurs through a close-contact melting (CCM) process, in which the ice is melted, and the meltwater is squeezed out by the exerted force applied on the thermal head. During the drilling, a thin water film is formed to separate the thermal head from the surrounding ice. For the hot-point drill, the rate of penetration (ROP) is influenced by several variables, such as thermal head shape, buoyancy corrected force (BCF), thermal head power (or temperature) and ice temperature. In this study, we developed a model to describe the CCM process, where a constant power or temperature on the working surface of a thermal head is assumed. The model was developed using COMSOL Multiphysics 5.3a software to evaluate the effects of different variables on the CCM process. It was discovered that the effect of thermal head shape and the cone angle of conical thermal head on ROP is less significant, whereas the increase in the BCF and the power (or temperature) of the thermal head can continuously enhance the ROP.

In this study, the interaction between a spherical solid particle descending in a fine-bubble plume and the fine-bubbles around the particle is investigated experimentally. The fine-bubbles, which are formed by water electrolysis and have an average diameter of 0.037 mm, rise via buoyancy to form bubble plumes. The average bubble velocity in the conduit is 0.05 mm/s. A spherical solid particle with a diameter of 11.1 mm and a density of 1130 kg/m3 is dropped into the fully-developed bubble plume. The particle falls in a meandering motion in the fine-bubble plume. In this process, the terminal velocity of the particles falling in the fine-bubble plume is almost identical to that of a particle falling in the quiescent water. Additionally, after the fine-bubbles have been separated from the particle surface, they flow into the wake of the particle, forming a stagnant region. The particle wake diameter increases threefold, as compared with its value in the downstream direction of the fine-bubble plume.

The present work focuses on the study of mixed convection of a purely viscous shear-thinning fluid in a horizontal annular eccentric duct. The inner and outer cylinders are heated with constant and uniform heat flux densities. The objective of this work is to study the effect of the variation of eccentricity, rheological behavior of the fluid as well as the thermodependency of the rheological parameters on the reorganization of the flow and thermal stratification caused by the buoyancy forces. At the entrance of the heating zone, the dynamic regime is assumed to be established and the temperature profile uniform. The conservation equations are solved numerically using a finite difference method with implicit schemes. A secondary azimuthal flow, induced by natural convection, develops downstream of the inlet section. This flow creates a stratification of the thermal field on a given section of the duct, which intensifies downstream from the entrance. On the other hand, the decrease in consistency with increasing temperature near the heated walls produces a centrifugal radial flow towards the walls. The presence of an eccentricity induces in turn a significant effect on the main dynamic field and the stratification of the thermal field. Two cases of upward and downward eccentricity are treated. These show that an upward shift increases the stratification of the thermal field, while the stratification begins to weaken from a certain amount of eccentricity in the case of downward shift. This represents an important result in terms of possible industrial applications. We may indeed conclude that an appropriate choice of downward eccentricity can reduce the thermal stratification, observed experimentally in the case of a concentric heated annular duct [1], when this stratification is undesirable. The choice of this eccentricity depends on rheological and thermal properties of the fluid.

The mathematical model for nanomaterial convection through a complex geometry in presence of Kelvin force has been reported in this article. To involve the feature of nanomaterial, single phase model was selected which has nice accuracy for low concentration of iron oxide. Impose of magnetic source next to inner hot wall generates the new force which helps the nanofluid migration. The mathematical equations were solved via CVFEM. The accuracy of method was checked with comparing results against old publication. The styles of flow and isotherms with change of Ra, MnF were examined in results. Augment of Kelvin force converts the sole vortex to two eddies with opposite directions in low Ra and thermal plume appears inside the domain. Rise of buoyancy makes the strength of eddy to augment and Kelvin force makes the streamline values to increase. Nu enhances about 179. 36% with rise of Kelvin forces when Ra = 1e5. Nu augments about 11.13% with dispersing nanoparticles in to H2O. Rise of Ra makes Nu to grow about 119.97% when MnF = 4000.

With the development of industrial technology, heat transfer at the microscale has attracted more and more attention. In this work, 200 µm platinum wire and 150 µm nickel-chromium wire were used as experimental objects, which the power was provided by DC power with the range of 15.6 W to 56.2 W. Distilled water was used as the experimental liquid. Various bubbles on the micro wire were observed and the heat mechanism was analyzed. A variety of bubble attachment phenomena were captured on the 200 µm platinum wire, including the adhesion during bubble detachment, the rotation of small attached bubbles on the surface of large bubble, multiple bubbles circling at the top of the same bubble, and different attached bubble departure phenomena. Marangoni force in the vertical direction triggered the formation of bubble attachment. In addition, the effects of surface tension, adhesion force and buoyancy force on the circling of the bubble were also considered. The analysis of the bubble sweeping on the 150 µm nickel-chromium wire was analyzed. The results showed that the static bubble would interact with the sweeping bubbles on the other side, thereby changing the heat transfer mechanism, which was not discussed in detail before. The bubble jet flow generated by thermocapillary convection on the vertical direction of the bubble surface was the main influencing factor, which would change the microlayer encountered in front of the bubble. The effects of bubble diameter and liquid subcooling on the sweeping velocity were also studied. The results showed that the larger the bubble diameter was, the lower the sweeping velocity would be achieved while the liquid subcooling temperature had less impact on the velocity of sweeping bubble.

The previous studies related to the thermal-driven flow within enclosures had been summarized in the present work. Various geometries of enclosures like square, rectangular and triangular had been summarized. Besides, enclosures filled with different fluids had been taken into consideration like traditional and nanofluids as well as porous medium, Newtonian and non-Newtonian fluids and multilayer systems. The governing equations of heat transfer and fluid flows had been presented for different cases. Different numerical models like homogeneous, inhomogeneous and thermal non-equilibrium model, Darcy, Darcy extended–Forchheimer model, etc., had been summarized. The influence of various dimensionless parameters like Rayleigh, Darcy, Bejan and Hartmann number, nanofluid loading, diverse thermal cases of the applied boundary conditions, angle of inclination, the number for undulations, the existence of inner body and many others parameters acting and influencing hardly up on both of the entropy generation and the heat transfer was illustrated. The present review illustrates the physical mechanism behind the buoyancy thermally driven flow in terms of figures of contours as well as the Nusselt numbers profiles.