Thermal Buoyancy Effects Research Articles

Analysis of elasticity on the flow of blood through the transient permeable channel with an interaction of radiation

In general, the development of arterial diseases is characterized by the heat and solutal transport phenomena of blood. Also, the transport of macromolecules with mixed gasses via the arterial wall enhances due to the growth of the atherogenetic process. Therefore, an analysis is presented on the time-dependent conducting flow of viscoelastic fluid of blood flow through the artery for the inclusion of the heat source and thermal radiation. Free convective flow due to the combined effect of thermal and solutal buoyancy is affected by the transient permeability of the medium. The present biomechanical problem is designed by the aforesaid assumptions and the transformation of the non-dimensional for is obtained using the proposed substitution of the variables. Further, the perturbation technique along with the numerical method is deployed for the solution of the designed problem. The parametric behavior of the flow phenomena is obtained graphically and the numerical simulation of the rate coefficients is deliberated via tables. Finally, the important results are presented as dual characteristic in the fluid velocity is rendered in different regions for the impact of elasticity parameter, i.e. near the sheet region it attenuates whereas the reverse trend is observed far away from it.

Modeling of Multiphase Flow, Superheat Dissipation, Particle Transport, and Capture in a Vertical and Bending Continuous Caster

A new model of particle entrapment during continuous casting of steel is presented, which includes the effects of multiphase flow from argon gas injection and thermal buoyancy from superheat in the strand. The model simulates three different capture mechanisms, including capture by solidified hooks at the meniscus, entrapment between dendrites, and engulfment by the surrounding of large particles. The fluid flow and bubble capture results are validated with plant measurements, including nail board dipping tests and ultrasonic tests, respectively, and good agreement is seen. Results suggest that the superheat has a negligible effect on the flow in the mold region. However, higher (30 K) superheat causes a more complex flow in the lower strand by creating multiple recirculation zones due to the thermal buoyancy effects. This causes less penetration deep into the strand, which leads to fewer and shallower particle captures. Lower (10 K) superheat may enable significant top surface freezing, leading to very large internal defect clusters. Lower superheat also leads to deeper meniscus hooks, which sometimes (0.003%) capture large (1 mm) bubbles. Capture bands occur near the transition line from vertical to curved, due to the downward fluid velocity balancing the particle terminal velocity, enabling capture in the relative stagnation region beneath the longitudinal recirculation zone. These findings agree with plant observations.

Cilia beating modulated radiating ternary nanofluids flow in a corrugated asymmetric channel with electromagnetohydrodynamic and momentum slip

AbstractA mathematical model is presented to study the cilia beating modulated radiated ternary nanofluid flow between two cilia‐carpeted walls propagating with an applicable phase difference to show the asymmetric nature of the pumping. To regulate further the cilia‐based pumping, electroosmosis, and magnetohydrodynamics mechanisms are considered in this model. Ternary nanofluid is considered with TiO2, SiO2, and Al2O3 nanoparticles dispersed in pure blood. The momentum slip condition is employed to derive the model solution. Thermal radiation and buoyancy effects are also taken into consideration. Mathematica NDSolve is utilized to simulate the numerical results for the velocity, temperature, and concentration profiles under the influence of the emerging parameters. Moreover, entropy generation and variation in Nusselt and Bejan numbers are computed for better thermal analysis. A comparative thermal analysis for unary, binary, and ternary nanofluids is also made. The results indicate that the trihybrid nanofluid offers an 8.5% increase in the heat transfer rate as compared to the conventional fluid. Furthermore, the Brownian motion is responsible for the enhancement in fluid temperature, while electric double layer thickness helps in increasing heat transfer irreversibility around the channel lower wall. It is found that the total entropy of the system at the channel upper and lower space is minimized in the case of ternary fluid rather than conventional fluid. The promising results of the present model reflect huge applications in biomedical engineering, particularly in designing thermal cilia‐based microfluidic devices.

Study on the fluid flow and heat transfer characteristics of a horizontal elliptical cylinder under thermal buoyancy effect

Thermal buoyancy can be used as a tool for flow control and vortex shedding elimination. In the present study, a numerical investigation of the thermal buoyancy effect on the laminar flow and heat transfer characteristics of an elliptical cylinder with an isothermal surface was performed. The Richardson number varied from 0.0 to 1.5 for each angle of attack, while the Prandtl number is constant at 0.71. The Reynolds number was 1000 based on the major diameter of the cylinder, with an axis ratio (minor to major diameter ratio) of 0.25. The momentum and temperature equations were solved using a pressure-based in-house code with a second-order temporal and spatial accuracy. An initial examination showed that the thermal buoyancy led to the elimination of the vortex shedding or decrease in the flow separation area and also improved the aerodynamic performance of the cylinder. Additionally, it was discovered that insulation of the underside of the cylinder improved aerodynamic performance while reducing thermal energy consumption when compared to a fully isothermal cylinder. Other studies' findings indicated that the drag coefficient increased as the angle of attack increased; however, the Richardson number had little effect on the drag coefficient. The considered problem exhibited three distinct flow patterns: a vortex shedding region, a steady separated flow, and attached flow. Furthermore, a study on the heat transfer characteristics revealed that increasing the Richardson number increased the mean Nusselt number; however, for α > 15o, the convection heat transfer behavior was altered.

Concurrent model for sharp and progressive columnar to equiaxed transitions validated by directional solidification experiments processed in microgravity conditions

Columnar and equiaxed structures, which occur during solidification of metallic alloys, influence the texture and properties of castings, welded joints and additively manufactured components. During transient solidification, where grain refiner particles provide the predominant nucleation mechanism, a Columnar to Equiaxed Transition (CET) occurs when conditions that had originally favoured directional columnar growth change to those favouring equiaxed. Constitutional undercooling ahead of the columnar front can permit equiaxed nucleation and growth. By carrying out experiments in microgravity conditions, liquid flows due to thermal and solutal buoyancy effects are suppressed. In these diffusion-controlled conditions, we have observed examples of both sharp (clear) and progressive (gradual) CET. The experimental outcomes, especially the observation of a progressive CET, has highlighted the need for a continuum model that allows for competitive columnar and equiaxed structure development; hence, the Concurrent Columnar to Equiaxed Transition (C2ET) model is proposed. The C2ET is thermally transient and relies on the well-known concept of extended growth for impingement mechanics; thereby, greatly reducing numerical complexity. Importantly, the proposed approach removes the need for a specific equiaxed-blocking criterion, which is often proposed as an essential requirement in other CET models. The C2ET model is validated by four experimental solidification scenarios: two velocity jumps and two thermal-gradient decreases. The velocity jumps induced sharp CETs; whereas, thermal-gradient decreases gave progressive CETs. The C2ET model gave good agreement for the columnar and equiaxed transition zones for both sharp and progressive CET. Results are compared with the classic Hunt model. Unlike Hunt’s model, the C2ET model predicted all macrostructure transitions faithfully using a single (or consistent) set of nucleation input parameters across all four scenarios. Since, the same level of grain refinement was used in each experiment, a consistent set of nucleation parameters was expected. The validated approach can enable effective simulation at lower computational cost for industrial processes that rely on a solidification processing step.

Theoretical and experimental investigation of AC and DC arc propagation on polluted insulators surface

To acquire new knowledge of pollution flashover mechanism of insulators, the mechanical characteristics of local arc propagation on suspension insulators surface during AC and DC pollution flashover are analyzed in this paper. The results indicate that the propagation of local arc mainly depends on the electrostatic force and thermal buoyancy. The thermal buoyancy plays a leading role in the bigger arc current, resulting in the obvious phenomenon of arc wandering. Moreover, the effect of thermal buoyancy on AC arc is less than that on DC arc at the same arc current. DC arc wandering is more significant than that of AC. With the decrease in air pressure (the increase in altitude height), the effect of thermal buoyancy rises. As a result, it will cause severe arc-levitating at high altitudes, making the flashover distance of insulators less than its creepage distance. Therefore, the arc-levitating is one of the key factors for the decrease in flashover voltage of polluted insulators in high-altitude regions.

Thermo-solutal dual stratified convective magnetized fluid flow from an exponentially stretching Riga plate sensor surface with thermophoresis

A theoretical study is presented for the coupled thermo-solutal free convection two-dimensional boundary layer flow of a of a magnetized fluid from an exponentially stretched magnetic sensor (Riga plate) surface. Heat generation/absorption, nonlinear thermal radiation and thermophoretic body force effects are included. Furthermore, thermal and solutal stratification are also featured in the boundary layer model. The derived nonlinear partial differential equation system with associated wall (sensor surface) and free stream boundary conditions is transformed to system of ordinary differential equations (ODE) via applicable similarity variables. A numerical solution is developed with the efficient Runge–Kutta–Fehlberg (RKF) technique with a shooting numerical method, in MATLAB software using the RKF-45 method. The graphical profiles were represented to examine the impacts of physically parameterics on the important physical stream features. Streamline and isotherm plots are also included for thermal buoyancy effect (Grashof number).Validation of solutions with earlier simpler models is included. A relatively well agreement is achived between model prediction and the benchmark value. Values for skin friction factor, Nusselt number and Sherwood numbers are also tabulated to quantify momentum, heat and mass (species) transfer characteristics at the sensor surface. The results indicate that the significant depletion in temperature accompanies accumulation in magnetization and thermal stratification parameters. Significant accumulation in mixed convection and Riga plate electrode width parameter enhance the concatenation profiles. Nusselt number grows substantially as thermal stratification and radiative parameter increases. Sherwood number grows substantially as solutal stratification, thermophoresis and Schmidt parameter increases. The present study generalizes previous models to include simultaneously exponential stretching, thermophoresis, thermal and solutal (mass) stratification and heat source/sink effects.

Behaviour of a Non-Newtonian Fluid in a Helical Tube Under the Influence of Thermal Buoyancy

Abstract This work is an evaluative study of heat transfer in the helical-type heat exchanger. The fluid used is non-Newtonian in nature and is defined by Oswald’s model. The work was performed numerically by solving each of the Navier–Stokes equations and the energy equation using the package ANSYS-CFX. Following are the aspects that have been dealt with in this paper: the effects of thermal buoyancy, fluid nature and the tube shape on the heat transfer, and the fluid comportment. The interpretation of the obtained results was done by analyzing the isotherms and the streamlines. The mean values of the Nusselt number were also obtained in terms of the studied parameters. The results of this research enabled us to arrive at the following conclusion: the intensity of thermal buoyancy and the nature of the fluid affect the heat transfer distribution but keep the overall rate of heat transfer the same.

Newtonian Heating Effect on Heat Absorbing Unsteady MHD Radiating and Chemically Reacting Free Convection Flow Past an Oscillating Vertical Porous Plate

In this article, we have discussed in detail the effect of Newtonian heating on MHD unsteady free convection boundary layer flow past an oscillating vertical porous plate embedded in a porous medium with thermal radiation, chemical reaction and heat absorption. The governing PDEs of the model together with related initial and boundary conditions have been solved numerically by the finite element method. The dimensionless velocity, temperature and concentration profiles are analyzed graphically due to the effects of key parameters in the concerned model problem. Computed results for the skin friction coefficient, Nusselt number and Sherwood number are put in tabular form. It is observed that the thermal and mass buoyancy effects support the velocity whilst a reverse effect is noticed when the strength of the magnetic field is increased. The velocity and temperature enhances with an increase in the Newtonian heating and thermal radiation whilst a reverse effect is observed with an increase in the Prandtl number and heat absorption parameter. Increasing Schmidt number and chemical reaction parameter tends to depreciate both velocity and concentration. The Newtonian heating, thermal radiation and magnetic field tends to decrease in the skin friction. The Nusselt number increases with increasing Newtonian heating and heat absorption parameters. An increase in the Schmidt number and chemical reaction rate tends to improve the Sherwood number.

Breakdown Characteristics of Rod–Plane Gap in an Ethanol Flame

The wildfires nearby high-voltage transmission lines are frequently reported to induce outages in many countries. Less research addresses the role played by individual characteristic factor (i.e., high temperature, intense heat, etc.) of wildfires in triggering phase-to-ground flashovers of transmission lines. Aiming at this, systematic laboratory experiments are performed to reveal the breakdown characteristics of rod-(ring-like) plane gap in clean ethanol flames. By setting different scenarios of pure air gap, nonbridged and bridged gaps by ethanol flames, the breakdown voltage, average breakdown strength, and leakage current with various gap spacings are measured and analyzed. Results show that the flame centerline positions at a height of 4.0–6.0 cm are regarded as the main reaction zone. The breakdown processes of rod–plane gaps in ethanol flames are the combined results of diverse discharge patterns. Compared with the pure air case, the breakdown voltages are decreased by 27.8%–70.5% in the nonbridged gap and by 68.3%–77.1% in the bridged gap by ethanol flames, and their corresponding levels of average breakdown strengths decrease to 45.7% and 27.6%, respectively. Under the alternating current (ac) electric field, the coupled effects of electric field force, thermal buoyancy, and ionic wind together promote the formations of streamer channels in ethanol flames. Finally, the high-temperature flame itself is found to be the most dominant factor in influencing the breakdown.

Entropy analysis in an unsteady MHD flow of a radiating fluid through a vertical channel filled with a porous medium.

This paper investigates the blended effects of thermal buoyancy, magnetic field and radiative heat flux on the unsteady flow of an electrically conducting fluid through a permeable walled passageway filled with a penetrable medium. The model non-linear partial differential equations are obtained and solved analytically using separation of variable techniques and numerical for fluid velocity and fluid temperature. The two functions are then used to obtain entropy generation rate, skin friction, Nusselt number and the Bejan number. The effects of manifold novel parameters on the overall flow structure and thermal putrefaction are displayed graphically and discussed. One of the crucial findings of the current model is that entropy generation rate diminished temporally and with an upsurge in a magnetic field but amplified with a soar in porous medium permeability, thermal buoyancy and fluid Prandtl number.

Effect of beam oscillation on the fluid flow during laser welding

In the present study, a finite element based numerical model is developed to evaluate heat transfer and fluid flow during the laser welding process with a moving heat source. The developed model solves the fully coupled equations of incompressible fluid flow and heat transfer. In this study, laser beam welding involving non-oscillating to oscillating beam is compared with both conditions under similar heat input per unit length i.e., same power and welding speed. Dimensionless coefficients for mass and heat transport were used to analyse the effects of Marangoni flow and thermal buoyancy. Varying combinations of radius and frequency of oscillations are studied at a constant circumferential velocity.

Heat source and sink effects on periodic mixed convection flow along the electrically conducting cone inserted in porous medium.

The system of partial differential equations governing the unsteady hydromagnetic boundary-layer flow along an electrically conducting cone embedded in porous medium in the presence of thermal buoyancy, magnetic field, heat source and sink effects are formulated. These equations are solved numerically by using an implicit Finite-Difference Method. The effects of the various parameters that are source/sink parameter, porous medium parameter, Prandtl number, mixed convection parameter and magnetic Prandtl number on the velocity, temperature profiles, transverse magnetic field are predicted. The effects of heat source and sink parameter on the time-mean value as well as on transient skin friction; heat transfer and current density rate are delineated especially in each plot. The extensive results reveal the existence of periodicity and show that periodicity becomes more distinctive for source and sink in the case of the electrically conducting cone. As the source and sink contrast increases, the periodic convective motion is invigorated to the amplitude and phase angle as reflect in the each plot. The dimensionless forms of the set of partial differential equations is transform into primitive form by using primitive variable formulation and then are solved numerically by using Finite Difference Scheme which has given in literature frequently. Physical interpretations of the overall flow and heat transfer along with current density are highlighted with detail in results and discussion section. The main novelty of the obtained numerical results is that first we retain numerical results for steady part and then used in unsteady part to obtain transient skin friction, rate of heat transfer and current density. The intensity of velocity profile is increased for increasing values of porosity parameter Ω, the temperature and mass concentration intensities are reduced due heat source effects.

Study on Effects of Thermal Resistance and Thermal Buoyancy on Oxygen Flow Patterns during Underground Coal Gasification

The controllable growth of the cavity is the basis by which underground coal gasification (UCG) can achieve stable production, and the oxygen flow path and velocity are important factors in determining the expansion rate of the cavity. In this paper, a mathematical model of UCG in horizontal channels was developed, and the effects of multiple factors, including temperature, pressure, flow velocity, and the size of the cavity, on the flow pattern, path, and velocity field distribution of oxygen in the cavity were investigated by using COMSOL Multiphysics software. The results showed that temperature and pressure were the influencing factors of thermal buoyancy. In the established model, oxygen formed a counter-clockwise air vortex at the gas injection port under normal pressure, and the range of the air vortex and the gas flow rate increased with the increase in temperature. In the high-temperature area located in the center of the cavity, the phenomenon of oxygen throttling occurred, and the oxygen flow velocity increased. When the maximum temperature in the cavity was over 850 °C, gas back-mixing occurred at the end of the cavity. Under pressurized conditions, the air vortex at the inlet and back-mixing phenomenon at the outlet disappeared. The flow velocity and the cross-sectional area of the cavity determined the thermal resistance. In the model, the flow velocity was between 0.045 and 0.40 m/s and there were both airflow vortices and back-mixing. In addition, with the expansion of the cavity, back-mixing progressively decreased at the outlet and the airflow vortex changed from counter-clockwise to clockwise.

Non-Coaxially Rotating Motion in Casson Martial along with Temperature and Concentration Gradients via First-Order Chemical Reaction

The effect of non-coaxial rotation on the transport of mass subjected to first-order chemical reaction is studied analytically. The effects of thermal radiation, buoyancy, constructive and destructive chemical reactions along with Casson fluid in rotating frame are discussed. Time evolution of primary and secondary velocities, energy and solute particles are analyzed. The behavior of flow under the variation of intensity of magnetic field is also investigated. Evolutionary behavior of primary velocity is opposite to the evolutionary behavior of secondary velocity. The impact of buoyant force on primary velocity is opposite to the role of buoyant force on the secondary velocity. The evolutionary behavior of temperature is also examined and a remarkable enhancement in temperature is noticed. Thermal radiation causes the fluid to be cooled down as heat energy is escaped by thermal radiation. Evolutionary behavior of concentration is also analyzed and an increasing of concentration versus time is noted. Destructive chemical reaction results a remarkable reduction in the concentration and vice versa for generative chemical reaction.

Effect of Wave Instabilities in Thermal and Solutal Mixing of Double-Diffusive Opposed Jets Impinging in a Passive-Mixer

Abstract Two-dimensional (2D) numerical experiments are performed to investigate the flow instabilities and mixing of different nonisothermal counterflowing jets in a passive-mixer. The fluid is modeled as a binary mixture with thermal and solutal buoyancy effects considered through the Boussinesq approximation. The streams are arranged in a thermal and solutal buoyancy aiding configuration. Computations are carried out for three different ratios of the upper jet bulk velocity to the lower jet bulk velocity (VR), namely, VR = 0.5, 1.0, and 2. Within the parametric domain of RiT and RiC defined by region (RiT + RiC) ≤ 3, the instability causing transition from steady to unsteady flow regime is observed for VR = 1 and 2, while no transition is found to occur at VR = 0.5. Using Landau theory, it is established that the transition from steady to unsteady flow regime is a supercritical Hopf bifurcation. A complete regime map identifying the steady and unsteady flow regimes, within the parametric space of this study, is obtained by plotting the neutral curves of RiC and RiT (obtained using Landau theory) for different values of VR. Proper orthogonal decomposition (POD) analysis of the unsteady flows at VR = 1 establishes the presence of standing waves. However, for VR = 2, the presence of degenerate pairs in the POD eigenspectrum ascertains the presence of traveling waves in the unsteady flows. The standing wave unsteady flow mode is found to yield the highest rate of mixing.

A New Heat Dissipation Model and Convective Two-Phase Nanofluid in Brittle Medium Flow over a Cone

A time-dependent convectional flow of a two-phase nanofluid over a rotating cone with the impact of heat and mass rates is elaborated in this article. The instability in the flow field is induced by the cone angular velocity that depends on the time. The Navier–Stokes self-similar solution and the energy equations are obtained numerically. Here, the achieved solution is not only for Navier–Stokes equations but also for the equations of the boundary layers. In this work, the concentration, Brownian motion, and thermal buoyancy effects have important significance. We have assumed viscous dissipation with heat-absorbing fluid. Similarity answers for spinning cones with divider temperature boundary conditions give an arrangement of nonlinear differential conditions that have been handled numerically. The MATLAB methodology BVP4C is used to resolve the reduced structure of nonlinear differential equations numerically. Observation for skin friction and Nusselt number is also taken into account. Velocity and temperature impact is depicted graphically, while the outward shear stress values and heat allocation rate are included in tables.

Combining EnergyPlus and CFD to predict and optimize the passive ventilation mode of medium-sized gymnasium in subtropical regions

Thermal stratification is considered to be common in large space buildings, which constitutes a complex indoor thermal environment of gymnasiums. In consideration of the combined effect of wind pressure and thermal buoyancy on natural ventilation efficiency, this work combining the building energy modeling (BEM) and computational fluid dynamics (CFD) to evaluate the impact of climatic conditions and building forms on natural ventilation of medium-sized gymnasiums in subtropical regions. The ventilation efficiency at peak time (13:00-15:00) is significantly higher than that at valley time (08:00), which indicates that thermal buoyancy plays a positive role in air change rate (ACH) and wind velocity conditions, and the thermal pressure form with low-inlet and high-outlet has a higher climate adaptability. Using the Adaptive Predicted Mean Vote (APMV) to evaluate the comfort of the combining results, and to effectiveness predict the passive ventilation mode in each quarter. There is a large operating time range (71.35% ∼ 100%) for a medium-sized gymnasium in the subtropical regions to apply the passive ventilation mode in transition seasons of spring and autumn, and the thermal comfort condition of the sports area is better than that of the spectator area. On the other hand, the operating time range shows 36.96%–64.93% in summer as the peak time comfort exceeds the upper limit of the standard value. Since the sports crowd has stronger thermal endurance than the sitting crowd, it is possible to achieve a higher range of energy saving by effectively controlling the operation time and power consumption of air conditioning measures in non-event time (no consideration of the use in spectator area) in hot summer.

Characteristics of self–ignition and smoldering of coal dust layer under inclination conditions

In many actual industrial scenes, the coal body and coal dust are not always in the tiled state, but in the inclined situation. A two – dimensional transient numerical model of coal oxidation reaction was established to study the self–ignition and smoldering characteristics of coal dust layers in the presence of inclination angle. The effects of gravity and thermal buoyancy were considered to solve the coupling problem of coal oxidation reaction and air flow. The results show that the growth coefficients of minimum ignition temperature of coal dust with thicknesses of 6.4 mm, 12.7 mm, 19.1 mm and 25.4 mm are 0.117 °C/°, 0.0467 °C/°, 0.033 °C/° and 0.033 °C/°, respectively. With the increase of hot plate temperature, the inclination of coal dust can greatly shorten the ignition delay time (IDT). The ΔIDT of coal dust inclination of 60° is 128 s/°C greater than that of 0°. The movement of high temperature point of coal dust in self–heating development stage is from the inside to the outside, and that in decaying stage is from the outside to the inside. In addition, some interesting phenomena in the process of coal self–ignition and smoldering have also been found. The influence of coal dust inclination on its self–heating is mainly reflected in the location of high temperature points and the spread of smoldering. This study is helpful to understand the characteristics of coal dust self–ignition and smoldering spread in inclined situation, and can guide the prevention of coal dust self–ignition.

A model for predicting the effects of buoyancy driven convection on solidification of binary alloy with nanoparticles

Convection during the solidification of a binary alloy with nanoparticles governs the nanoparticle, species and temperature distribution in the domain and thus the solidification rate and morphology. Hence, a numerical model to predict the effect of convection on solidification with nanoparticles is essential. In this work, a volume averaged enthalpy method-based model is developed to predict the nanoparticle, solute and temperature distribution during micro-scale solidification of binary alloy with nanoparticles. In addition to nanoparticle transport due to Brownian motion based diffusion, the effects of thermal buoyancy, solutal buoyancy and nanoparticle driven convection are considered in the model. The effect of nanoparticles on the solidification process is also incorporated into the model. From single dendrite simulations, it is observed that convection plays a significant role in dendrite growth and nanoparticle distribution. Also, it is observed that the effect of nanoparticle driven convection is insignificant. Microstructure simulations with large number of dendrites show that the effect of convection on the dendrite growth is diminished due to the reduction in space available for flow in the interdendritic region. It is also found that the presence of convection increases the homogeneity of nanoparticle distribution in the final microstructure.