Convection Phenomena Research Articles

Validation and application of numerical modeling for in-vessel melt retention in corium pools

In a hypothetical severe accident scenario in light water reactors (LWR), after the loss of primary coolant, the melted core moves to the lower plenum of the reactor pressure vessel and accumulate there. The melted core cautiously releases the decay heat which forms a pool of melted core, called corium, and turbulent natural convection starts. The decay heat transferred by the corium to the vessel may result in vessel failure due to the focusing effect in the absence of an effective cooling system. A numerical analysis is carried out using STAR-CCM+ commercial software to investigate the ability of the existing turbulence models and Algebraic Heat Flux Model (AHFM) to simulate the natural convective heat transfer phenomena in the corium pools. Efforts have been made to predict the BALI experimental results which are designed to simulate the heat transfer in corium pools, in the framework of severe accident studies. For BALI experiments various test campaigns are run varying the internal Rayleigh number, the viscosity of the simulant fluid, and test facility height. Various such experimental cases are verified and compared with numerical simulations. Temperature profiles along a vertical line, surface heat flux along the curved surface, average Nusselt number on the top, and curved surfaces are estimated. The effect of wall boundary conditions on heat transfer is analyzed. The numerical analysis is extended to Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) geometries. The CFD analysis predicts the stratified zone and upper mixing zone very well, and the temperature profiles and average heat transfer values are in agreement with the experiments.

Free convection heat transfer with surface radiation from infrared suppression system and estimation of cooling time

• The thermo-fluid characteristics of the full-scale infrared suppression (IRS) system used in the marine gas turbine as an exhaust system are numerically investigated here. • The combined impact of conduction, convection, and radiation on the cooling of the IRS system is intended to be explored. • Various relevant parameters are taken for the analysis, viz., Rayleigh number (Ra), number of funnels, funnel wall temperature, diameter ratio (DR), funnel overlapping (OL), and emissivity of the surface (ε). • A generalized correlation for Nusselt number is developed, considering surface emissivity and the number of funnels as an independent parameter that can be used for academic and industrial applications. • The novelty of the work lies in finding the heat transfer rate and cooling time of the IRS device with varying numbers of funnels (3 to 6 funnels) subjected to the combined effect of free convection and surface radiation. A numerical study is performed on Infrared Suppression (IRS) system having finite wall thickness to observe the combined effect of free convection and surface radiation. The main focus of this work is to obtain thermo-fluid characteristics of natural convection phenomena from the IRS system due to conjugate heat transfer and surface radiation. In addition, the time required to cool down the hot IRS system to atmospheric temperature is also estimated. For the numerical analysis, different governing equations (Navier-Stokes equation, energy equation, and radiation equation) are solved using the finite volume method (FVM) based solver of ANSYS Fluent v 15. Several pertinent parameters, viz., the number of funnels, Rayleigh number, inner surface temperature, geometric ratio, funnel overlapping, and surface emissivity, are varied to elucidate the heat transfer behavior. It is evident from the research that surface radiation has a significant influent on total heat transfer and should not be ignored. In addition, the total heat transfer rate rises with the number of funnels. Both non-dimensional induced mass flow rate and total heat transfer rate have maximum value for zero overlapping cases. At constant temperature contribution of radiative heat transfer varies from 10 to 38% with the rise in emissivity. The inclusion of radiation with convection lowers the cooling time significantly compared to convection alone.

Numerical study of convection phenomenon in hybrid nanofluid flow over vertical rotating cylinder

Hybrid nanofluid is a novel nanotechnology fluid created by distributing two distinct nanoparticles into conventional energy transfer fluid. The thermal properties of aluminum alloys, namely AA7072 and AA7075 with engine oil (base fluid) of hybrid nanofluid flow induced over a vertical stretchable rotating cylinder are investigated in this study. The focus here is the analysis of energy transportation in the presence of Joule heating, thermal energy source/sink, thermal radiation and activation energy. Moreover, all energy constraints are analyzed through graphical outcomes, which are computed numerically through bvp4c built-in MATLAB. The significant observation made is that as the Reynolds number and the magnetic field parameter enhance, the flow field declines. Additionally, the convective transport of the thermal energy increases for a higher magnitude of resistive heating and buoyant motion of the fluid. It is also noted that the activation energy in the system decreases for mass diffusion.

Analysis of forced convection phenomenon in boundary layer flow of nanofluid over power-law rotating disk

In this paper, the thermal analysis is performed on the three-dimensional flow of viscous nanofluid over a rotating disk. The disk rotation is assumed as of the form [Formula: see text] where [Formula: see text], thus, there are different azimuthal velocities of the disk at different radii, and it increases with the increase in [Formula: see text]. Also, the effect of nanoparticles on the thermal conductivity of fluid is investigated by thermophoresis and Brownian motion features. For the governing problem, suitable similarity transformations are employed that convert the produced partial differential equations to ordinary differential equations. The flow field and energy transport are presented graphically and discussed in detail for the torsional exponent [Formula: see text], [Formula: see text] and [Formula: see text], respectively. It is noted that the temperature and concentration profile rise for the thermophoresis parameter while the opposite trend showed for Brownian motion. Moreover, the power-law index parameter [Formula: see text] declines the flow field temperature and concentration profiles.

Probability Forecasting of Short-Term Short-Duration Heavy Rainfall Combining Ingredients-Based Methodology and Fuzzy Logic Approach

Highly convection-related short-duration heavy rainfall (SDHR), defined as rainfall greater than 20 mm h−1 of a whole hour, causes severe damage every year in China. An objective forecasting method is developed to provide guidance products for the short-term probability of SDHR. Representative parameters of environmental moisture content, instability, and dynamical forcing are selected as predictors based on the ingredients-based methodology. The predictors are selected by comparing their ability to discriminate between SDHR and both no rainfall and ordinary rainfall with hourly rainfall records and the NCEP reanalysis dataset during the warm seasons of 2002 and 2009. A fuzzy logic approach is obtained for the calculation of SDHR probability. Intervals of intensities are obtained based on specific percentiles and various weight settings examined. The probabilistic SDHR forecasts during the 2015 warm seasons with the NCEP GFS dataset are obtained, and forecasts are evaluated by using an operational used spatial verification method. Results show that the reference operational SDHR forecasts are surpassed by the 00–12 h period objective SDHR forecasts measured with the maximum critical success index (CSI), and even the average CSI (CSIave) for the top groups is better than the reference. The guidance SDHR products are skillful within 60 h. Although the weights vary significantly, the short-term patterns of the SDHR probability are mainly determined by the environmental conditions. The objective forecasting method is ingredients-based but is combined with fuzzy logic algorithms. The new approach provides a feasible exploration of the convective weather phenomenon.

Simultaneous mapping of buoyancy-induced flow and temperature fields using thermographic PIV

ABSTRACT We report one of the first experimental investigation on simultaneous mapping of whole field temperature and velocity data to demonstrate the potential of thermographic PIV (or LIP/PIV) in the context of natural convection for a back-wall heated open square cavity. As part of the validation exercise, LIP/PIV-based results are compared with corresponding numerically simulated and interferometry-based whole field data. The study holds its importance as it successfully captures the flow physics associated with the buoyancy-driven convective phenomena in the cavity at varying temperature differences using a single imaging technique instead of two separate sets of optical diagnostics.

Zirconium oxide film deposition properties to build transparent electronic devices in conjunction with tin dioxide

ZrO2 thin films are deposited by the sol–gel dip-coating technique where the thermal annealing temperature and the number of deposited layers determine the properties of this material concerning the application in field-effect transistor (FET) devices in conjunction with SnO2 layer, also produced by sol–gel dip-coating. The best annealing temperature for the ZrO2 film was found as 400°C, along with a small number of layers and the position of the layer inside the oven, which determines the dominant heat transport mechanism of conduction or convection phenomena. The electrical insulating property of ZrO2 layers was confirmed with the current in the order of pA, even for layers about 100 nm thick. The temperature of 400°C also leads to a lower leak current through the gate in the device, which is three orders of magnitude lower than the source-drain current (in the order of a tenth of [Formula: see text]A).

Mass Transfer Effects on Mixed Convective MHD Flow of Second Grade Fluid Past a Vertical Infinite Plate with Viscous Dissipation and Joule Heating

An attempt is made to study an unsteady, two-dimensional, laminar, mixed convective Magnetohydrodynamic (MHD) flow of an incompressible visco-elastic fluid (Waltersfluid model) past an infinite vertical plate. The reduced governing equations are solved analytically using two-term harmonic and non-harmonic functions. The effects of different pertinent parameters are discussed with the help of graphs and tables. The novelty of the present study is to account for the effects of viscous and joules dissipative heat and a linear first-order chemical reaction of diffusive species and mixed convective flow phenomena on an infinite vertical plate subjected to time-dependent suction velocity and a transverse magnetic field acting at a distance. The important findings reported herein are: increasing values of chemical reaction parameter cause low velocity and concentration, a decline in concentration profile is seen for the higher values of Schmidt number, Prandtl number contributes to more active convection. The application of the present study may be seen in combustion systems, nuclear reactors, and chemical processes. Before concluding the considered problem, our results are validated with previous results and are found to be in good agreement.

LSM and DTM-Pade approximation for the combined impacts of convective and radiative heat transfer on an inclined porous longitudinal fin

The novel exploration designates temperature variation through an inclined porous longitudinal fine with convective radiative phenomenon. The Rosseland approximation is considered for the radiation heat exchange. Using non-dimensional terms, the existing system of stated problem is reduced to ordinary form. To resolve the nonlinear ordinary system of heat transfer, the analytical approaches, differential transform method (DTM) with Pade approximant has been workout. Furthermore, simulations are developed to examine the outcomes of convincing flow parameters like radiative-conduction parameter, porosity parameter, and conduction-convection variable on the thermal distribution for different modes of heat transfer like nucleate boiling and radiative heat transfer. The results show that an increase in the conduction-convection variable, porosity variable, and radiation number causes a reduction in the thermal field. On the other hand, augmentation change is appeared due to heat generation constant. The thermal behavior of the fin declines with escalating angle.

Mixed convection phenomenon in packed beds: A comprehensive review

This review summarises first the studies that have been performed to date for establishing a suitable model for mixed convection phenomenon in packed beds with working fluids include some simplifying assumptions such as local thermal equilibrium ( L T E ) condition, neglect of channelling effect, and neglect of thermal dispersion effect. It was found that most of these studies are based on Darcy’s model for the flow and on an averaged single equation model ( L T E ) for the energy equation with using the Boussinesq approximation to represent the variation in fluid density. Theoretical, numerical, or experimental studies, or studies which combine these approaches, are reviewed and for various 2- or 3-dimensional geometries such as horizontal, vertical or inclined parallel plate channels, ducts, cylinders, annuli, etc. In addition, the literature showed that there has been resealable efforts exerted to report works on the non-Darcian mixed convection, in which the effects of boundary, inertia, porosity variation, and inclusion of thermal dispersion under local thermal equilibrium condition ( L T E ) or under thermal non-equilibrium condition ( L T N E ). • This article reviews firstly the studies done about mixed convection phenomenon in packed beds using Darcy and (LTE) energy models. • Studies that include non-Darcian effects such as boundary, inertia, and thermal dispersion, are then summarised. • Works about utilising (LTNE) energy model are lastly reported.

Effect of the Moving Wall to the Thermal and Dynamic Field in a Square Cavity with a Heat Source

This study deals with convective heat transfer in a square cavity with a localized heat source at its center. The form factor of this configuration is (h/H=0.6) which is the ratio between the height of the source and the height of the cavity. The Rayleigh number is Ra=106 that indicated a laminar flow in this cavity. The numerical simulation gives a good result compared to the experimental data, which justifies the choice of our models used in this study. The objective is to make this natural convection a forced convection by the mobility of the upper wall. This made it possible to change the concentration of the state parameters in this cavity. This result can be used to make an orientation of the heat in this cavity. We note the use of four (4) speeds of the mobile wall in order to see the influence of the speed of this north wall on the convection phenomenon.

Bejan’s thermal and mass flow visualization in micropolar fluid

Micropolar fluids are greatly utilized in biomedical applications as in biological; physical, chemical processes, lubrication systems, porous media and hydrodynamic-fluid problems and many others. An attempt is made to demonstrate the thermal and mass flow phenomena in micropolar fluids through the heat and mass functions concepts. The present physical model is devised based on the used geometry and free convection phenomena and gives the coupled nonlinear differential equations. Crank–Nicolson numerical scheme has been applied to disclose the silent flow and thermal features of micropolar fluid about a vertical cylinder. Physical behaviors of various parameters are discussed in terms of flow profiles and visualization patterns. Tabular solutions are presented to illustrate the importance of engineering quantities. Thermal and velocity contours are suppressed in the vicinity and enhanced away from the cylinder. Heatlines become closer to the heated surface, whereas masslines move far from the heated cylinder under free convection conditions for the rising Prandtl number. Guarantee of the obtained solutions are presented and shows an excellent agreement and this fact confirms the numerical accuracy.

Mathematical Modeling of Recursive Drug Delivery with Diffusion, Equilibrium, and Convection Coupling

In this work, a mathematical model to describe drug delivery from polymer coatings on implants is proposed. Release predictability is useful for development and understanding of drug release mechanisms from controlled delivery systems. The proposed model considers a unidirectional recursive diffusion process which follows Fick’s second law while considering the convective phenomena from the polymer matrix to the liquid where the drug is delivered and the polymer–liquid drug distribution equilibrium. The resulting model is solved using Laplace transformation for two scenarios: (1) a constant initial condition for a single drug delivery experiment; and (2) a recursive delivery process where the liquid medium is replaced with fresh liquid after a fixed period of time, causing a stepped delivery rate. Finally, the proposed model is validated with experimental data.

Convective heat transfer in I-shape heat sink under the action of Lorentz force via LBM

Via the non-orthogonal multiple-relaxation time Lattice Boltzmann Method, the natural convection phenomenon inside I-shape heat sink under the effect of Lorentz force is investigated. The cavity is filled with MWCNT-Fe3O4/water hybrid nanofluid, and three cold and three hot tubes exist inside the cavity at the same time. The influences of Rayleigh number (103–106), Hartmann number (0–100), magnetic field tilt angle θ (0 – π/2), and different cold and heat pipe arrangements (Case 1–Case 5) on the heat transfer and fluid flow are numerically simulated. The calculated results are shown by the variation of flow lines, isotherms and the average Nusselt number. The results show that the amplitude of the flow function is always the largest for Case 5, which is the best arrangement from the fluid motion point of view, while Case 2 has the largest Nusselt number for different Rayleigh numbers, magnetic field inclination angle θ and different Hartmann numbers, which is the best arrangement from the heat transfer efficiency point of view. However, with different cold and hot pipe arrangements, the average Nusselt number increases when the Rayleigh number and the magnetic field inclination angle θ increase. In contrast, as Hartmann number increases, the average Nusselt number decreases. The local Nusselt number of the cold and hot tubes is further investigated, comparing the discrepancy of the heat transfer performance near each wall surface.

Rayleigh-Bénard double-diffusive mixed convection in two-dimensional rectangular cavities filled with non-Newtonian fluids

The present work investigates numerically and analytically Rayleigh-Bénard double-diffusive mixed convection inside horizontal rectangular cavities filled with non-Newtonian fluids and subjected to uniform heat and mass fluxes along the sliding horizontal walls, while the vertical ones are considered insulated and impermeable. The problem is found to be governed by: Peclet number Pe, thermal Rayleigh number RaT, Lewis number Le, buoyancy ratio N, and power-law behavior index n. Both solutions show a good agreement for inclusive ranges of governing parameters; thus, validating the analytical approach and the numerical code. To gain further insight into convection phenomena, the dominance regions of convective regimes, namely natural, mixed, and forced convection are defined, where ratios RaT/Pe1.57, RaT/Pe1.92, and RaT/Pe2.27 accurately delineate the three regimes for n = 0.6, n = 1.0, and n = 1.4, respectively. As a result, the effects of governing parameters are amply discussed in each governing regime. Increasing Peclet number enhances flow intensity while heat and mass transfer rates increase only until reaching a higher plateau. For thermal Rayleigh number, increasing it intensifies fluid circulation while heat and mass transfer increase only for a given range of Pe values, where after a critical value Pecr the opposite happens as shear and buoyancy forces compete driving the transfer rates down. Such results are due to Pe strengthening forced convection contribution in the overall convection and RaT enhancing natural convection. As for Lewis number, a given range correlated with n strongly enhances natural regime contribution, which increases flow intensity while the outcome on heat transfer depends on the associated shear force magnitude. Further, increasing Le enhances mass transfer until reaching a higher plateau due to boosted forced convection contribution. In contrast, buoyancy ratio is found to strengthen natural convection leading to similar outcomes as for RaT. Finally, the rheological behavior of non-Newtonian fluids leads to some unforeseen effects on flow characteristics directly related to the dominant convective regime. The critical Peclet number Pecr signaling the change in n effect and leading to buoyancy forces reducing transfer rates instead of enhancing them is presented as a function of the remaining governing parameters RaT, Le, N, and n.

Inclined surface slip flow of nanoparticles with subject to mixed convection phenomenon: Fractional calculus applications

The interest of researchers towards the nanofluids is noticed in recent years due to leading applications in thermal systems and industrial framework. Referring to such motivations, current study explores the role of velocity slip effects for the mixed convection flow of nanofluid endorsed due to inclined surface. The Casson base fluid model for which the thermal impact needs to be improved. The analysis is observed when the role of velocity slip is important. The modeling of unsteady free convective flow problem yields partial differential system. The Atangana-Baleanu (AB) and Caputo-Fabrizio (CF) fractional operators are implemented in order to simulates the computation of problem. The graphical presentations are prepared in order to check the physical dynamic of parameters.

Enhancing the performance of floating photovoltaic system by using thermosiphon cooling method: Numerical and experimental analyses

The increasing global energy demand, limited fossil fuel resources, and environmental issues have optimized renewable energy utilization. Solar energy is an unlimited, accessible, and eco-friendly energy source. However, the broad land requirement is one of the critical problems in developing solar power plants. Floating photovoltaic (PV) is an innovative idea to overcome this problem. This research aims to develop floating PV systems by adding a passive cooling, namely a thermosiphon. The thermosiphon is a passive heat exchanger based on the free convection phenomenon. Experimental analysis has been carried out to estimate the increase of electrical power output in a thermosiphon-floating PV system, compared to their conventional floating counterparts. Experimental results showed that the power output of floating PV systems is about 4.52% higher than that of ground PV installations. Furthermore, introduction of thermosiphon cooling PV increases the power output of floating PV panels by about 7.86% in comparation to ground PV generation, or by about 3.34% relative to floating PV systems. A comprehensive numerical investigation has also been conducted, to study the system's temperature contour and fluid flow characteristics. The transient calculations have been performed with all fluid's properties as temperature (piecewise-linear) functions, and laminar conditions have been assumed. Numerical results showed that the thermosiphon cooling system effectively dissipates heat from PV to the environment. This proposed thermosiphon cooling method effectively increased the electrical energy of floating PV units, with no-external energy required for cooling.

Mixed convection around two vertically aligned horizontal cylinders: A numerical, experimental, and modeling investigation on the effect of local conditions on heat transfer

Experimental and numerical studies were performed to assess the effects of buoyancy driven flows from lower cylinders in a tube array on the heat transfer of higher-elevation cylinders. First, PIV and LIF experiments were performed for a single 13.4 mm diameter cylinder submerged in water at atmospheric conditions to measure velocity and temperature fields. Then the effect of buoyancy from a lower heater was studied by arranging two horizontal cylinders at different vertical displacements from each other (P/D = 1.5, 2.15, 6.45). Wall temperatures were measured as a function of pitch and power. CFD analysis was performed for the same configurations using an LES model. Using a combination of CFD and experimental measurements, mechanistic models for velocity and temperature behaviour in the plume were developed. These models are used to determine the impact of buoyant plumes from cylinders at lower elevations on heat transfer from heated cylinders at higher elevations. Since the plumes from cylinders at lower elevation are imposed on the natural convection from higher-elevation cylinders there is a combined effect from the momentum and temperature of the lower plume as it interacts with the natural convection phenomena from the upper-elevation heater. A model to predict the NuD of the upper cylinder in the case of horizontal vertically aligned pair of tubes was developed based on superposition of the convection caused by the lower cylinder and the natural convection from the higher-elevation cylinder. This model was found to predict the cylinder NuD with an RMS error of 2.8 and an absolute deviation of 2.31 as compared to the experimental data obtained in this study. It was also found to agree well with independent data available in literature.

Multi view interferometric tomography measurements of convective phenomena in a differentially-heated nanofluid layer

ABSTRACT Three-dimensional reconstructions of thermal field in a differentially heated cylindrical cavity with dilute concentrations of nanofluids as the working medium have been reported. Experiments have been conducted in a specially designed tomography cell filled with dilute concentrations of Al2O3 nanofluids with ethylene glycol dimethacrylate (EGD) as the basefluid. The two horizontal walls of the cell have been differentially heated to apply the temperature potential across the fluid layer. Multiple projection data in the form of interferometric images have been recorded at every 20° view angle by slowly turning the cell over the range of 0 to 360°. The two-dimensional temperature fields retrieved through the quantitative analysis of interferometric images recorded at various view angles have been used to reconstruct the complete three-dimensional temperature field at select horizontal planes of the fluid layer. Iterative multiplicative algebraic reconstruction technique (MART) has been employed for tomography reconstructions. Based on the tomography reconstructions, possible fluid movement in the test cavity and the strong dependence of convective field on varying nanofluid concentrations and cavity temperature differences have been explained. Distribution of local temperature fields near the cavity wall surfaces revealed the phenomena of breakdown of large length scale fluid structures into multiple roll-like patterns with increasing concentration of nanoparticles. The distribution of the local Nusselt number, on select horizontal planes near the top and bottom wall surfaces of the cavity, highlighted the possible role of nanoparticles enhancing the heat transfer rates. An overall energy balance in the cavity was also to be seen based on the reconstructions of the local profiles of Nusselt numbers.

Mixed convective flow of blood biofluids containing magnetite ferroparticles past a vertical flat plate: shapes-based analysis

The blood circulatory system, which is made up of an opaque network of blood vessels, is responsible for transporting important nutrients and oxygen to the body’s active tissues as well as removing waste materials. The transmission of ions, cells, gases, and macromolecules between both the blood and tissue is handled by this system. Blood flow dynamics have an important role in the onset and development of cardiovascular illnesses. Here, in this article, the authors have presented the magnetohydrodynamic flow of magnetite blood-based nanofluid which plays a significant role in cancer therapeutics. Furthermore, by producing higher temperatures around tumors, these types of nanoparticles play an intriguing function in the killing of malignant cells while causing no harm to nearby strong cells. The mixture of blood and ferroparticles are analyzed past a vertical flat plate. Additionally, the ferroparticles have considered in three different shapes which include platelet, cylinder, and brick. The stagnation point flow along with mixed convection phenomena has been taken into account. The presence of non-Newtonian nanofluid flow has been considered incompressible and laminar. Suitable similarity transformations are considered to transform the system of partial differential equations into ordinary differential equations. The transformed system has been solved analytically and then compared with numerical technique, and have found a great agreement between both techniques. The results showed that the presence of hemoglobin molecules in red blood cells that are magnetic in nature, it concludes that blood is the magnetic fluid which can be controlled by applying a suitable magnetic field during surgeries. The greater magnetic parameter has thickened the thermal boundary layer of the blood based ferroparticles. Thus, an augmenting impression is found for assisting flow, while a reducing impact is found for opposing flow. Additionally, the greatest impact of magnetic parameter is found for platelet-shaped ferroparticles. The augmented mixed convection parameter speeds up the buoyancy forces which have accelerated the fluid motion, while retarded the thermal profile.