Natural Convection Heat Transfer Rate Research Articles

Optimization of Heating and Cooling System Locations by Taguchi’s Method to Maximize or Minimize the Natural Convection Heat Transfer Rate in a Room

Optimization of Heating and Cooling System Locations by Taguchi’s Method to Maximize or Minimize the Natural Convection Heat Transfer Rate in a Room

Heat Transfer Augmentation: Experimental Study with Nanobubbles Technology

The experimental research on heat transfer characteristics is an ever-ending scheme since the life of all thermoelectronic devices relies on the effective management of thermal energy. In some cases, the gradient of temperature for heat transfer is to be minimum to avoid energy loss, but also there are numerous applications where the requirement of heat transfer to be maximum and could be achieved with a higher temperature difference between the heat transfer medium. In our current research, distilled water-ethylene glycol heat transfer fluid (HTF) was tested with different inlet mass flow rates and temperature as the hot fluid. Atmospheric air was chosen as the cold fluid. The natural convection heat transfer rate between hot and cold fluid streams was analyzed with and without the generation of micronanobubbles in the hot fluid. It was observed that compared to the base heat transfer fluid, the nanobubbles heat transfer fluid resulted in a 10–12% increase in heat transfer rate at hot fluid inlet temperatures of 28°C, 30°C, 32°C, 34°C, and 36°C. The method of generation of nanobubbles in HTF and their behavior are also highlighted.

Mixed convection heat transfer in a two-dimensional annular cavity using an off-lattice Boltzmann method

In the present work, a characteristic-based off-lattice Boltzmann method is used to study mixed convection heat transfer in a two-dimensional concentric annular cavity. First, the numerical method is verified using two benchmark cases: the Taylor–Couette flow and mixed convection in a concentric annular cavity. Thereafter, the effects of strength and the direction of rotation of the cylinders on the flow and heat transfer characteristics are analyzed. To this end, four different configurations of the cylinders, namely, the counter-rotating-cylinders, the co-rotating-cylinders, outer-rotating-cylinder with a stationary inner-cylinder, and inner-rotating-cylinder with a stationary outer-cylinder are considered. Further, a range of Rayleigh numbers, Ra=104,105 and 106 and a range of Reynolds numbers, Re=0 to 104 are considered for a Prandtl number of, Pr=0.71. It is observed that irrespective of the Reynolds number of the flow, the forced convection always results in a lower heat transfer rate. At a lower Rayleigh number of Ra=104, heat transfer in the natural convection is always higher than the mixed convection heat transfer for all the four rotation configurations considered. Both the co-rotating-cylinders and inner-cylinder-rotating configurations always result in a lower heat transfer rate than the natural convection heat transfer rate. However, up to certain Reynolds numbers, the counter-rotating-cylinders and outer-rotating-cylinder configurations result in a higher heat transfer rate for Ra=105 and 106. Beyond this Reynolds number, the heat transfer rate begins to decrease with an increase in the Reynolds number. Compared to other configurations, the co-rotating-cylinders configuration behaves similar to forced convection at lower Reynolds numbers. The profiles of the overall Nusselt number as a function of the Richardson number are studied for all four configurations, and the map thus obtained could be divided into three different zones. The distinguishing features of each of these zones are explained.

Convection heat transfer analysis in rats and mice

Heat transfer is a vitally important mechanism in thermal control of living beings. It is no different for rats and mice, which are widely used for academic, pharmaceutical and medicinal research. The objective of the present study was to estimate the convective heat transfer in rats and mice confined in a vivarium at the UFRN’s Institute of Tropical Medicine. Twenty rats and twelve mice were used for the experiment, where the following measurements were made: surface skin temperature, body length, and weight. The study was able to determine mathematically an approximation of the natural and forced convection heat transfer rates produced by rats and mice, comparing them under the condition of 22°C in a controlled vivarium environment.

Physical–numerical parameters in laminar simulations of natural convection on three-dimensional square plates

PurposeThe purpose of this study is to analyze the influence of the main physical–numerical parameters in the computational evaluation of natural convection heat transfer rates in isothermal flat square plates in the laminar regime. Moreover by experimentally validate the results of the numerical models and define the best parameter settings for the problem situation studied.Design/methodology/approachThe present work is an extension of the study by Verderio Junior et al. (2021), differing in the modeling, results analysis and conclusions for the laminar flow regime with . The analysis of the influence and precision of the physical–numerical parameters: boundary conditions, degree of mesh refinement, refinement layers and κ – ω SST and κ – ε turbulence models, occurred from the results from 48 numerical models, which were simulated using the OpenFOAM® software. Comparing the experimental mean Nusselt number with the numerical values obtained in the simulations and the analysis of the relative errors were used in the evaluation of the advantages, restrictions and selection of the most adequate parameters to the studied problem situation.FindingsThe numerical results of the simulations were validated, with excellent precision, from the experimental reference by Kitamura et al. (2015). The application of the κ – ω SST and κ – ε turbulence models and the boundary conditions (with and without wall functions) were also physically validated. The use of the κ – ω SST and κ – ε turbulence models, in terms of cost-benefit and precision, proved to be inefficient in the problem situation studied. Simulations without turbulence models proved to be the best option for the physical model for the studies developed. The use of refinement layers, especially in applications with wall functions and turbulence models, proved unfeasible.Practical implicationsUse of the physical–numerical parameters studied and validated, and application of the modeling and analysis methodology developed in projects and optimizations of natural convection thermal systems in a laminar flow regime. Just like, reduce costs and the dependence on the construction of experimental apparatus to obtain experimental results and in the numerical-experimental validation process.Social implicationsExclusive use of free and open-source computational tools as an alternative to feasible research in the computational fluid dynamics area in conditions of budget constraints and lack of higher value-added infrastructure, with applicability in the academic and industrial areas.Originality/valueThe results and discussions presented are original and new for the applied study of laminar natural convection in isothermal flat plate, with analysis and validation of the main physical and numerical influence parameters.

STUDY AND VALIDATION OF MESHES IN TURBULENT ISOTHERMAL PROBLEMS OF NATURAL CONVECTION IN FLAT PLATES

The study of natural convection on flat plates is of great interest in the areas of Engineering, both for the simplicity of geometry and the wide variety of applications. In the study and definition of a numerical model, an ideal mesh configuration is the one that best represents physically, with minimal numerical influence and with the lowest computational cost, the problem addressed. The influence of two mesh configurations (non-uniform staggered and entirely uniform), at different refinement levels, was studied to evaluate natural convection heat transfer rates in flat plates of AR = 5; in isothermal conditions, in turbulent regime with the κ−ω SST RANS model and using free and open-source software OpenFOAM®. The physical-numerical methodology applied, and the numerical results obtained were validated from experimental results in the literature. The non-uniform staggered mesh configuration proved to be more adequate in precision, and computational cost to the problem situation studied. The entirely uniform mesh proved to be infeasible due to the high number of elements and computational cost demanded.

Physical–numerical parameters in turbulent simulations of natural convection on three-dimensional square plates

PurposeThis paper aims to study, experimentally validate and select the main physical and numerical parameters of influence in computational numerical simulations to evaluate mean heat flux by natural convection on square flat plates.Design/methodology/approachSeveral numerical models were built to study the influence of physical and numerical parameters about the predictions of the natural convection heat transfer rates on the surface of a flat plate with aspect ratio = 1, in isothermal conditions, turbulent regime and using the free and open-source software OpenFOAM®. The studied parameters were: boundary conditions (using or not using wall functions in properties ε, κ, νt and ω), degree of mesh refinement, refinement layers and turbulence models [κ – ε and κ – ω Shear Stress Transport (SST)]. From the comparison of the values of the mean Nusselt number, obtained from numerical simulations and literature experimental results, the authors evaluated the precision of the studied parameters, validating and selecting the most appropriate to the analyzed problem situation.FindingsThe validation and agreement of the numerical results could be proven with excellent precision from experimental references of the technical scientific literature. More refined meshes with refinement layers were not suitable for the studies developed. The κ – ε and κ – ω SST turbulence models, in meshes without refinement layers, proved to be equivalent. Whether or not to use wall functions in turbulent boundary conditions proved to be irrelevant as to the accuracy of results for the problem situation studied.Practical implicationsUse of the physical and numerical parameters is studied and validated for various applications in natural convection heat transfer of technology and industry areas.Social implicationsUse of free and open-source software as a research tool in the Computational Fluid Dynamics (CFD) area, especially in conditions without large financial resources or state-of-the-art infrastructure.Originality/valueTo the best of the authors’ knowledge, this work is yet not available in existing literature.

Heat Transfer by Natural Convection in a Square Enclosure Containing PCM Suspensions

Research on using phase change material (PCM) suspension to improve the heat transfer and energy storage capabilities of thermal systems is booming; however, there are limited studies on the application of PCM suspension in transient natural convection. In this paper, the implicit finite difference method was used to numerically investigate the transient and steady-state natural convection heat transfer in a square enclosure containing a PCM suspension. The following parameters were included in the simulation: aspect ratio of the physical model = 1, ratio of the buoyancies caused by temperature and concentration gradients = 1, Raleigh number (RaT) = 103–105, Stefan number (Ste) = 0.005–0.1, subcooling factor (Sb) = 0–1.0, and initial mass fraction (or concentration) of PCM particles (ci) = 0–0.1. The results showed that the use of a PCM suspension can effectively enhance heat transfer by natural convection. For example, when RaT = 103, Ste = 0.01, ci = 0.1, and Sb = 1, the steady-state natural convection heat transfer rate inside the square enclosure can be improved by 70% compared with that of pure water. With increasing Sb, the Nusselt number can change nonlinearly, resulting in a local optimal value.

Natural Convection Heat Transfer between Two Concentric and Eccentric Oblate Hemi Spheroids with Different Elliptical Ratios (Dept.M)

In this paper, !.he natural convection heal transfer between a pair of concentric and eccentric oblate hemi-spheroids with different elliptical ratios was: depicted experimentally and numerically. Four pairs of oblate hemi-spheroids having elliptical ratios of 0.0, 0.25, 0.5, 075 and 3 diameter ratio of 2 for each pair were cut using the wire-cut JSEDM CNC machining. The inner hemi-spheroid was heated while the outer one was exposed to the room temperature. The experimental tests SC3Jl the effect of the orientation modes (horizontal, inclined, ver1ical inclined revolved and vertical revolved) on the natural- convection heat transfer between the inner and outer surfaces of cach pair. A mathematical model was devoted for the case study to investigate the effect of the eccentricity on the thermal performance of such cases. The model, was solved numerically using FLUENT CFD solver. The CFD result, were validated via corresponding experimental data. The temperature and velocity contours of the air gab were displayed to indicate the ring movement of the buoyancy driven air plume. The comparison among the results of the present attempt and those by the literature implied that; a notable Increase in the natural convection heat transfer rates could be satisfied through the gabs among successive layers of hemi-spheroids having higher elliptical ratio rather than the hemi-spheroids ones. Also, the positive vertical eccentricity shows remarked increase in natural convection heat transfer rates rather than both concentric; and negative eccentric cases.

Impact of cavity chord shape on the free convection heat transfer rate in a triangular cavity

AbstractIn this study, the effect of different chord shapes of a triangular cavity has been investigated on the natural convection heat transfer characteristics inside the cavity using the commercial ANSYS‐FLUENT software. Heat transfer phenomena have been regarded as very significant due to the widespread application of natural convection heat transfer in different engineering fields. According to the performed studies, the chord with partitions can considerably enhance the natural convection heat transfer rate inside the cavity. Four different types of partitions including zigzag, two types of rectangular and sinusoidal were considered on the chord of the triangular cavity. The dimensionless parameters investigated in the results include Rayleigh number, Prandtl number, and Nusselt number. The results comprise the isotherm lines, streamlines, velocity lines, local and average Nusselt numbers analyzed with regard to the chord type and Rayleigh number variation. The results revealed that the temperature trend is almost the same for all configurations. Furthermore, the average Nusselt number value in zigzag and sinusoidal is higher in comparison with other cases due to the larger heat transfer area. Finally, by examining the streamlines, isotherm and velocity lines, average and local Nusselt numbers, it was founded that the cavities with zigzag and sinusoidal chord shape have the better performance in terms of heat transfer rate.

Heating performance enhancement of a new design trombe wall using rectangular thermal fin arrays: An experimental approach

Abstract It has been nowadays recognized that addressing energy use in buildings can reduce the fossil fuels usage and CO2 emission. Trombe wall is a widely applicable passive solar design option that can significantly reduce the fossil fuel consumption in buildings. This paper experimentally dealt with the effect of applying vertical thermal fin on the absorber of Trombe wall with new design. Three types of aluminum, brass and copper fins were investigated. The experiments were carried out at arid climate of Yazd, Iran. The results showed that when the thermal fin is used the performance efficiency of the Trombe wall increases up to 3% in terms of stored energy within the Trombe wall and 6% in terms of natural convection heat transfer rate inside the channel. However, adopting more thermal fins on the absorber could not ensure higher heating efficiency in terms of stored energy for all cases. Furthermore, copper fin led to maximum heating efficiency and highest average room temperature among three fin types.

Computational fluid dynamics analysis on natural convective heating of bottled liquid food during pasteurization: Effect of container orientation

AbstractThe aim of this work was to determine natural convective heat transfer rates in bottled liquid food pasteurized using different container orientations; conventional vertical, inverted vertical, and horizontal bottle positions. For this purpose, a computational fluid dynamic (CFD) model was applied to predict the temperature distribution, flow pattern, and quality changes in non‐Newtonian fluid foods for three orientations. The numerically predicted temperatures were successfully validated against experimental data and the model allowed to identify the critical point during the thermal process. Results showed that the fluid flow developed in a horizontal orientation provided a better mixing of liquid food and, hence, a more rapid heating of the slowest heating zone compared to a vertical position. Moreover, the horizontal orientation achieved a 47.2% reduction of processing time and quality losses decreased (45.5–46.4%) with respect to a vertical position. These results suggest that a horizontal position could be considered as an interesting alternative for food processors since processing times can be reduced improving the final quality of the product.Practical ApplicationsPasteurization is a heat treatment process applied to a food product with the purpose of destroying disease‐producing microorganisms, inactivating spoilage‐causing enzymes, and reducing spoilage microorganisms. Overcooking causes detrimental effects in terms of the final product quality. Therefore, providing an adequate process with a desired sterility is one of the challenges to canning industry. In this work, we investigate how to improve, through container orientation modification, the natural convective heat transfer rates during pasteurization of fluid food. These results were obtained by the numerical simulation of the thermal process using CFD analysis which allowed to determine the temperature history and velocity field in the bottled liquid food. From the numerical results, the set of operating conditions that enhance the quality and the safety of the final product was determined, thus minimizing expensive and time‐consuming pilot test‐runs.

Performance of SiO2-water nanofluids for single bubble-based nucleate pool boiling heat transfer

Experiments on single bubble-based pool boiling have been performed under saturated bulk conditions to understand the influence of suspended nanoparticles on bubble dynamics and the associated temperature gradients field. Water and two concentrations of silica-water nanofluid (0.005 and 0.01% (V/V)) have been subjected to isolated nucleate pool boiling on a ITO-coated borofloat heater by supplying constant heat flux. Experimental measurements have been made in a purely non-intrusive manner by employing refractive index-based rainbow schlieren deflectometry technique. Bubble dynamic parameters, such as bubble departure diameter and departure frequency, wait and growth times etc. and the temperature gradients in the bulk fluid have been mapped simultaneously in the form of schlieren images. Results on bubble dynamic parameters revealed that these parameters show more variability and assume skewed normal distribution in the case of nanofluids whereas water experiments showed insignificant variations in these parameters. Of the significant changes, the bubble departure diameter reduced, departure frequency and growth rate increased with increasing concentration of nanofluids. Schlieren images corresponding to 0.005 and 0.01% (V/V) nanofluids showed more spreading out of the superheat layer adjacent to the heater substrate than that observed in the case of water experiments. Time-averaged values of natural convection and evaporative heat transfer rates associated with single bubble-based boiling indicated that both these rates decrease in the presence of dispersed silica nanoparticles. Reduction in the overall heat transfer rate in the case of nanofluids has been explained on the basis of direct experimental measurements. In this direction, the observed phenomenon has been attributed to the role played by the suspended nanoparticles in diffusing the thermal gradients (reflected in the form of the broadening of the superheat layer) adjacent to the heater substrate along with substantially increasing the wait time of bubble inception in the case of nanofluids as compared to water-based experiments conducted under same wall superheat conditions.

Mixed convection flow caused by an oscillating cylinder in a square cavity filled with Cu–Al2O3/water hybrid nanofluid

The aim of this paper is to examine the effects of Cu–Al2O3/water hybrid nanofluid and Al2O3/water nanofluid on the mixed convection inside a square cavity caused by a hot oscillating cylinder. The governing equations are first transformed into dimensionless form and then discretized over a non-uniform unstructured moving grid with triangular elements. The effects of several parameters, such as the nanoparticle volume fraction, the Rayleigh number, the amplitude of the oscillation, and the period of the oscillation of the cylinder are investigated numerically. The results indicate that the motion of the oscillating cylinder toward the top and bottom walls increases the average Nusselt number when the Rayleigh number is low. Furthermore, the presence of Al2O3 and Cu–Al2O3 nanoparticles leads to an increase in the values of the average Nusselt number Nuavg for cases of low values of the Rayleigh number. It is found that the natural convection heat transfer rate of a simple Al2O3/water nanofluid is better than that of Cu–Al2O3/water hybrid nanofluid.

Experimental and numerical study of natural convection heat transfer from arrays of zigzag fins

In this study natural convection heat transfer from arrays of zigzag fin is studied numerically and experimentally. The length and width of the fins are L = 75 mm and H = 25 mm. Fin thickness and fin spacing are t = 2 mm and s = 6 mm, and zigzag arrangement is made with different pitches along the rectangular fins length. Finite volume scheme is used to solve three-dimensional equations of laminar flow and heat transfer numerically. For ten different zigzag fin geometries, both constant temperature and constant heat flux boundary conditions at the base plate are examined numerically and results have been compared with solid rectangular fin array. Rectangular and zigzag fin arrays with 4 and 14 triangles are made and tested experimentally at different heat inputs, and air flow field and natural convection heat transfer rate for each case are determined. Experiments are done in a thermally insulated room with specific equipment to control test conditions and measure necessary data. Results of experiments and numerical solutions are compared for various power input. By comparison with solid rectangular fin, 22 triangles zigzag fin array has the same heat transfer but with 50% reduction in mass for constant base temperature. For experiments for different input power, natural convection heat transfer coefficient of 4 triangles zigzag fin array is increased 36%–50% in comparison with solid rectangular fin array with 50% reduction in mass.

Thermal Analysis and Thickness Optimization of Two-Cavity Thermal Insulator in Buildings

This work presents an analytical and computational study on the use of two separated air cavities, located in common walls and roofs, as an application for thermal insulation in buildings. The aspect ratios of the two cavities were varied from 10 to 100. Firstly, the thermal analysis is performed, under the assumption that air in the two cavities is motionless, to determine the influence of cavity aspect ratio. The results show that the thermal diffusivity of buildings’ wall and roof components is sensitive to the presence of first and second air cavities as well as to the variation of cavity aspect ratio. In addition, the natural convection thermal analysis across the two separated air cavities, for isothermal case, was performed to assess the influence of aspect ratio on various natural convection parameters. The results show that the Rayleigh number decreases with small range of aspect ratio, but remain nearly constant in high range of aspect ratio. The optimum aspect ratio of the two separated air cavities (i.e., optimum cavities thicknesses) is determined. At each air cavity, the optimum cavity aspect ratio will be at a location where Nusselt number $$Nu = 1$$ . At this location, the natural convection heat transfer rate approximates pure conduction heat transfer rate, and thus, the cavities can work optimally as thermal insulators. The results revealed that the optimum aspect ratios for the two wall and two roof air cavities are 50 and 52.6 corresponding to optimum thicknesses of 0.02 and 0.019 m, respectively.

Natural convective heat transfer characteristics of Rayleigh-Benard convection of Al<sub>2</sub>O<sub>3</sub>-water nanofluids

Transient three-dimensional numerical computations are implemented to clarify the natural convective heat transfer characteristics of the Rayleigh-Benard convection of Al2O3-water nanofluids induced in a shallow vertical cylindrical enclosure. The thermophysical properties of nanofluids, assumed to be a single-phase fluid in the numerical computations, are estimated using the experimental correlation equations reported by Khanafer and Vafai (2011). When the average Nusselt numbers of nanofluids are plotted against the Rayleigh number defined by the thermophysical properties of water, computations for four different volume fractions of nanoparticles were below the average Nusselt number curve experimentally reported by Silveston (1958). The diagram also reveals that the increase of nanoparticles in the base fluid delays the generation of Rayleigh-Benard convection. However, the average Nusselt numbers of nanofluids almost agreed with the average Nusselt numbers of Silveston without depending on the volume fraction of nanoparticles when plotted against the Rayleigh number defined by the thermophysical properties of nanofluids. Thus, the natural convection heat transfer rates of Al2O3-water nanofluids can be considered to be similar to the general fluids reported by Silveston as long as experimental thermophysical properties are employed.

수직 원형관내 자연대류 열전달에서 기하구조의 영향

Natural convection heat transfer rates in vertical pipes were measured varying the diameter, length, and roughness of vertical cylinder. To achieve high Rayleigh number with relatively small test rig, mass transfer experiments instead of heat transfer were performed based on the analogy. Prandtl number was 2,014. The length of vertical cylinder was 0.1m, 0.3m, and 0.5m, which correspond to GrL , , and . To each length of vertical cylinder, the heat transfer rates were measured varying the iameter 0.005m, 0.01m, and 0.03m. The heat transfer rate for a short length pipe(0.1m) agreed with the prediction from Le Fevre correlation developed for a vertical plate for all diameter. The heat transfer rate decreases as the diameter and the length of the pipe increases. The heat transfer rate inside of vertical cylinder is affected by roughness only for a laminar flow regime.

Natural Convection Heat Transfer About a Vertical Cone Embedded in a Tridisperse Porous Medium

This work studies the natural convection heat transfer about a vertical cone embedded in a tridisperse porous medium with constant wall temperature. The three-velocity three-temperature formulation is used to derive the governing partial differential equations. The coordinate transformation and the order-of-magnitude analysis are then used to obtain the nonsimilar boundary layer partial differential equations. The cubic spline collocation method is then used to solve the nonsimilar boundary layer partial differential equations. The effects of two inter-phase heat transfer parameters, three modified thermal conductivity ratios, and two permeability ratios on the heat transfer and flow characteristics of the tridisperse porous medium are studied. Results show that increasing the three modified thermal conductivity ratios or the two permeability ratios tends to increase the natural convection heat transfer rate of the vertical cone in a tridisperse porous medium. Moreover, the thermal non-equilibrium phenomena between the three phases of the tridisperse porous medium are affected by the streamwise coordinate and the two inter-phase heat transfer parameters. The thermal non-equilibrium effects between the three phases of the tridisperse porous medium become significant as the two inter-phase heat transfer parameters or the streamwise coordinates are small.

Multi-objective optimization of natural convection in a cylindrical annulus mold under magnetic field using particle swarm algorithm

In the continuous casting process, natural convection occurs in mold containing a liquid metal. Natural convection in the melt causes the impurities to move and this phenomenon can lead to poor product. Therefore, by reducing natural convection, the quality of the product is improved. In this paper, 3D numerical simulation and multi-objective optimization of natural convection in a cylindrical annulus mold filled with molten potassium under a magnetic field is carried out. The inner and outer cylinders are maintained at uniform temperatures and other walls are thermally insulated. Two objective functions including the natural convection heat transfer rate (average Nusselt number) and magnetic field strength have been considered simultaneously. The multi-objective particle swarm optimization algorithm (MOPSO) has been employed. Four decision variables are the Hartmann number, inclination angle, and magnetic field angles. For the optimization process, the calculations of three-dimensional Navier–Stokes, energy, and electrical potential equations are combined with MOPSO. Using the numerically evaluated objective functions, the optimum frontier is estimated by a second order polynomial based on objective functions.