Radiation Convection Heat Transfer Research Articles

Scale-integrated simulation of coupled radiation and convection in metal foam layer under high-flux irradiation

Coupled radiation-convection heat transfer inside a unit metal foam layer with high-flux irradiation is studied using improved scale-integrated simulation. It integrates the discrete-scale CFD simulation with that of continuum-scale via the multi-domain concept with thermal radiation effect considered. Conservations of mass flux and energy are satisfied on these specified subdomain interfaces, and a special defined transition zone with geometry-dependent functional quantities is introduced within the continuum-scale subdomain. This zone is adjacent to the discrete-scale subdomain which ensures a reasonable transition. Absorbed radiative fluxes and radiative heat transfer within the whole domain are calculated with Monte Carlo method and treated as heat sources. The problem under consideration has been investigated via the discrete-scale simulation, the continuum-scale simulation, and the scale-integrated simulation. The accurateness of scale-integrated simulation has been fully validated for predicting both flow field and heat transfer characteristics in comparison with the outcomes of discrete-scale simulation and other investigations. Deviations between the outcomes of continuum-scale simulation and discrete-scale simulation are found to be mainly caused by inconsistent local absorbed radiative heat fluxes distribution and impinging effect around the entry region. Up to 70% and 60% reduction of computation time and memory footprints are achieved when scale-integrated strategy is employed for the CFD simulation compared with the discrete-scale simulation. Meanwhile, the detailed fluctuated characteristics of the local concerned region and overall heat transfer performance can be precisely reflected. It may break up the computation limitations of the discrete-scale simulation in studying the energy transport process in foam based solar receiver applications.

Thermal state of the energy-releasing globular particle at convective-radiative heat transfer

Thermal state of the energy-releasing globular particle at convective-radiative heat transfer

Methodology for assessment of the fire-resistant quality of reinforced-concrete floors protected by fire-retardant coatings

In the work, the tests have been analysed for fire-resistant quality of hollow-core reinforced-concrete floors with fire-retardant plaster covering. A two-dimensional physical model and computer model have been developed in the ANSYS FLUENT software environment, which includes a system of equations and boundary conditions, taking into account the thermal conductivity in the coating and concrete, as well as complex convection-radiation heat transfer in air voids. A one-dimensional multilayer mathematical model has been developed, equivalent to a two-dimensional model of the thermal state of hollow-core reinforced-concrete floor with fire-retardant plaster covering, with the specific these layers’ thicknesses. A methodology has been developed in the article, based on solving the direct and inverse heat conduction problems, with the help of which it is possible to assess the fire-resistant quality of hollow-core reinforced-concrete floors with and without fire-retardant coatings. The developed methodology was applied to determine the thermophysical (heating constant and specific heating capacity per unit volume) and fire-retardant characteristics of the studied plaster covering according to the results of tests for fire-resistant quality of reinforced-concrete floors with this fire-retardant coating. The conclusion was made about the effectiveness of this coating and the boundaries of using the fire-retardant coatings to ensure normalizable values for the limit of the fire-resistant quality of hollow-core reinforced-concrete floors.

Effect of uniform crystal rotation on convective and radiation-convective heat transfer in the Czochralski method

The conjugate heat transfer in different regimes of heat transfer in the system “crystal – environment – walls of the growth vessd”, geometrically similar to the simplified scheme of the upper part of the thermal node in the Czochralski method, is studied numerically by the finite element method. The system of equations of thermo-gravitational and mixed convection in variables of vortex, stream function and temperature is solved. In addition, the uniform rotation of crystals is taken into account. Calculations are carried out at convective and radiation-convective heat transfer with fixed crystal length. Calculations of radiation fluxes are carried out on the basis of the zonal method under the following assumptions: the calculated area is limited by a closed system of surfaces; all surfaces of the system are grey, diffuse- emitting and diffuse-reflecting; the surfaces are divided into zones within which the radiation properties and temperature can be considered constant; the medium filling the growth chamber is diathermic. The calculations were performed with the Prandtl number equal to 0.68 (argon) and the Grashof number of 16000, typical of the real process. The effect of uniform crystal rotation from 1 to 25 rpm on radiation-convective heat transfer is studied. It is shown that under the action of rotation the spatial form of convective flows loses stability. Secondary vortices appear. As a result, the cooling efficiency of the crystal increases significantly.

Gas radiative effects on gas-filled flat plate solar collectors

A standard thermal solar collector can be used for both hot water production and air heating purposes. Gas-filled solar collectors represent a new emerging design approach with enhanced characteristics. In this research, numerical modeling is utilized to study radiative effects of the participating gases on the performance of solar collectors. The coupled radiative–convective heat transfer in the solar collector is considered and the collector cavity is considered as a radiatively participating medium. The finite volume method has been adopted to solve the governing equations and discrete ordinates method is used for radiative transfer. After validating the model used in this study, it is used to obtain the heat transfer characteristics of a flat-plate solar collector with real solar conditions of the city of Kerman, Iran, in summer at a wide range of air absorption coefficients. According to the results, by increasing the absorption coefficient of the air, the temperature of the absorber plate is reduced and the air temperature is increased, but the increase of air temperature is much higher than the reduction of absorber temperature. Hence, it is concluded that it is possible to use participating gases in the solar air heaters to enhance the performance of the collector.

Convective-radiative double-diffusion heat transfer in power-law fluid due to a stretching sheet embedded in non-Darcy porous media with Soret–Dufour effects

A numerical model is developed to study the combined effects of Soret and Dufour on mixed convection heat and mass transfer in a Power-law fluid saturated with Darcy–Forchheimer porous medium in the presence of nonlinear thermal radiation, chemical reaction and Ohmic dissipation. The governing boundary layer equations are solved numerically by using shooting method with Runge–Kutta Fehlberg integration scheme. The influence of Soret and Dufour numbers, chemical reaction, thermal Grashof number and solutal Grashof number on velocity, temperature and concentration fields are studied. The effects of some important physical parameters on skin-friction, Nusselt number and Sherwood number are studied in detail.

Analysis of the heat transfer for processes of the cylinder heating in the heat-treating furnace on the basis of solving the inverse problem

This paper presents results of experimental test concerning heating of a cylinder in the furnace for heat and thermochemical treatment. Tests were conducted for heating up to temperature of 550 °C, what corresponds to nitriding processes. Calculation model including Chebyshev polynomials enabling determination of temperature distribution in the cylinder is presented.Two variants of calculation model were examined; they differ in terms of the assumed boundary condition. Analysis of regularization of the solution to the inverse problem done by the change of time step (self-regularization) was made. Based on the solution of the non-linear inverse problem for the heat equation and on the gas temperature measurement read from the furnace retort, temperature distributions on the boundary of the cylinder, combined radiation convection heat transfer coefficients (CHTC) and heat flux on the surface of the cylinder were calculated. Due to a great variability of temperature for the processes under consideration, the calculation model includes the variation of the heat conduction coefficient and of the specific heat of the material the cylinder is made of depending on temperature change. Five heating processes differing in parameters of heating rate were analysed. Impact of measurement errors on distributions of temperature on the edge of the cylinder and of the CHTC was determined. Obtained temperature distributions in the element were verified by a check measurement. Maximal error of the obtained results did not exceed 0.5%. Data presented herein give practical knowledge enabling supervising the temperature distribution in the cylindrical element being processed. These problems are of crucial meaning for processes of heat and thermochemical treatment due to formation of proper surface and subsurface layers.

Development of the physical and mathematical model of the baking process of the dough pieces in bakery ovens

The object of research is the physical and mathematical models designed to describe the heat and mass transfer inside the porous material during baking. In order to improve the quality and at the same time reduce energy consumption in production, as well as improve the technical and economic indicators of the operation of furnaces, the duration and safety of their operation, the designs of furnace units are being improved, new ones are being developed and their thermal conditions are being optimized. One of the biggest problems is the task of replacing obsolete oven designs with new ones, with automatic regulation of the thermal regime of baking, which will ensure high quality bread while reducing fuel, steam, electricity and human resources. Since the quality of products, in particular, taste, aroma, porosity, gloss, appearance and other indicators of bakery products, largely depends on the design of the furnace unit, the thermal and hygrothermal conditions of the working chamber, as well as its proper operation. These factors affect the loss during baking, which can vary from 6 to 12 %, which affects the yield of bread. This paper presents a physical and mathematical model of the process of baking dough pieces in baking ovens using the example of the industrial oven K-BOM-25 (Ukraine) developed by the author.A mathematical model of the process of baking bread in the gas channels of the baking chamber is given taking into account radiation-convective heat transfer, mass transfer taking into account the introduction of water vapor to moisten the dough pieces and turbulence of the multiphase flow. The dependence of the multiphase flow turbulence is formulated on the basis of the Euler equations averaged over Reynolds. This model allows with sufficient accuracy and detail to take into account the technological conditions and design features of modern conveyor baking ovens. And it also allows for extensive parametric studies of conjugate heat transfer in them with access to the final indicator – the quality of finished products.

Effect of Thermal Conductivity and Emissivity of Solid Walls on Time-Dependent Turbulent Conjugate Convective-Radiative Heat Transfer

In the present study, the conjugate turbulent free convection with the thermal surface radiation in a rectangular enclosure bounded by walls with different thermophysical characteristics in the presence of a local heater is numerically studied. The effects of surface emissivity and wall materials on the air flow and the heat transfer characteristics are the main focus of the present investigation. The conjugate convective heat transfer for the fluid (air), described in terms of linear momentum, continuity, and energy equations combined with k-e turbulence model, is predicted by using the finite difference method. The results for the isotherms, streamlines, and average Nusselt numbers along the heat source are presented. The numerical experiments show that an increase in thermal conductivity of solid walls illustrates the enhancement of heat transfer. Eventually, the main result obtained in this work provides a good technical support for the development and research of energy-efficient building materials.

Thermohydrodynamic characteristics of combined double-diffusive radiation convection heat transfer in a cavity

This work deals with the numerical analysis of a radiating gas flow caused by both temperature and buoyancy concentration gradients in a square cavity; in this regard, the set of governing equations, including conservation of mass, momentum, species, and energy are solved by a numerical technique. In terms of radiation, since the fluid is considered as a semitransparent medium, the radiative term in the energy equation appears and is calculated by numerical solving of the radiative transfer equation (RTE). Furthermore, all of the surrounding cavity walls are considered to be opaque, gray, and diffuse with constant emissivity. All of the flow equations are solved by the finite difference method (FDM) and the RTE by the discrete ordinate one (DOM). In the present study, an attempt is made to verify the optical thickness effects on flow, thermal behavior, and mass transform in a cavity flow, such that reciprocating trends were seen in this manner. Our numerical results show that the thermal field in double-diffusive convection flow reaches very fast its steady-state situation in comparison to the concentration distribution. Besides, it is found that the thermohydrodynamic characteristics of a double-diffusive convection flow of a radiating gas are much affected by optical thickness.

The Influence of Surface Radiation on the Passive Cooling of a Heat-Generating Element

Low-power electronic devices are suitably cooled by thermogravitational convection and radiation. The use of modern methods of computational mechanics makes it possible to develop efficient passive cooling systems. The present work deals with the numerical study of radiative-convective heat transfer in enclosure with a heat-generating source such as an electronic chip. The governing unsteady Reynolds-averaged Navier–Stokes (URANS) equations were solved using the finite difference method. Numerical results for the stream function–vorticity formulation are shown in the form of isotherm and streamline plots and average Nusselt numbers. The influence of the relevant parameters such as the Ostrogradsky number, surface emissivity, and the Rayleigh number on fluid flow characteristics and thermal transmission are investigated in detail. The comparative assessment clearly emphasizes the effect of surface radiation on the overall energy balance and leads to change the mean temperature inside the heat generating element. The results of the present study can be applied to the design of passive cooling systems.

Investigation of a computer CPU heat sink under laminar forced convection using a structural stability method

In the present study, the thermal performance of an air-cooled, flat-plate heat sink in forced convection was performed using a three-dimensional analytical model. To validate the present model, this solution is compared with numerical and experimental results that agree. This research studies the influence of heat sink thermal performance and of environmental parameters on the heat sink’s stability against thermal stress, which is essential for manufacturers’ goals. The effective parameters on heat transfer rate from the surface of heat sink were investigated. Reynolds and Nusselt numbers, heat transfer coefficient, fins height, and fins number were studied under various air flow velocities. In addition, the thermal stress (structural stability method) was investigated. Enhancement of the airflow velocity caused the Nusselt number value to incline, which increases the heat transfer coefficient. Increasing the Reynolds number from 3.80*103 to 2.28*104 leads to less thermal stress by 47.36%, less deformation by 50%, less surface temperature by 28.57%, and, consequently, less geometry failure. Results depicted that pure convection has less heat transfer by 1.66% compared with convection-radiation heat transfer, and enhancing the fins number and fins height reduces the surface temperature by 25% and 20%, respectively. As a result, the chosen range of the Reynolds number is totally suitable for this high-temperature device that does not allow thermal stress to deform the fins of the heat sink.

Investigation of Simultaneous Effects of Surface Roughness, Porosity, and Magnetic Field of Rough Porous Microfin Under a Convective–Radiative Heat Transfer for Improved Microprocessor Cooling of Consumer Electronics

The ever-increasing demand for high-processing compact electronic systems has unequivocally called for improved microprocessor performance. However, increasing microprocessor performance requires increasing power and on-chip power density, both of which are associated with increased heat dissipation. Electronic cooling using fin has been identified as a reliable cooling approach in miniaturized electronic systems. However, an investigation into the thermal behavior of fin would help in the design of miniaturized, effective heatsinks for reliable microprocessor cooling. The aim of this paper is to investigate the simultaneous effects of surface roughness, porosity, and magnetic field on the performance of a porous microfin under a convective-radiative heat transfer mechanism. The developed thermal model considers variable thermal properties according to linear, exponential, and power laws, and is solved using the Chebychev spectral collocation method. A parametric study is carried out using the numerical solutions to establish the influences of porosity, surface roughness, and the magnetic field on the microfin thermal behavior. The results of the simulation establish that thermal efficiency of the microfin is significantly affected by the porosity, magnetic field, geometric ratio, nonlinear thermal conductivity parameter, thermogeometric parameter, and the surface roughness of the microfin. Furthermore, the study establishes that the fin efficiency ratio which is the ratio of efficiency of the rough fin to the efficiency of the smooth fin is found to be greater than unity when the rough and smooth fins of equal geometrical, physical, thermal, and material properties are subjected to the same operating condition.

Inverse boundary design problem of combined radiation convection heat transfer in a duct with diffuse-spectral design surface

In the present work, an optimization technique is applied for inverse boundary design problem of radiative convective heat transfer of laminar duct flow by numerical method. The main goal is to verify how the solution of inverse problem is affected by the spectral behavior of the boundary surfaces. The conjugate gradient method is used to find the unknown temperature distribution over the heater surface to satisfy the prescribed temperature and heat flux distributions over the design surface. The bottom boundary surface (including design surface) is diffuse-spectral, while the top wall (heater surface) behaves as gray one. The variation of emissivity with respect to the wavelength is approximated by considering a set of spectral bands with constant emissivity and then the radiative transfer equation is solved by the discrete ordinates method for each band. The performance of the present method is evaluated by comparing the results with those obtained by considering a diffuse-gray design surface. Finally an attempt is made to investigate the spectral behavior of the design surface on the calculated temperature distribution over the heater surface.

Numerical simulation of mixing and heat transfer in an integrated centrifugal microfluidic system for nested-PCR amplification and gene detection

Nucleic acid amplification via polymerase chain reaction (PCR) is one of the essential and powerful methods used in a myriad of bio-assays in clinical laboratories. Application of microfluidic devices in biologically-related processes like PCR can result in the usage of less volume of reactant samples and reduce the processing time. By implementing PCR systems on centrifugal microfluidic platforms, automation and portability can be easily achieved. Although several methods have been developed, most of them are still dealing with challenges of the required high processing time. This study presents the numerical simulation of a fully automated PCR system with the goal of enhancing the mixing quality of the agents comprising primers and samples and accelerating the thermocycling processes through applying a hybrid convective-radiative heat transfer mechanism resulting in an optimized time consumption. In this work, sample manipulation units such as siphon valves and capillary valves are precisely designed and optimized and the desired time of the operation is reduced considerably. In addition, the blending quality of sample and reactants – primers, polymerase enzyme and nucleotides in this case – is enhanced by employing two serpentine micromixers. The efficiency of these micromixers was improved considering rotational speed and geometrical parameters of the corresponding sections of the microfluidic device. Furthermore, instead of using a common convection-based thermocycling process, the novel presented system is designed based on a hybrid convective-radiative heat transfer mechanism and time consumption is significantly minimized and managed by conducting numerical optimizations.

Radiation-convective heat transfer from the crystals in methods of pulling from melts

The radiation-convective heat transfer from the crystals in methods of pulling from melts such as Czochralski, Stepanov and floating zone is investigated numerically in the conjugate statement of the problem. The calculations are performed in the ranges of Grashof numbers from 1000 to 25000, with the gas Prandtl number equal to 0.68. Calculations of convective heat transfer are carried out by finite element method, and radiation fluxes are calculated by the zonal method. The relative role of thermal conductivity, convective heat transfer and radiation heat transfer is investigated.

Conditions for Explosive Disintegration of Inhomogeneous Water Droplets on High-Temperature Heating

Experimental investigations of the characteristic stages of the processes of heating, evaporation, and explosive disintegration of inhomogeneous water droplets (with a commensurate graphite inclusion) in a high-temperature (600–1200 K) gaseous medium are carried out. Three methods of heating droplets differing in the dominating mechanism of heat transfer are used: heating in a muffle tube furnace (thermal radiation), in a stream of heated air (radiative-convective heat transfer), and in a stream of high-temperature combustion products of a typical liquid fuel (radiative-convective heat transfer). Characteristic values of each heat flux component ate determined for the conditions of experiments, as well as their dependences on temperature. It is shown that the highest values of the radiative heat flux (determining one from the viewpoint of the origination of the effect of explosive fragmentation of droplets) correspond to the schemes of heating in a stream of combustion products and in a tubular muffle furnace. The threshold values of the gaseous media temperatures at which a stable explosive disintegration of evaporating inhomogeneous droplets is realized (Tg > 850 K for conditions of heating in a stream of heated air, Tg > 800 K for the tubular muffle furnace, and Tg > 600 K for a stream of combustion products) have been obtained experimentally. With the use of thermocouple measurements the assumption on accumulation of the energy of thermal radiation near the liquid–solid particle interface and on the resulting formation of an additional source of liquid fi lm heating has been confirmed, which leads to the overheating of the liquid and to explosive disintegration of the droplet.

INFLUENCE OF CHARACTERISTICS VORTEX BURNER FOR HEAT TRANSFER EFFICIENCY IN THE FURNACE BOILER

In Ukraine, a significant number of domestic and industrial boilers are in operation. In this paper, we present the results of a numerical study of the processes of burning gaseous fuels in a furnace of a steam water-tube boiler DE-10/14. The burner device GMG-7 with a capacity of 728 m3/h of natural gas provides a swirling short and wide torch. The fuel-air mixture is formed by premixing 15% of the air, with a primary twist factor n = 2.4, and a secondary burner twist ratio n = 1.6, and an excess air factor αa = 1.10. The option of installing blades in the primary air duct with an angle φ1 = 45° was considered, and in the secondary air channel with an angle φ2=60°. The burner embrasure is conical with an opening angle of 60°. Twist of primary and secondary air – one way. As a result of the research, the temperature and velocity distributions of gases in the combustion chamber, the density of heat flows on the screen tubular surfaces, and the concentrations of the combustion components were determined. The mathematical model of radiation-convective heat transfer in the gas path of the boiler is formed on the basis of the Reynolds-averaged Navier-Stokes equations with allowance for gravity and neglect of compressibility. The model is the equation of continuity, momentum transfer, energy and chemical components of the gas mixture recorded in a stationary form. The equations are closed by the Newton law for the pressure tensor, the Fourier law for the heat flux, the Fick law for the mass flow, the Clapeyron-Mendeleyev law for the thermodynamic state of the gas mixture, the equations of the Launder-Spaulding turbulence model and the Magnusen-Hertager turbulent combustion model. The simulation is performed by the control volume method. At a slope angle of the blades of the register φ2=60° in the secondary air flow path and φ1 = 45° in the primary air duct, a wide opening of the V-shaped flame near the cutoff of the burner at a distance of 0.5 to 0.6 m is observed. Combustion of the gas – in narrow jets of the open torch at a distance of 1.0 – 1.5 m. The temperature of the gases in the streams of the torch is about 1500 – 1700°C. In this case, narrow jets move near the screen surfaces of the heat exchange tubes. It is established that the structure of the flame at the angle of installation of the blades φ2 = 45° is symmetrical and stable, and the dimensions of the torch correspond more to the geometry of the combustion volume. At the same time, no flare is observed on the screen side surfaces and the bottom of the furnace, tightening the torch in a convective beam. However, the temperature of the gases and the density of the heat flow are not high enough – the average values are about q=100 kW/m2 (at qcp = 76.8 kW/m2).

Turbulent natural convection combined with surface thermal radiation in a square cavity with local heater

PurposeThe purpose of this paper is to conduct a numerical analysis of transient turbulent natural convection combined with surface thermal radiation in a square cavity with a local heater.Design/methodology/approachThe domain of interest includes the air-filled cavity with cold vertical walls, adiabatic horizontal walls and isothermal heater located on the bottom cavity wall. It is assumed in the analysis that the thermophysical properties of the fluid are independent of temperature and the flow is turbulent. Surface thermal radiation is considered for more accurate analysis of the complex heat transfer inside the cavity. The governing equations have been discretized using the finite difference method with the non-uniform grid on the basis of the special algebraic transformation. Turbulence was modeled using the k–ε model. Simulations have been carried out for different values of the Rayleigh number, surface emissivity and location of the heater.FindingsIt has been found that the presence of surface radiation leads to both an increase in the average total Nusselt number and intensive cooling of such type of system. A significant intensification of convective flow was also observed owing to an increase in the Rayleigh number. It should be noted that a displacement of the heater from central part of the bottom wall leads to significant modification of the thermal plume and flow pattern inside the cavity.Originality/valueAn efficient numerical technique has been developed to solve this problem. The originality of this work is to analyze unsteady turbulent natural convection combined with surface thermal radiation in a square air-filled cavity in the presence of a local isothermal heater. The results would benefit scientists and engineers to become familiar with the analysis of turbulent convective–radiative heat transfer in enclosures with local heaters, and the way to predict the heat transfer rate in advanced technical systems, in industrial sectors including transportation, power generation, chemical sectors and electronics.

Experimental and numerical investigation of natural convection in tilted square cavity filled with air

In the present work, the natural convection mechanism of air in a square cavity with active side and inferior walls has been investigated both experimentally and numerically. The experimental method developed by some of the authors has been tested against new boundary condition. In addition, the combined radiative-convective heat transfer method has been investigated. The position relative to the gravity vector and the temperature difference between hot and cold walls were analysed: four different inclination angles (ϑ = 0, ϑ = π/6, ϑ = π/3, ϑ = π/2) of the cavity and five different temperatures (104 ≤ Ra ≤ 105) of the hot walls were tested for a total of 20 different cases. The velocity fields and the effective temperature of the walls have been measured by, respectively, a Particle Image Velocimetry (PIV) system and a set of thermocouples. It has been assessed that both the average and maximum velocities increase with the increasing of the Rayleigh number and of the tilting angle. The velocity field is characterized by two vortices, with a symmetry axe passing from the cavity centre when ϑ = 0; by increasing the tilting angle the symmetry tends to disappear leading to the presence of only one vortex for ϑ = π/2. Numerical simulations with both pure convection and combined radiative-convective model have been performed. For the model validation, both the maximum and the average values of the velocity field on the cavity have been taken into account. The results are in good agreement with the experimental measurements. The addition of the radiance contribution does not influence the results.