Contribution Of Radiative Heat Transfer Research Articles

An isothermal constant surface area horizontal helical coil under natural convection and radiative heat loss: A CFD study

This paper presents a numerical analysis of a horizontally oriented constant surface area isothermal helical coil under natural convection heat transfer and surface radiation. Numerical computationsare carried out in the laminar regime of Rayleigh number (104 ≤ Ra ≤ 108), surface emissivity (0 ≤ ε ≤ 1), and geometrical parameters of the helical-coil, like diameter of the coil (8 ≤ D/d ≤ 24), pitch (3 ≤ p/d ≤ 7.5), and coil height (40 ≤ H/d ≤ 60) to study the heat transfer characteristics. This study is used for effectively designing the helical coils, which are typically manufactured at an extremely high temperature. The numerical computation has been validated by comparing the average Nusselt number (Nu) with the previous experimental results. Temperature-dependent fluid properties are incorporated to achieve precise results over a wide range of temperatures. The effect of the geometrical parameters on the heat transfer characteristics has been depicted graphically in terms of average Nu, relative convection and radiation heat transfer rate, and the temperature profile. It is found that the relative contribution of radiative heat transfer is 87.8% at an emissivity of 0.9 and Ra of 10(Ali, 19944), whereas, for an emissivity of 0.1 and Ra of 10(Moawed, 20058), the relative contribution of radiation heat transfer is only 6.4%. Lastly, a correlation has been developed for the average Nu based on all the pertinent parameters which can be beneficial to the engineers in the industry and also academic researchers. The developed correlation has been used to predict the cooling curve of the helical coil in certain situations.

Method for calculating radiative energy transfer in the “back and forth” approximation

Abstract. This paper presents a method for calculating the radiative energy transfer in the back and forth approximation for the case of cylindrically symmetric currents. The key element of the method is the scheme for calculating radiative heat transfer in a medium with opacity that strongly varies in space and time. The paper discusses the possibility of improving accuracy of solving a difference equations’ system by making the approximation of absorption coefficients more accurate. The numerical technique proposed for multi-parameter computing experiments makes it possible to obtain the radiation energy density as a quadrature of plasma opacity and emissivity. In two-temperature model of gas dynamics and magnetohydrodynamics, this determines the contribution of radiative heat transfer into the energy balance of the plasma electronic component. Numerical comparison of the proposed method with some diffusion methods was implemented via test examples simulating spatial inhomogeneity of the radiation field. It is shown that the calculations performed according to the methodology considered in this paper, give a qualitatively correct match with the analytical solution of the problem, in contrast to calculations performed by the method of diffuse approximation. The obvious advantage of the back and forth approximation is that integrality is inherent in its structure.

Natural Convection and Radiative Heat Transfer From Constant Surface Area Vertical Helical Coils: Effect of Pitch and Height

Abstract In this work, numerical simulations are carried out to delineate the natural convection and surface radiation heat transfer characteristics of vertically oriented isothermal helical coils having a constant surface area. Numerical computations using the finite-volume method are carried out in the laminar regime for the following non-dimensional parameter ranges: Rayleigh number (104 ≤ Ra ≤ 108), surface emissivity of the coil (0 ≤ ɛ ≤ 1), pitch to the rod-diameter of the coil (3 ≤ p/d ≤ 7.5), and coil-height to the rod-diameter (40 ≤ H/d ≤ 60). Temperature-dependent fluid properties have been implemented to obtain accurate results. The impact of Ra and ɛ on both convective and radiative heat losses is discussed in detail. At a high Ra of 108, when H/d varies from 40 to 60, the mass flowrate inducted through the coil reduces from 40.6% at p/d = 3 to 11.4% at p/d = 7.5. As a result, the relative strength of convection heat loss declines with a rise in H/d. For a higher emissivity of the coil surface of 0.9 and a lower Ra of 104, heat transfer by convection contributes only 12.66% of the total heat transfer. In contrast, the contribution of radiative heat transfer is only 7.46% for a lower emissivity of the coil surface of 0.1 and a higher Ra of 108.

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.

Модель Шустера – Шварцшильда для цилиндрически симметричных течений излучающего газа

The paper presents an algorithm for calculating radiative heat transfer in an emitting and absorbing gas based on the Schuster – Schwarzschild approximation for the case of a cylindrically symmetric flow. In this approximation, the radiation energy density is calculated as a quadrature of the absorption coefficient and the emissivity of the plasma. This quadrature is used to calculate the contribution of radiative heat transfer to the energy balance of the gas flow. An algorithm is described for constructing a one-dimensional adaptive difference mesh for the numerical calculation of the radiative energy transfer equation, which makes it possible to obtain a high-precision numerical solution on a mesh of a smaller dimension than the original “gas-dynamic” mesh. On test examples simulating the spatial inhomogeneity of the radiation field, a numerical comparison of the proposed method with some diffusion-type methods on the initial gas-dynamic and adapted computational meshes is carried out.

Radiative heat transfer in FLiBe molten salt participating medium in a vertical heated tube under forced and mixed convection laminar flows

The contribution of radiative heat transfer (RHT) to convective heat transfer in a heated vertical pipe with laminar flow and constant wall temperature, for FLiBe molten salt, is investigated computationally. The combined effects of conduction in the fluid, forced convection, buoyancy, temperature-dependent physical properties, and thermal radiation are investigated. The P1 approximation is employed in the discretization of the Radiative Transfer Equation (RTE). The COMSOL Multiphysics software is used to generate the numerical solutions. The theoretical analysis developed here demonstrates that only for intermediate values of the optical thickness, i.e. τD~O1, the participating media effects are expected to be important. This analysis is confirmed computationally and a value of τD≈4 is shown to lead to the highest increase in overall heat transfer behavior. Under forced convection, the Nusselt ratio Nutotal/Nuno-rad reaches a peak value of 1.76 at τD=4 and z/D=200, and is less than 1.1 for τD<0.1andτD>60. Under aiding-flow mixed convection, Nutotal/Nuno-rad reaches a peak of 1.34 at τD=4.2 and is less than 1.1 for τD<0.4andτD>36. Under opposing-flow mixed convection, Nutotal/Nuno-rad reaches a peak of 1.81 at τD=4.2 and is less than 1.1 for τD<0.2andτD>50. RHT effects on the overall heat transfer are most pronounced in opposing mixed convection, where Nutotal/Nuno-rad is 2.26 observed at z/D=50. The sensitivity to wall emissivity is evaluated; in forced convection, the peak Nutotal/Nuno-rad is 1.66 at ε = 0 (reflective wall) and 2.04 at ε =1 (absorptive wall). The sensitivity to pipe diameter is also discussed. Overall, for a vertical heated tube, radiative heat transfer effects lead to enhancement of heat transfer by as high as a factor of two, and they depend on the optical thickness of the flow, mixed convection environment (Gr/Re2 and direction of flow relative to the gravitational force), surface emissivity, and entrance effects.

Thermal Analysis of a Microhotplate - An Experimental Study

In the present work, experimental and numerical thermal analysis of a microhotplate were performed to evaluate the heat transfer mechanism at small Rayleigh number ( $Ra$ ). The convective and radiative heat transfer coefficient values were evaluated for microhotplate mounted over two different mounting positions (namely model-1 & model-2). At different heat generation ( $Q$ ) values, it was found that the average convective coefficient value was less than $10\frac {W}{m^{2}K}$ , irrespective of the microhotplate mounting position. Comparison of both models at a similar $Q$ ( $\simeq 0.69W$ ) value, reveals that the radiative heat transfer contribution was 33% & 68% for model-1 and model-2 respectively. Based on the results obtained in the present study, a correlation was developed using regression analysis for the convective Nusselt number in terms of Rayleigh number ( $8.84\times 10^{-4}\le \:Ra\:\le 4.94\times 10^{-3}$ ), and the convective Nusselt number obtained from the correlation and present study are within ±17%.

Influence of micropores on structural instability of the combustion wave

The article shows examples of visualization of the process of heat transfer by radiation in unstable combustion modes of porous powder materials, which are in good agreement with the results of high-speed video recording and micropyrometry. The mathematical model and the results of calculating the structure of the combustion wave in the Ni-Al system are presented. The contribution of radiative heat transfer at an adiabatic combustion temperature in the range of 810 % and the effect of its trigger shutdown with decreasing temperature were revealed.

Evaluation of bed-to-tube surface heat transfer coefficient for a horizontal tube in bubbling fluidized bed at high temperature

Experiments have been conducted in a bubbling fluidized bed to determine the bed-to-tube surface heat transfer coefficient to a horizontal tube at high bed temperatures (400–950 °C). The aim was to study the influence of the bed temperature, the particle size, the bed material and the superficial gas velocity. A tube (do = 6 mm) through which water is flowing was placed close to the middle of the bed with a fixed bed height of 17 cm. Silica sand, crushed and beneficiated ilmenite and ground steel converter slag were used as bed materials. Air was used as fluidization gas. The mean particle diameter of the used bed materials was in the range 123–327 μm. The estimated heat transfer coefficients were compared with correlations from literature describing the convective bed-to-tube heat transfer coefficient (all of which have been derived at temperatures below 400 °C), with the addition of the radiative heat transfer contribution. The heat transfer coefficient increased with increasing bed temperature, decreasing particle size and increasing superficial gas velocity up to a certain value. These trends were observed for all bed materials although significant differences were still observed in the estimated heat transfer coefficients. Ilmenite was the material for which the highest heat transfer coefficient was estimated. The experiments verified high heat transfer coefficients (768–1858 W/(m2K)) to the outside tube surface operating at a superficial gas velocity of 0.15 m/s, which aligned very well with two of the heat transfer correlations and reasonably well with a third. The remaining three correlations were not as accurate at the examined conditions. The contribution from the convective heat transfer is significantly higher than the estimated radiative heat transfer to the tube.

Radiation Matters in Fixed‐Bed CFD Simulations

AbstractFixed‐bed reactors often operate at elevated temperatures, where radiation can be a significant heat‐transfer mechanism. Particle‐resolved CFD models fixed‐bed reactors on a very detailed macroscopic level. In this study, the contribution of radiative heat transfer is investigated in a 500‐mm bed of 7‐hole pellets. At industrially relevant temperatures (250 – 800 °C) and with a steam‐reforming gas‐phase mixture, the S2S and DOM radiation models were applied. Neglecting radiation results in temperatures being up to 6 % lower. In this case, the main driver is surface‐to‐surface (S2S) radiation. Additional modeling recommendations are given.

Heat transfer in counterflow fluidized bed of oxide particles for thermal energy storage

The potential for inert oxide particles as a heat transfer and thermal energy storage (TES) media in concentrating solar power (CSP) depends in part on particle receiver designs that provide high wall-to-particle heat transfer rates. This paper presents a novel continuous-flow approach to achieve high heat transfer coefficients hw for particle receivers by fluidizing net-downward-flowing particles in a narrow vertical channel bounded by an external irradiated/heated wall and a parallel interior wall with a metal mesh opening that allows the upward-flowing fluidizing gas to exit at the top of the channel. To demonstrate the high hw of this flow configuration, a fluidized bed in a 10 cm × 10 cm × 0.64 cm deep channel was heated through an external aluminosilicate wall with mid-IR quartz lamps that provided external wall heat fluxes up to 20 W cm−2. Extensive heat transfer measurements with fluidized Carbo Accucast ID50 particles (diameters between 150 and 350 μm) in steady-state continuous downward flow and in transient batch mode assessed total hw as functions of particle bed temperatures Tb, bed solids volume fractions αb, and superficial gas velocities Ug. Results showed that the narrow-channel fluidized bed can achieve overall hw as high as 1000 W m−2 K−1. The highest hw were measured at upward Ug between 2 and 4 times the minimum bed fluidization velocities, Umf, which decreased to0.12 m s−1 for the mean particle diameter at Tb=600°C. Increasing Ug further above Umf decreased hw due to an associated decrease in αb. hw increased strongly with Tb in part, because gas-phase conductivity and the radiative heat transfer contribution increased with Tb. The extensive measurements were fit to a modified version of the Nusselt number correlation by Molerus (1992). For αb⩾0.1, the Molerus correlation with adjusted dependence on excess fluidization velocity (Ug-Umf) provided an excellent fit to the measured convective fraction of hw (with <10% error). Adding the radiation component with the Molerus correlation provides an effective tool for calculating hw for this counterflow fluidized bed configuration. A simple analysis explored the impact of such high hw for an indirect receiver design with angled external walls to spread solar aperture fluxes. Results from the analysis indicated that total hw=1000 W m−2 K−1 can enable solar collection efficiencies approaching 90% with external wall temperatures Tw,ext≈1020°C. This potential performance motivates further exploration of this fluidized bed configuration for particle receivers for CSP applications.

A comparative study of 2-D and 3-D conjugate natural convection from a vertical rectangular fin array with multilayered base subjected to distributed high heat flux

This paper presents a comparative numerical study of 2-D and 3-D steady, laminar conjugate natural convection heat transfer from a vertical rectangular aluminium fin array subjected to a distributed high heat flux related to the cooling of a 105 W LED street lighting luminaire. The effect of radiation is also considered in the aforementioned models. The 2-D model uses the Stream function-Vorticity-Temperature approach. The finite-difference methodology is used to discretize the governing equations. The discretization method in the 3-D model is finite-volume. The solution algorithm is SIMPLE (Semi-Implicit Method for Pressure-linked Equations). A detailed parametric study based on a realistic lamp data is conducted to observe the effect of heat flux, fin spacing, fin height, fin width, fin thickness and fin material (aluminium, brass, copper, mild steel) on the fluid (air) velocity and temperature distributions, heat transfer and LED junction temperature. A novel finding of this work is the revelation of asymmetric flow and thermal fields in the successive channels of the fin array, as predicted by the 3-D model. In contrast with the existing studies in which fin base is treated as isothermal, an optimal fin spacing is found to exist corresponding to the maximum heat transfer coefficient in the present study. The difference between 2-D and 3-D model-predicted average heat transfer coefficients increases with increase in fin height to fin spacing ratio. The contribution of radiation heat transfer is about 8–14% of the total heat transfer in the range of dimensions of the fin array considered in this work. A penalty function based optimization study is carried out to maximize the average heat transfer coefficient while maintaining the LEDs below the safe limit of 353 K. Finally, correlations for predicting the average heat transfer coefficient in the vertical rectangular fin array heat sink, and the maximum LED temperature as a function of luminaire wattage and geometric parameters of the fin array have been presented.

Experimental Study of Heat Transfer Enhancements from Array of Alternate Rectangular Dwarf Fins at Different Inclinations

The main objective of this experimental work is to investigate and compare heat transfer enhancement of alternate dwarf fin array at different angles of inclination. In this study, the steady state heat transfer from the full length fin arrays and alternate dwarf fin arrays are measured in natural convection and radiation environment. Largest increase in the Nusselt number was achieved with alternate dwarf fin at angle of orientation 90°, which shows about 28% enhanced heat transfer coefficient as opposed to the full-length fin array with 25% saving in material. In case of non-black FAB, contribution of radiation heat transfer is found to be very small nearly within 1% of the heater input. After coating lamp black contribution of radiation heat transfer is found to increase to about 3–4% of the heater input in the range of temperatures considered in this study.

Laplace Transformation for an Optically Thick Gray Gas in a Vertical Channel with Magnetic Body Force: Rosseland Approximation

A Couette flow of an optically thick gray gas flowing in a vertical channel is analyzed analytically with consideration of the buoyancy effects and thermal radiation. The Rosseland diffusion flux approximation is employed to simulate the radiative heat transfer contribution. An impulsive movement and a uniformly accelerated movement of one plate are considered with the second plate being stationary. The numerical results for some special cases are compared with the exact solutions and are found to be in good agreement. The effects of the flow parameters on the nondimensional velocity and skin friction are revealed.

Experimental study of the thermal conductivity of polyurethane foams

Polyurethane foams are widely used in energy conservation field and thermal conductivity is one of the most important properties. To reveal and optimize the thermal insulation performance of PU foams, the thermal conductivity of five PU foam samples formed by blowing agents of CP, CP+IP, CP+245fa and CP+245fa+LBA are measured using transient plane source method under various environment. Influences of temperature, humidity, water uptake, alternate high and low temperature, long time storage at high temperature and gas pressure of the atmosphere on the thermal conductivity of PU forms are investigated comprehensively. The mechanism of temperature that affects the thermal conductivity of PU foams is discussed. Fourier transform infrared spectroscopy is adopted to measure the spectral extinction coefficients of these five samples. With the spectral extinction coefficient, the radiative thermal conductivity is calculated from Rosseland model. Then the contributions of radiative heat transfer to the effective thermal conductivity are decomposed. The thermal conductivity of five foams increases non-monotonically with temperature. When stored in moist air, thermal conductivity can increase as high as 10–18%. Radiative thermal conductivity contributes 3.6–4.1% at −40°C, 7.3–9.0% at 20°C and 9.1–11.8% at 70°C to the effective thermal conductivity.

On the possible role of thermal radiation in containment thermal–hydraulics experiments by the example of CFD analysis of TOSQAN T114 air–He test

One of the experiments of ERCOSAM–SAMARA (E–S) projects (TOSQAN T114) is examined from the viewpoint of the radiative heat transfer (RHT) contribution to the overall heat exchange. E–S projects and T114 test were focused on investigation of light gas stratification in severe accident containment atmosphere and stratification break-up after the activation of mitigation systems. The first from two phases of T114 test is considered during which helium is quasistatically injected into the upper part of the TOSQAN vessel having isothermal walls and initially filled by air. The developing free convection removes most of the heat acquired, but not all. Thus stable local deviations in calculated temperatures were obtained in simulations that were interpreted as the deficiencies of the physical heat-transfer model. The modeling of RHT was included in full CFD simulation that resulted in a better agreement in local temperatures. The results of comparative calculations performed without/with RHT modeling are described in the paper. The RHT model implemented in the used CFD code (ANSYS FLUENT) was tested on known analytical solutions. The RHT contribution in T114 test was also estimated analytically to demonstrate independently that it may be noticeable in this experiment. The same estimations may be valid for stagnant zones of severe accident containment. All that shows the need in further detailing of the role of RHT in gas-structure heat exchange: as for interpretation of some containment tests performed in pressure vessel as for containment modeling.

Modeling the thermal radiation properties of thermal barrier coatings based on a random generation algorithm

Thermal barrier coatings (TBCs) are porous media in which many different pores and cracks are induced by different manufacturing procedures. Many studies have been conducted to investigate the impact of microstructures of TBCs on thermal conductivity; nevertheless, the influence of microstructures on the radiative properties of TBCs has not drawn significant attention. In addition, the working condition of thermal barrier coatings is at high temperatures at which the contribution of radiative heat transfer plays a very crucial role. Therefore, it is necessary to study the radiative properties of TBCs to characterize their insulation performance. In this work, the microstructures of air-plasma- sprayed (APS) 8wt% yttria stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) are constructed by the quartet structure generation set (QSGS) algorithm. A finite-difference-time-domain (FDTD) method is carried out to simulate radiative heat transfer through TBCs. Three parameters—average pore size, directional growth probability Di (especially horizontal growth probability D13) and porosity—have been investigated to study the microstructural effect on the radiative properties of TBCs. The reflectance of freestanding 50-μm-thick thermal barrier coatings is studied using Lumerical FDTD Solutions in the wavelength range from 1 to 6μm at normal incidence. The absorption and scattering coefficient as a function of wavelength are extracted using the four-flux model. The results will help us to characterize the radiative heat transfer process across the TBCs and provide us with a theoretical guide to design TBCs with a high thermal insulation property.

Heat transfer in cellulose-based aerogels: Analytical modelling and measurements

A simple analytical approach for estimating the total heat transfer inside new cellulose-based aerogels has been investigated. The model accounts for the characteristic solid matrix at the nanometric scale by using a cellular representation of the nanofoam porous structure.The radiation-conduction heat transfer is taken into account. Previous analytical correlation for the fluid phase is used to model the conduction heat transfer in gas. New analytical formulations based on mean free path theory combined with phonon tracking approach are proposed to model the conduction heat transfer in the solid phase at the nanometric scale. The contribution of radiation heat transfer is obtained from Rayleigh scattering approach combined to the Rosseland approximation.These analytical relations validated experimentally are expected to be useful for researchers aiming at developing new insulating organic aerogels since they permit to determine conduction-radiation equivalent conductivity as a function of cell dimensions, phonon and optical properties of cellulose.

Model of the Structure of Fibrous Heat-Insulating Materials for Analyzing Combined Heat Transfer Processes

A model of the structure of a highly porous fibrous material is suggested within the framework of which deformation of a setting semifinished item is considered. An algorithm is suggested for calculating the effective thermal conductivity and its components. It accounts for heat transfer through a solid phase and a gas, as well as by radiation. The Mie theory is used to estimate the radiative heat transfer, which led to a somewhat underestimated result in determining the effective thermal conductivity. To refine the contribution of radiative heat transfer, it is suggested to determine the optical properties of materials by solving the inverse problem of radiation transfer, the initial data for which are furnished by experimentally measured values of the coefficients of radiation transmission through a set of samples of different thickness. As a result, the radiation absorption and diffusion coefficients of a fibrous heat-insulating material have been determined. The dependence of the effective thermal conductivity of a material on temperature has been obtained, which actually coincides with the results of experimental investigations.

A Numerical Study of Combined Forced Convection and Gas Radiation From a Circular Cylinder in Cross Flow

A numerical study of combined forced convection and gas radiation from a circular cylinder is carried out. The fluid is assumed to be radiatively participating. Simulations are carried out for both laminar and turbulent flows. The Reynolds numbers considered are 100 and 500 for laminar flow and 7190, 21,580, 35,950, and 50,350 for turbulent flow. To check the effect of three-dimensionality on heat transfer characteristics, numerical simulations are also carried out for three-dimensional geometry. All the thermophysical properties of the working fluid are taken as those of air and are assumed to be constant for the range of temperature considered in the present study. In order to understand the effects of absorption and scattering of thermal radiation, a detailed parametric study is carried out by varying the absorption and scattering coefficients from 0 to 60 (m−1). It is observed that the contribution of radiative heat transfer on total heat transfer is significant when the flow is laminar. The radiative Nusselt number decreases with the increase of absorption coefficient.