Wall Emissivity Research Articles

Collapse of axionic domain wall and axion emission.

We examine the collapse of an axionic domain wall bounded by an axionic string. It is found that the collapse proceeds quickly and axionic domain walls disappear. However axions are emitted in the collapse and its energy density increases during the radiation-dominated era and contributes significantly to the present mass density of the Universe. In particular the axion emitted from the wall can account for the dark matter in the Universe for ${\mathit{F}}_{\mathit{a}}$\ensuremath{\gtrsim}${10}^{10}$ GeV.

Natural convection of a radiating fluid in a square enclosure with perfectly conducting end walls

Combined natural convection and radiation in an asymmetrically heated square enclosure is studied numerically with both adiabatic and perfectly conducting end walls. The momentum and energy equations are solved by a control volume based finite difference algorithm which is coupled with the discrete ordinates method for radiative heat transfer calculations. The changes in the flow patterns and temperature distributions due to the presence of radiation in an enclosure with conducting end walls are compared with those for the case of an enclosure with adiabatic end walls, and significant differences are noted. The flow field is stronger, and the heat input along the hot wall and the end walls are greater for the conducting end wall case. The effects of optical thickness, scattering and wall emissivity on the flow and temperature fields and heat transfer rates are analysed.

Film cooling effectiveness in a particle-laden flow

A computational method is developed to evaluate the cooling effectiveness of a tangential slot in a particleladen flow. The physical problem analyzed involves the tangential injection of a coolant at a wall subjected to convective heating from a turbulent boundary layer and radiative heating from hot particles. Previous published analyses did not include radiative or particle effects. The boundary-layer equations, including energy and species conservation, are solved using a finite difference scheme. The particle transport equation is also solved by a finite difference technique. A two-dimensional formulation for the solution of the radiative equation of transfer is developed using the spherical harmonics method. This formulation is different from those previously published in that it solves for an azimuthally integrated form of the intensity. This approach reduces the number of unknowns in the solution and is therefore more computationally efficient. Particle optical properties are obtained using the Mie scattering theory. A parametric study is performed to determine the effects of wall emissivity and other parameters on cooling effectiveness and particle deposition on the wall.

COMBINED CONVECTION AND RADIATION IN SIMULTANEOUSLY DEVELOPING FLOW AND HEAT TRANSFER WITH NONGRAY GAS MIXTURES

Abstract Combined convection and radiation in simultaneously developing laminar flow and heat transfer in a smooth tube is numerically considered with the P-I approximation and the exponential wideband model. The fluid is a mixture of carbon dioxide, water vapor, and nitrogen, The bulk mean temperature variation, temperature profiles, and Nusselt numbers are shown for a uniform inlet temperature and several constant wall temperatures. Nusselt numbers for simultaneously developing flow are compared with those for thermally developing flow. The effects of the mole fraction of the nongray gases and the wall emissivity on convective and radiative Nusselt numbers are explored.

Estimation of Effect of Thermophysical Properties on Temperature of Water Wall in Furnace of Thermal Electric Power Plant.

In the analysis of water wall temperature in a thermal electric power plant, we must grasp many thermophysical properties correctly, such as wall emissivity and absorption coefficient of gas. However, their measurement during operation of the plant is very difficult and we cannot grasp them accurately. For this reason, we estimate the effect of thermophysical properties on the temperature of the water wall and thermal efficiency.

PHYSICAL CHARACTERISTICS OF WALL AND VOLUME EMISSIONS OF X-RAY IN LASER-CAVITY TARGET

We present the studies of physical mechanisms of wall and volume emissions of X-ray for different type cavity targets on "Shen-guang" facility. The wavelength 1.053 μm, pulse shape-Gauss type, energy ～500-700J/beam, pulse width ～600-1000ps laser are used to irradiate cylindrical Hohlraum from two ends. The typical target consists of three cylindrical sections of which two ends are absorption-conver-sion regions of laser energy (source region), and the median is imploding region, which is heated by radiation energy from source region. The experiment results of X-ray wall and volume emission and some analyses are presented.

Radiant and convective heat transfer for flow of a radiation gas in a heated/cooled tube with a grey wall

An analytical investigation is presented of the influence of radiative heat transfer on the complex heat exchange problem involving flow of an optically active (radiating) gas inside a tube of diffuse grey properties. The method used is based on Hottel's formulation of zone division, and involves the transformation-zone approach, where radiation gas emission is replaced with an equivalent surface emission. Separable-kernel and surface transformation techniques give a set of non-linear differential equations treated by the Runge-Kutta method with Hamming modification. The solutions are governed by several independent parameters such as the wall and radiation gas emissivities, inlet and exit gas temperatures, length diameter ratio of the tube, uniform and non-uniform heat flux and variable convective heat transfer coefficient at the inner surface. The results apply both to heating and cooling situations.

Composite heat transfer in a variable porosity medium bounded by an infinite flat plate

Heat transfer by simultaneous radiation and convection in a variable porosity medium bounded by an infinite flat plate is determined by solving momentum and energy equations. The problem is investigated for two different cases, viz., presence of porous medium and absence of porous medium. In the presence of porous medium both variable and constant porosity situations are studied. Similarity solutions are obtained for an isothermal and impermeable wall. The Kozeny-Blake expression connecting porosity and permeability is incorporated in the analysis for unified treatment. A parameter survey is made to study the effect of optical thickness, porous parameter, albedo scattering and ratio of conduction to radiation using Runge-Kutta-Gill method. The heating and cooling cases are considered for variable porosity situation. The results show that both convective and radiative components decrease with increase in porous parameter, and the inhomogeneity of the medium enhances heat transfer rate. The increase in optical thickness leads to decrease in both convective and radiative fluxes. For a particular value of wall emissivity (e w =0.5932116) the radiative flux is reduced to zero.

Towards a generalized model for vertical walls to gas—solid fluidized beds heat transfer—II. Radiative transfer and temperature effects

The radiative heat transfer is combined with the heterogeneous model accounting for conduction and convection proposed in the first part of this paper in order to determine heat transfer coefficients in high-temperature fluidized beds. The effects of parameters such as particle size, emissivities of particle and walls, bed and wall temperatures, heat flux direction, etc., are investigated systematically and the relative importance of each component at different conditions is considered. A comparison of the predicted results with the experimental data in the literature shows a reasonable agreement in a large range of particle diameters and temperatures.

Multidimensional Modeling of Radiative Heat Transfer in Scattering Media

There exist various numerical techniques for computing radiative transfer in combustion systems. The discrete transfer model is applied to solve the radiative heat transfer problem in two- and three-dimensional rectangular enclosures containing absorbing-emitting and scattering media. Results obtained with this technique are compared with other well-established methods. Geometries and surface and gas properties were used in a wide variety of situations to understand the performances of the method better. Required computer times and number of iterations are also reported as a function of number of rays, size of the grid, wall emissivity, and gas scattering coefficient. 16 refs., 4 figs.

Discrete ordinates solutions for radiatively participating media in a cylindrical enclosure

The radiative transfer equation is solved by the S-N discrete ordinates method in the two-dimensional r-z coordinates system. The walls of the enclosure are diffuse, and the participating medium absorbs, emits, and anisotropically scatters the radiative energy. Diffuse wall incidence, isothermal medium emission, and collimated incidence problems are considered. Effects of the scattering phase functions on average incident radiation and net radiative heat fluxes are studied. In addition, the effects of scattering albedo, optical thickness, and the wall emissivity are briefly discussed.

Convective-radiative heat transfer in a square duct with a centered circular core

Combined laminar forced convection and radiation heat transfer have been numerically investigated for a gray, absorbing, emitting, nonscattering gas in the entrance region of a square duct with a centered, circular core. The thermal boundary condition imposed on the square duct and the core is a uniform temperature, both axially and peripherally. The method of moments is applied to considered the radiation contribution. The momentum equation, the three-dimensional energy equation, and the two-dimensional irradiaton equation are discretized and numerically solved by the control-volume-based finete-difference method in the boundary-fitted coordinate system. This numerical scheme is found to be simple, accurate, and efficient. The effects of four major parameters influencing combined convective and radiative heat transfer, namely, the geometry parameter, α ∗ , as well as radiation-conduction, N, optical thickness, τ b , and wall emissivity, ϵ w , are discussed in detail. The results for the total Nusselt number variations indicate that heat transfer is enhanced by thermal radiation. The predicted results for pure convective heat transfer are in good agreement with the available data published in the open literature.

Three dimensional discrete ordinates modelling of Radiative transfer in a geometrically complex furnace

Discrete·ordinates S2' 54, 56 and 58 approximations arc used to study radiative transfer in athree-dimensional furnace with complex geometry including cooling-pipes in the combustion chamber. Theability of the discrete-ordinates method to model highly-directional shadowing effects caused by the internalpipes is illustrated. Predicted values for incident wall flux and net radiative transfer to the pipes are shown tocompare well with experimental data. The large energy sink produced by the pipes minimizes the sensitivityof results to variations in wall emissivity. The 54 approximation is shown to produce results of adequateaccuracy except in cases where detailed modeling of the pipe shadows requires the higher order 56 and 58approximations. Results from mesh densities of 76.000, 388,000 and 815,000 cells show different mesh sizesdo not significantly alter incident flux or net heat transfer predictions,

Effect of Radiation on Diffusion Flame Behavior over a Combustible Solid

The effects of two-dimensional gas radiation on the flame behavior over a solid fuel plate are explored. The theoretical model includes the elliptic type of momentum, energy and species equations with a second-order single-step chemical reaction model of the Arrhenius type. The discrete ordinates method is employed to solve the radiative transfer equation. Numerical calculations are performed for a wide range of mass absorption coefficients and wall emissivities to characterize the flame behavior. When the gas radiation is stronger, the radiative heat loss causes the flame to retreat downstream, causes shrinkage of the flame size, and causes a decrease in the maximum flame temperature. The flame configuration is thus strongly affected by the gas radiation process.

Simulation of Rapid Thermal Processing Equipment and Processes

ABSTRACTWe present finite element simulations of fluid flow, heat transfer, and chemical reactions in axisymmetric rapid thermal processing (RTP) configurations. A new approach to simulating radiation heat transfer between lamps, substrates, and system walls is described. The method accounts for multiple reflections and readily allows the inclusion of temperature, radiation wavelength, and materials specific emissivity parameters. The influence of system geometry, lamp power profile, substrate and wall emissivity parameters, and process gas flow upon RTP performance characteristics is illustrated through examples. Transient flow and heat transfer simulations are used to identify operating conditions where flow recirculations are avoided. The further use of physically based models in the design and optimization of RTP systems is discussed.

Three-dimensional radiation in absorbing-emitting-scattering media using the modified differential approximation

The modified differential approximation (MDA) first proposed by Olfe and recently extended by Modest is applied to a three-dimensional absorbing-emitting-scattering medium bounded by non-black walls. Numerical techniques are developed to solve for the radiation fields due to wall emission and medium emission. The techniques are evaluated in terms of the computer memory requirement, CPU time and numerical accuracy. For a model problem, the modified differential approximation is compared against the zone method, the P 1 approximation, the P, approximation, S 4 and S 6. MDA is shown to be superior to the P 1 and P 3 approximations and to compare favorably with S N solutions for all conditions of optical depth.

The boundary condition of the PN-approximation used to solve the radiative transfer equation

The arbitrariness of the boundary condition applied to the P 1-approximation is explored by extending the conventional Marshak boundary condition to include an arbitrary constant. The influence of the constant on the results of the P 1-approximation is investigated numerically for one- and multi-dimensional radiative heat transfer problems. Based on these numerical investigations, optimized values for the constant are recommended according to the wall emissivity of the radiating system. The accuracy of the P 1-approximation can be improved significantly when optimized values are employed in the boundary condition.

Combined radiation and convection heat transfer in simultaneously developing flow in ducts with semi-circular and right triangular cross sections

In this paper, the interaction of radiation and forced convection in simultaneously developing laminar flow through semicircular and right triangular ducts with isothermal non-black wall is investigated. The three dimensional momentum and energy equations are discretized by the method of lines and solved numerically by the marching method. The method of momentum is employed to consider the radiation contribution which models the radiation in the partial differential equation, instead of the partial integrodifferential equation. The effects of three major parameters, radiation-conduction parameter,N, optical thickness, τ b , and wall emissivity, e w , in the entry region of these irregular geometry ducts are discussed. The numerical results in terms of the variation of the bulk temperature and the mean Nusselt number indicate that the thermal radiation not only enhances the heat transfer rate, but also changes the characteristics of the convective heat transfer. Furthermore, the results compare very well with the available data published in the open literature for the pure convective case.

Analysis of heat transfer in arbitrary shaped ducts: Interaction of forced convection and radiation

Combined convective-radiative heat transfer in the hydrodynamically and thermally developing region of rectangular and equilateral-triangular ducts is numerically investigated in this paper. The radiative heat transfer contribution is modeled by a differential method, the method of moments. The three-dimensional governing equations of momentum and energy are discretized by an explicit finite-difference algorithm, the method of lines (MOL). This system of discretized algebraic equations are then solved by the fourth-order Runge-Kutta method. The effects of three major parameters affecting combined heat transfer, namely, conduction-radiation parameter, N, optical thickness, τ b, and wall emissivity, ϵ w, are analyzed in detail. This numerical scheme is found to be simple, accurate and efficient, and it is especially suited for combined heat transfer. The predicted results for pure convective heat transfer are in good agreement with available data published in open literature.

CONVECTIVE AND RADIATIVE HEAT TRANSFER IN THE ENTRANCE REGION OF AN ELLIPTIC DUCT WITH FINS

Abstract This paper investigates combined laminar forced convection and radiation heat transfer of an absorbing-emitting gas in the entrance region of an elliptic duct with internal fins. The fins are continuous and longitudinal with zero thickness. The thermal conditions imposed on the elliptic duct are uniform temperature, both axially and peripherally. The method of moments, which models radiation in a partial differential equation, is employed to consider the radiation contribution. The three-dimensional energy equation and two-dimensional irradiation equation are diseretized and numerically solved by the control-volume based finite-difference method in the boundary-fitted coordinate system. The effects of six major parameters on the thermal behavior in the entrance region of the elliptic duct is discussed. These parameters are radiation-conduction, optical thickness, wall emissivity, Picltt number, fin height, and the minor axis to major axis ratio of the elliptic duct. The total Nusselt number variations show that heat transfer is enhanced by both thermal radiation and juts. Furthermore, the results are compared with the available data in the literature for pure forced convection with fins. We prove that radiation plays a significant role in the enhancement of heat transfer.