Wall Emissivity Research Articles

ANALYSIS OF TWO-PHASE RADIATION IN THERMALLY DEVELOPING POISEUILLE FLOW

Combined conductive and radiative heat transfer in a thermally developing two-phase Poiseuille flow in a cylindrical duct is studied here. A two-phase radiative transfer equation (RTE) considering radiation by both gas and particles is taken into account. A complexform of nonlinear integrodifferential RTE is solved by the discrete ordinates method (DOM, or so called SN method) in axisymmetric geometry. After such validation, namely, the solution in a two-dimensional channel flow between two flat plates is compared with that solved by the zone method, the program is then applied to fully developed gas-particle two-phase flow in a cylindrical duct. A parametric study is performed for gas and particle absorption coefficients, particle number density, particle emissivity, and wall emissivity. The results show a significant effect of two-phase radiation on the thermal characteristics. However, in all cases, it was found that conduction is predominant near the wall.

Natural Convection Cooling of Vertical Rectangular Channels in Air Considering Radiation and Wall Conduction

Temperature distributions on the surfaces of vertical channels formed by parallel plates heated uniformly and symmetrically and cooled by conduction, radiation, and natural convection in air are determined numerically and experimentally. Effects of wall separation, thickness, thermal conductivity, and emissivity on the wall temperature distribution are determined. Both cases of controlled and uncontrolled channel edge leading and exit edge temperatures are examined. Optimum channel widths and correlations for the maximum wall temperature rise are offered for both the controlled and uncontrolled edge temperature conditions.

THERMALLY DEVELOPING POISEUILLE FLOW AFFECTED BY RADIATION

A combined convective and radiative heat transfer problem in thermally developing Poiseuille flow in a cylindrical tube is analyzed. A complex form of the nonlinear integrodifferential radiative transfer equation is solved by the discrete ordinates method in an axisymmetric geometry. To check its accuracy, the solution obtained by the discrete ordinates method is compared with that solved by the integral method. A parametric study is also performed for the conduction-to-radiation parameter, optical thickness, wall emissivity, scattering albedo, and linear anisotropic scattering coefficient. The results show a significant effect of the radiation on the thermal characteristics.

Effect of Narrow Band Nonuniformity on Unsteady Heat Up of Water vapor under Radiation-Conduction Combined Heat Transfer.

Effect of narrow band nonuniformity on unsteady heat up process of water vapor under radiation-conduction combined heat transfer is examined by comparing the result of numerical simulations with and without incorporation of narrow band nonuniformity. The authors propose a rational and comprehensive computational approach for incorporating the narrow band nonuniformity into numerical simulations of radiative heat transfer when the considered field is nonisothermal. Results of examination exhibited that the contribution of radiative heat transfer to the heat up rate of water vapor may be almost twice overestimated, if the narrow band nonuniformity effect is neglected. Separate analyses of radiative energy attributed to wall emission and gas emission clarified that the absorption of wall emission is overestimated and, on the contrary, the absorption of radiation energy emitted by water vapor itself is underestimated if the narrow band nonuniformity is neglected. The reason why such over- or under-estimation is induced is understood by examining the influence of line overlap parameter on the transmittance averaged within a narrow band. Smaller value of line overlap parameter {gamma}/d means more violent narrow band nonuniformity. The broken lines show the narrow band transmittance for flat incident power spectrum, and the solid lines show that for themore » radiative emission from the absorbing gas itself. It is also clarified that the disregard of the narrow band nonuniformity give rise to serious error in the estimation of absorption rate of wall and gas emission even in the case where the disregard of narrow band nonuniformity bring little change to the temperature distribution. The results illustrated in this paper suggest that the narrow band nonuniformity should not be neglected.« less

Radiative Heat Transfer in the Combustion Chamber of a Diesel Engine

Radiation is considered to be an important mode of heat transfer in diesel engines. It is mainly caused by a presence of soot particles in combustion gases. Spatial distribution of radiative heat flux on walls of the combustion chamber can be an important factor of thermal load of some parts of the engine. The paper presents a numerical method used for solution of integral equations resulting from mathematical description of variation of radiative heat flux on the walls of the combustion chamber. The charge in the chamber has been divided into the burned and the unburned zones. Shape and volume of the burned zone as well of the combustion chamber are varying according to variation in a crank angle during the whole combustion process. Comparison between numerical results obtained for opaque and semi-transparent burned zone is made. Influence of a degree of the burned zone transparency, the crank angle value and wall emissivity variation on the radiative heat flux distribution on different parts of the chamber surface has been presented.

Inverse radiation problem in one-dimensional semitransparent plane-parallel media

The discrete ordinates method is used to develop a solution to an inverse radiation problem of source term in one-dimensional semitransparent plane-parallel media with opaque and specularly reflecting boundaries. It is assumed that, with the exception of the inhomogeneous source term, all aspects of the radiation transport problem are known. A method is developed to determine the inhomogeneous source term from specified incident radiation intensities on the boundaries. The inverse problem is solved using conjugate gradient method that minimizes the error between the incident radiation intensities calculated and the experimental data. The effects of single-scattering albedo, scattering asymmetry parameter, wall emissivity, the diffuse fraction of reflectivity, and the optical thickness on the accuracy of the inverse are investigated. The results show that the source term can be estimated accurately, even with noisy data.

Radiation heat transfer in catalytic monoliths

AbstractNumerical simulations of heat‐ and mass‐transfer and heterogeneous reactions in catalytic monoliths are reported with the focus on the influence of radiation heat transfer on the thermal behavior of the monolith. Appropriate heterogeneous kinetics and boundary conditions were calculated for two cases including: an automobile catalytic converter in which carbon monoxide is oxidized over a platinum (Pt) catalyst and a catalytic combustor for gas turbine power generation in which methane is oxidized over a palladium oxide (PdO) catalyst. Surface oxidation rates of carbon monoxide are based on measurements of Pt catalyst activity in a catalytic flow reactor, and rates for methane oxidation are based on reaction kinetics for a supported PdO catalyst assigned from differential reactor measurements. These simulations show that in the high aspect ratio passageways (length to diameter) in catalytic monoliths, radiation heat transfer can play a major role in the energy balance on the catalytic surfaces, which determines the transient warm‐up behavior and the steady‐state location of catalyst light‐off. For gray surfaces, however, the predicted steady‐state and transient behaviors are not sensitive to the emissivity of the monolith walls.

Turbulent natural convection coupled with thermal radiation in large vertical channels with asymmetric heating

Numerical and experimental investigations on turbulent natural air convection coupled with thermal radiation have been performed in a vertical rectangular channel with one-sided heated wall. A radiation model has been developed with two new features: an analytical method for computing the view factor and the macro-elements method for improving the numerical efficiency. Numerical studies have shown that the CFD code FLUTAN combined with the radiation model developed is an efficient and accurate numerical tool to investigate the flow and heat transfer behaviour in systems considered. It has shown that at intermediate and high wall emissivities thermal radiation contributes significantly to the total heat transfer by natural air convection, even at low temperatures of the heated wall. Based on the experimental and numerical results the following semi-empirical correlation has bene developed for describing the heat transfer of turbulent natural convection coupled with thermal radiation in a vertical, rectangular channel with one-sided heated wall: Nu t-0.1·Ra 1 3 1+(1+2·l) 0.919·R s 1+0.513·R s

A Bayesian Analysis of Steam Generator Tube Wall Emissivity for Heat Treatment

A Bayesian Analysis of Steam Generator Tube Wall Emissivity for Heat Treatment

An Approximate Solution to Radiative Transfer in Two-Dimensional Rectangular Enclosures

The application of a new approximate technique for treating radiative transfer in absorbing, emitting, anisotropically scattering media in two-dimensional rectangular enclosures is presented. In its development the discontinuous nature of the radiation intensity, stability of the iterative solution procedure, and selection of quadrature points have been addressed. As a result, false scattering is eliminated. The spatial discretization can be formed without considering the chosen discrete directions, permitting a complete compatibility with the discretization of the conservation equations of mass, momentum, and energy. The effects of anisotropic scattering, wall emission, and gray-diffuse surfaces are considered for comparison with results available in the literature. The computed numerical results are in excellent agreement with those obtained by other numerical approaches.

An efficient computational technique for radiative heat transfer in axisymmetric enclosures

One of the most important mechanisms of heat transfer in furnaces and other similar equipment is radiation. Radiative heat transfer can be rigorously described by the radiative transfer equation, which is a six-dimensional integro-differential equation in a phase space. The most popular and widely adopted techniques for solving the radiative transfer equation are the P1 approximation and the S4 method. Although the P1 approximation solves the radiative transfer equation with a decent amount of computer time, it yields somewhat inaccurate results for the cases of optically thin media. On the contrary, the S4 method yields accurate results for all ranges of optical thickness but it requires a large amount of computer time and memory. Moreover, the number of iterations in the S4 method, and consequently the computer time, increases as the value of the scattering albedo increases when the extinction coefficient is not small. In the present work, a new algorithm is devised to solve the radiative heat transfer equation efficiently. This algorithm consists of separating the radiation intensity into two parts i.e., wall emission and medium emission according to the modified differential approximation. Then the wall emission is treated by the S4 method, which can satisfy the wall boundary conditions exactly, and the medium emission is treated by the P1 approximation. The scattering term which increases the iteration number in the S4 method is taken care of by the P1 approximation. The present algorithm solves the radiative heat transfer equation accurately and efficiently for all ranges of optical thickness when compared with conventional techniques, and may be adopted in the analysis of many engineering systems with a significant amount of radiative heat transfer such as pulverised coal-fired furnaces.

Feasibility of neutron spectrometry diagnostic for the fuel ion density in DT tokamak plasmas

The neutron wall emission (WE) background is a crucial element of neutron spectrometry as a fuel density diagnostic for burning deuterium-tritium fusion plasmas in tokamaks. WE spectra have been studied based on MCNP neutron transport calculations applied to the conditions of the ITER vessel according to the preliminary EDA design taking into account the multiple nuclide composition of the plasma facing wall multilayered structure. The results of calculated signal and background intensities suggest that neutron spectrometry is a feasible density diagnostic on ITER for burning plasmas for aim set at the 10% accuracy level (random and systematic).

NUMERICAL ANALYSIS OF CONDUCTION. CONVECTION, AND RADIATION IN A GRADUALLY EXPANDING CHANNEL

The problem with simultaneous conductive, connective, and radiative heat transfer is analyzed in a two-dimensional gradual expansion channel flow. The main objective is to generate a program to attack the radiation in a curvilinear coordinate system with momentum and energy equations involved. The governing equations are transformed into generalized coordinates. Whereas the SIMPLE algorithm developed in a body-fitted coordinate system is used to obtain the flow field solution, the finite volume method is applied to solve the radiative transfer equation. The accuracy and performance of the finite volume method for the radiation are validated in a well-known combined heat transfer problem by using a curvilinear grid system. Then detailed thermal characteristics are investigated in a gradually expanding channel through various parameters such as Prandtl number, conduction to radiation parameter, wall emissivity, and scattering albedo. Results show that radiation plays a significant role in the gradually expa...

THERMAL CONVECTIVE INSTABILITY IN TRANSLUCENT POROUS MEDIA WITH RADIATIVE HEAT TRANSFER

Abstract The onset of thermal convection in a translucent porous layer is considered. Attention is focused on the effect of radiative heat transfer on the critical Rayleigh-Darcy number and the convection cell shape. If we consider the contribution of radiative heat transfer, the basic temperature profile is non-linear and the thermal convective instability is influenced by the ratio of conduction to radiation heat flux, the temperatures at the boundary surfaces, and radiative parameters such as wall emissivity, scattering albedo and extinction coefficient as well as the usual Rayleigh-Darcy number. Effects of these parameters on the onset of convective instability are investigated with the help of linear stability theory employing the Darcy's law and the radiative transport equation simplified by the P1 approximation. The increased effective thermal conductivity due lo the radiation inhibits the onset of convection and causes increased critical Rayleigh-Darcy number and decreased convection cell size. Th...

Coupled radiative and conductive thermal transfers across transparent honeycomb insulation materials

Combined radiative and conductive heat transfer across transparent, square-celled, honeycomb insulation is investigated. The governing equations for one-dimensional heat transfer are formulated using an exponential kernel approximation. The resulting non-linear equation is linearised by least-squares approximation and solved analytically. Computational results for conductive, radiative and total heat transfers through the transparent insulation material (TIM) of three bounding plate conditions: black-black, selective-black and selective-selective are presented. For the case of selective bounding plates, there is a strong coupling of conductive and radiative heat transfer because of the intermediate wall emissivity of the TIM. The effect of cell aspect ratio, wall emissivity and the absorber temperature on the total heat transfer is also studied and it is pointed out that the following configurations of TIM have the lowest heat loss coefficients: (1) TIM with black end plates and cellular walls of high emissivity; (2) TIM with selective end plates and cellular walls fully transparent to IR radiation.

PARAMETRIC ANALYSIS OF RADIATIVE-CONVECTIVE HEAT TRANSFER AROUND A CIRCULAR CYLINDER IN A CROSS FLOW USING THE FINITE VOLUME RADIATION SOLUTION METHOD

The radiative-cancective heat transfer around a circular cylinder in a cross flow of a radiating gas has been numerically analyzed using the finite volume radiation solution method in a nonorthogonal coordinate system. The cross-flow Reynolds number based on the cylinder diameter is 40, and the fluid Prandtl number is assumed to be 0.7. The radiative heat transfer coupled with convection is reasonably predicted by the finite volume radiation solution method. Distributions of the local Nusselt number are investigated according to the variation of radiation parameters such as conduction-to-radiation parameter, optical thickness, scattering albedo, and cylinder wall emissivity.

Neutron wall emission in tokamaks and plasma diagnostics

The wall emission (WE) background accompanying observations of fusion plasma neutrons is a limiting factor in high accuracy neutron diagnostics. It is an important issue to solve for envisaged burning plasma experiments such as with ITER. This work represents the first study of the spectral shape and magnitude of the WE spectrum. The salient features are defined and analyzed with respect to their parametric dependencies based on model descriptions of the plasma neutron source and calculations of the neutron transport. Simple analytical expressions for neutron scattering are used to display the dependence on reaction kinematics, nuclear structure and plasma conditions. Monte Carlo calculations are used for comprehensive descriptions of the WE spectrum distinguishing between single and different multiple scattering contributions. Special consideration is paid to the errors in predicting WE spectra and to the practicality for adopting measurement based descriptions. Illustrations are given describing the impact of the WE background on essential neutron diagnostic measurements proposed for ITER and the means for controlling and correcting for such interference. Specific improvements in the neutron scattering data base relevant in describing WE spectrum are identified.

A study of thermophoretic particle deposition in a particle laden stagnation flow including the effect of radiative heat transfer

A study of thermophoretic particle deposition has been carried out for a particle laden stagnation flow considering the effect of radiative heat transfer. Energy, concentration and radiative transfer equations are all coupled and have been solved iteratively assuming that absorption and scattering coefficients were proportional to the local concentration of particles. Radiative heat transfer was shown to strongly affect the profiles of temperature and particle concentration. e. g., radiation increases the thickness of thermal boundary layer and wall temperature gradients significantly. As the wall temperature gradients increase, the particle concentration at the wall decreases due to thermophoretic particle transport. The deposition rate that is thermophoretic velocity times particle concentration at the wall decreases as the effects of radiation increases. The effects of optical thickness, conduction to radiation parameter and wall emissivity have been determined. The effects of anisotropic scattering are shown as insignificant.

Effect of wall emissivities on radiation heat transfer in glass tank forehearths

In an earlier work, we had proposed a two-band, non-grey radiative transfer model for heat transfer in forehearths with simultaneous optically thick and thin approximations for molten glass interiors and at boundaries. Here using the same model, the radiative interaction of the top-crown and bottom-refractory walls with interior layers of shallow molten glass is studied by varying the wall emissivities. The forehearth exit temperature profiles for higher wall emissivities (0.9) show better conditioning of the glass for white flint glasses (optically thin).

Active millimeter-wave pyrometer

A 135 GHz heterodyne receiver with a rotatable graphite waveguide/mirror system has been implemented on a waste remediation direct-current arc furnace for internal surface temperature measurements. The linear temperature measurement range extends from <1° to approximately 15,000 °C relative to ambient with a simultaneous capability to monitor surface reflectivity with the local oscillator leakage. Reliable and robust operation on a continuous 24 h basis in a smoky, dirty furnace environment is demonstrated for a total of five furnace runs reaching a maximum temperature of 2200 °C. Complete temperature profile measurements with approximately 5 cm spatial resolution clearly documented thermal gradients on the slag melt surface and refractory walls and ceiling for all operating regimes of the furnace. The unique active probing capability of this instrument provided additional real-time information on melt surface turbulence, changing furnace wall emissivity, and millimeter-wave optic losses inside the furnace.