Contribution Of Radiative Heat Transfer Research Articles

Heat and sweat transport in fibrous media with radiation

The paper is concerned with heat and sweat transport in porous textile media with a non-local thermal radiation and phase change. The model, based on a combination of these classical heat transfer mechanisms (convection, conduction and radiation), is governed by a nonlinear, degenerate and strongly coupled parabolic system. The thermal radiative flow is described by a radiation transport equation and characterized by the thermal absorptivity and emissivity of fibre. A conservative boundary condition is introduced to describe the radiative heat flux interacting with environment. With the conservative boundary condition, we prove the global existence of positive/non-negative weak solutions of a nonlinear parabolic system. A typical clothing assembly with a polyester batting material sandwiched in two laminated covers is investigated numerically. Numerical results show that the contribution of radiative heat transfer is comparable with that of conduction/convection in the sweating system.

Uncertainties Quantification of Effective Thermal Conductivity for Ceramic Fiber Blanket

In the present paper, a probabilistic propagation model for assessing the uncertainty of the effective thermal conductivity was developed based on a combined conduction and radiation heat transfer model of a ceramic fiber blanket composite. The Monte Carlo technique was used to cope with the uncertainties in the material density, radiative properties, and boundary temperatures observed in experimental tests. The calculated effective thermal-conductivity distribution for the sample was compared with the experimental measurements performed on multiple samples, and the predicted mean values were in good agreement with the measured data. The result validates the thermal predictive model and demonstrates the suitability of the stochastic model containing statistical distributions in the input variables. Statistical information also indicates that the uncertainty effect can be enlarged at high temperatures. Response sensitivity analysis between the random inputs and the effective thermal conductivity demonstrates that the randomness in the hot-side temperature, the cold-side temperature, and extinction coefficient of the sample has a significant influence on the variability of thermal-conductivity properties. The extinction coefficient becomes more and more important with an increase of temperature due to the dominant radiative heat transfer contribution at high temperature. The analysis provides good insight into the scattering control in the experimental measurement and theoretical prediction of the effective thermal conductivity of a ceramic fiber composite.

Computational Fluid Dynamics Simulation of Regression Rate in Hybrid Rockets

Computational fluid dynamics has been applied to the simulation of hybrid rockets using as the oxidizer and hydroxyl-terminated polybutadiene or high-density polyethylene as the fuel. Simulations have been carried out using predefined fuel and oxidizer mass flow rates, and calculating the fuel mass flow rate as a function of the wall heat flux. In this second case, the calculated regression rate has been compared to the average value derived from the reference experiments; no tuning coefficients have been introduced. Computational fluid dynamics results are compared with the corresponding experiments in the literature. In those tests where regression rate is calculated as a function of the wall heat flux, the fuel grain is divided into segments and its average value is plotted as a function of the average oxidizer mass flux in the combustion chamber. Computational fluid dynamics regression rate is underestimated by 30% for high-density polyethylene and 50% for hydroxyl-terminated polybutadiene if only convective heat exchange is accounted for. For hydroxyl-terminated polybutadiene at relatively low gaseous oxygen, the radiative heat transfer contribution to the total heat flux is not negligible, and this is confirmed by computational fluid dynamics: when radiative heat flux is taken into account, the error on regression rate is reduced to 15% and its value is overestimated. Computational fluid dynamics does not correctly predict regression rate modulus, but it correctly calculates the parameter of the regression rate formulation .

Evaluation of radiation heat transfer in porous medial: Application for a pebble bed modular reactor cooled by CO2 gas

This work analyses the contribution of radiation heat transfer in the cooling of a pebble bed modular reactor. The mathematical model, developed for a porous medium, is based on a set of equations applied to an annular geometry. Previous major works dealing with the subject have considered the forced convection mode and often did not take into account the radiation heat transfer. In this work, only free convection and radiation heat transfer are considered. This can occur during the removal of residual heat after shutdown or during an emergency situation. In order to derive the governing equations of radiation heat transfer, a steady-state in an isotropic and emissive porous medium (CO2) is considered. The obtained system of equations is written in a dimensionless form and then solved. In order to evaluate the effect of radiation heat transfer on the total heat removed, an analytical method for solving the system of equations is used. The results allow quantifying both radiation and free convection heat transfer. For the studied situation, they show that, in a pebble bed modular reactor, more than 70% of heat is removed by radiation heat transfer when CO2 is used as the coolant gas.

Experimental Investigation of Radiation Effects on Natural Convection in Horizontal Channels Heated From Above

Radiation heat transfer affects natural convection of air inside an open-ended cavity with a heated horizontal upper plate and an unheated lower parallel plate. Its effect is the heating of the lower plate, which heats the adjacent fluid layer and could determine secondary motions. In this paper, an experimental study is carried out to describe the effect of high value of surface emissivity on air flow in an open-ended cavity. The investigation is performed by means of wall temperature profiles, smoke visualization, and air temperature measurements. Results are obtained for an emissivity of the horizontal plates equal to 0.8, for aspect ratios between 10.0 and 20.0. By means of flow visualization and local air temperature measurements in the cavity as a function of time, remarkable secondary motions in the cavity are observed for the highest considered surface heat flux (Ra=8.91×103, 6.45×104, and 1.92×105). Measurement of the air temperature in the cavity also shows that radiation causes and damps secondary motions at the same time. Mean air temperature profiles as a function of the vertical coordinate, at different locations along the longitudinal axis, confirm both the main flow path inside the cavity and radiation effect on convective heat transfer. Finally, correlations for average Nusselt numbers and dimensionless maximum wall temperatures, in terms of Rayleigh number and channel aspect ratio, are proposed for natural convection with or without radiative heat transfer contribution for 2.26×103≤Ra≤1.92×105 and 10≤2L/b≤20.

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated. After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.

Effect of chemical vapor infiltration on erosion and thermal properties of porous carbon/carbon composite thermal insulation.

A highly porous carbon/carbon composite, known as carbon bonded carbon fiber (CBCF) and used as thermal insulation, was densified by chemical vapor infiltration (CVI). The erosion resistance, thermal conductivity and thermal expansion coefficient were measured with interest to utilization of the CVI densified composite as erosion protection in furnaces that employ inert gas quenching. It was found that the erosion rate decreased exponentially as bulk density increased from 0.18 to 1.00 Mg·m −3 and this was related to the Ryshkewitch and Duckworth relationship which relates mechanical properties to the porosity of ceramic materials. The thermal conductivity of the CVI densified composite both parallel and perpendicular to the fiber array was greater than that of CBCF in the measured range between 1000 and 2000°C. The measured values of thermal conductivity showed reasonable agreement with values calculated from a model structure. For CBCF and CVI densified composite the contribution of solid conduction was calculated to be dominant compared to the contribution of radiative heat transfer and heat transfer taking a path through the gas present in the pores of the composite. As a function of temperature thermal conductivity decreased for the CVI densified composite whereas an increase was found for CBCF. This opposite behavior is a result of the intrinsic properties of the dominant component in the composites: fibers in CBCF and pyrolytic carbon deposit in the case of CVI densified composite. Both materials exhibited anisotropy in the thermal conductivity; the thermal conductivity parallel to the fiber array being about 2.8 times that in the perpendicular direction in the case of CBCF. The thermal expansion coefficient in both directions increased as CBCF was densified by CVI; the increase was greater in the direction perpendicular to the fiber array which increased from 1.8×10 −6 K −1 for CBCF to 5.3×10 −6 K −1 for the CVI densified composite (1.10 Mg·m −3). The thermal expansion anisotropy was consistent with a c-axis radial orientation of the pyrolytic carbon around the fibers in the CVI densified composite.

Radiative heat transfer in a circulating fluidized bed coal combustor

In order to investigate the contribution of radiative heat transfer to total heat transfer in a circulating fluidized bed coal combustor, heat transfer experiments were carried out. The test facility has the dimension of 6.0 m in height and 0.2 m×0.2 m rectangular cross-section. Anthracite coal was used for the fuel, and the heat transfer tubes were installed near the wall at three different axial locations. The radiation probe for the direct measurement of radiative heat flux from the bed was installed at the middle of the combustor. The bed temperature was in the range of 650–850°C, superficial gas velocity was 3.0–5.0 m/s and measured suspension density was 20–30 kg/m 3. The heat transfer coefficients obtained in this operating range were about 200–300 W/m 2 K. The suspension emissivity determined from the radiation probe was in the range of 0.7–0.98 and the contribution of radiative component to total heat flux was found to be 15–36% at the operating temperature ranges.

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

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...

Radiative heat transfer in porous uranium dioxide

Due to the low thermal conductivity and high emissivity of UO 2, it has been suggested that radiative heat transfer may play a nontrivial role in the transport of heat through the pores of UO 2 fuel. This possibility was investigated computationally, and the contribution of radiative heat transfer through pores to the overall heat transport in porous UO 2 was quantified. A two-dimensional unit cell model was employed to approximate a porous UO 2 fuel system. The heat transfer through unit cells representing a wide variety of fuel conditions was calculated using finite element analysis. Conduction through both the solid fuel matrix and pore gas, and radiative exchange within the pore were modeled. Several pore compositions were investigated. Porosity, pore size, shape and orientation, temperature, and temperature gradient were parametrically studied to determine their influence on the radiative transfer mechanism.

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.

Experimental techniques for the measurement of radiative and total heat transfer in gas fluidized beds: A review

In a number of chemical and metallurgical applications, gas fluidized beds are used at high temperatures. Under such operating conditions, a significant portion of the total thermal energy is transferred in the radiative mode. Several efforts have been made from time to time to experimentally establish the contribution of radiative heat transfer in relation to the total heat transfer by employing different experimental techniques. These efforts have been categorized in two general classes and are briefly described in this review. Relative merits and demerits of these methods are discussed, and a critical appraisal of the different alternatives is given. The dependence of radiative heat transfer on different operating and system parameters is examined, and a comparative presentation is given of the results obtained by different investigators. Specific needs for future research work are pointed out.

Film Boiling Heat Transfer from a Horizontal Circular Plate Facing Downward

The two-dimensional, steady-state, laminar film boiling heat transfer from a horizontal circular plate facing downward to a stagnant saturated liquid is studied theoretically based on that the flow of vapor beneath the circular plate is induced by the hydrostatic pressure gradient due to the change in the vapor film thickness. In the present analysis, the effect of plate edge and contribution of radiation heat transfer are taken into account. The boundary-layer equations for the vapor film are solved by the integral method and the solution is obtained for water at atmospheric pressure. The vapor film thickness, coefficients of convection heat transfer and contribution of radiation heat transfer are examined. The present analysis is compared with the experimental data in terms of total wall heat flux.

Bottom Surface Combustion of Polymethylmethacrylate Disks—II. Diffusion Flame Radiative Properties

Abstract The purpose of this work is to calculate the radiative properties and fluxes at each location through a fully developed “laminar” diffusion flame over the bottom surface of disks of a polymeric material, Le., polymethylmethacrylate. The temperature, species and soot concentration profiles having been experimentally determined, the radiative properties of the combustion zone have been obtained using a spectral method adapted to this particular scale test and configuration. The results show that for combustion driven by natural convection, the contribution of radiative heat transfer to the burning rate is a “negative” contribution: the solid surface emits much more heat by radiation than it receives from the gas phase. This effect of surface re-radiation allows one to interpret the previously observed linear correlation between combustion rate and characteristic dimension of the samples. In the case of experiments with additional imposed radiative fluxes, the contribution to the burning rate becomes “positive” and this contribution increases logically with the increase of the additional flux. In parallel, an enhancement of the heat blocking effect of the convective flux can be observed.

Radiation heat transfer in foam insulation

The contribution of radiation heat transfer to the overall conductivity of foam insulations has been examined. The absorption and scattering coefficients as well as the phase function were measured for foam and glassfiber insulation. A simple absorption coefficient can be derived from transmissivity measurements over a range of sample thickness. When this absorption coefficient is used in the diffusion equation to predict the radiative flux for foams, the error in the calculated radiant flux is of the order of ten percent for foams. For fibrous insulation, transmissivity measurements with an integrating sphere are necessary.

Diffusion Flame Radiative Properties in Gas-Solid Combustion

Abstract The main purpose of this work is to calculate the radiative properties and the radiative fluxes at each location through a fully developed turbulent boundary layer over a solid reacting surface with a diffusion flame. The radiative properties of gas combustion products are obtained using a spectral method especially well adapted for small scale tests. The initial temperature and species profiles needed are given by the resolution of the conservation equations without radiation. The computed emissivities for various gas mixtures including or not soot are in agreement with previous results. The computational method has been used for different systems: polyethylene or polystyrene with O2 or air. The results confirm that the contribution of radiation heat transfer to burning rate is negligible compared to convective heat transfer in a laboratory scale test. Radiative heat transfer tends to increase combustion rate only for high soot volume fraction. The values obtained for the various fluxes are in good agreement with the experimental measurements and observations made previously.

Measurements of Thermal Conductivities and Heat Capacities for As<sub>2</sub>S<sub>3</sub> and As<sub>2</sub>Se<sub>3</sub> Glasses

Temperature dependences of thermal conductivities and heat capacities for As2S3 and As2Se3-glasses were measured by continuous heating method in the temperature range from the room temperature to the transition temperature range. The following conclusions were obtained.(1) By continuous heating method, thermal conductivities and heat capacities were measured easily and in a short time.(2) Thermal conductivities of these glasses increased slowly with rising temperature and, above the transition range, the conductivities slightly decreased with increase of temperature.(3) Heat capacities of these glasses increased with rising temperature, and in the transition range the temperature dependence showed a sudden change and had a maximum.(4) Temperature dependences of specific heats were determined from the observed heat capacities and specific gravities.(5) The contribution of radiation heat transfer to the total heat transfer was estimated. It is reasonable to consider the heat to be transfered only by conduction, since the contribution of radiation was negligible small in the present work.