Thermal Radiative Transfer Research Articles

Research of Macroscopic Regularities of Heat and Mass Transfer at the Ignition Condition of a Liquid High-Energy Material by an Immersed Source with a Limited Energy Capacity

We carried out a numerical and experimental investigation of heat and mass transfer at the ignition condition of a liquid high-energy material by a typical immersed source with a limited energy capacity, being a small, intensely heated metallic particle. The numerical research is made on the basis of a model taking into account a group of interrelated physicochemical processes (thermal conductivity, diffusion, convection, mixing, and radiative heat transfer) with phase transitions (evaporation of the liquid and crystallization of the particle's material). We established such terminal conditions for the immersion energy source that prevent inflammation of the high-energy material.

MHD and Mixed Convection Flow of Maxwell Fluid on Heat Transfer near a Stagnation Point Flow

The effects of thermal radiation and heat transfer of a Maxwell fluid near a mixed convection stagnation point flow over a moving surface in the presence of MHD has been studied. The governing differential equations are transformed into a set of coupled non-linear ordinary differential equations and then solved with a numerical technique using appropriate boundary conditions for various physical parameters. The numerical solution for the governing non-linear boundary value problem is based on applying the fourth-order Runge-Kutta method coupled with the shooting technique using appropriate boundary conditions for various physical parameters. The effects of various parameters like the viscosity parameter, radiation parameter, mixed convection parameter, Deborah number, magnetic parameter and Prandtl number on the velocity and temperature profiles as well as on the local skin-friction coefficient and the local Nusselt number are presented and discussed.

Thermal radiation heat transfer in one- and two-dimensional enclosures using the spectral collocation method with full spectrum k-distribution model

Thermal radiation transfer in one-dimensional parallel plates and two-dimensional rectangle enclosures filled with non-gray gases, namely CO2, H2O, or their mixtures, is calculated using the spectral collocation method with full spectrum k-distribution model (SCM–FSK). The k-distribution model is a promising model for treating the spectral properties of an absorbing–emitting medium, representing an alternative to line-by-line calculations which reduces the number of radiative transfer equation (RTE) evaluations from the order of a million to the order of ten without any significant loss of accuracy. Moreover, the full spectrum k-distribution model, as a global model, can be easily combined with other numerical methods to solve the RTE. The spectral collocation method (SCM) which is used to solve partial differential equations is combined with FSK for the present problems. In order to test the accuracy and efficiency of SCM–FSK, several gas radiation transfer problems involving isothermal/non-isothermal and homogeneous/inhomogeneous non-gray gases are examined. The results obtained by SCM–FSK are assessed by comparing the predictions with line by line results. These comparisons indicate that SCM–FSK is sufficiently accurate and convenient for engineering calculations.

Thermal radiation and magnetic field effects on unsteady mixed convection flow and mass transfer over a porous stretching surface with heat generation

The effects of thermal radiation and mass transfer on an unsteady hydromagnetic boundary layer mixed convection flow along a vertical porous stretching surface with heat generation are studied. The fluid is assumed to be viscous and incompressible. The governing partial differential equations are transformed into a system of ordinary differential equations using similarity variables. Numerical solutions of these equations are obtained by using the Runge-Kutta fourth order method along with the shooting technique. Velocity, temperature, concentration, the skin-friction coefficient, Nusselt number and Sherwood number for variations in the governing thermo physical parameters are computed, analyzed and discussed.

Superconductor Stability Revisited: Impacts from Coupled Conductive and Thermal Radiative Transfer in the Solid

Standard stability calculations following the Stekly, adiabatic or dynamic stability models apply purely solid thermal conduction mechanism, derive predictions under (quasi-) stationary and adiabatic conditions and assume ideal (mostly centrosymmetric) location of disturbances. Instead, the present paper takes into account also thermal radiative heat transfer in the superconducting solid, pool boiling, and considers the impact of random location and intensity of disturbances on the stability problem. The analysis is based on interplay between Monte Carlo radiative transfer calculations and a rigorous Finite Element method to calculate the resulting transient temperature field and stability functions. The combined Monte Carlo/Finite Element method is applied to 1G filament and 2G thin-film-coated high temperature superconductors. Results are strongly different from solutions achieved with standard, solely solid conduction thermal transport. It is not realistic, even in thin films, to assume uniform conductor temperature under transient disturbances. This may have significant consequences for design and simulation of performance of superconducting fault current limiters. There are doubts whether superconducting fault current limiters under any operation conditions could work in either pure flux flow or Ohmic resistive states.

Development of a Three-Dimensional Urban Energy Model for Predicting and Understanding Surface Temperature Distribution

The Model for Urban Surface Temperature, a three-dimensional approach, is developed for a realistically complex city with considerations of the energy exchange processes at the urban surface. The discrete transfer method and Gebhart absorption factor method are used for the shape factor estimation and multiple reflection calculation, respectively. The surface energy balance model is evaluated against existing field measurements that pertain to idealized urban geometry. It performs well in terms of predicting surface temperature and heat fluxes by allowing for detailed urban surface properties and meteorological conditions. The compressed row storage scheme is applied to calculate the transfer of surface thermal radiation, which dramatically reduces the computational requirements. This strategy permits the rigorous consideration of multiple reflections in a realistic urban area with hundreds of buildings. The result illustrates that considering only the first reflection is a good approach when the urban area is comprised of typical urban materials, e.g. materials with high emissivity and low albedo, because relatively accurate computational results can be obtained rapidly by avoiding the multiple reflection calculation.

Optimization of radiative heat transfer in hyperbolic metamaterials for thermophotovoltaic applications

Using our recently developed method we analyze the radiative heat transfer in micron-thick multilayer stacks of metamaterials with hyperbolic dispersion. The metamaterials are especially designed for prospective thermophotovoltaic systems. We show that the huge transfer of near-infrared thermal radiation across micron layers of metamaterials is achievable and can be optimized. We suggest an approach to the optimal design of such metamaterials taking into account high temperatures of the emitting medium and the heating of the photovoltaic medium by the low-frequency part of the radiation spectrum. We show that both huge values and frequency selectivity are achievable for the radiative heat transfer in hyperbolic multilayer stacks.

Toward a Full Simulation of the Basic Oxygen Furnace: Deformation of the Bath Free Surface and Coupled Transfer Processes Associated with the Post-Combustion in the Gas Region

The present article treats different phenomena taking place in a steelmaking converter through the development of two separate models. The first model describes the cavity produced at the free surface of the metal bath by the high-speed impinging oxygen jet. The model is based on a zonal approach, where gas compressibility effects are taken into account only in the high velocity jet region, while elsewhere the gas is treated as incompressible. The volume of fluid (VOF) method is employed to follow the deformation of the bath free surface. Calculations are presented for two- and three-phase systems and compared against experimental data obtained in a cold model experiment presented in the literature. The influence on the size and shape of the cavity of various parameters and models (including the jet inlet boundary conditions, the VOF advection scheme, and the turbulence model) is studied. Next, the model is used to simulate the interaction of a supersonic oxygen jet with the surface of a liquid steel bath in a pilot-scale converter. The second model concentrates on fluid flow, heat transfer, and the post-combustion reaction in the gas phase above the metal bath. The model uses the simple chemical reaction scheme approach to describe the transport of the chemical species and takes into account the consumption of oxygen by the bath and thermal radiative transfer. The model predictions are in reasonable agreement with measurements collected in a laboratory experiment and in a pilot-scale furnace.

Coupled thermal radiation and natural convection heat transfer in a cavity with a heated plate inside

This paper reports a fundamental theoretical study made to understand the interaction of surface radiation and natural convection in an air filled cavity with a centrally placed thin heated plate. The vertical walls of the cavity are cooled while the horizontal ones are insulated. The thin plate is assumed to be isothermal and is placed horizontally or vertically. The governing equations were solved using a finite volume method on a uniformly staggered grid system. The effects of the pertinent parameters, viz., Rayleigh number (105⩽Ra⩽107), plate length (0.25⩽D⩽0.75) and emissivity (0⩽ε⩽1) are investigated in detail. In general the results indicate a better homogenization of temperature field within the cavity by radiation. It is also found that the contribution of the convective mechanism to the overall heat transfer increases with emissivity when the plate is horizontally placed whereas decreases when it is vertically placed. This study demonstrates that any model representing a situation of this kind that ignores surface radiation leads to erroneous predictions.

An Efficient and Time Accurate, Moment-Based Scale-Bridging Algorithm for Thermal Radiative Transfer Problems

We present physics-based preconditioning and a time-stepping strategy for a moment-based scale-bridging algorithm applied to the thermal radiative transfer equation. Our goal is to obtain (asymptotically) second-order time accurate and consistent solutions without nonlinear iterations between the high-order (HO) transport equation and the low-order (LO) continuum system within a time step. Modified equation analysis shows that this can be achieved via a simple predictor-corrector time stepping that requires one inversion of the transport operator per time step. We propose a physics-based preconditioning based on a combination of the nonlinear elimination technique and Fleck--Cummings linearization. As a result, the LO system can be solved efficiently via a multigrid preconditioned Jacobian-free Newton--Krylov method. For a set of numerical test problems, the physics-based preconditioner reduces the number of GMRES iterations by a factor of 3$\sim$4 as compared to a standard preconditioner for advection-diffusion. Furthermore, the performance of the proposed physics-based preconditioner is insensitive to the time-step size.

An implicit Monte Carlo method for thermal radiation transport

The equations of thermal radiative transfer are mathematical models that describe the physical processes of photons scattering through and being absorbed in and emitted by a high-energy background material. These processes are important research objects of inertial confinement fusion (ICF). Numerical simulation is an indispensable method for this transportation equation. As an important method in the field of particle transportation, the Monte Carlo method has been widely employed in the fields of linear transportation of neutrons and high energy photons. However, traditional Monte Carlo method does not hold true when it is applied to the simulation of thermal radiation. In this research, an implicit Monte Carlo method based on calculable modeling and numerical simulation is studied. An integral transport equation that is suitable to Monte Carlo simulation is derived. A three-dimensional simulation code is developed, by which some thermal radiative transportation problems are simulated. The results of numerical experiments support that the implicit Monte Carlo method is applicable for thermal radiation transfer simulating. The work is expected to provide an important calculation method and tool for the thermal radiative transfer simulations of ICF.

Similarity Solution of the Influence of the Thermal Radiation and Heat Transfer on Steady Compressible Boundary Layer Flow

This paper determines the influence of the radiation and heat transfer on the compressible boundary layer flow using similarity solutions approach. The Roseland approximation is used to describe the radiative heat flux in the energy equation and the compressible boundary layer equations are transformed using Stewartson transformation. Similarity (invariant) solutions for the governing partial differential equations system are constructed. The shooting method is employed to transform the resulting non-linear boundary value problem into initial value problem, which is solved numerically. The effects of various parameters on the velocity and temperature profiles as well as the Falkner skan exponent and Prandtl number are shown graphically.

Radiative heat transfer study on silica aerogel and its composite insulation materials

This paper studies the thermal radiative transfer in silica aerogel and silica aerogel composite insulation materials (a xonotlite–aerogel composite and a ceramic fiber–aerogel composite). The spectral transmittances of silica aerogel, xonotlite-type calcium silicate, and ceramic fiber insulation materials–all considered semi-transparent mediums capable of absorbing, emitting, and scattering thermal radiation–are measured with a Fourier transform infrared spectrometer (FTIR) at different infrared wavelengths ranging from 2.5 to 25μm. The spectral transmittances are used to determine the specific spectral extinction coefficient and the specific Rosseland mean extinction coefficient of each sample. The radiative conductivity of each sample, deduced from the overall thermal conductivity measured using the transient hot-strip (THS) method, is compared against diffusion approximation predictions by using the measured spectral extinction coefficient. The results show that the spectral extinction coefficients of the samples are strongly dependent on the wavelength, particularly in the short wavelength regime (<10μm). The total Rosseland mean extinction coefficients of the samples all decrease with increasing temperature. The radiative conductivities are almost proportional to the cube of temperature, decreasing as sample densities increase.

AN ANALYTIC RADIATIVE-CONVECTIVE MODEL FOR PLANETARY ATMOSPHERES

We present an analytic 1-D radiative-convective model of the thermal structure of planetary atmospheres. Our model assumes that thermal radiative transfer is gray and can be represented by the two-stream approximation. Model atmospheres are assumed to be in hydrostatic equilibrium, with a power law scaling between the atmospheric pressure and the gray thermal optical depth. The convective portions of our models are taken to follow adiabats that account for condensation of volatiles through a scaling parameter to the dry adiabat. By combining these assumptions, we produce simple, analytic expressions that allow calculations of the atmospheric pressure-temperature profile, as well as expressions for the profiles of thermal radiative flux and convective flux. We explore the general behaviors of our model. These investigations encompass (1) worlds where atmospheric attenuation of sunlight is weak, which we show tend to have relatively high radiative-convective boundaries, (2) worlds with some attenuation of sunlight throughout the atmosphere, which we show can produce either shallow or deep radiative-convective boundaries, depending on the strength of sunlight attenuation, and (3) strongly irradiated giant planets (including Hot Jupiters), where we explore the conditions under which these worlds acquire detached convective regions in their mid-tropospheres. Finally, we validate our model and demonstrate its utility through comparisons to the average observed thermal structure of Venus, Jupiter, and Titan, and by comparing computed flux profiles to more complex models.

Laboratory scale analysis of the influence of different heat transfer mechanisms on liquid nitrogen vaporization rate

The prediction of the potential hazards associated to accidental liquefied natural gas (LNG) spills has motivated a number of different studies including experimental and numerical approaches. Most of these studies focus on dispersion predictions, however there is limited information regarding source term of it: liquid spill and vaporization. There is a need of further improvements on the understanding of these phenomena and the quantification of the most important parameters that can affect them.The vaporization of cryogenic liquids is governed by the heat transfer phenomena including conduction, convection and thermal radiation mechanisms. The present work investigates the contribution of each of these heat transfer modes to the vaporization rate of cryogenic liquid nitrogen (LN2) contained in a Dewar flask using well controlled and instrumented laboratory scale experiments. LN2 vaporization rate was measured with individually controllable contributions from convective (generated by an electric fan) and thermal radiative (generated by light bulb) heat transfer in the presence of a baseline conductive heat transfer rate.In both cases of convection and radiation analysis the experimental study showed that they can play a significant role in the vaporization rate of LN2. It was observed that the radiative heat absorbed by the LN2 during the vaporization experiment represents only 50%–65% of the incident radiation that would reach the LN2 pool surface if no vapour was present. Convective heat transfer generated by the fan was shown to have had the most significant contribution to the total heat transfer. As expected, this contribution was significantly higher than the one from bulb radiation. The experimental data also showed that the liquid level in the Dewar play a key role in the resulting amount of convective heat transfer. This could be attributed to the fact that lower liquid level the side walls of the Dewar were high enough to hold a layer of vapour and limit air motion directly above the liquid surface, thus limiting the heat transfer by convection.

Physical properties of silver-coated silica clusters: A computer experiment

The effect on the physical properties of a silica cluster after silver coating has been studied by the molecular dynamics method. It has been established that silver atoms make a sphere-like cluster shape, increase mechanical stability and give a stable positive value of the surface tension. They increase the number of electrons able to interact with the electromagnetic field, and, hence, enhance the efficiency of thermal radiation transfer by clusters.

A Consistent, Moment-Based, Multiscale Solution Approach for Thermal Radiative Transfer Problems

We present an efficient numerical algorithm for solving the time-dependent grey thermal radiative transfer (TRT) equations. The algorithm utilizes the first two angular moments of the TRT equations (Quasi-diffusion (QD)) together with the material temperature equation to form a nonlinear low-order (LO) system. The LO system is solved via the Jacobian-free Newton-Krylov method. The combined high-order (HO) TRT and LO-QD system is used to bridge the diffusion and transport scales. In addition, a “consistency” term is introduced to make the truncation error in the LO system identical to the truncation error in the HO equation. The derivation of the consistency term is rather general; therefore, it can be extended to a variety of spatial and temporal discretizations.

EFFECTS OF THERMAL RADIATION AND MASS DIFFUSION ON FREE CONVECTION FLOW NEAR A VERTICAL PLATE WITH NEWTONIAN HEATING

The effects of thermal radiation and mass transfer on unsteady natural convection flow of an optically dense viscous incompressible fluid near a vertical plate with Newtonian heating have been investigated. Both physically important boundary conditions of uniform wall concentration (UWC) and uniform mass flux (UMF) are considered. Rosseland diffusion approximation is used to describe the radiative heat flux in the energy equation. The governing dimensionless boundary layer equations are solved analytically using the Laplace transform technique. The effects of mass to thermal buoyancy ratio parameter (N), Prandtl number (Pr), Schmidt number (Sc), and the radiation parameter (R) as well as time (t) on the velocity field and skin friction are determined. It is found that velocity increases for aiding flows and it decreases for opposing flows in the cases of both UWC and UMF. The skin friction is reduced with increasing species concentration in the presence of aiding flows for both UWC and UMF. Also, the velocity is greater in the case of UWC than the case of UMF at an early time, whereas the velocity is slightly greater in the case of UMF than that of UWC at a later time in the vicinity of the plate.

The Asymptotic Telegrapher’s Equation (P1) Approximation for Time-Dependent, Thermal Radiative Transfer

We develop the asymptotic P 1 approximation for the time-dependent thermal radiative transfer equation for a multidimensional general geometry. Careful derivation of the asymptotic P 1 equations, directly from the time-dependent Boltzmann equation, yields a particle velocity that is closer (v≈0.91c) to the exact value of c but is based on an asymptotic analysis rather than diffusion theory (infinite velocity) or conventional P 1 theory (which gives rise to the Telegrapher’s equation, ). While this approach does not match the exact value of c as does the P 1/3 method, the latter method is an ad hoc approach that has not been justified on theoretical grounds. This article provides the theoretical justification for the almost-correct value of c that yields improved results for the well-known (one-dimensional) Su-Olson benchmark for radiative transfer, for which we obtain a semi-analytic solution in the case of local thermodynamic equilibrium. We found that the asymptotic P 1 approximation yields a better solution than the diffusion, the classic P 1, and the P 1/3 approximations, yielding the correct steady-state behavior for the energy density and the (almost) correct particle velocity.

Near-field thermal radiation transfer controlled by plasmons in graphene

It is shown that thermally excited plasmon-polariton modes can strongly mediate, enhance and \emph{tune} the near-field radiation transfer between two closely separated graphene sheets. The dependence of near-field heat exchange on doping and electron relaxation time is analyzed in the near infra-red within the framework of fluctuational electrodynamics. The dominant contribution to heat transfer can be controlled to arise from either interband or intraband processes. We predict maximum transfer at low doping and for plasmons in two graphene sheets in resonance, with orders-of-magnitude enhancement (e.g. $10^2$ to $10^3$ for separations between $0.1\mu m$ to $10nm$) over the Stefan-Boltzmann law, known as the far field limit. Strong, tunable, near-field transfer offers the promise of an externally controllable thermal switch as well as a novel hybrid graphene-graphene thermoelectric/thermophotovoltaic energy conversion platform.