Showing papers on "Thermal radiation published in 2008"

Near-field Thermal Radiation Between Two Closely Spaced Glass Plates Exceeding Planck’s Blackbody Radiation Law

Abstract: This work reports experimental studies on radiative heat flux between two parallel glass surfaces. Small polystyrene particles are used as spacers to maintain a micron-sized gap between two optical flats. By carefully choosing the number of particles and performing the measurement in a high-vacuum environment, the experiment is designed to ensure that the radiative heat flux is the dominant mode of heat transfer. The experimental results clearly demonstrate that the radiative heat flux across micron-sized gaps can exceed the far-field upper limit given by Planck’s law of blackbody radiation. The measured radiative heat flux shows reasonable agreement with theoretical predictions.

Near-field radiative heat transfer between a sphere and a substrate

Abstract: Near-field force and energy exchange between two objects due to quantum electrodynamic fluctuations give rise to interesting phenomena such as Casimir and van der Waals forces and thermal radiative transfer exceeding Planck's theory of blackbody radiation. Although significant progress has been made in the past on the precise measurement of Casimir force related to zero-point energy, experimental demonstration of near-field enhancement of radiative heat transfer is difficult. In this work, we present a sensitive technique of measuring near-field radiative transfer between a microsphere and a substrate using a bimaterial atomic force microscope cantilever, resulting in ``heat transfer-distance'' curves. Measurements of radiative transfer between a sphere and a flat substrate show the presence of strong near-field effects resulting in enhancement of heat transfer over the predictions of the Planck blackbody radiation theory.

High‐Spatial‐Resolution Surface‐Temperature Mapping Using Fluorescent Thermometry

Abstract: The characterization of temperature and thermal properties is of particular importance in micro- and nanotechnology. Considering the highly increased density of structures and the increased power dissipation per unit area associated with miniaturization, good thermal design is of great importance for device reliability and performance. Locating hot spots, for example, on a microelectronic circuit, can be of great value in evaluating a design, optimizing the performance, and performing failure analysis. [1,2] Apart from the industrial applications of micro- and nanoscale thermometry, fundamental questions of the thermal behavior, for example, thermal transfer at a scale comparable to the phonon wavelength, [3] could be more effectively addressed with improved characterization tools. The common approach for mapping temperature on the microscale is based on infrared microscopy, which relies on the analysis of the thermal radiation that is emitted from any material. IR microscopy is a well-established technique and can be used with relative ease for temperature mapping on large scales. However, the technique suffers from a diffractionlimited resolution, giving it an optimal spatial resolution of around 5 mm. [2,4,5] Nanoscale scientists typically use scanning thermal microscopy (SThM) for high-resolution measurements. Since the invention of the scanning probe microscope at the beginning of the 1980s, [6] several scanning probes for thermal characterization have been developed. The thermal probes used are generally based on either thermocouple or thermistor elements. [7–11] Other approaches have proposed bimaterial cantilevers or fluorescent particles as temperaturesensing probes. [12–14] The highest spatial resolution obtained

The effect of thermal radiation on the heat and mass transfer flow of a variable viscosity fluid past a vertical porous plate permeated by a transverse magnetic field

Abstract: The effect of temperature-dependent viscosity on free convective flow past a vertical porous plate is studied in the presence of a magnetic field, thermal radiation, and a first-order homogeneous chemical reaction. Boundary layer equations are derived and the resulting approximate nonlinear ordinary differential equations are solved numerically by the shooting method. A parametric study of all parameters involved is conducted, and a representative set of numerical results for the velocity and temperature profiles as well as the skin-friction parameter and the Nusselt and Sherwood numbers is illustrated graphically to show typical trends of the solutions. The dynamic viscosity in this study is taken as a function of the temperature although the Prandtl number is considered constant.

Role of fluctuational electrodynamics in near-field radiative heat transfer

Abstract: The objective of this paper is to discuss the role of fluctuational electrodynamics in the context of a generalized radiative heat transfer problem. Near-field effects, including the interference phenomenon and radiation tunneling, are important for applications to nanostructures. The classical theory of radiative transfer cannot be readily applied as the feature size approaches the dominant wavelength of radiative emission. At all length scales, however, propagation of radiative energy is properly represented by the electromagnetic wave approach, which requires the solution of the Maxwell equations. Fluctuational electrodynamics provides a model for thermal emission when solving a near-field radiation heat transfer problem, and the fluctuation–dissipation theorem provides the bridge between the strength of the fluctuations of the charges inside a body and its local temperature. This paper provides a complete and systematic derivation of the near-field radiative heat flux starting from the Maxwell equations. An illustrative example of near-field versus far-field radiation heat transfer is presented, and the length scale for transition from near- to far-field regime is discussed; the results show that this length scale can be as large as three times than predicted from Wien's law.

MHD mixed convection–radiation interaction along a permeable surface immersed in a porous medium in the presence of Soret and Dufour’s Effects

Abstract: This work is focused on the numerical modeling of steady, laminar, heat and mass transfer by MHD mixed convection from a semi-infinite, isothermal, vertical and permeable surface immersed in a uniform porous medium in the presence of thermal radiation and Dufour and Soret effects. A mixed convection parameter for the entire range of free-forced-mixed convection is employed and the governing equations are transformed into non-similar equations. These equations are solved numerically by an efficient, implicit, iterative, finite-difference scheme. The obtained results are checked against previously published work on special cases of the problem and are found to be in excellent agreement. A parametric study illustrating the influence of the thermal radiation coefficient, magnetic field, porous medium inertia parameter, concentration to thermal buoyancy ratio, and the Dufour and Soret numbers on the fluid velocity, temperature and concentration as well as the local Nusselt and the Sherwood numbers is conducted. The obtained results are shown graphically and the physical aspects of the problem are discussed.

Where is the radiation edge in magnetized black hole accretion discs

Abstract: General relativistic (GR) magnetohydrodynamic (MHD) simulations of black hole accretion find significant magnetic stresses near and inside the innermost stable circular orbit (ISCO), suggesting that such flows could radiate in a manner noticeably different from the prediction of the standard model, which assumes that there are no stresses in that region. We provide estimates of how phenomenologically interesting parameters like the ‘radiation edge’, the innermost ring of the disc from which substantial thermal radiation escapes to infinity, may be altered by stresses near the ISCO. These estimates are based on data from a large number of three-dimensional GRMHD simulations combined with GR ray tracing. For slowly spinning black holes (a/M < 0.9), the radiation edge lies well inside where the standard model predicts, particularly when the system is viewed at high inclination. For more rapidly spinning black holes, the contrast is smaller. At fixed total luminosity, the characteristic temperature of the accretion flow increases between a factor of 1.2 and 2.4 over that predicted by the standard model, whilst at fixed mass accretion rate, there is a corresponding enhancement of the accretion luminosity which may be anywhere from tens of per cent to order unity. When all these considerations are combined, we find that, for fixed black hole mass, luminosity and inclination angle, our uncertainty in the characteristic temperature of the radiation reaching distant observers due to uncertainty in dissipation profile (around a factor of 3) is greater than the uncertainty due to a complete lack of knowledge of the black hole's spin (around a factor of 2) and furthermore that spin estimates based on the stress-free inner boundary condition provide an upper limit to a/M.

Monte Carlo simulation of radiative heat transfer and turbulence interactions in methane/air jet flames

Abstract: A Photon Monte Carlo method combined with a composition PDF method is employed to model radiative heat transfer in combustion applications. Turbulence–radiation interactions (TRIs) can be fully taken into account using the proposed method. Sandia's Flame D and artificial flames derived from it are simulated and good agreement with experimental data is found. The effects of different TRI components are investigated. It is shown that, to predict the radiation field accurately, emission TRI must be taken into account, while, as expected, absorption TRI is negligible in the considered nonsooting methane/air jet flames if the total radiation quantities are concerned, but non-negligible for evaluation of local quantities. The influence of radiation on the turbulent flow field is also discussed.

Photoconductivity of intrinsic graphene

Abstract: We examine the photoconductivity of an intrinsic graphene associated with far- and midinfrared irradiation at low temperatures. The model under consideration accounts for the excitation of the electron-hole pairs by incident radiation, the interband generation-recombination transitions due to thermal radiation, and the intraband energy relaxation due to acoustic phonon scattering. The momentum relaxation is assumed to be caused by elastic scattering. The pertinent collision integrals are adapted for the case of the massless energy spectrum of carriers that interact with the longitudinal acoustic mode and the thermal radiation. It is found that the photoconductivity is determined by interplay between weak energy relaxation and generation-recombination processes. Due to this, the threshold of nonlinear response is fairly low.

A Review of Heat Transfer Physics

Abstract: With rising science contents of the engineering research and education, we give examples of the quest for fundamental understanding of heat transfer at the atomic level. These include transport as well as interactions (energy conversion) involving phonon, electron, fluid particle, and photon (or electromagnetic wave). Examples are development of MD and DSMC fluid simulations as tools in nanoscale and microscale thermophysical engineering. nanoscale thermal radiation, where the characteristic structural size becomes comparable to or smaller than the radiation (electromagnetic) wavelength. laser-based nanoprocessing, where the surface topography, texture, etc., are modified with nanometer lateral feature definition using pulsed laser beams and confining optical energy by coupling to near-field scanning optical microscopes. photon-electron-phonon couplings in laser cooling of solids, where the thermal vibrational energy (phonon) is removed by the anti-Stokes fluorescence; i.e., the photons emitted by an opti...

A Numerical Tackling on Sakiadis Flow with Thermal Radiation

Abstract: Momentum and energy laminar boundary layers of an incompressible fluid with thermal radiation about a moving plate in a quiescent ambient fluid are investigated numerically. Also, it has been underlined that the analysis of the roles of both velocity and temperature gradient at infinity is of key relevance for our results.

Finite Element Simulation of Heat Transfer

Abstract: Part 1: Steady conduction1. Mathematical formulation.2. The finite element method.3. Isoparametric elements.Part 2: Transient conduction, non-linearities, convective heat transfer4. Transient conduction.5. Non-linearites.6. Convective heat transfer.Part 3: Coupled problems7. Radiative heat transfer in cavities.8. Fluid-structure interaction in a pipe.9. Metallurgical phase change.10. Thermal and electrical phenomena.

Local Heat Transfer Coefficients of a High-Frequency Synthetic Jet during Impingement Cooling over Flat Surfaces

Abstract: It has been shown that synthetic jets can enhance heat transfer in air-cooling during natural convection heat transfer. Those meso scale devices are expected to be one of the methods of choice for cooling confined space, low heat-generating electronics. The present study focuses on the local and global heat transfer coefficients of a high-frequency meso scale synthetic jet. The experiments have been completed with synthetic jets, which are 12.5 mm in diameter and 2 mm thick with a square orifice of 1 mm. A synthetic jet has been driven at the resonance frequency of 4500 Hz, and voltage was between 30 V and 50 V. Earlier studies have focused on understanding the effect of voltage and driving frequency on the average heat transfer effect, while the current study aims for determining local heat transfer. A microscopic infrared thermal imaging technique was used to acquire local temperature distributions, and the data were analyzed for local convective and radiative heat transfer coefficients. Four square hea...

Radiation effect on MHD mixed convection flow about a permeable vertical plate

Abstract: This study investigates mixed convection heat transfer about a permeable vertical plate in the presence of magneto and thermal radiation effects. The effects of the mixed convection parameter, the radiation–conduction parameter, the surface temperature parameter, the magnetic parameter and the suction/injection parameter on the local skin friction and local heat transfer parameters are presented and analyzed.

Thermal disc emission from a rotating black hole: X-ray polarization signatures

Abstract: Thermal emission from the accretion disc around a black hole can be polarized, due to Thomson scattering in a disc atmosphere. In Newtonian space, the polarization angle must be either parallel or perpendicular to the projection of the disc axis on the sky. As first pointed out by Stark and Connors in 1977, General Relativity effects strongly modify the polarization properties of the thermal radiation as observed at infinity. Among these effects, the rotation of the polarization angle with energy is particularly useful as a diagnostic tool. In this paper, we extend the Stark and Connors calculations by including the spectral hardening factor, several values of the optical depth of the scattering atmosphere and rendering the results to the expected performances of planned X-ray polarimeters. In particular, to assess the perspectives for the next generation of X-ray polarimeters, we consider the expected sensitivity of the detectors on board the planned POLARIX and International X-ray Observatory missions. We assume the two cases of a Schwarzschild and an extreme Kerr black hole with a standard thin disc and a scattering atmosphere. We compute the expected polarization degree and the angle as functions of the energy as they could be measured for different inclinations of the observer, optical thickness of the atmosphere and different values of the black hole spin. We assume the thermal emission dominates the X-ray band. Using the flux level of the microquasar GRS 1915+105 in the thermal state, we calculate the observed polarization.

Experimental and theoretical studies on high-temperature thermal properties of fibrous insulation

Abstract: In the present paper, an experimental apparatus has been developed to measure heat transfer through high-alumina fibrous insulation for thermal protection system. Effective thermal conductivities of the fibrous insulation were measured over a wide range of temperature (300–973 K) and pressure (10−2–105 Pa) using the developed apparatus. The specific heat and the transmittance spectra in the wavelength range of 2.5–25 μm were also measured. The spectral extinction coefficients and Rosseland mean extinction coefficients were obtained from transmittance data at various temperatures to investigate the radiative heat transfer in fibrous insulation. A one-dimensional finite volume numerical model combined radiation and conduction heat transfer was developed to predict the behavior of the effective thermal conductivity of the fibrous insulation at various temperatures and pressures. The two-flux approximation was used to model the radiation heat transfer through the insulation. The experimentally measured specific heat and Rosseland mean extinction coefficients were used in the numerical heat transfer model to calculate the effective thermal conductivity. The average deviation between the numerical results for different values of albedo of scattering and the experimental results was investigated. The numerical results for ω=1 and experimental data were compared. It was found that the calculated values corresponded with the experimental values within an average of 13.5 percent. Numerical results were consistent with experimental results through the environmental conditions under examination.

Thermal radiation interaction with unsteady MHD flow past a vertical porous plate immersed in a porous medium

Abstract: A study of unsteady MHD free convection flow through a porous vertical flat plate immersed in a porous medium in presence of magnetic field with radiation has been analyzed. Introducing a time dependent suction to the plate, a similarity procedure has been adopted by taking a time dependent similarity parameter. In this analysis we consider a Darcy-Forchhemier model and the corresponding momentum and energy equations have been solved numerically, for cooling and heating of the plate by employing Nachtsheim-Swigert iteration technique along with the sixth order Runge-Kutta integration scheme. Non-dimensional velocity and temperature profiles are then presented graphically for different values of the parameter entering into the problem. During the process of numerical computations the skin-friction coefficient (viscous drag) and the rate of heat transfer (Nusselt number), which are of physical interest, are sorted out and presented in the form of tables. Keywords : Thermal radiation, MHD, Unsteady, Suction, Porous medium DOI: 10.3329/jname.v3i1.924 Journal of Naval Architecture and Marine Engineering 3(2006) 7-14

Unsteady hydromagnetic free convection flow of a dissipative and radiating fluid past a vertical plate with constant heat flux

Abstract: The effect of thermal radiation absorption on an unsteady free convective flow past a vertical plate is studied in the presence of a magnetic field and constant wall heat flux. Boundary layer equations are derived, and the resulting approximate nonlinear ordinary differential equations are solved analytically using asymptotic technique. A parametric study of all parameters involved is conducted, and a representative set of numerical results for the velocity and temperature profiles as well as the skin-friction parameter are illustrated graphically to show typical trends of the solutions.