Thermal radiation

About: Thermal radiation is a research topic. Over the lifetime, 12290 publications have been published within this topic receiving 197186 citations. The topic is also known as: heat radiation.

Particle Acceleration in Supernova Remnants and the Production of Thermal and Nonthermal Radiation

Abstract: Efficient cosmic-ray production can have a significant effect on the evolution and X-ray emission from SNRs. Using hydrodynamic simulations coupled to diffusive shock acceleration, we produce thermal and nonthermal forward-shock photon spectra. For a given ambient density and explosion energy, we find that the position of the forward shock at a given age is a strong function of the acceleration efficiency, providing a signature of cosmic-ray production. Using an approximate treatment for the ionization state of the plasma, we investigate the effects of slow versus rapid heating of the postshock electrons on the ratio of thermal to nonthermal X-ray emission at the forward shock. We also investigate the effects of magnetic field strength on the observed spectrum for efficient cosmic-ray acceleration. The primary effect of a large field is a flattening of the nonthermal spectrum in the soft X-ray band. X-ray spectral index measurements may thus be indicators of the postshock magnetic field strength. The predicted gamma-ray flux from inverse Compton (IC) scattering and neutral pion decay is strongly affected by the ambient conditions, and for the parameters used in our examples, the IC emission at E ~ 1 TeV exceeds that from pion decay, although at both lower and higher energies this trend is reversed for cases of high ambient density. More importantly, high magnetic fields produce a steepening of the electron spectrum over a wide energy range, which may make it more difficult to differentiate between IC and pion-decay emission solely by spectral shape.

Heat Transfer in High-Temperature Fibrous Insulation

Abstract: The combined radiation/conduction heat transfer in high-porosity, high-temperature fibrous insulations was investigated experimentally and numerically. The effective thermal conductivity of fibrous insulation samples was measured over the temperature range of 300-1300 K and environmental pressure range of 1.33 x 10(exp -5)-101.32 kPa. The fibrous insulation samples tested had nominal densities of 24, 48, and 72 kilograms per cubic meter and thicknesses of 13.3, 26.6 and 39.9 millimeters. Seven samples were tested such that the applied heat flux vector was aligned with local gravity vector to eliminate natural convection as a mode of heat transfer. Two samples were tested with reverse orientation to investigate natural convection effects. It was determined that for the fibrous insulation densities and thicknesses investigated no heat transfer takes place through natural convection. A finite volume numerical model was developed to solve the governing combined radiation and conduction heat transfer equations. Various methods of modeling the gas/solid conduction interaction in fibrous insulations were investigated. The radiation heat transfer was modeled using the modified two-flux approximation assuming anisotropic scattering and gray medium. A genetic-algorithm based parameter estimation technique was utilized with this model to determine the relevant radiative properties of the fibrous insulation over the temperature range of 300-1300 K. The parameter estimation was performed by least square minimization of the difference between measured and predicted values of effective thermal conductivity at a density of 24 kilograms per cubic meters and at nominal pressures of 1.33 x 10(exp -4) and 99.98 kPa. The numerical model was validated by comparison with steady-state effective thermal conductivity measurements at other densities and pressures. The numerical model was also validated by comparison with a transient thermal test simulating reentry aerodynamic heating conditions.

Nanoscale thermal radiation between two gold surfaces

Abstract: In this letter, we measured the nanoscale thermal radiation between a microsphere and a substrate which were both coated with thick gold films Although gold is highly reflective for thermal radiation, the radiative heat transfer between two gold surfaces was demonstrated to be significantly enhanced at nanoscale gaps beyond the blackbody radiation limit due to the tunneling of non-resonant evanescent waves The measured heat transfer coefficient between two gold surfaces agreed well with theoretical prediction At a gap d = 30 nm ± 5 nm, the heat transfer coefficient between two gold surfaces was observed to be as large as ∼400 W/m2·K, much greater than the blackbody radiation limit (∼5 W/m2·K)