Showing papers on "Thermal radiation published in 2014"
TL;DR: A nanoscale analog of a field-effect transistor that is able to control the flow of heat exchanged by evanescent thermal photons between two bodies and opens up new perspectives concerning the development of contactless thermal circuits intended for information processing using the photon current rather than the electric current.
Abstract: Using a block of three separated solid elements, a thermal source and drain together with a gate made of an insulator-metal transition material exchanging near-field thermal radiation, we introduce a nanoscale analog of a field-effect transistor that is able to control the flow of heat exchanged by evanescent thermal photons between two bodies. By changing the gate temperature around its critical value, the heat flux exchanged between the hot body (source) and the cold body (drain) can be reversibly switched, amplified, and modulated by a tiny action on the gate. Such a device could find important applications in the domain of nanoscale thermal management and it opens up new perspectives concerning the development of contactless thermal circuits intended for information processing using the photon current rather than the electric current.
422 citations
TL;DR: In this paper, a similarity transformation is used to reduce the governing momentum and energy equations into non-linear ordinary differential equations, and the resulting differential equations with the appropriate boundary conditions are solved by shooting iteration technique together with fourth-order Runge-Kutta integration scheme.
334 citations
TL;DR: In this article, the boundary layer flow of a non-Newtonian fluid accompanied by heat transfer toward an exponentially stretching surface in presence of suction or blowing at the surface is investigated.
328 citations
TL;DR: In this article, the problem of laminar fluid flow which results from a permeable stretching of a flat surface in a nanofluid with the effects of heat radiation, magnetic field, velocity slip and convective boundary conditions have been investigated.
Abstract: The problem of laminar fluid flow which results from a permeable stretching of a flat surface in a nanofluid with the effects of heat radiation, magnetic field, velocity slip and convective boundary conditions have been investigated. The transport equations used in the analysis took into account the effect of Brownian motion and thermophoresis parameters. The solution for the velocity, temperature and nanoparticle concentration depends on parameters viz. thermal radiation parameter
211 citations
TL;DR: It is shown that the thermal bistability could find broad applications in the domains of thermal management, information processing, and energy storage and can be used to store heat and thermal information for arbitrary long times.
Abstract: We predict the existence of a thermal bistability in many-body systems out of thermal equilibrium which exchange heat by thermal radiation using insulator-metal transition materials. We propose a writing-reading procedure and demonstrate the possibility to exploit the thermal bistability to make a volatile thermal memory. We show that this thermal memory can be used to store heat and thermal information (via an encoding temperature) for arbitrary long times. The radiative thermal bistability could find broad applications in the domains of thermal management, information processing, and energy storage.
162 citations
TL;DR: In this article, the effects of thermal radiation using the nonlinear Rosseland approximation are investigated and a numerical analysis in connection with the boundary layer flow induced in a quiescent fluid by a continuous sheet stretching with velocity uw (x) ∼x 1/3 with heat transfer is performed.
161 citations
TL;DR: A platform for near-field heat transfer on-chip is demonstrated and it is shown that it can be the dominant thermal transport mechanism between integrated nanostructures, overcoming background substrate conduction and the far-field limit.
Abstract: Near-field heat transfer recently attracted growing interest but was demonstrated experimentally only in macroscopic systems. However, several projected applications would be relevant mostly in integrated nanostructures. Here we demonstrate a platform for near-field heat transfer on-chip and show that it can be the dominant thermal transport mechanism between integrated nanostructures, overcoming background substrate conduction and the far-field limit (by factors 8 and 7, respectively). Our approach could enable the development of active thermal control devices such as thermal rectifiers and transistors.
148 citations
TL;DR: In this article, the authors introduce general principles for achieving maximal violation of detailed balance in thermal radiation, and validate these principles by direct numerical calculations on thermal emitters constructed from magneto-optical photonic crystals.
Abstract: We introduce general principles for achieving maximal violation of detailed balance in thermal radiation. We validate these principles by direct numerical calculations, based on fluctuational electrodynamics, on thermal emitters constructed from magneto-optical photonic crystals. Such a capability to maximally violate the detailed balance provides different opportunities for the design of thermal absorbers and emitters.
131 citations
TL;DR: In this article, a different application of Rosseland approximation for thermal radiation is introduced in the two-dimensional stagnation-point flow of viscous nanofluid due to solar energy.
Abstract: Radiation effects in the two-dimensional stagnation-point flow of viscous nanofluid due to solar energy are investigated. Heat transfer subject to thermal radiation, Joule heating, viscous dissipation and convective boundary conditions is considered. A different application of Rosseland approximation for thermal radiation is introduced in this study. The governing equations are simplified through the boundary layer assumptions and then transformed into non-dimensional forms by appropriate transformations. The resulting differential systems are solved numerically through fourth-fifth order Runge–Kutta method (RK45) using a shooting technique. The influences of different parameters are explained through graphs for velocity, temperature and concentration and numerical values of local Nusselt and Sherwood numbers. A comparative analysis of the solutions is performed through previous studies in some limiting cases. Both the temperature and wall temperature gradient are increasing functions of the radiation parameter. The excessive movement of nanoparticles in the base fluids results in the deeper absorption of solar radiations in the liquids.
123 citations
TL;DR: This article addresses the boundary layer flow and heat transfer in third grade fluid over an unsteady permeable stretching sheet by considering the transverse magnetic and electric fields in the momentum equations.
Abstract: This article addresses the boundary layer flow and heat transfer in third grade fluid over an unsteady permeable stretching sheet. The transverse magnetic and electric fields in the momentum equations are considered. Thermal boundary layer equation includes both viscous and Ohmic dissipations. The related nonlinear partial differential system is reduced first into ordinary differential system and then solved for the series solutions. The dependence of velocity and temperature profiles on the various parameters are shown and discussed by sketching graphs. Expressions of skin friction coefficient and local Nusselt number are calculated and analyzed. Numerical values of skin friction coefficient and Nusselt number are tabulated and examined. It is observed that both velocity and temperature increases in presence of electric field. Further the temperature is increased due to the radiation parameter. Thermal boundary layer thickness increases by increasing Eckert number.
112 citations
TL;DR: In this article, the effects of thermal radiation in the energy equation are considered and the problem is first modeled and then written in dimensionless form, which is then solved by using the Laplace transform technique.
Abstract: In this article, the unsteady boundary layer magnetohydrodynamic (MHD) free convection flow past an oscillating vertical plate embedded in a porous medium with constant mass diffusion and Newtonian heating condition is analysed. By considering the effects of thermal radiation in the energy equation, the problem is first modeled and then written in dimensionless form, which is then solved by using the Laplace transform technique. The expressions for velocity, temperature and concentration fields are obtained and plotted graphically to see the influence of embedded parameters. The results for skin friction, Nusselt number and Sherwood number are also shown in tables. Further a table is included for the comparison of our results with those present in the literature.
TL;DR: It is shown that thermal radiation is an attractive route for photon energy upconversion, with efficiencies higher than those of state-of-the-art energy transfer upconverted under continuous wave laser excitation.
Abstract: The efficiency of many solar energy conversion technologies is limited by their poor response to low-energy solar photons. One way for overcoming this limitation is to develop materials and methods that can efficiently convert low-energy photons into high-energy ones. Here we show that thermal radiation is an attractive route for photon energy upconversion, with efficiencies higher than those of state-of-the-art energy transfer upconversion under continuous wave laser excitation. A maximal power upconversion efficiency of 16% is achieved on Yb(3+)-doped ZrO2. By examining various oxide samples doped with lanthanide or transition metal ions, we draw guidelines that materials with high melting points, low thermal conductivities and strong absorption to infrared light deliver high upconversion efficiencies. The feasibility of our upconversion approach is further demonstrated under concentrated sunlight excitation and continuous wave 976-nm laser excitation, where the upconverted white light is absorbed by Si solar cells to generate electricity and drive optical and electrical devices.
TL;DR: In this paper, the authors considered the 10 stellar-mass black hole candidates for which the spin parameter has already been estimated from the analysis of the disk's thermal spectrum under the assumption of the Kerr background, and translated the measurements reported in the literature into constraints on the spin parameters-deformation parameter plane.
Abstract: In a previous paper, one of us (C. Bambi) described a code to compute the thermal spectrum of geometrically thin and optically thick accretion disks around generic stationary and axisymmetric black holes, which are not necessarily of the Kerr type. As the structure of the accretion disk and the propagation of electromagnetic radiation from the disk to the distant observer depend on the background metric, the analysis of the thermal spectrum of thin disks can be used to test the actual nature of black hole candidates. In this paper, we consider the 10 stellar-mass black hole candidates for which the spin parameter has already been estimated from the analysis of the disk's thermal spectrum under the assumption of the Kerr background, and we translate the measurements reported in the literature into constraints on the spin parameter-deformation parameter plane. The analysis of the disk's thermal spectrum can be used to estimate only one parameter of the geometry close to the compact object; therefore, it is not possible to get independent measurements of both the spin and the deformation parameters. The constraints obtained here will be used in combination with other measurements in future work with the final goal of breaking the degeneracy between the spin and possible deviations from the Kerr solution and thus test the Kerr black hole hypothesis.
TL;DR: In this article, a novel numerical method called the Thermal Discrete Dipole Approximation (T-DDA) is proposed for modeling near-field radiative heat transfer in three-dimensional arbitrary geometries.
Abstract: A novel numerical method called the Thermal Discrete Dipole Approximation (T-DDA) is proposed for modeling near-field radiative heat transfer in three-dimensional arbitrary geometries. The T-DDA is conceptually similar to the Discrete Dipole Approximation, except that the incident field originates from thermal oscillations of dipoles. The T-DDA is described in details in the paper, and the method is tested against exact results of radiative conductance between two spheres separated by a sub-wavelength vacuum gap. For all cases considered, the results calculated from the T-DDA are in good agreement with those from the analytical solution. When considering frequency-independent dielectric functions, it is observed that the number of sub-volumes required for convergence increases as the sphere permittivity increases. Additionally, simulations performed for two silica spheres of 0.5 μm-diameter show that the resonant modes are predicted accurately via the T-DDA. For separation gaps of 0.5 μm and 0.2 μm, the relative differences between the T-DDA and the exact results are 0.35% and 6.4%, respectively, when 552 sub-volumes are used to discretize a sphere. Finally, simulations are performed for two cubes of silica separated by a sub-wavelength gap. The results revealed that faster convergence is obtained when considering cubical objects rather than curved geometries. This work suggests that the T-DDA is a robust numerical approach that can be employed for solving a wide variety of near-field thermal radiation problems in three-dimensional geometries.
TL;DR: In this article, a local thermal non-equilibrium model is adopted to solve the steady state heat and mass transfer problems of porous media solar receiver, where the fluid entrance surface is subjected to concentrated solar radiation, and CH4/H2O mixture is adopted as feeding gas.
TL;DR: In this paper, a physically based model demonstrates that the pressure dependence of transparency to infrared radiation leads to a common tropopause pressure that is probably applicable to many planetary bodies with thick atmospheres.
Abstract: In many planetary atmospheres, including that of Earth, the base of the stratosphere—the tropopause—occurs at an atmospheric pressure of 0.1 bar. A physically based model demonstrates that the pressure-dependence of transparency to infrared radiation leads to a common tropopause pressure that is probably applicable to many planetary bodies with thick atmospheres. A minimum atmospheric temperature, or tropopause, occurs at a pressure of around 0.1 bar in the atmospheres of Earth1, Titan2, Jupiter3, Saturn4, Uranus and Neptune4, despite great differences in atmospheric composition, gravity, internal heat and sunlight. In all of these bodies, the tropopause separates a stratosphere with a temperature profile that is controlled by the absorption of short-wave solar radiation, from a region below characterized by convection, weather and clouds5,6. However, it is not obvious why the tropopause occurs at the specific pressure near 0.1 bar. Here we use a simple, physically based model7 to demonstrate that, at atmospheric pressures lower than 0.1 bar, transparency to thermal radiation allows short-wave heating to dominate, creating a stratosphere. At higher pressures, atmospheres become opaque to thermal radiation, causing temperatures to increase with depth and convection to ensue. A common dependence of infrared opacity on pressure, arising from the shared physics of molecular absorption, sets the 0.1 bar tropopause. We reason that a tropopause at a pressure of approximately 0.1 bar is characteristic of many thick atmospheres, including exoplanets and exomoons in our galaxy and beyond. Judicious use of this rule could help constrain the atmospheric structure, and thus the surface environments and habitability, of exoplanets.
TL;DR: In this article, the convection and radiation effects in the analysis of performance of a porous triangular fin with temperature-dependent thermal conductivity were considered and solved by differential transformation method (DTM).
TL;DR: In this article, a comprehensive analysis of megnetohydrodynamic (MHD) steady boundary layer flow has been discussed through the linear stretching surface, where the effect of thermal radiation are also taken into account while fluid consists of nanoparticles with convective boundary conditions.
Abstract: In the present article, a comprehensive analysis of megnetohydrodynamic (MHD) steady boundary layer flow have been discussed through the linear stretching surface. The effect of thermal radiation are also taken into account while fluid consists of nanoparticles with convective boundary conditions. The resulting expressions for temperature and nanoparticle equations are coupled. The simplified non-linear equations are tackled with the help of homotopy analysis method (HAM). Main objective of this article is to inspect the effects of emerging parameters which appears in solution. Interesting results are shown graphically. The dimensionless heat transfer rates and dimensionless concentration rate are also plotted against flow control parameters.
TL;DR: In this article, a two-dimensional numerical analysis of combined heat transfer (transient natural convection, surface thermal radiation and conduction) in an air-filled square enclosure having heat-conducting solid walls of finite thickness and a local heat source in conditions of convective heat exchange with an environment has been carried out.
TL;DR: In this paper, the impact of the thermal radiation field on thermal comfort, building energy consumption, and air-conditioning control is discussed, and various options that need to be considered in the efforts to mitigate the impacts of thermal radiant field on the occupants' thermal comfort and building's energy consumption.
Abstract: Thermal comfort is determined by the combined effect of the six thermal comfort parameters: temperature, air moisture content, thermal radiation, air relative velocity, personal activity and clothing level as formulated by Fanger through his double heat balance equations. In conventional air conditioning systems, air temperature is the parameter that is normally controlled whilst others are assumed to have values within the specified ranges at the design stage. In Fanger’s double heat balance equation, thermal radiation factor appears as the mean radiant temperature (MRT), however, its impact on thermal comfort is often ignored. This paper discusses the impacts of the thermal radiation field which takes the forms of mean radiant temperature and radiation asymmetry on thermal comfort, building energy consumption and air-conditioning control. Several conditions and applications in which the effects of mean radiant temperature and radiation asymmetry cannot be ignored are discussed. Several misinterpretations that arise from the formula relating mean radiant temperature and the operative temperature are highlighted, coupled with a discussion on the lack of reliable and affordable devices that measure this parameter. The usefulness of the concept of the operative temperature as a measure of combined effect of mean radiant and air temperatures on occupant’s thermal comfort is critically questioned, especially in relation to the control strategy based on this derived parameter. Examples of systems which deliver comfort using thermal radiation are presented. Finally, the paper presents various options that need to be considered in the efforts to mitigate the impacts of the thermal radiant field on the occupants’ thermal comfort and building energy consumption.
TL;DR: In this paper, a numerical analysis of natural convection and surface thermal radiation in a cubical cavity having heat-conducting solid walls of finite thickness with a heat source located at the bottom of the cavity in conditions of convective heat exchange with an environment has been carried out.
TL;DR: In this article, a conservative analytical estimate of the effects of radiation heat loss is derived and validated against detailed numerical simulations, and a solver with a graphical interface is provided in the Supplemental material to allow implementation of these analytical results.
TL;DR: In this article, the steady boundary layer flow of an Eyring-Powell model fluid due to an exponentially shrinking sheet was examined and the heat transfer process in the presence of thermal radiation was considered.
TL;DR: In this paper, the authors studied the Casimir-Lifshitz force and the radiative heat transfer in a system consisting of three bodies held at three independent temperatures and immersed in a thermal environment, the whole system being in a stationary configuration out of thermal equilibrium.
Abstract: We study the Casimir-Lifshitz force and the radiative heat transfer in a system consisting of three bodies held at three independent temperatures and immersed in a thermal environment, the whole system being in a stationary configuration out of thermal equilibrium. The theory we develop is valid for arbitrary bodies, i.e., for any set of temperatures, dielectric, and geometrical properties, and describes each body by means of its scattering operators. For the three-body system we provide a closed-form unified expression of the radiative heat transfer and of the Casimir-Lifshitz force (both in and out of thermal equilibrium). This expression is thus first applied to the case of three planar parallel slabs. In this context we discuss the nonadditivity of the force at thermal equilibrium, as well as the equilibrium temperature of the intermediate slab as a function of its position between two external slabs having different temperatures. Finally, we consider the force acting on an atom inside a planar cavity. We show that, differently from the equilibrium configuration, the absence of thermal equilibrium admits one or more positions of minima for the atomic potential. While the corresponding atomic potential depths are very small for typical ground-state atoms, they may become particularly relevant for Rydberg atoms, becoming a promising tool to produce an atomic trap.
TL;DR: In this paper, the authors investigated the unsteady boundary layer flow of a nanofluid over a heated stretching sheet with thermal radiation and found that the heat transfer rate at the surface increases in the presence of Brownian motion but reverse effect occurs for thermophoresis.
Abstract: This paper investigates the unsteady boundary layer flow of a nanofluid over a heated stretching sheet with thermal radiation. The transport model employed includes the effects of Brownian motion and thermophoresis. The unsteadiness in the flow field is caused by the time-dependence of the stretching velocity, free stream velocity and the surface temperature. The unsteady boundary layer equations are transformed to a system of non-linear ordinary differential equations and solved numerically using a shooting method together with Runge–Kutta–Fehlberg scheme. The clear liquid results from this study are in agreement with the results reported in the literature. It is found that the heat transfer rate at the surface increases in the presence of Brownian motion but reverse effect occurs for thermophoresis.
01 Jan 2014
TL;DR: In this paper, the impact of the thermal radiation field on thermal comfort, building energy consumption, and air-conditioning control is discussed, and various options that need to be considered in the efforts to mitigate the impacts of thermal radiant field on the occupants' thermal comfort and building's energy consumption.
Abstract: Thermal comfort is determined by the combined effect of the six thermal comfort parameters: temperature, air moisture content, thermal radiation, air relative velocity, personal activity and clothing level as formulated by Fanger through his double heat balance equations. In conventional air conditioning systems, air temperature is the parameter that is normally controlled whilst others are assumed to have values within the specified ranges at the design stage. In Fanger’s double heat balance equation, thermal radiation factor appears as the mean radiant temperature (MRT), however, its impact on thermal comfort is often ignored. This paper discusses the impacts of the thermal radiation field which takes the forms of mean radiant temperature and radiation asymmetry on thermal comfort, building energy consumption and air-conditioning control. Several conditions and applications in which the effects of mean radiant temperature and radiation asymmetry cannot be ignored are discussed. Several misinterpretations that arise from the formula relating mean radiant temperature and the operative temperature are highlighted, coupled with a discussion on the lack of reliable and affordable devices that measure this parameter. The usefulness of the concept of the operative temperature as a measure of combined effect of mean radiant and air temperatures on occupant’s thermal comfort is critically questioned, especially in relation to the control strategy based on this derived parameter. Examples of systems which deliver comfort using thermal radiation are presented. Finally, the paper presents various options that need to be considered in the efforts to mitigate the impacts of the thermal radiant field on the occupants’ thermal comfort and building energy consumption.
TL;DR: In this article, the mixed convection boundary layer flow of nanofluids on a stagnation-point flow over a permeable stretching/shrinking sheet subject to thermal radiation, heat source/sink, viscous dissipation and chemical reaction by using numerical method.
TL;DR: In this paper, a mathematical analysis of MHD flow and heat transfer from a warm, electrically conducting fluid to melting surface moving parallel to a constant free stream in the presence of thermal radiation is presented.
TL;DR: In this paper, a new effective model, spherical hollow cube model, is proposed based on the structures of aerogels and the prediction equation for the apparent thermal conductivity is theoretically derived.
TL;DR: The findings of this study generate a better understanding of the human body’s microclimate, which is important in fields such as thermal comfort, HVAC, or indoor air quality, and can be used by CFD users for the validation of their simulations.
Abstract: The human body is surrounded by a micro‐climate which results from its convective release of heat. In this study, the air temperature and flow velocity of this micro‐climate were measured in a climate chamber at various room temperatures, using a thermal manikin simulating the heat release of the human being. Different techniques (Particle Streak Tracking, thermography, anemometry, and thermistors) were used for measurement and visualization. The manikin surface temperature was adjusted to the particular indoor climate based on simulations with a thermoregulation model (UCBerkeley Thermal Comfort Model). We found that generally, the micro‐climate is thinner at the lower part of the torso, but expands going up. At the head, there is a relatively thick thermal layer, which results in an ascending plume above the head. However, the micro‐climate shape strongly depends not only on the body segment, but also on boundary conditions: the higher the temperature difference between the surface temperature of the manikin and the air temperature, the faster the air flow in the micro‐climate. Finally, convective heat transfer coefficients strongly increase with falling room temperature, while radiative heat transfer coefficients decrease. The type of body segment strongly influences the convective heat transfer coefficient, while only minimally influencing the radiative heat transfer coefficient.