Showing papers on "Thermal radiation published in 2019"

Observation of thermal Hawking radiation and its temperature in an analogue black hole.

Abstract: The entropy of a black hole1 and Hawking radiation2 should have the same temperature given by the surface gravity, within a numerical factor of the order of unity In addition, Hawking radiation should have a thermal spectrum, which creates an information paradox3,4 However, the thermality should be limited by greybody factors5, at the very least6 It has been proposed that the physics of Hawking radiation could be verified in an analogue system7, an idea that has been carefully studied and developed theoretically8–18 Classical white-hole analogues have been investigated experimentally19–21, and other analogue systems have been presented22,23 The theoretical works and our long-term study of this subject15,24–27 enabled us to observe spontaneous Hawking radiation in an analogue black hole28 The observed correlation spectrum showed thermality at the lowest and highest energies, but the overall spectrum was not of the thermal form, and no temperature could be ascribed to it Theoretical studies of our observation made predictions about the thermality and Hawking temperature29–33 Here we construct an analogue black hole with improvements compared with our previous setup, such as reduced magnetic field noise, enhanced mechanical and thermal stability and redesigned optics We find that the correlation spectrum of Hawking radiation agrees well with a thermal spectrum, and its temperature is given by the surface gravity, confirming the predictions of Hawking’s theory The Hawking radiation observed is in the regime of linear dispersion, in analogy with a real black hole, and the radiation inside the black hole is composed of negative-energy partner modes only, as predicted The spectrum of Hawking radiation is measured in an analogue black hole composed of rubidium atoms, confirming Hawking’s prediction that Hawking radiation is thermal with a temperature given by the surface gravity

Magneto-hydrodynamic flow and heat transfer of a hybrid nanofluid in a rotating system among two surfaces in the presence of thermal radiation and Joule heating

Abstract: In this paper, the researchers explore heat transfer and magneto-hydrodynamic flow of hybrid nanofluid in a rotating system among two surfaces. The upper and lower plates of the system are assumed penetrable and stretchable, respectively. The thermal radiation and Joule heating impacts are considered. A similarity technic is applied to alter governing energy and momentum equations into non-linear ordinary differential ones that contain the convenient boundary conditions and used the Duan-Rach Approach (DRA) to solve them. Influences of assorted parameters including rotation parameter, suction/blowing parameter, radiation parameter, Reynolds number, hybrid nanofluid volume fraction, and magnetic parameter on temperature and velocity profiles are examined. Also, a correlation for the Nusselt number has been developed in terms of the acting parameters of the present study. The outcomes indicate that Nusselt number acts as an ascending function of injection and radiation parameters, as well as volume fraction of nanofluid.

A hybrid investigation on numerical and analytical solutions of electro-magnetohydrodynamics flow of nanofluid through porous media with entropy generation

Abstract: The purpose of this paper is to present the investigation of the pressure-driven flow of aluminum oxide-water based nanofluid with the combined effect of entropy generation and radiative electro-magnetohydrodynamics filled with porous media inside a symmetric wavy channel.,The non-linear coupled differential equations are first converted into a number of ordinary differential equations with appropriate transformations and then analytical solutions are obtained by homotopic approach. Numerical simulation has been designed by the most efficient approach known homotopic-based Mathematica package BVPh 2.0 technique. The long wavelength approximation over the channel walls is taken into account. The obtained analytical results have been validated through graphs to infer the role of most involved pertinent parameters, whereas the characteristics of heat transfer and shear stress phenomena are presented and examined numerically.,It is found that the velocity profile decreases near to the channel. This is in accordance with the physical expectation because resistive force acts opposite the direction of fluid motion, which causes a decrease in velocity. It is seen that when the electromagnetic parameter increases then the velocity close to the central walls decreases whereas quite an opposite behavior is noted near to the walls. This happens because of the combined influence of electro-magnetohydrodynamics. It is perceived that by increasing the magnetic field parameter, Darcy number, radiation parameter, electromagnetic parameter and the temperature profile increases, and this is because of thermal buoyancy effect. For radiation and electromagnetic parameters, energy loss at the lower wall has substantial impact compared to the upper wall. Residual error minimizes at 20th order iterations.,The proposed prospective model is designed to explore the simultaneous effects of aluminum oxide-water base nanofluid, electro-magnetohydrodynamics and entropy generation through porous media. To the best of author’s knowledge, this model is reported for the first time.

Heat transfer enhancement in hydromagnetic alumina–copper/water hybrid nanofluid flow over a stretching cylinder

Abstract: In the current study, heat transfer performances and flow characteristics of alumina–copper/water (Al2O3–Cu/H2O) hybrid nanofluid over a stretching cylinder are explored under the influence of Lorentz magnetic forces and thermal radiation. The Roseland’s flux model is employed for the impact of thermal radiations. The governing flow problem comprises of nonlinear ordinary differential equations, which are transformed into nondimensional form via suitable similarity transforms, Boussinesq and boundary layer approximations. Results of heat and fluid flow as well as convective heat transfer coefficient and skin friction coefficient under influence of embedding parameters are displayed and discussed through tables and graphs. To check its heat transfer performance, a comparison of hybrid nanofluid with base fluid and single material nanofluids is also made and found that hybrid nanofluids are more effective in heat transfer than conventional fluids or single nanoparticles-based nanofluids.

A near-field radiative heat transfer device.

Abstract: Recently, several reports have experimentally shown near-field radiative heat transfer (NFRHT) exceeding the far-field blackbody limit between planar surfaces1–5. However, owing to the difficulties associated with maintaining the nanosized gap required for measuring a near-field enhancement, these demonstrations have been limited to experiments that cannot be implemented in large-scale devices. This poses a bottleneck to the deployment of NFRHT concepts in practical applications. Here, we describe a device bridging laboratory-scale measurements and potential NFRHT engineering applications in energy conversion6,7 and thermal management8–10. We report a maximum NFRHT enhancement of approximately 28.5 over the blackbody limit with devices made of millimetre-sized doped Si surfaces separated by vacuum gap spacings down to approximately 110 nm. The devices use micropillars, separating the high-temperature emitter and low-temperature receiver, manufactured within micrometre-deep pits. These micropillars, which are about 4.5 to 45 times longer than the nanosize vacuum spacing at which radiation transfer takes place, minimize parasitic heat conduction without sacrificing the structural integrity of the device. The robustness of our devices enables gap spacing visualization by scanning electron microscopy (SEM) before performing NFRHT measurements. An enhancement of 28.5 times beyond the blackbody radiation limit is demonstrated between two plates 110 nm apart in a chip-scale device.

Natural convection MHD flow due to MoS2–Ag nanoparticles suspended in C2H6O2H2O hybrid base fluid with thermal radiation

Abstract: In this article, natural convection 3D magneto hydrodynamic flow and heat transfer of MoS2–Ag/ ethylene glycol-water (50–50%) hybrid Nano fluid along a vertical stretching surface in the attendance of variable thermal conductivity, non-linear thermal radiation and nanoparticle shape factor has been analyzed using Runge–Kutta Fehlberg fifth order (RKF 5) numerical method. The impact of changing different parameters and nanoparticles shape, named Bricks, Cylinders, Platelets and Blades on temperature and velocity distribution has been explored. Outputs demonstrate Lorentz force produced by increasing Hartman number (Ha) causes reduction in velocity profile. Increasing thermal radiation and shape factor caused increase in temperature profile and Nusselt number (Nu). The impact of hybrid nanoparticles on increasing Nusselt number is more than nanoparticle. Furthermore, validation of obtained results indicates the high accuracy of solving method employed in this paper.

Effects of nanofluid and radiative heat transfer on the double-diffusive forced convection in microreactors

Abstract: Understanding transport phenomena in microreactors remains challenging owing to the peculiar transfer features of microstructure devices and their interactions with chemistry. This paper, therefore, theoretically investigates heat and mass transfer in microreactors consisting of porous microchannels with thick walls, typical of real microreactors. To analyse the porous section of the microchannel, the local thermal non-equilibrium model of thermal transport in porous media is employed. A first-order, catalytic chemical reaction is implemented on the internal walls of the microchannel to establish the mass transfer boundary conditions. The effects of thermal radiation and nanofluid flow within the microreactor are then included within the governing equations. Further, the species concentration fields are coupled with that of the nanofluid temperature through considering the Soret effect. A semi-analytical methodology is used to tackle the resultant mathematical model with two different thermal boundary conditions. Temperature and species concentration fields as well as Nusselt number for the hot wall are reported versus various parameters such as porosity, radiation parameter and volumetric concentration of nanoparticles. The results show that radiative heat transfer imparts noticeable effects upon the temperature fields and consequently Nusselt number of the system. Importantly, it is observed that the radiation effects can lead to the development of a bifurcation in the nanofluid and porous solid phases and significantly influence the concentration field. This highlights the importance of including thermal radiation in thermochemical simulations of microreactors.

Consequences of activation energy and binary chemical reaction for 3D flow of Cross-nanofluid with radiative heat transfer

Abstract: In view of ecological concern and energy security, execution of refrigeration system should be enriched which can be done by improving the characteristics of working liquids. The nanoliquids have gained interest in industrial and engineering fields due to their outstanding thermophysical features. Researchers used nanoliquids as working liquid and detected substantial variations in thermal performance. In the present research work, our intention is to explore the impact of nonlinear thermal radiation and variable thermal conductivity on 3D flow of cross-nanofluid. Moreover, heat sink–source, chemical processes and activation energy are implemented. Zero mass flux relation with thermophoresis and Brownian motion mechanisms are scrutinized. The required system of ordinary ones is achieved by implementing appropriate transformations. The achieved system of ordinary ones is computed numerically by implementing bvp4c scheme. Graphs are plotted to explore the impact of various physical parameters on concentration, temperature and velocity fields. It is detected from obtained graphical data that thermophoresis and Brownian motion mechanisms significantly affect heat transport mechanism. Furthermore, graphical analysis reveals that concentration of cross-nanofluid enhances for augmented values of activation energy.

Effects of Radiative Electro-Magnetohydrodynamics Diminishing Internal Energy of Pressure-Driven Flow of Titanium Dioxide-Water Nanofluid due to Entropy Generation.

Abstract: The internal average energy loss caused by entropy generation for steady mixed convective Poiseuille flow of a nanofluid, suspended with titanium dioxide (TiO2) particles in water, and passed through a wavy channel, was investigated. The models of thermal conductivity and viscosity of titanium dioxide of 21 nm size particles with a volume concentration of temperature ranging from 15 °C to 35 °C were utilized. The characteristics of the working fluid were dependent on electro-magnetohydrodynamics (EMHD) and thermal radiation. The governing equations were first modified by taking long wavelength approximations, which were then solved by a homotopy technique, whereas for numerical computation, the software package BVPh 2.0 was utilized. The results for the leading parameters, such as the electric field, the volume fraction of nanoparticles and radiation parameters for three different temperatures scenarios were examined graphically. The minimum energy loss at the center of the wavy channel due to the increase in the electric field parameter was noted. However, a rise in entropy was observed due to the change in the pressure gradient from low to high.

Natural convection of water-based carbon nanotubes in a partially heated rectangular fin-shaped cavity with an inner cylindrical obstacle

Abstract: This study investigates the heat augmentation and hydromagnetic flow of water-based carbon nanotubes (CNTs) inside a partially heated rectangular fin-shaped cavity. A thin heated rod is placed within the cavity to create a resistance or to provide a source for heat transfer. The obstacle is tested for the heated case, while the right side of the horizontal tip is tested for three different temperatures (adiabatic, cold, and heated). The left vertical side of the cavity is partially heated with temperature Th, and the rest of the sides are kept cold at temperature Tc except the right tip. Two different thermal boundary conditions (prescribed temperature and adiabatic) are employed on the fin tip. The CNTs and water are assumed to be in thermal equilibrium with no-slip velocity. The magnetic field and thermal radiation are introduced in the momentum and energy equations, respectively. The governing equations are obtained in dimensionless form by means of dimensionless variables. The numerical computation is performed via the finite element method using the Galerkin approach. The substantial effects of emerging parameters on the streamlines, isotherms, dimensionless velocities, and temperature are reported graphically and discussed. In the case of a cold or adiabatic fin-tip, a drop to minimum is found in the dimensionless temperature. The components of velocity are perceived maximum at a vertical corner while minimum at the horizontal corner. It is demonstrated that the local Nusselt numbers are increased by introducing both solid volume fraction of CNTs and radiation effects, while the Nusselt number noticed maximum at the corners.

One-Chip Near-Field Thermophotovoltaic Device Integrating a Thin-Film Thermal Emitter and Photovoltaic Cell.

Abstract: Thermal radiation transfer between two objects separated by a subwavelength gap (near-field thermal radiation transfer) can be orders of magnitude larger than that in free space, which is attractin...

Thermal radiation control from hot graphene electrons coupled to a photonic crystal nanocavity

Abstract: Controlling thermal radiation is central in a range of applications including sensing, energy harvesting, and lighting. The thermal emission spectrum can be strongly modified through the electromagnetic local density of states (EM LDOS) in nanoscale-patterned metals and semiconductors. However, these materials become unstable at high temperature, preventing improvements in radiative efficiency and applications such as thermophotovoltaics. Here, we report stable high-temperature thermal emission based on hot electrons (>2000 K) in graphene coupled to a photonic crystal nanocavity, which strongly modifies the EM LDOS. The electron bath in graphene is highly decoupled from lattice phonons, allowing a comparatively cool temperature (700 K) of the photonic crystal nanocavity. This thermal decoupling of hot electrons from the LDOS-engineered substrate opens a broad design space for thermal emission control that would be challenging or impossible with heated nanoscale-patterned metals or semiconductor materials. Efficient control of thermal radiation is at the core of device design for a variety of applications. Here, the authors demonstrate a high-temperature thermal emitter with selective emission from a graphene-silicon photonic crystal nanocavity.

Near-complete violation of Kirchhoff's law of thermal radiation with a 0.3 T magnetic field.

Abstract: The capability to overcome Kirchhoff’s law of thermal radiation provides new opportunities in energy harvesting and thermal radiation control. Previously, design towards demonstrating such capability requires a magnetic field of 3 T, which is difficult to achieve in practice. In this work, we propose a nanophotonic design that can achieve such capability with a far more modest magnetic field of 0.3 Tesla, a level that can be achieved with permanent magnets. Our design uses guided resonance in low-loss dielectric gratings sitting on a magneto-optical material, which provides significant enhancement on the sensitivity to the external magnetic field.

CVFEM approach for EHD flow of nanofluid through porous medium within a wavy chamber under the impacts of radiation and moving walls

Abstract: In current investigation, ferrofluid circulation and energy transport inside a wavy-walled porous enclosure are modeled considering radiation and EHD effects. The finite volume method is employing for simulation of EHD circulation structures and thermal transmission. Properties of working fluid depend on the electric field and nanosized solid particles concentration. Impacts of thermal radiation, nanoparticles shape, and volume fraction are considered in governing equations. Distributions of unknown functions are received for various voltage, permeability, radiation parameters, nanoparticles’ shape and concentration. Results have shown that platelet form leads to the strongest convective circulation. An amplification of electric force characterizes a diminution of the boundary layer thickness. Greater permeability of the porous medium characterizes the strongest convective circulation and thermal transmission.

Impact of thermal radiation on electrical MHD rotating flow of Carbon nanotubes over a stretching sheet

Abstract: The main objective of this article is to study the inventive conception of the electrical Magneto hydrodynamics (MHD) rotational flow of Single and Multi-Walled Carbon nanotubes (SWCNTs/MWCNTs) base on the fluids (water, engine oil, ethylene glycol and kerosene oil). The thermal radiation impact is taken to be varying the purpose, to see the concentration as well as the temperature modifications between the nanofluid and the surfaces. Kerosene oil is taken as based nanofluids because of its unique attention due to their advanced thermal conductivities, exclusive features and applications. The fluid flow is assumed in steady state. The basic Navier Stocks equations have been transformed through similarity variables in the form of nonlinear differential equations. The solution of the problem has been obtained through Homotopy Analysis Method (HAM). Results obtained for single and multi-wall carbon nanotubes are compared. Plots have been presented in order to examine how the velocities and temperature profile get affected by various flow parameters. The numerical outputs of the physical properties are shown trough tables. The impact of Skin fraction coefficient and Nusselt number are shown in tables.The main objective of this article is to study the inventive conception of the electrical Magneto hydrodynamics (MHD) rotational flow of Single and Multi-Walled Carbon nanotubes (SWCNTs/MWCNTs) base on the fluids (water, engine oil, ethylene glycol and kerosene oil). The thermal radiation impact is taken to be varying the purpose, to see the concentration as well as the temperature modifications between the nanofluid and the surfaces. Kerosene oil is taken as based nanofluids because of its unique attention due to their advanced thermal conductivities, exclusive features and applications. The fluid flow is assumed in steady state. The basic Navier Stocks equations have been transformed through similarity variables in the form of nonlinear differential equations. The solution of the problem has been obtained through Homotopy Analysis Method (HAM). Results obtained for single and multi-wall carbon nanotubes are compared. Plots have been presented in order to examine how the velocities and temperature profil...

Infrared-Radiation-Enhanced Nanofiber Membrane for Sky Radiative Cooling of the Human Body.

Abstract: Extreme heat events are mainly responsible for weather-related human mortality due to climate change. However, there is a lack of outdoor thermal management for protecting people from extreme heat events. We present a novel infrared-radiation-enhanced nanofiber membrane (NFM) that has good infrared resonance absorption and selectively radiates thermal radiation of the human body through the atmosphere and into the cold outer space. The NFM comprises polyamide 6 (PA6) nanofibers and randomly distributed SiO2 submicron spheres and has sufficient air permeability and thermal-moisture comfortability because of its interconnect nanopores and micropores. We measure the sky radiative cooling performance under a clear sky, and PA6/SiO2 NFM produces temperatures that are about 0.4-1.7 °C lower than those of commercial textiles when covering dry and wet hands and temperatures 1.0-2.5 °C lower than the ambient temperature when thermal conduction and convection are isolated in a closed device. Our processed PA6/SiO2 NFM combines sky radiative cooling with thermal management of the human body very well, which will promote the development of radiative cooling textiles.

Impact of Nonlinear Thermal Radiation on MHD Nanofluid Thin Film Flow over a Horizontally Rotating Disk

Abstract: Nanoscience can be stated as a superlative way of changing the properties of a working fluid. Heat transmission features during the flow of nanofluids are an imperative rule from the industrial and technological point of view. This article presents a thin film flow of viscous nanofluids over a horizontal rotating disk. The impact of non-linear thermal radiation and a uniform magnetic field is emphasized in this work. The governing equations were transformed and solved by the homotopy analysis method and the ND-Solve technique. Both analytical and numerical results are compared graphically and numerically, and excellent agreement was obtained. Skin friction and the Nusselt number were calculated numerically. It is concluded that the thin film thickness of nanofluids reduces with enhanced values of the magnetic parameter. In addition, the nanofluid temperature was augmented with increasing values of the thermal radiation parameter. The impact of emerging parameters on velocities and temperature profiles were obtainable through graphs and were deliberated on in detail.