Control Of Radiative Heat Transfer Research Articles

Measured spectral, directional radiative behavior of corrugated surfaces

Spacecraft thermal control is entirely reliant upon radiative heat transfer with its surroundings for temperature regulation. Current methods are often static in nature and do not provide dynamic control of radiative heat transfer. As a result, modern spacecraft thermal control systems are typically ‘cold-biased’ with radiators that are larger than necessary for many operating conditions. Deploying a variable radiator as a thermal control technique in which the projected surface area can be adjusted to provide the appropriate heat loss for a given condition can reduce unnecessary heat rejection and reduce power requirements. However, the radiative behavior of the apparent surface representing the expanding/collapsing radiator changes in addition to the projected surface area size. This work experimentally quantifies the spectral, directional emissivity of an apparent surface comprised of a series of V-grooves (e.g. corrugated surface), as a function of angle and highlights its emission characteristics that trend toward black behavior. The experimental setup for quantifying this apparent radiative surface behavior is described and utilized to show the influence of surface geometry, direction and wavelength. Experiments on test samples were performed at 573 K. The experimental design is validated and demonstrated using fully oxidized, nearly diffuse, copper, corrugated test samples. The results presented in this work demonstrate, for the corrugated oxidized copper surfaces tested, that (1) higher emissivity values correspond to higher wavelengths in the spectral range of 2.5 to 15.4 µm (2) apparent emissivity values increase with decreasing V-groove angle resulting in less spectral variation in emissivity and greater blackbody like behavior, (3) azimuth dependence can be relatively small despite the obvious pattern associated with a corrugated surface, and (4) as the V-groove angle decreases, higher emissivity values are associated with θ→0∘ and ϕ→0∘. Results provide a foundation for future radiator design, improved spacecraft thermal control methods, and improved emissivity testing methods for patterned or angular surfaces.

Strain-induced control of radiative heat transfer between nanoparticles in a plasmonic cavity

Graphene enables the versatile modulation and significant enhancement of their optical properties by external strains, which plays a pivotal role in various applications, such as active thermal modulation. In this work, we investigate the near-field radiative heat transfer (NFRHT) between two nanoparticles mediated by a strain-induced plasmonic cavity consisting of the two parallel graphene sheets (GSs) under external mechanical strain. By adjusting the strain modulus, the ellipsoidal surface plasmon polaritons supported by strain-induced graphene are found to significantly impact the NFRHT between nanoparticles. The integral and interlayer twist can synergistically manipulate the reflection properties of the cavity, allowing the anisotropic coupling of nanoparticles with the cavity in different directions. Under large strain modulus and suitable twisted angles, the NFRHT can be enhanced up to 5 orders of magnitude. In addition, the chemical potential of graphene can be tuned to provide additional ways to modulate the heat transfer between nanoparticles. These findings may open up promising avenues for highly efficient thermal management, thermal communication, and energy harvesting.

Plasmon-Enhanced Greenhouse Selectivity for High-Temperature Solar Thermal Energy Conversion.

The greenhouse effect arises when thermal radiation is forced to undergo absorption and re-emission many times before escaping, while sunlight transmits largely unimpeded. Although this effect is responsible for global warming, it is generally weak in solid-state materials because radiation can be easily overpowered by other modes of heat transfer. Here, we report on the use of infrared plasmonic nanoparticles to enhance the greenhouse effect in transparent mesoporous materials. Local surface plasmon resonances in transparent conducting oxide nanoparticles (TCO NPs) selectively shorten the mean free path of thermal photons while maintaining high solar transmittance. The addition of a small amount of TCO NPs (<0.1% by volume) nearly halves the heat losses at 700 °C. This leads to an experimentally demonstrated effective thermal emittance of ∼0.17 at 700 °C, which is the lowest reported value to date, among all selective surfaces and transparent insulating materials measured at 650 °C or above. Our results show that plasmon-enhanced greenhouse selectivity (PEGS) is a promising mechanism for spectral control of radiative heat transfer, and more specifically, for conversion of minimally concentrated sunlight into high-temperature heat.

Electric field control of radiative heat transfer in a superconducting circuit

Heat is detrimental for the operation of quantum systems, yet it fundamentally behaves according to quantum mechanics, being phase coherent and universally quantum-limited regardless of its carriers. Due to their robustness, superconducting circuits integrating dissipative elements are ideal candidates to emulate many-body phenomena in quantum heat transport, hitherto scarcely explored experimentally. However, their ability to tackle the underlying full physical richness is severely hindered by the exclusive use of a magnetic flux as a control parameter and requires complementary approaches. Here, we introduce a dual, magnetic field-free circuit where charge quantization in a superconducting island enables thorough electric field control. We thus tune the thermal conductance, close to its quantum limit, of a single photonic channel between two mesoscopic reservoirs. We observe heat flow oscillations originating from the competition between Cooper-pair tunnelling and Coulomb repulsion in the island, well captured by a simple model. Our results highlight the consequences of charge-phase conjugation on heat transport, with promising applications in thermal management of quantum devices and design of microbolometers.

Active Temporal Control of Radiative Heat Transfer with Graphene Nanodisks

The ability to dynamically control the radiative transfer of heat at the nanoscale holds the key to the development of a diverse number of technologies, ranging from nanoscale thermal-management systems to improved thermophotovoltaic devices. Recently, graphene has emerged as an ideal material to achieve this goal, since it can be electrically doped to support surface plasmons, collective oscillations of the conduction electrons. These resonances produce large and spectrally narrow optical cross sections, which dictate the emission and absorption properties of the graphene nanostructure and, thus, the heat that it radiatively exchanges with other objects and the environment. For attainable levels of doping, the plasmons supported by graphene nanostructures naturally lie in the midinfrared part of the spectrum, which is the most relevant wavelength range for radiative heat transfer under realistic temperatures. Furthermore, these resonances are actively tunable, thus providing full dynamic control over the heat transfer. Motivated by this great potential, we present a comprehensive analysis of the temporal evolution of the radiative heat transfer between arrangements of graphene nanodisks, showing that it is possible to exploit the tunability of these structures to obtain actively controlled heat transfer scenarios not possible with conventional passive nanostructures. The results of this work provide a framework for achieving fully dynamical control over nanoscale radiative heat transfer and thus provide fundamental insights into this process.

Magnetically Tunable Near-Field Radiative Heat Transfer in Hyperbolic Metamaterials

Active control of radiative heat transfer still remains open due to its exciting application potential. In view of this, we present a theoretical demonstration of dynamically tunable near-field radiative heat transfer (NFRHT) between two multilayer hyperbolic metamaterials (consisting of alternating layers of a magneto-optical (MO) material and a dielectric) using an external magnetic field. We show that magnetization-induced hyperbolic modes play a significant role in radiative heat transfer and allow a highly tunable heat flux. Moreover, the hybridization of intrinsic and magnetization-induced hyperbolic modes significantly enhances the radiative heat transfer. In particular, we find that polarization conversion due to nonzero off-diagonal elements enables the spectral heat transfer coefficient for the TE polarization to be as large as that for the TM polarization, which is vastly different from the case of two closely spaced bulk MO plates. In addition, to quantitatively characterize the thermal modulation, we show a negative thermal magnetoresistance effect in this system, which approaches $\ensuremath{-}59.6\mathrm{%}$ when the magnetic field intensity reaches 10 T. Our findings may be beneficial for active noncontact thermal management at the nanoscale, and facilitate a deeper understanding of the mechanisms of MO hyperbolic metamaterials in terms of NFRHT.

Control of Net Radiative Heat Transfer With a Variable-Emissivity Accordion Tessellation

Abstract Origami tessellations have been proposed as a mechanism for control of radiative heat transfer through the use of the cavity effect. This work explores the impact of a changing projected surface area and varying apparent radiative properties on the net radiative heat transfer of an accordion fold comprised of V-grooves. The net radiative heat transfer of an accordion tessellation is obtained by a thermal energy balance at the cavity openings with radiative properties of the cavities given as a function of various cavity parameters. Results of the analytical model are experimentally confirmed. An accordion tessellation, constructed of stainless-steel shim stock, is positioned to achieve a specified fold angle and placed in a vacuum environment while heated by Joule heating. A thermal camera records the apparent temperature of the cavity openings for various fold angles. Results are compared to apparent temperatures predicted with the analytical model. Analytically and experimentally obtained temperatures agree within 5% and all measurements fall within experimental uncertainty. For diffusely irradiated surfaces, the decrease in projected surface area dominates, causing a continuous decrease in net radiative heat transfer for a collapsing accordion fold. Highly reflective specular surfaces exposed to diffuse irradiation experience large turn-down ratios (7.5× reduction in heat transfer) in the small angle ranges. Specular surfaces exposed to collimated irradiation achieve a turn down ratio of 3.35 between V-groove angles of 120 deg and 150 deg. The approach outlined here may be extended to modeling the net radiative heat transfer for other origami tessellations.

Dynamic optical control of near-field radiative transfer.

Dynamic control of radiative heat transfer is of fundamental interest as well as for applications in thermal management and energy conversion. However, realizing high contrast control of heat flow without moving parts and with high temporal frequencies remains a challenge. Here, we propose a thermal modulation scheme based on optical pumping of semiconductors in near-field radiative contact. External photo-excitation of the semiconductor emitters leads to increases in the free carrier concentration that in turn alters the plasma frequency, resulting in modulation of near-field thermal radiation. The temporal frequency of the modulation can reach hundreds of kHz limited only by the recombination lifetime, greatly exceeding the bandwidth of methods based on temperature modulcation. Calculations based on fluctuational electrodynamics show that the heat transfer coefficient between two silicon films can be tuned from near zero to 600 Wm-2K-1 with a gap distance of 100 nm at room temperature.

Anisotropic Thermal Magnetoresistance for an Active Control of Radiative Heat Transfer

The discovery that the near-field radiative heat transfer enables to overcome the limit set by Planck’s law holds the promise to have an impact in different nanotechnologies that make use of thermal radiation, and the challenge now is to find strategies to actively control and manipulate this near-field thermal radiation. Here, we predict a huge anisotropic thermal magnetoresistance (ATMR) in the near-field radiative heat transfer between magneto-optical particles when the direction of an external magnetic field is changed with respect to the heat current direction. We illustrate this effect with the case of two InSb particles where we find that the ATMR amplitude can reach values of up to 800% for a magnetic field of 5 T, which is many orders of magnitude larger than its spintronic analogue. This thermomagnetic effect could find broad applications in the field of ultrafast thermal management as well as magnetic and thermal remote sensing.

A Large‐Area Smart Window with Tunable Shading and Solar‐Thermal Harvesting Ability Based on Remote Switching of a Magneto‐Active Liquid

AbstractSwitchable windows provide intriguing opportunities for addressing the challenges of modern building skins. In particular, indoor comfort and the control of radiative heat transfer into and out of the building require adaptive and tunable solutions for shading and emissivity. Here, a switchable, ultrathin suspended particle device (SPD) for large‐area integration with smart facades is presented. The system is based on a fluidic window, manufactured at low cost from a laminate of structured, rolled glass and a thin cover with high surface strength. Loading the circulating fluid with magnetic nanoparticles enables active shading and solar‐thermal energy harvesting, whereby the loading state and, hence, the optical properties of the liquid can be controlled through remote switching in a particle collector‐suspender device. In the fully shaded state, a typical harvesting efficiency of 45% of the incoming solar power is obtained. For an average solar irradiance of 1000 W m−2 during 800 h a−1, this corresponds to a solar thermal harvesting capacity in the range of 360 kWh a−1 m−2. In comparison to alternative SPD concepts, this enables high flexibility and compatibility with established production lines. In addition, there is no need for further electrical contact, transparent conductive layers, or electrolytes.

Control of radiative heat transfer in high-temperature environments via radiative trapping—Part I: Theoretical analysis applied to pressurized oxy-combustion

Burning fuels with pure oxygen offers many benefits, including higher efficiency, a CO2-rich flue gas that is suitable for carbon capture, and improved flame stability. However, the extremely high flame temperature that occurs during combustion in pure oxygen is typically assumed to lead to extreme levels of radiative heat flux that are beyond the tolerable limits of boiler materials. This paper presents a unique approach to control wall heat flux under extreme temperatures in particle-laden reacting flows. Fundamental studies were carried out to understand the radiative heat transfer behavior of such systems when temperature and absorption coefficient profiles are dictated by the diffusive–convective characteristics of the system, such as the case of a non-premixed combustion reactor. The results show that if the optical thickness of the particle-laden gas medium is sufficiently large, a considerable amount of emissive power coming from the high temperature sources can be trapped in the medium and the net heat flux on the wall can be managed. An average-temperature approximation (ATA) method is developed to conveniently approximate the wall heat flux when trapping of radiation occurs in an optically dense medium. The ATA method can be utilized to design systems that require manageable wall heat flux via radiative trapping under extremely high flame temperatures.

Fundamental study on control of radiative heat transfer between human body and surrounding walls based on wall surface emittance

Fundamental study on control of radiative heat transfer between human body and surrounding walls based on wall surface emittance

Model hierarchy-based optimal control of radiative heat transfer

We present a model hierarchy multilevel optimisation approach to solve an optimal boundary control problem in glass manufacturing. The process is modelled by radiative heat transfer and formulated as an optimal control problem restricted by partial differential algebraic equations (PDAEs) and additional control constraints. We consider a sequence of model approximations given by space-time dependent non-linear PDAEs of ascending accuracy. The different models allow for a model hierarchy-based optimisation approach, where the models are shifted automatically as the optimisation proceeds. We present a realisation of a multilevel generalised SQP method within the fully space-time adaptive optimisation environment Kardos using linearly implicit methods of Rosenbrock type and multilevel finite elements. We apply the optimal control algorithm to a glass cooling problem and present numerical experiments for the model hierarchy-based approach in two spatial dimensions and for a fully space-time adaptive optimisation in three spatial dimensions.

Optimal control of radiative heat transfer in glass cooling with restrictions on the temperature gradient

SUMMARYThis paper is motivated by an optimal boundary control problem for the cooling process of molten and already formed glass down to room temperature. The high temperatures at which glass is processed demand to include radiative heat transfer in the computational model. Since the complete radiative heat transfer equations are too complex for optimization purposes, we use simplified approximations of spherical harmonics coupled with a practically relevant frequency bands model. The optimal control problem is considered as a partial differential algebraic equation (PDAE)‐constrained optimization problem with box constraints on the control. In this paper, we augment the objective by a functional depending on the state gradient, which forces a minimization of thermal stress inside the glass. To guarantee consistent and grid‐independent values of the reduced objective gradient at the end of the cooling process, we pursue two approaches. The first includes the temperature gradient with a time‐dependent linearly decreasing weight. In the second approach, we augment the objective functional by the final state tracking and final state gradient optimization. To determine an optimal boundary control, we apply a projected gradient method with the Armijo step size rule. The reduced objective gradient is computed by the continuous adjoint approach. The arising time‐dependent PDAEs are numerically solved by variable step size one‐step methods of Rosenbrock type in time and adaptive multilevel finite elements in space. We present two‐dimensional numerical results for an infinitely long glass block and compare the two different approaches derived to ensure consistency at the end of the cooling process. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Adaptive computation for boundary control of radiative heat transfer in glass

This paper is concerned with an optimal boundary control of the cooling down process of glass, an important step in glass manufacturing. Since the computation of the complete radiative heat transfer equations is too complex for optimization purposes, we use simplified approximations of spherical harmonics including a practically relevant frequency bands model. The optimal control problem is considered as a constrained optimization problem. A first-order optimality system is derived and decoupled with the help of a gradient method based on the solution to the adjoint equations. The arising partial differential–algebraic equations of mixed parabolic–elliptic type are numerically solved by a self-adaptive method of lines approach of Rothe type. Adaptive finite elements in space and one-step methods of Rosenbrock-type with variable step sizes in time are applied. We present numerical results for a two-dimensional glass cooling problem.

Radiative Heat Transfer Properties of Electro-Controllable Fluids

Abstract Active control of radiation heat transfer in liquids can be accomplished with the use of a class of fluids referred to here as electro-controllable (EC) fluids. EC fluids in general consist of a colloidal suspension of polarizable, micron-size particles dispersed in a carrier fluid with an appropriate dielectric constant. When an electric field is applied, the particles redistribute in the fluid, changing from a uniformly dispersed configuration to a tightly organized chain formation that follows the lines of the electric field, thus causing a change in the thermal radiation transport. In an example application, experiments are conducted and models are developed for thermal radiation transmittance through a composite window featuring a central layer of EC fluid. The specific EC fluids of this study are made of micron-sized Zeolite particles suspended in a light Silicone oil carrier fluid of appropriate dielectric strength. The incident thermal radiation ranged in wavelength between 500 nm and 800 nm, and the strength of the applied electric field ranged from 100 V/mm to 500 V/mm. The models are applicable for both the dispersed organizational state and the field induced chained state. Absorption was demonstrated to be the fundamental radiation transport property enabling the control process. The EC fluid transmittance predicted by these models are compared to the data obtained by experimental measurement demonstrate very good agreement.

Tuned Infrared Emission From Lithographically-Defined Silicon Surface Structures

AbstractPhotonic bandgap structures have received much attention as optical and infrared filters with controllable narrow-band absorbance. There is a need, however, for the same kind of control of the thermal emittance of surfaces for applications ranging from control of radiative heat transfer to gas absorption spectroscopy. We report on the fabrication of photonic bandgap structures on silicon surfaces using standard lithographic techniques. Substrate resistivity varied from n− to n+ and in some cases background surface emissivity was suppressed with a high reflectivity coating such as aluminum. We have measured the infrared reflectance and emittance of these patterned surfaces. Peak absorption wavelength and spectral purity (linewidth) correlate with photonic bandgap feature size and spacing as well as surface conductivity. We demonstrate band emission with a sharp short wavelength cut-off from these structures when heated.

Directional Control of Radiation Heat Transfer from Solid Surface Using Grating Composed of Parallel Elliptical Cylinders -Directional Control of Energy Concetration-

A grating composed of elliptical cylinders (GEC), specially designed, is applicable to control of radiation heat transfer from a heated surface, as reported in our previous work. In this study, an analysis of radiation heat transfer is performed for a physical model in which the GEC is placed in front of a heated black-base surface and the major axes of the elliptical cylinders are inclined as a certain angle from the normal to the row of elliptical cylinders. Numerical solutions are obtained. Variations of the direction and the radiative energy concentration with slant angle of the major axis are shown for some parameters. It is verified that the GEC is able to widely change the direction of radiation heat transfer from the heated surface.

Directional Control of Radiation Heat Transfer from Solid Surface Using Grating Composed of Parallel Elliptical Cylinders.

A grating composed of elliptical cylinders (GEC), specially designed, is applicable to control of radiation heat transfer from a heated surface, as reported in our previous work. In this study, an analysis of radiation heat transfer is performed for a physical model in which the GEC is placed in front of a heated black-base surface and the major axes of the elliptical cylinders are inclined at a certain angle from the normal to the row of elliptical cylinders. Numerical solutions are obtained. Variations of the direction and the radiative energy concentration with the slant angle of the major axis are shown for some parameters. It is verified that the GEC is able to widely change the direction of radiation heat transfer from the heated surface. An analysis taking account of radiative energy inward from the environment is also made, and the numerical results are discussed.

Control of radiation heat transfer through a composite window featuring ER fluid: A conceptual investigation

Radiation heat transfer control through the application of an electric field upon an Electroheological (ER) fluid based composite material is an innovative new area of research. A conceptual experiment has been conducted to study radiation heat transfer through a composite window featuring an ER fluid. The composite window is composed of two thin glass plates with a layer of ER fluid contained between them. The glass walls were transparent except for a very thin coating of an electric-conductive film which enabled the inside of the glass surfaces to serve as electrodes. The ER fluid was contained between the glass surfaces and consisted of a suspension of micron sized crystalline zeolite particles in a silicon oil. This study has demonstrated the unique capability of ER fluids to regulate and control radiation heat transfer via transmittance measurements. A semi-empirical model is developed from the experimental data to correlate the dependence of radiation transmission through ER fluids based on several physical parameters (fv, V*, and L). This model agrees reasonably well with the measured data. The results obtained in this study are very important to those concerned with the development of a thermally smart material for heat transfer control.