Radiative Heat Exchange Research Articles

Analysis of the heat exchange processes of the remote measurement device of mechanical values

The object of research is a model of the thermal state of a remote measurement device, determined by the operating conditions and caused by the need to confirm the operation of the device in ambient conditions according to the requirements of the specification. During the research, the physical formulation of the problem of determining the thermal state of a remote measurement device under various operating conditions and their mathematical description were used. The choice of strategy is based on the solution of technological problems that ensure the optimal process of control. The thermal and mathematical model of the thermal mode of the device is developed to calculate the multidimensional temperature fields of the elements of the device. The model of the thermal mode of the remote measurement device is obtained. The proposed method allows to determine the temperature regimes of the nodes in the preparation mode and the main work at different ambient temperatures. In particular, determine the emitting power at the design state and changes over time. The model takes into account the geometric parameters of the main structural elements of the device. At the same time, in order to maintain the heat balance and the adequacy of the model to the main elements of the device, additional characteristics of these components are introduced in the mathematical model. So, the radiation heat exchange between the external surfaces of the device and the compartment structures surrounding it is taken into account. The regularity of changes in ambient temperature depending on the operation mode of the device is taken into account. The non-stationary thermal state of a remote measurement device is considered, taking into account the radiation-convective heat exchange of the external surfaces of the device with the environment. According to the conducted studies, the thermophysical characteristics of the materials used in the model are determined with two options of the temperature conditions of operation of the remote measurement device. The model is implemented in the STAR-CCM program code based on the control volume method. This technique simplifies the development of a numerical three-dimensional model from the control volumes of a remote measurement device for computer analysis. These studies are an integral part in the design, development and use of a remote measurement device based on a gyro sensor in various temperature conditions.

Coupled harmonic oscillator model to describe surface-mode mediated heat transfer

When two objects made of a material that supports surface modes are brought in close proximity to each other such that the vacuum gap between them is less than the thermal wavelength of radiation, then the coupling between the surface modes provides an important channel for the heat transfer to occur, which is different from that mediated by long range propagating electromagnetic waves. Indeed, the heat transfer then exceeds Planck’s blackbody limit by several orders of magnitude and consequently has been used for several energy applications, such as near-field thermophotovoltaic systems. This near-field radiative heat exchange has been traditionally and successfully described using fluctuational electrodynamics principles. Here, we describe an alternate coupled harmonic oscillator model approach, which can be used to model the coupling between surface modes and hence the resultant near-field heat transfer. We apply this theory to estimate the near-field heat transfer for the configurations of two metallic nanoparticles and two planar metal surfaces and compare the result with predictions from fluctuational electrodynamics theory.

Determination of the heat state in the five-layer hollow sphere at convective and radiative heat exchange with the environment

The exact analytical expressions for the temperature field in a five-layer thermal sensitive hollow sphere using the model of the stationary nonlinear heat conduction with temperature-dependent coefficients of heat conduction and heat exchange has been obtained. The heat sources are present in the second and fourth layers. The inner surface has steady temperature and the outer one is subjected to the convective and radiative heat exchange. The influence of the thermal sensitivity of the material of sphere components on the temperature distribution has been investigated. Cite as: V. O. Horoshko, “Determination of the heat state in the five-layer hollow sphere at convective and radiative heat exchange with the environment,” Prykl. Probl. Mekh. Mat. , Issue 16, 141–146 (2018) (in Ukrainian), http://doi.org/10.15407/10.15407/apmm2018.16.141-146

Effects of crosswind and burner aspect ratio on flame characteristics and flame base drag length of diffusion flames

Experimental investigations were conducted to characterise the impacts of crosswind and burner aspect ratio on the flame evolution characteristics and flame base drag length of gas diffusion flames on rectangular burners. The burners have the same surface area of approximately 100 cm2. The tests to capture the flame base drag length were conducted three times for each condition with the differences between the original and repeated tests being less than 6%. The thermocouple readings were corrected for the effect of radiative and convective heat exchange with the surroundings. Overall, 84 independent test conditions were conducted on 4 different burner aspect ratios, 3 fuel supply rates and 7 crosswind conditions. The changing behaviour of the flame with different burner aspect ratios, heat release rates and crosswind speeds were carefully analysed. The appearance of “blue flames” in the upstream edge of the main diffusion flames just above the burner in relatively strong winds was analysed. Unlike the flame tilt angle and flame height which either increase (the former) or decrease (the later) monotonically with the increase of wind speed, the flame base drag length was found to increase with the wind speed firstly until a critical point and then decrease with further increase of the crosswind for a given heat release rate. This is thought to be due to the competing influence of thermal buoyancy and wind induced inertial forces. The transition point for the maximum flame base drag length with regard to crosswind was found to decrease with the increasing aspect ratio of the burner for a given heat release rate. A new physics-based correlation considering decay phase with the crosswinds was proposed for the flame base drag length incorporating all important physical factors including inertia force, fire induced thermal buoyancy, Froude number, dimensionless heat release rate and fuel/air density ratio. The proposed formulations were found to correlate well with the current measurements of gas burner fires as well as some published data in the literature for pool fires on the ground which were not used in their derivation.

Friction Force and Radiative Heat Exchange in a System of Two Parallel Plates in Relative Motion: Corollaries of the Levine–Polevoi–Rytov Theory

It has been shown that the fundamental results obtained in the works by Levine–Polevoi–Rytov (1980) and Rytov (1990) adequately describe the rate of radiative heat exchange and frictional force in a system of two thick parallel plates in relative motion, in full agreement with the results obtained by other authors later. A numerically calculated friction force for Drude metals turns out to be higher by a factor of 107 than the early result obtained by Polevoi. In addition, the friction force significantly increases with increasing the conductivity of the plates or increasing the relaxation time of electrons with decreasing temperature.

A transparent insulation façade enhanced with a selective absorber: A cooling energy load and validated building energy performance prediction model

Solar thermal façade concepts for building integration have a relevant contribution in the field of renewable solar energy exploitation. This study is focused on an experimental analysis of a transparent insulation façade (TIF) concept and the options for the prediction of its performance using building energy simulation (BES) modelling. A TIF prototype based on a transparent insulation material (TIM) and a selective absorber is tested and compared with a nonselective type of absorber. An experimental study was conducted involving full-scale climatic chamber and dynamic outdoor tests. The proposed TIF concept with a selective absorber and its solar effective layers provides the same thermal performance as conventional types of thermal insulation. As the TIM integration is potentially limited by overheating phenomena, a maximum cooling energy load obtained by dynamic outdoor conditions provides its real outdoor performance. The results show that a significant difference is measured when compared with the nonselective absorber type. Finally, a detailed comparative investigation of the thermal performance of both TIFs was conducted using the BES method. When applying the low-e functionality of the solar absorber, significant limitations were found specifically concerning the surface radiant heat exchange with the low emissivity surface affecting the air cavity enclosed zone.

The study of the thermal state of the metal in the production of the hot rolled strips in «Deform 3D»

The mathematical model of the thermal state of the sheet metal has been developed, taking into account radiation heat exchange and convection between the metal surface and the environment, the impact of descaling, heat transfer at the contact of metal with rollers, and also the heating of the metal due to plastic deformation. The model uses the results of mechanical tests obtained on the installation of the Gleeble 3800 under plane strain compression. Simulation of the rolling process is performed in the software complex «Deform 3D». The results of mathematical modeling adequately reflect the study of the thermal state of the metal produced in a sheet rolling shop. The model can be used to study the thermal state of the metal in the hot rolling production of high-quality steel grade.

A generalized model of human body radiative heat exchanges for optimal design of indoor thermal comfort conditions

Human thermal sensation depends heavily on radiative exchanges between the human body and the surrounding environment. Because these exchanges play a crucial role in the thermal balance of the human body, about 35% of the process, human thermal sensation should draw the attention of planners when designing both indoor environments and equipment.The present study aims to contribute to this field by proposing a procedure for delineating the optimal comfort conditions for occupants in most of the articulate and realistic configurations of actual indoor environments. Specifically, this procedure enables accurate assessment of the radiant field surrounding a subject in a given indoor realistic environment and considers its variability with space and time along with the presence of high-intensity radiant sources. The proposed simulation tool contains a set of algorithms in which the degree of complexity depends on the level of accuracy for modelling the radiative heat transfer between the occupants and surrounding environment.The feasibility of these algorithms for designers and researchers has also been checked for a single room characterised by the presence of windows in two different exposures. This configuration implies a complex pattern of the sun entering the room, which in turn determines relevant spatial modifications of the indoor comfort thermal conditions. Such complex situations are effectively interpreted by the proposed model.This analysis provides useful indications for suitable design of layouts of the confined space and the size of an effective heating, ventilating, and air conditioning system to limit the discomfort felt inside the room.

Investigating the performance of reflective insulation and low emissivity paints for the energy retrofit of roof attics

The energy retrofitting and conversion of attic spaces into liveable dwelling units is a widely diffused practice in EU countries, and it leads to an increment in urban density, without having to construct new buildings.Roof surfaces are the parts of buildings that are exposed the most to the sun in summer season, and for this reason, they are responsible for high cooling loads, which can determine overheating phenomena in the dwellings below the attics.Different strategies that involve the use of extra-insulation or an increase in the solar reflectance of the outer layer can be adopted to resolve this problem. However, another solution that is sometimes adopted is the introduction of long-wave infra-red reflective insulations, which are generally applied below the roof tiles, facing an air cavity. This strategy allows the thermal resistance of a roof to be increased, as a result of a reduction in the radiation heat exchange, without having to increase the insulation thickness.In this study, an in-field experimental campaign has been carried out in Turin, Italy. The analysis was aimed at investigating the thermal behaviour of traditional roof adopting reflective insulation, made up of aluminium sheets, and of a new strategy, based on a treatment with low emissivity paint below the roof tiles.The experimental data were used to verify numerical 1D heat transfer simulations to account for the effect of the radiation heat exchange in the airspace below the roof tiles. The verified simulation models were used to analyse the effects of the insulation levels and of the emissivity properties of the reflective layer on the roof heat exchange.A reduction in the indoor summer heat gains of between ∼10% and ∼53% was found for roofs treated with refective insulation, depending on their emissivity properties, while the contribution to reducing the winter heat losses was negligible.The obtained results are encouraging and highlight that reflective insulation, used together with Low-E paints, represents a promising solution that can be used to help design attic retrofitting interventions, with the aim of achieving a better thermal performance in the summer period, and to address the requirements of indoor space saving.

Friction and radiative heat exchange in a system of two parallel plates moving sideways: Levin–Polevoi–Rytov theory revisited

It is shown that the fundamental results obtained in the works by Levin et al. (1980) and Polevoi (1990), based on the fluctuation-electromagnetic theory by Levin and Rytov, adequately describe the rate of radiative heat exchange and the frictional force in a system of two parallel thick plates in relative lateral motion. A numerically calculated friction force for Drude metals and thin gaps turns out to be by a factor 107 higher than earlier obtained by Polevoi and increases with decreasing temperature (increasing relaxation time of electrons).

Challenges in Modeling Turbulent Heat Fluxes to Snowpacks in Forest Clearings

Abstract Forest clearings are common features of evergreen forests and produce snowpack accumulation and melt differing from that in adjacent forests and open terrain. This study has investigated the challenges in specifying the turbulent fluxes of sensible and latent heat to snowpacks in forest clearings. The snowpack in two forest clearings in the Canadian Rockies was simulated using a one-dimensional (1D) snowpack model. A trade-off was found between optimizing against measured snow surface temperature or snowmelt when choosing how to specify the turbulent fluxes. Schemes using the Monin–Obukhov similarity theory tended to produce negatively biased surface temperature, while schemes that enhanced turbulent fluxes, to reduce the surface temperature bias, resulted in too much melt. Uncertainty estimates from Monte Carlo experiments showed that no realistic parameter set could successfully remove biases in both surface temperature and melt. A simple scheme that excludes atmospheric stability correction was required to successfully simulate surface temperature under low wind speed conditions. Nonturbulent advective fluxes and/or nonlocal sources of turbulence are thought to account for the maintenance of heat exchange in low-wind conditions. The simulation of snowmelt was improved by allowing enhanced latent heat fluxes during low-wind conditions. Caution is warranted when snowpack models are optimized on surface temperature, as model tuning may compensate for deficiencies in conceptual and numerical models of radiative, conductive, and turbulent heat exchange at the snow surface and within the snowpack. Such model tuning could have large impacts on the melt rate and timing of the snow-free transition in simulations of forest clearings within hydrological and meteorological models.

Influence of gray particle assumption on the predictive accuracy of gas property approximations

In this study, influence of gray particle assumption on the predictive accuracy of gas property models is investigated for conditions typically encountered in industrial coal-fired furnaces. The aim is (i) to identify how the share of gas radiation is influenced by the presence of particles and particle properties and (ii) to determine the effect of gray particle assumption on the predictive accuracy of gas property approximations. For that purpose, predictive accuracy of a simple gas property model is benchmarked against that of Spectral Line-Based Weighted Sum of Grey Gases Model (SLW) in the presence of gray/non-gray particles with different ash compositions, particle loads and boundary conditions. Input data required for the radiation code and its validation are provided from two combustion tests previously carried out in a 300 kWt Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) test rig burning low calorific value Turkish lignite with high volatile matter/fixed carbon (VM/FC) ratio in its own ash. Comparisons reveal that gray particle assumption leads to over-estimation of particle radiation, which leads to under-estimation of gas radiation share in total radiative heat exchange. This under-estimation is found to be reflected on the predictive accuracy of gas property models, that is, a simple gas property model can be found to be “accurate” if particles are assumed to be gray although that is not the case. Furthermore, share of particle radiation in total radiative heat exchange is demonstrated to be strongly dependent on the spectral nature of particle properties. The results show that accurate gas property models such as SLW are needed to represent the spectral behavior of combustion gases even at high particle loads.

Unventilated Air Layers with a Reflective Coating in the Building Envelope

Heat exchange through infrared radiation in air layers located inside building envelopes may be significantly modified in case of use of aluminium foil coatings therein. The intensity of conductive, convective and radiative heat exchange in these structures depends on the thickness of the air layer and the temperature difference on its surfaces. Generally speaking, application of aluminium foil in air layers of a building envelope improves its thermal insulation capacity. However, assessment of efficiency of such a solution and determination of the thermal resistance value of a given structure is often incorrectly determined and assumed for design calculations. The article analyzes the instructions and principles of determination of thermal insulation capacity of unventilated air layers. Provisions of two standards have been compared with results of tests of air layer parameters. The effect of different factors on heat exchange and insulation capacity of air layers with reflective coatings has been considered and assessed.

New Aspects of the Fluctuation-Electromagnetic Theory of Friction and Radiative Heat Exchange of Levine–Polevoi–Rytov in a System of Two Parallel Plates

It has been shown that the fundamental results obtained in the works by Levine, Polevoi, and Rytov (1980) and Polevoi (1990) adequately describe the rate of radiative heat exchange and dissipative frictional force in a system of two parallel plates in relative lateral motion. Within the framework of the Levine–Polevoi–Rytov theory, a new expression has been obtained for the force of fluctuation-electromagnetic friction of two plates in the terms of local dielectric permittivities and magnetic permeabilities of the plate materials.

Heat transfer in syntactyc carbon porous materials

Heat transfer in the syntactic carbon porous materials is considered. A number of assumptions regarding the microstructure of these materials and the corresponding mechanisms of heat transfer are made. Relations to estimate the effective density, heat capacity and thermal conductivity of the materials taking into account the radiant heat exchange between the matrix elements are given. The behavior of these parameters depending on the structure, properties, and components of syntactic porous material is analyzed.

Modeling Landscape Change Effects on Stream Temperature Using the Soil and Water Assessment Tool

Stream temperature is one of the most important factors for regulating fish behavior and habitat. Therefore, models that seek to characterize stream temperatures, and predict their changes due to landscape and climatic changes, are extremely important. In this study, we extend a mechanistic stream temperature model within the Soil and Water Assessment Tool (SWAT) by explicitly incorporating radiative flux components to more realistically account for radiative heat exchange. The extended stream temperature model is particularly useful for simulating the impacts of landscape and land use change on stream temperatures using SWAT. The extended model is tested for the Marys River, a western tributary of the Willamette River in Oregon. The results are compared with observed stream temperatures, as well as previous model estimates (without radiative components), for different spatial locations within the Marys River watershed. The results show that the radiative stream temperature model is able to simulate increased stream temperatures in agricultural sub-basins compared with forested sub-basins, reflecting observed data. However, the effect is overestimated, and more noise is generated in the radiative model due to the inclusion of highly variable radiative forcing components. The model works at a daily time step, and further research should investigate modeling at hourly timesteps to further improve the temporal resolution of the model. In addition, other watersheds should be tested to improve and validate the model in different climates, landscapes, and land use regimes.

Dynamics of magnetic flux tubes and IR-variability of young stellar objects

We simulate the dynamics of slender magnetic flux tubes (MFTs) in the accretion disks of T Tauri stars. The dynamical equations of our model take into account aerodynamic and turbulent drag forces, and the radiative heat exchange between the MFT and ambient gas. The structure of the disk is calculated with the help of our MHD model of the accretion disks. We consider the MFTs formed at distances of 0.027−0.8 au from the star with various initial radii and plasma betas β0. The simulations show that MFTs with a weak magnetic field (β0 = 10) rise slowly with speeds less than the speed of sound. MFTs with β0 = 1 form an outflowing magnetized corona above the disk. Strongly magnetized MFTs (β0 = 0.1) can cause outflows with velocities 20 – 50 km s−1. The tubes rise periodically over times from several days to several months according to our simulations. We propose that periodically rising MFTs can absorb stellar radiation and contribute to the IR-variability of young stellar objects.

Photonic structures improve radiative heat exchange of Rosalia alpina (Coleoptera: Cerambycidae)

The insect cuticle serves a multitude of purposes, including: mechanical and thermal protection, water-repelling, acoustic signal absorption and coloration. The influence of cuticular structures on infrared radiation exchange and thermal balance is still largely unexplored. Here we report on the micro- and nanostructured setae covering the elytra of the longicorn beetle Rosalia alpina (Linnaeus, 1758) (Coleoptera: Cerambycidae) that help the insect to survive in hot, summer environments. In the visible part of the spectrum, scale-like setae, covering the black patches of the elytra, efficiently absorb light due to the radiation trap effect. In the infrared part of the spectrum, setae of the whole elytra significantly contribute to the radiative heat exchange. From the biological point of view, insect elytra facilitate camouflage, enable rapid heating to the optimum body temperature and prevent overheating by emitting excess thermal energy.

Radiative Heat Shuttling.

We demonstrate the existence of a shuttling effect for the radiative heat flux exchanged between two bodies separated by a vacuum gap when the chemical potential of photons or the temperature difference is modulated. We show that this modulation typically gives rise to a supplementary flux which superimposes to the flux produced by the mean gradient, enhancing the heat exchange. When the system displays a negative differential thermal resistance, however, the radiative shuttling contributes to insulate the two bodies from each other. These results pave the way for a novel strategy for an active management of radiative heat exchanges in nonequilibrium systems.

The Effect of Radiative Cooling on Reducing the Temperature of Greenhouses.

Currently, greenhouses are widely used for the cultivation of various crops. However, in tropical and subtropical regions, undesired near-infrared radiation (NIR) causes heat loads inside the greenhouse. Recent works have demonstrated that radiative cooling, releasing energy via radiative heat exchange where the heat is dumped directly into outer space, can be achieved by using silica particles designed to emit in the infrared atmospheric transparency window. The purpose of this study is to improve the plastic greenhouse cladding to regulate the temperature inside the greenhouse, mainly by passive cooling. Low-density-polyethylene (LDPE)-based formulations with anti-fogging agent, UV stabilizer, and silica particles were prepared by the melt blending technique and were formed into a double film by extrusion molding. Experimental results showed that under 35 °C ambient conditions, the inner temperature of the simulated greenhouse with the newly developed cladding was 4 to 5 °C less than that of the greenhouse with the commercial agricultural polyethylene (PE) film.