Radiative Heat Exchange Research Articles

Effect of scale on the modeling of radiation heat transfer in packed pebble beds

Thermal radiation is important in high temperature packed pebble bed, which is still poorly understood. The present work is to analyze the effect of spatial scale in modeling thermal radiation of packed pebble beds. The long-range model (full integral scale), short-range model (partial integral scale) and microscopic models (sub-particle scale) are compared and analyzed with reference to existing correlations. In high temperature packed pebble beds, the long-range model takes into account all possible radiation between surrounding spheres, even those that are not direct Voronoi neighbors, whereas the short-range model considers only a portion nearby. It is found that when solid conductivity is much greater than the effective thermal conductivity of radiation (ks≫kr or Λ>10), the long-range model provides better results than the short-range model in predicting the radiative heat exchange. The short-range model overestimates solid conductivity at low temperatures (lower than 1215°C) when ks~O(kr) (or Λ<10) while underestimating radiative heat exchange. It therefore still provides predictions for total heat exchange that is in good agreement with experimental data in cases where the errors cancel out. Moreover, the short-range radiation model is more computationally efficient than the long-range model and microscopic model to compute view factor between particles of Voronoi neighbors.

Comparison of selective transmitters for solar thermal applications.

Solar thermal collectors are radiative heat exchangers. Their efficacy is dictated predominantly by their absorption of short wavelength solar radiation and, importantly, by their emission of long wavelength thermal radiation. In conventional collector designs, the receiver is coated with a selectively absorbing surface (Black Chrome, TiNOx, etc.), which serves both of these aims. As the leading commercial absorber, TiNOx consists of several thin, vapor deposited layers (of metals and ceramics) on a metal substrate. In this technology, the solar absorption to thermal emission ratio can exceed 20. If a solar system requires an analogous transparent component-one which transmits the full AM1.5 solar spectrum, but reflects long wavelength thermal emission-the technology is much less developed. Bespoke "heat mirrors" are available from optics suppliers at high cost, but the closest mass-produced commercial technology is low-e glass. Low-e glasses are designed for visible light transmission and, as such, they reflect up to 50% of available solar energy. To address this technical gap, this study investigated selected combinations of thin films that could be deposited to serve as transparent, selective solar covers. A comparative numerical analysis of feasible materials and configurations was investigated using a nondimensional metric termed the efficiency factor for selectivity (EFS). This metric is dependent on the operation temperature and solar concentration ratio of the system, so our analysis covered the practical range for these parameters. It was found that thin films of indium tin oxide (ITO) and ZnS-Ag-ZnS provided the highest EFS. Of these, ITO represents the more commercially viable solution for large-scale development. Based on these optimized designs, proof-of-concept ITO depositions were fabricated and compared to commercial depositions. Overall, this study presents a systematic guide for creating a new class of selective, transparent optics for solar thermal collectors.

STATIONARY HEAT EXCHANGE IN THE BLAST FURNACE SMELTING ZONE IRRIGATED WITH MELT

The problem of stationary heat exchange in the blast furnace smelting zone irrigated with melt, considering features of process of a fi ltration of pig-iron and slag through coke nozzle is formulated. It is shown, that in this zone there is a simultaneous heating by gas of streams of coke, pig-iron and slag, each of which is in a heat exchange condition among themselves. The processes of mutual heat exchange are carried out as convection and radiation. Convection provides heat interchange among gas and all the heated up materials, and also between slag and coke as slag fl owing down on coke partially coats its surface. Radiant heat exchange develops between coke, pig-iron and slag. Its intensity increases with the growth of temperature of heated up materials. It is fi xed, that mutual heat exchange among streams of coke, pig-iron and slag in an irrigated zone of a blast furnace makes considerable impact on formation of a temperature fi eld in this zone. The knowledge of thermal eff ects distribution of chemical reactions of direct restoration of iron, silicium and manganese are necessary for calculation of heat interchange in the fi eld of bosh and a blast-furnace well on height of an irrigated zone and a fraction of a surface of the coke, coated with slag.

A mathematical model of a solar air thermosyphon integrated with building envelope

A mathematical model comprising of analytical and two dimensional network procedures is proposed for predicting steady heat flow and heat transport in a solar heated thermosyphon integrated with building envelope. The detail mathematical solution method is presented for solving partial differential heat flow and transport equations by performing two dimensional energy balances at surface and air nodes through conjugate heat exchange and heat transport analysis of a solar thermosyphon. The mathematical solution procedure is devised for conduction and radiation heat exchange between surface nodes to improve the accuracy of traditional analytical solution for predicting buoyancy-induced mass flow rate of air flowing through a solar thermosyphon. The matrix inversion solution of mathematical model is unconditionally stable, with accuracy dependent on magnitude of conductance terms and number of nodes in the grid. Only Δy is chosen as aspect ratios (Δx = L, L/H and W/H) are defined by the geometry of the thermosyphon. The conduction and convection conductance terms are based on discretisation height Δy, thermal capacity conductance (mcp) is based on air-gap length Δx, whilst integrated radiation conductance terms are based on both height Δy and width Δx of the grid. The proposed model has compared the results obtained from analytical method, two dimensional network method for a single set of environmental condition with given geometry of thermosyphon. The proposed model is validated with experimental results obtained from outdoor experimental setup comprising of thermosyphon based photovoltaic solar wall system.

Influence of the gray gases number in the weighted sum of gray gases model on the radiative heat exchange calculation inside pulverized coal-fired furnaces

The influence of the number of gray gases in the weighted sum in the gray gases model on the calculation of the radiative heat transfer is discussed in the paper. A computer code which solved the set of equations of the mathematical model describing the reactive two-phase turbulent flow with radiative heat exchange and with thermal equilibrium between phases inside the pulverized coal-fired furnace was used. Gas-phase radiative properties were determined by the simple gray gas model and two combinations of the weighted sum of the gray gases models: one gray gas plus a clear gas and two gray gases plus a clear gas. Investigation was carried out for two values of the total extinction coefficient of the dispersed phase, for the clean furnace walls and furnace walls covered by an ash layer deposit, and for three levels of the approximation accuracy of the weighting coefficients. The influence of the number of gray gases was analyzed through the relative differences of the wall fluxes, wall temperatures, medium temperatures, and heat transfer rate through all furnace walls. The investigation showed that there were conditions of the numerical investigations for which the relative differences of the variables describing the radiative heat exchange decrease with the increase in the number of gray gases. The results of this investigation show that if the weighted sum of the gray gases model is used, the complexity of the computer code and calculation time can be reduced by optimizing the number of gray gases.

Influence of Radiation and Convective Types of Heat Exchange on the Necessary Duration of Temperature Measurement by Thermocouples

Results of research of influence of radiation heat exchange on duration of execution of measurement high темератур are given by thermocouples, influence of the free convection on results of temperature measurement is also considered by the thermocouples set in protective sleeves with a gap. Researches showed that radiation heat exchange leads to lowering of duration of heating of the thermocouple to 30% whereas the free convection in a protective sleeve practically doesn’t exert impact on process of heating up of the sensor.

Mock-up Study of the Effect of Wall distance on the Thermal Rating of Power Cables in Ventilated Tunnels

Power transmission through power cables installed in ventilated tunnels is increasingly used worldwide and offers a complex thermal environment. Established correlations currently in use have been deduced from a nonfully developed turbulent flow and the effect of the proximity to a tunnel wall is not accounted for. This paper details the experimental and numerical investigations conducted on a mock-up of the heat transfer from a single cable in a fully developed turbulent airflow, with emphasis on the effect of the cable spacing from the tunnel wall. Nusselt numbers have been compared for different spacing and Reynolds numbers. The overall heat transfer is found to be meaningfully lower than in previous studies as only a third of the heat transfer is found, possible explanation of these differences are given in this paper. A threshold value is observed for the heat transfer while the cable rating depreciation is more gradual. The radiative heat exchange is found to be up to 30% of the total heat loss. An improved cooling law for the case of a single cable in tunnel taking into account the proximity of the wall has been developed from this study and confronted to the experimental and simulated data.

Utilization of waste heat from rotary kiln for burning clinker in the cement plant

Cement subsector next to the glass industry is counted among one of the most energy-intensive industries, which absorbs approx. 12-15% of the total energy consumed by the industry. In the paper various methods of energy consumption reduction of in the cement industry are discussed. Cement production carries a very large emissions of greenhouse gases, where CO2 emissions on a global scale with the industry than approx. 5%. Great opportunity in CO2 emissions reduction in addition to the recovery of waste heat is also alternative fuels co-firing in cement kilns [1], [2]. In the cement sector interest in fitting-usable waste energy is growing in order to achieve high rates of savings and hence the financial benefits, as well as the environment ones [3]. In the process of cement production is lost irretrievably lot of energy and reduction of these losses on a global scale gives a visible saving of consumed fuel. The aim of this study is to investigate the possibility of waste heat use in Rudniki Cement Plant near to Czestochowa. After analyzing of all waste heat sources will be analyzed the heat emitted by radiation from the surface of the rotary kiln at the relevant facility. On the basis of thermal-flow calculations the most favorable radiative heat exchanger will be designed. The calculations based on available measurements provided by the cement plant, a thermal power of the heat exchanger, the heat exchange surface, the geometry of the heat exchanger, and other important parameters will be established. In addition the preliminary calculations of hydraulic losses and set directions for further work will be carried out. Direct benefits observed with the introduction of the broader heat recovery technology, is a significant increase in energy efficiency of the industrial process, which is reflected in the reduction of energy consumption and costs. Indirectly it leads to a reduction of pollution and energy consumption.

Cooling Performance Test and Analysis of a Radiant-convective Air Conditioning Terminal Device with Parallel Pipes

In the paper, an air-conditioning terminal device with parallel pipes was brought out and studied. A test rig was set up to measure the cooling performances of the device in various working conditions. Then the radiation heat transfer percentage of the device was calculated. And the comparison between the device and the radiant cooling ceiling, whose heat transfer also relies on radiation and natural convection, was conducted in respect of cooling capacity. The results showed that the proportion of the radiation heat exchange rate to the total heat exchange rate was around one third, and the cooling capacities per square meter of the device surface can reach as high as 143.8W/m2 when the inlet cooling media temperature was 8̊C and the flow rate kept as 240L/h. The total cooling capacity under this working condition of the device is 631.0W. And the cooling capacity per square meter of the device surface increases by 30% over the value of the radiant cooling ceilings at the same inlet cooling media temperature.

Modular reactor model for the solar thermochemical production of syngas incorporating counter-flow solid heat exchange

Recent progress in thermochemical reactor concepts, among them batch, particle and counter-rotating configurations, shows different approaches to heat recuperation and gas separation. An idealized physical analysis in search of best-efficiency potentials is required, that covers multiple degrees of freedom in configuration space from batch to quasi-continuous concepts.A modular and generic solar thermochemical reactor model is thus presented that describes two-step redox reactions of solid pieces of reactant moving in counter flow between reduction and oxidation chambers to produce CO and H2 from CO2 and H2O. Solid heat recuperation is implemented through radiation heat exchange between reduced and oxidized elements moving in opposite directions. The model can be adapted to a wide range of reactor concepts and is validated through a comparison of results with models presented in the recent literature.The model is demonstrated in an upper-bound performance analysis with an example of a ceria system, where a heat exchanger efficiency of about 80% is reached. Using the minimum thermodynamic energy for the vacuum pump and gas separation, the physical potential of cycle efficiency at the modeled temperatures at a reduction oxygen partial pressure of 10−3atm is 22% and may be increased to 33% at a partial pressure of 10−5atm. With assumptions on practical energy requirements the efficiency is reduced to 16% at 10−3atm. In order to quantify the dependence of the heat exchanger efficiency on internal heat transfer characteristics, heat propagation by radiation and conduction through the reactive material and insulation is modeled in an example case of a porous medium.This model provides the basis for future analyses of different reactor concepts including parameter studies of the number of heat exchanger chambers, the residence time and reaction temperatures.

Lightweight Green Roofs’ Thermal Response under Freezing Conditions

This paper presents thermal response study of lightweight extensive green roofs with lightweight mineral wool growing media in winter period. Green roof heat and mass transfer model was developed based on short- and long-wave radiation heat exchange, surface convection and vegetation evapotranspiration. The model also considers latent heat accumulation in lightweight mineral wool growing media using effective heat capacity method. Experimental results as well as results of numerical simulations proved that in the winter heat loses of green roofs are smaller compared to the reference roof construction of the same composition and the same U-value. Heat stored in the green roof growing media in the form of latent heat, by water freezing or melting, additionally reduces the daily peak heat flux as well as daily heat losses. For continental climate conditions the effect of latent heat accumulation is larger in colder days and during night time conditions resulting in significant decrease of building peak heat demand.

Influence of the Geometric Parameter on the Regimes of Natural Convection and Thermal Surface Radiation in a Closed Parallelepiped

We have performed a numerical analysis of the stationary regimes of thermogravitational convection and thermal surface radiation in a closed differentially heated parallelepiped. The mathematical model formulated in dimensionless natural velocity–pressure–temperature variables was realized numerically in the control volume approach. Analysis of the radiative heat exchange was carried out on the basis of the surface radiation approach with the use of the balance method in the Polyak variant. We have obtained three-dimensional temperature and velocity fields, as well as dependences for the mean Nusselt number reflecting the influence of the geometric parameter, the Rayleigh number, and the reduced emissive factor of the walls on the flow structure and the heat transfer.

Cooling and energy saving potentials of shade trees and urban lawns in a desert city

The use of urban vegetation in cities is a common landscape planning strategy to alleviate the heat island effect as well as to enhance building energy efficiency. The presence of trees in street canyons can effectively reduce environmental temperature via radiative shading. However, resolving shade trees in urban land surface models presents a major challenge in numerical models, especially in predicting the radiative heat exchange in canyons. In this paper, we develop a new numerical framework by incorporating shade trees into an advanced single-layer urban canopy model. This novel numerical framework is applied to Phoenix metropolitan area to investigate the cooling effect of different urban vegetation types and their potentials in saving building energy. It is found that the cooling effect by shading from trees is more significant than that by evapotranspiration from lawns, leading to a considerable saving of cooling load. In addition, analysis of human thermal comfort shows that urban vegetation plays a crucial role in creating a comfortable living environment, especially for cities located in arid or semi-arid region.

Evenand odd-parity kinetic equations of particle transport. 3: Finite analytic scheme on tetrahedra

A finite analytic (not finite-difference) scheme is derived for the numerical solution of evenand odd-parity transport equations of neutral particles. This discrete algebraic equation is constructed by cross-linking analytic solutions of differential equations in tetrahedral cells. Such a scheme makes it possible to simulate the three-dimensional transport of neutrons and photons in heterogeneous absorbing, scattering, and multiplying media (simulate problems of nuclear reactors, radiation shielding, radiative heat exchange, and radiation gas dynamics) without restrictions on the optical depth of the cell (the product of the beam extinction coefficient by the cell chord) and on the value of the jump in the extinction coefficient under the particle transition from one cell to another. A change in the sign of the extinction coefficient is allowed. The scheme is combined with efficient iterative methods for solving deterministic transport problems in which the angular (direction-of-flight) variable is discretized using the discrete-ordinate (Sn) approximation.

General correlations among geometry, orientation and thermal performance of natural convective micro-finned heat sinks

The interest in micro-technologies has increased in the last decades, because of the low volumes and high performance granted by their application. Micro-fins can find application in several fields, such as power electronics, concentrating photovoltaics and LED. Although micro-technologies have been widely applied in cooling, there is still a lack of knowledge on the thermal behavior of micro-finned heat sinks under natural convective conditions. In the present study, the correspondences between fin geometries and heat transfer coefficients, as well as the effects of the orientation, are experimentally investigated using silicon micro-finned heat sinks with different geometries. The heat sinks are made of 5cm×5cm squared silicon wafer and the fin height ranges between 0.6mm and 0.8mm, the spacing between 0.2mm and 0.8mm and the thickness between 0.2 and 0.8mm. Power loads higher than those considered in previous works are studied. The experimental setup is validated using a software simulation and the Nusselt number correlation available in literature. The influence of the fin thickness on this parameter is analyzed and a modified correlation is proposed. Also, the effect of the radiative heat exchange on the overall heat transfer is considered and commented. An analysis of the uncertainty is conducted and reported too.

Comparison of software models for energy savings from cool roofs

A web-based Roof Savings Calculator (RSC) has been deployed for the United States Department of Energy as an industry-consensus tool to help building owners, manufacturers, distributors, contractors and researchers easily run complex roof and attic simulations. RSC simulates multiple roof and attic technologies for side-by-side comparison including reflective roofs, different roof slopes, above sheathing ventilation, radiant barriers, low-emittance roof surfaces, duct location, duct leakage rates, multiple substrate types, and insulation levels. Annual simulations of hour-by-hour, whole-building performance are used to provide estimated annual energy and cost savings from reduced HVAC use.While RSC reported similar cooling savings to other simulation engines, heating penalty varied significantly. RSC results show reduced cool roofing cost-effectiveness, thus mitigating expected economic incentives for this countermeasure to the urban heat island effect. This paper consolidates comparison of RSC's projected energy savings to other simulation engines including DOE-2.1E, AtticSim, Micropas, and EnergyPlus. Also included are comparisons to previous simulation-based studies, analysis of RSC cooling savings and heating penalties, the role of radiative heat exchange in an attic assembly, and changes made for increased accuracy of the duct model. Radiant heat transfer and duct interaction not previously modeled is considered a major contributor to heating penalties.

Laws of Radiation From Large Gas Volumes and Claculation of Heat Transfer in Steam Boiler Boxes

The progress in the development of methods for calculating heat transfer in torch furnaces, fireboxes, and combustion chambers is analyzed. In the XIX-XX centuries calculations of radiative heat exchange were followed the methods based on the laws of radiation from black body, disclosed in the XVII-XX. It is substantiated that there is crisis of methods for calculating heat transfer in torch furnaces, fire boxes and combustion chambers. In the early XXI century the author of this manuscript disclosed the laws of radiation from torch gas volumes, described in the article.

Численно-аналитическое моделирование и идентификация нестационарного теплового процесса с учетом радиационного теплообмена

Численно-аналитическое моделирование и идентификация нестационарного теплового процесса с учетом радиационного теплообмена

Conditions for the use of infrared camera diagnostics in energy auditing of the objects exposed to open air space at isothermal sky

AbstractConvective and radiation heat transfer take place between various objects placed in open air space and their surroundings. These phenomena bring about heat losses from pipelines, building walls, roofs and other objects. One of the main tasks in energy auditing is the reduction of excessive heat losses. In the case of a low sky temperature, the radiation heat exchange is very intensive and the temperature of the top part of the horizontal pipelines or walls is lower than the temperature of their bottom parts. Quite often this temperature is also lower than the temperature of the surrounding atmospheric air. In the case of overhead heat pipelines placed in open air space, it is the ground and sky that constitute the surroundings. The aforementioned elements of surroundings usually have different values of temperature. Thus, these circumstances bring about difficulties during infrared inspections because only one ambient temperature which represents radiation of all surrounding elements must be known during the thermovision measurements. This work is aimed at the development of a method for determination of an equivalent ambient temperature representing the thermal radiation of the surrounding elements of the object under consideration placed in open air space, which could be applied at a fairly uniform temperature of the sky during the thermovision measurements as well as for the calculation of radiative heat losses.

High temperature dependence of thermal transport in graphene foam

In contrast to the decreased thermal property of carbon materials with temperature according to the Umklapp phonon scattering theory, highly porous free-standing graphene foam (GF) exhibits an abnormal characteristic that its thermal property increases with temperature above room temperature. In this work, the temperature dependence of thermal properties of free-standing GF is investigated by using the transient electro-thermal technique. Significant increase for thermal conductivity and thermal diffusivity from ∼0.3 to 1.5 W m−1 K−1 and ∼4 × 10−5 to ∼2 × 10−4 m2 s−1 respectively is observed with temperature from 310 K to 440 K for three GF samples. The quantitative analysis based on a physical model for porous media of Schuetz confirms that the thermal conductance across graphene contacts rather than the heat conductance inside graphene dominates thermal transport of our GFs. The thermal expansion effect at an elevated temperature makes the highly porous structure much tighter is responsible for the reduction in thermal contact resistance. Besides, the radiation heat exchange inside the pores of GFs improves the thermal transport at high temperatures. Since free-standing GF has great potential for being used as supercapacitor and battery electrode where the working temperature is always above room temperature, this finding is beneficial for thermal design of GF-based energy applications.