Entransy flux of thermal radiation and its application to enclosures with opaque surfaces

Entransy is a new concept developed in recent years to measure the transport ability of heat at a temperature in conduction and convection. This paper develops the concept of entransy flux for thermal radiation in enclosures with opaque surfaces. The entransy balance equation and entransy dissipation function are derived. The minimum principle of radiative entransy loss is developed. The potentials and the heat fluxes distribution which meet the Stefan–Boltzmann’s law and the energy balance equation would make the radiative entransy loss minimum if the net heat flux of each surface or the thermal potentials of the surfaces are given. The extremum entransy dissipation principles (EEDP) for thermal radiation are developed. The minimum radiative entransy dissipation leads to the minimum average radiative thermal potential difference for prescribed total heat exchange and the maximum radiative entransy dissipation leads to the maximum heat exchange for prescribed average radiative thermal potential difference. The minimum and maximum principle can be concluded into the minimum thermal resistance principle (MTRP) for thermal radiation by defining the thermal resistance with the entransy dissipation. The EEDP or MTRP is proved to be reliable when they are used to optimize some radiative heat transfer problems, and a comparison is made between the minimum principle of entropy generation and the EEDP.     

Modeling of thermal radiation in a participating media with conjugate heat transfer

Modeling a combination of thermal radiation and conjugate heat transfer in a three-dimensional rectangular domain which has a participating media CO₂ flowing through is done numerically in OpenFOAM. The rectangular duct has a vertical step (facing forward to the inlet) which is located at a distance from the inlet (the distance is same as the height of the inlet section). The domain is divided into two regions (namely solid and fluid). Carbon dioxide, a highly absorbing fluid with extinction, is used here as the participating medium. The ability of the code is verified to analyze the thermal radiation in a participating media with conjugate heat transfer. The study was carried out for a constant Reynolds number 250 and a contraction ratio of 0.5. The study focused primarily on the importance of adding thermal radiation on to thermal analysis and the reason behind the Nusselt number variation on different regions of solid–fluid interface. It also discussed the effect of radiative properties, such as optical thickness and linear scattering albedo, on the average convective Nusselt Number.     

Modeling of laminar flame propagation through organic dust cloud with thermal radiation effect

In this research, a mathematical model is performed to analyze the structure of flame propagation through a two-phase mixture consisting of organic fuel particles and air. In contrast to previous analytical studies, thermal radiation effect is taken into consideration, which has not been attempted before. In order to simulate of the dust combustion phenomenon, it is assumed that the flame structure consists of four zones: preheat, vaporization, reaction and post flame (burned). Furthermore, radiative heat transfer equation is employed to describe the thermal radiation exchanged between these zones. The obtained results show that the induced thermal radiation from flame interface into the preheat and vaporization zones plays a significant role in the improvement of vaporization process and burning velocity of organic dust mixture, compared with the case in which the thermal radiation factor is neglected. According to present results, flame structure variables such as the burning velocity, mixture temperature, mass fraction of volatile fuel particles and gaseous fuel mass fraction strongly depend on radiative heat transfer. These predictions have reasonable agreement with published experimental data.     

Modeling the behavior of selenium in Pulverized-Coal Combustion systems

The behavior of Se during coal combustion is different from other trace metals because of the high degree of vaporization and high vapor pressures of the oxide (SeO₂) in coal flue gas. In a coal-fired boiler, these gaseous oxides are absorbed on the fly ash surface in the convective section by a chemical reaction. The composition of the fly ash (and of the parent coal) as well as the time-temperature history in the boiler therefore influences the formation of selenium compounds on the surface of the fly ash. A model was created for interactions between selenium and fly ash post-combustion. The reaction mechanism assumed that iron reacts with selenium at temperatures above 1200 °C and that calcium reacts with selenium at temperatures less than 800 °C. The model also included competing reactions of SO₂ with calcium and iron in the ash. Predicted selenium distributions in fly ash (concentration versus particle size) were compared against measurements from pilot-scale experiments for combustion of six coals, four bituminous and two low-rank coals. The model predicted the selenium distribution in the fly ash from the pilot-scale experiments reasonably well for six coals of different compositions.     

Modeling and analysis of the dynamic behavior of hot water systems with tanks in series

A generalized model for hot water systems with tanks in series is presented in which the loss of heat through an insulation lagging is considered, and the variation of the ambient temperature is taken into account. Using a Laplace transformation, an exact solution is presented which, under certain conditions, reduces to an approximate solution. The conditions for convergence to the approximate solution are discussed, and figures are presented comparing it with the exact solution for several different sets of conditions.     

辐射传热时有限热源多级连续卡诺热机系统的最大输出功

对高低温热源均为有限热源、工质与高温热源间辐射传热、工质与低温热源间线性传热的多级连续卡诺热机系统最大输出功的最优温度曲线进行了研究,利用最优控制理论和"伪线性传热"模型得出了其最优温度曲线为驱动流体温度随流速和过程时间成单调递减变化,求出了其最大输出功,并与低温热源为无限热源下的结果进行了比较.     

Modeling of NOx formation in diesel engines using finite-rate chemical kinetics

A fast, physics-based model to predict the temporal evolution of NO_x in diesel engines is investigated using finite-rate chemical kinetics. The temporal variation of temperature required for the computation of the reaction rate constants is obtained from the solution of the energy equation. NO_x formation is modeled by using a six step mechanism with eight species instead of the traditional equilibrium calculations based on the Zeldovich mechanism. Fuel combustion chemistry is modeled by a single-step global reaction. Effects of various stages of combustion on NO_x formation is included using a phenomenological burning rate model. The effects of composition and temperature on the thermophysical properties of the working fluid are included in the computations. Comparison with measured single-cylinder engine-out NO shows good agreement with experimental data. The validated model is then used to demonstrate the impact of various operating parameters such as injection timing and EGR on engine-out NO_x. This fast, robust model has potential applications in model-based real-time control strategies seeking to reduce feed gas NO_x emissions from diesel engines.     

Effect of storage thermal behavior in seasonal storage solar heating systems

An analytical methodology has been developed to investigate the effects of storage operational strategies (or, equivalently, stratification) on the performance of a seasonal storage solar heating system with a water storage. The method is based on a relative comparison between a thermally stratified and well-mixed storage system representing probable extreme outcomes of the subsystem-to-storage loop control strategies. The effects are incorporated into a set of performance reduction factors that describe maximum changes in the solar collector yield, storage losses and solar fraction due to storage operational mode. The study indicates that the storage thermal behavior could in the worst case affect the yearly solar fraction by a factor of 2, but most likely a maximum value from 1.35 to 1.6 could be expected.     

Modeling of coupled spectral radiation,thermal and carrier transport in a silicon photovoltaic cell

Photovoltaic conversion efficiency of a crystalline silicon cell is investigated as a function of its temperature and taking into account complete thermal and irradiation operating conditions. The spectral radiative transfer problem is solved through a gray per band approach and a separated treatment of the collimated and diffuse components of radiation fluxes. The heat transfer modeling includes local heat sources due to radiation absorption and thermal emission, non-radiative recombinations and excess power release of photogenerated carriers. Continuity equations for minority carriers are solved to provide the current–voltage characteristic. A detailed analysis of the electrical and thermal behaviors demonstrates that proper adjustment and control of both thermal and surroundings radiative operating conditions are likely to provide guidelines for the improvement of photovoltaic cell performances.     

Effect of sun radiation on the thermal behavior of distribution transformer

Performance and life of oil-immersed distribution transformers are strongly dependent on the oil temperature. Transformers, working in regions with high temperature and high solar radiation, usually suffer from excessive heat in summers which results in their early failures. In this paper, the effect of sun radiation on the transformer was investigated by using experimental and analytical methods. Transformer oil temperature was measured in two different modes, with and without sun shield. Effects of different parameters such as direct and indirect solar radiation on the thermal behavior of the transformer were mathematically modeled and the results were compared with experimental findings. Agreements between the experimental and numerical results show that the model can reasonably predict thermal behavior of the transformer. It was found that a sun shield has an important effect on the oil temperature reduction in summer which could be as high as 7 °C depending on the load ratio. The amount of temperature reduction by sun shield reduces as the load ratio of transformer increases. By installing a sun shield and reducing oil temperature, transformer life could be increased up to 24% in average.     

Modeling and simulation of wetted porous thermal barriers operating under high temperature or high heat flux

Porous media with high water content can be successfully used as thermal barriers to operate under high exposure temperatures and/or high heat fluxes. Modeling and simulation of such systems presents difficulties and challenges, which are pointed and worked out in this work. Liquid water and water vapor transfers are considered, including the capillary effects for the liquid phase, as well as the air transfer inside the porous medium. Heat transfer model includes conduction, radiation, enthalpy convection, sensible heating and phase change. A realistic model is considered at the exposed boundary in what concerns mass transfer: the outflow mass transfer is dictated by the water effusion and not by the convection transfer mechanism between the exposed surface and the environment. A set of numerical aspects is detailed, concerning both the numerical modeling and the solution of the discretization equations, which are crucial to obtain successful simulations. Some illustrative results are presented, showing the potential of the wetted porous media when used as thermal barriers, as well as the capabilities of the presented physical and numerical models to deal with such systems.     

Natural convective flow past a suddenly started vertical plate with uniform mass and heat flux in the presence of radiation,thermal diffusion

An attempt has been made to investigate the problem of a natural convective radiative flow past an impulsively moving vertical plate with uniform mass and heat flux in the existence of the thermal diffusion effect. The resulting governing equations are solved by the Laplace transform technique in closed form. Effects of radiation, Prandtl number, Soret number, Grashof number, modified Grashof number, and Schmidt number are studied on temperature field, concentration field, velocity field, plate temperature, plate concentration, skin friction, and are demonstrated through graphs. The present study reveals that an intensification of the thermal radiation effect causes a downfall in the fluid temperature, plate temperature, and skin friction, but a contradictory outcome is spotted for plate concentration. One of the significant findings of this study includes that the increasing thermo-diffusion effect hikes the concentration and frictional resistance of the field.     

Inverse Radiation Problem of Boundary Incident Radiation Heat Flux in Semitransparent Planar Slab with Semitransparent Boundaries

INTRODUCTI0NInverseradiati0nproblemshavedefinedasubjectofinterestf0rthepast3Oyears0nsoandthereex-istsac0nsiderablebody0fknowledgesurroundingthesubjectthathasbeenextensivelyreviewedinaseries0fpapersbyM.C.rmick[1-4].Theyarecon-cernedwiththedeterminati0noftheradiativepr0p-ertiesandthetemperaturedistributionsofmediaus-ingvari0ustypesofradiationmeasurements.Despitetherelativelylargeinterestexpressedininverseradia-tionproblems,mostoftheworkfocusedontheinverseestimati0noftemperaturedistributions…     

Chilled air production in cool–thermal discharge systems from ice melting under constant heat flux and melt removal

A new device of complete melt removal to produce the chilled air in cool–thermal discharge systems is operated with constant heat discharge flux from ice melting. Heat-transfer efficiency performance was considerably improved, compared with the device without melt removal, using the approximation solution of energy balances with integral boundary-layer analysis. The effect of the melt removal can effectively enhance the heat transfer rate, leading to improved performance. Numerical examples of inlet ambient air temperatures with the specified heat fluxes were illustrated and the results of the air mass velocity and the outlet chilled air temperature were also delineated.     

The effect of solar radiation on dynamic thermal performance of floor heating systems

This paper presents a numerical investigation of transient heat transfer in floor heating systems using a three-dimensional explicit finite difference model. The study focused on the influence of the cover layer and incident solar radiation on floor temperature distribution and on energy consumption. Complete and partial (area) carpets were considered as well as hardwood cover layers over concrete or gypcrete thermal storage. Experimental and simulation results for an outdoor testroom reveal that solar beam radiation can cause a local floor surface temperature in the illuminated area 8°C higher than that in the shaded area. Partial carpet cover further increases floor surface temperature differences up to 15°C when solar radiation is absorbed. Solar radiation stored in the floor thermal mass was found to reduce heating energy consumption significantly (30% or more). Increase of thermal mass thickness from 5 cm to 10 cm did not lead to higher energy savings with conventional proportional-integral control. Advanced control algorithms need to be developed to maximize energy savings while maintaining good thermal comfort.     

Nonlocal behavior in thermal lagging

Nonlocal behavior during thermal lagging is studied in this work to accommodate the effect of the thermomass (TM) of the dielectric lattices. Perfect correlations are established, with the lagging time in the TM model equivalent to the phase lag of the heat flux vector and the length parameter in the TM model equivalent to the correlating length describing the nonlocal response. Simultaneous existence of the nonlocal (in space) and lagging (in time) behaviors give rise to a new type of thermal waves, which can be many times faster than the Cattaneo–Vernotte waves. Nonlinear behaviors of the phase lag and the correlating length are considered in the general model to study their influences on suppressing the sharp wavefronts. Dominating parameters are extracted to characterize the nonlocal response with thermal lagging.     

Composite heat transfer with thermal radiation in non-grey medium part I: Interaction of radiation with conduction

The energy equation including thermal radiation is a non-linear high order integro-differential equation and the spectroscopic constants involved are usually complex functions of frequency. Accordingly, it is formidable to solve the equation rigorously. On this basis many investigators have introduced the assumption of the grey gas that spectroscopic constants are independent of wavelength. This assumption, however, might smear the essential feature of radiative heat transfer. Alternatively dividing a spectral band into parts of center and wings and estimating an appropriate effective absorption coefficient in each part, the opaque (Rosseland) approximation is applicable to the central part in a band and the transparent approximation to the part of wings. Such an analytical procedure reduces to simple treatment despite of taking into account of non-grey behaviour. The current study considers a simple interaction problem between conduction and radiation excluding the convection in the mediums. Numerical calculations are performed on carbon monoxide and carbon dioxide.     

Mirror booster systems with artificial radiation sources

A classification is proposed of mirror booster systems (MBS) with artificial radiation sources (MBSARS) covering practically all fields of their application. Examples of successfully operated MBSARS are presented for each application field, including solar simulators, engineering and test facilities, and testbeds.     

Interaction of solar radiation with plant systems

The interaction of solar radiation with plant systems is described in terms of a modified form of the Kubelka-Munk model for light transmission through scattering media. This model is used to predict upward and downward radiation flux densities in plant systems as a function of leaf optical properties, depth in the system, and reflectance of the soil surface. The model can be used for monochromatic radiation or wavebands of desired width. The results of the Kubelka-Munk model are incorporated into a set of simultaneous energy budget equations in order to predict the profile of leaf temperature within plant systems. Measurements testing the reliability of the above theoretical treatment are discussed.     

Stochastic modeling of finite-rate chemistry effects in hydrogen-air turbulent jet diffusion flames with helium dilution

Stochastic simulations of turbulent hydrogen-air jet diffusion flames at three different dilution rates with helium are implemented using the ‘one-dimensional turbulence’ (ODT) model. The approach is based on one-dimensional unsteady solution of boundary layer equations to represent molecular processes and a stochastic implementation of turbulent advection. The 1D scalar and streamwise momentum profiles represent radial profiles within the flames; while, the unsteady evolution of the solution is interpreted as a downstream evolution of the radial scalar and streamwise momentum profiles. Multiple realizations of jet simulations are used to compute conditional statistics of major species, NO, and temperature. The ODT computations are implemented with a five-step reduced mechanism for hydrogen combustion and an optically-thin radiation model. Computed conditional statistics of temperature, major and minor species are compared to the experimental data from a set of documented flames at Sandia National Labs. Reasonable qualitative and quantitative agreement between computed and measured statistics is found, including very good predictions of NO mean and RMS profiles. Both computation and experiment exhibit the role of dilution in enhancing finite-rate chemistry effects, which vary as a function of downstream distance and fuel dilution.