Evolution of weak discontinuities in a radiating magnetofluid

Summary The evolutionary behaviour of first and higher order weak discontinuities in a radiating magnetofluid is analysed using the singular surface theory. The fluid is assumed to be a neutral gas with infinite electrical conductivity and it is penetrated by a magnetic field normal to the propagation direction of the wave front. The effects of radiative heat transfer are treated by the use of a differential approximation for a grey gas including effects of radiative heat flux, pressure and energy density.     

Phase-function normalization for accurate analysis of ultrafast collimated radiative transfer

The scattering of radiation from collimated irradiation is accurately treated via normalization of phase function. This approach is applicable to any numerical method with directional discretization. In this study it is applied to the transient discrete-ordinates method for ultrafast collimated radiative transfer analysis in turbid media. A technique recently developed by the authors, which conserves a phase-function asymmetry factor as well as scattered energy for the Henyey-Greenstein phase function in steady-state diffuse radiative transfer analysis, is applied to the general Legendre scattering phase function in ultrafast collimated radiative transfer. Heat flux profiles in a model tissue cylinder are generated for various phase functions and compared to those generated when normalization of the collimated phase function is neglected. Energy deposition in the medium is also investigated. Lack of conservation of scattered energy and the asymmetry factor for the collimated scattering phase function causes overpredictions in both heat flux and energy deposition for highly anisotropic scattering media. In addition, a discussion is presented to clarify the time-dependent formulation of divergence of radiative heat flux.     

Radiative heat transfer in splash and dilution zones of the pressurized oxy-fuel combustion CFB considering particle-dependent scattering

Radiative heat transfer is dominant in splash and dilution zones of the oxy-fuel combustion pressurized circulating fluidized bed (PCFB). However, the particle radiation fields interact with each other due to the high particle concentration. Moreover, the particle concentration distribution, the flue gas components and pressure change drastically due to pressurized oxy-fuel combustion technology. In this paper, the radiative heat transfer in splash and dilution zones of the oxy-fuel combustion PCFB is investigated when particle-dependent scattering is considered. The results show that the maximum error of the particle radiation model is less than 1% in predicting the incident heat flux at the wall when particle-dependent scattering is considered. The particle concentration distribution has a significant impact on the radiative heat transfer in splash and dilution zones of oxy-fuel combustion PCFB. The simplified particle concentration distribution model cannot capture the zero-source term phenomenon in the core region and has a large error in the annular region. In addition, the mechanism and importance of the flue gas composition and pressure on the radiative heat transfer in splash and dilution zones of oxy-fuel combustion PCFB are analyzed.     

Analytical solution of radiative transfer in the coupled atmosphere-ocean system with a rough surface

Using the computationally efficient discrete-ordinate method, we present an analytical solution for radiative transfer in the coupled atmosphere-ocean system with a rough air-water interface. The theoretical formulations of the radiative transfer equation and solution are described. The effects of surface roughness on the radiation field in the atmosphere and ocean are studied and compared with satellite and surface measurements. The results show that ocean surface roughness has significant effects on the upwelling radiation in the atmosphere and the downwelling radiation in the ocean. As wind speed increases, the angular domain of sunglint broadens, the surface albedo decreases, and the transmission to the ocean increases. The downward radiance field in the upper ocean is highly anisotropic, but this anisotropy decreases rapidly as surface wind increases and as ocean depth increases. The effects of surface roughness on radiation also depend greatly on both wavelength and angle of incidence (i.e., solar elevation); these effects are significantly smaller throughout the spectrum at high Sun. The model-observation discrepancies may indicate that the Cox-Munk surface roughness model is not sufficient for high wind conditions.     

The reduction of computational time of the Monte Carlo method,applied to radiative heat transfer with variable properties,by using a fixed properties loop

The Monte Carlo method has the excellent feature of easy adaptation to problemens such as radiative heat transfer with variable properties, and problems of system of heat transfer including radiation, with complex geometries. However, this method has a deficiency that it requires a large computational time when the energy equation is non-linear. In this paper, a modified Monte Carlo method, named the DPEF method, is suggested to overcome this deficiency, by adding an iterative loop of fixed properties to the conventional method, without sacrificing the advantageous of the Monte Carlo method. An analytical example of this new method, as applied to a model of radiative heat transfer in a furnace with variable properties, is given. It is found that the computational time is reduced by half of that of the conventional Monte Carlo method, and moreover, the stability in iteration process is found to be improved.     

A coupled numerical analysis of shield temperatures, heat losses and residual gas pressures in an evacuated super-insulation using thermal and fluid networks: Part I: Stationary conditions

This paper describes numerical simulations, using thermal networks, of shield temperatures and radiative and conductive heat losses of a super-insulated cryogenic storage tank operating at 77 K. Interactions between radiation and conductive heat transfer modes in the shields are investigated, by calculation of local shield temperatures. As a new method, fluid networks are introduced for calculation of stationary residual gas pressure distribution in the evacuated multilayer super-insulation. Output from the fluid network is coupled to the iterative thermal network calculations. Parameter tests concern thickness and emissivity of shields, degree of perforation, residual gas sources like desorption from radiation shields, spacers and container walls, and permeation from the inner container to the evacuated insulation space. Variations of either a conductive (thickness of Al-film on Mylar) or a radiative parameter (thermal emissivity) exert crosswise influences on the radiative or conductive heat losses of the tank, respectively.     

Transient Coupled Heat Transfer in Multilayer Non-gray Semitransparent Media with Reflective Foils

In this paper, transient coupled radiative and conductive heat transfer in multilayer thermal insulation (MTI), which consists of non-gray semitransparent materials and reflective foils, is investigated for re-entry aerodynamic heating. The governing equation for combined radiation and conduction heat transfer in MTI is solved using a finite volume numerical method, whereas the radiative transfer equation (RTE) is solved using a finite difference method, in which the spectral scattering and absorption coefficients are determined using the Mie theory and the scattering phase function is modeled by the Delta-Eddington approximation. A comparison is carried out between numerical results for coupled heat transfer in non-gray MTI and those obtained by the two-flux model in which the medium is assumed to be gray. Finally, the numerical optimization of MTI is discussed.     

Regular solution of inverse optimal design problems for axisymmetric systems of radiative heat transfer

Inverse optimal design problems are considered for axisymmetric systems of heat transfer with only radiative heat transfer. In these problems, the geometry of thermal system is preassigned, and it is required to find the optimal distribution of temperature on the heater surface, which provides for the preassigned distribution of radiative heat flux (at a given temperature) on the surface being heated. Variational methods of regularization are employed for solving these problems, namely, Tikhonov’s regularization method and parametric regularization. Minimization problems are solved numerically by gradient methods. The conjugate problem method is used for calculating the gradient of discrepancy functional. Examples of solution of model problems are given.     

Radiative heat transfer in participating media — A review

This paper presents an overview of various exact analytic and approximate numerical methods for the solution of radiative heat transfer problems in participating media. Review of each method is followed by its strengths and limitations. Importance of radiative heat transfer analysis and difficulties in the solution of radiative transfer problems have been emphasized.     

Characterization of Thermal Radiative Properties of Nanofluids for Selective Absorption of Solar Radiation

Nanofluids are used in device cooling, heat pipes, and other applications. Researchers in thermal engineering have extensively investigated the thermal conductivity and enhanced heat transfer of nanofluids. This study investigates the thermal radiative properties of nanofluids and discusses the characteristics of the selective absorption of solar radiation of nanofluids. Several kinds of nanoparticles are prepared using the two-step method, and the effects of dispersants, mass fractions, and nanoparticle materials on the radiative properties of nanofluids within the wavelength range of 300 nm to 2500 nm are analyzed. Dispersants can reduce the transmittance of water by <5 % within the visible spectrum. ZnO- and AlN-water nanofluids selectively absorb solar radiation, whereas ZrC- and TiN-water nanofluids absorb most of the solar radiation applied to them. The findings of this study are beneficial to research on the application of nanofluids in solar energy utilization.     

Radiative transfer model: matrix operator method

A radiative transfer model, the matrix operator method, is discussed here. The matrix operator method is applied to a plane-parallel atmosphere within three spectral ranges: the visible, the infrared, and the microwave. For a homogeneous layer with spherical scattering, the radiative transfer equation can be solved analytically. The vertically inhomogeneous atmosphere can be subdivided into a set of homogeneous layers. The solution of the radiative transfer equation for the vertically inhomogeneous atmosphere is obtained recurrently from the analytical solutions for the subdivided layers. As an example for the application of the matrix operator method, the effects of the cirrus and the stratocumulus clouds on the net radiation at the surface and at the top of the atmosphere are investigated. The relationship between the polarization in the microwave range and the rain rates is also studied. Copies of the FORTRAN program and the documentation of the FORTRAN program on a diskette are available.     

Computation of hypersonic flow and radiation of viscous chemically reacting gas in a channel modeling a section of a scramjet

This work is devoted to a consideration of flow and combustion of a hydrogen-air mixture in a channel modeling a section of a supersonic combustion ramjet (scramjet). Fields of concentrations, pressure, and temperature are obtained. Based on them, the thermal radiation of gas within a scramjet combustor is computed. The density of the radiative heat flux to the chamber wall is computed by two methods, i.e., in a P1 approximation of the spherical harmonics method and in an approximation of the plane layer. It has been shown that the radiative heat flux contribute significantly to the total heating of the jet wall.     

The Clausius inequality corrected for heat transfer involving radiation

The objective of this paper is to prove that the Clausius inequality must be re-stated to have general applicability for heat transfer involving radiative fluxes. The integrand (đQ/T) of the Clausius expression applies to heat conduction and convection, but does not hold for most radiative transfer scenarios, with the exception of reversible infinitesimal net blackbody radiation transfer. In other cases involving radiative transfer, the equality holds for a cycle even though irreversible heat addition by radiative transfer occurs. This is without the erroneous presumption of entropy destruction anywhere in the cycle. Thus, the Clausius inequality indicates reversibility for a cycle that includes an irreversible process. Further, in some radiative cases the quantity đQ/T, where T is the boundary temperature, is not the entropy transfer at the system boundary, and in fact, primarily represents entropy production within the system. It is also clear that in another case considered, the quantity đQ/T had no physical meaning whatsoever. Consequently, the Clausius expression has been re-stated so that it is applicable to cycles with processes involving any form of heat transfer. A new integrand (đQ_cc/T + đS_Net,Rad) is presented, allowing the Clausius inequality to generally apply to all heat transfer scenarios. The work in this paper emphasizes the need to re-state other fundamental equations allowing applicability to all heat transfer processes, and draws attention to the unique character of radiative entropy calculations.     

Investigation of transient conduction–radiation heat transfer in a square cavity using combination of LBM and FVM

In this paper, the effect of surface radiation in a square cavity containing an absorbing, emitting and scattering medium with four heated boundaries is investigated, numerically. Lattice Boltzmann method (LBM) is used to solve the energy equation of a transient conduction–radiation heat transfer problem and the radiative heat transfer equation is solved using finite-volume method (FVM). In this work, two different heat flux boundary conditions are considered for the east wall: a uniform and a sinusoidally varying heat flux profile. The results show that as the value of conduction–radiation decreases, the dimensionless temperature in the medium increases. Also, it is clarified that, for an arbitrary value of the conduction–radiation parameter, the temperature decreases with decreasing scattering albedo. It is observed that when the boundaries reflect more, a higher temperature is achieved in the medium and on boundaries.     

Analysis of radiative heat-loss effects in thermal explosion of a gas using the integral manifold method

Semenov's classical model of thermal explosion in a combustible gas mixture is modified to include radiative (rather than conductive) heat-loss effects and gas-density changes. A geometrical asymptotic technique (the method of integral manifolds - MIM) is exploited to perform a qualitative analysis of the governing equations. The strength of this method lies in the compact, clear geometrical/analytical rendition and classification of all possible dynamical scenarios, in terms of the physico-chemical parameters of the system. It is found that there are two main dynamical regimes of the system: cooling regimes and fast explosive regimes. Peculiarities of these dynamical regimes are investigated and their dependence on physical system parameters is analyzed. A criterion for the occurrence of thermal explosion is disclosed. An estimate for the maximum mixture temperature is also derived analytically. It is found that, under certain operating conditions, the dynamics are such that the initial explosive stage of the process essentially behaves adiabatically before succumbing to the dominance of the radiative heat loss that brings the system down to the ambient temperature.     

Natural convection of a radiating fluid in a square enclosure with perfectly conducting end walls

Combined natural convection and radiation in an asymmetrically heated square enclosure is studied numerically with both adiabatic and perfectly conducting end walls. The momentum and energy equations are solved by a control volume based finite difference algorithm which is coupled with the discrete ordinates method for radiative heat transfer calculations. The changes in the flow patterns and temperature distributions due to the presence of radiation in an enclosure with conducting end walls are compared with those for the case of an enclosure with adiabatic end walls, and significant differences are noted. The flow field is stronger, and the heat input along the hot wall and the end walls are greater for the conducting end wall case. The effects of optical thickness, scattering and wall emissivity on the flow and temperature fields and heat transfer rates are analysed.     

Design and modeling of multiple layered TBC system with high reflectance

Ceramic thermal barrier coatings (TBCs) are playing an increasingly critical role in advanced gas turbine engines due to their ability to sustain further increases in operating temperatures. However, these increases in temperature could raise considerable issues associated with increased radiative heat transfer into the TBC systems. This study was conducted to design a ceramic based multiple layered TBC system with high reflectance to radiation. Mathematical modeling was used to calculate the potential temperature reduction on the substrate surface when the multiple layered TBC is applied. The result of the simulation shows that a temperature reduction up to 90 °C is possible when utilizing the designed multiple layered TBC coatings.     

氧化膜对耐热不锈钢试验喷管传热特性的影响

建立了耐热不锈钢试验喷管轴对称非稳态传热数值计算模型以及不锈钢在高水蒸气含量燃气中的氧 化动力学模型,研究了氧化膜对喷管传热特性的影响.考虑了燃气时壁面的辐射换热及材料热物性参数随温度的变化,分别计算了结构材料无氧化膜、氧化膜表面光滑、氧化膜表面粗糙情况下的内喷管温度分布,通过理论计算值验证了数值计算模型的合理性与可靠性.结果表明,耐热不锈钢表面氧化膜的低导热系数使喷管热阻增大,但较大的氧化膜表面粗糙度强化了传热,2种因素综合作用决定了氧化膜对喷管传热特性的影响,氧化膜起到了隔热作用,计算结果可为表面热应力腐蚀数值模拟提供数据支持.     

发动机试验室的通风（空调）热负荷计算

为了确定一种常见的水冷式柴油发动机试验室内的通风（空调）热负荷值,采用一种简化的等效体表面换热模型,用以计算出该发动机的对流和辐射换热负荷（简称“等效计算法”）;并对采用不同计算（或估算）方法得出的换热负荷值进行比较分析,推荐了在一定条件下发动机散热系数的经验取值范围,供此类暖通设计者参考。