Infrared radiative heat transfer in nongray nonisothermal gases

In the present paper, both nongray and nonisothermal behaviors of an infrared emittingabsorbing gas have been taken into account in radiative transfer analyses through the use of the nonisothermal band absorptance. Consideration is given specifically to a simple system consisting of a radiating medium bounded by two infinite parallel black surfaces of different temperatures. Solutions are presented for the cases of radiative equilibrium and combined conduction and radiation. Results based on different methods of evaluating the nonisothermal band absorptance are also compared among themselves. Differences in several fundamental features are exhibited in the nongray nonisothermal solutions as compared to those with nongray but isothermal properties.     

Combined nongray radiative and conductive heat transfer in multiple glazing taking into account specular reflection and absorption

The problem of combined nongray radiative and conductive heat transfer in multiple glazing subjected to solar irradiation is analyzed. A spectral solar model proposed by Bird and Riordan is used to calculate direct and diffuse solar irradiance. The radiation element method by ray emission model, REM², is used to analyze the spectral dependence of radiative heat transfer. Specular reflection at boundary surfaces is taken into account. The spectral dependence of radiation properties of glass such as specular reflectivity, refraction angle, and absorption coefficient is taken into account. The steady‐state temperature and heat flux distributions in the glass layer are obtained and the insulating efficiency of multiple glazing is examined. The overall heat transfer coefficients predicted by the present method are compared with those based on the JIS method. The values obtained by the present method are slightly lower than those obtained by the JIS method. To investigate the spectral variation of radiative heat flux attenuated in the glass layer, the spectral heat flux at the room‐side surface and incident radiation are compared. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 712–726, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10125     

Spectral Monte Carlo models for nongray radiation analyses in inhomogeneous participating media

A spectral line-by-line (LBL) method is developed for photon Monte Carlo simulations of radiation in participating media. The performance of the proposed method is compared with that of the stochastic full-spectrum k-distribution (FSK) method in both homogeneous and inhomogeneous media, and in both traditional continuum media and media represented by stochastic particle fields, which are frequently encountered in combustion simulations. By using random-number relations, the LBL method does its own spectral reordering, in effect similar to the FSK reordering. Both LBL and FSK methods result in the same level of statistical errors for similar numbers of photon bundles. It is shown that the FSK approach is superior to the LBL approach in both memory and CPU efficiency in all test cases. However, the CPU efficiency is not prominent when spectral calculations are not the dominant source of overall CPU-time cost. Due to a lack of correlation of absorption coefficients in inhomogeneous media, the FSK approach may produce substantial errors, which are avoided by the LBL method. The LBL database can be constructed once and for all, while the FSK database has to be reconstructed every time the reference state changes. Thus, the LBL method is more suitable for quickly-evolving media.     

气体光谱数据库对气体辐射特性的影响

利用最新光谱数据库提出了基于数据库的普朗克平均吸收系数计算方法和逐线法,并针对一维平行平板间等温辐射传热问题探讨了逐线法(line-by-line,LBL)、统计窄谱带模型(statistical narrow-band model,SNB)和统计窄谱带关联K模型(statistical narrow-band correlated-K model,SNBCK)计算原理、计算精度和三模型间的偏离变化。结果表明,三种模型的结果吻合较好,几个数据库都比较准确。对于逐线法,表明可采用HITRAN2012(high resolution TRANsmission spectroscopic database)光谱数据库替代HITEMP2010(high-TEMPerature spectroscopic absorption parameters spectroscopic database)数据库来提高计算效率。     

Growth and evaporation of aerosol particles in the presence of adsorbable gases

Influence of adsorption of noncondensable molecules on the surface of phase transition on the rate of growth (evaporation) of aerosol particles (drops) is investigated theoretically. Combined manifestation of the effects that influence the rate of the growth of particles in the opposite directions is considered: decrease in surface tension on adsorption of a foreign gas and blocking of the surface of phase transition by the molecules adsorbed. The effect of the adsorption of the molecules of a noncondensable (in particular, buffer) gas on the rate of homogeneous nucleation and also on the growth of particles in chemical deposition is discussed.     

Condensation of vapor in the presence of non-condensable gas in condensers

This paper presents a set of differential and algebraic equations that model heat and mass transfer in condensers in which a mixture of water vapor and non-condensable gas is cooled. The model has been used to predict the condensation rate, the bulk temperatures of the coolant and the gas–vapor mixture, and the surface temperatures of the condenser wall. The predicted results for counter flow tube condensers are compared with three sets of published experimental data for system in which air is the non-condensable gas. It is found that the predicted condensation rates and coolant bulk temperatures agree very well with all the three sets of experimental data, the predicted wall temperatures agree reasonably well with the experimental results, and the agreement between the predictions and the experimental results on the bulk temperature of the air–vapor mixture is excellent for one set of the experimental data, reasonable for the second set of experimental data, but poor for the third set of experimental data. It is suggested that the poor agreement between the predicted and measured bulk temperatures of the mixture for the third set of experimental data arises from the experimental errors. The results from this study show that when modeling vapor condensation in the presence of a non-condensable gas, a simple model for the mixture channel alone may not be sufficient since neither the temperature nor the heat flux at the wall can be assumed to be constant. The results also show that the wall temperature in the coolant channel can be quite high, and careful modeling of the heat transfer in the coolant channel is needed in order to achieve good agreement between the model predictions and the experimental results.     

Calcium carbonate scaling in a plate heat exchanger in the presence of particles

Scale formation of CaCO₃ in a plate heat exchanger is investigated in the presence of various types of added particles under isothermal conditions. The parameters examined include the degree of supersaturation, the type of particles, the flow velocity, the particle concentration and the direction of flow inside the heat exchanger. The key result of this work is the strong synergistic effect of fine aragonite particles on the deposition rate and the morphology of the deposits. On the contrary, the presence of fine titanium oxide and of relatively large calcite particles does not seem to affect the main scaling characteristics of CaCO₃.     

Assessment of the axisymmetric radiative heat transfer in a cylindrical enclosure with the finite volume method

The radiative heat transfer in an axisymmetric enclosure containing an absorbing, emitting, and scattering gray medium is investigated by using the finite volume method (FVM). Especially, formulations with the cylindrical base vectors are introduced and its characteristics is discussed by comparing with other solution methods in the finite volume category. By considering the three-dimensional procedure, the angular redistribution term, which appears in such curvilinear coordinates as axisymmetric and spherically symmetric ones, can be treated efficiently without any artifice usually introduced in the conventional discrete ordinates method (DOM). After a mathematical formulation and corresponding discretization equation for the radiative transfer equation (RTE) are derived, final discretization equation is introduced by using the directional weight, which is the key parameter in the FVM since it represents the inflow or outflow of radiant energy across the control volume faces depending on its sign. The present approach is then validated by comparing the present results with those of previous works. All the results presented in this work show that the present method is accurate and valuable for the analysis of cylindrically axisymmetric radiative heat transfer problems.     

Thermal flow of fluid with magnetic particles in the presence of magnetic field

A magnetic fluid is a stable colloidal solution composed of magnetic particles, surfactant, and a carrier fluid. Magnetic fluids have numerous applications. In this study, we used the computational fluid dynamics model to simulate the behavior of a magnetic fluid in a 2-D square under different conditions such as different positions and intensities of the magnetic source. The preliminary model was established and used in conjunction with experimental data obtained from a present study, in order to determine the influence of particle size and mass fraction on fluid behavior. Our results show that particles with a smaller size have better ability to dissipate heat, and a larger mass fraction would provide a stronger driving force which leads to the velocity and temperature profile. We anticipate that our model would be useful to develop newer applications of magnetic fluids and magnetic flow.     

Computational study of heat transfer in solar collectors with different radiative flux models

Two‐dimensional steady incompressible laminar Newtonian viscous convection‐radiative heat transfer in a rectangular solar collector geometry is considered. The ANSYS FLUENT finite volume code (version 17.2) is used to simulate the thermo‐fluid characteristics. Extensive details of computational methodology are given to provide engineers with a framework for simulating radiative‐convection in enclosures. Mesh‐independence tests and validation are conducted. The influence of aspect ratio, Prandtl number ( Pr), Rayleigh number ( Ra) and radiative flux model on temperature, isotherms, velocity, and pressure is evaluated and visualized in colour plots. In addition, local convective heat flux is computed, and solutions are compared with the MAC solver for various buoyancy effects achieving excellent agreement. The P1 model is shown to better predict the actual influence of solar radiative flux on thermal fluid behaviour compared with the limited Rosseland model. With increasing Ra, the hot zone emanating from the base of the collector is found to penetrate deeper into the collector and rises symmetrically dividing into two vortex regions with very high buoyancy effect. With increasing Pr there is a progressive incursion of the hot zone at the solar collector base higher into the solar collector space and simultaneously a greater asymmetric behaviour of the dual isothermal zones.     

Preparation and thermochromic property of tungsten-doped vanadium dioxide particles

Tungsten (W)-doped vanadium dioxide (VO₂) particles were successfully synthesized by hydrolysis of vanadyl sulfate mingled with a small amount of sodium tungstate dihydrate and subsequent calcination. XPS analyses demonstrated that the vanadium oxide was composed of stoichiometric VO₂, and the doped W content in VO₂ particles could be easily controlled by the concentration of W precursor. XRD patterns indicated that all the W-doped VO₂ particles were monoclinic crystals, and the W dopant almost had no influence on the crystal structure of VO₂ particles. DSC analyses displayed that the critical phase transition temperature of VO₂ particles could be reduced to room temperature or even lower by increasing the doped W fraction. UV–vis–NIR spectra showed that the W-doped VO₂ particles embedded acrylic coating had very good thermochromic performance.     

Effect of gas radiation on radiative heat transfer in urban street canyon model

In this paper, we describe the results of numerical simulation of radiative heat transfer between the human body and an urban street canyon (building walls, pavement, and the sky) in the presence of participating non‐gray gas mixtures consisting of H₂O and CO₂. The ambient temperature in typical summer conditions and the concentration of gas mixtures during summer in Tokyo were assumed. Further, the parallel infinite plane model and simple urban street canyon model were used. The results show that the participating gas significantly affects the infrared radiation field in an urban street canyon. The radiation flux emitted by the participating gas is approximately 35% of the total radiation flux incident on the human body surface. This causes a homogenization of the infrared radiation field surrounding the human body. Gas radiation plays an important role in the heat transfer between the human body and the environment under hot and humid summer conditions. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20258     

Coagulation kernel of particles in a turbulent gas flow

On the base of modern probability density functions approach turbulent coagulation of particles in gravitational field is investigated. Spectral presentation of second velocity moments of gas phase is used for calculation of intensity of particles relative chaotic motion. Closed diffusion equation for two-particle distribution in space is obtained. Boundary condition taking into account coefficients of new particle formation and momentum restitution during two particles collision is found. Formula for calculation of turbulent coagulation kernel of particles in gravity field is gain. Influence of cloud turbulence and turbulence in a pipe flow on intensity of droplets coagulation is studied. Strong effects of relative turbulent diffusion between droplets, droplets inertia and droplets gravitational settling on intensity of coagulation are found out. Connection between internal structure of turbulence type and coagulation rate is illustrated. Obtained results are right for polydisperse additions in turbulent flows.     

Determination of thermal radiative properties of packed-bed media containing a mixture of polydispersed particles

Thermal radiative characteristics of packed beds containing a mixture of polydispersed SiO₂, ZnO, and C particles are determined numerically by employing the Monte Carlo technique, which is validated with the experimentally measured overall transmittance. A radiative heat transfer model is formulated for a pseudo-continuum multi-component medium of Mie-scattering particles. Good agreement is achieved by incorporating approximate phase functions that reproduce the experimentally observed preference for forward scattering.     

Estimating the agricultural demand for natural gas and liquefied petroleum gas in the presence of measurement error in the data

The paper begins by discussing the importance of accurate estimates of the price elasticity of demand and some of the problems frequently encountered in obtaining these estimates. To these problems is added that associated with inaccuracy in the measurement of the dependent variable and one or more of the independent variables that affect the quantity demanded. Two diagnostics, i.e. the regression coefficient bounds and the bias correction factor, have been introduced to assess the effect that such measurement error has on the estimated coefficients of demand relationships. The use of these diagnostics will aid in assessing the integrity of the estimates obtained. In considering the demand for natural gas and the demand for liquefied petroleum gas by farmers in the USA, both the quantity demanded and the price data available for demand model estimation purposes contain measurement error. The regression coefficient bounds diagnostic was used to indicate a range over which the true price responsiveness of farmers to changes in energy prices lies. The results suggest that each 1% increase (decrease) in the price of energy will result in a decrease (increase) of between 0.41 and 0.17% in the quantity of natural gas demanded and a decrease (increase) of between 0.48 and 0.07% in the quantity of liquefied petroleum gas demanded. The bias correction factor was computed to evaluate the magnitude of the underestimation of the responsiveness of the quantity of natural gas and liquefied petroleum gas demanded to a change in the number of acres irrigated. For natural gas, the under-estimation was 26.5%, whereas, for liquefied petroleum gas, it was 9.5%.     

Thermal performance of an MHD radiative Oldroyd‐B nanofluid by utilizing generalized models for heat and mass fluxes in the presence of bioconvective gyrotactic microorganisms and variable thermal conductivity

The current paper analyzes the thermal and concentration attributes with the temperature‐dependent mass diffusion coefficient and thermal conductivity for the flow of an Oldroyd‐B nanoliquid over a stretchable configuration using the Buongiorno model under the application of boundary layers. The mechanisms of heat and mass transport are modeled by using the revised definitions of heat and mass fluxes. Mathematical expressions for the conservation laws are transformed into ordinary differential expressions by making the appropriate changes. The resulting complexly structured expressions are handled via an optimal homotopy procedure. The impact of influential variables on the desired solutions is plotted, tabulated, and discussed in detail. Comparative analysis of the thermal wall flux coefficient, concentration flux coefficient, density magnitude of the motile microorganisms, and reduced dimensionless stresses with already published research as a limiting case of this exploration is presented for the validity of the proposed scheme, and an excellent agreement is observed, which confirms the reliability of the homotopic solution.