共查询到20条相似文献,搜索用时 15 毫秒
1.
In this paper we clarify directional characteristics of thermal radiation reflection on rough metal surfaces and establish a technique for determining the parameters for heat transfer computation of radiation energy exchange among surfaces. Directional distribution of bidirectional reflectance ρ of surfaces of root‐mean‐square roughness Σ = 0.1 to 1 µm to the irradiation of a visible laser of wavelength λ = 0.6328 µm and to that of an infrared laser of λ = 3.39 µm is investigated experimentally. The optical roughness (Σ/λ) ranges from 0.028 to 1.27. A measure of the magnitude of specular reflection is presented. A model for describing the ρ distribution is presented, and the experimental results of the ρ distribution are analyzed quantitatively and systematically to determine the values of the specular reflection component Rs and the perfect‐diffuse reflection component Rd of the hemispherical reflectance RH, which are input parameters for radiation heat transfer computation. © 2002 Scripta Technica, Heat Trans Asian Res, 31(2): 76–88, 2002; DOI 10.1002/htj.10008 相似文献
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The thermal and radiative environment of human life space is characterized strongly by solar radiation and by the radiation from surrounding walls as well as by air temperature and humidity. To consider the environmental radiation quantitatively, absorptance spectra and total emittance of the surfaces should be measured properly. In this work we measure spectra of normal incidence absorptance of such surfaces in a near‐ultraviolet through infrared region of wavelength of 0.30–11 μm. On the basis of measured spectra we evaluate solar absorptance of the surfaces to respond to the needs of radiation characteristics of the surfaces in the fields of architecture, life science, and solar engineering. Objective surfaces of the measurement are surfaces of indoor and outdoor environments: cloths, indoor wall materials, painted surfaces, ceramics, bricks, outdoor wall materials, road materials, leaves, etc. We also observe the transition of the absorptance spectra of a water–wet cement surface in a drying process. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20356 相似文献
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In recent literature, the analysis of a combined convective flow over a cone has received a lot of attention. To explore the convection effects of flow over a cone in greater detail, in this investigation, we have considered a cone with a rough surface, which is entirely a new flow problem. Recent studies have shown the influence of roughness on fluid flow over several geometries, but flow over a rough conical surface has not been studied so far. In addition, we have analyzed the effects of nanoparticles, magnetohydrodynamic (MHD), and suction/blowing, which could have significant impacts on characteristics of fluid flow over the cone with a rough surface. Initially, the governing equations, which are partial differential equations with a high degree of nonlinearity, are nondimensionalized through Mangler's transformations. Later, linear equations are obtained via the method of quasilinearization, which is then solved numerically through finite difference approximations. The roughness of the cone's surface has notable effects on fluid flow, that too away from the origin. In fact, the roughness increases the friction at the cone's surface. Furthermore, the magnetic field applied at the wall increases the surface friction. Thus, the combination of roughness and MHD helps delay the boundary layer separation. On the other hand, the suction reduces the temperature of the fluid and increases the energy transport strength, while the thermophoresis parameter exhibits the contrary nature. Therefore, the combined consideration of these two could enhance energy transport strength in several industrial applications. 相似文献
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Qunzhi Zhu Hyunjin Lee Zhuomin M. Zhang 《Frontiers of Energy and Power Engineering in China》2009,3(1):60-79
Radiative properties of rough surfaces, particulate media and porous materials are important in thermal engineering and many
other applications. These properties are often needed for calculating heat transfer between surfaces and volume elements in
participating media, as well as for accurate radiometric temperature measurements. In this paper, recent research on scattering
of thermal radiation by rough surfaces, fibrous insulation, soot, aerogel, biological materials, and polytetrafluoroethylene
(PTFE) is reviewed. Both theoretical modeling and experimental investigation are discussed. Rigorous solutions and approximation
methods for surface scattering and volume scattering are described. The approach of using measured surface roughness statistics
in Monte Carlo simulations to predict radiative properties of rough surfaces is emphasized. The effects of various parameters
on the radiative properties of particulate media and porous materials are summarized. 相似文献
5.
介绍了地面辐射供暖的发展概况、系统形式、构成特点及设计施工方法,通过工程实际提出目前地面辐射供暖的设计施工中存在的几个问题及改进方法,阐述了地面辐射供暖的优缺点和发展方向。 相似文献
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In this paper, it is proved that the intemal porous structure of alumina-silicate refractory fiber has fractal characteristics, which is reconstructed by the computer and the reconstructed structure further proved to have fractal characteristics. Based on the reconstructed structure, the network-thermal-resistance model is established to calculate the thermal conductivity of the fiber. It is shown that the calculated results agree well with the previous experimental ones, proving the correctness of the method. 相似文献
7.
Yuying LIU Xinxin ZHANG Department of Thermal Engineering School of Jet Propulsion Beijing University of Aeronautics Astronautics Beijing Department of Thermal Engineering School of Mechanical Engineering University of Science Technology Beijing Beijing 《热科学学报(英文版)》2005,14(4):368-373
A temperature measurement model of radiation thermometry for the surface covered by participating media was developed. The model was based on the radiation heat transfer of participating media and principles of radiation pyrometers, and solved by integral formulation of discrete ordinate method on spectral waveband. The influence of water mist on the indicated temperature of Raytek MR1SB one/two color pyrometer was discussed. Mie theory was used to calculate the radiative properties of water mist. In order to verify the model, a laboratory temperature measurement experiment was executed. The result shows that temperature of radiation thermometry is sensitive to the spectral response wavelength of pyrometer, and the simulated temperature of pyrometer agrees well with the experimental measurements on a suitable wavelength. The simulated temperature was lower than the real temperature of surface for one-color pyrometer, and it could be higher or lower than the real one for two-color pyrometer with the influence of participating media. 相似文献
8.
A theoretical study was conducted to investigate the wetting behavior of liquid meniscus on a vertical plate with three‐dimensional random characteristics of heterogeneity and roughness. The thermodynamic stable condition was derived by considering the minimum of system free energy. The local stable condition leads to a result similar to that obtained for a plate with two‐dimensional characteristics, that is, the system has many meta‐stable states. For the stable condition of the whole system, a relation was derived between the macroscopically observed contact angle and the surface characteristics. The product of cosine of the contact angle and liquid surface tension is equal to the energy difference for the liquid to wet the plate by apparent unit area. If the liquid wets the solid surface reversibly, there is only one contact angle observed macroscopically. This fact suggests that the contact angle hysteresis is caused by the irreversible motion when the liquid advances or recedes on the solid surface. The well‐known Cassie's and Wenzel's contact angles are explained as those corresponding to a thermodynamically stable condition when the liquid wets the solid reversibly. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 371–382, 2001 相似文献
9.
Transient thermal investigation of a fully wet porous convective–radiative rough cylindrical pin fin
The microelectromechanical systems technologies frequently produce rough surfaces, and the repercussion of roughness on the thermal performance is more prominent in structures of smaller dimensions. In this regard, the present article intends to examine the unsteady thermal behavior of a fully wet, porous, and rough micropin-fin structure under convective–radiative conditions. Here, a pin fin of a cylindrical profile has been chosen. The problem is modeled by incorporating the roughness parameters in the perimeter and cross-sectional area of the pin fin. Further, the study of the porous structure has been carried out by implementing the Darcy model. The resulting partial differential equation is nonlinear and of the second order which has been solved by employing the finite difference method. The impact of the roughness parameter, wet porous parameter, dimensionless time, convective parameter, base radius-to-length ratio, radiative parameter, thermal conductivity parameter, power index, and ambient temperature on the thermal performance and efficiency of rough micropin-fin structures has been established graphically. According to the findings, for rise in roughness, the rough micropin fin has more thermal drop rate and less efficiency than the smooth one. Further, the work is beneficial in the field of microelectronics, especially in the design of micropin-fin structures. 相似文献
10.
Machireddy Gari Gnaneswara Reddy Kondlalli Ganesh Kumar Sabir Ali Shehzad Tariq Javed Tehmina Ambreen 《亚洲传热研究》2019,48(6):2262-2275
This article considers the flow and heat transfer of a single and multi‐walled carbon nanotube over a sensor surface. For this persistence, a mathematical forming is established with the aspects of thermal radiation. In addition, the stimuli of magnetic properties and variable thermal conductivity are presented. By means of noteworthy conversions, nonlinear PDEs are altered into nonlinear ODEs and elucidated via a numerical approach in virtue of the Runge‐Kutta fourth order method scheme. The repercussion of countless variables of flow and energy transfer characteristics are portrayed and conferred. These upshots portray that the enhancement of heat is bounteous in a single‐wall nanotube when compared with multiwall nanotubes. Further, the velocity field is contracted for enhancing the values of . 相似文献
11.
We are presenting a note on comparative study of diffuse component of solar radiation on tilted surface with different angle of inclination. The monthly mean daily diffuse radiation have been calculated on inclined surfaces from the data of horizontal surface using the Circumsolar and Isotropic model and the anisotropic model of Klucher and Hay at Lucknow (Latitude 26.75°, Longitude 80.85°), Uttar Pradesh, India. These calculated results are compared with our measured value. The comparison shows that Klucher model who considered the effect of cloudy sky conditions gives comparatively good estimations particularly at low inclination angles. 相似文献
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It is necessary to know the spectral emissivity of a heater to ensure efficient heating. This paper proposes a useful method for spectral evaluation of radiation energy emitted from far infrared heaters. The measurement device consists of one radiometer and two band‐pass filters. In order to confirm the experimental results, numerical simulations were also carried out with the spectral emissivity of the heaters and the respective spectral sensitivities of the radiometers. The results obtained by these methods show that the far infrared heater radiates high amounts of energy in the far infrared region even at high temperature. It means far infrared heaters can transmit power effectively to materials which exhibit high emissivity in the far infrared region. This method can classify a type of far remote infrared heaters with the near infrared/far infrared energy ratio. By using this data, the authors also proposed a simple identification method of spectral emissivity of far infrared heaters. This inverse method can obtain a rough value of the spectral emissivity of commercial heaters while maintaining the heating conditions. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20396 相似文献
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The present work addresses the magnetohydrodynamic (MHD) flow and heat transfer over a permeable stretching sheet. Analysis has been carried out in the presence of thermal radiation and ohmic dissipation. The velocity and thermal slip effects are given main attention. The Rosseland approximation is used to describe the radiative heat flux in the energy equation. The non‐linear partial differential equations are reduced to a set of non‐linear ordinary differential equations which are solved analytically by the homotopy analysis method (HAM). The effects of emerging physical parameters on the velocity and temperature profiles are interpreted. Numerical data for skin friction coefficient and Nusselt number have been tabulated for various values of the parameters. The results have been compared with the known exact solution from the literature in a limiting sense. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20367 相似文献
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《International Journal of Hydrogen Energy》2019,44(48):26624-26636
The impact of roughness on nonlinear mixed convective nanofluid flow past a sphere is analysed in the presence of nonlinear density variations. This study is found to be innovative as it investigates the effects of nanoparticles, nonlinearity and surface roughness on mixed convective flow past a sphere with three diffusive components. The problem is modelled in the form of nonlinear partial differential equations that are dimensional in nature. This set of equations is transformed to dimensionless form by applying non-similar transformations. The technique of Quasilinearization is employed to linearize the transformed set of equations and then the implicit finite difference scheme is used for further simulation to get the required numerical solutions. The graphical presentation of numerical results exhibit that the friction, heat, mass and nanoparticles mass transfer rates at the surface of sphere increase along with the fluid's velocity due to the roughness of the surface, while the fluid's temperature reduces, significantly. The steep jump in the fluid's velocity near the wall is observed due to the surface roughness. The present analysis reveals that separation of boundary layer can be delayed with the proper selection of roughness and mixed convection parameters. Also, the third diffusing component, namely, liquid oxygen influences the fluid flow significantly. That is, the introduction of liquid oxygen diffusion into the liquid hydrogen diffusion diminishes the species concentration boundary layer, while it increases the corresponding mass transfer rate. 相似文献
16.
Spectral control of thermal radiation emitted from rectangular microcavities (0.5×0.5×0.5µm3) was investigated through emission experiments at high temperatures. The microcavities were fabricated on a mirror‐finished Ni metal surface. Through measurement of the normal spectral emittance, the maximum emittance was obtained around a wavelength of 0.87µm, which was very close to that (0.894µm) estimated from the cavity resonance theory. The emittance reached a maximum value of 0.95, and then decreased drastically with increasing wavelength, from the cut‐off wavelength. For a longer wavelength range from 1.7µm, it was equal to the emittance of the mirror‐finished surface. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20248 相似文献
17.
A numerical analysis has been done for opposing mixed convection resulting due to wall movement and buoyancy induced by a clockwise fluid motion in a differentially heated cavity. The effect of Prandtl number (Pr) and wall surface emissivity (?) has been investigated for different values of Richardson number. The net radiation method has been employed to simulate the effect of surface radiation. The energy equation along with its nonlinear boundary condition is treated with the Newton‐Raphson scheme. As momentum and energy equations are coupled with each other through their source terms, an iterative solution procedure is employed. A vorticity‐stream function formulation of the momentum equation has been adopted and solved by using an underrelaxation parameter of 0.45. The effect of Prandtl number with respect to the transformation of a multi‐cellular structure of streamline into a unicellular structure has been analyzed. For the same Richardson number (Ri) with an increase in Prandtl number, the flow and heat transfer phenomena changes from a buoyancy‐induced dominated flow to a shear‐induced dominated flow, which leads to some exciting results with respect to wall movement as well. Furthermore, the role of surface radiation in this respect has been emphasized. Nusselt number variation with the Prandtl number and surface emissivity has also been presented. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21003 相似文献
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The transmission properties of water and sodium chloride water solutions with various salt concentrations were investigated. This paper concerns the measurement of the spectral transmittance and the calculation of the total transmittance for water and NaCl water solutions. The spectral transmittance of the NaCl water solutions over the infrared and nearby regions increased with increasing salt concentration for constant specimen thickness and air mass. The spectral transmittance was found to be useful for precisely calculating the thermal performance of a solar pond, and by using an effective absorption coefficient, a five-partition method dividing the wavelengths into five bands by which the total transmittance up to a water depth of 3 m could be calculated, was found to be important as a practical method of solar pond technology. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 26(1): 1–15, 1997 相似文献