Heat Transfer Across Opaque Fibers

2012 ◽  
Vol 134 (7) ◽  
Author(s):  
Krishpersad Manohar ◽  
Gurmohan S. Kochhar ◽  
David W. Yarbrough
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Thermal Conductivity ◽  
Fiber Diameter ◽  
Radiation Heat Transfer ◽  
Physical Parameters ◽  
Density Range ◽  
Radiation Heat ◽  
Apparent Thermal Conductivity ◽  
Air Conduction

Predicting the thermal conductivity of loose-fill fibrous thermal insulation is a complex problem, when considering the combined conduction, convection, and radiation heat transfer within a scattering, emitting, and absorbing medium. A piecewise model for predicting the overall apparent thermal conductivity of large diameter opaque fibrous materials was developed by considering the radiation heat transfer, solid conduction and air conduction components separately. The model utilized the physical parameters of emissivity, the density of the solid fiber material, the percentage composition and range of fiber diameter, and the mean fiber diameter to develop specific equations for piecewise contribution from radiation, solid fiber conduction, and air conduction toward the overall effective thermal conductivity. It can be used to predict the overall apparent thermal conductivity for any opaque fibrous specimen of density (ρ), known thickness (t), mean temperature (T), and temperature gradient (ΔΤ). Thermal conductivity measurements were conducted in accordance with ASTM C518 specifications on 52 mm thick, 254 mm square test specimens for coconut and sugarcane fibers. The test apparatus provided results with an accuracy of 1%, repeatability of 0.2%, and reproducibility of 0.5%. The model was applied to and compared with experimental data for coconut and sugarcane fiber specimens and predicted the apparent thermal conductivity within 7% of experimental data over the density range tested. The model also predicted the optimum density range for both coconut and sugarcane fibers.

Design Studies for a Solar Reactor Based on a Simple Radiative Heat Exchange Model

2005 ◽  
Vol 127 (3) ◽  
pp. 425-429 ◽  
Author(s):  
C. Wieckert
Keyword(s):  
Heat Transfer ◽  
Solar Power ◽  
Radiation Heat Transfer ◽  
Heat Transfer Analysis ◽  
Physical Parameters ◽  
Concentrated Solar Power ◽  
Radiation Heat ◽  
Small Aperture ◽  
Lower Cavity ◽  
Solar Reactor

A high-temperature solar chemical reactor for the processing of solids is scaled up from a laboratory scale (5kW concentrated solar power input) to a pilot scale (200kW). The chosen design features two cavities in series: An upper cavity has a small aperture to let in concentrated solar power coming from the top. It serves as the solar receiver, radiant absorber, and radiant emitter to a lower cavity. The lower cavity is a well-insulated enclosure. It is subjected to thermal radiation from the upper cavity and serves in our application as the reaction chamber for a mixture of ZnO and carbon. Important insight for the definition of the geometrical parameters of the pilot reactor has been generated by a radiation heat transfer analysis based on the radiosity enclosure theory. The steady-state model accounts for radiation heat transfer within the solar reactor including reradiation losses through the reactor aperture, wall losses due to thermal conduction and heat consumption by the endothermic chemical reaction. Key results include temperatures of the different reactor walls and the thermal efficiency of the reactor as a function of the major geometrical and physical parameters. The model, hence, allows for a fast estimate of the influence of these parameters on the reactor performance.

A Micro-Insulation Concept for MEMS Applications

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Rui Yao ◽  
James Blanchard
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Radiation Heat Transfer ◽  
Thermal Conversion ◽  
Power Source ◽  
Radioactive Isotopes ◽  
Small Scale ◽  
Power Sources ◽  
Planar Surfaces ◽  
Apparent Thermal Conductivity

Small scale, thermally driven power sources will require appropriate insulation to achieve sufficiently high thermal conversion efficiencies. This paper presents a micro-insulation design, which was developed for a thermionic microbattery, which converts the decay heat from radioactive isotopes directly to electricity using a vacuum thermionic diode. The insulation concept, which is suitable for any small scale application, separates two planar surfaces with thin, semicircular posts, thus reducing conduction heat transfer and increasing the relative radiation heat transfer. In this case, the surfaces are silicon wafers and the columns are SU-8, a photoresist material. The experimental results indicate that this design is adequate for a practical power source concept, and they are supported by a numerical model for the effective thermal conductivity of the structure. The results show that a typical design of 20 columns/cm2 with a 200 μm diameter and a 10 μm wall thickness has an apparent thermal conductivity on the order of 10−4 W/m K at a pressure of 1 Pa. System models of a thermionic power source indicate that this is sufficiently low to provide practical efficiency.

scholarly journals The Effect of Radiation Heat Transfer in the Measurement of Thermal Conductivity for the Semitransparent Medium

1976 ◽  
Vol 19 (134) ◽  
pp. 973-979 ◽  
Author(s):  
Masaaki KURIYAMA ◽  
Kozo KATAYAMA ◽  
Yoshiyuki TAKUMA ◽  
Yasushi HASEGAWA
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Radiation Heat Transfer ◽  
Radiation Heat ◽  
Semitransparent Medium

Development of hybrid method for coupled conduction-radiation heat transfer in two-dimensional irregular enclosure considering thermo-radiative effects and varying thermal conductivity

2019 ◽  
Vol 30 (4) ◽  
pp. 1815-1837
Author(s):  
Mehdi Zare ◽  
Sadegh Sadeghi
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Flux ◽  
Heat Source ◽  
Radiation Heat Transfer ◽  
Isotropic Scattering ◽  
Two Dimensional ◽  
Radiation Heat ◽  
Content Type ◽  
Irregular Enclosure

Purpose This study aims to perform a comprehensive investigation to model the thermal characteristics of a coupled conduction-radiation heat transfer in a two-dimensional irregular enclosure including a triangular-shaped heat source. Design/methodology/approach For this purpose, a promising hybrid technique based on the concepts of blocked-off method, FVM and DOM is developed. The enclosure consists of several horizontal, vertical and oblique walls, and thermal conductivity within the enclosure varies directly with temperature and indirectly with position. To simplify the complex geometry, a promising mathematical model is introduced using blocked-off method. Emitting, absorbing and non-isotropic scattering gray are assumed as the main radiative characteristics of the steady medium. Findings DOM and FVM are, respectively, applied for solving radiative transfer equation (RTE) and the energy equation, which includes conduction, radiation and heat source terms. The temperature and heat flux distributions are calculated inside the enclosure. For validation, results are compared with previous data reported in the literature under the same conditions. Results and comparisons show that this approach is highly efficient and reliable for complex geometries with coupled conduction-radiation heat transfer. Finally, the effects of thermo-radiative parameters including surface emissivity, extinction coefficient, scattering albedo, asymmetry factor and conduction-radiation parameter on temperature and heat flux distributions are studied. Originality/value In this paper, a hybrid numerical method is used to analyze coupled conduction-radiation heat transfer in an irregular geometry. Varying thermal conductivity is included in this analysis. By applying the method, results obtained for temperature and heat flux distributions are presented and also validated by the data provided by several previous papers.

Collocation Spectral Method for the Transient Conduction–Radiation Heat Transfer With Variable Thermal Conductivity in Two-Dimensional Rectangular Enclosure

2015 ◽  
Vol 137 (3) ◽  
Author(s):  
Guo-Jun Li ◽  
Jian Ma ◽  
Ben-Wen Li
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Spectral Method ◽  
Radiation Heat Transfer ◽  
Variable Thermal Conductivity ◽  
Two Dimensional ◽  
Radiation Heat ◽  
Rectangular Enclosure ◽  
Collocation Spectral Method ◽  
Transient Conduction

The collocation spectral method (CSM) is further developed to solve the transient conduction–radiation heat transfer in a two-dimensional (2D) rectangular enclosure with variable thermal conductivity. The energy equation and the radiative transfer equation (RTE) are all discretized by Chebyshev–Gauss–Lobatto collocation points in space after the discrete ordinates method (DOM) discretization of RTE in angular domain. The treatment of variable thermal conductivity is executed using the array multiplication. The present method can deal with different boundary conditions with high accuracy, the Dirichlet one and mixed one, for example. Based on our new method, the effects of several parameters on heat transfer processes are analyzed.

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