Modeling Effective Thermal Conductivity of Thermal Radiation for Nuclear Packed Pebble Beds

2017 ◽  
Vol 140 (4) ◽  
Author(s):  
Hao Wu ◽  
Nan Gui ◽  
Xingtuan Yang ◽  
Jiyuan Tu ◽  
Shengyao Jiang
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Thermal Radiation ◽  
Effective Thermal Conductivity ◽  
Transfer Cells ◽  
Transfer Model ◽  
Radiation Model ◽  
Radial Temperature ◽  
Effective Heat ◽  
Good Agreement

In nuclear packed pebble beds, it is a fundamental task to model effective thermal conductivity (ETC) of thermal radiation. Based on the effective heat transfer cells of structured packing, a short-range radiation model (SRM) and a subcell radiation model (SCM) are applied to obtain analytical results of ETC. It is shown that the SRM of present effective heat transfer cells are in good agreement with the numerical simulations of random packing and it is only slightly higher than empirical correlations when temperature exceeds 1200 °C. In order to develop a generic theoretical approach of modeling ETC, the subcell radiation model is presented and in good agreement with Kunii–Smith correlation, especially at very high temperature ranges (over 1500 °C). Based on SCM, one-dimensional (1D) radial heat transfer model is applied in the analysis of the HTTU experiments. The results of ETC and radial temperature distribution are in good agreement with the experimental data.

Effective Thermal Conductivity of Submicron Powders: A Numerical Study

2016 ◽  
Vol 846 ◽  
pp. 500-505
Author(s):  
Wei Jing Dai ◽  
Yi Xiang Gan ◽  
Dorian Hanaor
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Thermal Resistance ◽  
Effective Thermal Conductivity ◽  
Granular Materials ◽  
Gas Phase ◽  
Numerical Study ◽  
Transfer Model ◽  
Size Of Particles ◽  
Contact Unit

Effective thermal conductivity is an important property of granular materials in engineering applications and industrial processes, including the blending and mixing of powders, sintering of ceramics and refractory metals, and electrochemical interactions in fuel cells and Li-ion batteries. The thermo-mechanical properties of granular materials with macroscopic particle sizes (above 1 mm) have been investigated experimentally and theoretically, but knowledge remains limited for materials consisting of micro/nanosized grains. In this work we study the effective thermal conductivity of micro/nanopowders under varying conditions of mechanical stress and gas pressure via the discrete thermal resistance method. In this proposed method, a unit cell of contact structure is regarded as one thermal resistor. Thermal transport between two contacting particles and through the gas phase (including conduction in the gas phase and heat transfer of solid-gas interfaces) are the main mechanisms. Due to the small size of particles, the gas phase is limited to a small volume and a simplified gas heat transfer model is applied considering the Knudsen number. During loading, changes in the gas volume and the contact area between particles are simulated by the finite element method. The thermal resistance of one contact unit is calculated through the combination of the heat transfer mechanisms. A simplified relationship between effective thermal conductivity and loading pressure can be obtained by integrating the contact units of the compacted powders.

Convective Heat Transfer Analysis of Open Cell Metal Foam for Solar Air Heaters

Solar Energy ◽  
2003 ◽  
Author(s):  
Nihad Dukhan ◽  
Pablo D. Quinones
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Metal Foam ◽  
Heat Transfer Analysis ◽  
Transfer Model ◽  
Aluminum Foams ◽  
Maximum Heat ◽  
Open Cell ◽  
Maximum Heat Transfer

A one-dimensional heat transfer model for open-cell metal foam is presented. Three aluminum foams having different areas, relative densities, ligament diameters, and number of pores per inch were analyzed. The effective thermal conductivity and the heat transfer increased with the number of pores per inch. The effective thermal conductivity of the foams can be up to four times higher than that of solid aluminum. The resulting improvement in heat transfer can be as high as 50 percent. The maximum heat transfer for the aluminum foams occurs at a pore Reynolds number of 52. The heat transfer, in addition, becomes insensitive to the flow regime for pore Reynolds numbers beyond 200.

Particle-Scale Investigation of Thermal Radiation in Nuclear Packed Pebble Beds

2018 ◽  
Vol 140 (9) ◽  
Author(s):  
Hao Wu ◽  
Nan Gui ◽  
Xingtuan Yang ◽  
Jiyuan Tu ◽  
Shengyao Jiang
Keyword(s):  
Heat Transfer ◽  
Thermal Radiation ◽  
Particle Diameter ◽  
Transfer Model ◽  
Wall Region ◽  
Surface Emissivity ◽  
Radiation Model ◽  
Particle Radiation ◽  
Radiation Exchange ◽  
Effective Particle

For the heat transfer of pebble or granular beds (e.g., high temperature gas-cooled reactors (HTGR)), the particle thermal radiation is an important part. Using the subcell radiation model (SCM), which is a generic theoretical approach to predict effective thermal conductivity (ETC) of particle radiation, particle-scale investigation of the nuclear packed pebble beds filled with monosized or multicomponent pebbles is performed here. When the radial porosity distribution is considered, the ETC of the particle radiation decreases significantly at near-wall region. It is shown that radiation exchange factor increases with the surface emissivity. The results of the SCM under different surface emissivity are in good agreement with the existing correlations. The discrete heat transfer model in particle scale is presented, which combines discrete element method (DEM) and particle radiation model, and is validated by the transient experimental results. Compared with the discrete simulation results of polydisperse beds, it is found that the SCM with the effective particle diameter can be used to analyze behavior of the radiation in polydisperse beds.

Analysis of Clumped-Pebble Shape on Thermal Radiation and Conduction in Nuclear Beds by Subcell Radiation Model

2020 ◽  
Vol 142 (3) ◽  
Author(s):  
Hao Wu ◽  
Nan Gui ◽  
Xingtuan Yang ◽  
Jiyuan Tu ◽  
Shengyao Jiang
Keyword(s):  
Heat Transfer ◽  
High Temperature ◽  
Thermal Radiation ◽  
Packed Bed ◽  
Radiation Model ◽  
The Core ◽  
Model Combining ◽  
Temperature Ranges ◽  
Pebble Flow ◽  
Good Agreement

Abstract The core of high-temperature gas-cooled reactor is a dense pebble bed of random packing filled with monosized fuel spheres. Subcell radiation model (SCM) is a generic analytical approach to calculate effective thermal conductivity (ETC) of thermal radiation. For the packed bed of monosized spheres operated in various conditions, it is proven that the SCM is still applicable in the particle size ranges of 1.2–60 mm and temperature ranges of 0–1200 °C. Based on the SCM, radiation-to-conduction ratio ξ is presented and radiation becomes an essential part at ξ>0.1 for the accurate evaluation. For the beds of nonoverlapping clumped-sphere particles, the model combining with discrete element method (DEM) and SCM is presented to study the heat transfer behaviors, including effects of particle shape, emissivity distribution and pebble flow with transient heat transfer. For the experimental nuclear pebble beds, the results of SCM are in good agreement with the empirical correlation and accord well with the experimental data under high temperature range.

Thermal radiation and conduction properties of materials ranging from sand to rock-fill

2011 ◽  
Vol 48 (4) ◽  
pp. 532-542 ◽  
Author(s):  
Marie-Hélène Fillion ◽  
Jean Côté ◽  
Jean-Marie Konrad
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Particle Size ◽  
Thermal Radiation ◽  
Effective Thermal Conductivity ◽  
Radiation Effects ◽  
Radiation Heat Transfer ◽  
Experimental Results ◽  
Radiation Heat ◽  
Rock Fill

This paper presents an experimental study on thermal radiation and the thermal conductivity of rock-fill materials using a 1 m × 1 m × 1 m heat transfer cell. Testing temperatures are applied by temperature-controlled fluid circulation at the top and bottom of the sample. Heat flux and temperature profiles are measured to establish the effective thermal conductivity λe, which includes contributions from both conduction and radiation heat transfer mechanisms. The materials studied had an equivalent particle size (d10) ranging from 90 to 100 mm and porosity (n) ranging from 0.37 to 0.41. The experimental results showed that thermal radiation greatly affects the effective thermal conductivity of materials with λe values ranging from 0.71 to 1.02 W·m−1·K−1, compared with a typical value of 0.36 W·m−1·K−1 for conduction alone. As expected, the effective thermal conductivity increased with particle size. An effective thermal conductivity model has been proposed, and predictions have been successfully compared with the experimental results. Radiation heat transfer becomes significant for d10 higher than 10 mm and predominant at values higher than 90 mm. The results of the study also suggest that the cooling potential of convection embankments used to preserve permafrost conditions may not be as efficient as expected because of ignored radiation effects.

scholarly journals Conjugate heat transfer performance of stepped lid-driven cavity with Al2O3/water nanofluid under forced and mixed convection

2021 ◽  
Vol 3 (6) ◽  
Author(s):  
Naveen Janjanam ◽  
Rajesh Nimmagadda ◽  
Lazarus Godson Asirvatham ◽  
R. Harish ◽  
Somchai Wongwises
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Mixed Convection ◽  
Conjugate Heat Transfer ◽  
Heat Transfer Performance ◽  
Pure Water ◽  
Interface Temperature ◽  
Transfer Model ◽  
Transfer Performance ◽  
Driven Cavity

AbstractTwo-dimensional conjugate heat transfer performance of stepped lid-driven cavity was numerically investigated in the present study under forced and mixed convection in laminar regime. Pure water and Aluminium oxide (Al2O3)/water nanofluid with three different nanoparticle volume concentrations were considered. All the numerical simulations were performed in ANSYS FLUENT using homogeneous heat transfer model for Reynolds number, Re = 100 to 500 and Grashof number, Gr = 5000, 13,000 and 20,000. Effective thermal conductivity of the Al2O3/water nanofluid was evaluated by considering the Brownian motion of nanoparticles which results in 20.56% higher value for 3 vol.% Al2O3/water nanofluid in comparison with the lowest thermal conductivity value obtained in the present study. A solid region made up of silicon is present underneath the fluid region of the cavity in three geometrical configurations (forward step, backward step and no step) which results in conjugate heat transfer. For higher Re values (Re = 500), no much difference in the average Nusselt number (Nuavg) is observed between forced and mixed convection. Whereas, for Re = 100 and Gr = 20,000, Nuavg value of mixed convection is 24% higher than that of forced convection. Out of all the three configurations, at Re = 100, forward step with mixed convection results in higher heat transfer performance as the obtained interface temperature is lower than all other cases. Moreover, at Re = 500, 3 vol.% Al2O3/water nanofluid enhances the heat transfer performance by 23.63% in comparison with pure water for mixed convection with Gr = 20,000 in forward step.

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