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.

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.

Radiation Heat Transfer in Fluidized Beds: A Comparison of Exact and Simplified Approaches

1994 ◽  
Vol 116 (3) ◽  
pp. 652-659 ◽  
Author(s):  
G. Flamant ◽  
J. D. Lu ◽  
B. Variot
Keyword(s):  
Heat Transfer ◽  
Fluidized Beds ◽  
Radiation Heat Transfer ◽  
Discrete Ordinates ◽  
Transfer Model ◽  
Radiation Heat ◽  
Flux Divergence ◽  
Transfer Coefficients ◽  
Variable Porosity ◽  
Radiation Exchange

Radiation heat transfer at heat exchanger walls in fluidized beds has never been examined through a complete formulation of the problem. In this paper a wall-to-bed heat transfer model is proposed to account for particle convection, gas convection, and radiation exchange in a variable porosity medium. Momentum, energy, and intensity equations are solved in order to determine the velocity, temperature, radiative heat flux profiles and heat transfer coefficients. The discrete-ordinates method is used to compute the radiative intensity equation and the radiative flux divergence in the energy equation. Both the gray and the non-gray assumptions are considered, as well as dependent and independent scattering. The exact solution obtained is compared with several simplified approaches. Large differences are shown for small particles at high temperature but the simplified solutions are valid for large particle beds. The dependency of radiative contribution on controlling parameters is discussed.

Combustion and Heat Transfer in 300MW Oxy-Fired CFBB

2011 ◽  
Vol 354-355 ◽  
pp. 369-375
Author(s):  
Chun Bo Wang ◽  
Xiao Fei Ma ◽  
Jiao Zhang ◽  
Jin Gui Sheng ◽  
Hong Wei Li
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Particle Diameter ◽  
External Heat ◽  
Transfer Model ◽  
Heat Transfer Characteristics ◽  
Operation Condition ◽  
Transfer Capability ◽  
The Heat Transfer Coefficient

A combustion and heat transfer model in oxy-fired CFBB was set. Particle diameter, voidage of the bed ,etc, was analyzed with 30%, 50%, and 70% oxygen. Take a 300MW CFBB for example, the heat transfer characteristics in furnace were numerical simulated. In the sparse zone, heat transfer coefficient is proportional to oxygen concentration at the same voidage of the bed; under the same operation condition, the heat transfer coefficient in CFB increases with the voidage of the bed at first, then it decreases. It was found the heat transfer capability decrease due to the higher concentration of oxygen. It is necessary to set an external heat exchanger to keep a normal combustion

Modeling Thermal Radiation With Focus on Particle Radiation in Grate Fired Furnaces Combusting MSW or Biomass: A Parametric Study

2013 ◽  
Author(s):  
Henrik Hofgren ◽  
Bengt Sundén
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Fly Ash ◽  
Thermal Radiation ◽  
Radiative Heat Transfer ◽  
Parametric Study ◽  
Radiative Heat ◽  
Size Distributions ◽  
Particle Radiation ◽  
Mass Size

This parametric study shows that thermal radiation from particles, fly ash and char, can be highly relevant for estimating the radiative heat flux to surfaces in grate fired furnaces, especially to the hot bed. The large effects of particle radiative heat transfer come from cases with municipal solid waste (MSW) as fuel whereas biomass cases have moderate effect on the overall radiative heat transfer. The parameters investigated in the study were the fuel parameters, representing a variety of particle loads and size distributions, emissivities of walls and bed, and the size of furnace. The investigations were conducted in a 3-D rectangular environment with a fixed temperature field, and homogeneous distribution of gases and particles. The choice of boundary emissivity was found to be much more or equally important as the particle radiation effects, dependent if biomass or MSW, respectively, was used as the fuel. The effect of particle radiation increased with increasing furnace size, mostly evident in the change of the radiative source term and the heat flux to the bed. Compared to previous studies of particle radiation in grate fired combustion, this study used realistic particle mass size distributions for fly ash. Estimates of char mass size distributions inside the furnace were conducted and used.

Heating and Ignition of Metal Particles in the Transition Heat Transfer Regime

2008 ◽  
Vol 130 (10) ◽  
Author(s):  
Salil Mohan ◽  
Mikhaylo A. Trunov ◽  
Edward L. Dreizin
Keyword(s):  
Heat Transfer ◽  
Temperature Difference ◽  
Particle Diameter ◽  
Accommodation Coefficient ◽  
Heat Transfer Model ◽  
Heat Transfer Analysis ◽  
Transfer Model ◽  
Metallic Particles ◽  
The Continuum ◽  
Transition Heat

This paper considers the heating and ignition of small metallic particles in hot gases for a range of Knudsen numbers, for which the continuum description of heat transfer is not valid. Modified Fuchs’ model for the transition heat transfer analysis was adapted to treat diatomic gas with properties changing as a function of temperature. The dimensionless heat transfer coefficient, Nusselt number, was calculated as a function of the particle diameter for the transition heat transfer regime. Heat transfer rates in the transition regime are somewhat different from one another for the cases of particle heating and cooling while the absolute values of the particle-gas temperature difference are the same. This effect does not exist for the continuum heat transfer model. It is observed that the applicability of the continuum heat transfer model for particles of different sizes depends on pressure and particle-air temperature difference. For example, for particles at 300K heated in air at 2000K, the continuum heat transfer model can be used for particle diameters greater than 10μm and 1μm at the pressures of 1bar and 10bars, respectively. Transition heat transfer model must be used for the analysis of heat transfer for nanosized particles. For calculating the ignition delay, the continuum model remains useful for particle diameters greater than 18μm and 2μm for 1bar and 10bars, respectively. The sensitivity of the transition heat transfer model to the accommodation coefficient is evaluated. It is found that for metallic particles, the accommodation coefficient has a relatively weak effect on the heat transfer rate.

Effects of Micro-Particle Diameters and Low Fluid Velocities on Effective Thermal Parameters of Micro-Particle Packed Bed as a Biothermal Reactor

2012 ◽  
Vol 516-517 ◽  
pp. 15-23 ◽  
Author(s):  
Yi Xu ◽  
Li Ping Wang ◽  
Yan Qian Zhong ◽  
Yi Hua Zheng
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Particle Diameter ◽  
Packed Bed ◽  
Thermal Parameter ◽  
Transfer Model ◽  
Filling Material ◽  
Experimental Conditions ◽  
Constant Wall Temperature ◽  
Fluid Velocities

The effects of micro-particle diameters (i.e. dp=0.4~1.1mm) and low fluid velocities (v=5ml/min, 3ml/min and 1ml/min) on the heat transfer behavior of water flowing through a micro-particle packed bed as a reactor of thermal biosensor were investigated experimentally under constant wall temperature conditions (i.e. 60°C). The effective thermal parameter is smaller with decreasing the particle diameter and fluid velocities. This is mainly due to the poor thermal conductivity of the filling materials which leads to a larger thermal resistance and hydraulic resistance. As such, it is very important to select a filling material with better thermal conductivity to enhance heat transfer, which is favorable to completely detect the heat created during the enzyme-catalyzed reaction. Comparing the correlations of both this work and those published in the literature, there are considerable discrepancies among them due to different experimental conditions. The two-dimensional heat transfer model that predicts the temperature distributions agree reasonably well with actual measurements except a slight over-prediction in the region close to the inlet.

2-D Transient Radiative Heat Transfer Analysis on the Slab in a Walking-Beam-Type Reheating Furnace

2014 ◽  
Vol 610 ◽  
pp. 993-997 ◽  
Author(s):  
Jun Bo Huang ◽  
Jiin Yuh Jang ◽  
Chien Nan Lin ◽  
Chao Hua Wang
Keyword(s):  
Heat Transfer ◽  
Heat Conduction ◽  
Thermal Radiation ◽  
Transient Heat Conduction ◽  
Heat Transfer Model ◽  
Transfer Model ◽  
Gas Temperature ◽  
Reheating Furnace ◽  
Hot Gas ◽  
Beam Type

A two-dimensional mathematical heat transfer model for the prediction of temperature distribution within the slab has been developed by considering the thermal radiation in the walking-beam-type reheating furnace chamber and transient heat conduction in the slab, respectively. The steel slabs are heated up through the preheating, heating, and soaking zones in the furnace. Heat transfer characteristics and temperature uniformity of the slab is investigated by changing hot gas temperature. Comparison with the in-situ experimental data show that the present heat transfer model works well for the prediction of thermal behavior of the slab in the reheating furnace. The skid mark severity decreases with an increase in hot gas temperature. Keywords: Reheating Furnace, Thermal Radiation, Transient heat conduction

Experimental Study on Natural Convection in an Asymmetrically Heated Inclined Channel With Radiation Exchange

2003 ◽  
Author(s):  
Qin Lin ◽  
Stephen J. Harrison
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Inclination Angle ◽  
Channel Length ◽  
Surface Emissivity ◽  
Inclined Channel ◽  
Length Space ◽  
Radiation Exchange

Heat transfer in an asymmetrically heated, inclined channel by natural convection and radiation exchange was experimentally investigated. Experiments were conducted on channels with small inclination angle (to horizontal) ranging from 18° to 30° and a wall surface emissivity of 0.29 to 0.95. The channel length/space ratio was between 44 and 220. In each test, a uniform heat flux was applied along the top wall of the channel, while the bottom wall was thermally insulated. Temperature profiles along both the top and bottom walls of the channel were recorded under different heat flux and channel length/space ratios. The dependency of maximum wall temperature and heat transfer on the channel spacing and surface emissivity was explored. As a result of this work, correlations of local and average Nusselt number, with modified channel Rayleigh number, were determined and proposed for channels at inclination angle of around 18° and surface emissivities of around 0.95. The proposed correlation will be valid for modified-Rayleigh number in the range of 10 < Ra” < 5.6 × 104 at asymmetric heat flux boundary conditions.

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