It's an unsolved problem to calculate the thermal radiation view factors among fuel pebbles a... more It's an unsolved problem to calculate the thermal radiation view factors among fuel pebbles as accurately and quickly as possible in the simulation of the temperature fields within the pebble-bed. In this study, a series of fully connected neural networks (FCNs) has been developed to realize the fast calculation of view factors. In order to verify the accuracy and effects of the networks, the neural networks are compared with the Monte Carlo (MC) algorithm. The results show that, in most cases, the relative errors of the FCN method can be controlled within 1.0%, and the prediction accurate probability is up to 99%. In comparisons of specific examples, the temperature errors of the FCN method and the MC method are less than 1 K within the range neural networks have covered. In addition, the time of neural networks for a single calculation is about 2–20 μs, which is even less than 10−4 of the time taken by the MC algorithm. In conclusion, neural networks can greatly improve comput...
Abstract In this work, the discharging process of the binary mixture composed of sphere and spher... more Abstract In this work, the discharging process of the binary mixture composed of sphere and sphere-paired particles in a two-dimensional silo was studied. High-speed camera and self-developed particle tracking velocimetry (PTV) program were used to capture the flow behaviors of all particles. The key parameters of mixed flow, including coordination number, horizontal displacement and mechanical energy loss in the discharge process, were highlighted. It was found that the increase of sphere-paired particles can decrease the average coordination number of particles during the discharging process. The analysis about the loss of mechanical energy and the horizontal displacement of particles indicated that sphere-paired particles preferentially squeezed out sphere particles from fast flow field above the outlet. Moreover, an empirical formula was proposed to assess the influence of the proportion of sphere-paired particles on the discharge flow rate. Sphere-paired particles tended to hinder the discharging process, which was caused by the rotation around their centroids and the angular deflection close to angle of the hopper.
Abstract The pebbles’ movement in the pebble beds of 10 MW high-temperature gas-cooled test react... more Abstract The pebbles’ movement in the pebble beds of 10 MW high-temperature gas-cooled test reactor (HTR-10) presents as a kind of circulating quasi-static dense pebble flow. The mixing of the upper and the lower fuel pebbles is important because the fast and good mixing is beneficial to flattening the power distribution and lowering the maximum temperature in the lower part of the pebble bed, where the temperature is higher than other region because of the higher burn-up of fuel pebbles. To figure out the effect of different three-dimensional (3-D) wall structures on the flow and mixing of pebbles, eight 3-D wall structures are designed and mono sized pebbles with same density are adopted to simulate by discrete element method (DEM). With phenomenological methods and different mixing indices, qualitative and quantitative analysis are performed and presented hereinafter. All selected wall structures have different degrees of effect on impeding the pebble flow near the wall, accelerating the cycling speed of pebbles at the core region and strengthening the mixing of the upper and lower pebbles in the pebble bed. Compared with the designed trapezoidal and plane structures, triangular and helicoidal structures perform better in accelerating the cycling speed for pebbles at the core region and strengthening the mixing of pebbles by analyzing cycle index (CI), coordination-Lacey’s rule mixing index (CLMI) and coordination-concentration based mixing index (CCMI). In addition, a phenomenon of saturation for the number of structural grooves is reported in this literature. That is, the effect of accelerating the cycling speed of pebbles at core zones and strengthening the mixing gets closer to a certain degree if we make the different grooves denser. The optimal structure to accelerating cycling speed for pebbles in core zones and improving mixing is triangular structure among the four kinds of structures, while the optimum solution for the number of grooves requires deeper investigation.
A particle-scale surface tension force model (STF) is proposed here to be incorporated in the smo... more A particle-scale surface tension force model (STF) is proposed here to be incorporated in the smoothed hydrodynamics particle (SPH) method. This model is based on the identification of interface geometry and the gradient of densities across the interface. A square bubble of single-phase and a square bubble immersed in fluids are simulated by the STF model accompanied with a combined kernel in SPH to validate their suitability to simulate the immersed bubble motion. Two cases of rising bubbles, i.e., a single rising bubble and a pair of rising bubbles, are simulated for demonstration. The rising velocity, density, surface tension force, interfacial curvature, the power of the STF, and the smoothing length of the rising bubble and surrounding fluids are all computed by the current STF model to study the characteristics of immersed bubble’s motion and coalescence. The current model provides a way to capture the interfacial interactions in two-phase flows at particle scales.
In order to study the pebble flow dynamics in the high temperature reactor core, the 2-D experime... more In order to study the pebble flow dynamics in the high temperature reactor core, the 2-D experiment system is set up based on the similarity criterion and the surface analysis quantitative experiment method is proposed. This paper introduces the theory basis of SAM (Surface Analysis Method) and its application in the pebble flow mechanics study. The experiment with different proportion of loading pebbles is carried out. Finally, The 2-D pebble flow characteristics including mean stream lines, velocity field distribution etc. are analyzed and compared with the results of the Marking Pebble Method.
Abstract A discrete particle model plays a fundamental role in modeling gas-particle flows. In th... more Abstract A discrete particle model plays a fundamental role in modeling gas-particle flows. In this chapter, besides the discrete element method – a discrete particle model, we introduce three additional discrete particle models: the generalized hard particle model (GHPM), soft sphere imbedded pseudo-hard particle model (SIPHPM), and the extended hard particle model – discrete element method coupled model (EHPM-DEM model), each has different features and advantages. GHPM has been developed aiming to solve the collision dynamics of rigid particles analytically. It is particularly suitable for dilute particle flow. On the contrary, SIPHPM is for pseudo-hard materials (only small deformations are allowed) of dense particle flows. EHPM-DEM is a hybrid model for complex or transiting particle flows, e.g. both dilute and dense particle flows, or intermittent dilute and dense particle flows. These three models are not limited to spherical particles but also suitable for particles of nonspherical shapes. They have potential to be extended for heat transfer modeling in particle flows.
Volume 8: Computational Fluid Dynamics (CFD); Nuclear Education and Public Acceptance, 2018
The influence of contact angle on bubble growth and detachment is investigated in this paper. The... more The influence of contact angle on bubble growth and detachment is investigated in this paper. The phase-change Lattice Boltzmann Method (LBM) which includes a SRT pseudo-potential LB model and a thermal LB model is used to simulate the flow boiling in vertical tube. To verify the correctness of the model, the coexistence curve obtained from the LBM simulations is compared to the analytical one. Then the relation between the diameter of bubble detachment and the wall superheat is compared with the empirical relation. The effect of contact angle on the bubble growth is investigated. The bubble growth with different contact angles is calculated. The bubble growth processes with different contact angles and the detached shape are shown in this paper. The bubble equivalent diameter and the length of contact line is investigated. The bubble equivalent diameter curve shows that bubble growth can be divided into two stages, initially isothermal growth stage, followed by isobaric growth stag...
Facing the dual pressure of environmental problems and energy issues, much effort has been made t... more Facing the dual pressure of environmental problems and energy issues, much effort has been made to find new energy alternatives like nuclear energy or upgrade the existing technologies such as petrochemical technology. To utilize the nuclear energy, many types of nuclear reactors have been developed worldwide after decades of research, among which the pebble bed high temperature gas-cooled reactor (pebble bedHTGR) has attracted wide attention and been regarded as a promising generation-IV reactor. It uses large fuel pebbles and helium gas as coolant. Inside the core, the pebble bed flow, a kind of extremely slow flow of pebbles, plays a critical role for the reactor core design and safety analysis. To gain a better understanding of theflow characteristics, we focus on various fundamental issues associatedwith pebble bed flow, such as the quasi-static packing structure, the flow velocity, the flow pattern, the resident time, and stagnant ratio, etc. In the work (Huang et al.), discha...
It's an unsolved problem to calculate the thermal radiation view factors among fuel pebbles a... more It's an unsolved problem to calculate the thermal radiation view factors among fuel pebbles as accurately and quickly as possible in the simulation of the temperature fields within the pebble-bed. In this study, a series of fully connected neural networks (FCNs) has been developed to realize the fast calculation of view factors. In order to verify the accuracy and effects of the networks, the neural networks are compared with the Monte Carlo (MC) algorithm. The results show that, in most cases, the relative errors of the FCN method can be controlled within 1.0%, and the prediction accurate probability is up to 99%. In comparisons of specific examples, the temperature errors of the FCN method and the MC method are less than 1 K within the range neural networks have covered. In addition, the time of neural networks for a single calculation is about 2–20 μs, which is even less than 10−4 of the time taken by the MC algorithm. In conclusion, neural networks can greatly improve comput...
Abstract In this work, the discharging process of the binary mixture composed of sphere and spher... more Abstract In this work, the discharging process of the binary mixture composed of sphere and sphere-paired particles in a two-dimensional silo was studied. High-speed camera and self-developed particle tracking velocimetry (PTV) program were used to capture the flow behaviors of all particles. The key parameters of mixed flow, including coordination number, horizontal displacement and mechanical energy loss in the discharge process, were highlighted. It was found that the increase of sphere-paired particles can decrease the average coordination number of particles during the discharging process. The analysis about the loss of mechanical energy and the horizontal displacement of particles indicated that sphere-paired particles preferentially squeezed out sphere particles from fast flow field above the outlet. Moreover, an empirical formula was proposed to assess the influence of the proportion of sphere-paired particles on the discharge flow rate. Sphere-paired particles tended to hinder the discharging process, which was caused by the rotation around their centroids and the angular deflection close to angle of the hopper.
Abstract The pebbles’ movement in the pebble beds of 10 MW high-temperature gas-cooled test react... more Abstract The pebbles’ movement in the pebble beds of 10 MW high-temperature gas-cooled test reactor (HTR-10) presents as a kind of circulating quasi-static dense pebble flow. The mixing of the upper and the lower fuel pebbles is important because the fast and good mixing is beneficial to flattening the power distribution and lowering the maximum temperature in the lower part of the pebble bed, where the temperature is higher than other region because of the higher burn-up of fuel pebbles. To figure out the effect of different three-dimensional (3-D) wall structures on the flow and mixing of pebbles, eight 3-D wall structures are designed and mono sized pebbles with same density are adopted to simulate by discrete element method (DEM). With phenomenological methods and different mixing indices, qualitative and quantitative analysis are performed and presented hereinafter. All selected wall structures have different degrees of effect on impeding the pebble flow near the wall, accelerating the cycling speed of pebbles at the core region and strengthening the mixing of the upper and lower pebbles in the pebble bed. Compared with the designed trapezoidal and plane structures, triangular and helicoidal structures perform better in accelerating the cycling speed for pebbles at the core region and strengthening the mixing of pebbles by analyzing cycle index (CI), coordination-Lacey’s rule mixing index (CLMI) and coordination-concentration based mixing index (CCMI). In addition, a phenomenon of saturation for the number of structural grooves is reported in this literature. That is, the effect of accelerating the cycling speed of pebbles at core zones and strengthening the mixing gets closer to a certain degree if we make the different grooves denser. The optimal structure to accelerating cycling speed for pebbles in core zones and improving mixing is triangular structure among the four kinds of structures, while the optimum solution for the number of grooves requires deeper investigation.
A particle-scale surface tension force model (STF) is proposed here to be incorporated in the smo... more A particle-scale surface tension force model (STF) is proposed here to be incorporated in the smoothed hydrodynamics particle (SPH) method. This model is based on the identification of interface geometry and the gradient of densities across the interface. A square bubble of single-phase and a square bubble immersed in fluids are simulated by the STF model accompanied with a combined kernel in SPH to validate their suitability to simulate the immersed bubble motion. Two cases of rising bubbles, i.e., a single rising bubble and a pair of rising bubbles, are simulated for demonstration. The rising velocity, density, surface tension force, interfacial curvature, the power of the STF, and the smoothing length of the rising bubble and surrounding fluids are all computed by the current STF model to study the characteristics of immersed bubble’s motion and coalescence. The current model provides a way to capture the interfacial interactions in two-phase flows at particle scales.
In order to study the pebble flow dynamics in the high temperature reactor core, the 2-D experime... more In order to study the pebble flow dynamics in the high temperature reactor core, the 2-D experiment system is set up based on the similarity criterion and the surface analysis quantitative experiment method is proposed. This paper introduces the theory basis of SAM (Surface Analysis Method) and its application in the pebble flow mechanics study. The experiment with different proportion of loading pebbles is carried out. Finally, The 2-D pebble flow characteristics including mean stream lines, velocity field distribution etc. are analyzed and compared with the results of the Marking Pebble Method.
Abstract A discrete particle model plays a fundamental role in modeling gas-particle flows. In th... more Abstract A discrete particle model plays a fundamental role in modeling gas-particle flows. In this chapter, besides the discrete element method – a discrete particle model, we introduce three additional discrete particle models: the generalized hard particle model (GHPM), soft sphere imbedded pseudo-hard particle model (SIPHPM), and the extended hard particle model – discrete element method coupled model (EHPM-DEM model), each has different features and advantages. GHPM has been developed aiming to solve the collision dynamics of rigid particles analytically. It is particularly suitable for dilute particle flow. On the contrary, SIPHPM is for pseudo-hard materials (only small deformations are allowed) of dense particle flows. EHPM-DEM is a hybrid model for complex or transiting particle flows, e.g. both dilute and dense particle flows, or intermittent dilute and dense particle flows. These three models are not limited to spherical particles but also suitable for particles of nonspherical shapes. They have potential to be extended for heat transfer modeling in particle flows.
Volume 8: Computational Fluid Dynamics (CFD); Nuclear Education and Public Acceptance, 2018
The influence of contact angle on bubble growth and detachment is investigated in this paper. The... more The influence of contact angle on bubble growth and detachment is investigated in this paper. The phase-change Lattice Boltzmann Method (LBM) which includes a SRT pseudo-potential LB model and a thermal LB model is used to simulate the flow boiling in vertical tube. To verify the correctness of the model, the coexistence curve obtained from the LBM simulations is compared to the analytical one. Then the relation between the diameter of bubble detachment and the wall superheat is compared with the empirical relation. The effect of contact angle on the bubble growth is investigated. The bubble growth with different contact angles is calculated. The bubble growth processes with different contact angles and the detached shape are shown in this paper. The bubble equivalent diameter and the length of contact line is investigated. The bubble equivalent diameter curve shows that bubble growth can be divided into two stages, initially isothermal growth stage, followed by isobaric growth stag...
Facing the dual pressure of environmental problems and energy issues, much effort has been made t... more Facing the dual pressure of environmental problems and energy issues, much effort has been made to find new energy alternatives like nuclear energy or upgrade the existing technologies such as petrochemical technology. To utilize the nuclear energy, many types of nuclear reactors have been developed worldwide after decades of research, among which the pebble bed high temperature gas-cooled reactor (pebble bedHTGR) has attracted wide attention and been regarded as a promising generation-IV reactor. It uses large fuel pebbles and helium gas as coolant. Inside the core, the pebble bed flow, a kind of extremely slow flow of pebbles, plays a critical role for the reactor core design and safety analysis. To gain a better understanding of theflow characteristics, we focus on various fundamental issues associatedwith pebble bed flow, such as the quasi-static packing structure, the flow velocity, the flow pattern, the resident time, and stagnant ratio, etc. In the work (Huang et al.), discha...
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