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Particle Transport and Entrainment during Reactor Accidents
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
Obviously, molecular diffusion is a very slow process, and when it comes to the core, it does not occur rapidly enough to be of major importance. However, because a reactor core can generate a great deal of heat, the flow rate through the core must be highly turbulent to remove the heat as efficiently as possible. Therefore, as soon as the flow becomes turbulent, a related process called turbulent diffusion or turbulent mixing comes into play. This process takes place much more rapidly than molecular diffusion, and it can sometimes affect the rate of mass and energy transfer in a dramatic way. In essence, turbulent diffusion can be thought of as the transfer of mass, energy, or momentum due to random temporal fluctuations in the velocity field. In this case, the turbulent eddies are the “particles” that we would like to observe. To see how this affects the transport process, suppose that we would like to apply the advective–diffusion equation to a turbulent flow. Turbulent eddies are produced because of the shear forces that particles of fluid exert on each other. These eddies can carry mass, energy, or momentum along with them (see Figure 28.7), and in general, they transport energy and momentum from regions of high temperature to regions of low temperature. In other words, they help to flatten the core temperature profile by transferring thermal energy from hotter channels to cooler ones.
Atmospheric Confinement: The Role of Boundary Surfaces
Published in Neil McManus, Safety and Health in Confined Spaces, 2018
Diffusion can occur in two modes: molecular diffusion and eddy or turbulent diffusion (Geankoplis 1972, Treybal 1980). Molecular diffusion occurs in fluids that are stagnant or undergoing laminar flow. Turbulent diffusion occurs when the fluid undergoes turbulent motion. In laminar flow the fluid flows in smooth streamlines. In turbulent flow, there are no orderly streamlines. Rather, large eddies or “chunks” of fluid of varying size move in seemingly random fashion. These eddies can move rapidly from one part of the fluid to another, including perpendicular to the direction of flow. As a result, the concentration of solute differs from one eddy packet to another. The rapid movement from one part of the fluid to another, combined with the transfer of relatively large amounts of solute per packet, enhances mass transfer from the interface. Eddy or turbulent diffusion is very rapid compared to molecular diffusion.
Environmental Analysis
Published in Connie Kelly Tang, Lei Zhang, Principles and Practices of Transportation Planning and Engineering, 2021
Turbulent diffusion refers to mixing movements through eddy flows as defined in fluid dynamics and characterized by chaotic changes in pressure and flow velocity along a flow path. Turbulent diffusion does not exist in a laminar flow regime where the medium (e.g., air) flows in parallel layers without interfering with each other (see Figure 5.6 for illustration).
Computational and experimental study of aerosol dispersion in a ventilated room
Published in Aerosol Science and Technology, 2022
George H. Downing, Yannis Hardalupas, Justice Archer, Henry E. Symons, Ulas Baran Baloglu, Daniel Schien, Bryan R. Bzdek, Jonathan P. Reid
To evaluate the fluid flow simulation, the turbulent kinetic energy (TKE) was measured at several locations in the room and compared with the calculated turbulent kinetic energy from the CFD simulation at the equivalent locations in the computational domain. Turbulent kinetic energy was used as a guideline to assess the CFD simulation since it is an important parameter for the determination of the turbulent diffusion of aerosols (Equation (3)). Evaluation of the air flow velocities across the room was not undertaken due to the simplified room geometry in the computational domain. The turbulent kinetic energy was estimated from the measured velocity magnitude using a RS-8880 hotwire anemometer.
A Review of Molten Salt Reactor Kinetics Models
Published in Nuclear Science and Engineering, 2018
Daniel Wooten, Jeffrey J. Powers
In many CFRs, particularly open core fast systems, the fuel salt flow is turbulent in nature. In these systems, DNPs not only are displaced by the bulk movement of the fluid but also may be affected by turbulent diffusion. Turbulent diffusion describes the overall lessening of concentration gradients by combined advective and diffusive transport in which the advective component is inherently chaotic and multidirectional. Figure 17 shows some perturbations of the DNP distribution for the fourth group of DNPs when including turbulent diffusion in the DNP modeling for the MOSART reactor.
Comparisons of Supercritical Loop Flow and Heat Transfer Behavior Under Uniform and Nonuniform High-Flux Heat Inputs
Published in Nuclear Science and Engineering, 2023
Dong Yang, Lin Chen, Yongchang Feng, Haisheng Chen
Figure 11 shows the radial variation in the thermophysical properties of case 27 at different axial positions. The analysis of each thermophysical property is similar to the above process. HTD occurs at L/R = 400, where the viscosity is the lowest in each section of the tube. The viscosity affects the turbulent diffusion term, which directly affects the intensity of the turbulence. Combining with the largest heat flux here, the wall temperature comes to a peak value.