Todd S. Mintz

Comparative Assessment of Turbulence Models for Prediction of Flow-Induced Corrosion Damages

The present study makes a comparative assessment of different turbulence models in simulating the flow-assisted corrosion (FAC) process for pipes with noncircular cross sections and bends, features regularly encountered in heat exchangers and other pipeline networks. The case study investigates material damage due to corrosion caused by dissolved oxygen (O2 ) in a stainless steel pipe carrying an aqueous solution. A discrete solid phase is also present in the solution, but the transport of the solid particles is not explicitly modeled. It is assumed that the volume fraction of the solid phase is low, so it does not affect the continuous phase. Traditional two-equation models are compared, such as isotropic eddy viscosity, standard k-e and k-ω models, shear stress transport (SST) k-ω models, and the anisotropic Reynolds Stress Model (RSM). Computed axial and radial velocities, and turbulent kinetic energy profiles predicted by the turbulence models are compared with available experimental data. Results show that all the turbulence models provide comparable results, though the RSM model provided better predictions in certain locations. The convective and diffusive motion of dissolved O2 is calculated by solving the species transport equations. The study assumes that solid particle impingement on the pipe wall will completely remove the protective film formed by corrosion products. It is also assumed that the rate of corrosion is controlled by diffusion of O2 through the mass transfer boundary layer. Based on these assumptions, corrosion rate is calculated at the internal pipe walls. Results indicate that the predicted O2 corrosion rate along the walls varies for different turbulence models but show the same general trend and pattern.Copyright

Numerical Evaluation of Corrosion Condition for Pressurized Water Reactor Secondary Cooling System

Flow-accelerated corrosion (FAC) in a pressurized water reactor (PWR) feedwater piping system is considered the leading degradation process that has been blamed for a number of accidents and incidents. One of the challenging issues in estimating FAC in piping is the hydrazine (N2 H4 ) oxygen (O2 ) reaction rate and its subsequent impact on the wall mass transfer. N2 H4 is injected in the system to maintain a stable pH in the water and also to act as an O2 scavenger. Previous research results indicate a varying degree of FAC rate dependence on the presence of N2 H4 in the system. The N2 H4 -O2 reaction is also a complex function of temperature and pipe material. The present paper presents a two-part analysis that uses computational fluid dynamics (CFD) tools to investigate the N2 H4 -O2 reaction and its subsequent impact on mass transfer. In the first part of the analysis, chemistry and flow of water with dissolved O2 and N2 H4 is simulated to assess different reaction mechanisms available in the literature. Results obtained from this study are compared with available experimental data for benchmarking. The numerical results were able to capture the general pattern of reaction rate as a function of temperature. Numerical simulations were also carried out to accommodate the surface effects on the reaction, but results indicate that such accommodations will yield useful results only if the geometrical extent of the near-wall-zone, where surface effects are prevalent, is well known. In the second part of the analysis, numerical simulations were carried out for a U-bend pipe. A number of restrictive assumptions were made to assess the dependence of O2 mass-transfer rate on N2 H4 -O2 reaction. Hydrodynamic results show the secondary flow pattern within the bend section. Results are also presented for the Sherwood number ratio at the pipe wall with and without reaction. Results indicate that the N2 H4 -O2 reaction decreases the O2 flow rate toward the wall. This calculation also shows that secondary flow in the bend affects the wall mass transfer pattern.Copyright

Historical Rail Accident Analyses Identifying Accident Parameters That Could Impact Transportation of Spent Nuclear Fuel

As the regulatory authority for transportation of spent nuclear fuel (SNF) in the United States, the Nuclear Regulatory Commission (NRC) requires that SNF transportation packages be designed to endure a fully engulfing fire with an average temperature of 800 °C (1,475 °F) for 30 minutes, as prescribed in Title 10 of the Code of Federal Regulations (CFR) Part 71. The work described in this paper was performed to support NRC in determining the types of accident parameters that could produce a severe fire with the potential to fully engulf a SNF transportation package. This paper describes the process that was used to characterize the important features of rail accidents that would potentially lead to a spent nuclear fuel transport package being involved in a severe fire. Historical rail accidents involving hazardous material and long duration fires in the United States have been analyzed using data from the Federal Railroad Administration (FRA) and the Pipeline and Hazardous Materials Safety Administration (PHMSA). Parameters that were evaluated from this data include, but were not limited to, class of track where the accident occurred, class of hazardous material that was being transported, and number of railcars involved in the fire. The data analysis revealed that in the past 34 years of rail transport, roughly 1,800 accidents have led to the release of hazardous materials resulting in a frequency of roughly 1 accident per 10 million freight train miles. In the last 12 years, there have only been 20 accidents involving multiple car hazardous material releases that led to a fire. This results in an accident rate of 0.003 accidents per million freight train miles that involved multiple car releases and a fire. In all the accidents analyzed, only one involved a railcar carrying Class 7 (i.e., radioactive) hazardous material (HAZMAT).Copyright