Hernan Tinoco

Numerical Simulation of Industrial Flows

Computational Fluid Dynamics (CFD) is a numerical methodology for analyzing flow systems that may involve heat transfer, chemical reactions and other related phenomena. This approach employs numerical methods imbedded in algorithms to solve general conservation and constitutive equations together with specific models within a large number of control volumes (cells or elements) into which the associated computational domain of the flow system has been divided to build up a grid. Numerical simulation of industrial flows using commercial CFD codes is now well developed in a number of technical fields. With the advent of powerful and low-cost computer clusters, events including both complex geometry and high Reynolds numbers, i.e. fully turbulent practical industrial applications, may today be accurately modeled. This technique constitutes a rather new tool for analyzing problems related to, for instance, design, performance, safety and trouble-shooting of industrial systems since time can now be treated fully as the primary independent variable. The first commercial general-purpose CFD code, built around a finite volume solver, the Parabolic Hyperbolic Or Elliptic Numerical Integration Code Series (PHOENICS), was released in 1981. Initially, the solver was conformed to work only with structured, monoblock, regular Cartesian grids but it was subsequently broadened to admit even structured body-fitted grids. The multi-block grid option was developed many years later within this code which still preserves this restricting structured grid topology. Another well known commercial CFD code, FLUENT, was brought out onto the market in 1983 as a structured software that bore a resemblance to PHOENICS, but aimed towards modeling of systems with chemical reactions, specifically those related to combustion. Hence, during the 1980s, CFD simulations were limited to rough time-independent models with very simplified geometry due to the grid-structured character of the software and the vast limitations in, at that time, normally available computer resources at the industry (see e.g. Tinoco & Hemstrom, 1990). It might be of some interest to point out that the top performance of a supercomputer at the end of the 1980s was of the order of 10 GFLOPS (10×109 FLoating point Operations Per Second). The computers normally available at the industry had a thousandth to a hundredth of that performance, i.e. 10-100 MFLOPS. Today, a computer cluster containing a couple of hundred CPUs has a capacity of the order of TFLOPS. At the beginning of the 1990s, important steps in software improvement took place through the development of grid-unstructured, parallelized algorithms (e.g. FLUENT UNS) that

Flow Mixing Inside a Control-Rod Guide Tube: Part I—CFD Simulations

Two broken control rods and a large number of rods with cracks were found at the inspection carried out during the refueling outage of the twin reactors Oskarshamn 3 and Forsmark 3 in the fall of 2008. As a part of an extensive damage investigation, time dependent CFD simulations of the flow and the heat transfer in the annular region formed by the guide tube and control rod stem were carried out, [1]. The simulations together with metallurgical and structural analyses indicated that the cracks were initiated by thermal fatigue. The knowledge assembled at this stage was sufficient to permit the restart of both reactors at the end of year 2008 conditioned to that further studies to be carried out for clarifying all remaining matters. Additionally, all control rods were inserted 14% to protect the welding region of the stem. Unfortunately, this measure led to new cracks a few months later. This matter will be explained in the second part of this work, [2]. As a part of the accomplished complementary work, new CFD models were developed in conformity with the guidelines of references [3] and [4]. The new results establish the simulation requirements needed to accomplish accurate conjugate heat transfer predictions. Those requirements are much more rigorous than the ones needed for flow simulations without heat transfer. In the present case, URANS simulations, which are less resource consuming than LES simulations, seem to rather accurately describe the mixing process occurring inside the control rod guide tube. Structure mechanics analyses based on the CFD simulations show that the cracks are initiated by thermal fatigue and that their propagation and growth are probably enhanced by mechanical vibrations.Copyright

Flow Mixing Inside a Control-Rod Guide Tube: Part II—Experimental Tests and CFD-Simulations

Alarge number of control rod cracks were detected during therefuelling outage of the twin reactors Oskarshamn 3 and Forsmark3 in the fall of 2008. The extensive damage investigationfinally lead to ...