Michael L. Hall

Diffusion, P1, and other approximate forms of radiation transport

Abstract Full transport solutions of time-dependent problems can be computationally very expensive. Therefore, considerable effort has been devoted to developing approximate solution techniques that are much faster computationally and yet are accurate enough for a particular application. Many of these approximate solutions have been used in isolated problems and have not been compared to each other. This paper presents two test problems that test and compare several approximate transport techniques. In addition to the diffusion and P1 approximations, we will test several different flux-limited diffusion theories and variable Eddington factor closures. For completeness, we will show some variations that have not yet appeared in the literature that have some interesting consequences. For example, we have found a trivial way to modify the P1 equations to get the correct propagation velocity of a radiation front in the optically thin limit without modifying the accuracy of the solution in the optically thick limit. Also, we will demonstrate nonphysical behavior in some published techniques.

A cell-centered lagrangian-mesh diffusion differencing scheme

Abstract A new cell-centered diffusion differencing scheme for the quadrilateral meshes associated with Lagrangian hydrodynamics codes is described. Computational comparisons of this scheme and existing schemes are given. It is shown that the new scheme is much more accurate than existing schemes when the mesh is significantly skewed. The new scheme is also more costly because there are special cell-edge unknowns in addition to the standard cell-center unknowns, and the associated diffusion matrix is asymmetric. The disadvantages of an asymmetric diffusion matrix are mitigated by a multigrid solution technique that is quite effective.

Status Report on the Throhput Transient Heat Pipe Modeling Code

Improvements have been made to the throhput code which models transient thermohydraulic heat pipe behavior. The original code was developed as a doctoral thesis research code by Hall. The current emphasis has been shifted from research into the numerical modeling to the development of a robust production code. Several modeling obstacles that were present in the original code have been eliminated, and several additional features have been added.

A sensitivity study of the effects of evaporation/condensation accommodation coefficients on transient heat pipe modeling

Abstract The dynamic behavior of liquid metal heat pipe models is strongly influenced by the choice of evaporation and condensation modeling techniques. Classic kinetic theory descriptions of the evaporation and condensation processes are often inadequate for real situations; empirical accommodation coefficients are commonly utilized to reflect nonideal mass transfer rates. The complex geometries and flow fields found in proposed heat pipe systems cause considerable deviation from the classical models. The THROHPUT code, which has been described in previous works, was developed to model transient liquid metal heat pipe behavior from frozen startup conditions to steady state full power operation. It is used here to evaluate the sensitivity of transient liquid metal heat pipe models to the choice of evaporation and condensation accommodation coefficients. Comparisons are made with experimental liquid metal heat pipe data. It is found that heat pipe behavior can be predicted with the proper choice of the accommodation coefficients. However, the common assumption of spatially constant accommodation coefficients is found to be a limiting factor in the model.

Integrated modeling of nuclear thermal rocket system safety and performance

The Advanced Nuclear Thermal Rocket Engine Simulator (ANTARES) environment, which consistes of an overall architecture employing state‐of‐the‐art computer hardware and an integrated physics simulation nucleus is presented. The ANTARES architecture is described, and its benefits are outlined. In an attempt to demonstrate the fidelity and ability of the physics simulator, a coupled physics methodology for use in simulating design basis accidents is presented. A preliminary high‐resolution, heat‐transfer model for use in ANTARES is also presented.

Transient Thermohydraulic Heat Pipe Modeling: Incorporating throhput into the cæsar Environment

The THROHPUT code, which models transient thermohydraulic heat pipe behavior, is being incorporated into the CAESAR computational physics development environment. The CAESAR environment provides many beneficial features for enhanced model development, including levelized design, unit testing, Design by ContractTM (Meyer, 1997), and literate programming (Knuth, 1992), in a parallel, object-based manner. The original THROHPUT code was developed as a doctoral thesis research code; the current emphasis is on making a robust, verifiable, documented, component-based production package. Results from the original code are included.