Next-generation nuclear reactors are poised to efficiently provide reliable power at enhanced safety levels for many years to come. Among the fundamental designs for these reactors is the very-high-temperature reactor (VHTR), which employs helium as the primary coolant and has the potential of reaching elevated temperatures such that chemical processing (e.g., electrolysis for hydrogen production) is viable alongside traditional electricity generation. This and other next-generation reactors also represent a strategic bridge for ultimately transitioning from fossil fuel-dominated energy portfolios to one where renewable options abound. For the VHTR, the ultimate goal of constructing a new plant will be preceded by additional fundamental research aimed at generating trusted models for behavior prediction under normal and accident scenarios. This paper presents a detailed computational fluid dynamics simulation of the lower plenum, where hot coolant from the core mixes together in a turbulent fashion before traveling to power conversion equipment. Because the flow is expected to enter the lower plenum across a wide range of temperatures and velocities, concerns exist when the mixing is incomplete. The potential hot spots on cylindrical support pedestals and the uniformity of the main outlet are the primary metrics of interest in this study, along with flow velocities and temperatures at different locations within the lower plenum. Three locations are probed in detail and suggest a large range exists for approach velocities and temperatures. A large degree of stratification is also seen on the surfaces of the support pedestals, suggesting a facility should accommodate testing for such behavior. The characterizations presented provide the valuable data needed in order to design appropriate experimental testbeds, where scaled modeling can be carried out in a manner meaningful in predicting the full-scale behavior. (C) 2019 Elsevier Ltd. All rights reserved.

Inlet conditions for a turbulent jet are known to affect the near field behavior but eventually lose their significance downstream. Metrics of importance are often derived from mean and fluctuating velocity components, but little has been done to explore inlet effects on transport of a scalar quantity (e.g., temperature). This paper aims to provide fundamental understanding in this regard and employs large eddy simulations (LES) of a nonisothermal round turbulent jet (Reynolds number of 16,000) with geometry and boundary conditions mimicked after a well-known experimental study. The jet inlet is first modeled with a standard Blasius profile and next by performing a simulation of the upstream flow modeled with either detached eddy simulations (DES) or LES for the second and third approaches, respectively. Only the model employing LES for both upstream nozzle and downstream jet is found to completely capture the root-mean-square (RMS) temperature behavior, namely, a distinct hump when normalized by the local mean centerline temperature at roughly five diameters downstream. Regarding the far field conditions, all three inlet conditions converge for the centerline values, but the radial distributions still portray non-negligible differences. Not surprisingly, the complete LES modeling approach agrees the best with experimental data for mean and RMS distributions, suggesting that the inlet condition plays a vital role in both the near and far field of the jet. The current effort is the very first LES study to successfully capture flow physics for a nonisothermal round turbulent jet in near and far field locations.

Despite the large repository of experimental and computational studies on the topic of turbulent jets, inconclusive and conflicting estimates prevail in regard to certain terms in the turbulence energy budget and the dependence (or lack of dependence) of these and other flow physics on the jet Reynolds number. No comprehensive study exists which adequately addresses these inconsistencies. The purpose of this study is to resolve these contradictions and ascertain the true dependence of the flow statistics on the jet Reynolds number. This is accomplished through high fidelity Large Eddy Simulations (LES), which are performed for a single isothermal round jet at three different Reynolds numbers, encompassing nearly two orders of magnitude. In each case, results are compared to well-accepted experimental and computational studies, and excellent agreement is found with experimental quantities either directly acquired or computed directly from raw data. A separate discrete eddy simulation of the flow in the nozzle upstream of the jet inlet is performed and is found to be crucial in quantifying the flow physics in the near field (e.g., virtual origin). Results show a definite Reynolds number dependence for nearly all third order terms and this is non-negligible especially for the mean convection and production terms in the turbulent kinetic energy budget.

The lower plenum of a High-Temperature Gas-Cooled Reactor (HTGR) consists of numerous jets from the core mixing together across a wide range of temperatures (differences as high as 300-400 K) and velocities (50-100 m/s). The flow from these jets enters the lower plenum and changes direction by 90 degrees and is then forced to navigate through a hexagonal array of support posts. Modeling this level of complexity is a significant computational challenge due to the high velocities and the small length and time scales at which mixing could occur. Efforts aimed at verification and validation of modeling approaches for the lower plenum are underway and will be aided by both experimental and computational studies. The current work focuses on non-isothermal Large Eddy Simulations motivated by a scaled experimental facility whose design is a small subset of the lower plenum, consisting of six jets in the presence of a crossflow, and seven support posts. This "unit cell" serves to mimic flow conditions in different regions of the lower plenum by adjusting velocities, temperatures, and orientations. The primary purpose of this LES-based effort is to provide a much-needed foundation for more applied modeling and experiments by providing insight into the turbulent mixing in the lower plenum of the HTGR. Results in this work reveal that each post in the lower plenum can experience entirely different thermal fluid signatures depending on its location within the unit cell and the jet/crossflow velocities and temperatures. As the jet to crossflow velocity ratio is increased, the penetration of the jets becomes more significant and the variance of the temperature signal on the surface of the posts can become quite large. A significant recirculation region just downstream of the unit cell is also apparent and can begin to impact the temperature signal near the furthest downstream support posts. Regions of the domain where detailed experimental data would be of great value are identified for three separate unit cell configurations representing a wide range of locations within the lower plenum.

Round turbulent jets have fundamental relevance in various engineering applications and are also of practical interest in the lower plenum of the High Temperature Gas-Cooled Reactors (HTGR). In the direction of developing an experimentally validated computational model for the lower plenum flow, a Large Eddy Simulation (LES) of an isothermal high Reynolds number confined jet has been studied. The enclosure within which the jet is confined has been selected large enough so that the results can be compared with well-known experimental studies available in the literature. The Sub-Grid Scale (SGS) model chosen within the LES framework is a variant of the dynamic Smagorinsky model. The effect of inlet flow profile and turbulent fluctuations on the evolution of the jet have been analyzed in detail. The mesh distribution was found to play a vital role in the magnitude and profile of the Reynolds stresses throughout the computational domain. Additionally, it is critically important to properly specify the turbulent fluctuations at the jet inlet in order to accurately predict key near field characteristics such as the potential core length. We perform a separate discrete eddy simulation of the flow in the nozzle upstream of the jet inlet to accurately determine the inlet turbulent fluctuations. The LES results of this study include both first order statistics (mean velocity field) and second order statistics (components of the Reynolds stresses). For each of these quantities, excellent agreement is obtained between our LES predictions and experimental measurements. This research lays the groundwork needed to develop a high-fidelity computational model of the complex mixing flow in the HTGR lower plenum.