Flavio Dal Forno Chuahy

Tristructural-isotropic (TRISO) fuel and other coated particle nuclear fuels have been selected as candidate fuels for a wide range of microreactor concepts for terrestrial use and space exploration. The fabrication of these fuels largely relies on spout fluidized bed chemical vapor deposition, a process in which a batch of fuel kernels is subjected to flows of fluidizing and pyrolyzing gases, which are precursors to the desired coating composition. Kernel sphericity, particle size distribution (PSD), and surface roughness are critical parameters that can affect fluidization, coating quality, and ultimately fuel performance. Understanding the impact of PSD of the initial fuel kernel could allow the use of kernels with a wider fuel specification. If possible, reducing requirements on sphericity and particle size uniformity without compromising coating performance could reduce process cost, time, and waste generation. Therefore, a better understanding of their impact on the fluidization and coating characteristics is needed. This work used computational fluid dynamics coupled with discrete element modeling to assess the impact of polydispersity of the initial kernel population on fluidization for two mean particle sizes: 250 and 400 um. The initial particle size is normally distributed with standard deviations varying from 12 um (matching experimental conditions) to 63 um. The results show that preferential fluidization effects are increasingly present with wider PSDs and are more prevalent for the smaller mean particle size of 250 um. Increasing the PSD leads to an increase in coating thickness variations for the smallest particles and smaller coating thickness variations for the largest particles. The results have important implications to fabrication efforts, where consistency of coating thickness is of importance.

The International Maritime Organization (IMO) introduced the Energy Efficiency Design Index (EEDI) in 2013, a regulatory framework of associated metrics for reducing emissions of CO₂ per tonne-mile from shipping by approximately 10% each decade. Therefore, decarbonizing the maritime sector requires the development of new fuel sources. Because of the extremely large physical size of the internal combustion engines present in shipping vessels, experimental iterative development of the engine and fuel system is cost-prohibitive. Thus, the ability to perform combustion system development in a scaled platform that can be more easily operated and modeled computationally is of interest. To that end, scaling relationships are needed to translate the results from a smaller engine to a larger counterpart. Scaling studies to date have been restricted to low scaling ratios, four-stroke light-duty engines, and under-resolved computational fluid dynamic simulations that likely do not accurately capture the physics of scaling. In this work, computational models of a 1:10 scale and a full-scale two-stroke crosshead low-speed marine engine were created and validated against experiments obtained in a real 1:10 scale engine installed at Oak Ridge National Laboratory (ORNL). The results of the small- and large-scale engine simulations were compared to analyze the effectiveness of the appropriate scaling laws under these extreme scaling ratio conditions.

Traditional selective catalytic reduction aftertreatment technologies used to reduce N O x are very limited at exhaust temperatures below 175 ° C . Therefore, under these low engine load conditions, having effective in-cylinder control of N O x emissions is important. Previous work by the authors explored the effect of fuel physical properties on the ability to control N O x in-cylinder. That work was limited to one direct injection near top dead center. Modern diesel high-pressure fuel systems have the capability of five or more injections in one engine cycle. A higher-volatility diesel fuel and high amounts of exhaust gas recirculation to delay ignition could provide an opportunity for reduction in engine-out N O x through an increased level of fuel premixing. By appropriately timing multiple short injections, a more optimal distribution of fuel in-cylinder may be achieved, which could reduce N O x while maintaining an efficient combustion phasing. A computational fluid dynamics model previously validated against experimental data was used to explore several injection strategies with increased levels of fuel premixing to assess the potential trade-offs between N O x and CO/unburned hydrocarbon (UHC) emissions and thus reduce reliance on the aftertreatment system for N O x control. The results show that the devised injection strategies resulted in an increased level of fuel premixing. However, none of the attempted injection strategies resulted in significant N O x reductions, and all strategies showed a significant increase in CO and UHC emissions.