Publications

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Thermal protection system designers rely heavily on computational simulation tools for design optimization and uncertainty quantification. Because high-fidelity analysis tools are computationally expensive, analysts primarily use low-fidelity or surrogate models instead. In this work, we explore an alternative approach wherein projection-based reduced-order models (ROMs) are used to approximate the computationally infeasible high-fidelity model. ROMs are preferable to alternative approximation approaches for high-consequence applications due to the presence of rigorous error bounds. This work presents the first application of ROMs to ablation systems. In particular, we present results for Galerkin and least-squares Petrov-Galerkin ROMs of 1D and 2D ablation system models.

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We present recent results toward the quantification of spray characteristics at engine conditions for an eight-hole counter-bored (stepped) GDI injector – Spray G in the ECN denomination. This computational study is characterized by two novel features: the detailed description of a real injector's internal surfaces via tomographic reconstruction; and a general equation of state that represents the thermodynamic properties of homogeneous liquid-vapor mixtures. The combined level-set moment-of-fluid approach, coupled to an embedded boundary formulation for moving solid walls, makes it possible to seamlessly connect the injector's internal flow to the spray. The Large Eddy Simulation (LES) discussed here presents evidence of partial hydraulic flipping and, during the closing transient, string cavitation. Results are validated by measurements of spray density profiles and droplet size distribution.

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We propose a novel approach to evaluate the relaxation time of vapor bubble growth in the context of the flash boiling of a superheated liquid. In alternative to the empirical correlation derived from superheated water experiments almost fifty years ago, the new model describes the thermally-dominated growth of vapor bubbles in terms that are dependent on the local Jakob number (the ratio of sensible heat to latent heat during phase change) and the number density of vapor bubbles. The model is tested by plugging the resulting relaxation time into the Homogenous Relaxation Model (HRM). Flash-boiling simulations carried out with HRM are compared with n-pentane (C5H12) injection and boil-off experiments conducted with a real-size, axial-hole, transparent gasoline injector discharging into a constant-pressure vessel. The long-distance microscopy images from the experiments, processed to derive the projected liquid volume (PLV) of the spray, provide a unique set of time-resolved validation data for direct fuel injection simulations. At conditions ranging from flaring to mild and minimal flash boiling, we show that switching to the new relaxation time improves the agreement with the measured PLV profiles with respect to the standard empirical model. Particularly at flaring conditions, the predicted increase in gas cooling caused by rapid vapor production is shown to be more consistent with the observed boil-off.

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Numerical simulations of internal nozzle flow that include transient needle valve motion offer the potential to better predict spray penetration, mixing and liquid breakup. For example, the level of gas initially inside the sac and holes, as well as the rate of needle movement, influence the initial fuel delivery rate and spray development, thereby affecting ignition position and combustion. In this study, needle movement and gas exchange inside operating transparent fuel injectors are imaged at high speed, and CFD simulations with fine resolution (2-micrometers) in the needle-seat area are performed to understand the impact of needle movement and initial gas in the sac on ramp-up in rate of injection. The injector bodies and sac geometries are replicas of the Engine Combustion Network Spray A and Spray D injectors. Imaging shows that gas is ingested into the injector at the beginning of needle movement, an unexpected results given the high injection pressure above the needle valve. Finite element analysis simulations accounting for the elastic properties of the metal seat and needle are performed to explain this result. As forces on the needle and seat are relieved at the beginning of injection, the sac volume enlarges while contact between sealing surfaces remains. Needle and nozzle wall measurements confirm that the needle tip may move roughly 5-10 micrometers before the passage opens at the needle seat to allow flow and pressurization of the sac. Measured needle movement from an experiment (optical or X-ray) must be corrected to achieve a different "needle gap" profile for simulations with no elasticity. This elasticity-corrected profile should be used for CFD simulations, otherwise, early and incorrect spray development will be predicted. Simulations with the corrected needle-lift profile and gas initially within the sac show that the mass flow rate at the start of injection includes cycling in flow rate caused primarily by sac pressure fluctuations, which are recommended for future Lagrangian CFD simulations.

We report on the development of a model framework to simulate spray flames from direct injection of liquid fuel into an automotive cylinder engine. The approach to this challenging problem was twofold. On one hand, the interface-capturing multiphase computer code CLSVOF was used to resolve the rapidly evolving, topologically convoluted interfaces that separate the liquid fuel from the gas at injection: the main challenges to address were the treatment of the high-pressure flow inside the injector, which required the inclusion of compressibility effects; and the computational framework necessary to achieve a Direct Numerical Simulation (DNS) level of accuracy. On the other hand, the scales of turbulent fuel mixing and combustion in the cylinder engine were ad- dressed by the high-performance computer code RAPTOR within the Large Eddy Simulation (LES) framework. To couple the two computational methods, a novel methodology was developed to de- scribe the dense spray dynamics in Raptor from the assigned spray size distribution and dispersion angle derived from CLSVOF. This new, independent Eulerian Multi-Fluid (EMF) spray module was developed based on the kinetic description of a system of droplets as a pressure-less gas; as we will show, it was demonstrated to efficiently render the near-nozzle coupling in mass, momentum, and energy with the carrier gas phase.

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We investigate the optical properties of ice crystals nucleated on atmospheric black carbon (BC). The parameters examined in this study are the shape of the ice crystal, the volume fraction of the BC inclusion, and its location inside the crystal. We report on new spectrometer measurements of forward scattering and backward polarization from ice crystals nucleated on BC particles and grown under laboratory-controlled conditions. Data from the Cloud and Aerosol Spectrometer with Polarization (CASPOL) are used for direct comparison with single-particle calculations of the scattering phase matrix. Geometrical optics and discrete dipole approximation techniques are jointly used to provide the best compromise of flexibility and accuracy over a broad range of size parameters. Together with the interpretation of the trends revealed by the CASPOL measurements, the numerical results confirm previous reports on absorption cross-section magnification in the visible light range. Even taking into account effects of crystal shape and inclusion position, the ratio between absorption cross-section of the compound particle and the absorption cross-section of the BC inclusion alone (the absorption magnification) has a lower bound of 1.5; this value increases to 1.7 if the inclusion is centered with respect to the crystal. The simple model of BC-ice particle presented here also offers new insights on the effect of the relative position of the BC inclusion with respect to the crystal's outer surfaces, the shape of the crystal, and its size.

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The breakup of liquid metals is of relevance to powder formation, thermal spray coatings, liquid metal cooling systems, investigations of accident scenarios, and model validation. In this work, a column of liquid Galinstan, a room-temperature liquid metal alloy, is studied in a shock-induced cross-flow. Backlit experiments are used to characterize breakup morphology and digital in-line holography is used to quantitatively measure the size and speed of secondary droplets. Two-dimensional simulations are also developed in order to help understand the underlying mechanisms that drive breakup behavior. Results show that although breakup morphologies are similar for water and Galinstan at the same Weber number, the breakup distance, secondary droplet size, and secondary droplet shapes differ. Evidence indicates that secondary droplet formation may be related to the Weber number, density ratio, the convective velocity and other effects.

Numerical simulation of sprays can potentially be used for assessing the performance and variability of engines. But today's simulations only provide average quantities, after calibration. Spray injection can be thought of as a multiscale problem with 4 levels (chamber, mixing layer, drop scale, and nozzle). Because of their complex interactions, increasing predictivity requires simulations to capture more of these scales. To assess the trade-offs in this regard, we perform a review of recent Diesel spray simulations. After highlighting the importance of the mixing layer in driving spray dynamics, we analyze various two-phase flow formalisms and show the potential benefits of a Eulerian spray formulation combined with Large Eddy Simulation (LES) to capture a variable-density mixing layer (VDML). We then present a framework and numerical methods to make this description operative and show how it performs on a realistic autoignition case called Spray A.

We report on the development of a model framework to simulate spray flames from direct injection of liquid fuel into an automotive cylinder engine. The approach to this challenging problem was twofold. On one hand, the interface-capturing multiphase computer code CLSVOF was used to resolve the rapidly evolving, topologically convoluted interfaces that separate the liquid fuel from the gas at injection: the main challenges to address were the treatment of the high-pressure flow inside the injector, which required the inclusion of compressibility effects; and the computational framework necessary to achieve a Direct Numerical Simulation (DNS) level of accuracy. On the other hand, the scales of turbulent fuel mixing and combustion in the cylinder engine were ad- dressed by the high-performance computer code RAPTOR within the Large Eddy Simulation (LES) framework. To couple the two computational methods, a novel methodology was developed to de- scribe the dense spray dynamics in Raptor from the assigned spray size distribution and dispersion angle derived from CLSVOF. This new, independent Eulerian Multi-Fluid (EMF) spray module was developed based on the kinetic description of a system of droplets as a pressure-less gas; as we will show, it was demonstrated to efficiently render the near-nozzle coupling in mass, momentum, and energy with the carrier gas phase.

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As part of a Laboratory Directed Research and Development project, we are developing a modeling-and-simulation capability to study fuel direct injection in automotive engines. Predicting mixing and combustion at realistic conditions remains a challenging objective of energy science. And it is a research priority in Sandia’s mission-critical area of energy security, being also relevant to many flows in defense and climate. High-performance computing applied to this non-linear multi-scale problem is key to engine calculations with increased scientific reliability.