Publications

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Here the analytical plate penetration relationship derived by De Chant [An explanation for the minimal effect of body curvature on hypervelocity penetration hole formation. Sandia National Laboratories, Albuquerque, NM, no. SAND2003-2696J. To appear in: International Journal of Solids and Structures] using an analogy between hydrodynamics and penetration processes is combined with the Grady-Kipp dynamic fragmentation model [International Journal of Impact Engineering 14 (1993) 427; Fragmentation of solids under dynamic loading. In: Wierzbicki, T., Jones, N. (Eds.), Structural Failure. Wiley, New York, 1989; Mechanisms of dynamic fragmentation: factors governing growth size, SAND84-2304C, Sandia National Laboratories, Albuquerque, NM, 1984] to estimate the size and distribution of fragments generated due to impact and penetration of thin plates by spherical projectiles. Fragment statistics estimated using this analytical model are then compared to fragmentation as predicted by CTH, a shock-physics hydrocode [International Journal of Impact Engineering 10 (1990) 351]. Agreement between analytically based model and CTH simulations shows good agreement. Extending the hydrodynamic based analogy used to develop the analytical plate penetration model to include fragmentation processes, we derive expressions relation fragmentation to plate impact and penetration. The functional form of these equations is also in good agreement with CTH simulation. Finally, a deterministic (as opposed to statistical) cumulative particle size distribution relationship is derived using mass conservation concepts and gives a power law distribution. The power law distribution requires the presence of a non-zero minimum fragment size rather than a continuous distribution that includes particles approaching zero size. The presence of a non-zero fragment size is in agreement with computational models and theoretically based cumulative lognormal fragment distributions. © 2004 Elsevier Ltd. All rights reserved.

The length scale of stress domain patterns formed at solid surfaces is usually calculated using isotropic elasticity theory. Because this length depends exponentially on elastic constants, deviations between isotropic and anisotropic elasticity can lead to large errors. Another inaccuracy of isotropic elasticity theory is that it neglects the dependence of elastic relaxations on stripe orientation. To remove these inaccuracies, we calculate the energy of striped domain patterns using anisotropic elasticity theory for an extensive set of surfaces encountered in experimental studies of self-assembly. We present experimental and theoretical evidence that elastic anisotropy is large enough to determine the stripe orientation when Pb is deposited on Cu(111). Our analytical and numerical results should be useful for analysis of a broad range of experimental systems. ©2005 The American Physical Society.

As interest in 3D face recognition increases the importance of the initial alignment problem does as well. In this paper we present a method utilizing the registered 2D color and range image of a face to automatically identify the eyes, nose, and mouth. These features are important to initially align faces in both standard 2D and 3D face recognition algorithms. For our algorithm to run as fast as possible, we focus on the 2D color information. This allows the algorithm to run in approximately 4 seconds on a 640X480 image with registered range data. On a database of 1,500 images the algorithm achieved a facial feature detection rate of 99.6% with 0.4% of the images skipped due to hair obstruction of the face.

A continuum-scale, evolutionary model of bubble nucleation, growth and He release for aging metal tritides is described which accounts for major features of the tritide database. Bubble nucleation, modeled as self-trapping of interstitially diffusing He atoms, occurs during the first few days following tritium introduction into the metal. Bubble growth by dislocation loop punching yields good agreement between He atomic volumes and bubble pressures determined from bulk swelling and 3He NMR data. The bubble spacing distribution determined from NMR is shown to remain fixed with age, justifying the separation of nucleation and growth phases and providing a sensitive test of the growth formulation. Late in life, bubble interactions are proposed to produce cooperative stress effects, which lower the bubble pressure. Helium generated near surfaces and surface-connected porosity accounts for the low-level early helium release. Use of an average ligament stress criterion predicts an onset of inter-bubble fracture in good agreement with the He/Metal ratio observed for rapid He release. From the model, it is concluded that He retention can be controlled through control of bubble nucleation.

Numerical methods are employed to examine the work, electric power input, and efficiency of electrokinetic pumps at a condition corresponding to maximum pump work. These analyses employ the full Poisson-Boltzmann equations and account for both convective and conductive electric currents, including surface conductance. We find that efficiencies at this condition of maximum work depend on three dimensionless parameters, the normalized zeta potential, normalized Debye layer thickness, and a fluid property termed the Levine number indicating the nominal ratio of convective to conductive electric currents. Efficiencies at maximum work exhibit a maximum for an optimum Debye layer thickness when the zeta potential and Levine number are fixed. This maximum efficiency increases with the square of the zeta potential when the zeta potential is small, but reaches a plateau as the zeta potential becomes large. The maximum efficiency in this latter regime is thus independent of the zeta potential and depends only on the Levine number. Simple analytical expressions describing this maximum efficiency in terms of the Levine number are provided. Geometries of a circular tube and planar channel are examined. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The diminished response of thermoluminescent phosphors over time is a well-documented challenge to thermoluminescent dosimetry. Wide ranges in fading rates for various phosphor types have been reported, making it necessary for many external dosimetry programs to perform individual studies on thermoluminescent fade. Sandia National Laboratories currently uses the Thermo 8802 LiF:Mg,Ti thermoluminescent dosimeter (TLD) in its personnel external dosimetry program. Doses received in the field are calculated by applying a fade algorithm published by the manufacturer to TLD readings. Since the algorithm was established by characterizing the diminished response of a TLD similar to the 8802, Sandia chose to model its fade study after the analysis done by Thermo. As a result, the parameters of each experiment were comparable, and data from the two studies were compared to determine whether or not the current algorithm should be modified specifically for use at Sandia. Cards were irradiated using an internal 90Sr/90Y source, and pre- and post-irradiation fading rates were monitored over a period of 18 wk. While significant fading was demonstrated, results closely matched those found in the original Thermo study.

The molten salt Flibe, a combination of lithium and beryllium fluorides studied for molten salt fission reactors, has been proposed as a breeder and coolant for fusion applications. The melting points of 2LiF-BeF2 and LiF-BeF2 are 460°C and 363°C, but LiF-BeF2 is rather viscous and has less lithium for breeding. In the Advanced Power Extraction (APEX) Program, concepts with a free flowing liquid for the first wall and blanket were investigated. Flinabe (a mixture of LiF, BeF2 and NaF) was selected for a molten salt design because a melting temperature below 350°C appeared possible and this provided an attractive operating temperature window for a reactor. To confirm that a ternary salt with a low melting temperature existed, several combinations of the fluoride salts, LiF, NaF and BeF2, were melted in a stainless steel crucible under vacuum. One had an apparent melting temperature of 305°C. The test system, preparation of the mixtures, melting procedures and temperature curves for the melting and cooling are presented along with the apparent melting points. Thermal modeling of the salt pool and crucible is reported in an accompanying paper.

It is generally accepted that thermal (Zeldo'vich) chemical kinetics dominate NO formation in diesel engines, so control of temperature is critical for reducing exhaust NOx emissions. Recent optical engine data revealed that when the start of injection (SOI) was retarded to very late timings, combustion luminosity decreased while exhaust NOx emissions increased, causing a "NOx bump." This data suggested that changes in radiative heat transfer from soot may affect in-cylinder temperatures and subsequent NOx formation. In this study, soot thermometry measurements of in-cylinder temperature and radiative heat transfer were correlated with exhaust NOx to quantify the role of radiative heat transfer on in-cylinder temperatures and NOx formation. The engine was operated at low-load conditions, for which the premixed burn was a significant fraction of the total heat release. Soot thermometry showed that radiative cooling reduced flame temperatures by 25-50 K for early SOI, reducing exhaust NOx by 12-25%. Near the NOx bump, radiative cooling was essentially absent, and the NOx reduction was thus removed. Radiative cooling alone, however, was found to be insufficient to be solely responsible for the observed changes in NOx emissions. Thermodynamic analysis showed that in the slow-mixing limit, compression-heating of burned gases for the large premixed-burn conditions near the NOx bump can also increase temperatures by about 25 K, for a further 12% increase in NOx. The data also shows that these two factors can contribute to other exhaust NO observations, such as the intake temperature optimum for minimizing NOx emissions. Finally, although it was not directly measured, the rate of post-flame mixing was identified and examined as a third primary factor affecting in-cylinder temperatures and NOx formation in diesel engines.Copyright © 2005 SAE International.

PDC drill bit performance has been greatly improved over the past three decades by innovations in bit design and how these designs are applied. The next leap forward is most likely to come from using high-speed, real-time downhole data to optimize the performance of these sophisticated bits on an application-by-application basis. By effectively managing conditions of lateral, axial and torsional acceleration, damage to cutting structures can be minimized for improved penetration rates. Avoiding these damaging vibrations is essential to increasing bit durability and improving overall drilling economics. This paper describes one set of independent drilling optimization results obtained as part of a series of controlled demonstrations of PDC bits. Sandia National Laboratories on behalf of the U. S. Department of Energy (DOE) managed this work. The effort was organized as a Cooperative Research and Development Agreement (CRADA) established between Sandia and four bit manufacturers with DOE funding and in-kind contributions by the industry partners. The goal of this CRADA was to demonstrate drag bit performance in formations with degrees of hardness typical of those encountered while drilling geothermal wells. The test results indicate that the surface weight-on-bit (WOB), revolutions per minute (RPM) and torque readings traditionally used to guide adjustments in the drilling parameters do not always provide the true picture of what is really taking place at the bit. Instead, a holistic approach combining traditional methods of optimization together with high-speed, real-time data enables far better decisions for improving bit performance and avoiding damaging situations that may have otherwise gone unnoticed.

The Z-Pinch Power Plant uses the results from Sandia National Laboratories' Z accelerator in a power plant application to generate energy pulses using inertial confinement fusion. A collaborative project has been initiated by Sandia to investigate the scientific principles of a power generation system using this technology. Research is under way to develop an integrated concept that describes the operational issues of a 1000 MW electrical power plant. Issues under consideration include: 1-20 gigajoule fusion pulse containment, repetitive mechanical connection of heavy hardware, generation of terawatt pulses every 10 seconds, recycling often thousand tons of steel, and manufacturing of millions of hohlraums and capsules per year. Additionally, waste generation and disposal issues are being examined. This paper describes the current concept for the plant and also the objectives for future research.

Optical parametric chirped-pulse amplifiers utilizing a 300-ps Nd:YAG pump system, a tunable 1.6-μm fiber signal, and KNbO3, KTA, RTP, or BBO nonlinear crystals were designed and built. Gain >109, and peak powers >30GW were obtained. © 2005 Optical Society of America.

An experimental investigation is made into the fluid mechanics and heat transfer of a circular cylinder immersed in a wall-bounded turbulent mixed-convection flow of water. The cylinder is oriented spanwise to the forced channel flow and within the thermal boundary layer of the heated lower wall. The flow channel is capped with a cold, near-adiabatic upper wall producing a fully turbulent gap Rayleigh number of 108. A low-speed crossflow is applied to advect the turbulent thermal plumes over the cylinder surface. We present spatially resolved cylinder-surface heat-flux data alongside 2-D PIV imaging of the streamwise and wall-normal velocity components for two flow conditions in the mixed-convection heat-transfer regime. The measured cylinder-wake flowfield reflects the complex coupling between the separated wake flow, the highly turbulent freestream and the buoyant wall and cylinder boundary layers. A method for measurement of spatially resolved surface heat fluxes based on the measured cylinder-surface temperature distribution and a well-posed two-dimensional solution to the conduction problem in the cylinder wall is presented. The resulting spatially resolved flux measurements show enhanced surface heat transfer, which results from the intense buoyancy generated free-stream turbulence and mixing in the cylinder wake. This work extends the literature on thermal convection with crossflow well into the turbulent regime and is, to our knowledge, the first investigation of surface heat-transfer to an object of engineering importance placed in this type of turbulent mixed-convection flowfield. The data are currently being utilized for validation of mixed-convection turbulence models at Sandia and comparisons between the computational and experimental results are presented.

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Forward-to-reverse bias step-recovery measurements were performed on In.07Ga.93N/GaN and Al.36Ga.64N/Al.46Ga.54N quantum-well (QW) light-emitting diodes grown on sapphire. With the QW sampling the minority-carrier hole density at a single position, distinctive two-phase optical decay curves were observed. Using diffusion equation solutions to self-consistently model both the electrical and optical responses, hole transport parameters tp = 758 {+-} 44 ns, Lp = 588 {+-} 45 nm, and up = 0.18 {+-} 0.02 cm2/Vs were obtained for GaN. The mobility was thermally activated with an activation energy of 52 meV, suggesting trap-modulated transport. Optical measurements of sub-bandgap peaks exhibited slow responses approaching the bulk lifetime. For Al.46Ga.54N, a longer lifetime of tp = 3.0 us was observed, and the diffusion length was shorter, Lp = 280 nm. Mobility was an order of magnitude smaller than in GaN, up = 10-2 cm2/Vs, and was insensitive to temperature, suggesting hole transport through a network of defects.

We report micro-Raman studies of self-heating in an AlGaN/GaN heterostructure field-effect transistor using below (visible 488.0 nm) and near (UV 363.8 nm) GaN band-gap excitation. The shallow penetration depth of the UV light allows us to measure temperature rise ({Delta}T) in the two-dimensional electron gas (2DEG) region of the device between drain and source. Visible light gives the average {Delta}T in the GaN layer, and that of the SiC substrate, at the same lateral position. Combined, we depth profile the self-heating. Measured {Delta}T in the 2DEG is consistently over twice the average GaN-layer value. Electrical and thermal transport properties are simulated. We identify a hotspot, located at the gate edge in the 2DEG, as the prevailing factor in the self-heating.

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Order-of-accuracy verification is necessary to ensure that software correctly solves a given set of equations. One method to verify the order of accuracy of a code is the method of manufactured solutions. In this study, a manufactured solution has been derived and implemented that allows verification of not only the Euler, Navier-Stokes, and Reynolds-Averaged Navier-Stokes (RANS) equation sets, but also some of their associated boundary conditions (BC's): slip, no-slip (adiabatic and isothermal), and outflow (subsonic, supersonic, and mixed). Order-of-accuracy verification has been performed for the Euler and Navier-Stokes equations and these BC's in a compressible computational fluid dynamics code. All of the results shown are on skewed, non-uniform meshes. RANS results will be presented in a future paper. The observed order of accuracy was lower than the expected order of accuracy in two cases. One of these cases resulted in the identification and correction of a coding mistake in the CHAD gradient correction that was reducing the observed order of accuracy. This mistake would have been undetectable on a Cartesian mesh. During the search for the CHAD gradient correction problem, an unrelated coding mistake was found and corrected. The other case in which the observed order of accuracy was less than expected was a test of the slip BC; although no specific coding or formulation mistakes have yet been identified. After the correction of the identified coding mistakes, all of the aforementioned equation sets and BC's demonstrated the expected (or at least acceptable) order of accuracy except the slip condition.

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Rate constants for the thermal dissociation of Si{sub 2}H{sub 6} are predicted with a novel transition state model. The saddle points for dissociation on the Si{sub 2}H{sub 6} potential energy surface are lower in energy than the corresponding separated products, as confirmed by high level ab initio quantum mechanical calculations. Thus, the dissociations of Si{sub 2}H{sub 6} to produce SiH{sub 2} + SiH{sub 4} (R1) and H{sub 3}SiSiH + H{sub 2} (R2) both proceed through tight inner transition states followed by loose outer transition states. The present 'dual' transition state model couples variational phase space theory treatments of the outer transition states with ab initio based fixed harmonic vibrator treatments of the inner transition states to obtain effective numbers of states for the two transition states acting in series. It is found that, at least near room temperature, such a dual transition state model is generally required for the proper description of each of the dissociations. Only at quite high temperatures, i.e., above 2000 K for (R1) and 600 K for (R2), does a single fixed inner transition state provide an adequate description. Similarly, only at quite low temperatures (below 100 and 10 K for (R1) and (R2), respectively) does a single outer transition state provide an adequate description. Pressure dependent rate constants are obtained from solutions to the multichannel master equation. These calculations confirm that dissociation channel (R2) is negligible under conditions relevant to the thermal chemical vapor deposition (CVD) processes. Rate constants for the chemical activation reactions, SiH{sub 2} + SiH{sub 4} {yields} Si{sub 2}H{sub 6} (R-1) and SiH{sub 2} + SiH{sub 4} {yields} H{sub 3}SiSiH + H{sub 2} (R3), are also evaluated within the dual transition state model. It is found that reaction R3 is the dominant channel for low pressures and high temperatures, i.e., below 100 Torr for temperatures above 1100 K.

In this article, we discuss in detail the addition of hydrogen atoms to diacetylene and the reverse dissociation reactions, H + C{sub 4}H{sub 2} {leftrightarrow} i-C{sub 4}H{sub 3} (R1) and H + C{sub 4}H{sub 2} n-C{sub 4}H{sub 3} (R2). The theory utilizes high-level electronic structure methodology to characterize the potential energy surface, Rice-Ramsperger-Kassel-Marcus (RRKM) theory to calculate microcanonical/J-resolved rate coefficients, and a two-dimensional master-equation approach to extract phenomenological (thermal) rate coefficients. Comparison is made with experimental results where they are available. The rate coefficients k{sub 1}(T, p) and k{sub 2}(T, p) are cast in forms that can be used in chemical kinetic modeling. In addition, we predict values of the heats of formation of i-C{sub 4}H{sub 3} and n-C{sub 4}H{sub 3} and discuss their importance in flame chemistry. Our basis-set extrapolated, quadratic-configuration-interaction with single and double excitations (and triple excitations added perturbatively), QCISD(T), predictions of these heats of formation at 298 K are 130.8 kcal/mol for n-C{sub 4}H{sub 3} and 119.3 kcal/mol for the i-isomer; multireference CI calculations with a nine-electron, nine-orbital, complete-active-space (CAS) reference wavefunction give just slightly larger values for these parameters. Our results are in good agreement with the recent focal-point analysis of Wheeler et al. (J. Chem. Phys. 2004, 121, 8800-8813), but they differ substantially for {Delta} H{sub f 298}{sup 0}(n-C{sub 4}H{sub 3}) with the earlier diffusion Monte Carlo predictions of Krokidis et al.

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Many accelerators at Sandia National Laboratories utilize the Rimfire gas switch for high-voltage, high-power switching. Future accelerators will have increased performance requirements for switching elements. When designing improved versions of the Rimfire switch, there is a need for quick and accurate simulation of the electrical effects of geometry changes. This paper presents an advanced circuit model of the Rimfire switch that can be used for these simulations. The development of the model is shown along with comparisons to past models and experimental results.

The Sandia Lightning Simulator at Sandia National Laboratories can provide up to 200 kA for a simulated single lightning stroke, 100 kA for a subsequent stroke, and hundreds of Amperes of continuing current. It has recently been recommissioned after a decade of inactivity and the single-stroke capability demonstrated. The simulator capabilities, basic design components, upgrades, and diagnostic capabilities are discussed in this paper.

An experimental investigation is made into the fluid mechanics and heat transfer of a circular cylinder immersed in a wall-bounded turbulent mixed-convection flow of water. The cylinder is oriented spanwise to the forced channel flow and within the thermal boundary layer of the heated lower wall. The flow channel is capped with a cold, near-adiabatic upper wall producing a fully turbulent gap Rayleigh number of 10{sup 8}. A low-speed crossflow is applied to advect the turbulent thermal plumes over the cylinder surface. We present spatially resolved cylinder-surface heat-flux data alongside 2-D PIV imaging of the streamwise and wall-normal velocity components for two flow conditions in the mixed-convection heat-transfer regime. The measured cylinder-wake flowfield reflects the complex coupling between the separated wake flow, the highly turbulent freestream and the buoyant wall and cylinder boundary layers. A method for measurement of spatially resolved surface heat fluxes based on the measured cylinder-surface temperature distribution and a well-posed two-dimensional solution to the conduction problem in the cylinder wall is presented. The resulting spatially resolved flux measurements show enhanced surface heat transfer, which results from the intense buoyancy generated free-stream turbulence and mixing in the cylinder wake. This work extends the literature on thermal convection with crossflow well into the turbulent regime and is, to our knowledge, the first investigation of surface heat-transfer to an object of engineering importance placed in this type of turbulent mixed-convection flowfield. The data are currently being utilized for validation of mixed convection turbulence models at Sandia and comparisons between the computational and experimental results are presented.

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The WLF equation is typically used to describe the dependence of polymer mobility on temperature at atmospheric pressure. Tests at different pressures would at least require different WLF parameterization. Completely different tests, for example, probing the temperature dependence of mobility at constant density, would require even greater modifications. By performing molecular dynamics simulations on simple chain molecules equilibrated at different thermodynamic states, we have shown that the mobility depends in a more general sense on the potential energy density of the system. That is, mobilities for any equilibrated state collapse onto one master curve when plotted against the potential energy density. Moreover, this relationship can be fit by either a 'generalized' WLF equation or by a power-law relationship observed in critical phenomena. When this mobility relationship is used within a rheologically simple, thermodynamically consistent, viscoelastic framework, quantitative agreement is seen between experimental data and theoretical predictions on a range of tests covering enthalpy relaxation to mechanical yield to physical aging.

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In this study, we describe the extension of the 2-d preliminary design bluff body drag estimation tool developed by De Chant1 to apply for 3-d flows. As with the 2-d method, the 3-d extension uses a combined approximate Green's function/Gram-Charlier series approach to retain the body geometry information. Whereas, the 2-d methodology relied solely upon the use of small disturbance theory for the inviscid flow field associated with the body of interest to estimate the near-field initial conditions, e.g. velocity defect, the 3-d methodology uses both analytical (where available) and numerical inviscid solutions. The defect solution is then used as an initial condition in an approximate 3-d Green's function solution. Finally, the Green's function solution is matched to the 3-d analog of the classical 2-d Gram-Charlier series and then integrated to yield the net form drag on the bluff body. Preliminary results indicate that drag estimates computed are of accuracy equivalent to the 2-d method for flows with large separation, i.e. less than 20% relative error. As was the lower dimensional method, the 3-d concept is intended to be a supplement to turbulent Navier-Stokes and experimental solution for estimating drag coefficients over blunt bodies.

The Method of Nearby Problems is employed to generate exact solutions to equations 'nearby' the steady and unsteady Burgers equation. Burgers equation is chosen because of the existence of exact solutions, and these exact solutions are discussed. Legendre polynomials are used to derive the exact solutions to the nearby problems, and the application of Legendre polynomials for both 1D and 2D problems is also discussed. Results are presented for the steady-state Burgers equation corresponding to a viscous shock wave for Reynolds numbers of 8, 16, and 512. The low Reynolds number cases are well approximated by 10th order Legendre polynomial fits, while the high Reynolds number case is not. The unsteady Burgers equation corresponding to coalescence of two viscous shock waves at a Reynolds number of 8 is also examined. Preliminary results indicate that further investigation is required to accurately capture this 2D solution.

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We performed calculations to investigate the classical theories of chain branching and thermal--run--away that lead to the rapid oxidation of fuels. Mathematically, both theories infer the existence of eigenvalues with positive real parts i.e., explosive modes. We found in studies of homogeneous hydrogen--air and the methane--air mixtures that when ignition is initiated by a sufficiently high initial temperature, the transient response of the system exhibits two stages. The first stage is characterized by the existence of explosive modes. The ensuing second stage consists of fast exponential decay modes that bring the system to its equilibrium point. We demonstrated with two examples that the existence of explosive modes is not a necessary condition for the existence of a premixed flame. Homogeneous ignition calculations for mixtures with an initial concentration of radical species suggest that the diffusive transport of radical species is probably responsible for the lack of explosive modes in premixed flames.

Modal analysis of three-dimensional structures frequently involves finite element discretizations with millions of unknowns and requires computing hundreds or thousands of eigenpairs. In this presentation we review methods based on domain decomposition for such eigenspace computations in structural dynamics. We distinguish approaches that solve the eigenproblem algebraically (with minimal connections to the underlying partial differential equation) from approaches that tightly couple the eigensolver with the partial differential equation.

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Low-temperature combustion concepts that utilize cooled EGR, early/retarded injection, high swirl ratios, and modest compression ratios have recently received considerable attention. To understand the combustion and, in particular, the soot formation process under these operating conditions, a modeling study was carried out using the KIVA-3V code with an improved phenomenological soot model. This multi-step soot model includes particle inception, surface growth, surface oxidation, and particle coagulation. Additional models include a piston-ring crevice model, the KH/RT spray breakup model, a droplet wall impingement model, a wall heat transfer model, and the RNG k-{var_epsilon} turbulence model. The Shell model was used to simulate the ignition process, and a laminar-and-turbulent characteristic time combustion model was used for the post-ignition combustion process. A low-load (IMEP=3 bar) operating condition was considered and the predicted in-cylinder pressures and heat release rates were compared with measurements. Predicted soot mass, soot particle size, soot number density distributions and other relevant quantities are presented and discussed. The effects of variable EGR rate (0-68%), injection pressure (600-1200 bar), and injection timing were studied. The predictions demonstrate that both EGR and retarded injection are beneficial for reducing NO{sub x} emissions, although the former has a more pronounced effect. Additionally, higher soot emissions are typically predicted for the higher EGR rates. However, when the EGR rate exceeds a critical value (over 65% in this study), the soot emissions decrease. Reduced soot emissions are also predicted when higher injection pressures or retarded injection timings are employed. The reduction in soot with retarded injection is less than what is observed experimentally, however.

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Two-color resonant four-wave-mixing spectroscopy (TC-RFWM) is used to investigate ground-state energy transfer of hydroxyl radical in atmospheric-pressure flames. Two amplified distributed-feedback dye lasers produce 50-ps, nearly transform-limited, infrared (IR) and ultraviolet pulses. The infrared pump laser is tuned to individual rovibrational transitions of OH X {sup 2}{pi}{sub 3/2} (v{prime}=1, N{prime}) {l_arrow} X {sup 2}{pi}{sub 3/2} (v{double_prime}=0, N{double_prime}), and the ultraviolet pulse probes either the directly pumped or collisionally populated intermediate levels via A{sup 2}{Sigma}{sup +} (v*=1, N*) {l_arrow} X{sup 2}{pi}{sub 3/2}(v{prime}=1, N{prime}). By time-delaying the probe pulse with respect to the pump pulse, and appropriately constraining the polarizations of each of the four fields taking part in the wave-mixing process, we are able to independently and unambiguously measure the moments of the rotational angular momentum distribution in single rotational levels of the ground state. We present measurements of population, alignment, and orientation decay in X {sup 2}{pi}{sub 3/2} for several flame conditions. These experiments provide data necessary for the development of accurate models for diagnostic techniques using saturating laser pulses.

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