Isaac W. Ekoto

Response of supersonic turbulent boundary layers to local and global mechanical distortions

A series of experiments were conducted to investigate the response of a Mach 5 turbulent boundary layer (Reθ ≈ 38,000) to periodic diamond roughness elements in combination with a series of curvature-driven favorable pressure gradients. Flow visualizations were obtained using schlieren photography and quantitative data were obtained using planar particle image velocimetry. It is found that streamwise variations in the Reynolds-averaged normal and shear stresses occur due to the shock waves and expansions fans generated by the roughness elements. The Reynolds-averaged shear stress in particular, was found to vary by up to 35% in the streamwise direction, in comparison to the equivalent zero-pressuregradient case. Overall, results indicate that the favorable pressure gradients considered have a dominant stabilizing effect on the flow, reducing Reynolds-averaged normal and shear stresses by up to 50%. Profiles of the shear stress indicate appreciable structural changes within the boundary layer, with the peak shear stress moving closer to the wall with increasing favorable pressure gradient strength.

Supersonic Boundary Layers with Periodic Surface Roughness

In the present study, the effects of large-scale periodic surface roughness on a high-speed (M = 2.86), high Reynolds number (Re θ ≈ 60,000), supersonic turbulent boundary layer was examined. Two roughness topologies (square and diamond) were compared with an aerodynamically smooth wall. The measurements included planar contours of the mean and fluctuating velocity, pitot pressure profiles, pressure-sensitive paint, and schlieren photography. The local strain-rate distortion parameters for the square roughness pattern were small (∼-0.01), and the mean and turbulent flow properties followed the canonical rough-wall boundary-layer trends. The diamond-shaped roughness topology produced a pattern of attached oblique shocks and expansion waves that led to strong distortion parameters. The distortions varied from -0.3 to 0.4 across the roughness elements, which resulted in localized extra turbulence production that generated large periodic variations in the turbulence levels across individual roughness elements that spanned the boundary-layer thickness; for example, the Reynolds shear stress varied by ∼100%. This result demonstrated a mechanism for altering the turbulence in supersonic boundary layers.

UHC and CO Emissions Sources From a Light-Duty Diesel Engine Undergoing Late-Injection Low-Temperature Combustion

Low load carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions sources are examined in an optically accessible, light-duty diesel engine employing a late-injection, low-temperature combustion strategy. The study focus is to identify the cause of the rapid degradation in emissions and efficiency as injection timing is retarded. The in-cylinder progression of mixing and combustion processes is examined through ultraviolet planar laser-induced fluorescence (UV PLIF) imaging of hydrocarbon spatial distributions. Spectrally-resolved, deep-UV LIF measurements are also used to construct late-cycle spatial distributions of CO, C2 , and polycyclic aromatic hydrocarbons within the clearance volume. Engine-out emissions measurements and numerical results from both detailed chemistry homogeneous reactor and multidimensional simulations complement the measurements. The measured spatial distributions show that while most fuel accumulates on the bowl-pip during high-temperature heat-release, much of it is transported into the squish-volume by the reverse squish flow. Homogeneous reactor simulations further show that expansion cooling quenches reactions, preventing the transition to high-temperature heat-release for mixtures with an equivalence ratio below 0.6. Lean squish-volume mixtures, coupled with wall heat losses, severely inhibit squish volume fuel oxidation. Further retarding injection timing exacerbates quenching, resulting in a two-fold increase in UHC emissions and a 33% increase in CO, primarily from the squish-volume.Copyright

Optical Investigation Into Wall Wetting From Late-Cycle Post-Injections Used for Diesel Particulate Filter Regeneration

Wall wetting phenomena were investigated in a light-duty diesel optical engine operating under typical diesel particulate filter regeneration conditions through the use of liquid spray imaging during late-cycle post-injections. Three post-injection timings were explored: 1) an ‘early’ timing (44.5°aTDC) where high ambient temperatures and densities were expected to decrease the liquid penetration, 2) a ‘conventional’ timing (78.5°aTDC) that is typically used to produce the necessary aftertreatment regeneration exhaust conditions, and 3) a ‘late’ timing (133.5°aTDC) where in-cylinder flows generated by exhaust valve opening-induced blowdown can disrupt the liquid penetration. In addition to a 2007 US certification diesel fuel, a palm-derived B20 biodiesel blend and soy-derived B100 biodiesel were examined since liquid spray impingement is thought to worsen for biodiesel blends due to the higher fuel distillation temperature, density, and viscosity. No significant wall wetting was observed for the early post-injection. However, considerable impingement occurred for the conventional and late post-injections. Liquid penetration and the persistence of liquid fuel in the cylinder were found to increase with biodiesel content, while exhaust blowdown flows were ineffective in reducing the severity of wall wetting. Negligible distortion of jet structure was observed for the liquid spray at the late post-injection. Short pulse durations decreased the severity of liquid penetration with the soy-derived biodiesel during the early post-injection SOI, but were otherwise ineffective.Copyright

Large-Scale Hydrogen Jet Flame Radiant Fraction Measurements and Modeling

Analytic methods used to establish thermal radiation hazard safety boundaries from ignited hydrogen plumes are based on models previously developed for hydrocarbon jet fires. Radiative heat flux measurements of small- and medium-scale hydrogen jet flames (i.e., visible flame lengths < 10 m) compare favorably to theoretical calculations provided corrections are applied to correct for the product species thermal emittance and the optical flame thickness. Recently, Air Products and Chemicals Inc. commissioned flame radiation measurements from two larger-scale hydrogen jet flames to determine the applicability of current modeling approaches to these larger flames. The horizontally orientated releases were from 20.9 and 50.8 mm ID pipes with a nominal 60 barg source pressure and respective mass flow rates of 1.0 and 7.4 kg/s. Care was taken to ensure no particles were entrained into the flame, either from the internal piping or from the ground below. Radiometers were used to measure radiative heat fluxes at discrete points along the jet flame radial axis.The estimated radiant fraction, defined as the radiative energy escaping relative to chemical energy released, exceeded correlation predictions for both flames. To determine why the deviation existed, an analysis of the data and experimental conditions was performed by Sandia National Laboratories’ Hydrogen Safety, Codes and Standards program. Since the releases were choked at the exit, a pseudo source nozzle model was needed to compute flame lengths and residence times, and the results were found to be sensitive to the formulation used. Furthermore, it was thought that ground surface reflection from the concrete pad and steel plates may have contributed to the increased recorded heat flux values. To quantify this impact, a weighted multi source flame radiation model was modified to include the influence of planar surface radiation. Model results were compared to lab-scale flames with a steel plate located close to and parallel with the release path. Relative to the flame without a plate, recorded heat flux values were found to increase by up to 50% for certain configurations, and the modified radiation model predicted these heat fluxes to within 10% provided a realistic steel reflectance value (0.8) was used. When the plate was heavily and uniformly oxidized, however, the reflectance was sharply attenuated. Model results that used the surface reflectance correction for the larger-scale flames produced good agreement with the heat flux data from the smaller of the two flames if an estimated reflectance of 0.5 was used, but was unable to fully explain the under predicted heat flux values for the larger flame.Copyright

Development of a Raman Spectroscopy Technique to Detect Alternate Transportation Fuel Hydrocarbon Intermediates in Complex Combustion Environments

Spontaneous Raman spectra for important hydrocarbon fuels and combustion intermediates were recorded over a range of low-to-moderate flame temperatures using the multiscalar measurement facility located at Sandia/CA. Recorded spectra were extrapolated to higher flame temperatures and then converted into empirical spectral libraries that can readily be incorporated into existing post-processing analysis models that account for crosstalk from overlapping hydrocarbon channel signal. Performance testing of the developed libraries and reduction methods was conducted through an examination of results from well-characterized laminar reference flames, and was found to provide good agreement. The diagnostic development allows for temporally and spatially resolved flame measurements of speciated hydrocarbon concentrations whose parent is more chemically complex than methane. Such data are needed to validate increasingly complex flame simulations.

Planar Measurements of Supersonic Boundary Layers with Curvature Driven Pressure Gradients

,θ ) turbulent boundary layers to streamline curvature driven pressure gradient distortions. The pressure gradients included a zero, a weak favorable, a strong favorable and a strong adverse. To quantify the flow structure, the following measurements were accomplished: highly resolved planar measurements of mean and fluctuating velocity components with particle image velocimetry, surface pressure contours via pressure sensitive paint, pitot pressure profiles and schlieren photography. In addition to quantifying the mean and turbulent shear stress fields, the PIV data were also processed to provide the production of the axial, transverse and shear stresses. In the favorable pressure gradient regions of the wind tunnel models, a reduction in the turbulence levels (up to 40%) was seen. Corresponding reductions of 50% and 45% in the axial and shear stress production, respectively, were also observed. The reductions in the turbulent stresses, and the production thereof, were due to the manipulation of velocity gradients by the streamline curvature driven gradients. In the adverse pressure gradient region, the trend was reversed.

Analyses in support of risk-informed natural gas vehicle maintenance facility codes and standards :

Safety standards development for maintenance facilities of liquid and compressed gas fueled large-scale vehicles is required to ensure proper facility design and operation envelopes. Standard development organizations are utilizing risk-informed concepts to develop natural gas vehicle (NGV) codes and standards so that maintenance facilities meet acceptable risk levels. The present report summarizes Phase I work for existing NGV repair facility code requirements and highlights inconsistencies that need quantitative analysis into their effectiveness. A Hazardous and Operability study was performed to identify key scenarios of interest. Finally, scenario analyses were performed using detailed simulations and modeling to estimate the overpressure hazards from HAZOP defined scenarios. The results from Phase I will be used to identify significant risk contributors at NGV maintenance facilities, and are expected to form the basis for follow-on quantitative risk analysis work to address specific code requirements and identify effective accident prevention and mitigation strategies.

Calorimetry and Atomic Oxygen Laser-Induced Fluorescence of Pulsed Nanosecond Discharges at Above-Atmospheric Pressures

The conversion efficiency of secondary electrical energy into thermal energy was measured in air using an optically accessible spark calorimeter for high-voltage (28 kV peak) pulsed nanosecond discharges with secondary streamer breakdown (SSB) and similar low-temperature plasmas (LTP) without. Initial pressures were varied between 1 and 5 bar absolute, with the anode/cathode gap distances likewise varied between 1 and 5 mm. Secondary electrical energy was measured using an in-line attenuator, with the thermal energy determined from pressure-rise calorimetry measurements. The SSB probability at each initial pressure and gap distance was also recorded. The calorimetry measurements confirm that, similar to inductive spark discharges, SSB discharges promote ignition by increasing the local gas temperature. LTP discharges, on the other hand, had very little local gas heating, with electrical-to-thermal conversion efficiencies of ~1 %. Instead, the LTP was found to generate substantial O-atom populations — measured using two-photon laser-induced fluorescence near the anode where electric field strengths were strongest — that persisted for 100’s of microseconds after the discharge. The influence of 10 repetitive pulses spaced 100 µs apart was also evaluated for a fixed 5 mm electrode gap distance, with the conditional SSB probability for each pulse evaluated using an available photodiode, with the SSB probability found to have increased for each successive pulse. The influence of chemical and thermal preconditioning by the preceding LTP pulse was evaluated, with the increase in SSB occurrence attributed predominantly to mild gas heating that decreased number densities between the electrodes and hence the gas resistance for the subsequent pulse.

Mechanically Distorted Supersonic Boundary Layers

θ ) supersonic turbulent boundary layers were examined. Large localized distortions ( d = -0.5 to 0.4) were generated from the shock and expansion structure produced by diamond roughness elements. Weak ( dmax = 0.05) and strong global favorable pressure gradients ( dmax = 0.25) were studied. The results were compared to similar flows with canonical surface patterns (smooth and square roughness). The measurements included planar contours of the mean and fluctuating velocity, Pitot pressure profiles, pressure sensitive paint and schlieren photography. The canonical flows followed established trends. However, their inclusion provides (1) a basis for comparison for the non- canonical flows and (2) new high-speed experimental data with turbulence. The diamond roughness element produced substantially different flows that were characterized by strong local distortions ( d = -0.5 to 1.8 across the element) and highly varying turbulence properties, where the shear stress levels varied by ~100%. The present data showed that combining the global pressure gradient to the local gradients associated with the diamond roughness element produced regions of flow over a rough surface with turbulence levels reduced to 70% of the undisturbed zero pressure boundary layer. These data and trends have important implications in controlling the turbulence in high-speed boundary layers.