Archives—October 2010

Laser Ignition and Associated Diagnostics for Natural Gas Fueled Engines

Thursday October 28, 2010 4:00 – 5:00 p.m.
ME-EM Building, Room 112

Sreenath Gupta
Argonne National Laboratory

Lean-burn operation is very popular with natural gas fueled stationary engines as it offers simultaneous low-NO,emissions and high engine efficiencies, while not requiring the use of any aftertreatment devices. Though enginesoperating on lean-burn operation are capable of even better performance, they are currently limited by the inability tosustain reliable ignition under lean conditions. Addressing such an issue, Argonne has evaluated the use of laser ignitionas an alternative to the conventional Capacitance Discharge Ignition (CDI). Initial tests in a static chamber and a rapidcompression machine have shown laser ignition to be capable of ignition at very higher pressures and leaner conditionsthan those ignitable using CDI. Subsequent tests in a lean burn single cylinder engine have shown NO, reductions up to70% for a given efficiency, or efficiency improvements up to 3% for a given NO, emission. Though efforts continueworldwide to reduce laser ignition to practice, further improvement in fiber optics’ performance and laser technologyare required.In the meantime, two diagnostics were developed as a part of this effort:The first diagnostic relies on measuring the flame chemiluminescence, which has been acknowledged in premixedturbulent flames to be correlated to local? global heat release rate. Results from tests conducted in a single-cylinderengine equipped with a EGR system show that over 90% of the luminous signal can be attributed to CO2 emission.Also, the effect of dilution on ’2CO chemiluminescence intensities was studied, by varying the global equivalence ratio(0.6 — 1.0) and by varying the Exhaust Gas Recirculation rate. The flame luminosity signals, did not correlate on a crankangle resolved basis with temperatures and heat release rates obtained from in-cylinder pressure measurements.However, the peak luminous signals correlated very well with both peak cycle heat release rates as well as peak cycletemperatures. Such observations point towards the potential use of flame chemiluminescence to monitor peak bulk gastemperatures as well as peak heat release rates in natural gas fired reciprocating engines.A second diagnostic based on Laser Induced Breakdown Spectroscopy (LIBS) was developed to measure in-cylinderequivalence ratio. Following the laser induced spark ignition event, the broadband emission from the spark kernel wascollected and spectrally resolved. The peaks corresponding to H,,, N and 0 atoms were measured for a range ofconditions with global equivalence ratios ranging between 0.6 and 1.0, and for Exhaust Gas Recirculation fractions up to29%. The (H,,/O) and (H,,/N) peak intensity ratios from the spectral scans correlated extremely well (R’ > 0.97) with localoxygen based equivalence ratios. From the flow relations, it appears that for homogenous intake charge, such a LIBSdiagnostic enables estimation of EGR rate with the knowledge of the global equivalence ratio.


Flow Investigations: Enabling New Thermal System Designs

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A very significant knowledge and technology gap exists between modern-day refrigeration and air-conditioning systems and the ones required for extreme thermal applications like space exploration. Amitabh Narain’s cutting edge research is expected to enable development of new technologies and systems of interest to NASA, the US Air Force, and the electronics/avionics cooling industry.


On-going Studies of Chemical Aging of Nitrate Ester Solid Propellants and Health Monitoring Sensor Development

Thursday October 14, 2010 4:00 – 5:00 p.m.
ME-EM Building, Room 112

Brian McDonald
U.S. Army

The primary aging mechanism of nitrate ester propellants include several steps that begin with thethermal decomposition of the nitrate ester and lead to, if unmitigated, the evolution ofseveral gas speciesto include C02, and N02. These gases may lead to internal fissuring of the propellant or autocatalyticdecomposition and combustion. Stabilizers are added to the propellants to neutralize the decompositionproducts and minimize the gas production. These stabilizers are consumed with aging with the servicelife of a particular propellant system determined as the time that the stabilizer mass fraction is consumedbelow a minimum acceptable level. The U.S. Army WDI directorate has an on-going technologyprogram for the development of a service life monitoring system that incorporates non-destructive sensorsfor the monitoring of nitrate ester aging. This presentation presents an update of sensor propellant agingtest results, and the results of a series of nitrate ester humidity aging results that have been completed todate. The effects of aging under controlled humidity on the thermal decomposition of two nitrateester/RDX propellants are examined. Propellant samples are accelerated aged at 60 C and 70 C atvarious levels of relative humidity. Differential Scanning Calorimetry is used to study the post agedsamples to determine the activation energy of the peak exotherm and peak temperature of these events.Nitrate ester stabilizer depletion rates are presented as a function of aging and humidity. The results ofthe study show that that moisture level during aging irreversibly alters the activation energy of theprimary heat release events and significantly influences the stabilizer depletion rates. In addition,humidity effects on the deterioration of the mechanical properties of the nitrate ester are investigated.The acidic by-products of the thermal decomposition of the nitrate ester plasticizers and hydrolyzedbinder combine with the available excess moisture leading to the acidic catalyzed hydrolyticdecomposition of the binder polymer reducing the cross-link density, elastic modulus and the strainendurance of the composite material. Dynamic Mechanical Analysis and polymer solvent swell testingare used to investigate the effect that the temperature and humidity induce aging has on the rheology ofthe polymer composite to include changes in cross link density, loss and storage modulus, and the glasstransition temperature. A correlation of the change in these properties with the magnitude of the relivehumidity of the aging environment is demonstrated. On-going propellant aging test results using carbonnano-tube gas sensors as a health monitoring device are presented.


Atomisitic and Multiscale Modeling of Surface Effects on the Mechanical Behavior and Properties of Nanomaterials

Thursday October 7, 2010 4:00 – 5:00 p.m.
ME-EM Building, Room 112

Harold S. Park
Boston University

Surface dominated nanostructures such as nanowires have recently garnered significant attention due to theirunique physical properties. In this presentation, I will discuss recent efforts using various computationalmodeling techniques to gain further insights into unique mechanical behavior and properties that result fromthese surface effects.Using atomistic modeling, I will discuss recent predictions of novel shape memoiy and pseudoelastic behaviorobserved in FCC and intermetallic nanowires that are not observed in the corresponding bulk materials. I willdiscuss the important role of nanoscale surface stress effects in enabling the shape memory response, and I willdiscuss the novel atomistic deformation mechanisms that are observed during the mechanical deformation ofshape memory nanowires.The second part of the talk will discuss recent developments in multiscale, fmite element method-basedmodeling to capture surface effects on the mechanical behavior and properties of both FCC metal andsemiconducting nanowires. I will discuss this approach, the surface Cauchy-Bom model, then demonstrate itsability to capture size, surface and boundary condition effects on the elastic properties of the nanowires.Specific attention will be made to compare the obtained results to those where surface effects are neglected suchthat an understanding of how surface effects impact the elastic properties of nanowires across various lengthscales can be obtained.