Robert A. Sawchuk

Influence of hydrogen addition to fuel on temperature field and soot formation in diffusion flames

Overventilated, coflowing axisymmetric laminar diffusion flames of ethylene, propane, and butane were used to study the influence on soot of hydrogen addition to the fuel. The flame temperatures were measured by CARS along the flame axis as well as at off-axis radial locations. CARS spectra taken in heavily sooting regions exhibited poor fits due to C 2 absorbtion of part of the fundamental band of the nitrogen spectrum. It was found that C 2 absorbtion was confined to frequencies greater than 2313 cm −1 . We, therefore, implemented a strategy that fitted only the CARS spectra in the frequency range smaller than 2313 cm −1 . Measured soot concentrations and the flame temperature data with and without hydrogen and helium dilution were evaluated and the relative influences of dilution and direct chemical interaction on soot formation, as a result of hydrogen addition, are presented. It is shown that when hydrogen or helium is added to the fuel as a diluent in moderate quantities, the changes in the temperature field of the coflow diffusion flames are negligible. When allowance is made for the influence of dilution, addition of hydrogen to the fuel side of an ethylene diffusion flame reduces the soot formation. For propane and butane flames, hydrogen addition does not show any influence on soot formation apart from the dilution effect.

Precision of multiplex CARS temperatures using both single-mode and multimode pump lasers

The noise level in single-pulse resonant nitrogen CARS spectra is shown to decrease with increasing pump laser bandwidth. This is the reverse of the trend observed for nonresonant CARS spectra. The precision of single-pulse CARS temperature measurements is shown to be dramatically increased by performing a weighted fit of theoretical and experimental CARS spectra using the measured detector noise coefficients as weighting parameters. The inclusion of collisional narrowing and cross-coherence in the CARS theory calculations and their effect on best fit temperatures are discussed. These temperatures, measured in a flat-flame burner, are compared with those obtained by Na line-reversal.

Noise in single-shot broadband coherent anti-Stokes Raman spectroscopy that employs a modeless dye laser.

The noise in single-shot coherent anti-Stokes Raman (CARS) spectroscopy that employs a broadband modeless dye laser (MDL) is examined and the results are compared with those of a conventional dye laser. The noise of the dye-laser, the nonresonant CARS, and the resonant N(2) CARS signals are determined. The use of a MDL is shown to result in substantially reduced CARS noise when the CARS signal is generated with a single-mode pump laser, but only a marginal reduction of noise is observed with a multimode pump source The noise measurements are compared with theoretical predictions that are based on models that assume modes of random amplitudes and phases in the multimode laser sources. The combination of a MDL and a single-mode pump laser is shown to increase the precision of single-shot N(2) CARS temperature measurements.

In-Situ Real-Time Characterization of Particulate Emissions from a Diesel Engine Exhaust by Laser-Induced Incandescence

Diesel engines face tightening particulate matter emissions regulations due to the environmental and health effects attributed to these emissions. There is increasing demand for measuring not only the concentration, but also the size distribution of the particulates. Laser-induced incandescence has emerged as a promising technique for measuring spatially and temporally resolved particulate volume fraction and size. Laser-induced incandescence has orders of magnitude more sensitivity than the gravimetric technique, and thus offers the promise of real-time measurements and adds the increasingly desirable size and morphology information. The usefulness of LII as a diagnostic instrument for the precise measurement of particulate concentration and primary particle size has been demonstrated. Measurements have been performed in the exhaust of a single cylinder DI research diesel engine. Simultaneous gravimetric filter measurements were made for direct comparison with the LII technique. Quantitative LII is shown to provide a sensitive, precise, and repeatable measure of the particulate concentration over a wide dynamic range. LII and gravimetric measurements are shown to correlate well over a wide range of operating conditions. A novel method for determining the primary particle size is shown to be precise enough to distinguish particle sizes for different engine operating conditions, and subsequently the number density of primary particles was determined. LII has also been shown to be sensitive in differentiating the PM performance between four different fuels. The LII technique is capable of real-time particulate matter measurements over any engine transient operation. The wide dynamic range and lower detection limit of LII make it a potentially preferred standard instrument for particulate matter measurements. INTRODUCTION From an environmental perspective, there is an urgent need to decrease the total emissions from transportation engines. The undesirable exhaust emissions include CO2, NOx, and particulate matter (PM). CO2 is a recognized greenhouse gas, and as a result of the Kyoto Protocol, industrialized countries have committed to reducing emissions of CO2. This can be primarily achieved by reductions in fuel consumption, and diesel engines offer the highest efficiency for road-going vehicles. The concession is that the emissions reduction systems for other pollutants are not as well developed for diesel engines as they are for spark-ignited engines. Demand for improved environmental performance has led to increasingly restrictive emission regulations for diesel-powered vehicles throughout Europe, North America, and Japan. Proposed regulations indicate that this trend to lower emissions levels will continue for the foreseeable future. Although PM is regulated for environmental reasons, from an operational point of view, particulate formation is not desirable. A significant portion of atmospheric particulates arises from combustion of fuels in various engines and furnaces. In urban areas, mobile sources are major contributors to ambient PM concentrations. The particulate emissions from diesel engines are in the form of complex aerosols consisting primarily of soot and volatile organics. For regulatory purposes, particulate matter emissions are defined as the mass of the matter that can be collected from a diluted exhaust stream on a filter kept at 52°C. This includes the organic compounds that condense at lower temperatures, but excludes the condensed water. This measurement provides the timeaveraged PM emissions over the period during which the particulates are collected on the filter, making measurements of the transient behavior of PM emissions impractical. Since the collected PM and other

Multichannel light detectors and their use for CARS spectroscopy.

A broadband CARS system designed to measure flame temperature from hot nitrogen CARS spectra is described, and a source of temperature error is identified and attributed to nonlinear behavior of the optical multichannel diode array detector. Detector sensitivity, linearity, uniformity, and noise are discussed. Data are presented for the Tracor-Northern TN-1223-4GI and TN-6132 detectors. It is shown that the nonlinearity observed with CARS signals is a function of radiation density at the detector input plane. This nonlinearity is associated with saturation in the microchannel plate intensifier and should be observable with short duration high intensity light pulses other than CARS, and with intensified detectors (both diode array and vidicon) other than the Tracor-Northern. The nonlinearity observed with the TN-1223-4GI detector can be eliminated by reducing the radiation density. At maximum sensitivity the detector shot noise contributes a significant component to the shot-to-shot variation of the spectral profile of the CARS signal.

Nonlinearity and image persistence of P-20 phosphor-based intensified photodiode array detectors used in CARS spectroscopy

Several self-scanning photodiode arrays (IPDA) used for CARS spectroscopy are shown to exhibit a greater image persistence than has generally been realized, and to exhibit a falloff in sensitivity that is logarithmic with decreasing output signal. These effects are attributed to the P-20 phosphor based intensifiers used in the IPDAs and are probably generic to all such detectors. A strategy for minimizing the image persistence in CARS spectroscopy is presented. A prototype detector incorporating a much faster rare earth phosphor is evaluated and shown to be more suited to single pulse CARS measurements in turbulent combustion than the IPDAs incorporating P-20 phosphors.

Concurrent Quantitative Laser-Induced Incandescence and SMPS Measurements of EGR Effects on Particulate Emissions from a TDI Diesel Engine

A comparison of scanning mobility particle sizer (SMPS) and laser-induced incandescence (LII) measurements of diesel particulate matter (PM) was performed. The results reveal the significance of the aggregate nature of diesel PM on interpretation of size and volume fraction measurements obtained with an SMPS, and the accuracy of primary particle size measurements by LII. Volume fraction calculations based on the mobility diameter measured by the SMPS substantially over-predict the space-filling volume fraction of the PM. Correction algorithms for the SMPS measurements, to account for the fractal nature of the aggregate morphology, result in a substantial reduction in the reported volume. The behavior of the particulate volume fraction, mean and standard deviation of the mobility diameter, and primary particle size are studied as a function of the EGR for a range of steady-state engine speeds and loads for a turbocharged direct-injection diesel engine. Both the SMPS and LII techniques demonstrate good repeatability and consistency with each other. Increasing the EGR results in a sharp rise in the volume fraction of particulates for all engine speeds and loads. At all speed and load conditions the primary particle size decreases with increasing EGR. (Less)

An improved CARS spectrometer for single‐shot measurements in turbulent combustion

A broadband coherent anti‐Stokes Raman spectroscopy (CARS) system optimized for single shot measurements in turbulent flames is described. The CARS spectrometer incorporates an intensified photodiode array (IPDA) with a P46‐phosphor based intensifier that is shown to have an order of magnitude less image persistence than P20‐phosphor based IPDAs, and thus largely eliminates the temperature errors that can result from this image persistence. The low and high signal nonlinearity of IPDAs incorporating both P20 and P46 phosphor based intensifiers is described. The CARS signal is dispersed with a single element concave holographic diffraction grating, which is fiber‐optically coupled. To provide the necessary dynamic range (approximately 1000:1) to handle CARS spectra whose temperatures can vary from 300 K to adiabatic flame temperatures a fiber‐optic splitter was employed and the nonlinearity of the IPDA detectors was characterized. A method of determining the nonlinearity was developed that was convenient a...

Effect of EGR on Heavy-Duty Diesel Engine Emissions Characterized With Laser-Induced Incandescence

The regulations governing diesel engine particulate matter (PM) and oxides of nitrogen (NOx ) emissions are becoming increasingly stringent. New instrumentation is urgently needed to make accurate and precise measurements of PM emissions from low-emitting engines and emission control systems in a reasonable amount of time. Laser-induced incandescence (LII) is a technique for making temporally resolved measurements of soot volume fraction. LII offers real-time particulate concentration measurements over several orders of magnitude, and adds desirable information about particulate size and surface area. In this study, the exhaust gas recirculation (EGR) system of a heavy-duty diesel engine was tuned at eight speed/load conditions using quantitative LII. Soot concentrations measured by LII correlated strongly with measurements taken using the standard gravimetric technique and an AVL smoke meter.Copyright