Steven D. Woodruff

Measurements and analysis of temperature histories and size changes for single carbon and coal particles during the early stages of heating and devolatilization

Abstract A novel system is described for monitoring rapid changes in particle size and temperature during coal devolatilization at heating rates representative of high-intensity combustion environments. The system incorporates an electrodynamic balance and a pulsed radiation source to isolate and rapidly heat individual particles. A high-speed two-dimensional photodiode array and a single wavelength radiation pyrometer are applied to record changes in particle size and temperature. Dynamics of volatile evolution and particle swelling are also recorded using high-speed cinematography. Measurements of temperature and size changes are reported for carbon spheres and a HVA bituminous coal at heating rates on the order of 105 K/s. Measured temperature histories are compared with theoretical estimates of the temperature response of radiatively heated coal and carbon particles. The analysis considers the development of the transient temperature distribution inside the particles by numerical solution of the Fourier equation. Measurements and model predictions for 135-μm-diameter carbon spheres were in excellent agreement using property data correlations commonly applied in modeling coal devolatilization and combustion behavior. Model predictions for coal particles, however, significantly underestimated (on the order of 50%) the observed heating rates for 115-μm-diameter coal particles using the same property correlations. It is concluded that coal particles heat significantly faster than is predicted using commonly employed approaches to model heat transfer with assumptions routinely applied to coal. Potential reasons for this may include inadequate understanding of relevant coal thermodynamic and heat transfer properties as well as failure to account for particle shape factors. Heat transfer analyses employing spherical particle assumptions and commonly used coal property correlations can lead to large errors in predicted temperature histories and associated devolatilization rates.

Lean-Burn Stationary Natural Gas Reciprocating Engine Operation with a Prototype Miniature Diode Side Pumped Passively Q-switched Laser Spark Plug

To meet the ignition system needs of large bore lean burn stationary natural gas engines a laser diode side pumped passively Q-switched laser igniter was developed and used to ignite lean mixtures in a single cylinder research engine. The laser design was produced from previous work. The in-cylinder conditions and exhaust emissions produced by the miniaturized laser were compared to that produced by a laboratory scale commercial laser system used in prior engine testing. The miniaturized laser design as well as the combustion and emissions data for both laser systems was compared and discussed. It was determined that the two laser systems produced virtually identical combustion and emissions data.

Laser Spark Ignition: Laser Development and Engine Testing

Evermore demanding market and legislative pressures require stationary lean-burn natural gas engines to operate at higher efficiencies and reduced levels of emissions. Higher in-cylinder pressures and leaner air/fuel ratios are required in order to meet these demands. Contemporary ignition systems, more specifically spark plug performance and durability, suffer as a result of the increase in spark energy required to maintain suitable engine operation under these conditions. This paper presents a discussion of the need for an improved ignition source for advanced stationary natural gas engines and introduces laser spark ignition as a potential solution to that need. Recent laser spark ignition engine testing with natural gas fuel including NOx mapping is discussed. A prototype laser system in constructed and tested and the results are discussed and solutions provided for improving the laser system output pulse energy and pulse characteristics.Copyright

Lean-Burn Stationary Natural Gas Engine Operation With a Prototype Laser Spark Plug

An end pumped passively Q-switched laser igniter was developed to meet the ignition system needs of large bore lean burn stationary natural gas engines. The laser spark plug used an optical fiber coupled diode pump source to axially pump a passively Q-switched Nd:YAG laser and transmit the laser pulse through a custom designed lens. The optical fiber coupled pump source permits the excitation energy to be transmitted to the spark plug at relatively low optical power, less than 250 W. The Q-switched laser then generates as much as 8 mJ of light in 2.5 ns, which is focused through an asymmetric biconvex lens to create a laser spark from a focused intensity of approximately 225 GW/cm 2 . A single cylinder engine fueled with either natural gas only or hydrogen augmented natural gas was operated with the laser spark plug for approximately 10 h in tests spanning 4 days. The tests were conducted with fixed engine speed, fixed boost pressure, no exhaust gas recirculation, and laser spark timing advance set at maximum brake torque timing. Engine operational and emissions data were collected and analyzed.

Laser Spark Ignition of a Blended Hydrogen-Natural Gas Fueled Single Cylinder Engine

Charge dilution, due to the reduced combustion temperatures that it brings about, has long been proven as effective means of reducing Nitrogen Oxides (NOx ) emissions in reciprocating engines. The extent of this dilution is practically bounded on the lean side of stoichiometric conditions by engine misfire or the point at which the combustion process is no longer sufficiently reliable to sustain engine operation within some specified limit. Extending this misfire limit of an engine becomes a worth while goal as it brings about further reductions in NOx emissions. Much work has been dedicated to reaching this end and several techniques have proven viable in natural gas fueled engines. This work explores potential synergies between two proven techniques for NOx reductions in lean-burn natural gas fueled engines, hydrogen enrichment of the natural gas fuel and application of laser spark ignition. Independently both techniques have been shown to provide significant NOx emissions reductions through lean limit extension in spark ignited gaseous fueled reciprocating engines [1–11, 13–15]. Here hydrogen is blended with natural gas at five different levels ranging from 0% to 40% by volume in a single cylinder engine. The mixtures are fired using a conventional spark plug based ignition system and then again with an open beam path laser induced breakdown spark ignition system. NOx emissions measurements were made at different levels including misfire conditions for each level of hydrogen enrichment with both ignition systems. Data are presented and the emissions and engine performance of two configurations are compared to determine realizable benefits that arise from combining the two techniques.Copyright

Misfire, Knock and NOx Mapping of a Laser Spark Ignited Single Cylinder Lean Burn Natural Gas Engine

Evermore demanding market and legislative pressures require stationary lean burn natural gas engines to operate at higher efficiencies and reduced levels of emissions. Higher in-cylinder pressures and leaner air/fuel ratios are required in order to meet these demands. The performance and durability of spark plug ignition systems suffer as a result of the increase in spark energy required to maintain suitable engine operation under these conditions. Advancing the state of the art of ignition systems for these engines is critical to meeting increased performance requirements. Laser-spark ignition has shown potential to improve engine performance and ignition system durability to levels required meet or exceed projected requirements. This paper discusses testing which extends previous efforts [1] to include constant fueling knock, misfire, thermal efficiency, and NO x emissions mapping of a single cylinder lean burn natural gas engine. Tests are conducted using an open beam path laser spark ignition system and a conventional spark plug based system for contrast. Under the conditions tested, the laser-spark ignition system increased the total operating envelope of the engine by 46% when compared to the conventional ignition system. Due to a wider misfire margin using the laser spark system, NO x emissions were half the minimum value of the spark plug ignition system with no appreciable degradation in thermal efficiency. Hydrocarbon emissions were comparable for both systems. The results of the testing are discussed in detail.

Azimuthal polarization for Raman enhancement in capillary waveguides

Abstract. Hollow, metal-lined capillary waveguides have recently been utilized in spontaneous gas-Raman spectroscopy to improve signal strength and response time. The hollow waveguide is used to contain the sample gases, efficiently propagate a pump beam, and efficiently collect Raman scattering from those gases. Transmission losses in the waveguide may be reduced by using an azimuthally polarized pump beam instead of a linearly or radially polarized pump. This will lead to improved Raman signal strength, accuracy, and response time in waveguide-based Raman gas-composition sensors. A linearly polarized laser beam is azimuthally polarized using passive components including a spiral phase plate and an azimuthal-type linear analyzer element. Half-wave plates are then used to switch between the azimuthally polarized beam and the radially polarized beam with no change in input pump power. The collected Raman signal strength and laser throughput are improved when the azimuthally polarized pump is used. Optimization of the hollow waveguide Raman gas sensor is discussed with respect to incident pump polarization.

Measurements of coal particle shape, mass, and temperature histories: Impact of particle irregularity on temperature predictions and measurements

Individual coal and carbon particles were levitated in an electrodynamic balance (EDB) and characterized using high-speed diode array and video based imaging systems to determine particle surface area, volume, drag, mass and density. These same particles were then heated bidirectionally using a long pulsed Nd:YAG laser to simulate combustion level heating fluxes (heating rates on order of 10{sup 4} to 10{sup 5} K/s). Measurements of particle surface temperature, size and laser temporal power variation were made and recorded during each heating experiment. Measured temperature histories were compared with a heat transfer analysis that accounted for variations in particle shape, mass, density, and laser heating power. Results of this study indicate that with well characterized materials of known properties agreement between measurement and model of within 20 K is typical throughout an entire heating and cooling profile. Large particle to particle variations are observed in coal particle temperature histories during rapid heating. These variations can be explained in large part by accounting for particle to particle property (shape, mass and density) variations. Even when accounting for particle to particle shape and density variation, however, model predictions greatly underestimate observed temperature histories. It is concluded that these discrepancies are largely due to uncertainties in the thermal properties (heat capacity and thermal conductivity) typically used to model coal combustion behavior.

Laser spark plug numerical design process with experimental validation

This work reports the numerical modeling design procedure for a miniaturized laser spark plug. In previous work both side pumped and end pumped laser spark plugs were empirically designed and tested. Experimental data from the previous laser spark plug development cycles is compared to the output predicted by a known set of rate equations. The rate equations are used to develop interrelated inter cavity time dependent waveforms that are then used to identify key variables. These variables are then input to a set of secondary equations for determining the output pulse energy, output power, and output pulse width of the simulated laser system. The physical meaning and the operation of the rate equations is explained in detail. This paper concentrates on the process and decision points needed to successfully design a solid state passively Q-switched laser system, either side pumped or end pumped, that produces the appropriate output needed for use as a laser spark plug for internal combustion engines.

Optical diagnostics in gas turbine combustors

Deregulation of the power industry and increasingly tight emission controls are pushing gas turbine manufacturers to develop engines operating at high pressure for efficiency and lean fuel mixtures to control NOx. This combination also gives rise to combustion instabilities which threaten engine integrity through acoustic pressure oscillations and flashback. High speed imaging and OH emission sensors have been demonstrated to be invaluable tools in characterizing and monitoring unstable combustion processes. Asynchronous imaging technique permit detailed viewing of cyclic flame structure in an acoustic environment which may be modeled or utilized in burner design . The response of the flame front to the acoustic pressure cycle may be tracked with an OH emission monitor using a sapphire light pipe for optical access. The OH optical emission can be correlated to pressure sensor data for better understanding of the acoustical coupling of the flame. Active control f the combustion cycle can be implemented using an OH emission sensor for feedback.