• Energy Research

Fuel droplet characteristics in a flame holding region of an oil furnace (0.1MW) were investigated both in experiments and simulations. The size and shape of the recirculation flow under different conditions were examined and it was found that the simulation underestimated the size of the recirculation zone due to a κ-e turbulent model. The size classification technique of fuel droplets was implemented in order to understand the followable characteristics of droplet in a high-turbulence region. A droplet under 30μm can follow the recirculating flow, but that over 50μm penetrates the recirculation zone due to large momentum. A conceptual flow illustration was demonstrated.

Local chemiluminescence measurements of OH * , CH * , and C 2 * were made at the flamefront of premixed turbulent propane flames to study details of the reaction zone and flamefront structure. The relationship between excited OH, CH, and C 2 concentrations and the intensity of chemiluminescence emissions was investigated by PREMIX (GRI-Mech), and a strong correlation was proven. Cassegrain optics were developed to detect the intensity of these chemiluminescent emissions simultaneously at a single point to obtain flame spectroscopic data for a local point. The relationship between the scale of turbulence and the thickness of the flamefront was investigated using simultaneous measurement of the three chemiluminescences and laser Doppler velocimetry. Particle velocimetry was also used to demonstrate combustion flow characteristics of interest. It was found that local chemiluminescence can provide very useful spectroscopic data on the flamefront region. Time-series chemiluminescence can describe the timescale of the reaction and the flows at the flame front. Furthermore, the structure of the flamefront can be characterized by the duration of the chemiluminescence and by the patterns of its occurrence, which were found to correspond to the thickness of the flamefront and the Damkohler number. A new technique for measuring the local flamefront structure using local chemiluminescence measurements is proposed, and its performance is proven.

A microwave-assisted spark plug was used to extend the lean operating limit (lean limit) and reduce emissions of an engine burning methane-air. In-cylinder pressure data were collected at normalized air-fuel ratios ofλ=1.46,λ=1.51,λ=1.57,λ=1.68, andλ=1.75. For eachλ, microwave energy (power supplied to the magnetron per engine cycle) was varied from 0 mJ (spark discharge alone) to 1600 mJ. At lean conditions, the results showed adding microwave energy to a standard spark plug discharge increased the number of complete combustion cycles, improving engine stability as compared to spark-only operation. Addition of microwave energy also increased the indicated thermal efficiency by 4% atλ=1.68. Atλ=1.75, the spark discharge alone was unable to consistently ignite the air-fuel mixture, resulting in frequent misfires. Although microwave energy produced more consistent ignition than spark discharge alone atλ=1.75, 59% of the cycles only partially burned. Overall, the microwave-assisted spark plug increased engine performance under lean operating conditions(λ=1.68)but did not affect operation at conditions closer to stoichiometric.

Abstract Detailed spatial intensity profiles of OH(A), CH(A), and C2(d) chemiluminescence in the reaction zone were revealed in “two-dimensional” laminar premixed flames ( ϕ = 0.85 –1.5) using spatially resolved emission measurements with a resolution-enhanced Cassegrain-type optical probe. A numerical simulation of one-dimensional laminar premixed flames using the PREMIX code, with GRI-Mech 3.0 incorporating with reaction and quenching kinetics for OH(A), CH(A), and C2(d), was compared with the experimental result. The measured and simulated profiles showed reasonable agreement in shape, peak location, emission zone thickness, and peak-intensity variation in fuel-lean and stoichiometric flames. We found disagreement between the experimental and numerical results under fuel-rich conditions, which likely resulted from incomplete C2(d) chemistry used in the present study. Stoichiometry dependence of chemiluminescence peak-intensity ratio, OH(A)/CH(A), observed experimentally was clearly supported by the simulated result.

The spatially resolved chemiluminescence in the reaction zone of a laminar premised methane/air flamewas measured. The pressure dependence of the flame front structure and chemiluminescence spectra were examined up to 1.5 MPa. Local point measurements of chemiluminescence were obtained using the Cassegrain optics system that we have developed, which has a high spatial resolution of d=0.1 mm and L=0.8 mm. Two types of measurements were performed: a local point chemiluminescence measurement with an ICCD camera with monochromator and selected chemiluminescence intensity measurements to obtain its profile and the flame front thickness at the flame front. The chemiluminescence at different pressures was examined to identify the main OH*, CH*, and C2* spectra. Background spectra related to CO-O* were observed at pressures greater than 0.5 MPa. Detailed OH* and CH* spectra were measured at different pressures, and the results indicate that each vibrational and rotational peak is in the same location, regardless of the pressure. The chemiluminescence intensity ratio of OH*/CH* at different pressures was examined. Less dependence on pressure was observed, although the individual chemiluminescence intensities changed with increasing pressure. The CO-O* broadband spectrum was significant at 0.5 MPa: its intensity remained at the same level at pressures greater than 0.5 MPa. This background spectrum was not observed at 0.1 MPa. In order to examine these chemiluminescence intensity profiles with the reaction zone and their dependence on pressure, the peak locations of OH*, CH*, and C2* were measured. The OH* peak was located inside the CH* and C2* curves for all measured pressures.

The process of homogeneous charge compression ignition (HCCI) combustion was compared with stratified charge compression ignition (SCCI) and standard spark ignition (SI) using local spectrum analysis, with a transparent cylinder and piston. The engine was operated at 1000 rpm. The combustion characteristics were analyzed using the heat-release rate, burned-mass fraction, and time-series spectra. Local emission spectra were measured using Cassegrain optics, which have a measurement volume 0.1 mm in diameter and 0.8 mm in length. Regular gasoline was used for the SI engine and a 50:50 blend of iso-octane and n-heptane was used for the HCCI and SCCI engines. Detailed spectra were measured and compared to demonstrate the differences in the combustion process with time. The combustion characteristics of HCCI were distinguished from those of standard SI combustion using the emission spectrum, which measures the cool-flame region in front of the main hot flame. Cool-flame regions were found in both HCCI and SCCI. The cool-flame region in HCCI was about 12° crank angle in front of the main hot flame, while the two cool-flame regions in SCCI were 10 and 5° crank angle in front of the main flame. These cool flames have the spectral structure of formaldehyde bands regardless of combustion type. The characteristics of CO−O* emissions were well correlated with the ratio of heat release (ROHR), and chemiluminescence measurement was confirmed as a good method for analyzing engine combustion.

Abstract This paper aims to improve a knowledge base of methyl decanoate, a long alkyl-chain biodiesel surrogate fuel gaining popularity in engine combustion research. To this end, a comparative study on diesel and methyl decanoate combustion has been conducted with a focus on high temperature flame structures and soot distributions in an optically accessible single-cylinder light-duty common-rail diesel engine. The in-cylinder pressure trace and apparent heat release rate curves were well matched for both fuels when the same amount of fuel energy was supplied, which confirmed very similar combustion phasing. Planar laser induced fluorescence of hydroxyl radicals (OH-PLIF) and planar laser induced incandescence (PLII) as well as line-of-sight integrated chemiluminescence imaging of cool-flame signals and electronically excited OH (OH∗) were performed for various crank angles to capture the temporal and spatial development of diesel and methyl decanoate flames. The results show that both the cool-flame and OH radical signals are higher during methyl decanoate combustion with their wider distributions and larger in-cylinder volume fraction when compared to that of diesel, suggesting enhanced low- and high-temperature reactions due to oxygen in fuel. The oxygenated methyl decanoate with no aromatics in its molecular structure shows a lower soot formation rate than diesel as evidenced by delayed appearance of LII signals and lower overall intensity. This difference is significant even if the lower sooting propensity of methyl decanoate and thus less attenuation in the laser beam is considered. The rate of soot oxidation is also higher for methyl decanoate not only due to oxygen in fuel but also higher OH radicals surrounding smaller soot pockets compared to diesel.