Faculty Profiles - KAWAHARA Nobuyuki

Evaluation of the Flame Lift-off Length in Diesel Spray Combustion Based on Flame Extinction Reviewed

Ulugbek Azimov, Nobuyuki Kawahara, Eiji Tomita, Kazuya Tsuboi

JOURNAL OF THERMAL SCIENCE AND TECHNOLOGY 5 ( 2 ) 238 - 251 2010

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC MECHANICAL ENGINEERS

The interaction of spray and combustion processes forms a complex system of physical phenomena undergoing in IC engines. Studying this interaction is important to determine strategies for simultaneously reducing soot and NOx emissions from diesel engines. Spray combustion interactions are evaluated by the flame lift-off length - the distance from the injector orifice to the location of hydroxyl luminescence closest to the injector in the flame jet. Various works have been dedicated to successful simulations of lifted flames of a diesel jet by use of various combustion modeling approaches. In this work, flame surface density and flamelet concepts were used to model the diesel lift-off length. Numerical studies have been performed with the ECFM3Z model and n-Heptane fuel to determine the flame lift-off length under quiescent conditions. The numerical results showed good agreement with experimental data, which were obtained from an optically accessible constant volume chamber and presented at the Engine Combustion Network (ECN) of Sandia National Laboratories. It was shown that at a certain distance downstream from the injector orifice, stoichiometric scalar dissipation rate matched the extinction scalar dissipation rate. This computed extinction scalar dissipation rate correlated well with the flame lift-off length. For the range of conditions investigated, adequate quantitative agreement was obtained with the experimental measurements of lift-off length under various ambient gas O-2 concentrations and ambient gas densities.

DOI： 10.1299/jtst.5.238

Performance and emissions of a supercharged dual-fuel engine fueled by hydrogen-rich coke oven gas Reviewed

Murari Mohon Roy, Eiji Tomita, Nobuyuki Kawahara, Yuji Harada, Atsushi Sakane

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 34 ( 23 ) 9628 - 9638 2009.12

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

This study investigated the engine performance and emissions of a supercharged dual-fuel engine fueled by hydrogen-rich coke oven gas and ignited by a pilot amount of diesel fuel. The engine was tested for use as a cogeneration engine, so power output while maintaining a reasonable thermal efficiency was important Experiments were carried out at a constant pilot injection pressure and pilot quantity for different fuel-air equivalence ratios and at various injection timings without and with exhaust gas recirculation (EGR). The experimental strategy was to optimize the injection timing to maximize engine power at different fuel-air equivalence ratios without knocking and within the limit of the maximum cylinder pressure. The engine was tested first without EGR condition up to the maximum possible fuel-air equivalence ratio of 0 65. A maximum indicated mean effective pressure (IMEP) of 1425 kPa and a thermal efficiency of 39% were obtained. However, the nitrogen oxides (NOx) emissions were high. A simulated EGR up to 50% was then performed to obtain lower NOx emissions. The maximum reduction of NOx was 60% or more maintaining the similar levels of IMEP and thermal efficiency. Two-stage combustion was obtained; this is an indicator of maximum power output conditions and a precursor of knocking combustion. (C) 2009 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved.

DOI： 10.1016/j.ijhydene.2009.09.016

Fuel concentration measurement of premixed mixture using spark-induced breakdown spectroscopy Reviewed

N. Kawahara, E. Tomita, S. Takemoto, Y. Ikeda

SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY 64 ( 10 ) 1085 - 1092 2009.10

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

This study determined the local equivalence ratio of a CH(4)/air mixture in a laminar premixed flame using spark-induced breakdown spectroscopy (SIBS) with a fiber-coupled intensified charge coupled device (ICCD) spectrometer. Spectrally resolved emission spectra of plasma generated by a spark plug were investigated for their potential to measure local fuel concentrations in a premixed mixture. The influence of key parameters, such as the camera gate timing and spark energy, on the intensity of radical emission was illustrated. The intensity ratio of CN/NH had a greater sensitivity to the equivalence ratio than did that of CN/CH, and the local equivalence ratio could be obtained with high resolution by measuring the local intensity ratios of CN/NH. Moreover, a spark-plug sensor with an optical fiber was developed for application in spark-ignition engines. The atomic emission intensity during the breakdown and arc phases of spark discharge could be obtained using the fiber-optic spark-plug sensor. The H(alpha)/O intensity showed better linearity than the CN/NH intensity ratio in lean mixtures. The results presented here confirm the use of SIBS as a diagnostic tool for spark-ignition engines. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.sab.2009.07.016

Performance and emission comparison of a supercharged dual-fuel engine fueled by producer gases with varying hydrogen content Reviewed

Murari Mohon Roy, Eiji Tomita, Nobuyuki Kawahara, Yuji Harada, Atsushi Sakane

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 34 ( 18 ) 7811 - 7822 2009.9

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

This study investigated the effect of hydrogen content in producer gas on the performance and exhaust emissions of a supercharged producer gas-diesel dual-fuel engine. Two types of producer gases were used in this study, one with low hydrogen content (H(2) = 13.7%) and the other with high hydrogen content (H(2) = 20%). The engine was tested for use as a co-generation engine, so power output while maintaining a reasonable thermal efficiency was important. Experiments were carried out at a constant injection pressure and injection quantity for different fuel-air equivalence ratios and at various injection timings. The experimental strategy was to optimize the injection timing to maximize engine power at different fuel-air equivalence ratios without knocking and within the limit of the maximum cylinder pressure. Two-stage combustion was obtained; this is an indicator of maximum power output conditions and a precursor of knocking combustion. Better combustion, engine performance, and exhaust emissions (except NO.) were obtained with the high H(2)-content producer gas than with the low H(2)-content producer gas, especially under leaner conditions. Moreover, a broader window of fuel-air equivalence ratio was found with highest thermal efficiencies for the high H(2)-content producer gas. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2009.07.056

Visualization of auto-ignition and pressure wave during knocking in a hydrogen spark-ignition engine Reviewed

Nobuyuki Kawahara, Eiji Tomita

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 34 ( 7 ) 3156 - 3163 2009.4

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (international conference proceedings) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Visualizations of auto-ignition inside the end-gas region due to flame propagation and pressure waves that occur during knocking were carried out in a hydrogen spark-ignition engine using a high-speed camera. Our results demonstrated that auto-ignition in an endgas region that was compressed by the propagating flame front could be visualized using the high-speed color camera. A large amount of unburned mixture caused by the auto-ignited kernel explosion generated the strong pressure waves. Strong pressure wave oscillations induced by the initial auto-ignition could be visualized using a video camera with a speed of 250 kframes/s. The auto-ignition and pressure waves caused the thermal boundary layer to breakdown near the cylinder wall and piston head, therefore combustion of the lubricant oil grease was visible inside burned gas region. This auto-ignition and pressure waves may result in damage to the cylinder wall and piston head during engine knocking. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved

DOI： 10.1016/j.ijhydene.2009.01.091

Laser-induced radical generation and evolution to a self-sustaining flame Reviewed

J. L. Beduneau, N. Kawahara, T. Nakayama, E. Tomita, Y. Ikeda

COMBUSTION AND FLAME 156 ( 3 ) 642 - 656 2009.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE INC

Images and emission spectra of sparks produced by laser-induced breakdown of methane and propane air mixtures were investigated with a high degree of spatial and temporal resolution. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using an intensified high-speed camera and a single/multi-fiber Cassegrain optics system coupled to an ICCD spectrometer. Emission spectra of OH*, CN*, CH*, and C-2* radicals were also collected using spectra boxes. The results provided information about the different stages of the laser-induced breakdown, with a specific focus on the transition from a flame kernel to a self-sustaining flame. The plasma shape and emission spectrum were very reproducible. The differences in the size of the flame kernel and the evolution of the radical emissions were analyzed for mixtures that fired or misfired. The impact of the level of radicals in the flame kernel was a critical parameter for the firing process, starting around 1 mu s after the laser-induced breakdown. The transition from plasma cooling to the classical chemical reactions in the combustion zone was analyzed.
Even though the flame kernel size was directly linked to the spark energy, this was not a key parameter toward evolution to a self-sustaining flame. The Taylor blast wave theory was used to plot the location of the shock based on the evolution of the flame kernel size. The location was calculated using a laser-supported detonation model. A very good correlation was observed with the hot gas ignition process. Our results allowed us to obtain information about the process leading to firing or misfiring for similar environments, resulting in a better understanding of the laser breakdown phenomena and the means of utilizing this technique in an industrial context. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

DOI： 10.1016/j.combustflame.2008.09.013

High temporally resolved optical measurement for laser ignition process of laminar premixed mixtures Reviewed

Takashi Nakayama, Nobuyuki Kawahara, Eiji Tomita, Yuji Ikeda

Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B 74 ( 743 ) 1633 - 1640 2008.7

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

This paper describes laser ignition process in a laminar premixed mixture of propane and air. The plasma was generated by focusing a second harmonic and several-nanosecond pulse emitted from a Q-switched Nd : YAG laser. Minimum ignition energy (MIE) was measured in different equivalence ratio. The growth process in the plasma and the hot kernel (flame kernel) around the MIE were observed using high-speed schlieren photography and a multi-fiber coupled to an ICCD spectrometer. The kernel size, the kernel expansion velocity and the emission spectrum history form the plasma and the kernel were determined under the ignition case and misfire case. Further, temperature of the plasma and the kernel were determined from the CN molecular spectra by comparing experimental and simulated (LIFBASE) spectra. The main results of this study are as follows : (1) In laser ignition method, combustion reaction begins through the cooling process in which the ion recombines to the atom, furthermore the atom recombines to the molecular after breakdown. (2) By analyzing the CH emission intensity history, it was considered that combustion reaction occurs confirmed about 100 μs after laser pulse. (3) The temperature of the plasma and the kernel decrease with time and there were almost no difference between fire and misfire. (4) The minimum ignition energy is the smallest near the stoichiometric mixture.

DOI： 10.1299/kikaib.74.1633

Spatially, temporally, and spectrally resolved measurement of laser-induced plasma in air Reviewed

N. Kawahara, J. L. Beduneau, T. Nakayama, E. Tomita, Y. Ikeda

APPLIED PHYSICS B-LASERS AND OPTICS 86 ( 4 ) 605 - 614 2007.3

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

Images and emission spectra of sparks produced by laser-induced breakdown in air were investigated with a high degree of spatial and temporal resolution. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using a framing intensified charged coupled device (CCD) camera and a multi-fiber Cassegrain optics system coupled to an intensified CCD spectrometer. The results provided information about the different stages of laser-induced breakdown. The plasma shape and emission spectrum were very reproducible. Different ionization levels in the plasma kernels, which were observed using the high spatial resolution of the multi-fiber Cassegrain optics system, occurred during the plasma formation and cooling and at different locations within the plasma. This was due mainly to the thickness of the plasma relative to the laser wavelength, which created different ionization levels and energy absorption rates throughout. These observations were correlated with the plasma visualizations obtained with the framing ICCD camera. The plasma emission analysis permitted us to study the temperature evolution along the plasma during the laser-induced breakdown process. The analysis demonstrated the validity of a laser-supported wave model during the first stages of laser-induced breakdown and illustrated the weak dependence of the plasma temperature on the input energy.

DOI： 10.1007/s00340-006-2531-4

Auto-ignited kernels during knocking combustion in a spark-ignition engine Reviewed

Nobuyuki Kawahara, Eiji Tomita, Yoshitomo Sakata

PROCEEDINGS OF THE COMBUSTION INSTITUTE 31 ( 2 ) 2999 - 3006 2007

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE INC

In this study, auto-ignition of end-gas due to flame propagation and intensity oscillations caused by shockwaves that occur during knocking combustion were visualized in a compression-expansion engine using a high-speed video camera. Chemical luminescence emissions of the end-gas were detected to analyze the chemical reactions caused by the auto-ignition. Four main conclusions were drawn from this study. When the end-gas region was compressed due to the propagating flame front, auto-ignited kernels appeared near a negative curvature of the flame front. This negative curvature was related to low-temperature chemistry. The large amount of unburned mixture generated a strong pressure wave caused by the auto-ignited kernels explosion. Visualized images of a regular propagating flame front and auto-ignited kernels confirmed that the knocking intensity had a strong relationship with the mass fraction of the unburned mixture. Oscillations of OH* radicals were synchronized with the in-cylinder pressure oscillations, which were produced due to the resulting shockwave. Before auto-ignition of the end-gas occurred, weak OH* radicals and very weak HCHO* radicals appeared in the end-gas region due to low-temperature chemistry. The OH* radicals played an important role in the low-temperature kinetic reactions. This confirms low-temperature chemical reaction of auto-ignited kernel in the end gas region. OH* radicals are a good indicator of the transition from low-temperature chemistry to high-temperature auto-ignition. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

DOI： 10.1016/j.proci.2006.07.210

Cycle-resolved measurements of the fuel concentration near a spark plug in a rotary engine using an in situ laser absorption method Reviewed

Nobuyuki Kawahara, Eiji Tomita, Kenta Hayashi, Michihiko Tabata, Kouhei Iwai, Ryoji Kagawa

PROCEEDINGS OF THE COMBUSTION INSTITUTE 31 ( 2 ) 3033 - 3040 2007

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE INC

Cycle-resolved measurements of the fuel concentration near a spark plug in a commercial rotary engine were performed. An in situ laser infrared (IR) absorption method was developed using a spark plug sensor and a 3.392-mu m He-Ne laser as the light source. This wavelength coincided with the absorption line of hydrocarbons. The newly developed IR spark plug sensor had a higher signal-to-noise ratio than its previous version due to the optimization of its quartz lens and two optical fibers. The new sensor provided quantitative cycle-resolved fuel concentration measurements around the spark plug with a high temporal resolution. At lean preset air/fuel (A/F) ratios, fuel was mixed with the surrounding air gradually by the rotor motion in the plug hole of the rotary engine. Strong mixture inhomogeneities were measured during the compression stroke; the magnitude of these inhomogeneities decreased throughout the compression stroke. Cycle-resolved measurements were made to investigate the effects of the fuel concentration near the spark plug on the combustion characteristics of the commercial rotary engine. There was a strong correlation between the fuel concentration measured with the spark plug sensor and the combustion characteristics during the initial combustion period, which occurred faster when conditions were slightly richer than stoichiometric near the spark plug. The indicated mean effective pressure (IMEP) was slightly related to the A/F ratio near the spark plug. It was possible to measure the cycle-resolved A/F ratio near the spark plug and investigate its cycle-to-cycle fluctuations to achieve stable operation using the newly developed spark plug sensor. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

DOI： 10.1016/j.proci.2006.08.088

Combustion and Exhaust Emissions Characteristics in the Supercharged Engine Ignited with Gas Oil Fueled by Pyrolysis Gas Generated from Biomass Reviewed

軽油着火過給天然ガスエンジンにおける燃焼および排気特性 Reviewed

深谷信彦, 冨田栄二, 河原伸幸, 丸山慶士, 薦田哲男

自動車技術会論文集 38 ( 2 ) 125 - 130 2007

FUKATANI Nobuhiko, TOMITA Eiji, KAWAHARA Nobuyuki, MARUYAMA Keiji, KOMODA Tetsuo

Transactions of the Japan Society of Mechanical Engineers B 73 ( 730 ) 1337 - 1344 2007

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

Pyrolysis gas produced from woody and waste biomass is considered as one of very important fuels to establish a sustainable society. In this study, asupercharged single cylinder gas engine with micro pilot ignition by gas oil was used. This ignition system has an advantage of large energy source, long period for ignition and stable ignition compared to the spark ignition system. Four types of artificial pyrolysis gases that were compressed into high pressure vessels were used. Compositions of these gases were H_2 , CO, CO_2 , CH_4 and N_2. Combustion was visualized by using a high-speed color video camera from the bottom of the quartz piston. The pressure history was analyzed to obtain the rate of heat release in order to investigate combustion characteristics. Exhaust emissions of NO_x, HC, CO and smoke were measured. Many ignition kernels produced by the iginition of gas oil initiate flame propagation of gas/air mixture and then flames move toward the wall of cylinder. Effects of injection timing, gas composition and equivalence ratio on engine performance were studied.

DOI： 10.1299/kikaib.73.1337

Method for Predicting Scuffing on Poston Ring and Cylinder Liner in Marine Diesel Engines: Damage Prevention Using Observation of Combustion Flame:Damage Prevention Using Observation of Combustion Flame Reviewed

Takasu Isao, Kan Tadayuki, Takasaki Koji, Tomita Eiji, Kawahara Nobuyuki

JOURNAL OF THE MARINE ENGINEERING SOCIETY IN JAPAN 39 ( 4 ) 250 - 258 2004

Language：Japanese Publisher：The Japan Institute of Marine Engineering

Residual low-grade fuels are often used for marine diesel engines for reasons of economy. Some marine engines have been damaged due to the difficulty of combustion control of such fuels. Many factors affect scuffing of the piston ring and cylinder liner. Because of the complexity and difficulties, however, no one can predict scuffing. Problems have to be solved one by one. This study focuses on scuffing that is caused by the fuel itself. A spray flame was observed with a high-speed video camera in a constant volume vessel, in which ambient pressure and temperature were controlled. A previous study found that after-burning and long ignition delay were problems, so this analysis focused on combustion phenomena after the end of injection. A function of the period from the end of injec-tion till quenching of the flame and of the flame length at quench showed good correlation with the degree of scuffing damage. These two parameters are more valuable for predicting scuffing than are other methods before bunkering fuel in the ship.

DOI： 10.5988/jime.39.250

CiNii Article