NOx emission characteristics in turbulent hydrogen jet flames with coaxial air

The characteristics of NOx emissions in pure hydrogen nonpremixed jet flames with coaxial air are analyzed numerically for a wide range of coaxial air conditions. Among the models tested in simple nonpremixed jet flame, the one-half power scaling law could be reproduced only by the Model C using the HO₂/H₂O₂ reaction, implying the importance of chemical nonequilibrium effect. The flame length is reduced significantly by augmenting coaxial air, and could be represented as a function of the ratio of coaxial air to fuel velocity. Predicted EINOx scaling showed a good concordance with experimental data, and the overall one-half power scaling was observed in coaxial flames with Model C when flame residence time was defined with flame volume instead of a cubic of the flame length. Different level of oxygen mass fraction at the stoichiometric surface was observed as coaxial air was increased. These different levels imply that the coaxial air strengthens the nonequilibrium effect. This paper was recommended for publication in revised form by Associate Editor Haecheon Choi Hee-Jang Moon received his B.S. degree in Aeronautical Engineering from Inha University, Korea in 1986. He then received his M.S. and Doctoral degrees from Universite de Rouen, France in 1988 and 1991, respectively. Dr. Moon is currently a Professor at the School of Aerospace and Mechanical Engineering at Korea Aerospace University in Koyang, Korea. He serves on the Editorial Board of the Korean Society of Propulsion Engineers. His research interests are in the area of turbulent combustion, hybrid rocket combustion and nanofluids. Youngbin Yoon received his B.S. and M.S. degrees in Aerospace Engineering from Seoul National University, Korea in 1985 and 1987, respectively. He received a Ph.D. degree from the University of Michigan in 1994. Dr. Yoon is currently a professor at the School of Mechanical and Aerospace Engineering in Seoul National University, Korea. He is currently on the Editorial board and executive of ILASS-KOREA. The research areas of Dr. Yoon are liquid rocket injectors, combustion instability and control, ram and gas turbine combustor and laser diagnostics.     

The effect of chemical structure of dimethyl ether (DME) on NOx formation in nonpremixed counterflow flames

To clarify the effect of chemical structure of Dimethyl ether(DME) on NOx formation in nonpremixed counterflow flame, DME flame was investigated numerically to compare the flame structures and NOx emissions with C₂H₆ and Mixed-fuel. Numerically, the governing equations were solved using the Oppdif code coupled with CHEMKIN package, and DME flames were calculated by Kaiser’s mechanism, while the C₂H₆ flames and Mixed-fuel flames were calculated by the C₃ mechanism. These mechanisms were combined with the modified Miller-Bowman mechanism for the analysis of NOx. Numerical results of nonpremixed counterflow flames show that the EI_NO of DME nonpremixed flame is low as much as 50 % of the C₂H₆ nonpremixed flame. The cause of EI_NO reduction is attributed mainly to the characteristics of partial premixed flame due to the existence of oxygen atom in DME and partly to the O-C bond in DME, instead of C-C bond in hydrocarbon fuels. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Chang-Eon Lee received his B.S. and M.S. degrees in Mechanical Engineering from Inha University, Korea, in 1983 and 1985, respectively. Then he received his Ph.D. degree from Toyohashi National University of Technology, Japan in 1992. Dr. Lee is currently a Professor at the School of Mechanical Engineering at Inha University in Incheon, Korea. He serves as an Editor of the Journal of the Korean society of combustion and serves as an associate Editor of Transactions of the Korean society of mechanical engineers. Dr. Lee’s research interests include fluid mechanics, combustion and environmental pollution, and total energy.     

Numerical simulation on soot deposition process in laminar ethylene diffusion flames under a microgravity condition

A numerical study on soot deposition in ethylene diffusion flames has been conducted to elucidate the effect of thermophoresis on soot particles under a microgravity environment. Time-dependent reactive-flow Navier-Stokes equations coupled with the modeling of soot formation have been solved. The model was validated by comparing the simulation results with the previous experimental data for a laminar diffusion flame of ethylene (C₂H₄) with enriched oxygen (35% O₂ + 65% N₂) along a solid wall. In particular, the effect of surrounding air velocity as a major calculation parameter has been investigated. Especially, the soot deposition length defined as the transverse travel distance to the wall in the streamwise direction is introduced as a parameter to evaluate the soot deposition tendency on the wall. The calculation result exhibits that there existed an optimal air velocity for the early deposition of soot on the surface, which was in good agreement with the previous experimental results. The reason has been attributed to the balance between the effects of the thermophoretic force and convective motion. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Jae Hyuk Choi received his B.S. and M.S. degrees in Marine System Engineering from Korea Maritime University in 1996 and 2000, respectively. He then went on to receive a Ph.D. degrees from Hokkaido university in 2005. Dr. Choi is currently a BK21 Assistant Professor at the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. Dr. Choi’s research interests are in the area of reduction of pollutant emission (Soot and NOx), high temperature combustion, laser diagnostics, alternative fuel and hydrogen production with high temperature electrolysis steam (HTES). Junhong Kim received his B.S., M.S., and Ph. D degrees in Mechanical Engineering from Seoul National University in 1998, 2000, and 2004, respectively. His research interests include lifted flames, edge flames, and numerical simulation. Sang Kyu Choi received his B.S. degree in Mechanical Engineering from Seoul National University in 2004. He is a Ph. D student in the School of Mechanical Engineering, Seoul National University. His research interests include edge flames, oxy-fuel combustion, and numerical simulation. Byoung ho Jeon received his B.S degrees in Mechanical Engineering from kangwon University in 1998, and M.S., Ph. D. degrees in Mechanical Engineering from Hokkaido University in 2002, 2008, respectively. Dr Jeon is working at Korea Aerospace Research Institute from 2007. June. as Gasturbine engine developer. Jeon’s research interests are in the area of reduction of pollutant emission (Soot and Nox), High temperature combustion, combustion system (Furnace, Combine Generation system, IGCC, CTL), and Fire safety in building. Osamu Fujita received his B.S., M.S., and Ph. D. degrees in Mechanical Engineering from Hokkaido University in 1982, 1984, and 1987, respectively. Prof. Fujita is currently a Professor at the division of Mechanical and space Engineering at Hokkaido University in sapporo, Japan. Prof. Fujita’s research interests are in the area of reduction of pollutant emission (Soot and Nox), solid combustion, catalytic combustion, high temperature combustion, alternative fuel and fire safety in space. Suk Ho Chung received his B.S. degree in Mechanical Engineering in 1976 from Seoul National University, and his M.S. and Ph. D. degree in Mechanical Engineering in 1980 and 1983, respectively from Northwestern University. He is a professor since 1984 in the School of Mechanical and Aerospace Engineering, Seoul National University. His research interests cover combustion fundamentals, pollutant formation, and laser diagnostics.     

Improvement of flame stability and NOx reduction in hydrogen-added ultra lean premixed combustion

Lean premixed combustion is a well known method in gas turbine combustors that can reduce fuel consumption and decrease flame temperature. In lean premixed flames, flame instabilities can occur because the combustion takes place near the lean flammable limit. For the purpose of increasing flame stability, a small amount of hydrogen was added into a fuel, which has ultra low lean flammable limit. The extinction stretch rate increased and total equivalence ratio at extinction decreased with hydrogen addition; consequently, ultra lean premixed combustion was possible and flame stability could be achieved at low temperature conditions. The NO_x emission increased with hydrogen addition for the same stretch rate and equivalence ratio, but the extinction stretch rate and lean flammability limit was enlarged. Consequently, NO_x emission decreased with hydrogen addition in the near extinction conditions. Hydrogen addition could improve flame stability and reduce NO_x emission in ultra lean premixed combustion. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Dr. Eun-Seong Cho received his B.S. and M.S. degrees in Mechanical Engineering from Hanyang University, Korea, in 1996 and 1998, respectively. He then received his Ph.D. degree from Seoul National University, Korea, in 2005. He was a principal engineer of KD Navien research center and currently a research associate at Delft University of Technology, The Netherlands. His research interests include eco-friendly clean combustion technology, new and renewable energy systems. Prof. Suk Ho Chung received his B.S. degree from Seoul National University, Korea, in 1976 and Ph.D. degree in Mechanical Engineering from Northwestern University, USA, in 1983. He is a Professor since 1984 in the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests cover combustion fundamentals, pollutant formation, laser diagnostics, and plasma-assisted combustion.     

Measurement of three-dimensional flame structure by combined laser diagnostics

To investigate three-dimensional flame structures of turbulent premixed flame experimentally, dual-plane planar laser induced fluorescence (PLIF) of CH radical has been developed. This dual-plane CH PLIF system consists of two independent conventional CH PLIF measurement systems and laser beam from each laser system are led to parallel optical pass using the difference of polarization, and CH PLIF is conducted in two parallel two-dimensional cross sections. The newly-developed dual-plane CH PLIF is combined with single-plane OH PLIF and stereoscopic particle image velocimetry (PIV) to clarify the relation between flame geometry and turbulence characteristics. The laser sheets for single-plane OH PLIF and stereoscopic PIV measurement are located at the center of two planes for CH PLIF. The separation between these two planes is selected to 500 μm. The measurement was conducted in relatively high Reynolds number methane-air turbulent jet premixed flame. The experimental results show that various three-dimensional flame structures such as the handgrip structure, which has been shown by three-dimensional direct numerical simulations (DNS), are included in high Reynolds number turbulent premixed flame. It was shown that the simultaneous measurement containing newly-developed dual-plane CH PLIF is useful for investigating the three-dimensional flame structures. To analyze the flame structures quantitatively, the flame curvature was estimated by using the CH and OH PLIF images, and the probability density function (pdf) of the curvatures was compared with the results of DNS. It was revealed that the minimum radius of curvature of the flame front coincides with Kolmogorov length. However, the feature of pdf of the flame curvature is slightly different from result of DNS, if the curvature was estimated from experimental results in two-dimensional cross section. On the other hand, the feature of pdf of mean curvature that calculated from triple-plane PLIF results is similar to that obtained from three-dimensional DNS. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Takashi Ueda received his B.S. degree in Mechano-Aerospace Engineering from Tokyo Institute of Technology, Japan, in 2008. Mr. Ueda is currently in Master course at Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan. Mr. Ueda’s research interests include turbulent combustion. Masayasu Shimura received his B.S. degree in Mechano-Aerospace Engineering from Tokyo Institute of Technology, Japan, in 2005. He then received his M.S. from Tokyo Institute of Technology in 2006. Mr. Shimura is currently in Ph.D candidate at Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan. Mr. Shimura’s research interests include noise control of turbulent combustion. Mamoru Tanahashi received his B.S. degree in Mechanical Engineering, Science from Tokyo Institute of Technology, Japan, in 1990. He then received his M.S. and Dr. Eng. from Tokyo Institute of Technology in 1992 and 1996, respectively. Prof. Tanahashi is currently an Associate Professor of Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan. Prof. Tanahashi’s research interests include turbulence and combustion. Toshio Miyauchi received his B.S. degree in Mechanical Engineering from Tokyo Institute of Technology, Japan, in 1971. He then received his M.S. and Ph.D. degrees from Tokyo Institute of Technology in 1973 and 1981, respectively. Prof. Miyauchi is currently a Professor of Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan. Prof. Miyauchi’s research interests include turbulence and combustion.     

Development of rapid mixing fuel nozzle for premixed combustion

Combustion in high-preheat and low oxygen concentration atmosphere is one of the attractive measures to reduce nitric oxide emission as well as greenhouse gases from combustion devices, and it is expected to be a key technology for the industrial applications in heating devices and furnaces. Before proceeding to the practical applications, we need to elucidate combustion characteristics of non-premixed and premixed flames in high-preheat and low oxygen concentration conditions from scientific point of view. For the purpose, we have developed a special mixing nozzle to create a homogeneous mixture of fuel and air by rapid mixing, and applied this rapidmixing nozzle to a Bunsen-type burner to observe combustion characteristics of the rapid-mixture. As a result, the combustion of rapid-mixture exhibited the same flame structure and combustion characteristics as the perfectly prepared premixed flame, even though the mixing time of the rapid-mixing nozzle was extremely short as a few milliseconds. Therefore, the rapid-mixing nozzle in this paper can be used to create preheated premixed flames as far as the mixing time is shorter than the ignition delay time of the fuel. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Masashi Katsuki received his B.E. degree in Mechanical Engineering from Osaka University, Japan, in 1965. He received his Dr. Eng. from O. U. in 1985. Dr. Katsuki is currently a Visiting Professor at the Department of Environmental Engineering at Hoseo University in Chungnam, Korea. He was a Vice President of the Japan Society of Mechanical Engineers. Dr. Katsuki’s research interests include combustion, computational thermo-fluid dynamics, and molecular dynamics. Jin-Do Chung received his B. S., M.S. and Ph.D. degrees in Mechanical Engineering from Chungnam University, Korea in 1983, 1985 and 1990. He then received another Ph.D. in Environmental Engineering from Kanazawa University, Japan in 1996. After that he worked as Post-doc researcher for 1,6 year at KIMM and Senior researcher for 6years at KEPCO Research Center. Dr. Chung is currently a Professor at the Department of Environmental Engineering at Hoseo University in Asan, Korea. Dr. Chung’s research interests include thermal-fluid and environmental engineering. Jang-Woo Kim received his B. S. degree in Mechanical Engineering from Chungnam University, Korea, in 1990. He then received his M. S. and Ph. D. degrees from Kyushu University, Japan in 1994 and 1998, respectively. Dr. Kim is currently a Professor at the School of Display Engineering at Hoseo University in Asan, Korea. Dr. Kim’s research interests include CFD, aerodynamics, and display equipment technology. Seung-Min Hwang received the Ph.D. degree in Mechanical Engineering at Osaka University in 2005. After that he worked as visiting researcher for 3 years at CRIEPI (central research institute of electric power industry) and Osaka University in Japan. He is currently a Professor at the Graduate School of Venture at Hoseo University in Korea. His major research is thermal-fluid, energy issue and environment. Seung-Mo Kim received his Ph. D. degrees in Mechanical engineering from Osaka University, Japan, in 2004. Dr. Kim is currently a research Professor at Pusan Clean Coal Center at Pusan National University in Pusan, South Korea. Dr. Kim’s research interests include coal combustion, oxy-fuel combustion, coal gasification, coal de-watering, power generation plant system and energy issues. Chul-Ju Ahn received his B.S. degree in Mechanical Engineering from Hanyang University, Korea, in 1998. He then received his M.S. and Ph.D. degrees from Osaka University, Japan, in 2001 and 2006, respectively. Dr. Ahn is currently a Senior Research Engineer at Samsung Techwin CO. LTD. in Changwon, Korea. Dr. Ahn’s research interests include gas turbine engine, biomass gasification, and power system.     

Numerical modeling for the H2/CO bluff-body stabilized flames

This study investigates the nonpreximed H₂/CO-air turbulent flames numerically. The turbulent combustion process is represented by a reaction progress variables model coupled with the presumed joint probability function. In the present study, the turbulent combustion model is applied to analyze the nonadiabatic flames by introducing additional variable in the transport equation of enthalpy and the radiative heat loss is calculated using a local, geometry independent model. Calculation are compared with experimental data in terms of temperature, and mass fraction of major species, radical, and NO. Numerical results indicate that the lower and higher fuel-jet velocity flames have the distinctly different flame structures and NO formation characteristics in the proximity of the outer core vortex zone. The present model correctly predicts the essential features of flame structure and the characteristics of NO formation in the bluff-body stabilized flames. The effects of nonequilibrium chemistry and radiative heat loss on the thermal NO formation are discussed in detail.     

Numerical modeling of turbulent nonpremixed lifted flames

The present study has focused on numerical investigation on the flame structure, flame lift-off and stabilization in the partially premixed turbulent lifted jet flames. Since the lifted jet flames have the partially premixed nature in the flow region between nozzle exit and flame base, level set approach is applied to simulate the partially premixed turbulent lifted jet flames for various fuel jet velocities and co-flow velocities. The flame stabilization mechanism and the flame structure near flame base are presented in detail. The predicted lift-off heights are compared with the measured ones.     

Flow dynamics at the minimum starting condition of a supersonic diffuser to simulate a rocket’s high altitude performance on the ground

A numerical analysis was conducted to investigate and characterize the unsteadiness of the flow structure and oscillatory vacuum pressure inside of a supersonic diffuser equipped to simulate high-altitude rocket performance on the ground. A physical model including a rocket motor, vacuum chamber, and diffuser, which have axisymmetric configurations was employed. Emphasis was placed on investigating the physical phenomena of very complex and oscillatory flow evolutions in the diffuser operating very close to the starting condition, i.e. at a minimum starting condition, which is one of the major important parameters from a diffuser design point of view. This paper was recommended for publication in revised form by Associate Editor Jun Sang Park Hyo-Won Yeom received a B.S. degree in the department of Aerospace & Mechanical Engineering from Korea Aerospace University in 2007. He is currently a master candidate at the school of Aerospace & Mechanical Engi-neering at Korea Aerospace Uni-versity in Goyang-city, Korea. His research interests are in the area of numerical analysis for High-speed propulsion system. Sangkyu Yoon received a B.S. degree in the department of Aerospace & Mechanical Engineering from Korea Aerospace University in 2006 and M.S. degrees in the school of Aerospace & Mecha-nical Engineering from Korea Aerospace University in 2008. He currently works in Hanwha Corporation R&D Center. Hong-Gye Sung received a B.S. degree in the department of Aerospace Engineering from Inha University in 1984 and Ph.D. degree in Nuclear and Mechanical Engineering from The Pennsylvania State University in 1999. Dr. Sung has various research experiences in the fields of high-speed propulsion and rocket propulsion in Agency for Defense Development for 22 years (1984–2006). He is currently a professor at the School of Aerospace and Mechanical Engineering of Korea Aerospace University in Goyang, Korea. Dr. Sung’s research interests are in the area of propulsion, combustion, and its control.     

A study on separate learning algorithm using support vector machine for defect diagnostics of gas turbine engine

A separate learning algorithm with support vector machine (SVM) has been studied for the development of a defect-diagnostic algorithm applied to the gas turbine engine. The system using only an artificial neural network (ANN) falls in a local minima and its classification accuracy rate becomes low in case it is learning nonlinear data. To make up for this risk, a separate learning algorithm combining ANN with SVM has been proposed. In the separate learning algorithm, a sequential ANN learns selectively after classification of defect patterns and discrimination of defect position using SVM, resulting in higher classification accuracy rate as well as the rapid convergence by decreasing the nonlinearity of the input data. The results have shown this suggested method has reliable and suitable estimation accuracy of the defect cases of the turbo-shaft engine. This paper was recommended for publication in revised form by Associate Editor Dongsik Kim Tae-Seong Roh received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1984 and 1986. He then went on to receive his Ph.D. degree from Pennsylvania State University in 1995. Dr. Roh is currently a Professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of combustion instabilities, rocket and jet propulsions, interior ballistics, and gasturbine engine defect diagnostics. Dong-Whan Choi received his B.S. degree in Aeronautical Engineering from Seoul National University in 1974. He then went on to receive his M.S. and Ph.D. degrees from University of Washington in 1978 and 1983. Dr. Choi had served three years as a President of Korea Aerospace Research Institute since 1999. He is currently a Professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of turbulence, jet propulsions, and gasturbine defect diagnostics.     

Prediction of flame formation in highly preheated air combustion

Fundamental information about the ignition position and shape of a flame in highly preheated air combustion was obtained, and the suitability of the suggested reduced kinetic mechanism that reflects the characteristics of the highly preheated air combustion was demonstrated. Flame lift height and flame length with variations of premixed air temperature and oxygen concentration were measured by CH* chemiluminescence intensity, and were computed with a reduced kinetic mechanism. Flame attached near a fuel nozzle started to lift when preheated air temperature became close to auto-ignition temperature and/or oxygen concentration reduced. The flame lift height increased but the flame length decreased with decreasing preheated air temperature and flame length reversed after a minimum value. Calculated results showed good agreement with those of experiment within tolerable error. Flame shape shifted from diffusion flame shape to partial premixed flame shape with increasing lift height and this tendency was also observed in the computation results. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Gyung-Min Choi studied the areas of combustion engineering, heat recirculating combustion, and solid fuel gasification, receiving his Ph.D. degree in engineering from Osaka University in 2001. He served as a researcher at Japan Aerospace Exploration Agency and is now an associate professor in the School of Mechanical Engineering at Pusan National University.     

Defect diagnostics of SUAV gas turbine engine using hybrid SVM-artificial neural network method

A hybrid method of an artificial neural network (ANN) combined with a support vector machine (SVM) has been developed for the defect diagnostic system applied to the SUAV gas turbine engine. This method has been suggested to overcome the demerits of the general ANN with the local minima problem and low classification accuracy in case of many nonlinear data. This hybrid approach takes advantage of the reduction of learning data and converging time without any loss of estimation accuracy because the SVM classifies the defect location and reduces the learning data range. The results of test data have shown that the hybrid method is more reliable and suitable algorithm than the general ANN for the defect diagnosis of the gas turbine engine. This paper was recommended for publication in revised form by Associate Editor Tong Seop Kim Tae-Seong Roh received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1984 and 1986. He then went on to receive his Ph.D. degree from Pennsylvania State University in 1995. Dr. Roh is currently a Professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of combustion instabilities, rocket and jet propulsion, interior ballistics, and gas turbine engine defect diagnostics. Dong-Whan Choi received his B.S. degree in Aeronautical Engineering from Seoul National University in 1974. He then went on to receive his M.S. and Ph.D. degrees from University of Washington in 1978 and 1983. Dr. Choi served three years as a President of the Korea Aerospace Research Institute from 1999. He is currently a professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of turbulence, jet propulsion, and gas turbine defect diagnostics.     

Friction-induced ignition modeling of energetic materials

The heat released during the external frictional motion is a factor responsible for initiating energetic materials under all types of mechanical stimuli including impact, drop, or penetration. We model the friction-induced ignition of cyclotrimethylenetrinitramine (RDX), cyclotetramethylene-tetranitramine (HMX), and ammonium-perchlorate/ hydroxylterminated-polybutadiene (AP/HTPB) propellant using the BAM friction apparatus and one-dimensional time to explosion (ODTX) apparatus whose results are used to validate the friction ignition mechanism and the deflagration kinetics of energetic materials, respectively. A procedure to obtain the time-to-ignition for each energetic sample due to friction is outlined. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Min-cheol Gwak received his B.S. degree in Mechanical Engineering from Korea Aerospace University, Korea, in 2007. Now he is a graduate student of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests are ignition of high energy material and combustion phenomena. Tae-yong Jung received his B.S. degree in Mechanical and Aerospace Engineering from Seoul National University, Korea, in 2007. Now he is a graduate student of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests are solid propellant combustion and phase transformation. Professor J. Yoh received his BSME from UC Berkeley in 1992 and MSME from UCLA in 1995. His doctoral degree is in Theoretical & Applied Mechanics from the University of Illinois at Urbana-Champaign, 2001. His research interest is in high energy system design using high power lasers and condensed energetic materials.     

Large eddy simulation of turbulent premixed combustion flow around bluff body

Large eddy simulation (LES) of turbulent premixed flame stabilized by the bluff body is developed by using sub-grid scale combustion model based on the G-equation describing the flame front propagation. The basic idea of LES modeling is to evaluate the filtered-front speed, which should be enhanced in the grid scale by the scale fluctuations. The dynamic sub-grid scale models newly introduced into the G-equation are validated by the premixed combustion flow behind the triangle flame holder. The objective of this study is to investigate the validity of the dynamic sub-grid G-equation model to a complex turbulent premixed combustion such as bluff body stabilized turbulent premixed flames for realistic engineering application. A new turbulent flame speed model, introduced by the sub-grid turbulent diffusivity and the flame thickness, is also proposed and is compared with the usual model using sub-grid turbulent intensity and with the experimental data. The calculated results can predict the velocity and temperature of the combustion flow in good agreement with the experiment data.     

An experiment analysis of MILD combustion with liquid fuel spray in a combustion vessel

Mild combustion is characterized by its distinguished features, such as suppressed pollutant emission, homogeneous temperature distribution, reduced noise, and thermal stress. Recently, many studies have revealed the potential of MILD combustion in various power systems but most studies have been focused on gas phase fuel MILD combustion. Therefore, further study on MILD combustion using liquid fuel is needed for the application to a liquid-fueled gas turbine especially. In this work, we studied experimentally on the formation of liquid fuel MILD combustion under the condition of high dilution by burnt gas generated from a first premixed flame in two stages combustor which consists of the first premixed burner and secondary combustor. In particular, the effects of burnt gas velocity and oxygen level of burnt gas on the formation of liquid fuel MILD combustion were investigated. The results show that as the burnt gas velocity through the nozzle becomes higher, the color of flames was changed from yellow to pale blue and flames became very short. The OH radical measured by ICCD camera was uniformly distributed on the pale blue flame surface and its intensity was very low compared to conventional liquid diffusion flame. As burnt gas velocity is increased, local high-temperature region appeared to be diminished and the flame temperature became spatially uniform. And CO emission was sampled around 1 ppm and NOx emission was measured around 10 ppm under the overall equivalence ratio of 0.8 to 0.98 for 40 mm or less diameter of velocity control nozzle. This low NOx emission seems to be attributed to maintaining the average temperature in secondary combustor below the threshold temperature of thermal NOx formation. In view of the uniform temperature distribution, low OH radical intensity and low NOx emission data in the secondary combustor, formation of stable MILD combustion using kerosene liquid fuel could be verified at high burnt gas velocity.

     

Flow instability due to cryogenic cavitation in the downstream of orifice

Flow instability in LRE (liquid rocket engine) occurs due to various reasons such as flow interactions with valve, orifice and venturi, etc. The inception of cavitation, especially in the propellant feeding system, is the primary cause of mass and pressure oscillations because of the cyclic formation and depletion of cavitation. Meanwhile, the main propellant in a liquid rocket engine is the cryogenic fluid, which properties are very sensitive to temperature variation. And the change of propellant properties to temperature variation by thermodynamic effect needs to be properly taken into account in the flow analysis in order to understand basic mechanisms for cryogenic cavitation. The present study focuses on the formation of cryogenic cavitation by using the IDM model suggested by Shyy and coworkers. The flow instability was also numerically investigated in the downstream of orifice with a developed numerical code. Calculation results show that cryogenic cavitation can be a primary source of flow instability, leading to mass fluctuations accompanied by pressure oscillations. The prediction of cavitation in cryogenic fluid is of vital importance in designing a feeding system of an LRE. This paper was recommended for publication in revised form by Associate Editor Jun Sang Park Changjin Lee received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1983 and 1985. He then went on to receive his Ph.D. degree from University of Illinois at Urbana- Champaign in 1992. Dr. Lee is currently a Professor at the department of Aerospace Engineering at Konkuk University in SEOUL, Korea. His research interests are in the area of combustion instabilities of hybrid, liquid rocket and jet propulsions. Tae-Seong Roh received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1984 and 1986. He then went on to receive his Ph.D. degree from Pennsylvania State University in 1995. Dr. Roh is currently a Professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of combustion instabilities, rocket and jet propulsions, interior ballistics, and gas turbine engine defect diagnostics.     

Defect diagnostics of gas turbine engine using hybrid SVM-ANN with module system in off-design condition

A hybrid method of an artificial neural network (ANN) and a support vector machine (SVM) has been used for a health monitoring algorithm of a gas turbine engine. The method has the advantage of reducing learning data and converging time without any loss of estimation accuracy, because the SVM classifies the defect location and reduces the learning data range. In off-design condition, however, the operation region of the engine becomes wide and the nonlinearity of learning data increases considerably. Therefore, an improved hybrid method with the module system and the advanced SVM has been suggested to solve the problems. The module system divides the whole operating region into reasonably small-sized sections, and the advanced SVM has two steps of the classification. The proposed algorithm has been proven to reliably and effectively diagnose the simultaneous defects of the triple components as well as the defects of the single and dual components of the gas turbine engine in off-design condition. This paper was recommended for publication in revised form by Associate Editor Tong Seop Kim Tae-Seong Roh received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1984 and 1986. He then went on to receive his Ph.D. degree from Pennsylvania State University in 1995. Dr. Roh is currently a Professor at the department of Aerospace Engi-neering at Inha University in Incheon, Korea. His research interests are in the area of combustion instabilities, rocket and jet propulsions, interior ballistics, and gas turbine engine defect diagnostics. Dong-Whan Choi received his B.S. degree in Aeronautical Engineering from Seoul National University in 1974. He then went on to receive his M.S. and Ph.D. degrees from University of Washington in 1978 and 1983. Dr. Choi served three years as a President of Korea Aerospace Research Institute from 1999. He is currently a professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of turbulence, jet propulsions, and gas turbine defect diagnostics.     

Large eddy simulation of turbulent premixed flame in turbulent channel flow

Large eddy simulation of turbulent premixed flame in turbulent channel flow is studied by usingG-equation. A flamelet model for the premixed flame is combined with a dynamic subgrid combustion model for the filtered propagation flame speed. The objective of this work is to investigate the validity of the dynamic subgridG-equation model to a complex turbulent premixed flame. The effect of model parameters of the dynamic subgridG-equation on the turbulent flame speed is investigated. In order to consider quenching of laminar flames on the wall, wall-quenching damping function is employed in this calculation. In the present study, a constant density turbulent channel flow is used. The calculation results are evaluated by comparing with the DNS results of Bruneaux et al.     

Numerical study on no emission with flue gas dilution in air and fuel sides

Flue gas recirculation (FGR) is widely adopted to control NO emission in combustion systems. Recirculated flue gas decreases flame temperature and reaction rate, resulting in the decrease in thermal NO production. Recently, it has been demonstrated that the recirculated flue gas in fuel stream, that is, the fuel induced recirculation (FIR), could enhance much improved reduction in NO per unit mass of recirculated gas, as compared to conventional FGR in air. In the present study, the effect of dilution methods in air and fuel sides on NO reduction has been investigated numerically by using N3 and CO2 as diluent gases to simulate flue gases. Counterflow diffusion flames were studied in conjunction with the laminar flamelet model of turbulent flames. Results showed that CO2 dilution was more effective in NO reduction because of large temperature drop due to the larger specific heat of CO2 compared to N₂. Fuel dilution was more effective in reducing NO emission than air dilution when the same recirculation ratio of dilution gas was used by the increase in the nozzle exit velocity, thereby the stretch rate, with dilution gas added to fuel side.     

Eulerian particle flamelet modeling for combustion processes of bluff-body stabilized methanol-air turbulent nonpremixed flames

The present study is focused on the development of the RIF (Representative Interactive Flamelet) model which can overcome the shortcomings of conventional approach based on the steady flamelet library. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the RIF model can effectively account for the detailed mechanisms of NOx formation including thermal NO path, prompt and nitrous NOx formation, and reburning process by hydrocarbon radical without any ad-hoc procedure. The flamelet time of RIFs within a stationary turbulent flame may be thought to be Lagrangian flight time. In context with the RIF approach, this study adopts the Eulerian Particle Flamelet Model (EPFM) with mutiple flamelets which can realistically account for the spatial inhomogeneity of scalar dissipation rate. In order to systematically evaluate the capability of Eulerian particle flamelet model to predict the precise flame structure and NO formation in the multi-dimensional elliptic flames, two methanol bluffbody flames with two different injection velocities are chosen as the validation cases. Numerical results suggest that the present EPFM model has the predicative capability to realistically capture the essential features of flame structure and NOx formation in the bluff-body stabilized flames.