Flame structure and thermal NOx formation in hydrogen diffusion flames with reduced kinetic mechanisms

Structure and thermal NO_x formation of hydrogen diffusion flames are studied numerically, by adopting a counterflow as a model problem. Detailed kinetic mechanism having twenty-one step hydrogen oxidation is systematically reduced to a two-step mechanism while five-step thermal NO_x chemistry of the extended Zel'dovich mechanism is reduced to one-step. Results show that the extinction strain rates are much higher than those for hydrocarbon flames and the NO_x production can be controlled by increasing strain rates which results in the decrease of flame temperature significantly. Comparison between the results of the detailed and reduced mechanisms demonstrates that the reduced mechanism successully describes the essential features of hydrogen diffusion flames including the flame structure, extinction strain rate and NO_x production.     

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.     

Characteristics of propagating tribrachial flames in counterflow

The effect of fuel concentration gradient on the propagation characteristics of tribrachial (or triple) flames has been investigated experimentally in both two-dimensional and axisymmetric counterflows. The gradient at the stoichiometric location was controlled by the equivalence ratios at the two nozzles; one of which is maintained rich, while the other lean. Results show that the displacement speed of tribrachial flames in the two-dimensional counterflow decreases with fuel concentration gradient and has much larger speed than the maximum speed predicted previously in two-dimensional mixing layers. From an analogy with premixed flame propagation, this excessively large displacement speed can be attributed to the flame propagation with respect to burnt gas. Corresponding maximum speed in the limit of small mixture fraction gradient was estimated and the curvefit of the experimental data substantiates this limiting speed. As mixture fraction gradient approaches zero, a transition occurs, such that the propagation speed of tribrachial flame approaches stoichiometric laminar burning velocity with respect to burnt gas. Similar results have been obtained for tribrachial flames propagating in axisymmetric counterflow.     

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.     

An experimental study on the combustion characteristics of a low nox burner using reburning technology

The combustion characteristics of a low NO_x burner using reburning technology have been experimentally studied. The reburn burner usually has three distinct reaction zones which include the primary combustion zone, the reburn zone and the burnout zone by provided secondary air. NO_x is mainly produced in a primary combustion zone and a certain portion of NO_x can be converted to nitrogen in the reburn zone. In the burnout zone, the unburned mixtures are completely oxidated by supplying secondary air. Liquefied Petroleum Gas (LPG) was used as main and reburn fuels. The experimental parameters investigated involve the main/ reburn fuel ratio, the primary/secondary air ratio, and the injection location of reburn fuel and secondary air. When the amount of reburn fuel reaches to the 20-30% of the total fuel used, the overall NO reduction of 50% is achieved. The secondary air is injected by two different ways including vertical and parallel injection. The injector of secondary air is located at the downstream region of furnace for a vertical-injection mode, which is also placed at the inlet primary-air injection region for a parallel-injection mode. In case of the vertical injection of the secondary air flow, the NO_x formation of stoichiometric condition at a primary combustion zone is nearly independent of the reburn conditions (locations, fuel/air ratios) while the NO_x emission of the fuel-lean condition is considerably influenced by the reburn conditions. In case of the parallel injection of the secondary air, the NO_x emission is sensitive to the air ratio rather than the fuel ratio and the reburning process often coupled with the multiple air-staging and fuel-staging combustion processes.     

An experimental study on combustion processes and nox emission characteristics of the air-staged burner

The combustion processes and emission characteristics in air-staged burner have been experimentally studied. The light fuel oil doped with pyridine(C₅H₅N) is used to investigate the fuel NO_X emission characteristics. Experiments are carried out for a wide range of operating conditions of single-staged and multi-staged burner. The detailed discussions are made for the flame structure of the air-staged burner as well as effects of excess air ratios, staged air flow percentage, and spray conditions on flame pattern and NO_X emission characteristics.     

Improving retrieval precision of NO2 density in DOAS by accurately ascertaining low-frequency structure

The NO₂ density error retrieved by the traditional method of differential optical absorption spectroscopy (DOAS) was analyzed. The technique for reducing the error is proposed and used to obtain the NO₂ density of Chengdu through DOAS.     

A micromechanics analysis of the material removal mechanisms in the cutting of ceramic particle reinforced metal matrix composites

In previous investigations on the cutting of ceramic particle reinforced metal matrix composites using the finite element (FE) method, the particles are usually considered to be rigid. This is inconsistent with the actual situation and thus makes the FE predictions less practical. This paper proposes a micromechanics model to investigate the material removal mechanisms by considering the elasticity and fracture of the particles. It was found that the interaction position of a particle relative to the cutting edge greatly influences the particle and matrix fracture, particle-matrix debonding, surface integrity and cutting forces. There are two particle cracking mechanisms. One is caused by the direct contact of the cutting edge with the particle, and the other is due to the indirect tool-particle interaction through matrix. When the cutting path is below a particle, a smooth surface without subsurface damage can be achieved. When it is passing through a particle, particle fracture mostly occurs. If it is above a particle, subsurface damage is dominated by particle-matrix debonding. Cutting force peaks once the cutting edge is in contact with a particle and the cutting edge is easier to be damaged when the tool is passing through the upper part of the particle.     

Influence of glass substrate thickness in laser scribing of glass

The thickness of the glass substrate used in liquid crystal displays continues to be decreased from its original thickness of 1.1 mm for the purpose reducing size and weight. The aim of this study was to clarify the influence of the glass substrate thickness during laser scribing with crack propagation caused by laser heating followed by quick quenching. The laser scribe conditions for soda-lime glass substrates with thickness equal to or less than 1.1 mm were obtained in laser irradiation experiments. Two-dimensional thermal elasticity analysis was conducted with a finite element method based on the scribable conditions obtained in the experiment. The laser scribable conditions can then be estimated by the upper limit of the maximum surface temperature, T_max, and the lower limit of the maximum tensile stress, σ_tmax, in the cooling area, regardless of the glass substrate thickness. There is a substrate thickness with which the maximum tensile stress σ_tmax becomes the largest under each scribe condition. The substrate thickness with which σ_tmax becomes the largest is obtained at a faster scribe velocity for thinner glass substrate and at slower scribe velocity for thicker glass substrate. Owing to these relations, the crack depth also has almost the same tendency as σ_tmax.     

Low-Friction MoS x Coatings Resistant to Wear in Ambient Air of Low and High Relative Humidity

The investigation of the tribological performance of MoS₂-based coatings in air of high humidity is critical for the future use of such low-friction and high-wear-resistant coatings in ambient air. Sulfur-deficient MoS _x coatings with a basal plane (x = 1.3) and a random (x = 1.8) crystallographic orientation were produced by planar magnetron sputtering. The coefficient of friction and the wear loss of MoS_x coatings in comparison with TiN and amorphous TiB₂ coatings were investigated in bi-directional sliding fretting tests performed in ambient air of different relative humidity. The wear rate expressed as a volumetric loss per unit of dissipated energy was determined. From these results, the best friction and wear performance was achieved with basal-plane-oriented MoS _x coatings tested at a relative humidity in the range of 10-50%. A coefficent of friction of 0.06-0.08 and a wear rate of 4 × 10³ m³J^-1, at a normal load of 1 N and a fretting frequency of 10 Hz, were recorded for that type of MoS _x coatings.     

Numerical analysis of the flow and heat transfer characteristics for forced convection-radiation in entrance region of an internally finned tubes

The flow and heat transfer characteristics of combined forced convection and radiation in the entrance region of internally finned tubes are investigated numerically in this paper. The uniform flow is considered for an inlet flow condition. A three dimensional parabolic problem is solved by a marching-type procedure envolving a series of two dimensional elliptic problems in the cross-stream plane. The SIMPLER-algorithm and Raithby's pressure-velocity coupling method are employed to analyze the flow and heat transfer characteristics. For the calculation of radiative heat transfer, the P₁-approximation and the weighted sum of gray gases method (WSGGM) are used. The effects of fin height, number of fins, optical thickness, reference temperature, and Planck number on the flow and heat transfer characteristics are examined. It was found that the effect of fin-height on the heat transfer characteristic is more dominant than that of number of fins. The present results show that the optimal non-dimensional fin height and number of fins are 0.4 and 16, respectively.