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The effects of flow structure and flame dynamics on combustion instabilities in a lean premixed swirl combustor were numerically investigated using Large eddy simulation (LES) by varying the inlet geometry of combustor. The dynamic k sgs -equation and G-equation flamelet models were respectively employed as the LES subgrid models of turbulence and combustion. The divergent half angle (α) in the combustor inlet was varied systematically from 30° to 90° to quantify the effect of inlet geometry on the combustion instabilities. This variation caused considerable deformation in recirculation zones in terms of their size and location, leading to significant changes in flame dynamics. Analysis of unsteady pressure distributions in the combustor showed that the largest damping caused by combustion instabilities takes place at α = 45°, and the amplitude of acoustic pressure oscillation is largest at α = 30°. Examination of local Rayleigh parameters indicated that controlling flame-vortex interactions by modifying inlet geometry can change the local characteristics of combustion instabilities in terms of their amplification and suppression, and thus serve as a useful approach to reduce the instabilities in a lean premixed swirl combustor. These phenomena were studied in detail through unsteady analysis associated with flow and flame dynamics.

In this study confirmed the reduction effect of pollutant by applying Fi-EGR and FPI-EGR to hybrid combustion system realizing premixed flame and non-premxied flame at once. The results showed that NOx emission index decreased significantly in case of adopting EGR. Additionally, the hybrid combustion system with EGR resulted in a better performance compared to usual non-premixed combustion system such that it can reduce NO x emission at equivalent EGR ratios. Especially, in the case of 25% of FI-EGR ratio at hybrid combustion system that the ratio of non-premixed and premixed is 50 : 50, NOx emission index reduction rate was about 59% compared to NO x emission of non-premixed combustion system without EGR and in the case of 15% of FPI-EGR ratio at hybrid combustion system that the ratio of non-premixed and premixed is 70 : 30, NO x emission index reduction rate was about 48% compared to NO x emission of hybrid combustion system without EGR. Key words : Hybrid combustion, Exhaust gas recirculation (EGR), NO

The effects of the number of heat exchanger modules on thermal characteristics were experimentally studied in a premixed combustion system with a cross-flow staggered-tube heat exchanger. The various heat exchanger modules, from 4 to 8, combined with a premixed burner were tested to investigate the performance of the heat exchanger through the surface area of the heat exchanger at various equivalence ratios. Additionally, the performance of the heat exchanger was analyzed by applying entropy generation theory to the heat exchanger system. As a result, although the heat transfer rate increases with the increase of the equivalence ratio, the NOx and CO concentrations also increase due to the increasing flame temperature. In addition, the entropy generation increases with an increase of the equivalence ratio. Furthermore, the heat transfer rate and the effectiveness are increased with the increase of the number of the heat exchanger modules. Also, the effectiveness is sharply increased when the number of the heat exchanger modules is increased from 4 to 5. Consequently, the optimal operating conditions regarding pollutant emission, effectiveness and entropy generation in this experimental range are 0.85 for the equivalence ratio and 8 for the number of heat exchanger modules.

We examined the characteristics of NO x emission for CH4/air non-premixed flames using the exhaust gas recirculation(EGR) methods, which are the air-induced EGR(AI-EGR) and fuel-induced EGR(FI-EGR) methods. Our experimental results show that the NO x emission index(EI nox ) decreased with increasing EGR ratio. In the range needed to form a stable flame, the reduction rate of EInox for the FI-EGR method was approximately 29% when the EGR ratio was 20%, and the reduction rate for the AI-EGR method was approximately 28% with 25% of the EGR ratio. According to the flame structure based on numerical results, high temperature regions for the FI-EGR method were narrower and lower than those for the AI-EGR method at the same EGR ratio. Furthermore, based on the experimental results for swirl flames, the reduction rate of EI nox for the FI-EGR method was approximately 49% with 15% of the EGR ratio, while the maximum reduction rate for AI-EGR method was approximately 45% with 25% of the EGR ratio. Consequently, we verified that the FI-EGR method was more effective than the AI-EGR method in reducing NO x emission for non-premixed flames with EGR. We expect that the results of this study will provide fundamental information relating to hybrid combustion systems, which can be used in the design of combustion systems in the future.

This study investigated the characteristics of heat transfer and pressure drop for cross-flow heat exchanger of premixed combustion system. The premixed burner was in front of a heat exchanger, and the number of heat exchanger modules was changed to investigate the characteristics of NOx and CO emissions with various equivalence ratios. In addition, the effectiveness, entropy generation and pressure drop were calculated by various number of heat exchanger modules and the performance of heat exchanger was analyzed by the exergy loss.

The purpose of this study is investigated on the combustion and the thermal characteristics of porous media burners which are many using for a condensing boiler recently. In addition, results of this study will be used the fundamental information to decide the burner type which will be applied to the future development of EGR(Exhaust gas recirculation) condensing boiler. Two flat type of burners made of a the metal fiber(MF) and the ceramic(CM) were selected and examined, experimentally. As experimental results, the emitted CO concentration of CM was higher than that of MF. However, the NO concentration of MF was higher than that of CM. The efficiencies of both burners were increased as increasing the burner capacity. While the efficiency of MF was higher than that of CM, regardless of the burner capacity. In the experimental range, MF is appropriated for the burner material and 0.8 of equivalence ratio is an optimal operation condition, regarding of the proportional control, the thermal efficiency and emitted NO and CO concentration based on the regulations of KS B standard and EN 677 standard.

In this study, the heat transfer characteristics and NOx and CO emissions of a heat exchanger were investigated using a premixed burner. The experiments involved changing the distance between the burner and heat exchanger to 30, 40, and 50 mm with various equivalence ratios. The results showed that the NOx and CO emissions increased as the equivalence ratio was increased because the flame temperature increased, regardless of the distance between the burner and heat exchanger. In particular, the CO emission increased significantly as the distance between the burner and heat exchanger was decreased. The optimal equivalence ratio for the A-type heat exchanger (distance between the burner and heat exchanger: 30 cm) was 0.7 in the experimental range. In this case, the CO and NOx emissions were 94.5 ppm and 11.2 ppm, respectively, and the efficiency was 84.1%.

23 vol % 분석되었다. Abstract: To determine the test gases for gas appliances of new gas group, the interrelation between gas interchangeability, including thermal efficiency, of 13A gas appliances and present 13A test gases was experimentally investigated. The test results show that the Wobbe indices in the case of incomplete combustion and the flame lifting limit gas for the new gas group are respectively 2% higher than the upper limit and 1% lower than the lower limit of the Wobbe index range. The most suitable composition of R gas is 96 mol% of methane and 4 mol% of propane; LNG could be also used as R gas. Further, analysis results showed that the hydrogen concentration of flash back limit gas could be lowered from 30 vol% to 23 vol%.

We carried out numerical simulations to understand how CO2 dilution in either fuel or oxidizer stream changes the flame-vortex interactions in terms of hydrodynamic effects in CH4–air nonpremixed jet flames. The simulation used a time-dependent, axisymmetric computational model and a low Mach number approximation. Reaction rates were calculated from a two-step global reaction mechanism that considered six species. Studies were conducted with fixed initial velocities for three different cases of CO2 introduction: (1) without dilution, (2) dilution in a fuel stream, and (3) dilution in an oxidizer stream. A single vortex was generated by an axisymmetric jet driven of cold fuel, after a flame development was reached to quasi steady-state condition. The simulation shows that CO2 dilution in a fuel stream leads to a slightly increased vortex radius and more entrainment of surrounding fluids compared to the other dilution methods tested. Thus, dilution of CO2 in a fuel stream enhances the mixing inside a single vortex and increases the stretching of the flame surface. The vorticity transport equation budgets were examined to reveal the mechanisms of vortex formation in the presence of CO2. In the stage of vortex formation, vortex production due to baroclinic torque and vortex destruction due to volumetric expansion were found to be greater in the case of CO2 dilution in a fuel stream than in the other dilution cases. However, after vortex formation, there terms showed the opposite tendencies.

One-dimensional modeling of CH4/air premixed flame was conducted to validate the heat loss model and investigate NOx formation characteristics in the postflame region. The predicted temperature and NO concentration were compared to experimental data and previous heat loss model results using a constant gradient of temperature (100 K/cm). The following conclusions were drawn. In the heat loss model using steady-state heat transfer equation, the numerical results using the effective heat loss coefficient (heff) of 1.0 W/m 2 K were in very good agreement with the experiments in terms of temperature and NO concentration. On the other hand, the calculated values using the constant gradient of temperature (100 K/cm) were lower than that in the experiments. Although the effects of heat loss suppress NO production near the flame region, a significant difference in NO concentration was not found compared to that under adiabatic conditions. In the postflame region, however, there were considerable differences in NO emission index as well as the contribution of NO formation mechanisms. In particular, in the range of 0.8

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 EINO of DME nonpremixed flame is low as much as 50 % of the C2H6 nonpremixed flame. The cause of EINO 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.

Detailed analysis of NO formation routes and its contributions with strain rate in hydrogen/air flames were numerically investigated. LiG detailed reaction mechanism has been used for calculation, which is compared with experimental data in literature. It shows good agreement with experiment for both temperature and NO mole fraction. Three routes have been found important for NO formation in hydrogen flames. These are the Thermal route, NNH route and N2O route. Strain rate were varied to discuss the EINO reduction trend in hydrogen nonpremixed flames, which are analyzed by each NO formation routes. As a result , as the strain rate increase, EINO decrease sharply until strain rate 100s -1 and decrease slowly until strain rate 310s -1 again, after that EINO keeps nearly constant. It can be identified that EINO trend with the strain rate is well explained by a combination of variation of production rate of above Thermal, NNH and N2O route. Also result of Thermal-Mech. that includes only thermal NO reaction is compared with those of Full-Mech. . As a result, It can be identified that there was difference between the two results of calculation. It is attributed to result that Thermal-mech did not consider contributions of NNH and N2O route. From these result, we can conclude that NOx emission characteristics of hydrogen nonpremixed flames should consider contributions of above three routes simultaneously.

The effects of combustion instability on flow structure and flame dynamics with the inlet configurations in a model gas turbine combustor were investigated using large eddy simulation (LES). A G-equation flamelet model was employed to simulate the unsteady flame behaviors. As a result of mean flow field, the change of divergent half angle(α ) at combustor inlet results in variations in the size and shape of the central toroidal recirculation (CTRZ) as well as the flame length by changing corner recirculation zone (CRZ). The case of α = o 45 show smaller size and upstream location of CTRZ than those of o 90 and o 30 by the development of higher swirl velocity. The flame length in the case of α = o 45 is shorter than other cases, while the case of α = o 30 yields the longest flame length due to the decrease of effective reactive area with the absence of CRZ. Through the analysis of pressure fluctuation, it was identified that the case of α = o 45 shows the largest damping effect of pressure oscillation in all configurations and brings in the noise reduction of 2.97dB, compared to that of α = o 30 having the largest pressure oscillation. These reasons were discussed in detail through the analysis of unsteady phenomena related to recirculation zone and flame surface. Finally the effects of flame-acoustic interaction were evaluated using local Rayleigh parameter.

The NOx emission characteristics of DME in laminar coaxial jet and counterflow nonpremixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NOx emission of DME were compared with those of and . The DME flame was calculated using the Kaiser`s mechanism, while the and flames were calculated using the mechanism. These mechanisms were combined with the modified Miller-Bowman mechanism for the analysis of NOx. Experimental results show in coaxial jet flame that DME flame has the characteristics of partial premixed flame and the flame length decreases up to 1/3 than that of in the same condition of fuel mass flowrate. Then, the NOx emission of DME decreases to 40% approximately, comparing with that of . In the calculated results of counterflow nonpremixed flame, DME flame shows the decreases up to 50% approximately than those of and flames when the equivalent fuels are consumed per unit mass and time. Although the overall NOx reaction path of DME is similar with other hydrocarbon fuels, it can be identified that DME flame has a distinct NO reduction mechanism due to the reburning NO chemistry in fuel rich region. From these results, we can conclude that the different NOx emission characteristics of DME flame with other hydrocarbon fuels are attributed to not the temperature increase and the activation of NO reactions due to O atom in DME fuel but the rapid processes of pyrolysis/oxidation.

An analysis of the effects of on fundamental combustion characteristics was performed in Oxygen enriched condition by comparing the laminar burning velocities, flame structures, fuel oxidation paths. Fictitious was introduced to discriminate the chemical reaction effects of from the thermal effects. PREMIX code was utilized to evaluate the laminar burning velocities. OPPDIF code was utilized to investigate the flame structure and fuel oxidation path variation. The contributions of thermal effects on laminar burning velocities are dominant at lowly oxygen-enriched condition but those of chemical reaction effects become dominant at highly oxygen-enriched condition. Chemical reaction effects caused the additional flame temperature decrease besides thermal effects and oxygen-leakage increase in non-premixed flame. Specific fuel oxidation path and CO production path is enhanced in spite of overall decrement of fuel consumption rate by chemical reaction effects of.

A Promising new approach to achieve low pollutant emissions and improvement of flame stability is tested experimentally using a cyclone jet hybrid combustor employing both premixed and diffusion combustion mode. Three kinds of nozzle are tested for mixing enhancement of fuel and air. The LNG (Liquified Natural Gas) is used as a fuel. The combustor is operated by two methods. One is DC (Diffusion Combustion) mode generated swirl flow by air as general swirl combustor, and the other is HC (Hybrid Combustion) mode. The HC mode consists of diffusion jet flame of axial direction and premixed cyclone flame of tangential direction in order to stabilized the diffusion jet flame. The results showed that the flame stability of HC mode is significantly enhanced than that of DC mode through the change of mixing characteristics by modifications of fuel nozzle. In addition, the reductions of CO and NOx emission in HC mode, as compared with that for the DC mode, is large than about 50% in stable region. Also, even using the low calorific fuel as -blended gas, it is identified that the cyclone jet hybrid combustor has the high performance of flame stability.

The extinction behavior and the unsteady response of augmented reduced mechanism(ARM) have been investigated by adopting an OPPDIF code and a numerical solver for the flamelet equations. By comparing the performance of the ARM based on Miller and Bowman`s mechanism(MB-ARM) with that of the ARM based on GRI-Mech 3.0(GRI-3.0-ARM), it is identified that the MB-ARM is more suitable for the unsteady calculation because it is relatively less stiff than GRI-3.0-ARM during an ignition process. The steady results using the MB-ARM, which is modified to predict reasonably the extinction point of experiment, are in excellent agreement with those from full mechanism. Under the sinusoidal transient disturbances of scalar dissipation rate, the unsteady responses of the flame temperature and species concentrations using a modified MB-ARM show in very close agreement with those from full mechanism. It is presumed that above modified MB-ARM is very suitable for the unsteady simulation of turbulent flames because it gives not only a low computational cost but also a good prediction performance for flame structure, extinction point and unsteady response.

An experimental investigation was conducted on the structure of turbulent diffusion flames of hydrogen stabilized on a bluff body. A coaxial jet diffusion flame was formed using a cylindrical nozzle with an extremely thick rim, which works as a bluff body. In the present study, special attention was paid to the effect of combustion on the aerodynamic processes. The following results were obtained. (1) The combustion markedly enhances the penetration of the central fuel jet. (2) The laminarization phenomenon, reported in simple jet diffusion flames, was observed more conspicuously in this combustion field as well, and it was assumed to exert an important influence on the flame structure. (3) An accelerating flow field often exists in complicated flow fields with a reverse flow, where turbulence generation terms containing Reynolds normal stress become negative. This phenomenon sometimes exerts considerable effects on the turbulence behavior., ・rights：日本機械学会・rights：本文データは学協会の許諾に基づきCiNiiから複製したものである・relation：isVersionOf：http://ci.nii.ac.jp/naid/110002976848/

The present authors previously proposed a modification of the k-e turbulence model in the modelling of jet diffusion flames to represent the so-called laminarization phenomenon, caused by combustion in the low-turbulence region, and recently found, in experiments on diffusion flames stabilized on a bluff body, that this phenomenon also exerts an important influence on the structure of these flames with recirculation zone. In the present study, the above modified k-e model was applied to the modelling of this bluff-body diffusion flame in order to examine applicability of the modification and the ability of the k-e model to represent such a complex flow field. As a result of comparison of the calculated results to experimental ones, it was found that the consideration of the laminarization phenomenon enables the k-e model to provide preditions of encouraging quality, though their quantitative discrepancies are still discernible.