Yoshiaki Onuma

The Influence of the Preheated Air on Low NOx Combustion Using the Cyclone-Jet Combustor

As a means for a stable premixed combustion, there is a so-called cyclone combustor, which consists of a cylindrical chamber and fuel nozzles installed tangentially on the side wall. In this combustor an extremely stable flame can be obtained in the swirl flow, formed along the inner wall of the combustor. The authors utilized this combustor as a flame holder, to burn a high velocity jet flowing axially in the central part, and named this combustor a cyclone-jet combustor. In the present study, experiments on the air preheated by the cyclone-jet combustor were carried out. The maximum air temperature is 700 K, and the propane was used for fuel. With the air flow rate changed, the effects of the turbulence on the NOx formation were examined in non-premixed and premixed flames. It was shown that the increase of the air temperature increases the NOx emission, while the increase of the turbulence reduces the NOx, emission. The results suggest that the air preheated combustion using this combustor becomes very effective for the NOx reduction and the low fuel consumption, particularly under the strong turbulence.

An Experimental Study of Flame Characteristics of Jet Diffusion Flames in Cylindrical Furnaces (1st Report, The Effect of Inner Diameter of Furnace on NOx Emission Property)

The combination of burner and combustion chamber is one of important factors controlling flame characteristics, which has never been to our knowledge investigated systematically. In the present study, coaxial jet diffusion flames in cylindrical combustion chambers have been studied in terms of inner diameter of the combustion chamber, global equivalence ratio and turbulence in air flow. A fuel nozzle is composed of a stainless steel tube of an inner diameter of 2mm with a coaxial pilot burner of 3.19mm i.d., surrounded by two air coaxial tubes of 12mm i.d. and 30 mm i.d. The inner and outer air tubes are for higher and lower air flows, respectively, and the turbulence in air flow is changed by the velocity difference. The main fuel is propane. Hydrogen is used for the pilot flame, with a volumetric fuel ratio of 0.3. The wall of combustion chamber is made of heat-resistant glass to visualize the flames and the inner diameter is changed in a range between 95 mm and 182 mm to investigate the effect of size of furnace on flame characteristics. It has been found that the increase in the diameter of combustion chamber enhances the exhaust gas self-recirculation, as the result decreases the NOx emission. The increase in turbulence in air flow strengthens the entrainment of the exhaust gas transported by the recirculation vortex to the flame. The increase in the global equivalence ratio from 0.2 to 0.8 in the present study decreases the oxygen concentration of the exhaust gas and leads to the diluted combustion through the exhaust gas self-recirculation. It has been found that a proper combination of these factors can yield the low NOx combustion.

Lifting Phenomena due to Vortex-Flame Interactions of Jet Diffusion Flames.

Lifting phenomena caused by vortex-flame interactions have been investigated about diluted-propane jet diffusion flames forced by a speaker installed at the bottom of a fuel tube. Open jet diffusion flames were formed on a co-flow burner composed of the inner fuel tube and an outer air tube. Two type nozzles of rim thickness of 1 mm and 0.2 mm were set at the end of the fuel tube for each experiment and the effects of the rim thickness on the flame stability were investigated. Artificial vortices were imposed in the fuel flow through the speaker excitation to make clear vortex-flame interactions. The schlieren and OH radical imaging techniques were employed to unveil the interactions. Experiments in terms of the difference of rim thickness revealed that the ratio of the rim thickness to the quenching distance is important for lifting criteria as proposed by Takahashi and Schmoll, because the ratio of unity determines the limitation of whether the end of the flame base being in or out the recirculation zone behind the rim. Moreover, it revealed that the vortex-flame interactions are able to become the trigger of lifting, though the effect to lifting was questionable so far.

回流式燃焼装置による燃料ガス噴流の着火特性に関する研究(〔熱工学, 内燃機関, 動力など〕)

A circulating flow combustion device was developed for a spontaneous ignition experiment on fuel gas jets under high pressure. The combustion device is composed of a circular duct, a fan and electric heaters. Ethylene was injected perpendicular to a hot air stream of high pressure circulating through the duct. The ignition delay was measured and the effects of pressure, air temperature and fuel jet velocity were examined. The results show that the ignition delay time τ is expressed in terms of pressure Pa and air temperature Ta by the equation τ=APna exp (E/RTa), and the apparent activation energy E is constant, unrelated to pressure. In this study, E was 243 kJ/mol and the pressure exponent n was -1.3.

The local reaction rate in round-jet diffusion flames

A combustion model for turbulent diffusion flames is estimated frequently through the comparison of the simulated result with the experimental one. Usually, profiles of time-averaged concentration and temperature are used in that comparison, because the local reaction rate cannot be directly measured. However, since their profiles are also influenced largely by transport phenomena, it is difficult to estimate the combustion model properly with this method. Therefore, it is desirable to calculate the local reaction rate from experimental results and compare it with the simulated one. In the present study, from this point of view, the local reaction rate was tried to obtain by numerical calculation using measured values for a hydrogen jet diffusion flame. Then, it was suggested through the comparison of the obtained result with the simulated one that the method proposed here can provide reliable values for the local reaction rate.

The Local Reaction Rate in Round-Jet Diffusion Flames.

A combustion model for turbulent diffusion flames is estimated frequently through the comparison of the simulated result with the experimental one. Usually, profiles of time-averaged concentration and temperature are used in that comparison, because the local reaction rate cannot be directly measured. However, since their profiles are also influenced largely by transport phenomena, it is difficult to estimate the combustion model properly with this method. Therefore, it is desirable to calculate the local reaction rate from experimental results and compare it with the simulated one. In the present study, from this point of view, an attempt was made to obtain the lacal reaction rate by numerical calculation using measured values for a hydrogen jet diffusion flame. Then, it was suggested through the comparison of the obtained result with the simulated one that the method proposed here can provide reliable values for the local reaction rate.

Experimental and Numerical Studies of Triple Jet Diffusion Flames.

A coaxial jet diffusion flame is often used to experimentally analyze the phenomena in turbulent diffusion combustion and to examine the modeling technique for them. In this flame, however, a low turbulence region is inevitably formed in the periphery, which becomes a disadvantage in its use as a research object. In the present study, another coaxial air jet of high velocity was added to usual coaxial jet diffusion flames between the central fuel jet and the surrounding air flow. In this flame, called a triple jet diffusion flame, large turbulence is generated in the peripheral region of the flame by controlling the high-velocity air. In this report, the characteristics and usefulness of the flame are clarified by experiments and numerical simulations.

Modelling of Turbulent Diffusion Flames Stabilized on a Bluff Body.

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.

Ion current measurements in turbulent diffusion flames.

乱流拡散火災の研究において火災内局所の反応密度を知ることは重要であるが、現在これに対する簡単な良い方法はない。本研究はこの手段として陽イオン電流の仕様を試みた。まず陽イオン電流波形に現われるベース電流が反応とは無関係なことを確認した後、このベース電流の影響を除外した時間平均量 Ii を定義した。ついで Ii と火災内の温度およびCO, CO2 濃度分布とを比較し、 Ii が反応密度を表す特性値として有効なことを示した。

Experimental studies on the turbulent diffusion flame of ethylene/nitrogen mixture fuel.

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