Shoji Tanno

Performance of numerical spray combustion simulation

Abstract The aim of this study is to examine the performance of numerical spray combustion simulation. A numerical simulation for the prediction of local properties of heavy oil spray flames stabilized by a baffle plate is described. Time-averaged governing conservation equations are solved to estimate the combustion gas flow, gas composition and temperature fields in the experimental combustor. The κ-e turbulence model is used to describe the turbulent flow field. The behavior of fuel droplets in the turbulent combustion gas flow is calculated by the Lagrangian method. The combustion rate of fuel vapor is estimated by the eddy dissipation model. The effects of radiation are accounted for by the six-flux model of radiation. The performance of the simulation is examined by comparison with measured data. In the isothermal (cold) case, the calculated flow pattern is compared with the data measured by LDA, and it is clear that the calculated results show quantitative agreement with the measured data. In the combustion case, however, the simulation cannot predict well the measured profiles of temperature, O 2 and CO 2 concentrations near the baffle plate. It is inferred that this simulation cannot estimate accurately the interaction between the recirculation flow induced by the baffle plate and fuel droplets.

Effect of swirling flow on unburned ratio and nitrogen oxide concentration in a spray combustion system

Abstract The effect of swirling flow on the unburned ratio and NO concentration in exhausted gas was studied for slurry [coal-water mixture (CWM)] spray combustion with variations of swirl numbers. A numerical analysis for CWM combustion was performed for axisymmetric flow in a cylindrical geometry. First, to check the performance of three previous k - ϵ turbulence models modified with swirling flow, velocity components of isothermal swirling jets were measured by laser-Doppler anemometry (LDA) and compared with predicted results. The two modified models gave more reliable results than the conventional one. Next, as the swirl number could not be estimated by the angular momentum derived from the vane angle of the circular swirler, the reduction rate of the tangential momentum flux through the tube of the circular swirler was measured and calculated. Both measured and predicted results showed that when the swirl number S ′ given by the vane angle was 2.0, the effective swirl number S eff decreased by about 60% to S ′. To take the results mentioned above into consideration, effects of swirl number on both the exhausted NO concentration and unburned ratio were investigated. The predicted unburned ratio showed good agreement with the experimental results. Both experimental and calculated results showed that the optimum operating conditions controlling the exhausted NO concentration and unburned ratio in this spray combustion system were obtained when the swirl number S eff was about 0.5.

Analysis of Spray Flow Pattern and Spray Combustion Characteristics.

The analysis of spray flow was very important for the spray combus-tion simulation. In this study, the models for the spray pattern and the motion of spray dro-plets in the gas flow were investigated to improve the simulation accuracy. The spray pat-tern was expressed by the random sampling of spray angles from a normal probability de-nsity distribution. The droplet trajectories were calculated by Lagrangian method consider-ing the effect of the velocity fluctuation of turbulent gas flow.At first, the droplet trajectories for the hollow-cone and full-cone types were simulated successfully in the cold air flow. Next, the effect of the two-staged air ratio on the concen-trations of NO and soot in the exhausted gas of PWM (Pitch-Water mixture) spray combus-tion was investigated. As a result, the optimum combustion condition for the reduction of NO and soot concentrations was found out by the three-dimensional spray combustion simulation to estimate clearly the distribution such as gas velocity vectors, temperature, and gas species concentrations within the furnace.

Numerical analysis of turbulent flow in a duct introducing two-stage combustion air.

二段燃焼用空気の導入による炉内流動の変化を測定するために, 内径0.5m×全長2.0mのアクリル製ダクトを用いて, 二段燃焼用空気 (80Nm3/hr) をx/D=0.8の地点にダクトの全周から接線方向に導入した.このときの軸方向および接線方向平均速度をx/D=0.5, 1.5の二点においてレーザー・ドップラー・流速計により測定した.またk-e二方程式モデルに二段燃焼用空気導入モデルを組み込み円筒軸対称座標系でダクト内流動を数値解析し, 測定結果と比較・検討した.その結果, 解析値はダクト内流動を良好に表現し, 二段燃焼用空気導入モデルは適切であることが示された.さらに, 円筒軸対称座標系での解析の妥当性を検討するため, 上記と同一の導入位置, 空気量で実炉と同様に1本の導入管から二段燃焼用空気を導入し, 測定結果を解析結果と比較した.その結果, 流れは軸対称ではなく, 三次元解析が必要であるとの結論を得た.