Effect of Fuel Stage Proportion on Flame Position in an Internally-Staged Combustor

To study the effect of fuel stage proportion on flame position and combustion characteristics of the internally-staged combustor, a detailed numerical investigation is performed in the present paper. The prediction method of flame position is established by analyzing the variations of the distribution of intermediate components and the turbulent flame speed. Meanwhile, the flame position is simulated to verify the accuracy of the prediction method. It is demonstrated that the flame position prediction model established in this paper can accurately predict the flame position under different fuel stage proportions. On this basis, special attention is paid to analyze the variation of velocity field, temperature field, distribution of intermediate components and emissions under different fuel stage proportions. As the proportion of pilot fuel stage increases slightly, the mass fraction of fuel at the combustor dome increases. In addition, the combustion characteristics change significantly with the increase in the proportion of pilot stage fuels. The flame moves downstream and the high temperature area increases as the proportion of pilot fuel increases. In particular, when the proportion of pilot stage reaches 3%, the highest flame temperature is generated due to the most concentrated reaction area, resulting in the largest emission of NOx. At the same time, due to the most complete reaction, the minimum CO emission is produced. When the proportion of pilot fuel stage reaches 1%, the NOx emission is the lowest, and the highest CO emission is generated due to the incomplete reaction.

Numerical Study on the Influence of Blockage Ratio on Hydrogen Turbulent Premixed Flames in a Small Scale Obstructed Chamber

Although hydrogen is a clean and renewable fuel, there is still a need to understand and evaluate the potential risks posed in the event of an accidental explosion. This paper presents large eddy simulation (LES) numerical analysis for lean hydrogen premixed flames propagating inside a small laboratory combustion chamber with built in solid obstructions. The small-scale chamber is 0.625 litres in volume with three removable turbulence generating baffles and a square solid obstacle. A lean equivalence ratio of 0.7 is selected in this study. The LES model is utilised to investigate the influence of obstruction configuration and area blockage ratio on the flame characteristics and the generated combustion over-pressure. The LES turbulence technique is used with an in-house computational fluid dynamics (CFD) model for compressible flows. The numerical simulations are carried out with various arrangements of the baffle plates and a solid obstacle to examine the effects of the area blockage ratio and generated turbulence on the flame structure and generated over-pressure. Two different area blockage ratios of 0.24 and 0.5 are studied. Four configurations with different baffle arrangements are studied to examine the resulting turbulence effects on the generated over-pressure, flame position-time traces and flame transient speed following ignition. Direct comparisons are made between the different baffle/flow configurations to identify the various effects of an increased area blockage ratio. Numerical results showing the flame structure at various time windows after ignition are presented and compared with published experimental images. High speed laser induced fluorescence (LIF-OH) images of the reaction zones obtained from the experiments at a rate of 5 kHz provide the flame position data and convey the impact of the turbulence generated by the baffles and solid obstacle on the propagating flame structure [1]. The pressure is recorded at a rate of 25 kHz using a piezo-electric pressure transducer in the base plate of the chamber [2]. The rise in over-pressure as a result of increased turbulence due to additional baffles and an increased area blockage ratio is found to be consistent with experimental data. This is also found to be consistent for the flame position-time and speed-time traces across all configurations. Main points of interest such as the peak over-pressure, maximum rate of pressure rise and the flame propagation trends are also observed along with variations in flame speed as the flame interacts with the baffles and obstacles. Validation of the numerical results against available published experimental data conveys good agreement confirming the ability of the numerical model to predict numerical results for an increased area blockage ratio. Further numerical simulations are also carried out for flame/flow parameters where experimental data is unavailable due to physical limitations. Satisfactory agreement between numerical results and experimental data endorses further predictions for computational models in studying vented hydrogen explosions where there is an increased risk or limited experimental data.

Combustion Instability Characteristics Under Various Fuel and Air Flow Rates in a Partially Premixed Model Gas Turbine Combustor

In this study, the combustion instability characteristics are experimentally investigated in a partially premixed gas turbine model combustor. The combustor is operated with methane and preheated air as the fuel and oxidizer, respectively, at atmospheric pressure. The experiment is carried out at various equivalence ratios and flow rates of fuel and air to investigate the effect on the combustion instability frequency transition. According to the experimental results, the transition of the combustion instability frequency to higher longitudinal mode occurs because of the flow rate variation. To explain the frequency shift phenomenon, the concept of convection time is introduced, which is mostly affected by the flame position and exit velocity of the fuel-air mixture. The flame positions are measured using OH planar laser-induced fluorescence (OH-PLIF), and the flow field information is obtained using particle image velocimetry to calculate the convection time. The measurement results show that the injection velocities of fuel and air are also important factors in determining the combustion instability frequency as well as the equivalence ratio, which is a crucial parameter of the flame position. As a result, it is found that the decrease in convection time owing to a closer distance from the dump plane to the flame and a faster exit velocity of the fuel-air mixture causes the combustion instability frequency mode shift. Additionally, the structural characteristics of the flame are analyzed using high-speed OH-PLIF measurement. The differences in the flame structure between the stable and unstable flames in the 2nd and 3rd longitudinal modes are analyzed. The change in the unburned mixture is mainly observed and the relationship between the dynamic pressure, heat release rate, and length of the unburned region is also analyzed.

Lean-Premixed, Swirl-Stabilized Flame Response: Flame Structure and Response as a Function of Confinement

The effect of confinement (flame–wall interactions) on the response of a turbulent, swirl-stabilized flame is experimentally examined, with a focus on the shape and structure of these flames. A series of three cylindrical combustors of 0.11, 0.15, and 0.19 m diameter are used to vary the degree of confinement experienced by the flame. Using CH* chemiluminescence images, the shape of the flame in each combustor is described. These images are then further analyzed and reveal marked similarities in the geometry and location of these flames in a defined “flame base” region near the combustor inlet. This similarity in location of the flame base leads to a similarity in the response of this portion of the flame to imposed oscillations. In particular, the phase of the fluctuations in this region is shown to be the same in each confinement. The nature of the fluctuations in the mean flame position is also shown to be similar in each confinement. These results indicate that the geometry of the flame in the base region is not a function of confinement and that the flames are responding to the same convective mechanisms, and in the same manner, in this region of the flame.

Lean-Premixed, Swirl-Stabilized Flame Response: Flame Structure and Response As a Function of Confinement

The effect of confinement (flame-wall interactions) on the response of a turbulent, swirl-stabilized flame is experimentally examined, with a focus on the shape and structure of these flames. A series of three cylindrical combustors of 0.11, 0.15 and 0.19 m diameter are used to vary the degree of confinement experienced by the flame. Using CH* chemiluminescence images, the shape of the flame in each combustor is described. These images are then further analyzed and reveal marked similarities in the geometry and location of these flames in a defined ‘flame base’ region near the combustor inlet. This similarity in location of the flame base leads to a similarity in the response of this portion of the flame to imposed oscillations. In particular, the phase of the fluctuations in this region are shown to be the same in each confinement. The nature of the fluctuations in the mean flame position are also shown to be similar in each confinement. These results indicate that the geometry of the flame in the base region is not a function of confinement and that the flames are responding to the same convective mechanisms, and in the same manner, in this region of the flame.

Intelligent Multi-Soft Sensing for Flame Position of Steam Boilers

A new inference control system for the flame position in the combustion chamber of a power plant system boiler is presented. The system is based on enhanced multi-softsensing at three operational levels – basic level, providing estimates of all necessary technological variables; a separate Mill Fan (MF) level, and a total Dust Preparation System (DPS) level. The control system involves a subsystem for the stabilization of the position of the common MF ventilation rate momentum in a given threshold area in a burner horizon, which is supervised by an inference correction based on softsensed 2D flame position in the output section of the combustion chamber. A hybrid approach is accepted in softsensing, using fusion of the first principle models, statistical models, neural networks and fuzzy logic based models. Real experimental results are presented from TPP.

STAGNATION LAMINAR PREMIXED CH4/AIR FLAME SUBJECTED TO THE EQUIVALENCE RATIO OSCILLATIONS

The effect of the equivalence ratio oscillation on a premixed laminar CH4/air flame motion was studied experimentally with equivalence ratio oscillation frequencies of 2 to 15 Hz at lean equivalence ratio using stagnation flow field burner. Novel oscillator does the oscillation conditions and turbulence reduction method is used to suppress the velocity perturbation. The flame position variations at 2, 5, 10 and 15 Hz oscillation frequencies were significantly small when the amplitude of the equivalence ratio oscillation was zero. On the other hand, increase in amplitudes of the equivalence ratio oscillation increased the flame position variation significantly. The flame moved in sinusoidal shape and it can be clearly seen that the flame movement’s amplitude was proportional to the amplitudes of the equivalence ratio variations. This result showed that the velocity perturbation is significantly suppressed by turbulence reduction method in the examination range.