Investigation Of Flame Structure And Stability Of A Bluff-Body Stabilised Humid Air Non-Premixed Combustion

One of the characteristics in the humid air turbine (HAT) cycle, the steam injected gas turbine (STIG) cycle and integrated gasification combined cycle (IGCC) is water or steam injected into combustors. The flame stability is often adversely affected by the high efficiency and reduced NOx strategy.The flow characteristics and the flame structure with water or steam injected will be affected drastically. Therefore, it is necessary to study the mechanism of the characteristic flow field and flame structure in humid air combustion. The present paper reports on an experimental and computational investigation to determine the effect of humidity on the flow field and the flame stability limit in turbulent non-premixed flame, and examines the dynamical behavior of the unsteady aerodynamic flow structures observed on a bluff-body burner at both humid and non-humid air combustion states. Particle Image Velocimetry (PIV) is used to capture the instantaneous appearance of vortex structures and obtain the quantitative mean and turbulent velocity field. The commercial software-fluent is used to predict the bluff body diffision flame. In contrast, the correction of turbulent model is confirmed, and velocity field, temperature field and concentration fields for different humidity levels are further discussed.In order to get insight into the effect on flame structure of humidity, the characteristic velocity fields in non-humid bluff-body diffusion flame are primarily investigated. The results are obtained that the flame appearances and the structures are basically classified into three stable types (recirculation zone flame, transitional flame and central jet dominated flame) and one unstable type (partially quenching flame) before blowing-out. The air-to-fuel velocity ratio is the main controlling parameter. Two stagnation points, which are respective forward stagnation point and aft stagnation point, exist on the centerline. The disappearance of the stagnation point and the consistent positive axial velocities indicate that the central jet has penetrated the recirculation bubble. The statistic analysis of simultaneous velocity field indicates that the minimal velocities and their locations, the forward stagnation points have a basic Gaussian profile. With increased fuel velocity, the minimal velocity points and forward stagnation points move downstream and the magnitude of minimal velocity decreases. The comparison of characteristic flow field of disc bluff and tulip bluff show that the width of the recirculation zone for tulip case reduce, which results in the central fuel flow to penetrate the recirculation zone more easily in the tulip case.The flow characteristics and flame stability domain of humid air combustion are obtained. The results show that the humid air flame structures are similar to that observed in the non-humid air combustion. However, the addition of steam into airflow brings about a reduction in the flame stability. The specific velocity and location such as axial minimal velocity and stagnation point are chosen for comparison to elucidate the further flow characteristics. The comparison indicates that the recirculation zone shortens in the humid air case as a result of the weak flow recirculation. In addition, both central fuel penetration limit and partially quenching limit in the humid air case reduce. The increased viscosity may explain the fact that the annular humid air induced lower velocity and velocity gradient, due to which the recirculation strength will be reduced. The decrease of reverse mass flow will allow the fuel flow at fixed velocity to penetrate the recirculation zone more easily. Therefore, the penetration occurs at a lower fuel-to-air velocity ratio in the humid case. The decrease in the critical penetration limit is primarily attributed to a reduction in momentum of the humid air. The reduced partially quenching limits in the humid air case can be explained by both the triple flame theory and flamelet concept. The quantitative experimental data in humid air combustion by visualization PIV technique is ideal for validation of numeral prediction of this combustion.The numerical simulation based on Fluent is used to predict the turbulent bluff-body diffusion flame. The comparisons show an effective modification to improve representation of the recirculation zone is the use of the constant Cε2 =1.83 instead of standard value 1.92, which solves the discrepancy of standard k ?εin predicting the correct spreading rate, decay rate and length of the recirculation zone. Flamelet model is proved to have the most realistic advantage. Therefore, the k ?εmodel with value of Cε2 =1.83 in conjunction of the flamelet model is selected to predict the bluff body diffusion flame at different moisture contents (0 percent, 10 percent and 20 percent). The results indicate that the steam addition into the combustion field brings about the decrease in flame temperature, and major species CH4, O2, H2O, CO2 and CO are affected drastically. The minor species, O and OH decreases with increased moisture level.The humid air flame follows the general rules from the combustion flame state and controlling parameters. However, the flame instability is obvious due to the fact that the complex changes about flow field, chemical reaction, heat and mass transfer have taken place. So in the design, we can not rely on ordinary burner design standards. The combustion characteristics of humid air needed for a more pertinent and detailed study to satisfiy the demand of practice.