Swetaprovo Chaudhuri

Blowoff Characteristics of Bluff-Body Stabilized Conical Premixed Flames in a Duct with Upstream Spatial Mixture Gradients and Velocity Oscillations

An experimental study of flame blowoff phenomenon in a bluff body stabilized flame confined in a cylindrical duct is presented. Blowoff equivalence ratios were determined for two approach velocities and three different upstream equivalence ratio profiles (uniform, inner, and outer fuel enrichment) in the absence and presence of imposed upstream velocity oscillations. The results were compared with those for the unconfined flame configuration as well. It is found that the blowoff equivalence ratios exhibit somewhat different trends for different approach velocities. For the uniform mixture profile, blowoff equivalence ratio first increases with increasing excitation frequency and then decreases at higher frequencies for the approach velocity of 5 m/s. For the 11 m/s approach velocity, the trend is different, and the blowoff equivalence ratio continuously increases with increasing excitation frequency. The blowoff equivalence ratios are higher for the confined flame configuration. An empirical correlation of the blowoff data for the uniform mixture profile is presented in terms of Damköhler, Reynolds, and Strouhal numbers. Finally, analysis of the CH* chemiluminescence signal as blowoff condition is approached shows emergence of low-frequency oscillations that are connected with intermittency of the localized flame weakening and/or extinction near the flame stabilization region.

Blowoff dynamics of V-shaped bluff body stabilized, turbulent premixed flames in a practical scale rig

Near blowoff dynamics and blowoff characteristics of premixed flames stabilized by a triangular flame holder in the midspan of a rectangular duct were studied using high speed imaging at 500 fps and simultaneous PIV and OH PLIF. Near blowoff dynamics manifested by the onset of asymmetric vortex shedding and local extinction in the form of flame holes were observed. Observations are presented describing the final blowoff event and its precursor: asymmetric sinuous modes of flame motion. It has been hypothesized that partial or total extinction of flame in the shear layers is the major factor that determines the evolution of the asymmetric mode and the final blowoff event. This phenomenon is further evidenced by an observation of the presence of flame kernels within the recirculation zone which under stable conditions contain only combution products. Whether a flame can survive an almost total extinction is governed by the ability of a reacting wake during these times to reignite the extinguished shear layers. I. Introduction Ground-based and aero gas turbine engine applications routinely incorporate bluff body-stabilized flame holders for primary or secondary combustion in high speed flows. The bluff body is placed in the mid flow of a high-speed duct and produces a recirculating wake structure that allows combustion to stabilize and then propagate into the free stream. In stable conditions, the recirculating wake structure steadily entrains hot combustion products from the adjacent shear layers and carries them upstream to ignite cool reactants as they are mixed in the wake shear layers [1]. The difficulty in design and implementation of bluff-body combustors is a result of the small range of fuel/oxidizer mixing conditions that yield stable combustion for given airflow. Temporal or spatial changes in airflow or fuel flow frequently produce changes in the flame structure that can cause extinction or combustion instabilities. Therefore, for a particular design, the stability of these flames needs to be carefully characterized over the intended operating envelope to optimize the coordination of the subsystems that control air and fuel flow with the purposes of maximizing combustion efficiency, minimizing the need for relights during times of critical operation, and minimizing thermo-acoustic instabilities. In order to predict blow off early in the design stage of any combustor, the fundamental phenomena of blow off needs to be captured conceptually and analytically and used to optimize the hardware design. Investigations have been conducted for close to sixty years with the objectives of understanding the underlying

Transitional Blowoff Behavior of Wake-Stabilized Flames in Vitiated Flow

The flame holding and blowoff characteristics of bluff body stabilized premixed flames were studied in a rectangular duct with a triangular flame holder in the midspan of the duct cross section. Lean blowoff was first investigated at unvitiated and uniformly mixed flow conditions in order to characterize the baseline flame behavior and compare it with data from the literature. The lean blowoff margin was then characterized with upstream air vitiated at equivalence ratios of 0.15 and 0.33, without and with cooling, respectively. Further studies were performed with non-uniform fuel profiles (rich or lean in the center or asymmetric), thus imposing single and double fuel gradients near the flame holder. Fuel profiles were characterized using laser induced acetone fluorescence. Transitional blowoff behavior was documented with measurements of CH chemiluminescence emissions from the wake as well as high-speed imaging of flame dynamics. Simultaneous PIV and OH PLIF measurements were taken at transitional fuel-air ratios to capture flame edge and aerodynamic behavior as blowoff was approached to determine the driving mechanisms of final blow off. Post processing of the flame images and flow field revealed the interaction between the velocity field and flame sheet.

Modeling of Ceramic Particle Heating and Melting in a Microwave Plasma

A comprehensive model based on finite volume method was developed to analyze the heat-up and the melting of ceramic particles injected into a microwave excited laminar air plasma flow field. Plasma flow field was simulated as a hot gas flow generated by volumetric heat addition in the microwave coupling region, resulting in a temperature of 6000 K. Alumina and zirconia particles of different diameters were injected into the axisymmetric laminar plasma flow at different injection velocities and locations. Additionally, noncontinuum effects, variation of transport properties of plasma surrounding the spherical particles and absorption of microwave radiation in the ceramic particles were considered in the model. Model predictions suggest that zirconia and alumina particles with diameters less than 50 μm can be effectively melted in a microwave plasma and can produce more uniform melt states. Microwave plasma environment with the ability to inject particles into the plasma core provide the opportunity to create more uniform melt states as compared with dc arc plasmas that are influenced by characteristic arc root fluctuations and turbulent dispersions.

Blowoff Dynamics of Asymmetrically-Fueled, Bluffbody Flames

φξ = 0.5, 0.75, and 1.0. Fuel profiles were characterized using laser induced acetone fluorescence for unvitiated conditions. Transitional blowoff behavior was documented with PMT measurements of CH* chemiluminescence from the wake and high-speed imaging of broadband flame chemiluminescence, sampled from 5000 to 6600 frames per second. The high speed imaging revealed in greater detail the progression of lean blow out for uniform fueling, particularly the upstream propagation of the Benard-von Karman instability, local extinction, and finally the extinction of the flame sheet in the shear layer between the bulk flow and the recirculation zone. For asymmetrically fueled combustion, high-speed imaging revealed much greater vortex shedding coherence of the lean side flame sheet. At certain velocities and fuel gradients, the vortex shedding of the lean side of the flame holder coupled with the axial acoustic modes of the exhaust system, which resulted in thermoacoustic instability and early lean blowout.

Control of combustion in a thermally stabilized burner

Abstract A model has been developed with the assumption of a plug flow reactor that simulates heat transfer originating from the combustion of premixed reactants within a refractory tube. The work depicted in this article successfully implemented a transient one-dimensional coupled model of a plug flow reactor, which finds practical applications in a batch-type heat treatment furnace. All modes of heat transfer, namely conduction, convection, and radiation, were included in the model with single-step global chemistry for combustion of both methane and propane. Effects of variations of parameters such as inlet gas temperature, mass flux, and inlet fuel mass fraction on the gas and tube wall temperature profiles were studied to understand the shift in flame position along the tube length. It is established that the inlet gas temperature or the preheating level is one of the major governing parameters that determines the position of the flame in the tube. Similarly, reduction in mass flowrate shifted the flame location significantly upstream of the tube. It was also observed that the flame temperature exhibits a monotonic decay with depletion in mass flowrate. Results also show that for fixed inlet gas temperature and mass flowrate, the flame location shifts upstream for an increase in inlet fuel mass fraction. Thermally stabilized sustained combustion mode with optimum inlet gas preheating requirement was analysed as a function of mixture stoichiometry and mass flux. Open-loop and closed-loop control schemes were effectively implemented to establish the thermally stabilized scheme of heating and desired flame position within the tube. For open-loop control, with reduction in the mass flowrate (= 0.01 kg/s), an inlet gas temperature as low as 650 K can be maintained for sustaining combustion inside the tube. The joint variation of these inlet parameters has been well predicted by a correlation developed for a particular aspect ratio for high Reynolds number flows. The problem of absence of combustion and its solution towards stable flame position for tubes having smaller ratio of cross-sectional area to surface area have been addressed, which leads to the study for low Reynolds number flows. In the small Reynolds number flow cases, a thermally stabilized scheme has been successfully implemented that eliminates the necessity of preheating the gas prior to its combustion inside the tube.