Lean blowoff of bluff body stabilized flames: Scaling and dynamics

The availability of new lasers in the nearand mid infrared spectral region has led to the development of sensors for gas measurements that are now applied extensively in the process industries. Based on tunable diode laser absorption spectroscopy (TOLAS) molecules like O2, CH* H2O, CO, CO2, NH3, HC1 and HF can be detected in-situ with high selectivity and sensitivity in continuous, real time operation. Using sensitive detection techniques like wavelength modulation spectroscopy (WMS), often low ppb and ppm concentration measurements with Is integration time are feasible. Detection limits can be improved by using extractive sampling and a long multi-pass cell. TOLAS has become an accepted technique in the process industries for difficult measurement tasks, because it is compatible with high temperatures, pressures, dust levels and corrosive media. Gas concentrations, temperatures, velocities and pressures can be determined. TOLAS is used widely for continuous emission monitoring and process control with over 1,000 field instruments worldwide. In this article, after an introduction to the basics of TOLAS, several interesting applications and installations in various process industrial units with some examples from other industries are reviewed.

Tunable diode laser absorption spectroscopy (tdlas) in the process industries – a review

Blowoff dynamics of bluff body stabilized turbulent premixed flames

: This research was concentrated primarily on developments and applications of the filtered density function (FDF) for subgrid scale (SGS) modeling of turbulent reacting flows. During the past three years, this work addressed: (1) development of the joint velocity-scalar filtered mass density function (VSFMDF), (2) development of the joint frequency-velocity-scalar filtered mass density function (FVS-FMDF), and (3) implementation of the scalar filtered mass density function (SFMDF) and VSFMDF for large eddy simulation of complex turbulent flames. This is a final report of our activities sponsored by AFOSR under Grant FA9550-06-1-0015.

Filtered Density Function for Subgrid Scale Modeling of Turbulent Combustion

The wake characteristics of bluff-body-stabilized flames are a strong function of the density ratio across the flame and the relative offset between the flame and shear layer. This paper describes systematic experimental measurements and stability calculations of the dependence of the flow field characteristics and flame sheet dynamics upon flame density ratio, , over the Reynolds number range of 1000–3300. We show that two fundamentally different flame/flow behaviours are observed at high and low values: a stable, noise-driven fixed point and limit-cycle oscillations, respectively. These results are interpreted as a transition from convective to global instability and are captured well by stability calculations that used the measured velocity and density profiles as inputs. However, in this high-Reynolds-number flow, the measurements show that no abrupt bifurcation in flow/flame behaviour occurs at a given value. Rather, the flow field is highly intermittent in a transitional range, with the relative fraction of the two different flow/flame behaviours monotonically varying with . This intermittent behaviour is a result of parametric excitation of the global mode growth rate in the vicinity of a supercritical Hopf bifurcation. It is shown that this parametric excitation is due to random fluctuations in relative locations of the flame and shear layer.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112012002480

Density ratio effects on reacting bluff-body flow field characteristics

Abstract : Reduced Order Models (ROMs) and Computational Fluid Dynamics (CFD) codes are tools used to predict the extinction of flames behind bluff bodies. Accurate prediction of these models and codes is predicated on their validation with experimental data. This paper describes detailed experiments to obtain validation data for bluff body stabilized flames over a wide range of conditions. Included are non-reacting data from CFD and LDV, lean blowout and high speed images for three different flame holders. In our previous paper (Kiel 2006) it was asserted that the large vortices were a major driver of extinction. Those assertions are further supported here. It is concluded that the vortex dynamics and not geometry is the dominant mechanism for bluff body flame extinction. This conclusion is supported by the lean blowout data, by the high speed images and reference data from NACA.

A Detailed Investigation of Bluff Body Stabilized Flames

: Reduced Order Models (ROMs) and Computational Fluid Dynamics (CFD) codes are tools used to predict the extinction of flames behind bluff bodies Accurate prediction of these models and codes is predicated on their validation with experimental data This paper describes detailed experiments to obtain validation data for bluff body stabilized flames over a wide range of conditions Included are non-reacting data from CFD and LDV, lean blowout and high speed images for three different flame holders In our previous paper (Kiel 2006) it was asserted that the large vortices were a major driver of extinction Those assertions are further supported here It is concluded that the vortex dynamics and not geometry is the dominant mechanism for bluff body flame extinction This conclusion is supported by the lean blowout data, by the high speed images and reference data from NACA

A detailed investigation of bluff body stabilized flames (postprint)

Characterization of flame-shedding behavior behind a bluff-body using proper orthogonal decomposition

Abstract : This paper is the first in a series of papers studying the behavior of bluff body stabilized flames. In this research a combination of Laser Doppler Velocimetry (LDV), and High Speed Imaging are used to investigate these flames. LDV data taken over several non-combusting operating conditions detail the recirculation zone behind the bluff body as well as the effect of inlet conditions on the Karman Street vortex shedding that occurs. High speed images of combustion and equivalence ratio taken at blow out agree with assertions made by Ozawa (1971) and Zukoski (1957) on the transitional nature of the flame from "laminar" to "turbulent" at a Reynolds number of around 15,000. Lean blow out also correlate very well when using the correlation parameter set down by Dezubay (1950). Finally, high speed images also support assertions by Mehta and Soteriou (2003) Erickson et al. (2006) that under certain conditions Karman Street vortex shedding is not suppressed by momentum and baroclinic effects and are present in the flame near lean blowout.

Non-Reacting and Combusting Flow Investigation of Bluff Bodies in Cross Flow

In this study, we have analyzed premixed propane/air reacting in a duct, with the flame being anchored by a V-gutter flameholder. 3D Large Eddy Simulation (LES) analyses were performed for flow conditions of one atmosphere, 700K, and inlet velocity of 52.1 m/s (Reynolds number of 29,000 based on flameholder height). Three types of analyses were performed: 1) nonreacting flow with nonvitiated air, 2) reacting flow with nonvitiated air at various equivalence ratios leading to blowout, and 3) reacting flow with vitiated air at various equivalence ratios leading to blowout. The predictions were qualitatively compared to experimental measurements of time-averaged mean and rms velocities, Strouhal number, flame images, and lean blowout. Unfortunately, quantitative comparisons could not be made because of differences between flow conditions and geometry (open versus closed V-gutter) of the experiments and calculations. The predictions were able to capture the basic flame structure near blowout of a Kelvin Helmholtz shear layer off the lip of the V-gutter, followed by von Karman structures further downstream. Blowout predictions for reacting flow with nonvitiated air showed reasonable agreement with experimental measurements, although improvements are needed. Analysis of reacting flow with vitiated air showed that blowout occurred at a fuelflow that was 24% more than the nonvitiated case, caused by lower reaction rates for the vitiated case. The basic flame structure was similar for the nonvitiated and vitiated blowout scenarios.

Barry Kiel

Papers

A Detailed Investigation of Bluff Body Stabilized Flames

A detailed investigation of bluff body stabilized flames (postprint)

Characterization of flame-shedding behavior behind a bluff-body using proper orthogonal decomposition

Non-Reacting and Combusting Flow Investigation of Bluff Bodies in Cross Flow

LES Blowout Analysis of Premixed Flow Past V-gutter Flameholder