Mohy S. Mansour

The detailed flame structure of highly stretched turbulent premixed methane-air flames

Abstract The premixed stoichiometric turbulent methane flames are investigated on a piloted Bunsen burner with a nozzle diameter of 12 mm and mean nozzle exit velocities of 65, 50, and 30 m/s. Advanced laser diagnostics of the flow field using two-component and two-point laser Doppler anenometer (LDA), as well as of the scalar fields with 2-D Rayleigh thermometry and line Raman/Rayleigh laser-induced predissociation fluorescence (LIPF)-OH techniques, are applied to obtain both the instantaneous and mean flame structure in terms of velocity, temperature, and major species concentrations, as well as turbulent kinetic energy and length scales. In terms of their location on the combustion diagram, the three flames cover the entire range of the distributed-reaction-zones regime from the borderline to the well-stirred reactor regime to the flamelet regime. Measurements were from X/D = 2.5 above the nozzle exit plane to X/D = 12.5 downstream. Thus, a complete database is established for comparison with the numerical predictions. Within the mixing layer between the unburnt gas and the pilot flame, the instantaneous temperatures are much lower than the adiabatic flame temperature due to the short residence time and heat loss to the burner. With increasing residence time the mean flame temperature increases in the axial direction. The radial mixing of the turbulence generated with the shear layers between the nozzle jet stream and surrounding pilot stream is surpressed, such that the turbulence kinetic energy remains nearly constant on the centerline. From the two-dimensional (2D) temperature fields instantaneous iso-temperature contours are plotted showing broad regions where burnt and unburnt gas are partially mixed. These regions are interpreted in terms of the quench scale lq = (eτc3)1/2. The measured values of the flame brush thickness are proportional to the quench scale for the two high-velocity flames, whereas the low-velocity flame exhibits essential flamelet behavior.

An experimental and numerical study of a laminar triple flame

Abstract A lifted laminar axisymmetric diffusion flame is stabilized in the downstream region of a diluted methane jet that is surrounded by a lean methane-air co-flow and an outer co-flow of air. The flame shows a distinct triple flame structure in the stabilization region. It is investigated experimentally by PIV for the velocity field, OH-LIPF imaging, C2Hx-LIF imaging, and a 1D-Raman technique for major species concentrations, combined with a Rayleigh technique for temperature. This is complemented by numerical simulations solving the two-dimensional axisymmetric Navier-Stokes equations in the zero Mach number limit on an adapted mesh, coupled with balance equations for temperature and species. A simplified model for molecular transport properties was used with constant, but non-unity, Lewis numbers for all species. Chemistry is represented by a ten-step reduced mechanism for methane oxidation, which was derived starting from a 61-step elementary mechanism that includes the C1 and C2 chains. The agreement between the measured and the predicted flow field is very satisfactory. Owing to gas expansion, the velocity decreases immediately ahead of the flame and increases strongly at the flame front. Further downstream acceleration due to buoyancy is dominant and is predicted accurately. There is a good agreement between measurements and computations for flame shape and flame length. The measured OH-LIPF image and the computed OH concentrations indicate that OH is concentrated in the vicinity of stoichiometric mixture. The results from a newly developed C2Hx-LIF method are also supported by calculations. While these measurements were only qualitative, the temperature and mole fractions of the major species could be measured quantitatively with the combined Raman-/Rayleigh technique along a line and were found to agree well with the numerical predictions. It is found that the structure is a triple flame and is influenced essentially by two external parameters: heat exchange between the branches and heat loss at the curved flame front near the triple point.

Measurements of the turbulent burning velocity and the structure of premixed flames on a low-swirl burner

A method has been developed to accurately determine the turbulent burning velocity in planar turbulent premixed flames stabilized by a low swirl. Six lean methane/air flames have been investigated covering the flamelet as well as the thin reaction zones regime. The probability of finding the instantaneous flame front is measured together with the velocity field by combining simultaneously OH–laser-induced predissociative fluorescence with either Rayleigh thermometry or particle image velocimetry (PIV). It is demonstrated that the turbulent flame brush thickness is independent of v� /sL and that the turbulent burning velocity agrees with predictions from a model equation for the flame surface area ratio using the level set approach. The measurements of the two-dimensional flowfield show a recirculation zone downstream of the flame stabilization area. This exhibits a comparable flow pattern to stagnation point flames. However, mean strain rates are much lower since the flames stabilize close to the burner exit, and the flame is not influenced by the recirculation zone. A comparison of the root-mean-square velocities obtained from Laser-Doppler Amemometry measurements to those determined by PIV show a good agreement. It can be shown that turbulence is attenuated in the flame zone and only moderately increased behind it.

Investigation of flame broadening in turbulent premixed flames in the thin-reaction-zones regime

A two-plane two-dimensional Rayleigh thermometry technique is applied to investigate, the three-dimensional flame structure in highly stretched turbulent premixed flames beyond the flamelet regime. Experimental data of the preheat zone thickness, σ F , and flame temperature conditioned at the maximum temperature gradient, T o , are reported. Broadening of the preheat zone thickness is observed and found to be correlated well with the increase of turbulence intensity. This suggests a pure hydrodynamic straining effect in a chemically inert preheat zone in the thin-reaction-zones regime, which is consistent with the rate-ratio asymptotic analysis. Although a general decrease of T o is also found with increasing flame stretch, this temperature drop is subject to the nonadiabatic condition in the postflame region. Flame extinction would be expected when T o is approaching the ignition temperature ≥900 K.

Stability characteristics of lifted turbulent partially premixed jet flames

Abstract The stability characteristics of partially premixed turbulent lifted methane flames have been investigated and discussed in the present work. Mixture fraction and reaction zone behavior have been measured using a combined 2-D technique of simultaneous Rayleigh scattering, Laser Induced Predissociation Fluorescence (LIPF) of OH and Laser Induced Fluorescence (LIF) of C2Hx. The stability characteristics and simultaneous mixture fraction-LIPF-LIF measurements in three lifted flames with originally partially premixed jets at different mean equivalence ratio and Reynolds number are presented and discussed in this paper. Higher stability of partially premixed flames as compared to non-premixed flames has been observed. Lifted, attached, blow-out and blow-off regimes have been addressed and discussed in this work. The data show that the mixture fraction field on approaching the stabilization region is uniquely characterized by a certain level of mean and rms fluctuations. This suggests that the stabilization mechanism is likely to be controlled by premixed flame propagation at the stabilization region. Triple flame structure has been detected in the present flames, which is likely to be the appropriate model at the stabilization point.

Spatial-averaging effects in Raman / Rayleigh measurements in a turbulent flame☆

Abstract Spatial-averaging effects on laser-based measurements of scalar quantities in turbulent flames are investigated. Raman/Rayleigh measurements are made in the shear layers of a turbulent reverse flow reactor (RFR) with two different probe volumes to study the effects on the measured mean, rms, and probability density functions (pdfs). In addition, the errors in the measured variance are estimated by scalar spectrum analysis for an isotropic flow, using the Pao-Corrsin scalar spectrum. The data show that spatial averaging reduces the rms fluctuations of the species concentrations and Rayleigh signal. Also, the form of the pdf is affected in such a way that for a monomodal distribution, it may reduce any sharp peak, whereas for a bimodal distribution it reduces the peaks and fills the gap in between, i.e, reduces its bimodality. These errors are slightly higher for reacting species and the Rayleigh signal data than for nonreacting species. For the measurements reported here the effects are not large enough to change the overall shape of the pdf greatly. The analytical estimates agree quite well with the experimental data, and the theory shows that the effect of the length of the sensor on the variance depends on both the integral scale of the turbulence and the microscale of the scalar, which are related by the turbulence Reynolds number.

A Concentric Flow Conical Nozzle Burner for Highly Stabilized Partially Premixed Flames

A new burner design is presented in this work for highly stabilized partially premixed flames using gaseous fuel. The partially premixed flow is created in a concentric flow tube and the flames are stabilized by a relatively large diverging conical nozzle. The concentric flow conical nozzle burner (CFCN) creates highly stabilized flames at high Reynolds number, up to 60000. Three versions of the burner have been investigated and a maximum load of about 250 kW with 20 mm diameter nozzle has been achieved in partially premixed flame. Higher load is expected for larger burner size. The stability characteristics of the CFCN burner show that it is suitable for industrial applications. The burner turndown ratio is between 15 and 20. Fundamental research studies and modeling of the flames in CFCN are feasible because of the simple flow geometry of the burner. Therefore, four flames have been selected in the present work for detailed thermal reaction zone structure investigation and OH radical distribution using simultaneous two-dimensional imaging of Rayleigh scattering and Laser Induced Predissoeiation Fluorescence (LIPF). The reaction zone structure is relatively thin and may be classified in the thin reaction zones regime. The OH signal correlates well with temperature at the reaction zone. More fine structure in the preheat zone can be observed from the temperature images as compared with those of the OH radical. Stable flame structure with continuous reaction zone at high stretch conditions has been observed.

LINE RAMAN, RAYLEIGH, AND LASER-INDUCED PREDISSOCIATION FLUORESCENCE TECHNIQUE FOR COMBUSTION WITH A TUNABLE KRF EXCIMER LASER

We have applied a line UV Raman, Rayleigh, and laser-induced predissociation fluorescence technique for measurement of turbulent hydrocarbon flames. The species concentration of CO(2), O(2), CO, N(2), CH(4), H(2)O, OH, and H(2) and the temperature are measured instantaneously and simultaneously along a line of 11.4 mm, from which the gradients with respect to mixture fraction and spatial direction are obtained. The technique has been successfully tested in a laminar premixed stoichiometric methane flame and a laminar hydrogen diffusion flame. In addition the technique has been tested in a highly turbulent rich premixed methane flame. The data show that the technique can be used to provide instantaneous measurements of local profiles that describe the local flame structure in highly turbulent flames.

Two-plane two-dimensional Rayleigh thermometry technique for turbulent combustion

A two-plane two-dimensional Rayleigh thermometry technique is presented for measurements in turbulent hydrocarbon flames. UV Rayleigh thermometry is used in this technique for what is to the authors knowledge the first time. It shows the advantages of using LJY excimer lasers rather than visible lasers for Rayleigh scattering. The present technique is useful for studying the three-dimensional structure of the reaction zone in turbulent flames.

MIXING AND NOZZLE GEOMETRY EFFECTS ON FLAME STRUCTURE AND STABILITY

Flame stability and mean structure of partially premixed flames have been investigated under the effect of the level of partial premixing and nozzle cone angle. The stability curves and maps of the mean flame structure based on temperature and CO and O2 concentrations measurements in some selected partially premixed flames in the thin reaction zones regime are presented and discussed. More radial and axial mean profiles of temperature and CO and O2 concentrations are also presented for another set of flames at the same equivalence ratio and several nozzle cone angles. The data show that partially premixed flames are more stable than non-premixed and premixed flames. An optimum degree of partial premixing was achieved in the present burner, beyond which the flames are less stable. This optimum level was achieved when the dimensionless mixing length normalized by the nozzle diameter is equal to 5. At this level of partial premixing the structure is likely to form three interacting reaction zones of lean, rich and diffusion with expected triple flame structure. In partially premixed flames a stabilization core has been observed close to the conical nozzle that provides more heat source at the nozzle exit. This is responsible for stabilizing the flames at high Reynolds number. The data also show that the cone angle has a great influence on the flame stability. Increasing the cone angle leads to more air entrainment, breaking the stabilization core and hence reduces the flame stability. The cone, in all cases, provides protected environment at the early stage of reaction near the nozzle exit where intense turbulence is expected. This leads to highly stable flames as compared to similar burners without cone.