Lean Propane-air Flames Research Articles

Experimental investigation of OH*/CH* ratio variations in turbulent, disk stabilized, lean propane-air flames with inlet mixture preheat and stratification

An experimental investigation of OH* and CH* chemiluminescence emissions from bluff-body stabilized, stratified, lean propane-air flames, operated under elevated inlet temperatures has been performed in this work. Simultaneous band-pass filtered emissions, at inlet mixture temperatures ranging from 300 to 573 K, have been acquired to evaluate the OH*/CH* ratio in an effort to identify a possible correlation between the chemiluminescence ratio and the equivalence ratio, for these flame configurations. Moreover, the impact of preheat on the disposition of the radially stratified equivalence ratio gradient, issuing from an upstream premixing section has been investigated. The employed premixer/burner set up consists of three concentric disks that form two consecutive premixing cavities and lead to a radially stratified equivalence ratio gradient, characterized by its peak profile value, at the inlet to the burner zone. Band-passed filtered chemiluminescence measurements and Fourier Transform Infrared Spectroscopy (FTIR) analysis have been performed to assess the OH*/CH* ratio and the mixing fields supporting the flames. The results revealed an altered premixedness behavior of the employed arrangement with elevated inlet preheat, when compared to a non-preheated case. Additionally, both maximum and spatially averaged OH*/CH* ratio values have been studied in an effort to assess the equivalence ratio by using line of sight chemiluminscence measurements. The maximum chemiluminscence ratio values have been proven to better compensate for the spatial inhomogenieties encountered in the turbulent, stratified set ups studied, thus better estimating the effective equivalence ratio value that is burned in the reacting shear layer, closely corroborating counterpart laminar flame investigations.

An Investigation of Axisymmetric Disk Stabilized Propane-Air Flames Operating under Inlet Mixture Preheat and Stratification

ABSTRACT Turbulent, recirculating, lean propane-air flames with inlet mixture stratification and preheat have been simulated under stable and limiting burning conditions. The modeled burner setup comprises a supply tube enclosing three sequential disks producing two consecutive premixing cavities. Fuel is injected in the first cavity and is partially premixed with primary air flowing through this cavity system, resulting in a radially stratified equivalence ratio profile at the inlet of the afterbody disk flame stabilizer. Detailed velocity, turbulence, fuel-air mixing, and imaging data have been previously reported for inlet preheats from 300 to 573 K and for a range of fuel flow rates. The simulations were carried out with a finite-volume-based method, using the dynamic Smagorinsky subgrid model coupled with two combustion methodologies, a quasi-laminar reaction rate approach and the Thickened Flame Model approach. Propane oxidation was modeled with a 22-species skeletal scheme. OH* chemiluminescence distributions were also computed by post-processing quasi-steady state derived algebraic expressions, exploiting directly simulated species thus enabling comparisons with experimental images. The simulations were evaluated against velocity, turbulence, and temperature measurements as well as chemiluminescence images. These comparisons allowed for an assessment of the methodologies’ capability to reproduce important trends such as the notable extension of stable operation to ultra-lean mixtures, the appreciable effect on the near flame aerodynamic stretch and the variations in the local flame structure.

乱流予混合火炎で観察される選択拡散の痕跡(熱工学,内燃機関,動力など)

Effect of the preferential diffusion between oxygen and fuel molecules is observed as the cellular flame in laminar premixed flames. When the diffusion coefficient of the deficient species is larger, the deficient species diffuses preferentially to the reaction zone to cause the local stratification of the mixture ratio along the flame. Therefore, the cellular flame is established. If the preferential diffusion should occur in propane-air turbulent premixed flames, the wrinkle scale of rich flames would be smaller than that of lean flames, consequently, the turbulent burning velocity and/or the heat release rate of the former would be larger than those of the latter. Based on these considerations, traces of the preferential diffusion are sought from experimental results of propane-air turbulent premixed flames. Comparing the radius of curvature of the flamelet of rich and lean propane-air flames with the same laminar burning velocity, the radius of curvature of the flamelet of the former are smaller than those of the latter. The gas expansion ratio across the flamelet of the former is larger than that in the latter. The magnitudes of the flamelet velocity vector of the former are larger than the latter.

Characteristics of Combustion and Heat Transfer of Excess Enthalpy Flames Stabilized in a Stagnation Flow. 2nd Report, Heat Flux at High Flow Rate and Effects of Lewis Number.

Characteristics of an excess enthalpy flame stabilized in an axisymmetric flow impinging on the heat receiver wall (the stagnation surface) in a combustion chamber, and heat fluxes utilized from flames under the conditions of high flow rates near the extinction limits were experimentally examined. The effects of Lewis number of the deficient reactant on the characteristics of combustion were also discussed. (1) The excess enthalpy flames are stabilized near the heat receiver wall due to the effect of heat recirculation for the lean methane-air flames (Le≒1.0) and the lean propane-air flames (Le>1.0), and due to the effects of heat recirculation and the Lewis number for the lean hydrogen-air flames (Le<1.0), even in cases in which a large amount of heat is utilized from the flames. Thus, the heat flux utilized from excess enthalpy flames of methane, propane and hydrogen through the heat receiver wall increases rapidly with an increase in the stagnation velocity gradient. (2) When the flame is established outside the thermal boundary layer formed on the heat receiver wall, heat flux obtained from the flames through the heat receiver wall can be considered as the Convective heat transfer in the stagnation flow in which only the burned gas of high temperature flows. In contrast when the flame is stabilized inside the thermal boundary layer formed in the stagnation flow in which only the burned gas of high temperature flows, the heat flux transferring through the heat receiver wall increases rapidly with an increase in the stagnation velocity gradient, and the characteristics of convective heat transfer to the heat receiver wall are improved. The characteristics of heat transfer to the heat receiver wall are not dependent on the thickness of the thermal boundary layer, but are significantly affected by the flame location.