Burning velocity measurement of lean methane-air flames in a new nanosecond DBD microplasma burner platform

This paper presents the initial characterization of a new burner design to study the effect of non-thermal plasma discharge on combustion characteristics at atmospheric pressure. The burner allows stabilizing an inverted cone flame in a mixture flowing through a perforated plate designed as a microplasma reactor. The design principle of the microplasma reactor is based on the dielectric barrier discharge scheme which helps to generate a stable nonthermal plasma discharge driven by nanosecond high-voltage pulses in the burner holes. The consumed power and pulse energy have been calculated from simultaneously measurements of current and voltage of the electrical pulses. Time-resolved measurements of direct emission spectra for nitrogen second positive system N2(C-B) have been done to determine the rotational and vibrational temperatures of the plasma discharge. By fitting the spectra with SPECAIR simulation data, it was found that the rotational and vibrational temperatures are 480 K and 3700 K, respectively, for the discharge in methane-air mixture with an equivalence ratio of 0.5 at atmospheric pressure. The influence of a high-voltage (5 kV) pulsed nanosecond discharge on the laminar burning velocity of methane-air flame has been investigated over a range of equivalence ratios (0.55–0.75). The laminar burning velocity was calculated by the conical flame area method which has been validated by other published data. CH* chemiluminescence image analysis has been applied to accurately determine the flame area. The results show an increase of the burning velocity of about 100% in very lean (Φ= 0.55) flames as a result of the plasma discharge effect

On the mean stretch rate over a finite flame segment

In this work, an approach is suggested to determine the mean flame stretch rate for a finite segment of a thin flame, in particular, for the entire flame. The suggested approach does not require averaging of local stretch rate at multiple flame locations. It is evidenced that available theoretical predictions of the mean stretch rate, which rely on the strain rate of a material surface, can be invalid for turbulent flames with edges. It is shown that the mean stretch rate of a turbulent Bunsen flame is always negative and is not sensitive to the turbulence intensity, whether the flame is self- turbulized or turbulence is imposed externally

On extinction mechanism of lean limit methane-air flame in a standard flammability tube

Gas-dynamic and qualitative thermal structure of lean methane-air flames propagating upward in a standard flammability tube has been experimentally investigated at near-limit concentrations. Local burning velocity related to the cold and hot flame boundaries, and stretch rates have been measured along studied flames. The measured stretch rate attains its maximum value at the flame tip. It was experimentally found that flame temperature is minimum at the flame tip, which agrees with the observation that flame extinction starts from the leading point, but cannot be explained by the combined effect of stretch rate and preferential diffusion. In the coordinate system moving with the flame, a zone of stagnation of combustion products was observed in limit flame near its tip, while at higher methane concentrations stagnation zone does not exist. This observation suggests that limit flame extinction behavior is connected with the formation of the stagnation zone: leading edge of the flame can be cooled by heat conduction to the stagnation zone, which rises upward together with flame and is effectively cooled due to radiation heat loss. Simplified analysis of this effect in a single stretched flame is carried out in terms of coupling radiation losses with local stretch rate. It was found that flame is accompanied by a rising upward combustion products flow, resembling a hot bubble and located about 35 cm downstream of the flame

Experimental study of lean flammability limits of methane/hydrogen/air mixtures in tubes of different diameters

Lean limit flames in methane/hydrogen/air mixtures propagating in tubes of internal diameters (ID) of 6.0, 8.9, 12.3, 18.4, 25.2, 35.0, and 50.2 mm have been experimentally studied. The flames propagated upward from the open bottom end of the tube to the closed upper end. The content of hydrogen in the fuel gas has been varied in the range 0–40 mol%. Lean flammability limits have been determined; flame shapes recorded and the visible speed of flame propagation measured. Most of the observed limit flames in tubes with diameters in the range of 8.9–18.4 mm had enclosed shape, and could be characterized as distorted or spherical flame balls. The tendency was observed for mixtures with higher hydrogen content to form smaller size, more uniform flame balls in a wider range of tube diameters. At hydrogen content of 20% or more in the fuel gas, limit flames in largest diameters (35.0 mm and 50.2 mm ID) tubes had small, compared to the tube diameter, size and were lens -shaped. Regular open-front lean limit flames were observed only for the smallest diameters (6.0 mm and 8.9 mm) and largest diameters (35.0 and 50.2 mm ID), and only for methane/air and (90% CH4 + 10% H2)/air mixtures, except for 6 mm ID tube in which all limit flames had open front. In all experiments, except for the lean limit flames in methane/air and (90% CH4 + 10% H2)/air mixtures in the 8.9 mm ID tube, and all limit flames in 6.0 mm ID tube, visible flame speeds very weakly depended on the hydrogen content in the fuel gas and were close to- or below the theoretical estimate of the speed of a rising hot bubble. This observation suggests that the buoyancy is the major factor which determines the visible flame speed for studied limit flames, except that last mentioned. A decrease of the lean flammability limit value with decreasing the tube diameter was observed for methane/air and (90% CH4 + 10% H2)/air mixtures for tubes having internal diameters in the range of 18.4–50.2 mm. This effect has been attributed to the stronger combined effect of the preferential diffusion and flame stretch in narrower tubes for flames which resemble rising bubble

On the correlation of inverted flame blow-off limits with the boundary velocity gradient at the flame holder surface

Conductive heat losses from the base of a lean methane–air inverted flame stabilized behind the trailing edge of a thin rod have been experimentally evaluated. The results favor the view that the heat losses to the flame holder play a crucial role in the inverted flame stabilization and blow-off. Simple estimations have been performed, which indicate that the well-established correlation between the mixture composition and the boundary velocity gradient at the flame holder, usually considered as a proof of the flame stretch theory of blow-off, can be explained without involving the flame stretch concept. The suggested explanation of this correlation is based on the assumption that the heat loss to the flame holder is the main factor that determines the inverted flame blow-off behavior and on the similarity between the mechanisms of energy and momentum diffusion in gases (Pr≈ 1)

Experimental study of limit lean methane/air flame in a standard flammability tube using particle image velocimetry method

Lean limit methane/air flame propagating upward in a standard 50 mm diameter and 1.8 m length tube was studied experimentally using particle image velocimetry method. Local stretch rate along the flame front was determined by measured gas velocity distributions. It was found that local stretch rate is maximum at the flame leading point, which is in agreement with earlier theoretical results. Similar to earlier observations, extinction of upward propagating limit flame was observed to start from the flame top. It is stated that the observed behavior of the extinction of the lean limit methane/air flame can not be explained in terms of the coupled effect of flame stretch and preferential diffusion. To qualitatively explain the observed extinction behavior, it is suggested that the positive strain-induced flame stretch increases local radiation heat losses from the flame front. An experimental methodology for PIV measurements in a round tube is described

Anomalous blow-off behavior of laminar inverted flames of ultra-lean hydrogen-methane-air mixtures

An experimental study of rod-stabilized laminar inverted flames of ultra-lean hydrogen–methane–air mixtures has been performed. For mixtures with high hydrogen content, anomalous stabilization and blow-off behavior has been observed. Flames in those mixtures could be stabilized at equivalence ratios below the lean flammability limit for a zero-stretch planar flame. Stabilization of such flames was possible only when the mixture velocity exceeded some critical value. Flames were blown off when the mixture velocity was reduced below this value. The stand-off distance above the flame holder for those flames decreased and heat transfer from the flame base to the flame holder became more intense when the mixture velocity was increased. This is opposite to the regular behavior of inverted flames. These observed unusual phenomena were attributed to the combination of a strong flame stretch and preferential diffusion effects and to the negative value of the Markstein length in mixtures with high hydrogen content. According to the suggested explanation, increasing the velocity results in an increase of the flame stretch rate at the flame base. This, in turn leads to higher flame temperatures and higher burning velocities, making the survival of the flame below the flammability limits possible. Along with the anomalous blow-off behavior, normal blow-off occurring at increased velocity was observed for mixtures with high hydrogen content at the lowest tested equivalence ratios. The observation of the flame shape evolution showed that, when the normal blow-off limit is approached, a flame narrows slightly above the flame base, forming a "neck". The flame fronts merge at the "neck" location and flame breaks there, leading to complete flame extinction or leaving a very small flamelet near the flame holder. It is suggested that the flame local extinction in that case occurs as the result of the excessive flame stretch at the flame "neck" which leads to the flame fronts merging and to the incomplete reaction

Anomalous blow-off behavior of laminar inverted flames of ultra-lean hydrogen-methane-air mixtures

An experimental study of rod-stabilized laminar inverted flames of ultra-lean hydrogen–methane–air mixtures has been performed. For mixtures with high hydrogen content, anomalous stabilization and blow-off behavior has been observed. Flames in those mixtures could be stabilized at equivalence ratios below the lean flammability limit for a zero-stretch planar flame. Stabilization of such flames was possible only when the mixture velocity exceeded some critical value. Flames were blown off when the mixture velocity was reduced below this value. The stand-off distance above the flame holder for those flames decreased and heat transfer from the flame base to the flame holder became more intense when the mixture velocity was increased. This is opposite to the regular behavior of inverted flames. These observed unusual phenomena were attributed to the combination of a strong flame stretch and preferential diffusion effects and to the negative value of the Markstein length in mixtures with high hydrogen content. According to the suggested explanation, increasing the velocity results in an increase of the flame stretch rate at the flame base. This, in turn leads to higher flame temperatures and higher burning velocities, making the survival of the flame below the flammability limits possible. Along with the anomalous blow-off behavior, normal blow-off occurring at increased velocity was observed for mixtures with high hydrogen content at the lowest tested equivalence ratios. The observation of the flame shape evolution showed that, when the normal blow-off limit is approached, a flame narrows slightly above the flame base, forming a "neck". The flame fronts merge at the "neck" location and flame breaks there, leading to complete flame extinction or leaving a very small flamelet near the flame holder. It is suggested that the flame local extinction in that case occurs as the result of the excessive flame stretch at the flame "neck" which leads to the flame fronts merging and to the incomplete reaction

Structure and stability of premixed flames stabilized behind the trailing edge of a cylindrical rod at low Lewis numbers

Premixed flames stabilized behind the trailing edge of a semi-infinite cylindrical rod placed coaxially in a circular channel were investigated numerically within the diffusive-thermal model. Apart from the inverted flames, or V-flames, widely reported in the literature, the other kind of flames was observed for the Lewis number lower than unity. The main characteristic of such flames is confinement in the interior of a recirculating vortex formed behind the trailing edge. For a fixed Reynolds number, the flames of this kind exist within a finite range of the Damköhler number. Once the Damköhler number is fixed, they are observed for the Reynolds numbers above a critical value with no limit on large Re, assuming that flow remains laminar. Global linear stability analysis of the axisymmetric steady-state solutions of both kinds was performed. The ranges of the parameters where the axisymmetry-breaking bifurcation arises and the oscillatory behavior takes place were found. The results of the stability analysis were successfully compared with the direct two- and three-dimensional numerical simulations

Numerical and experimental studies of torus-like flame around the vortex filament in a premixed reactant flow

\u3cp\u3eIn this work we present numerical studies and experimental observations of premixed torus-like flames formed around the filament of a steady vortex in a flow of premixed reactants at lean conditions. The numerical results were obtained within the diffusive-thermal model while the experimental observations were carried out for pure methane-air and 50% hydrogen +50% methane-air mixtures. The parallels between such flames and the flame ball are observed owing to delivering of reactants to the curved flame front and removing of combustion products solely by diffusion.\u3c/p\u3