This paper presents a theory to predict the extinction limit of jet diffusion microflames. The constant-density approximation is introduced to separate flow field from combustion reaction; flow field then depends only on the jet Reynolds number, Re. An exact solution of the Navier-Stokes equation is adopted; the predicted flame shape based on the exact solution well agrees with experimental observation. An Arrhenius-type one-step global reaction is considered, adding the Damköhler number, Da, as an additional parameter to the Reynolds number. The extinction limit is obtained using an activation-energy asymptotics technique. The obtained extinction criterion, Da Re2 ≈ constant, reproduces experimental results that the jet exit velocity under the extinction condition is proportional to d-2, where d is the burner diameter.
This paper studied a normal scale lifted flame base (triple flame), which has remarkable leading edge characteristics, in order to investigate microflame stability. A simple theoretical analysis was performed using velocity profiles measured with particle image velocimetry (PIV) and temperature profiles measured with a thermocouple. Two-dimensional numerical calculations were also performed with a C1 chemistry reaction model. The maximum burning velocity change in the flame curvature can be reproduced by present theory considering flame stretch effects, but this velocity is not consistent with experiment results. However, numerical calculations with sufficient spatial resolution yield velocities consistent with experiments. These calculation results demonstrate that the reaction of CH3 formation with an H radical is accelerated at the leading edge of the flame. This reaction mechanism is identical to that for microflame stability, which is estimated through the usual numerical approach. Namely, the stability of the microflame is enhanced by the generated H radical. This stability mechanism is basically the same as that for the normal flame base.
Harvesting, transportation, energy conversion and the high-efficient utilization, cascade method and market formation besides become with the indispensable element in order to utilize the biomass resource. There are two type biogases; it is gasified gas from dried biomass by partially combustion and wet biogas from wet biomass by methane fermentation, especially from the livestock excrement resources. This paper discusses an experimental study for flame stabilization on microscopic scale with wet biogas (mainly 0.6CH4+0.4CO2). In this study, the microflame with the wet biogas fuels are formed by the diffusion flame on the coppered straight pipes of inner diameter 0.02mm ∼ 1.5mm. This study is obtained stability mapping on microscopic scale of formed microflame by wet biogas fuels. The flame stability limit conditions on microscopic scale of wet biogas is drawn with blow off and extinction flame double limit lines. It is suggested that minimum mixing spatial scale change by the each mixing ratio of the wet biogas.
The flame stability limits essentially define the fundamental operation of the combustion system. Recently the micro diffusion flame has been remarked. The critical conditions of the flame stability limit are highly dependent on nozzle diameter, species of fuel and so on. The micro diffusion flame of Methane/Propane and Hydrogen is formed by using the micro-scale nozzle of which inner diameter is less than 1mm. The configurations and behaviors of the flame are observed directly and visualized by the high speed video camera The criteria of stability limits are proposed for the micro diffusion flame. The objectives of the present study are to get further understanding of lifting/blow-off for the micro diffusion flame. The results obtained are as follows. (1) The behaviors of the flames are classified into some regions for each diffusion flame. (2) The micro diffusion flame of Methane/Propane cannot be sustained, when the nozzle diameter is less than 0.14 mm. (3) The diffusion flame cannot be sustained below the critical fuel flow rate. (4) The minimum flow which is formed does not depends on the average jet velocity, but on the fuel flow rate. (5) the micro flame is laminar. The flame length is decided by fuel flow rate.
Flame structure of micro scale methane-air premixed flames is investigated experimentally. First, the uppermost and lowest flow rates which propagating flame could be formed are examined with simple single burner. Propagating flame is not stabilized on the simple single burner whose diameter is less than 4mm, despite the flow rate is well controlled between the expected velocity gradient limits for blow off and flash back. In addition, all the extinction mechanism observed for the burner diameter less than 4mm is blow off. It is, consequently, considered that the flame formed on the burner whose diameter is less than 4mm has other extinction mechanism in addition to blow off and flash back caused by flow velocity gradient. Secondly, the flame formed on the burner with pilot flame is observed. The flame is stabilized even on the burner whose diameter is 0.3mm. However, shape of the flame formed on the burner whose diameter is less than 1mm and at around lowest flow rate is near spherical. It is similar to the appearance of micro diffusion flames. On the other hand, the flame formed on the burner whose diameter is less than 0.5mm is not considered as propagating flame, because typical laminar propagating flame has a structure more than 0.5mm thickness at this condition. Therefore, it is supposed that the flame formed on the burner whose diameter is submillimeter and at around lowest flow rate is dominated by the diffusion mixing of oxygen and methane from the premixture and high temperature heat flux from the pilot gas flow.
It is important for the research and development on combustors to measure temperature of burner and its surrounding wall. In the present study, the influence of flame on radiation thermometry is examined spectroscopically for the case of measuring solid wall temperature near flames using the multispectral CCD camera and the spectrometer. The influences of vapor and flame reaction are shown to be specific error factors owing to flame. The magnitudes of those influences are, however, found to be quite different; while influence of the vapor molecular band emission is below 10°C in the range of 500 to 600°C , the influence of emission of flame reaction can be over 100°C in the range of 700 to 800°C in the case of narrow band like 10 nm. Since flame reaction is due to aerosol in surrounds and hard to predict, the measurement wavelength should exclude around 590nm (Na) and around 765nm (K) in the case of radiation thermometry based on relatively shorter wavelength.