Consequently, the flow field cannot be treated as a jet satisfying the similarity. Since propane is heavier than air, when propane is injected vertically through a nozzle of d = O (1 mm), the fuel loses its momentum and then accumulates in a certain downstream region and flows downward toward the nozzle. Direct adoption of the previous experiment for the methane jet to propane fuel encountered difficulties due to buoyancy effect. The objective of the present study was to determine the propagation speed of tribrachial flame for propane fuel. Experimentally determined maximum propagation speed of tribrachial frame in methane free jet was 0.96 m/s, which is comparable to the maximum value of 1.09 m/s predicted theoretically. The effects of flame curvature and mixture fraction gradient on the propagation speed have been identified. The propagation speed has been determined from unsteady propagations of tribrachial flames when the methane jets were ignited at a downstream location using a pulse laser. In the previous experiment, the nozzle diameter used was d = 2.08 mm and the jet velocity u 0 was in the range of 1.3 < u 0 < 3.5 m/s. ![]() The propagating tribrachial flame in a jet is rather insensitive to ignition source and is free from external heat loss, eg, through material surface. The flow field has been extensively investigated previously and has a similarity solution. There are several advantages in adopting a jet configuration. The propagation speed of tribrachial flames of methane has been studied by conducting experiments in laminar jets. Īlthough the propagation speed of tribrachial flame has been rather extensively investigated numerically and theoretically, experimental determinations of the propagation speed, however, are rather limited. It has been theoretically and numerically predicted that the maximum speed of a tribrachial flame becomes the stoichiometric laminar burning velocity multiplied by the square root of density ratio of unburned to burnt mixture, resulting in the maximum propagation speed of about 1 m/s for such fuels as methane and propane. It has been found that the mixture fraction gradient and the flow redirection effect resulted from the heat release are the dominant factors that influence the propagation speed,. Theoretical and numerical studies have been extensively conducted to investigate the effect of Lewis number, mixture fraction gradient, ,, heat release, and buoyancy. ![]() For more accurate analysis, the information on the propagation speed of tribrachial flame is needed. The correlations of liftoff height with jet velocity, nozzle diameter, air dilution, inert dilution, and the dependence on Schmidt number of fuel in free jets have been successfully derived assuming the propagation speed having a constant value,. ![]() ![]() In laminar jets, the characteristics of liftoff height have been successfully analyzed based on the balance mechanism between the propagation speed of tribrachial flame and local flow velocity along the stoichiometric contour. Two of the important issues on tribrachial flame are its propagation speed and its role in mixing layers for flame stabilization. Extensive studies have been conducted to characterize tribrachial flames either in laminar jets, ,, or 2-D mixing layers, ,. When a mixture fraction gradient exists in a partially premixed condition, a flame edge could have the structure of tribrachial (or triple) flame, consisting of a lean and a rich premixed flame wings and a diffusion flame in between. The limiting maximum propagation speed under the microgravity condition is in good agreement with the theoretical prediction, ie, the ratio of maximum propagation speed to the stoichiometric laminar burning velocity is proportional to the square root of the density ratio of unburned to burnt mixture. Under the microgravity condition, the results showed that the propagation speed of tribrachial flame decreased with the mixture fraction gradient, in agreement with previous studies. Approximate solutions for the velocity and concentration accounting density difference and virtual origins have been used in determining the propagation speed of tribrachial flame and the concentration field was validated from the measurement of Raman scattering. We found in the present experiment that the displacement speed varied nonlinearly with axial distance because the flow velocity along the stoichiometric contour was comparable to the propagation speed of tribrachial flame. The propagation speed of tribrachial (triple) flames in laminar propane jets has been investigated experimentally under normal and micro gravity conditions.
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