Shorter Flame Length Research Articles

Effect of Fuel on Transition of Fuel Jet Diffusion Flame.

A numerical study on the transition from laminar to turbulent flow of two-dimensional fuel jet flames developed in a co-flowing air stream was made by adopting the flame surface model with infinite chemical reaction rate and unit Lewis number. The time-dependent compressible Navier-Stokes equation was solved numerically with the equation for mixture fraction using a finite difference method. The numerical calculation was performed for the pure methane fuel jet, diluted methane fuel jet and hydrogen fuel jet, with Reynolds numbers 2 000 and 4 000. The effect of fuel containing an inert gas on the structure and the stability of the flame jet was studied. It was found that the transition length of hydrogen fuel jet was longer than that of methane fuel jet. In the downstream turbulent region, the axial velocity decreased due to the density increase, resulting in shorter flame length. Moreover, the lateral expansion of fluctuating flame surface was suppressed due to the low density inside the flame.

Solid Propellant Flame Structure

AbstractPlanar Laser Induced Fluorescence (PLIF), UV/Vis Absorption, and thermocouple measurements were done for HNF, RDX, HMX, and XM39 deflagration with and without CO2 laser-support. RDX and especially HNF have very short self-deflagration flame length scales. HMX and XM39 have taller self-deflagration flames. XM39 has a marked dark zone with plateau temperature about 1400K. RDX's dark zone, present under laser supported deflagration, collapses when the external laser flux is removed. PLIF was used to measure the 2D NH, OH, and CN species profiles for these materials and OH temperature profile for RDX and HNF under non-laser supported conditions. The best spatial resolution for the RDX PLIF was about 4μm. Sandwiches of HNF and various binders were studied with PLIF and while obvious diffusion flames were present at low pressure, they are weak and are not expected to be burn rate controlling.

A Model of Low-Speed Concurrent Flow Flame Spread Over a Thin Fuel

Abstract A numerical model is developed to examine steady, laminar flame spread and extinction over a thin solid fuel in low-speed concurrent flow. The model incorporates an elliptic treatment of the upstream flame stabilization zone near the fuel burnout point, and a parabolic treatment of the downstream flame, which has a higher flow Reynolds number. This provides a more precise fluid-mechanical description of the flame than using parabolic equations throughout, and is the first time such an approach has been used in concurrent flame spread modeling. The parabolic and elliptic regions are coupled smoothly by matching boundary conditions. The solid phase consists of an energy equation with surface radiative loss and a surface pyrolysis relation. Calculations (with the flame spread rate being an eigenvalue) are performed for forced flow without gravitational influences in a range of velocities which are lower than those induced in a normal gravity buoyant environment. Steady spread with constant flame and pyrolysis lengths is found possible for thin fuels and this facilitates the adoption of a moving coordinate system attached to the flame. Both quenching and blowoffextinction are observed. As flow velocity or oxygen percentage is reduced, the flame spread rate, the pyrolysis length, and the flame length all decrease. The flame standoff distance from the solid and the reaction zone thickness, however, first increase with decreasing flow velocity, but eventually decrease very near the quenching extinction limit. The short, weak flames observed at low flow velocities and oxygen levels are consistent with available experimental data. The maximum flame temperature decreases slowly at first as flow velocity is reduced, then falls more steeply close to the extinction limit. At low velocities, surface radiative loss becomes a significant fraction of the total combustion reactions’ heat release. In addition, the shorter flame length causes an increase in the fraction of conduction downstream (into the gas phase) compared to conduction to the fuel. These heat losses lead to lower flame temperatures, and ultimately, a quenching extinction. This extinction mechanism differs from that of blowoff, where the flame is unable to be stabilized due to the high flow velocity.

Flame Length Measurements of Burning Fuel Puffs

Abstract An experimental study of burning of natural gas puffs generated by a fully modulated jet, has been conducted. The present study concentrated on the effects of duty-cycle and puff volume, or equivalently the injection time, on the flame length. It is observed that when individual puffs are well-separated, considerably shorter flame lengths, by as much as a factor of four, are obtained compared to a steady jet flame. Furthermore, the flame length of individual puffs scales with the initial volume of puff, in agreement with the previous studies of buoyant puffs in aqueous media. For a given puff volume, as the duty-cycle is increased beyond a given value, the flame length tends to rise rapidly due to the interaction among neighboring puffs. A dimensionless parameter is found which indicates the transition of puff characteristics.