On the Physics of Jet Diffusion Flames (original) (raw)
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Observations and predictions of jet diffusion flame behaviour
Atmospheric Environment (1967), 1987
A simple model has been developed for jet diffusion flames to estimate tlame height (kr)and angle of the flame to the vertical (an) The model is based upon the assumption that flame bchaviour is dominated by momentum effects. Buoyancy inthtcnccs on bchaviour are assumed to k negligible. Predictions of the model were assessed against flame parameters as observed in a wind tunnel and during tkld tests with an industrial flare. The wind tunnel studies involved experiments with hydrocarbon (methane, propane, ethylene, butane) diffusion Rames. Field experiments involved measuring hr and an of flames resulting from the combustion of acid gas-fuel gas mixtures possessing molecular weights of about 37 g mol-'.
A numerical/experimental study of the dynamic structure of a buoyant jet diffusion flame
Theoretical and Computational Fluid Dynamics, 1994
An overview of a joint numerical/experimental investigation of the dynamic structure of a low-speed buoyant jet diffusion flame is presented. The dynamic interactions between the flame surface and the surrounding fluid mechanical structures are studied by means of a direct numerical simulation closely coordinated with experiments. The numerical simulation employs the full compressible axisymmetric Navier-Stokes equations coupled with a flame sheet model.
Fire Science and Technology, 2010
A series of experiments was carried out in order to investigate the visible flame height of a jet flame with a high initial discharging velocity where the fuel-release time and mass flow rate of fuel were varied. The values of non-dimensional heat release rate (Q *) in present experiment were in the range of 10 7 < Q * < 10 8. Even if the value of Q * was larger than 10 6 , when the non-dimensional number (R M) employed by Heskestad which implies the ratio of gas release momentum to the momentum made by a purely buoyant diffusion flame was less than 0.1, the flame height with a comparatively high initial discharging velocity increased with the heat release rate. In other words, whether flame height depends on the Q *2/5 or not can be decided by the range of momentum ratio (R M) regardless of the range of non-dimensional heat release rate (Q *). This can be also decided by the fire Froude number employed by Delichatsios.
Buoyancy effects in strongly pulsed turbulent diffusion flames
Combustion and Flame, 2004
The effects of buoyancy in pulsed turbulent jet diffusion flames were investigated by conducting experiments in both microgravity and normal gravity. In all cases the flames were fully modulated; that is, the fuel flow was completely shut off between pulses. Unheated ethylene fuel was injected using a 2-mm-diameter nozzle into a combustor with an oxidizer coflow at ambient pressure. Microgravity conditions (10 −4 g) were achieved for 2.2 s in drop tower tests. Flames with short injection times and high duty cycle exhibit a marked increase in the ensemble-averaged flame length due to the removal of buoyancy. For other injection conditions, including steady state injection, the flame length is not strongly impacted by buoyancy. The significant increases in flame length with injection duty cycle are consistent with the duty cycle near the flame tip of microgravity flames exceeding that of normal gravity flames. The celerity of isolated compact flame puffs is approximately 40% less in microgravity than in normal gravity. An analytical argument indicates that duty cycle near the flame tip can significantly exceed that at injection due to the combination of a puff growth and the decrease in the celerity of the flame puffs with downstream distance. This effect is predicted to be significantly greater in the absence of buoyancy and for shorter injection times, in qualitative agreement with the experiments. The cycle-averaged centerline temperatures were generally higher in the microgravity flames than in normal gravity, especially at the flame tip where the difference was as much as 200 K.
Investigation of an excited jet diffusion flame at elevated pressure
Journal of Fluid Mechanics, 1989
Experiments have been carried out with the objective of studying the relationship between flow structure, flow excitation and the reaction process in the near field of a low-speed coflowing jet diffusion flame. The effect of axial forcing and increasing pressure on the structure and controllability of the flame has been studied in an attempt to elucidate some of the underlying mechanisms of control. The experiments were conducted in a variable-pressure flow facility which permits the study of reacting flows at pressures ranging from 10 to 1000 kPa (0.1 to 10 atm.). The flame was excited by adding a small-amplitude, periodic fluctuation to the central fuel jet exit velocity. The flow was visualized using an optical scheme which superimposes the luminous image of the flame on its schlieren image, giving a useful picture of the relationship between the luminous soot-laden core flow and the edge of the surrounding hot-gas envelope. Phase-conditioned velocity measurements were made with ...
Proceedings of the Combustion Institute, 2019
The spread of flames over the surface of a solid combustible material in an opposed flow is different from the mass-burning (or fuel-regression) in a pool fire. However, the progress of the flame front over a solid fuel includes both flame-spread and fuel-regression, but the difference between these two processes has not been well clarified. In this work, experiments using cylindrical PMMA samples were conducted in normal gravity and in microgravity are analyzed to identify the transition from opposed flame-spread to fuel-regression under varying conditions, including sample size, opposed flow velocity, pressure, oxygen concentration, external radiation, and gravity level. For a thick rod in normal gravity, as the opposed flow increases to 50~100 cm/s, the flame can no longer spread over the fuel surface but stay in the recirculation zone downstream of the cylinder end surface, like a pool fire flame. The flame spread first transitions to fuel regression at a critical leading-edge regression angle of ≈ 45 o , and then, flame blow-off occurs. Under large opposed flow velocity, a stable flat blue flame is formed floating above the rod end surface, because of vortex shedding. In microgravity at a low opposed flow (<10 cm/s), pure fuel-regression was not observed. This work aims to clarify the differences between the flame-spread and fuelregression in the progress of the flame and provide a better understanding of blow-off phenomena on solid fuels.
Computational and experimental study of a forced, timevarying, axisymmetric, laminar diffusion flame
Symposium ( …, 1998
Forced, time-varying flames are laminar systems that help bridge the gap between laminar and turbulent combustion. In this study, we investigate computationally and experimentally the structure of an acoustically forced, axisymmetric laminar methane-air diffusion flame in which a cylindrical fuel jet is surrounded by a coflowing oxidizer jet. The flame is forced by imposing a sinusoidal modulation on the steady fuel flow rate. Rayleigh scattering and spontaneous Raman scattering of the fuel are used to generate the temperature profile. Particle image velocimetry (PIV) is used to measure the fuel tube exit velocity over a cycle of the forcing modulation. CH flame emission measurements have been done to predict the excitedstate CH (CH*) levels. Computationally, we solve the transient equations for the conservation of total mass, momentum, energy, and species mass with detailed transport and finite-rate C 2 chemistry submodels to predict the pressure, velocity, temperature, and species concentrations as a function of the two independent spatial coordinates and time. The governing equations are written in primitive variables. Implicit finite differences are used to discretize the governing equations and the boundary conditions on a nonstaggered, nonuniform grid. Modified damped Newton's method nested with a Bi-CGSTAB iteration is utilized to solve the resulting system of equations. Results of the study include a detailed description of the fluid dynamic-thermochemical structure of the flame at a 20-Hz frequency. A comparison of experimentally determined and calculated temperature profiles and CH* levels agree well. Calculated mole fractions of species indicative of soot production (C 2 H 2 , CO) are compared against those levels in the corresponding steady flame and are observed to increase in peak concentration values and spatial extent. Analysis of acetylene production rates reveals additional significant production in the downstream region of the flame at certain times during the flame's cyclic history.
Reaction kernel structure and stabilizing mechanisms of jet diffusion flames in microgravity
Proceedings of the Combustion Institute, 2002
The structure of axisymmetric and two-dimensional, laminar, methane jet diffusion flames in normal earth gravity (1g) and zero gravity (0g) has been simulated numerically. Computations of the time-dependent full Navier-Stokes equations with buoyancy were performed using an implicit, third-order accurate numerical scheme and a detailed C 2 -chemistry model. An optically thin media radiation model for heat losses from CO 2 , H 2 O, CH 4 , and CO is included. Observations of the flames were also made at the NASA Glenn 2.2-s drop tower. The parametric computational results are as follows: (1) In a quasi-quiescent oxidizing environment in 1g, buoyancy induced the entrainment of the surrounding oxidant into the flame base and enhanced the convective-diffusive oxygen influx and key radical reactions, thereby forming a peak reactivity spot, that is, reaction kernel, responsible for flame stabilization.