Buoyant Flames Research Articles

Entrainment measurements in turbulent buoyant jet flames and implications for modeling

Air entrainment rates were measured along turbulent buoyant jet flames having heat release rates up to 45 kW and nozzle diameters 12 mm and 23 mm. We used the Ricou-Spalding measurement technique which involves supplying just enough ambient air to an enclosure around the flame to produce zero pressure drop across an exit orifice plate. The experimental enclosure, the orifice plate and the pressure lines were water-cooled because of the large radiative output from the fires. The entrainment rates at various positions in the turbulent flame for the larger nozzle (23 mm dia) were independent of the total heat release rate (20 kW, 31 kW and 45 kW) and increased with the 5/2 power of the distance from the nozzle exit. Similar behavior was observed for the smaller nozzle but with a virtual origin some distance above the nozzle exit. Implications of the present results for mathematical modeling of turbulent burning are discussed in conjunction with the heat release rates along turbulent buoyant jet flames.

A mathematical model for lean hydrogen-air-steam mixture combustion in closed vessels

The lean hydrogen-air-mixture combustion model described in this paper accounts for nonadiabatic, axisymmetric, buoyant flame propagation in a sphere or a cylinder. Turbulent burning velocity effects and heat losses associated with fan-stirred mixtures, equipment obstructions, and/or a water spray in the vessel are explicitly included in the model. Flame quenching and post-combustion cooldown are also modeled. Calculated results are compared with data from a variety of experiments conducted in vessels ranging in volume from 5 to 2,000 m3. Comparisons with pressure data recorded during the accidental hydrogen deflagration in the Three Mile Island Unit 2 reactor building are also presented. Results indicate that peak pressures and burn times can be simulated quite well in many cases without using any arbitrarily adjusted parameter values. In other cases, such as the intermediated-scale cylindrical vessel tests, the nominal flame surface are had to be increased by a factor of 2–3 to achieve good agreement.

Species Concentrations and Turbulence Properties in Buoyant Methane Diffusion Flames

Past measurements of mean velocities and temperatures in buoyant turbulent, axisymmetric methane diffusion flames burning in still air have been extended to include mean species concentrations (CH4, N2, O2, CO2, H2O, CO, and H2) and turbulence quantities. The new measurements were used to evaluate a Favre-averaged, k-ε-g turbulence model of the process—with all empirical constants fixed by measurements in noncombusting flows. Use of the laminar flamelet method to treat scalar properties yielded reasonably good predictions of mean properties. Turbulence predictions were less satisfactory, generally underestimating fluctuation levels and Reynolds stresses in highly buoyant regions of the flows. Measurements indicated significant anisotropy of turbulence properties in the same regions. These findings suggest the need for multistress closure to adequately model turbulence properties in buoyant flames.

Laser Doppler Anemometry Measurement in and around a Turbulent Buoyant Flame

Abstract A necessary element in the development of predictive procedures for the spread of fires in enclosures is a satisfactory description of the turbulent source. The absence of an established experimental data base inhibits the necessary model development specific to fires. Their buoyant, low source momentum mixing characteristics are quite different from the more widely investigated jet flames and require the application of both particular models and measurement techniques. This paper describes velocity measurements in and around a buoyant flame on a 25 cm diameter porous refractory burner, using LDA with photon correlation signal processing. This technique makes all parts of the flame accessible wilhout recourse to any artificial seeding. The air entrainment region, in which velocities are very low and susceptible to disturbance, is readily, and perhaps uniquely, accessible. Closer to the flame. the large scale inhomogeneities which accompany buoyant flame motion pose important questions of seeding ...

Tracer Studies of Jets and Diffusion Flames in Cross-Flow

Abstract SO2 was used as a tracer gas in an experimental study of turbulent air jets and propane diffusion flames in cross-flow. Measurements of the SO2 concentration field were used to obtain data on the shape and size of the bent-over plumes in the vortex zone, between 60 and 220 diameters downwind of the discharge. Data on the variation of plume cross sectional area with distance led to estimates of the variation of entrainment with distance. It appears that for turbulent diffusion flames in cross-flow, in which both initial jet momentum and flame buoyancy are significant influences, the trajectory can be expressed by a power law of the form [Ztilde]=M X¯ n in which the distances are nondimensionalized by the momentum length scale and n is a function of the Froude number. Even though the wind tunnel was too short to permit it, the SO2 tracer technique seems to be useful also for studies of the buoyant plume several hundred discharge diameters downwind of the flame.

Measurements of a conserved scalar in turbulent jet diffusion flames

The light scattering technique is used to measure the statistics of a conserved scalar in a hydrogen jet diffusion flame. Measurements of the means, root mean square fluctuations and probability density functions are made for flames at two Froude numbers. The results are presented as conventional and as Favre quantities. Conventional means are near sampled composition measurements but Favre means drop considerably faster on the centreline. The probability density functions reveal that in the buoyant turbulent flame the profiles are intermittent and far from Gaussian downstream. Favre rather than conventional probability density functions in the flames are found to resemble isothermal data.

Influence of Buoyancy on Turbulent Hydrogen/Air Diffusion Flames

Abstract A model which treats the axisymmetric turbulent mixing of co-flowing streams, including non-equilibrium chemistry, is applied to the prediction of buoyant H2/air diffusion flame properties. The model, which employs the Donaldson/Gray eddy viscosity formulation, is first tested against the nonbuoyant laboratory H2/air flame data of Kent and Bilger. It is shown that centerline temperatures and species mole fractions can be predicted to good accuracy. However, the calculated rate of radial transport of energy and mass is greater than that measured in the laboratory experiments. The results of a series of parametric calculations on buoyant flames and comparisons with buoyant flame length data show that: (1) flame properties scale with nondimensional distance for Froude numbers (Fr) greater than about 106, (2) buoyancy significantly affects temperature decay rates downstream of the location of maximum temperature (after all the H2 has burned), and (3) the predicted influence of Fr on buoyant flame lengths is consistent with the available data.

Water-Spray Wellhead Fire Protection Systems For Offshore Structures

Water-spray systems offer a simple way of cooling and protecting wellheads exposed to high temperatures. Field tests were conducted to determine the capability of water sprays in protecting wellheads from overheating when exposed to large gas, condensate, and crude-oil flares. Results demonstrate the ability of a properly designed, self-contained, water-spray system to protect offshore-platform wellheads during a fire. Introduction Offshore-platform fires have revealed the vulnerability of wellhead flanges to high temperatures. After relatively short periods of exposure to direct flame or radiation from a nearby fire, shut-in wellheads began to leak. In some cases, these leaking wells then became new sources of fuel for the fire. Laboratory experiments have closely defined the operating temperature limit (about 850 degrees F) of API-specification wellhead flanges and standard bolting, thereby confirming field observations. In addition, these tests revealed the limited improvement in high-temperature "time to leak" performance that can be achieved with high-temperature, creep-resistant bolting. Even with exotic bolting materials, API-specification flanges leaked within a few hours at temperatures above 1,200 degrees F. Protection of wellheads against high temperatures can be achieved in a variety of ways, such as the use of insulation, flame barriers, or water sprays. The purpose of this investigation was to determine the design criteria for a self-contained, on-board, water-spray system. Such a system would respond automatically to fire on the platform and would protect the wellheads from overheating for an intermediate period of time (about 36 hours). During that time, it is anticipated the fire would burn out or that additional fire-fighting equipment would arrive to combat a prolonged fire, Using the information developed by this investigation, water-spray systems can be designed for multiwell production platforms requiring this type of fire protection. The limited objective of this type of system protection. The limited objective of this type of system must be kept in mind. The purpose is not to extinguish the fire or to protect the entire structure. Rather, the water spray is intended to cool and thereby assure the pressure-containment capability of the wellheads. pressure-containment capability of the wellheads. Background Flame temperatures and total heat transfer rates are not absolutes for large gas and oil flares. The flame temperature is a function of heat content and volatility of the fuel, the quantity of air or oxidant available, and the mixing characteristics of the flame. In turn, heat transfer to an object in or near a large flare is a dynamic phenomenon dependent on flame temperature, target temperature and surface emissivity, luminosity and size of the flare, local convection at the target surface, and view factor. Theoretical flame temperatures are unrealistically high. Typical flame temperatures for gas or oil flares range from 1,500 to 2,500 degrees F. Total heat transfer to a cold target (200 to 300 degrees F) engulfed in thick, luminous, buoyant flames has been measured at 12,000 to 46,000 Btu/sq ft/hr. This compares with heat transfer rates of 50,000 to 100,000 Btu/ sq ft/hr achieved in boiler furnaces .3 Convective heat transfer to a surface engulfed in flame primarily depends on flame temperature and turbulence at the surface. Hence, its contribution to total heat transfer is relatively independent of fuel, provided the fuel does not wet or blacken the surface. However, radiant heat transfer is highly dependent on flame luminosity. JPT P. 323

Prediction of the Height of Turbulent Diffusion Buoyant Flames

Abstract A mathematical model of a turbulent diffusion buoyant flame based on a number of simplifying assumptions is presented. The model predicts the general characteristics of a free-burning fire in both the region where combustion is occurring and the hot gas plume formed above the combustion section. The solution yields the mass-flow rate as a function of height which can be directly related to the excess air. It was found that the height at which 400% excess air has been entrained corresponds to the visible flame height according to data taken in our laboratory as well as that presented by a number of other workers.

Radiative heat transfer from a turbulent diffusion buoyant flame with mixing controlled combustion

A mathematical model of a turbulent buoyant diffusion flame is postulated with a number of simplifying assumptions in order to calculate the radiative emission from the flame. The radiative interchange between the flame and a plane surrounding its base is determined assuming the substance emitting radiation in the flame is a grey emitter. From this radiative distribution it is possible to determine the radiative heat flux to the liquid fuel which is vaporizing to feed the flame. A graphical solution is presented which yields the rate of burning of a liquid fuel of given physical properties in a fixed diameter fuel source. The predicted burning rates agree reasonably well with some data obtained from large open pan liquid fires.