Average Burning Rate Research Articles

Multiple fire interactions: A further investigation by burning rate data of square fire arrays

This paper presents the first effort to explore the spatial distributions of the burning rates in group fires consisting of a large number of fire points, by analyzing burn-out time data from experimental square fire arrays ranging from 3×3 to 15×15. A new concept termed fire layer is introduced and defined to characterize the spatial locations of fire points by which the complex spatial variations of burning rates, under different conditions, are analyzed and physically interpreted. Analysis shows that the fire layer burning rates vary from outer to inner in definite nonlinear modes. This indicates that the two fire interaction effects, heat feedback enhancement and air supply restriction, involve distinct spatial fluctuations in fire arrays. The spatial fluctuations of the two interaction effects are significantly affected by the two major parameters, fire spacing and fire array size. Definite spatial regions and parameter ranges for the spatial fluctuations and high competitions of the two interaction effects are clearly distinguished. It is demonstrated that the average burning rates of all fire layers involve consistent variations versus fire spacing or fire array size, especially with high comparability to the entire fire array. It is found that by varying fire spacing, the average burning rates for all fire layers vary linearly versus the fire area ratio, within the same ranges as the entire fire array, while there exist different fluctuation modes of fire layer burning rates with respect to fire array size. Furthermore, analysis shows that the burning rates of all fire layers will be significantly affected by fire merging when it occurs. Finally, a new approach is presented to simulate fire propagation among discrete fuel sources, by which the positive effect of the surrounding new fire points on the burning rates of the original ones is definitely indicated.

Microgravity droplet combustion: effect of tethering fiber on burning rate and flame structure

Droplets tethering on fibers has become a well established technique for conducting droplet combustion experiments in microgravity conditions. The effects of these supporting fibers are frequently assumed to be negligible and are not considered in the experimental analysis or in numerical simulations. In this work, the effect of supporting fibers on the characteristics of microgravity droplet combustion has been investigated numerically; a priori predictions have then been compared with published experimental data. The simulations were conducted using a transient one-dimensional spherosymmetric droplet combustion model, where the effect of the supporting fiber was implicitly taken into account. The model applied staggered convective flux finite volume method combined with high-order implicit time integration. Thermal radiation was evaluated using a statistical narrow band radiation model. Chemical kinetics and thermophysical properties were represented in rigorous detail. Tether fiber diameter, droplet diameter, ambient pressure and oxygen concentration were varied over a range for n-decane droplets in the simulations. The results of the simulations were compared to previously published experiments conducted in the Japan Microgravity Center (JAMIC) 10 second drop tower and the NASA Glenn Research Center (GRC) 5.2 second drop tower. The model reproduces closely nearly all aspects of tethered n-decane droplet burning phenomena, which included droplet burning history, transient and average burning rate, and flame standoff ratio. The predictions show that the presence of the tethering fiber significantly influences the observed burning rate, standoff ratio, and extinction.

Correlations of Uncertainties of Composite Propellant Strand Burner Burning Rate Measurement for Quality Control

AbstractThis study acquired, classified, and analyzed more than 3500 repeated‐measured steady state strand burner burning rate data from our quality control data bank as well as from open literature. The large size of consistent data from our resource were employed for the construction of a model that correlates burning rate standard deviations with average burning rates for both within‐batch lots and among‐batch lots. An increase in standard deviations with burning rates was observed for both correlations. Both correlations exhibit an R2‐statistic larger than 0.82 within burning rate range of 3.4–38.6 mm s−1, and both correlations provide predictions in good agreements with some good quality published data. These two correlations may serve as feasible burning rate standard deviation tolerance reference when conducting composite propellants production quality control or burning rate data reproducibility checkup. Moreover, the confidence limits of parameters from the derived within‐batch correlation equation allow assessing the maximum pressure‐exponent uncertainties within selected burning rate range, thus provide insightful considerations to pressure‐exponent tolerance assignment for propellants under development or production.

Experimental studies on burning behaviour of liquid sodium in a shallow pool

A hypothetical leak of liquid sodium in a fast breeder type of reactor could result in potential fire hazards. It can be shown that for typical leak rates of 10,000–50,000 kg/h, a stable, shallow pool will be formed in a very short time compared to the duration of the fire ( Subramani et al., 2009). As a follow-up, in the present work, the experimental study of the burning characteristics of sodium pools of shallow depth (of 12 and 16 mm) under controlled environmental conditions is reported. It is found that the combustion of liquid sodium in a shallow pool can be very different when compared to the combustion in a deep pool. Estimated burning rates revealed that the average burning rate for a shallow pool is higher by a factor of two than the steady burning rate value of 40 kg/h m 2 commonly used in most of the sodium fire safety computer codes. If the prevailing wind conditions are such that the aerosols produced at the flame sheet are prevented from depositing on the pool, then even higher rates of burning can be expected.

The effect of variation in polymer properties on the rate of burning

AbstractThe sensitivity of the burning rate to variations in physical and chemical properties of synthetic polymers has been examined in order to understand the relative importance of the knowledge of these properties. The sensitivity analysis was performed using a numerical pyrolysis model called ThermaKin, which was employed to compute the rate of burning (expressed in terms of mass loss) of a one‐dimensional material object exposed to steady radiative heat. The results of the calculations indicate that the knowledge of decomposition reaction parameters (including the Arrhenius pre‐exponential factor, activation energy, heat, and char yield) is of key importance for prediction of the peak and average burning rates. Copyright © 2009 John Wiley & Sons, Ltd.

Effect of process parameters on pelletisation of herbaceous crops

Straw, sawdust and other herbaceous crops benefit from pelletisation, as the process increases the materials bulk density therefore reducing transportation and storage costs; providing better material feeding with less dust formation. This study investigated a pressure pelletisation method of switchgrass for five types of material preparation: raw cut switchgrass, raw shredded switchgrass, torrefied switchgrass, switchgrass combined with heavy pyrolysis oil and wheat straw. The effects of pelletisation pressure and temperature on the quality of pellets were evaluated in terms of density, mechanical strength and durability. Temperature had a greater effect than pressure on pellet quality, where at elevated temperatures, the lignin present in the biomass softened and acted as a binding agent. The cut switchgrass produced more desirable pellets over shredded switchgrass due to an additional binding effect of intertwined fibres. The torrefaction of grass was not an attractive pre-process as the pellets were very brittle and possessed little mechanical strength and reduced bulk density. At elevated temperatures, with a grass to tar ratio of 2:1, the pellets were twice as strong as pellets made by cut switchgrass. The increased durability was a result of lignin present in the biomass and the heavy oil dispersing inside the pelletiser, which filled in the gaps between the switchgrass fibres upon heating. Finally, the pellets were burnt in a fixed bed combustor and the ignition rate and average burning rates were evaluated. The results have provided an indication to how the pellets would perform compared to other fuels. Future work should focus on the standardisation of herbaceous crop pelletisation and in particular, upper and lower bound limits for the moisture content should be investigated, as moisture was found to have a significant impact on pellet quality.

Global burning rate of square fire arrays: Experimental correlation and interpretation

A quantitative description and interpretation of the global burning rate for square fire arrays consisting of multiple equidistant fires is presented. Altogether 38 experiments of n-heptane fire arrays (3 × 3–7 × 7, and 15 × 15) were performed, with the fire spacing varying within 5–50 cm and the fuel pans being 5 cm in diameter and 2 cm in height. The global average burning rate of fire arrays is extracted from burn-out time data, and is shown to be intimately connected with fire spacing and fire array size in a complex way. Analysis shows that the complexity arises from the competition between two fire interaction mechanisms: heat feedback enhancement and air entrainment restriction. When the fires are close to each other within certain ranges, the effect of heat feedback enhancement is more significant than the air entrainment restriction, inducing more intense burning compared with one single fire with the same fuel area. With increasing fire spacing, the heat feedback decays rapidly than the improvement of air entrainment. The global average burning rate shows complex fluctuations with increase of fire array size, and especially the fire array of 5 × 5 involves remarkable competition between the two mechanisms. Further, it is found that in the cases of fire spacing 0.2–0.5 m, the global average burning rate is linearly correlated with a well-defined dimensionless fire area ratio which characterizes the fuel load. Analysis shows that the linear correlation results from competition between the two fire interaction mechanisms. In such cases, the global burning rate is not apparently affected by the phenomena of fire merging and fire whirls. The deviation from linear correlation in other cases is also discussed. Finally, three regimes, each possessing specific fire interaction mechanisms, are distinguished for the variation of the global average burning rate within the whole range of fire area ratio.

Effect of air preheating and fuel moisture on combustion characteristics of corn straw in a fixed bed

Experiments were carried out on a one-dimensional bench combustion tests rig. The effect of air preheating and moisture level in the fuel on combustion characteristics of corn straw was investigated. The bed temperature distribution and the mass loss of fuel and gas components such as O 2, CO, CO 2 and NO were measured in the bed. The average burning rate and ignition front propagation velocity increased with increasing primary air preheating temperature. The total burning time was shorter under the higher primary air preheating temperature and the higher primary air preheating temperature produced a lower ignition front flame temperature in the bed. The variation of the flue gas O 2, CO and CO 2 concentrations with time was more intensive at a higher primary air preheating temperature during the ignition front propagation period and the char oxidation period. With the increase of the fuel moisture, average burning rate and ignition front propagation velocity decreased. As the fuel moisture was less than 30.71%, with the increase of the fuel moisture, residual mass loss rate decreased and ignition front flame temperature increased at a fixed air flow rate. Drier fuels resulted in fuel-rich combustion and higher CO concentration. The NO concentration decreased with increasing the moisture level in the fuel.

Effect of Corn Stalk Length on Combustion Characteristics in a Fixed Bed

The effect of corn stalk length on combustion characteristics in a fixed bed was investigated. The corn stalk was 20−70 mm long. Experiments were carried out using a one-dimensional bench combustion test rig. The bed temperature distribution and the mass loss of fuel and gas components, such as O2, CO, CO2, and NO, were measured in the bed. It was found that the average burning rate and ignition front propagation velocity decreased with an increasing fuel particle length. The total burning time was shorter for shorter particles, and shorter fuel particles produced a higher ignition front temperature in the bed under the same operating conditions. The high-temperature zone for shorter particles was closer to the grate than that for longer fuel particles. The variation with time of the flue gas O2, CO, and CO2 concentrations was less intensive for longer particles during the ignition front propagation period and the char oxidation period. Shorter fuel particles resulted in fuel-rich combustion and higher CO...

An Epidemiological and Risk Analysis of Virginia Workers' Compensation Burn Claims 1999 to 2002: Identifying and Prioritizing Preventive Workplace Interventions

This study examines workers' compensation burn claims from Virginia to assess risk factors and costs associated with occupational burn injuries. Virginia workers' compensation burn claims for the period of 1999 to 2002 were analyzed. Claim rates were determined by using the Bureau of Labor Statistics' Current Population Survey for the working population of Virginia. There were 5810 burn claims reported for the 4-year period examined, with the average burn rate estimated to be 4.3 per 10,000 workers. The total cost of claims was found to average $11,705,939 per year and $8059 per claim. There was over a sixfold increase for burn rates on weekends relative to weekdays. Burns are a common injury experienced by workers and are often severe. Assessing personnel issues affecting weekend employees may lead to valuable preventive interventions to reduce burn risk.

The influence of low atmospheric pressure on carbon monoxide of n-heptane pool fires

Qualitative theoretical analysis about air pressure influence upon the gas concentration of a fire plume was given, different scale n-heptane pool fires were conducted in a small and a standard compartment room in Lhasa and Hefei, respectively. The experimental results show that, in Lhasa, the average mass burning rates in the small room and the standard room both decrease, burning time increases at about 53% in small room and 45% in standard room more than in Hefei. Whereas for maximum changes of CO concentration, in the small room, in Lhasa, CO concentrations reach about twice bigger peak values at larger increase rates than in Hefei. While in the standard room, in Lhasa and Hefei, there are no significant changes for CO concentration, which agrees well with the theoretical analysis results.

Combustion characteristics of different parts of corn straw and NO formation in a fixed bed

Experiments with five samples of corn straw were carried out on a one-dimensional bench combustion test rig. The bed temperature distribution and the mass loss of fuel and gas components such as O 2, CO, CO 2 and NO were measured in the bed. The combustion of corn straw occurred in two stages, ignition front propagation and char oxidation. The average burning rate increased with an increase in the primary air flow until a critical point was reached, beyond which a further increase in the primary air flow resulted in a decreased burning rate. The mean concentration of NO reached a minimum value and then increased with increased primary air flow. The time taken for the drying front to reach the bottom of the bed was 800 s, 700 s, and 500 s; the temperatures in the high bed temperature zones were 900–935 °C, 800–850 °C and 700–743 °C; and the maximum concentrations of NO were 725 ppmv, 1287 ppmv, and 2730 ppmv, for whole corn stalks, hollow corn stalks and flaked corn stalks, respectively. The maximum concentrations of CO and NO were quite different between samples. There was only one peak in the distribution of NO concentration for sample B, but there were two peaks for whole corn stalks and sample A.

An analytical study of droplet combustion under microgravity: Quasi-steady transient approach

An analytical model based on an assumption of combined quasi-steady and transient behavior of the process is presented to exemplify the unsteady, sphero-symmetric single droplet combustion under microgravity. The model used in the present study includes an alternative approach of describing the droplet combustion as a process where the diffusion of fuel vapor residing inside the region between the droplet surface and the flame interface experiences quasi-steadiness while the diffusion of oxidizer inside the region between the flame interface and the ambient surrounding experiences unsteadiness. The modeling approach especially focuses on predicting; the variations of droplet and flame diameters with burning time, the effect of vaporization enthalpy on burning behavior, the average burning rates and the effect of change in ambient oxygen concentration on flame structure. The modeling results are compared with a wide range of experimental data available in the literature. It is shown that this simplified quasi-steady transient approach towards droplet combustion yields behavior similar to the classical droplet theory.

Burn-out time data analysis on interaction effects among multiple fires in fire arrays

This paper gives an experimental and methodological investigation on the behaviors of square fire arrays which are composed of 3 × 3 to 7 × 7 n-heptane fires initiated from fuel pans of 5 cm in diameter and 2 cm in height. It is intended to develop a burn-out time (BOT) data analysis method to analyze the interaction effects (which may induce fire merging and fire whirls) among the multiple fires. In 26 fire tests the fire point spacing D varied from 20 to 50 cm for each array size and in several cases shear flow was added from one side of the array. By considering the flame height L a reasonable critical condition for initiation of fire merging was implied to be D/ L = 0.29 − 0.34, which is independent of the fire array size and fire point spacing. By burn-out time data the Interaction index I ( m) and Interaction link index A ( m, n) were defined to characterize the fire interactions. The assumptions essential to solve the equation system of I ( m) = ∑ n A ( n, m) were examined in detail, whereby the equation system was solved. The analysis showed that the burn-out time data analysis realizes a quantitatively reasonable comparison of the fire interaction effects, thus indicating that it is reasonable to regard the burn-out time as a measure for the average burning rate for each specific fire point. An apparent criterion of BOT( m)/BOTR = 0.5 was summarized to identify whether any fire point m will be completely involved in fire merging (where BOTR is the burn-out time of the free burning reference fire point). It was implied that the interaction effect imposed on any fire is mainly ascribed to its adjacent four fires. The effects of shear flow to fire burning and occurrences of fire whirls were also discussed.

Straw combustion in a fixed bed combustor

Straw and herbaceous energy crops are key biomass materials for greenhouse gas neutral energy production. Combustion of straw and two herbaceous crops was investigated in a fixed-bed reactor for a range of air flow-rates (234–1170 kg/m 2 h). The fixed bed tests simulate the moving bed combustion where the distance along a grate corresponds to the time on the fixed bed. Measured temperatures, gas composition and mass loss were used to evaluate the combustion characteristics in terms of ignition front speed, burning rate, percentage of mass loss and the equivalence ratio. The average burning rates of herbaceous fuels reached a peak of 220–250 kg/m 2 h at air flow rates of 700–900 kg/m 2 h. An overlap of the two distinct periods of combustion was observed at high air flow rates, as the air supply was sufficient to simultaneously burn the char above the ignition front. When uncut straw was burned, less uniform packing of the long fibres increased the effect of air channelling and led to irregular propagation of the ignition front.

Single droplet combustion of decane in microgravity: experiments and numerical modelling

This paper presents experimental data on single droplet combustion of decane in microgravity and compares the results to a numerical model. The primary independent experiment variables are the ambient pressure and oxygen mole fraction, pressure, droplet size (over a relatively small range) and ignition energy. The droplet history (D2 history) is non-linear with the burning rate constant increasing throughout the test. The average burning rate constant, consistent with classical theory, increased with increasing ambient oxygen mole fraction and was nearly independent of pressure, initial droplet size and ignition energy. The flame typically increased in size initially, and then decreased in size, in response to the shrinking droplet. The flame standoff increased linearly for the majority of the droplet lifetime. The flame surrounding the droplet extinguished at a finite droplet size at lower ambient pressures and an oxygen mole fraction of 0.15. The extinction droplet size increased with decreasing pressure. The model is transient and assumes spherical symmetry, constant thermo-physical properties (specific heat, thermal conductivity and species Lewis number) and single step chemistry. The model includes gas-phase radiative loss and a spherically symmetric, transient liquid phase. The model accurately predicts the droplet and flame histories of the experiments. Good agreement requires that the ignition in the experiment be reasonably approximated in the model and that the model accurately predict the pre-ignition vaporization of the droplet. The model does not accurately predict the dependence of extinction droplet diameter on pressure, a result of the simplified chemistry in the model. The transient flame behaviour suggests the potential importance of fuel vapour accumulation. The model results, however, show that the fractional mass consumption rate of fuel in the flame relative to the fuel vaporized is close to 1.0 for all but the lowest ambient oxygen mole fractions.

The Thick And Thin Of Burning Nano-clay- Nylon Composites

The flammability properties of pure Nylon 6 and clay additives of 2 and 5% in thicknesses ranging from 1.6 to 24 mm were examined. Data were obtained over a range of radiant heat fluxes (17 to 55 kW/m2). The heats of combustion did not change with loading, and were 28 +/- 1 kJ/g. The critical heat flux for ignition also did not significantly change as it decreased from 17.7 to 16.0 for pure nylon to 5% clay addition. However, the addition of the clay could increase the ignition time by 30 to 100% over the pure nylon. This is believed to be due to the increased char residue and decrease in mass loss rate accordingly. The residual char-like yield was nearly identical to the clay loadings. The overall average mass loss rate was reduced by up to 50% for 5% clay over pure nylon for a given heat flux and thickness. For the clay nano-composites, the average burning rate increased as the thickness decreased. A theoretical model qualitatively explains the effect of thickness.

A diffusion model for particle mixing in a packed bed of burning solids

Particle mixing caused by grate movement in a packed bed of solids is an important process for biomass combustion and waste incineration. In this paper, a diffusion model for particle mixing in a burning bed is proposed and the related diffusion coefficient is measured. The diffusion model was incorporated into a combustion model for waste incineration in an actual full-scale bed and numerical calculations were carried to assess the effect of different mixing levels on the burning characteristics of the furnace. In-bed measurement of temperature, oxygen concentration and particle movement was also made using a special electronic device. It is found that the modelled flame front reaches the bed bottom at an earlier stage for a higher level of particle mixing; the average burning rate ranges from 0.05 to 0.13 kg/m 2 s and the mass loss rate for a higher mixing level can be twice of that for a lower mixing level. However, excessive mixing can cause significant delay in ignition or even extinction of the bed combustion; the obtained local air to fuel stoichiometric ratio covers a range from sub-stoichiometric (0.6 for the highest mixing level) to super-stoichiometric (1.6 for the lowest mixing level); the carbon in ash ranges from 3.5 to 10.5%; the most reasonable range of the particle-mixing (diffusion) coefficient is from 1.8 to 6.0 cm 2/min for a full-scale bed, according to the calculation.

A Qualitative Reactivity and Isotopic Assessment of Fuels for Lead-Alloy-Cooled Fast Reactors

Various methods have been proposed to transmute and thus consume the current inventory of transuranic waste from spent light water reactor (LWR) fuel and plutonium from weapons. We discuss the neutronics performance of nonfertile, fertile metallic, and fertile nitride fuels loaded with 20 to 30 wt% LWR-grade plutonium plus minor actinides and burned in an open-lattice lead-alloy–cooled fast reactor, with an emphasis on the fuel cycle life and spent fuel isotopic content. As a comparison, similar fuel was also studied in a sodium-cooled fast reactor. Our calculations show that the average actinide burn rate for fertile-free fuel is similar for both the sodium- and lead-bismuth–cooled cases, ranging from 1.02 to 1.16 g/MWd, compared to a typical LWR actinide generation rate of 0.303 g/MWd. In addition, our calculations show that the effective full-power days (EFPDs) of operation (or equivalent reactivity-limited burnup) using fertile fuel can extend beyond 20 yr, and the average actinide burn rate is similar for both the sodium- and lead-bismuth–cooled cases, ranging from 0.5 to 0.9 g/MWd. Using the same parameters (i.e., a large pitch-to-diameter ratio, same linear power, and fissile/fertile loading, etc.), the lead-alloy-cooled cases had an EFPD that was 18% to several times greater than their sodium-cooled counterparts. However, tight sodium-cooled lattices are equivalent to the looser lead-alloy lattices in terms of beginning-of-life excess reactivity.

Detailed modeling of an isolated, ethanol droplet combustion under microgravity conditions

Simulations of ethanol droplet combustion in microgravity have been conducted using a transient, finite-element, reactive flow model that includes a detailed description of gas-phase chemical kinetics and transport as well as non-luminous radiation. The results of the simulations are compared with a wide range of experimental data available in the literature. The experimental targets included ground-based, suspended droplet experiments, drop-tower experiments, and space-based experiments from the Fiber-Supported Droplet Combustion-2 project. The presented model is shown to reproduce closely nearly all aspects of ethanol droplet burning phenomena, such as droplet burning history, average burning rates, radiative extinction behavior, and kinetics of water accumulation in the liquid phase. Both the experimental and computational results reported in the present study suggest that for ethanol droplets burning in air at 1 atm, the radiative extinction diameter has a value of ∼4 mm.