Steady-state Burning Rate Research Articles

Combustion of hydrogen peroxide gel and micron-aluminum mixtures

A newly reported oxidizer, hydrogen peroxide encapsulated in silica xerogel (hydrogen peroxide gel), is prepared and applied to the combustion of aluminum particles in this study. Linear and mass burning rates of the highly viscous mixture of hydrogen peroxide gel and micron-aluminum (μAl) were measured as a function of mixture composition, pressure, and particle size. Steady-state burning rates were obtained at room temperature using an optical vessel for a pressure range of 3.6–12.0 MPa in argon, an oxidizer-to-fuel weight ratio (O/F) from 0.8 to 2.0, and particle diameters between 1 and 12 μm. The homogeneous mixture of hydrogen peroxide gel and μAl was able to be ignited over a wide O/F range of 0.8–2.0, even at low pressures (e.g., 3.6 MPa). At a nominal pressure of 3.6 MPa, mass burning rates ranged between 4.45 g·cm−2·s−1 (O/F = 0.8) and 1.26 g·cm−2·s−1 (O/F = 2.0), which corresponded to linear burning rates of 35.1 and 8.5 mm·s−1, respectively. In the pressure range of 3.6–10.0 MPa, linear burning rates of the oxidizer-rich mixture (O/F = 1.7) were linearly increased with pressure increasing and the pressure exponent (n) was 1.05. The combustion reaction was kinetically controlled within this pressure range. However, when the pressure beyond ∼10 MPa, there was a transition in pressure dependence and the burning rate tended to be constant. In addition, burning rates of the mixture were inversely proportional to Al particle diameter, increasing by 3.06 times when the particle diameter was decreased from 11.02 to 1.84 μm at stoichiometric ratio.

Energy balance in medium-scale methanol, ethanol, and acetone pool fires

Several series of measurements were made to characterize medium-scale pool fires steadily burning in a well-ventilated, quiescent, open environment. Time-averaged local measurements of radiative and total heat flux were made in steadily burning methyl alcohol (methanol; CH3OH), ethyl alcohol (ethanol; C2H5OH), and acetone ((CH3)2 CO) pool fires. The fuel lip height in a water-cooled stainless-steel burner was maintained at 10 mm. Schmidt-Boelter heat flux gauges were used to measure the radiative emission to the surroundings. The total heat flux directed towards the pool surface was measured using a Gardon gauge positioned just above the pool surface. A previously developed method was used to calculate the convective heat flux to the pool surface, allowing estimation of the radiative flux, which agreed within experimental uncertainty with a previous measurement in the methanol pool fire. The steady-state mass burning rate was measured using a load cell, and the heat release rate was measured in the exhaust using calorimetry. The energy balance for each of the fires was determined. The results showed that both radiation and convection play significant roles in these pool fires. Radiation was the dominant mechanism of heat feedback to the fuel surface accounting for 68% to 88% of the energy, while enthalpy convected in the plume represented 68% to 78% of the fire's total energy, far exceeding radiative emission to the surroundings.

Microwave plasma enhancement of multiphase flames: On-demand control of solid propellant burning rate

This effort explores the microwave-supported plasma enhancement of an aluminized, ammonium perchlorate composite solid propellant flame. The technique is enabled through novel alkali metal doping, which provides increased levels of ionization and allows efficient microwave energy deposition to the flame structure and subsequent perturbation of the steady-state propellant burning rate. Three potential modes of energy deposition are identified in composite propellants: (1) plasma-enhancement promoted by the presence of sodium, (2) strong absorption by high temperature metallic oxide products, and (3) direct absorption of energy by the propellant burning surface condensed phase. Equilibrium calculations are used to identify propellant compositions with high free-electron populations, and solution of the Boltzmann equation using the BOLSIG + code is used to show the significant effects of sodium doping on microwave absorption of the flame. Experimental emission spectroscopy and imaging of the microwave-enhanced propellant flame structure show evidence of these mechanisms. Propellant burning rates can be increased by up to 60% through the application of 1 kW, continuous 2.46 GHz microwave radiation. Propellant doping with 3.5% by weight of sodium nitrate shows significant improvement in the effectiveness of microwave application in modifying the propellant burning rate. This technique provides high levels of dynamic propellant combustion control from a solid-state system. The on-demand control of energetic material burning rates through low level doping and microwave irradiation is a promising technique which could lead to the development of a new class of ‘smart’ dynamically controllable energetic materials.

Unsteady chemical kinetics behavior of AP/HTPB propellant with micro-scale model

ABSTRACTThe heterogeneous AP/HTPB propellant is a mixture of periodical units comprised of two AP (ammonium perchlorate, NH4ClO4) particles separated by a HTPB (hydroxyl-terminated polybutadiene) layer. So, the flame structure and the respond of the burning rate of such a material is resulted by an incredibly complex set of concurrent reactions taking place in the gas, liquid, and solid phases of a heterogeneous mixture, especially under unstable environment conditions. This article starts with a one-dimensional unstable chemical kinetics model to fix the respond of the burning rate to unstable conditions. Then, a two-dimensional micro-scale combustion model of ‘sandwich’ AP/HTPB propellant under unsteady-state conditions is explored to investigate the characteristic of microflame structure of this material and parameter distributions. It is demonstrated that for the periodic sandwich geometry a decrease in the environment pressure from 3 MPa to 2 MPa can sharply decrease the transient burning rate that is lower than the steady-state burning rate under corresponding pressure. Moreover, the temperature of the burning surface decreases when the depressurization process proceeds and the extinguishment will be considered to occur as the temperature of the burning surface is less than critical extinguishment temperature of 700 K. It is obtained from the two-dimensional, unsteady-state model that, when the depressurization rate is higher, the temperature of the burning surface decreases more sharply and the extinguishment phenomenon may occur.

Use of immersed conductive objects to enhance the burning rate of hydrocarbon pool fires

This study investigates the ability of thermally conductive nonflammable objects to enhance the steady state burning rate of hydrocarbon pool fires with the ultimate goal of designing a burner for faster clean-up of hazardous spills in offshore and other remote environments. In the initial set of small-scale experiments, a vertical 1cm diameter solid aluminum cylinder with varying lengths is placed centrally in a 10cm diameter pan fed with a continuous supply of hexane burning in quasi-steady state. The upper part of the cylinder, engulfed in the flame, heats up, and conducts heat to the lower submersed part, that transmits heat to the liquid below the pool surface. Tests reveal that the submersed lower part of the cylinder heats up sufficiently to sustain nucleate boiling, significantly increasing the burning rate, when compared to the baseline pool fire, where vaporization is achieved solely by radiative and convective heat transfer at the pool surface. The effects of cylinder length and multiple cylinders on burning rate are also characterized. To determine if the concept can also be applied to oil spill clean-up, a set of large scale tests is performed by burning crude oil/water mixtures in a 1-meter diameter pool. These tests show that burners with thermally conductive cylinders can be designed to effectively dispose of crude oil and oil/water mixtures.

Experimental study of burning of methanol fed porous spheres in grid generated turbulent field

Experimental investigation of the effect of flow turbulence on the steady state burning of methanol is reported. A vertical air tunnel has been mounted with a grid at its exit plane in order to generate turbulence in the free jet stream. The flow field has been characterized using a hot-wire anemometer. Mean and fluctuating flow velocities and integral scale have been measured at an axial location of around 2 times its exit diameter (D). Three types of grids have been used. Classical porous sphere experiments have been carried out to analyze the steady-state burning rate of methanol over the surface of an inert sphere having constant diameter. Experiments have been done at atmospheric pressure under ambient temperature and normal gravity conditions. A porous sphere is positioned at an axial location of 2D, where the approaching flow has been characterized in detail. Results show that the burning rate as well as the flame stability are greatly influenced by the free stream turbulence. The ratio of turbulent time scales and the chemical time scales for grid mounted cases have been estimated from the integral scale, root mean square velocity fluctuation, flame stand-off distance and the vapor blowing velocity. Empirical expression relating the normalized burning rates to diffusion scale based Damköhlar number has been presented. A correlation for Sherwood number as a function of Reynolds number and turbulent intensity has also been proposed.

Predicting the heat release rates of liquid pool fires in mechanically ventilated compartments

In this paper we perform predictive simulations of liquid pool fires in mechanically ventilated compartments. We show that steady state burning rates are accurately predicted using a detailed model for the liquid phase heat transfer. The effect of lowered oxygen vitiation on the burning rate of pool fires is correctly captured. Simulations were done using the Fire Dynamics Simulator and the experiments considered were conducted in the OECD PRISME project. The main difference between the present study and previous simulation studies is the use of a detailed liquid evaporation model and the direct calculation of the vitiation and thermal environment interactions through the CFD solver.

Assessement of FDS 6 Simulation Results for a Large-Scale Ethanol Pool Fire

ABSTRACTA numerical study is presented on the prediction of the burning rate (and thus the heat release rate) of a large-scale ethanol pool fire (0.81 m × 0.70 m) using the Fire Dynamics Simulator (FDS 6). The steady-state burning rate is reasonably well predicted and the results are insensitive to the mesh resolution when cells of less than 5 cm are used. Furthermore, it is shown numerically that the lip height could increase the burning rate up to 50% in comparison to the flush configuration (i.e., no lip) as found in a previous experimental study. Finally, a simplified 1D calculation confirms that the incident radiative flux on the fuel surface is the dominant heat transfer mode and contributes by approximately 70% to the fuel burning rate.

Assessment of the Burning Rate of Liquid Fuels in Confined and Mechanically-Ventilated Compartments using a Well-Stirred Reactor Approach

The objective of this work is to provide a ‘support tool’ to assess the burning rate of a pool fire in a well-confined and mechanically-ventilated room using a single-zone model based on conservation equations for mass, energy and oxygen concentration. Such configurations are particularly relevant for nuclear facilities where compartments are generally sealed from one another and connected through a ventilation network. The burning rates are substantially affected by the dynamic interaction between the fuel mass loss rate and the rate of air supplied by mechanical ventilation. The fuel mass loss rate is controlled by (i) the amount of oxygen available in the room (i.e. vitiation oxygen effect) and (ii) the thermal enhancement via radiative feedback from the hot gas to the fuel surface. The steady-state burning rate is determined by the ‘interplay’ and balance between the limiting effect of oxygen vitiation and the enhancing effect of radiative feedback. An extensive sensitivity study over a wide range of fuel areas and mechanical ventilation rates shows that a maximum burning rate may be obtained. For the studied HTP (Hydrogenated Tetra-Propylene) pool fires, the maximum burning rate is up to 1.75 times the burning rate in open air conditions.

Combustion of micro aluminum–water mixtures

An experimental investigation on the combustion behavior of micro-sized aluminum (μAl)–water mixtures was conducted. It was easily ignited and self-deflagrated on μAl and liquid water when using a paper shell tube. Linear burning rates of quasi-homogeneous mixtures of μAl and liquid water as a function of pressure, mixture composition, density and environment gas medium were measured. Steady-state burning rates were obtained at room temperature using a windowed vessel for a pressure range of 1–80bar in a nitrogen atmosphere, particle size of 0.5×30×30μm and overall mixture equivalence ratios from 0.67 to 2.0. The pressure exponent was obtained as 0.47 at room temperature and compared to the case of nano-sized aluminum (nAl) and liquid water. When a wire was inserted into the sample, for increasing local heat transfer, burning rates were found to be faster.

Functionalized Graphene Sheet Colloids for Enhanced Fuel/Propellant Combustion

We have compared the combustion of the monopropellant nitromethane with that of nitromethane containing colloidal particles of functionalized graphene sheets or metal hydroxides. The linear steady-state burning rates of the monopropellant and colloidal suspensions were determined at room temperature, under a range of pressures (3.35-14.4 MPa) using argon as a pressurizing fluid. The ignition temperatures were lowered and burning rates increased for the colloidal suspensions compared to those of the liquid monopropellant alone, with the graphene sheet suspension having significantly greater burning rates (i.e., greater than 175%). The relative change in burning rate from neat nitromethane increased with increasing concentrations of fuel additives and decreased with increasing pressure until at high pressures no enhancement was found.

Analysis of the possibility of steady-state operation of a small-size solid-propellant thruster

The possibility of steady-state operation of small-size thrusters is discussed. A more detailed study of the known frequency equation makes it possible to compose a more detailed “chart of instability” for the physical formulation considered. It turns out that combustion in a certain range of parameters v and r (characterizing the dependence of the steady-state burning rate on pressure and the dependence of the surface temperature on initial temperature) is stable in a small-size thruster and unstable in a large-size motor.

A transient pyrolysis model based on the B-number for gravity-assisted flame spread over thick PMMA slabs

We have developed a transient pyrolysis model based on the modified mass transfer number determined from experiments on the steady-state burning rate of vertical PMMA slabs. It allowed satisfactory concordance with experiments on upward flame spread. A good agreement for the rate of spread was also observed for inclination angles below the critical angle.

Combustion characteristics of nanoaluminum, liquid water, and hydrogen peroxide mixtures

An experimental investigation of the combustion characteristics of nanoaluminum (nAl), liquid water (H 2O (l)), and hydrogen peroxide (H 2O 2) mixtures has been conducted. Linear and mass-burning rates as functions of pressure, equivalence ratio ( Φ), and concentration of H 2O 2 in H 2O (l) oxidizing solution are reported. Steady-state burning rates were obtained at room temperature using a windowed pressure vessel over an initial pressure range of 0.24 to 12.4 MPa in argon, using average nAl particle diameters of 38 nm, Φ from 0.5 to 1.3, and H 2O 2 concentrations between 0 and 32% by mass. At a nominal pressure of 3.65 MPa, under stoichiometric conditions, mass-burning rates per unit area ranged between 6.93 g/cm 2 s (0% H 2O 2) and 37.04 g/cm 2 s (32% H 2O 2), which corresponded to linear burning rates of 9.58 and 58.2 cm/s, respectively. Burning rate pressure exponents of 0.44 and 0.38 were found for stoichiometric mixtures at room temperature containing 10 and 25% H 2O 2, respectively, up to 5 MPa. Burning rates are reduced above ∼5 MPa due to the pressurization of interstitial spaces of the packed reactant mixture with argon gas, diluting the fuel and oxidizer mixture. Mass burning rates were not measured above ∼32% H 2O 2 due to an anomalous burning phenomena, which caused overpressurization within the quartz sample holder, leading to tube rupture. High-speed imaging displayed fingering or jetting ahead of the normal flame front. Localized pressure measurements were taken along the sample length, determining that the combustion process proceeded as a normal deflagration prior to tube rupture, without significant pressure buildup within the tube. In addition to burning rates, chemical efficiencies of the combustion reaction were determined to be within approximately 10% of the theoretical maximum under all conditions studied.

A theoretical and numerical evaluation of the steady-state burning rate of vertically oriented PMMA slabs

The steady state burning rate of vertically oriented slabs of poly-methyl-methacrylate (PMMA) is numerically investigated. Model predictions are compared with measurements and results of the laminar boundary layer (LBL) theory. The numerical model provides a solution of the Favre-averaged Navier–Stokes equations coupled with sub-models for turbulence, combustion, soot production and radiation. The modelling of condensed phase processes is based on the one-dimensional heat transfer equation and pyrolysis is treated as a phase change using the latent heat approach. Results show that the pyrolysing region can be divided into three regions. In the laminar part of the flow (Gr x < 4.3 × 107), the predicted normalised burning rate, ṁ″ p x/μ∞, is a power-law function of Gr x with an exponent close to that of the LBL theory, surface re-radiation being the primary source of discrepancies. From the LBL theory for free flow, it is demonstrated that the local burning rate is inversely proportional to the shear velocity gradient. This is globally confirmed by numerical model results. At Gr x = 4.3 × 107 the change in slope of the burning rate observed experimentally, which indicates the end of the laminar flow region, is reproduced numerically. From Gr x = 2.5 × 109 model results show that the surface mass flux of pyrolyzate increases with x, in agreement with experimental data in literature.

Experimental observations on the steady-state burning rate of a vertically oriented PMMA slab

A series of experiments is performed to determine the local burning rate of vertically oriented burning slabs of PMMA (poly(methyl methacrylate)) under steady-state conditions. The burning rate is essential for the determination of flame temperatures, flame lengths, and burnout, all critical parameters to establish fire growth. The experimental procedure consists of a postprocessing of digitized images obtained with three CCD cameras placed on the sides of the PMMA slabs. The cameras are used to establish the displacement of the regressing surface as a function of time. The PMMA/gas interface is characterized by a bubbling layer that scatters light, making the bubbles visible. The video images are processed by establishing a gray-level intensity threshold that defines the depth of the bubbling layer. The local burning rate is then computed from the time derivative of the surface regression. Local burning rates for PMMA slabs, 30 cm high, 3 cm thick, and 2.5, 5, 10, 15, and 20 cm in width are obtained. Three-dimensional effects appear for 2.5-cm-wide slabs. For wider slabs, the local burning rate along the centre line is found not to be width-dependent and to be in good agreement with literature data. Results show that the pyrolyzing surface divides vertically into four regions. In the lowest region ( 0 < x < 4 cm ), the local burning rate increases with the distance from the leading edge, x. For 4 < x < 10 cm , the local burning rate follows a power-law decay. For slabs wider than 2.5 cm, the exponent approximates −0.35. This is higher than that predicted by the two-dimensional laminar boundary layer theory, i.e., −0.25. For 2.5-cm-wide slabs, the local burning rate decreases faster, with a power law dependency of −0.8. From x = 10 to 18 cm, the local burning rate is approximately constant, which is believed to correspond to the laminar/turbulent transition region. In the upper region, the flow becomes fully turbulent and the burning rate increases weakly with x.

Combustion of nano-aluminum and liquid water

An experimental investigation on the combustion behavior of nano-aluminum (nAl) and liquid water has been conducted. In particular, linear and mass-burning rates of quasi-homogeneous mixtures of nAl and liquid water as a function of pressure, mixture composition, particle size, and oxide layer thickness were measured. This study is the first reported self-deflagration on nAl and liquid water without the use of any additional gelling agent. Steady-state burning rates were obtained at room temperature (∼25 °C) using a windowed vessel for a pressure range of 0.1–4.2 MPa in an argon atmosphere, particle diameters of 38–130 nm, and overall mixture equivalence ratios ( ϕ) from 0.5 to 1.25. At the highest pressure studied, the linear burning rate was found to be 8.6 ± 0.4 cm/s, corresponding to a mass-burning rate per unit area of 6.1 g/cm 2 s. The pressure exponent at room temperature was 0.47, which was independent of the overall mixture equivalence ratio for all of the cases considered. The mass-burning rate per unit area increased from ∼1.0 to 5.8 g/cm 2 s for an equivalence ratio range of 0.5–1.25. It varied inversely to particle diameter, increasing by 157% when the particle diameter was decreased from 130 to 50 nm at ϕ = 1.0.

Ballistic properties and burning behaviour of an ammonium perchlorate/guanidine nitrate/sodium nitrate airbag solid propellant

An experimental investigation on the determination of ballistic properties and burning behavior of a composite solid propellant for airbag application was conducted. The experimental results were obtained using a high-pressure optical strand burner. Steady-state burning rates were determined for a pressure range of 20.8–41.5 MPa and initial propellant temperatures of −30 to +80 °C. For the pressure and temperature ranges tested, the temperature sensitivity was on the order of 1 × 10 −3 K −1. The pressure exponent was found to be a function of the initial propellant temperature and was 0.75 at 25 °C. The activation energy and the pre-exponential factor of the Arrhenius equation are 2.735 kcal/mol and 15.06 cm/s, respectively. The pressure deflagration limit for this propellant was found to be in the range of 8.37–8.72 MPa. During combustion, small condensed-phase spherical particles were ejected from the burning surface. The size of the particles decreased with either increasing the pressure or the initial propellant temperature. For pressures below 41.4 MPa, average particle size was on the order of 900 μm, and at 84.4 MPa, the bead size was much smaller, on the order of 300 μm. A chemical analysis on these particles using both the ESEM and the X-ray diffraction method indicated that the material of the beads was mostly sodium chloride with a small amount of silicon-containing compounds.

A laser technology for measurement of solid propellant transient burning rates during rapid depressurization

A laser technique of transient burning rate measurement of solid propellant has been investigated. A photocell detects a laser beam passing through the propellant strand, and the cross-section of the laser beam is regulated so that the laser energy varies linearly along the height of the strand. Using a photo-controlled depressurization system, it is desirable to set up the starting time of depressurization in advance, and the initial rate of depressurization can be easily reproduced. Experimental results show that: (1) for a short time after depressurization begins, the burning rate remains unchanged; (2) during the depressurization process, the burning rate, while continuously decreasing in magnitude, exhibits low frequency oscillations; (3) the amplitude and the frequency of the burning rate oscillations, referenced to a smooth rate–time curve, increases as the depressurization rate increases; (4) for a short time after depressurization, the transient burning rates is higher than the steady state burning rate at the corresponding pressure, and then the transient burning rates oscillate around the quasi-steady rate. The experimental dynamic burning rates are compared with those available by microwave and capacitance methods. The discrepancies in the three experimental results have been discussed.

Smoke spread experiment in a multi-storey building and computer modelling

The CESARE-SMOKE model is a network model which can be used to estimate smoke spread in a multi-storey building when subjected to fire. The model calculates time-dependent temperature and species concentrations at locations remote from the room of fire origin. Spatial variation of physical parameters in a single enclosure with large aspect ratio can also be estimated. This paper describes a series of full-scale fire experiments that were designed to investigate the validity of the model. The experiments, involving steady state burning rates and a number of ventilation conditions, were conducted in a four-storey building. Temperature, pressure, flow velocity, smoke density and species concentrations were measured in various parts of the building. The chimney effect and its influence on temperature distribution in a stair shaft were observed. Comparisons are made between the experimental results and the model predictions. The experimental results are also compared with the predictions of the CFAST two-zone model. Suggestions are made for further improvements to the network model.