Change In Burning Rate Research Articles

Fluctuating Flame from Suspending Ferrimagnetic Core/Shell Al@Fe3O4 Nanoparticles in a Magnetic Field

AbstractCore–shell fuel@oxidizer nanocomposite can combust in oxygen‐starved environment. A ferrimagnetic reactive particle, being successfully engineered, allows for manipulation using an external magnetic field and facilitates target heat or gas production. This research reports on interesting flame dynamics of newly synthesized core–shell Al@Fe3O4 nanoparticles under the influence of a magnetic field. Serving as an oxidizer and a functional ferrimagnetic component, iron oxide nanoparticles (IONPs) are grown in situ on nano‐sized Al (n‐Al) particles. Electron microscopic images demonstrate nearly monodispersed IONPs ≈7 nm decorating the surface of n‐Al. X‐ray diffractogram and X‐ray photoelectron spectroscopy confirm the formation of Fe3O4. Thermal analysis suggests the as‐prepared core–shell particles predominantly go through a solid‐state reaction mechanism that exhibits 30% lower activation energy compared to physically‐mixed nanocomposite (215.0 vs 310.8 kJ mol−1). The core/shell particles can be ignited and combust under laser irradiation with or without the effect of a magnetic field. When suspended in the middle of the tube by the magnetic field, an interesting combustion process is observed, highlighting a grow‐shrink‐grow flame resulted from the interactions between the combustion products and the external magnetic field, as well as backfiring at the bottom of the sample without major changes in burning rate and ignition delay.

On‐demand microwave growth of porosity within a granular composite energetic material: Void formation via a dielectric loss phase change binder additive for propellant burning rate control

AbstractThis study demonstrates, for the first time ever, the ability to grow, in an on‐command fashion, porosity within a granular composite energetic material to effect a change in energy output rate. Specifically, the study investigates the change in burning rates of ammonium perchlorate composite propellants as a result of porosity created in situ via microwave field‐driven volatilization of the low boiling point binder additive, ethylene glycol. Theoretical mass densities were measured before and after microwave irradiation finding that the maximum observed %TMD change for tested propellants is 6 %. Propellants were burned at 1.72 MPa to 6.89 MPa pressures, finding that for all propellants, microwave irradiation produced a change in ballistic characteristics. Most propellant formulations demonstrate acceptable burning rate parameters for use within rocket motors; some exhibited a large change in their pressure exponent as well as slope breaks attributed to the onset of convective burning, while microwave irradiation produced no change in burning rate or density in reference propellants without the additive. Microwave heating simulation results are presented to gain insight into the thermal environment of the propellant during microwave irradiation. These results provide valuable insight into propellant formulations that can have their burning rates (and thus the thrust profile for motor grains) altered after casting via microwave irradiation.

Spatially Directed Pyrolysis via Thermally Morphing Surface Adducts

AbstractCombustion is often difficult to spatially direct or tune associated kinetics—hence a run‐away reaction. Coupling pyrolytic chemical transformation to mass transport and reaction rates (Damköhler number), however, we spatially directed ignition with concomitant switch from combustion to pyrolysis (low oxidant). A ‘surface‐then‐core’ order in ignition, with concomitant change in burning rate,is therefore established. Herein, alkysilanes grafted onto cellulose fibers are pyrolyzed into non‐flammable SiO2 terminating surface ignition propagation, hence stalling flame propagating. Sustaining high temperatures, however, triggers ignition in the bulk of the fibers but under restricted gas flow (oxidant and/or waste) hence significantly low rate of ignition propagation and pyrolysis compared to open flame (Liñán's equation). This leads to inside‐out thermal degradation and, with felicitous choice of conditions, formation of graphitic tubes. Given the temperature dependence, imbibing fibers with an exothermically oxidizing synthon (MnCl2) or a heat sink (KCl) abets or inhibits pyrolysis leading to tuneable wall thickness. We apply this approach to create magnetic, paramagnetic, or oxide containing carbon fibers. Given the surface sensitivity, we illustrate fabrication of nm‐ and μm‐diameter tubes from appropriately sized fibers.

Spatially Directed Pyrolysis via Thermally Morphing Surface Adducts.

Combustion is often difficult to spatially direct or tune associated kinetics - hence a run-away reaction. Coupling pyrolytic chemical transformation to mass transport and reaction rates (Damköhler number), however, we spatially directed ignition with concomitant switch from combustion to pyrolysis (low oxidant). This establishes a 'surface-then-core' order in ignition with concomitant change in burning rate. Herein, alkysilanes grafted onto cellulose fibers are pyrolyzed into non-flammable SiO2 terminating ignition propagation and further pyrolysis. Sustaining high temperatures, however, triggers ignition in the bulk of the fibers but under restricted gas flow (oxidant and/or waste) hence significantly reduced ignition propagation and pyrolysis compared the surface (Liñán's equation). This leads to inside-out degradation and, under felicitous choice of conditions, formation of graphitic tubes. Given the dependence on temperature, imbibing fibers with an exothermically oxidizing synthon (MnCl2) or a heat sink (KCl) abets or inhibits pyrolysis tuning tube-wall thickness. We apply this approach to create magnetic, paramagnetic, or oxide containing carbon fibers. The magnetic fibers are used for rapid filtration of oil from water.

Experimental study on influence of air gap on upward flame spread over discrete fuel

In a real fire scenario, the distribution of fire loads may not be continuous. In this paper, PMMA blocks (8 cm × 10 cm × 1 cm) are used as experimental materials to carry out upward flame spread experiments with different air gaps (0–18 cm) to analyze changes in burning rate, flame shape, flame height and pyrolysis front position. It is found that as the air gap increases, the burning rate and flame height both increase first and then decrease, and the turning point of this trend is predicted to be between 6 and 8 cm. In addition, it is founded that the pyrolysis reaction is accelerated due to the additional air entertainment provided by the air gap when the pyrolysis front passes through the air gap. However, this effect becomes insignificant for a larger air gap.

An analysis of heat feedback effects of different height embedded plates on promotion of pool fire burning using a variable B number

In this study, heat feedback effects of embedded plates with various heights on the enhancement of the pool fire burning were investigated through a variable Spalding B number. The study showed that the embedded plate had a significant effect on the increase in flame height and pool fire burning rate, but longer plate inhibited the effect, where a critical point arose. Besides, by increasing plate height, the solid temperature increased with a high growth rate before the critical point, while the growth rate decreased after the critical point. Below the liquid surface, the plate temperature of ethanol was generally lower as compared with n-heptane due to the lower boiling point of ethanol. While above the liquid surface, it was the opposite due to its less soot production with higher flame temperature. Secondly, the flame height had a significant positive effect on the heat feedback energy from solid radiation and solid-liquid nucleate boiling. For n-heptane fire, as its latent heat of gasification (hfg) was about 1/2.6 of ethanol, the nucleate boiling dominated total solid heat feedback. Finally, the solid heat feedback led the modified heat of gasification (Lm) to change oppositely, which first decreased then increased with the increasing plate height. Thus the B number decreased after a rise that made the same change in burning rate for both fires. Ethanol pool fire had a lower burning rate as compared with n-heptane fire because of its smaller values of B number affected by larger original heat of gasification.

The Role of Fuel Bed Geometry and Wind on the Burning Rate of Porous Fuels

The vast majority of wildland fires occur in windy conditions. However, most operational wildland fire models do not account for changes in burning rate or duration due to wind as no simple model exists. To gain some understanding of how wind and fuel bed properties interact to influence the burning rate and duration of wildland fuels, a relatively simple fuel bed, wood cribs, was first considered. The burning rate of twenty three crib designs was measured in a wind tunnel under a range of windspeeds from 0 to 0.7 m/s. Fuel element thickness varied from 0.32 cm to 1.27 cm and fuel bed width from 12.7 cm to 60.96 cm. A range of crib porosities was tested as well covering the loosely-packed to densely-packed regime. A clear threshold behavior of the burning rate was seen depending on fuel bed geometry. For fuel beds with element length to thickness ratio (l/b) less than 30, the burning rate increased with wind. However, for fuel beds with element length to thickness ratio larger than 30, the burning rate actually decreased with wind. This change in burning rate was linked to a visual change in burning behavior. When the burning rate increased, the wind and flames were observed to penetrate the internal portions of the fuel bed and the crib would burn uniformly. When the burning rate decreased, the wind and flames did not penetrate the entire fuel bed and the burning front would most often propagate from the upwind edge to the downwind edge. It appeared that for these fuel bed geometries the wind was forced around the fuel, preventing any horizontal or, more importantly, vertical flow through the bed. These results are likely most applicable to isolated, small clumps of elevated fuel where the wind has the opportunity to divert around the fuel bed. Future work will include experiments that force the airflow through the fuel bed.

Highly energetic nitramines: A novel platonizing agent for double-base propellants with superior combustion characteristics

The burning rate-pressure relation of solid propellant is an exponential relation; this is means catastrophic combustion process in case of sudden increase in operating pressure. This study reports on novel approach to minimize the dependence of burning rate on combustion pressure using highly energetic nitramines (RDX). Nitramine-based double base propellants with RDX content up to 20 wt% were manufactured by solventless extrusion technology. The impact of RDX content on burning rate, specific impulse, exhaust velocity, and thrust were evaluated using small-scale ballistic evaluation rocket motor. Specific impulse (Is) was enhanced by 20%. The action burning time (tb) was increased by 14%. The total thrust impulse (IFT) was significantly improved by 22%. One of the main outcomes of this study is that burning rate pressure exponent (n) was decreased significantly from 0.35 to 0.05. This novel ballistic performance finding means minimal change in burning rate with pressure variation. These findings confirmed that RDX has a dual function as energetic filler and platonizing agent. RDX blowing effect could alter the combustion mechanism by pushing the luminous flame from the burning surface. Furthermore, RDX decomposition can strengthen the platonization action. This is the first time ever to report on burning rate platonization using RDX. It can be concluded that many advantages have been achieved with one shot.

Combustion characteristics of extruded nitramine double-base propellants

AbstractThe burning rate-pressure relation of solid propellant is an exponential relation; this is means catastrophic combustion process in case of sudden increase in operating pressure. This study reports on novel approach to minimize the dependence of burning rate on the operating pressure using highly energetic nitramines (RDX). Nitramine-based double base propellants with RDX content up to 20 wt % were manufactured by solventless extrusion technology. The impact of RDX content on burning rate, specific impulse, exhaust velocity, and thrust were evaluated using small-scale ballistic evaluation rocket motor. Specific impulse (Is) was enhanced by 20 %. The action burning time (tb) was increased by 14 %. The total thrust impulse (IFT) was significantly improved by 22%. One of the main outcomes of this study is that burning rate pressure exponent (n) which was decreased significantly from 0.35 to 0.05. This novel ballistic performance means minimal change in burning rate with pressure variation. These novel findings confirmed that RDX has a dual function as energetic filler and as a platonizing agent. This is the first time ever RDX has been reported to act as a platonizing agent. It can be concluded that many advantages have been achieved with one shot.

Study of effect of binders and loading pressures on the performance of the time delay pyrotechnic compositions

ABSTRACTAn experimental investigation was carried out to determine the effect of binders and loading pressures on burning performance of B/BaCrO4 and Si/PbO/Pb3O4 delay compositions. The consolidated density and percent theoretical maximum density (%TMD) of these compositions were also studied with different binders and at multiple loading pressures. Carboxyl methyl cellulose (CMC), dextrin, and fish glue with varying wt. % were used as binders. It was observed that the burning rate of these delay compositions was inversely proportional to the binder content. The burning rate of B/BaCrO4 delay composition was 71.0 mm/s without binder. The burning rate decreased to 38.1 mm/s by adding 3.0 % fish glue. When 1.0 % CMC was added to the mixture, the burning rate decreased to 61.8 mm/s. By adding 3.0 % dextrin to the delay composition, the burning rate decreased to 38.2 mm/s. The burning rate of Si/PbO/Pb3O4 delay mixture was 38.6 mm/s without binder. The burning of this mixture decreased to 16.4 mm/s by adding 1.0 % fish glue. The loading pressures were varied from 103 to 414 MPa. The effect of loading pressures on the burning rate of both the delay compositions was marginal. The burning rate of B/BaCrO4 delay mixture decreased with the increase in loading pressure. On contrary, the change in burning rate of Si/PbO/Pb3O4 pyrotechnic delay composition was minimal by varying the loading pressures. Results also revealed that loading pressures of 345 and 348 MPa produced the minimum standard deviation in burning rate of B/BaCrO4 and Si/PbO/Pb3O4 compositions. The consolidated density and %TMD of both mixtures increased by adding binders and increasing the loading pressures.

The effect of decorated graphene addition on the burning rate of ammonium perchlorate composite propellants

Catalysts are often added to solid propellant formulations to tailor burning rates. Nanocatalysts can increase propellant burning rates over standard catalyst sizes due to the increase in surface area per unit weight. However, the increased surface area that the binder must wet can be prohibitive if large amounts of nanocatalysts are used. Additionally, agglomeration of the nanocatalyst can result in micron-scale particles, reducing catalyst effectiveness. In this study a nanoscale iron oxide catalyst has been used to decorate graphene. By decorating graphene with the catalyst, nanoscale features are kept but the catalyst is stabilized to reduce agglomeration. Changes in burning rate between the catalyzed and uncatalyzed propellants are investigated. The effect on burning rate of encapsulating the catalyst inside the fine AP crystals compared to propellants where it is added to the binder is also investigated. We also compare propellants with decorated graphene and propellants with undecorated graphene catalysts. The three comparisons are made for two different graphene preparation methods. It is found that the highest burning rates occur in propellants where the graphene is decorated with catalyst and encapsulated in the fine AP. The next highest burning rates occur in propellants with decorated graphene that is physically mixed into the propellant. The lowest burning rates are found in propellants where the graphene is undecorated, where there is little difference between encapsulated or physically mixed graphene blanks, or compared to a baseline propellant. Burning rates are found to be similar between graphene preparation methods considered.

Reducing Agglomeration of Ammonium Perchlorate Using Activated Charcoal

AbstractAmmonium perchlorate is the most widely employed oxidizer for composite solid propellants. When exposed to atmosphere, it absorbs moisture and agglomerates. It is usually vacuum dried in order to avoid this agglomeration. When ammonium perchlorate that has been exposed to atmosphere for a certain period of time, is used in making a composite solid propellant, the burning rate is different because of the change in particle size distribution due to its agglomeration. This change in burning rate will change the thrust‐time profile from that of what it is designed for. As one goes to a finer ammonium perchlorate particle size this problem becomes more evident. Experimental studies aimed at reducing the agglomeration of ammonium perchlorate by coating it with activated charcoal. Ammonium perchlorate coated with 1 % activated charcoal showed almost no agglomeration, even when the particle size of ammonium perchlorate is approx. 1 μm. The burning rates also remained unchanged when ammonium perchlorate coated with 1 % activated charcoal was employed in propellant composition, after it has been exposed to the atmosphere for a period of 1 h.

Transport phenomena within the liquid phase of a laboratory-scale circular methanol pool fire

The effects of altering the lower thermal boundary condition of a methanol pool from −5°C to 50°C was investigated within a 90mm diameter and 12mm deep quartz burner under steady state burning condition in a quiescent air environment. Both the burning rate and the flame height were observed to increase by 15% with increasing bottom temperature over this range of bottom boundary conditions. The temperature and velocity within the liquid were measured by a single thermocouple traversed through the pool and PIV, respectively, in order to better understand the transport of mass and energy in the liquid. Temperature measurements revealed a distinct two-layer vertical thermal structure with the upper layer of the pool being almost uniform and near the boiling temperature of the fuel, while the lower layer experienced an increasing temperature gradient as the bottom boundary temperature was lowered. The thickness of the thermally uniform layer increased as the bottom temperature was increased. The measured fluid velocity showed a complementary two-layer structure with the upper layer being dominated by a pair of counter-rotating vortices that kept this portion of the liquid well mixed and transferred heat from the hot pool wall to the pool center, while the flow in the lower layer was uniformly low in value and vertical. A model was presented to aid in understanding the energy transfer within the liquid phase. In the lower layer, the Peclet Number was in the order of unity and required that the energy transfer throughout the liquid phase to be modeled as a combination of conduction and convection. Using this physical model, the change in burning rate over the full 55°C change in bottom temperature was predicted within 2%, thereby supporting the proposed mechanism for energy transfer into the pool’s depth.

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.

Acoustic excitation of droplet combustion in microgravity and normal gravity

This experimental study focused on methanol droplet combustion characteristics during exposure to external acoustical perturbations in both normal gravity and microgravity. Emphasis was placed on examination of excitation conditions in which the droplet was situated (1) at or near a velocity antinode (pressure node), where the droplet experienced the greatest effects of velocity perturbations, or (2) at a velocity node (pressure antinode), where the droplet was exposed to minimal velocity fluctuations. Acoustic excitation had a significantly greater influence on droplet-burning rates and flame structures in microgravity than in normal gravity. In normal gravity, acoustic excitation of droplets situated near a pressure node produced only very moderate increases in burning rate (about 11–15% higher than for nonacoustically excited, burning droplets) and produced no significant change in burning rate near a pressure antinode. In microgravity, for the same range in sound pressure level, droplet burning rates increased by over 75 and 200% for droplets situated at or near pressure antinode and pressure node locations, respectively. Observed flame deformation for droplets situated near pressure nodes or antinodes were generally consistent with the notion of acoustic radiation forces arising in connection with acoustic streaming, yet both velocity and pressure perturbations were seen to affect flame behavior, even when the droplet was situated precisely at or extremely close to node or antinode locations. Displacements of the droplet with respect to node or antinode locations were observed to have a measureable effect on droplet burning rates, yet acoustic accelerations associated with such displacements, as an analogy to gravitational acceleration, did not completely explain the significant increases in burning rate resulting from the excitation.

Dynamic response of strained premixed flames to equivalence ratio gradients

Premixed flames encounter gradients of mixture equivalence ratio in stratified charge engines, lean premixed gas-turbine engines, and a variety of other applications. In cases for which the scales—spatial or temporal—of fuel concentration gradients in the reactants are comparable to flame scales, changes in burning rate, flammability limits, and flame structure have been observed. This paper uses an unsteady strained flame in the stagnation point configuration to examine the effect of temporal gradients on combustion in a premixed methane/air mixture. An inexact Newton backtracking method, coupled with a preconditioned Krylov subspace iterative solver, was used to improve the efficiency of the numerical solution and expand its domain of convergence in the presence of detailed chemistry. Results indicate that equivalence ratio variations with timescales lower than 10 ms have significant effects on the burning process, including reaction zone broadening, burning rate enhancement, and extension of the flammability limit toward learner mixtures. While the temperature of a flame processing a stoichiometric-to-lean equivalence ratio gradient decreased slightly within the front side of the reaction zone, radical concentrations remained elevated over the entire flame structure. These characteristics are linked to a feature reminiscent of “back-supported” flames—flames in which a stream of products resulting from burning at higher equivalence ratio is continuously supplied to lower equivalence ratio reactants. The relevant feature is the establishment of a positive temperature gradient on the products side of the flame which maintains the temperature high enough and the radical concentration sufficient to sustain combustion there. Unsteadiness in equivalence ratio produces similar gradients within the flame structure, thus compensating for the change in temperature at the leading edge of the reaction zone and accounting for an observed “flame inertia”. For sufficiently large equivalence ratio gradients, a flame starting in a stoichiometric mixture can burn through a very lean one by taking advantage of this mechanism.

Combustion of Liquid Fuels Spilled on Water. Prediction of Time to Start of Boilover

The combustion of a liquid fuel floating on water is a problem of interest because of its potential environmental and safety consequences. When a liquid fuel is burning under these conditions, the presence of the water may cause some particular effects due to heat transfer to the water. If the fuel layer is thin, heat losses to the water may cause quenching of the fuel burning. If the fuel layer is sufficiently thick, it is possible for the heat transferred to the water to induce nucleate boiling of the water and, in turn, splashing of the fuel above, a phenomenon referred to as thin-layer boilover. In this work, a one-dimensional, transient, heat transfer model of a burning liquid fuel floating on water is developed, and applied to predict the temperature histories in the fuel and water layers, and the time for the onset of boilover. The model includes in-depth radiation absorption but neglects convection within the liquid, and assumes that boilover occurs when the temperature at the fuel/water interface reaches a critical value above the water saturation temperature. Experimental measurements previously made by the authors with different multicomponent. and single composition fuels (crude and healing oil, and several parafines) were used to derive the heal flux at the fuel surface, and an average radiation absorption coefficient, which are two parameters needed as input in the model. It is shown that the model correctly predicts, within the uncertainty of the experiments, the influence of the major parameters of the problem, specifically the initial fuel layer thickness, the pool diameter, and fuel boiling point, on the temperature history of the fuel and water and time to the start of boilover. The effect of the fuel boiling point appears to be complex because a change in boiling point is accompanied by a change in burning rate, surface heat flux, in-depth absorption, and viscosity, all affecting the boilover phenomenon. The model represents a significant improvement from the quasi-steady heat conduction models previously developed by the present and other investigators of the boilover phenomenon.

On the quenching of premixed flames by water sprays: Influences of radiation and polydispersity

We analyze the process of premixed flame extinguishment by water sprays, as analytically as possible. Our main assumptions are as follows. o 1. Combustion is sustained by an overall Arrhenius reaction that yields water and carbon dioxide as products. 2. The activation energy is large. 3. The flame is planar and steady. 4. The water spray is polydispersed and viewed as a continuous medium exchanging heat and water vapor with the gas phase. 5. Combustion products and vaporized water radiate toward the surroundings according to the optically thin approximation. 6. Relistic emissive properties are retained. 7. Both phases have the same local velocity Using activation energy asymptotics and tabulations of two auxilliary functions, we obtain a relationship between the fractional changes in burning rate, the initial amount and size distribution of water droplets, and the gas properties/composition. This enables one to determine in a simple way how the burning velocity varies with the mixture strength and the spray properties. This is first illustrated in the case of monodisperse sprays, then of bimodal droplet-size distributions, for methane/air-like flames. In general, the amount of water needed to preclude flame propagation cannot be expressed in terms of such a simple moment of the droplet-size distribution as Sauter's diameter. The theory could be easily adapted to account for kinetic quenching.

Storage stability of solid rocket fuels. 2. Effect of oxidizer loadings

Ageing behaviour, leading to ballistic changes, has been studied as a function of oxidizer loading in polystyrene/ammonium perchlorate solid-propellants. The ageing studies were carried out at 100 °C in air. Change in burning rate decreased as the oxidizer loading increased from 75 to 80%. The change in thermal decomposition rates both at 230 and 260 °C also decreased as the oxidizer loading in the propellants increased. The shapes of the plots of the changes in burning rate and thermal decomposition rate (230 and 260 °C) at different storage times for different oxidizer-loaded propellants seem to be exactly similar. These results lead to the conclusion that the thermal decomposition of the propellant may be responsible for bringing about the ballistic changes during the ageing process. Infrared studies of the binder portion of the aged propellant indicate that peroxide formation takes place during the course of ageing and that peroxide formation for a particular storage time and temperature increases as the loading decreases.

Storage stability of solid rocket fuel. 1. Effect of temperature

Ageing behaviour of polystyrene (PS)/ammonium perchlorate (AP) propellent leading to ballistic changes has been studied. It follows a zero-order kinetic law. Ageing behaviour leading to change in burning rate ( r ́ ) in the temperature range of 60–200 ° C was found to remain the same. The dependence of the change of the average thermal decomposition (TD) rate at 230 and 260°C on the change in burning rate for the propellant aged at 100 ° C in air suggests that the slow TD of the propellant is the cause of ageing. The safe-life (for a pre-assigned burning-rate change limit) at 25 ° C in air has been calculated as a function of the rate of change.