Search

With good mechanical properties and high specific impulse, nitrate ester plasticized polyether (NEPE) is potentially well suited for future high-energy propulsion systems. In this study, ignition and combustion behaviors of NEPE propellants with different graphene oxide (GO) addition contents were investigated under rapid thermal stimulus using an optical rapid compression machine (RCM). Ignition sensitivity was studied by examining the pressure evolutions and high-speed images at different temperatures and pressures. Results show that the lowest ignition temperature for all NEPE propellants decreases with the increase of pressure. At fixed pressure NEPE propellant without GO (GO0) has the lowest ignition temperature among all propellants, indicating that addition of GO decreases the ignition sensitivity of NEPE propellant. Interestingly, with the increase of GO ratio (GO/(GO + CL-20)) from 0.5 % to 2.0 %, the lowest ignition temperature presents a nonmonotonic increase. The addition of 0.5 % and 1.5 % have excellent desensitized effects, which have the potential to be applied in future propulsion systems. Moreover, the hot spots for all samples locate left or right surface of propellant, strong convective heat transfer by the rapid movement of pistons in RCM occurs. Secondly, the burning rate of GO0 is around 11 mm/s at 40 bar and 940 K, as observed by high-speed images. Note that burning rates are increased by 30–45 % with the addition of GO. In addition, simultaneous thermal analysis (STA) experiments were used to reveal the overall reactivity of the samples and NEPE propellant combustion kinetic process under rapid thermal stimulus was illustrated. [ABSTRACT FROM AUTHOR]

Wood cribs pervade the fire research literature as the chosen fuel load for testing within the built environment. As such, the underpinning knowledge of fire behaviour in compartments was developed from experiments using wood cribs in small compartments. Despite the apparent incomparability of porous fuel-beds such as cribs to real solid fuels in the built environment, the role of the fuel mass transfer number (" B -Number") in defining the compartment fire dynamics has received little attention. In the case of large open-plan compartments, the burning processes are strongly dependant on the relationship of the fuel nature and compartment geometry. To address these limitations, the physical processes in-depth and external to a spreading wood crib fire in a compartment are examined. A theory to couple these processes to a compartment is proposed and analogised into the classical "Emmons problem", leading to a definition of a total mass transfer number for a wood crib. Comparing the theory against data from a large-scale experiment shows that the wood crib approximates steady-state burning in two regimes: a fuel-bed-controlled regime and a momentum-controlled regime. The fuel-bed-controlled regime occurs when the burning and spread rates are governed by the processes internal to the crib, and the fire behaviour is therefore defined by the crib geometry. This regime is characterised by a fire that travels or grows slowly, with small external heat fluxes. The momentum-controlled regime occurs when the fire is fully-developed and the external heat fluxes are very large. Burning rates are controlled by the residence time, with the compartment fire dynamics defined by complex transport processes associated with the momentum-driven flows external to the crib. Transitions from the fuel-bed-controlled regime to the momentum-controlled regime are driven by accelerations in the flame spread rate along the surface of the crib leading to an additional energy input mechanism that is used to raise the in-depth flame spread rate of the crib. It is hypothesised that the burning mechanisms of fuels with large mass transfer numbers, such as non-charring plastics, diverge significantly from wood cribs, and therefore extrapolating test data from wood cribs fires in compartments to real fuels must be done with extreme caution. Thus, the nature of the fuel is an important and unavoidable consideration when studying the fire dynamics of large open-plan compartments. [ABSTRACT FROM AUTHOR]

Diameter of liquid fuel surface is generally an important factor for pool fires. This work studies the effect of diameter of liquid fuel surface on the flame dynamics of the blue whirl formed on a concaved smooth metal surface with a 10-degree inclination. The results show that diameter of liquid fuel surface has a significant influence on the formation and evolution of the blue whirl. When the fuel surface diameter is in the range of 25 mm < d < 50 mm, a stable blue whirl can be formed. When d ≥ 50 mm, the blue whirl becomes unstable, that is a yellow whirling flame is generated in the center region of the envelope of the blue whirl, forming a transitional state. As the fuel surface diameter increases, both the circulation and flame width of the blue whirl show a near-linear growth. The liftoff height of the flame edge in the stable state of blue whirl varies linearly with fuel diameter, while remaining almost constant in the transitional state. Whereas the flame precession frequency of blue whirl increases linearly with the increase of circulation, the precession radius keeps nearly constant. The mass burning rate per unit area of the blue whirl decreases with increasing the fuel surface diameter. A radiative heat transfer model for the blue whirl is established based on radiation theory, and the modified burning rate expressions for the stable and transitional states of the blue whirl are obtained using the model and experimental measurements. Further scaling laws, based on dimensional analysis, are proposed for predicting the flame geometric features and flame precession frequency of the blue whirl. This work provides experiments of the flame dynamics at various fuel surface diameters and insights into the physics of the blue whirl. [ABSTRACT FROM AUTHOR]

The incorporation of aluminum (Al) into explosives represents a facility and efficient way to enhance the energy output through the secondary reaction between Al and the detonation products of explosive. The Al/O ratio and microstructure of HMX/Al play critical roles in combustion performance and energy output. Herein, a gradient structured HMX/Al composite has been designed and fabricated via 3D printing technology using well-dispersed HMX/Al-based ink. For HMX/Al lines, the second reaction heat increases from 196.5 J/g to 1644.8 J/g and the burning rate would decrease from 14.6 to 12.0 mm/s with Al content increased from 10 to 30 wt%. For gradient structured HMX/Al, the burning rate could be controlled by varying the component ratio and the burning rate of each component in the gradient structure was higher than that of the normal HMX/Al counterpart. Furthermore, a gradient evolution of pressure output was monitored for gradient structured HMX/Al cylinder ignited by the laser. These results indicate that the gradient structure as an effective approach can tune combustion performance and energy output of HMX/Al. [ABSTRACT FROM AUTHOR]

This paper shows how the combustion properties of single base nitrocellulose propellants are affected by degradation by ageing. A single base gunpowder sample containing methyl centralite and ethyl centralite as stabilizers was artificially aged by 10, 20 and 25 years. Closed vessel experiments were performed to evaluate the burning rate, the dynamic vivacity, the maximum pressure, the force, and the covolume of the gases produced. The propellant degradation was characterized by the Bergmann-Junk test, heat flow calorimetry test (HFC), high performance liquid chromatography (HPLC) and gel permeation chromatography (GPC). Gel permeation chromatography was especially useful to describe the degradation process. The results found for the burning rate suggest the existence of two phases during ageing: intensification and passivation. The intensification phase occurs while the propellant samples maintain the chemical stability and the passivation phase was observed for the unstable samples. [ABSTRACT FROM AUTHOR]

This study investigated the burning characteristics of PMMA with varied stretch rates under stagnation-point diffusion flames both theoretically and experimentally, focusing on the manner in which the surface energy balance and heat losses affected burning rate with varied stretch rates. The burning rate versus stretch rate was firstly investigated by introducing a modified Spalding B number, where the burning rate was proved to equal the buoyancy or forced flow induced stretch rate to the power of 0.5. At lower stretch rate, burning was largely affected by heat losses, associated with a lower B number, and decreased up to the quenching limit, similar to the microgravity results. At higher stretch rate, burning was controlled by flame radiation with a larger B number and increased burning rate. Additionally, ice bath increased the solid in-depth conduction heat loss due to the lower temperature maintained at the upper surface of the sample, and further influenced the energy balance with a larger heat losses fraction, leading to reduced B number and burning rate. This study provided an insightful investigation of the burning behaviors of stagnation-point diffusion flames with low stretch rates, which was used in a terrestrial environment to simulate microgravity flames. [ABSTRACT FROM AUTHOR]

The synthesis and development of novel energetic materials is a costly, challenging process. Rather than synthesizing new materials, cocrystallization provides the potential opportunity to achieve improved properties of existing energetic materials. This work examined the effects of cocrystallization on the deflagration of a 2:1 molar cocrystal of CL-20 and HMX as well as a 1:1 molar cocrystal of CL-20 and TNT. A hydrogen peroxide (HP) solvate of CL-20 as well as a polycrystalline composite of HMX and ammonium perchlorate (AP) were also studied. A physical mixture of each material was also tested for comparison. The burning rate of each material was measured as a function of pressure. Flame structure during self-deflagration was examined using planar laser-induced fluorescence (PLIF) of CN and OH. The burning rate of the HMX/CL-20 cocrystal and the CL-20/HP solvate closely matched that of CL-20, but the burning rate of the TNT/CL-20 cocrystal was between the burning rate of its coformers. All HMX/AP materials had a higher burning rate than either HMX or AP individually and the burning rate of a physical mixture was found to be a function of particle size. The differences in the burning rate of the physical mixtures and composite crystal of HMX/AP can be explained by changes in the flame structure observed using PLIF. Burning rates and flame structure of the cocrystals were found to closely match those of their respective physical mixtures when smaller particle sizes were used (approx. less than 100 µm). The results obtained demonstrate that the deflagration behavior of the coformers is not indicative of the deflagration behavior of the resulting physical mixture or cocrystal. However, changes in the resulting flame structure greatly affect the burning rate. [ABSTRACT FROM AUTHOR]

In the controllable combustion field, hydroxyl ammonium nitrate (HAN)-based electrically controlled solid propellants (ECSPs) is of growing interest in green fuels with safety, throttle- ability and at-will on-off capability. However, the current research is confined to the ignition characteristics and ablation process of basic ECSP formulations (75 wt% HAN ionic liquid oxidizer, 5 wt% ammonium nitrate co-oxidizer and 20 wt% polyvinyl alcohol binder), rather than working in functional additives. Herein, multiwall carbon nanotubes (MWCNT) and nano/micro aluminum powders were introduced in ECSPs, facilitating electrical conductivity and energy release. It indicates that the homogeneous dispersed MWCNT in polyvinyl alcohol (PVA) microspheres, preparing by the micro-segmented flow method, results in a 9.8% increase in electrical conductivity of the ECSPs. At 0.5 MPa, ECSPs containing 1%, 3% and 5% 5 µm aluminum powder reinforce the burning rate by 38.0%, 186.3%, 214.7% and mass ablation by 10.7%, 95.2%, 175.6% at 200 V, yet the large heat release by excess aluminum powder shifts the non-self-sustaining combustion of ECSPs to self-sustaining combustion at high pressure (above 0.8 MPa). The mechanism of promoting ECSPs combustion by liberating aluminum-nitric acid (released by HAN ionization) reaction were proposed. [ABSTRACT FROM AUTHOR]

The propellant synthesis community is constantly looking for green alternative monopropellants. Energetic ionic liquids have several attractive properties such as high energy content, high bulk density, low vapor pressure, high thermal stability, wide liquidus range, low corrosiveness, low toxicity, and ease of handling. The combustion characteristics of an energetic ionic liquid hydroxyethylhydrazinium nitrate (HEHN) was conducted in a pressurized chamber. The performance of HEHN was compared to that of the monopropellant Otto fuel II (OF-II) typically used for torpedo-propulsion. A liquid strand combustion study was performed in an atmosphere of air and nitrogen with chamber pressures varying from 10 to 90 bar. Regressing surface profiles and subsequent burning rates were obtained at different chamber pressures. A B-type thermocouple of 46 µm wire diameter was used to measure the monopropellant flame temperature of HEHN. Thermogravimetric analysis was performed to study the thermal decomposition of HEHN to understand its thermal stability and Fourier transform infrared spectroscopy (FTIR) was utilized to determine the possible reasons behind the high burning rates of HEHN. The gains in the specific impulse and density specific impulse coupled with enhanced burning rates and reasonable thermal stability are expected to establish HEHN as a frontrunner for propulsion and power-generation in oxygen-deficient scenarios. [ABSTRACT FROM AUTHOR]

In solid propellants, aluminum plays a key role in energy performance enhancement, but it also induces combustion problems such as low combustion efficiency and agglomeration. Herein, we study the effects of Al on the combustion properties of low-signal composite-modified double-base (CMDB) propellants. Experiments were conducted to determine the heat of explosion, the burning rates, and the combustion wave structures. The flame structure, the burning surface phenomenon and the morphology of the extinguished surface layer were observed. In addition, distribution of Al particles was calculated. It was found that using small Al particles or lowering the Al content will reduce the burning rate. This result was mainly cause by the "thermal sink" effect of Al. The Al agglomeration and ignition behaviors in the dark zone and the flame zone were observed at 1 and 3 MPa. It was found that propellants containing more than 10 wt.% Al or using Al particles smaller than 5 µm tended to generate large Al agglomerates. The extinguished burning surface presented "metal skeleton" structure under low pressures. The Al particles smaller than the HMX would be locally concentrated, so that it was easier for them to form large agglomerates. [ABSTRACT FROM AUTHOR]

For small-scale pool fires, Vali et al. [1] showed a pair of vortices in the liquid pool. The first vortex appeared just close to the sidewall of the container, and the second one emerged slightly away from the first vortex. Large-eddy simulations of small methanol pool fires coupled with liquid fuel convective flow were conducted using an in-house version of FireFOAM to investigate the above phenomenon. In this study, a three-dimensional liquid phase model is newly developed. The model incorporates the effects of thermocapillary Marangoni convection, buoyancy, shear stress, and evaporation. For the gas phase, the combustion model is the extended eddy dissipation concept model coupled with the laminar combustion model. This combustion model uses the viscous diffusion rate to consider laminar-turbulent transition. The predictions were in reasonably good agreement with the measured local mass burning rate, flame height and distributions of liquid temperature. The error of the mass burning rate was within 4%. The present predictions captured a pair of vortices in line with Vali et al.'s experiment [1]. Their sizes increased with increasing the liquid temperature. The Reynolds analogy could explain the sensible reason behind this trend. Shear stress and thermocapillary force caused convection in the liquid pool, and this convection formed a pair of vortices. Thermocapillary force was due to the different distributions of convective and radiative heat transfer. Sensitivity test for sub-models for the liquid phase demonstrated that their effects on the mass burning rate were all less than 5.1%. Conversely, the simulation assuming zero gravity only in the liquid phase resulted in almost 64% reduction in the mass burning rate. [ABSTRACT FROM AUTHOR]

Ammonium nitrate (AN, NH 4 NO 3) constitutes the key ingredient of monofuels and civilian-grade explosives, attracting scientific interests aimed at improving their operational and safety performance. This study investigates the combustion properties of redox crystals comprising ammonium nitrate and simple saccharides, with the infrared spectroscopy, X-ray diffraction and molecular modelling. Furthermore, the thermogravimetric measurements afford the isoconversional analysis that yields the overall activation energies of the decomposition process. In addition, the synthesised samples are subjected to elemental and sorption analyses. The results outline (i) the molecular inclusion of the solid fuels within the lattice clusters of AN, (ii) a comparable hygroscopicity behaviour, i.e., a minor increase in affinity towards the absorption of moisture, and (iii) an energetically improved decomposition (and regression) rate, relatively to pristine AN. These features manifest themselves in lower activation energies of redox crystals that enhance the deflagrating properties of these materials for possible application in aviation propellants, and minimise the environmental footprint, especially the emission of nitrogen oxide to the atmosphere, which arises because of inhomogeneities in AN-fuel mixtures commonly used in civilian explosives. [ABSTRACT FROM AUTHOR]

The development of halogen-free nitramine-based green solid propellants has attracted a significant interest due to their application in rapidly developing aerospace technologies. However, relatively low burn rates, and generation of high pressure in the combustion chamber limit applications of such propellants. The alleviation of these problems could be achieved by using suitable combustion catalysts. Herein, a new energetic nitrogen-rich open framework compound 1 was prepared and evaluated as an efficient Pb-free combustion catalyst for HMX-nitrocellulose-nitroglycerin solid composite propellant formulation. Compound 1 was synthesized by a straightforward one-step process, its structure was comprehensively characterized by x-ray crystallography, and it was found to be thermostable (decomposition at 289 °C), insensitive to mechanical impact and friction (60 J, 360 N), and showed high hydrolytic stability. A propellant formulation containing 3.0 wt.% of 1 exhibited 45% increase in its burning rate (under 6.0 MPa) versus catalyst-free reference propellant. In the pressure range of 3–10 MPa, the pressure exponent was calculated to be 0.32, which is more than two times lower than of the reference propellant. These impressive properties of catalyst 1 suggest that it has a great potential to be further used in the development of future highly energetic green solid propellants. [ABSTRACT FROM AUTHOR]

The dependence of the burning rate of a solid homogeneous energetic material on curvature of the burning surface is investigated theoretically and experimentally. For these purposes, we use wedge-shaped samples with different apex angles which allow realizing the curved burning surface with a controlled curvature. In order to stabilize combustion at low pressure in the experiments, we use the radiation assisted burning. We compare the experimental data with theoretical dependencies obtained in the framework of the Zel'dovich–Novozhilov's theory. [ABSTRACT FROM AUTHOR]

The dependence of the burning rate of a solid homogeneous energetic material on curvature of the burning surface is investigated experimentally for the first time. For this purpose we use a double-slit propellant grain which allows realizing the curved burning surface with a controlled curvature. We compare the experimental data with theoretical dependencies obtained in the framework of the Zel'dovich–Novozhilov's theory. [ABSTRACT FROM AUTHOR]

Abstract This paper presents an experimental investigation into the burning rates of pool fires with different lip heights in cross flows for which data are not previously reported. Square pool fires with side lengths (D) of 5, 10, 15 and 20 cm and dimensionless lip heights (h ⁎ = h / D) of 0.25, 0.5, 1.0, 1.5 and 2.0 were tested burning n-heptane using a system that maintains the fuel level. The mass burning rates were measured with cross flow air speeds of 0–∼3.0 m/s. Previous works have mostly been concerned with pool fires with similar lip heights (h ⁎) in still air for relative smaller pools, or in cross flows with very small lip heights (i.e., h ⁎ ≤ 0.1). It was found that the burning rate behavior differs with pool size and h ⁎. For small pools (D = 5 and 10 cm), the burning rate increased monotonically with cross flow air speed when h ⁎ was small, but first decreased then increased with large h ⁎. For larger pools (D = 15 and 20 cm), the burning rate first increased, then decreased, followed by a final increasing trend with small h ⁎; while for larger h ⁎, the burning rate first decreased and then increased with increasing cross flow air speed. These different trends were discussed in terms of the considerable change in the distance from the flame to the fuel surface, which affected the conduction, convection and radiation feedbacks to the fuel. It was observed that the flame was "pushed down" into the fuel-containing vessels by relatively strong cross flows, which further changed the heat feedback. A burning rate correlation was proposed to characterize the combined effects of cross flow air speed and lip height. [ABSTRACT FROM AUTHOR]

Abstract Energetic materials with a high iodine content and tunable reactivity are desirable for application as a biocidal agent. In this paper, aluminum/polyvinylidene fluoride (Al/PVDF) composites with different iodine content were prepared by an electrospray deposition method. Most of the iodine in the films are found to be fixed by PVDF and aluminum, which is released at 250 °C and 450 °C respectively. The heat release and burning rate of the iodine-containing films decreases with the increase of iodine content. With an iodine content of ≥40 wt.%, the film did not propagate. However, when fabricated in a laminate structure the threshold for iodine loading to sustain propagation increased to 67 wt.%. Evaluation of several multi-layered structured films indicated that an optimum single layer thickness of ∼25 µm produced the fastest reaction velocity, with loadings of up to 67 wt.% iodine. The thermal decomposition and oxidation of the laminated Al/PVDF films are also investigated. It appears thus that iodine which acts as a reaction retardant can be loaded in higher concentrations if it is physically separated from the primary energetic. In so doing, the primary energetic can maintain a continuous ignition threshold to propagate and enable the heat released from reaction to evolve gas phase iodine. [ABSTRACT FROM AUTHOR]

Abstract The stretched laminar flame model provides a convenient approach to embed realistic chemical kinetics when simulating turbulent premixed flames. When positive-only periodic strain rates are applied to a laminar flame there is a notable phase lag and diminished amplitude in heat release rate. Similar results have being observed with respect to the other component of stretch rate, namely the unsteady motion of a curved flame when the stretch rates are periodic about zero. Both cases showed that the heat release rate or consumption speed of these laminar-premixed flames vary significantly from the quasi-steady flamelet model. Deviation from quasi-steady behavior increases as the unsteady flow time scale approaches the chemical time scale that is set by the stoichiometry. A challenge remains in how to use such results predictively for local and instantaneous consumption speed for small segments of turbulent flames where their unsteady stretch history is not periodic. This paper uses a frequency response analysis as a characterization tool to simplify the complex non-linear behavior of premixed methane air flames for equivalence ratios from 1.0 down to 0.7, and frequencies from quasi-steady up to 2000 Hz using flame transfer functions. Various linear and nonlinear models were used to identify appropriate flame transfer functions for low and higher frequency regimes, as well as extend the predictive capabilities of these models. Linear models were only able to accurately predict the flame behavior below a threshold of when the fluid and chemistry time scales are the same order of magnitude. Other proposed transfer functions were tested against arbitrary multi-frequency stretch inputs and were shown to be effective over the full range of frequencies. [ABSTRACT FROM AUTHOR]

The thin-layer burning behaviors of gasoline, including the heat flux feedback to the burning surface, the penetrating thermal radiation, the temperature profile of liquid layer, and the burning rate were studied in experiments of thin-layer pool fires in square, fireproof glass trays. Experiments with four different tray sizes (side lengths of 30 cm, 40 cm, 50 cm and 60 cm) and four different initial liquid thicknesses of 6 mm, 9 mm, 12 mm and 15 mm were conducted. The results indicate that the heat flux feedback from the flame remained approximately constant, except during the ignition and extinguishment periods, and was also independent of the initial fuel thickness. The penetrating thermal radiation, on the other hand, increased with decreasing liquid layer thickness, gradually assuming rapid exponential growth. Furthermore, a boiling layer was formed during the initial burning period and its maximum depth was close to 3.0 mm. Four typical burning phases including pre-heating burning, steady burning, thin-layer burning and extinguishment were identified. The penetrating thermal radiation was the main cause of the burning rate decrease for thin-layer burning. These findings can provide a basis on which to build a real-time burning rate model for thin-layer burning. [ABSTRACT FROM AUTHOR]

Particle size and morphology of nitroguanidine (NQ) are two of the dominant factors that influence the thermal and combustion properties of triple-base propellants. However, the investigations of these effects and the corresponding mechanisms are insufficient. In this work, we examined the thermal properties and nonisothermal decomposition kinetics using differential scanning calorimetry (DSC) and thermogravimetry (TG). Combustion properties i.e. flame structure, melting layer and burning rate were determined on the bases of windowed strand burner (3 MPa), high-speed video camera, interrupted closed vessel, scanning electron microscope (SEM) and closed vessel, respectively. It was found that the decomposition process of triple-base propellant could be divided into two steps. The first step was mainly related to the decomposition process of nitroglycerin (NG) with higher activation energy and lower peak temperature, which was enhanced due to reactions between the added NQ and the decomposition product of NG (NO 2 ), while the second step mainly concerned the decomposition process of nitrocellulose (NC) with lower activation energy and higher peak temperature. The flame structure of triple-base propellants was heterogeneous and closely attached to the condensed phase without dark zone. When the NQ particle size was decreased, the active site number of NQ was increased, enhancing the intensity of the reaction between NO 2 and NQ; the decomposition peak temperature was decreased; the flame structure became more heterogeneous and brighter, resulting in more heat feedback from gas phase to the condensed phase. Thus, the burning rate was increased. On the other hand, the apparent activation energy and thickness of the melting layer of triple-base propellants were also increased with the decrease of NQ particle size, which meant that the sensitivities of burning rate to the conditions of gas phase were reduced. Therefore, the burning rate pressure exponent was decreased. [ABSTRACT FROM AUTHOR]

A study regarding the linear burning rates of strands of blended liquid monopropellants was conducted in a pressurized vessel. The blends consisted of the monopropellant isopropyl nitrate (IPN) and the desensitizer dibutyl sebacate (DBS) while an established monopropellant Otto fuel II (OF-II) was also tested. Additional experiments were performed to determine and compare the temperature profiles in the liquid and gas phases of the propellants under study using S-type thermocouples. The study was conducted in a quiescent atmosphere of air with the ambient pressure varying within the range of 10–100 bar. The liquid strands were created using quartz tubes with an inner diameter of 5 mm and a thickness 1 mm. Profiles of regressing surfaces and the derived burning rates were plotted with pressures varying from atmospheric to 100 bar for IPN-DBS blends and OF-II. Furthermore, the experimental results of IPN-DBS blends have been compared against the theoretical predictions of a semi-empirical model, with a reasonable match between the two. A blend of 90% IPN with 10% DBS (by mass) was found to be an excellent candidate to replace OF-II. [ABSTRACT FROM AUTHOR]

LMP-103S is a storable, green, ammonium dinitramide (ADN)-based liquid monopropellant with significant potential owing to its improved specific impulse (≥ 6%) and density (∼ 24%) in comparison to hydrazine, and its superior safety and handling characteristics. LMP-103S already has a significant history of thruster testing and flight heritage. However, its baseline combustion behavior is a surprisingly understudied topic. Constant-volume strand burner experiments with LMP-103S are reported herein for pressures between 0.69 and 34.5 MPa (100 – 5,000 psia). Linear burning rates were measured from transient pressure data and high-speed video. Visible and near-IR emission spectra were recorded as well. Phenomenological combustion behaviors are reported for the entire pressure range as deduced from these diagnostics, and two clear pressure regimes are observed. Hydrodynamic instabilities and supercritical combustion processes dominate the combustion behavior in the low- and high-pressure regimes, respectively. Supporting experiments include thermal cook-off of LMP-103S and impact sensitivity testing of ADN crystals extracted from LMP-103S solution. The data presented herein significantly expand upon those available in the literature for LMP-103S, and the phenomenological combustion behavior deduced from transient, high-speed video data improve upon the state-of-the-art knowledge regarding ADN-based liquid monopropellants. [ABSTRACT FROM AUTHOR]

This study experimentally investigates the burning dynamics of dodecane and heptane with a vertical-immersed copper rod on a turbulent water surface. The water surface turbulence is isotropic across the pool area created by an upward-directed axisymmetric submerged jet, with turbulence intensity ranging from 0.017 to 0.033 m/s. The copper rod (d r = 10 and 20 mm) is placed at the burning zone where the bottom section is immersed in both the fuel and water sublayer. The experiments reveal a strong dependence on burning behavior with the fuel type (heptane and dodecane are used) and the presence of turbulence. In quiescent conditions, the copper rod causes the burning rate to decrease by 28% for dodecane and increase by 13% for heptane because of the differences in nucleate boiling behaviors observed on the immersed section of the copper rod. With turbulence in the water sublayer, the nucleate boiling reduces, and the burning rate for both fuel types decreases because of the heat losses caused by the portion of the rod immersed in water. A mathematical model for the mass burning rate, including the effects of the immersed rod and turbulent water sublayer, is developed and validated using the experimental results. A parametric study using the model shows the influence of multiple immersed rods and the corresponding implications to a practical application of oil spill clean-up by burning. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this work, pillar candles are examined with eight varieties of candle wick having lengths from 1.0 mm to 19.5 mm and diameters from 1.4 mm to 3.2 mm using paraffin wax, beeswax, and soy wax as fuels. Experiments were undertaken to measure burning rate, flame height, flame width, and melt pool diameter using video and gravimetric analysis. Several properties of candle waxes were examined to understand their impact on burning rates and flame shapes. A model is proposed using the Spalding B-number with a novel wick efficiency model for burning rate accurate to ± 1.7 g/h (± 46%). The selection of wick was observed to have a strong impact on the burning rate of the candle flame. The Roper model for diffusion flames was found to provide a suitable estimate for the upper limit of flame lengths, and with the modification proposed in this work it can be used to predict candle flame lengths to within ± 10.4 mm. The width of candle flames was modelled using the Froude number and wax vapour specific gravity and was found to be accurate to within ± 1.69 mm (± 38%). The wick selection had no significant impact on the flame length or width external to its impact on burning rates. A numerical model for the diameter of the wax melt pool of a candle is presented and can be used to extend manufacturer data on melt pool sizes to other waxes. A simplified model is presented to relate the size of the melt pool to the heat release rate for various waxes. [ABSTRACT FROM AUTHOR]

This paper reports three-dimensional direct numerical simulations of ignition of a composite AP (ammonium perchlorate)–HTPB (hydroxyl-terminated polybutadiene) propellant subjected to a constant flux ϕ . The model includes solid heat transfer, gas-phase combustion with global kinetics and explicit description of propellant microstructure. Simulations show that ignition starts from AP particles because of primary AP/binder flame. Go/no-go computations reveal an unreported intermediate regime between go and no-go with apparent quenching followed by a delayed ignition. This delay is linked to a slow flame spreading from localized scattered hot spots on surface. In accordance with experiments, ignition delays deviate from classical ϕ − 2 scaling for high flux and low pressure conditions. For intense flux levels, simulations attest deradiation extinction upon flux termination meaning that there is a critical flux above which ignition is no longer possible. The role of AP particle shape and size distribution on ignition delay is studied and predicted to be limited. The effect of propellant surface conditions is also investigated and can lead to substantial effects on ignition delay. Finally, semi-transparent propellants are also considered and low absorption materials result in longer ignition times, reduced ϕ -dependence, and absence of deradiation extinction. This work eventually highlights the importance of microstructure-based details in the physics of composite propellant ignition and opens the way towards better understanding of the role of AP particles. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Effects of catalyst on the thermal decomposition were envisaged for AN and AN/KDN mixtures as an oxidizer and on the composite propellants based on them. AN/KDN based oxidizer samples were prepared in the ratios of 75/25 and 50/50 by a co-crystallization method. Propellants were formulated based on AN and AN/KDN mixtures as an oxidizer, and HTPB as a fuel binder with the addition of catalyst. The catalysts used for present studies are, (i) cupric (II) oxide (CuO) and (ii) copper–cobalt based metal oxide (Cu–Co * ). For all samples, 2% by weight of catalyst was added to the total weight of the samples. Thermal decomposition and burning rate studies were carried out for prepared propellant samples. All prepared oxidizers and propellants samples are subjected to DSC/TG studies using Simultaneous Thermal Analyzer (STA), while strand burners are used for burn rate determination of propellants in pressure regimes 100–4760 kPa and at sub-atmospheric pressures regimes (<100 kPa). Evolved gas analysis-QMS (EGA) was also performed to understand the decomposition mechanism of propellant samples. Propellants based on AN/KDN (50/50) in the presence of Cu–Co * showed the highest burning rate and also sustain combustion up to 40 kPa. Propellants based on CuO was also observed to enhance the burning rate but lesser as compared to the propellants based on Cu–Co * . [ABSTRACT FROM AUTHOR]

Simplified model propellant configurations, such as monomodal propellants, can be valuable in the development and validation of predictive numerical tools. These idealized experiments also yield insight into the effect of diffusion length scales on combustion, but comprehensive data covering a large range of diffusional length scales do not currently exist. Here, monomodal propellants with ammonium perchlorate (AP) particle sizes under 800 µm and AP pellets ported and filled with hydroxyl-terminated polybutadiene (HTPB) were used to systematically study the effect of diffusion length scales, or AP equivalent particle sizes) of up to 4.1 mm on flame structure. In general, burning rates increased with pressure and decreasing particle size, as expected. Burning rates for samples with particle sizes greater than 400 µm converged with AP monopropellant burning rate data above approximately 2 MPa, the AP low-pressure deflagration limit (LPDL). For a given pressure above the LPDL, burning rates eventually became constant for both increasing and decreasing particle sizes. Conversely, for a given pressure below the LPDL burning rate was shown to be a function of particle diameter. Flame structures above the composite propellants were observed using 5 kHz OH planar laser-induced fluorescence (PLIF). The transient flames were underventilated (jet-like) over the AP particles at 1 atm while lifted, inverted, and overventilated at 5 atm. Distinct diffusion flame structures were observed visually above the ported samples at 1 atm. Very luminous flames were observed at the interface between the AP and binder. The effect of strain rate on sample combustion was examined using an opposed flow burner; at 1 atm, sample burning rate was not affected by strain rate. At the largest strain rate, the sample self-extinguished after igniter shutoff, indicating that secondary diffusion flames are important in the opposed flow configuration. [ABSTRACT FROM AUTHOR]

There is contradictory literature about whether faster combustion may increase or decrease the likelihood of knock in spark ignition (SI) engines. Faster combustion allows less time for end-gas autoignition to occur, but also increases the end-gas pressure and temperature, which may reduce the time required for autoignition. By using the duration from ignition to 70% mass fraction burnt (MFB 0 − 70% ) as an explanatory variable, the hypothesis that “knocking cycles are the cycles with shorter MFB duration when they are compared with normal cycles” is proposed. In the experimental work, MFB 0 − 70% duration of normal cycles is calculated by the conventional method. For knocking cycles, which have non-uniform pressure, this conventional method cannot be used. Instead, it has been demonstrated that the MFB 0 − 70% duration can be estimated by a sine wave estimation (SWE) method with negligible errors. This MFB 0 − 70% duration is then used to represent the burning rate of knocking cycles. The proposed hypothesis is verified by the relationships between MFB 0 − 70% duration and the maximum rate of change of pressure. In the simulations, a multi-step adiabatic constant-volume zero-dimensional (MACZ) model is developed using Cantera software. In the model, the most recent detailed mechanism for gasoline surrogate, developed at the Lawrence Livermore National Laboratory (LLNL), is used. The MACZ model simulates the chemical kinetics of unburned air–fuel mixture from the start of compression stroke to the end of expansion stroke based on the recorded in-cylinder pressure. The simulation results match the experimental results fairly well. The simulation results suggest that increasing burning rate will promote knock, which is in agreement with the experimental results presented here. The contributions of burning rate to the knock are categorised by two factors described as “pre-dominant” steps and “post-dominant” steps. It is found that both factors have significant influences on the knock mechanism, even though the chemical reactions occurring in the pre-dominant steps are of low exothermicity. [ABSTRACT FROM AUTHOR]

In solid rocket propellants, nano-sized catalysts are expected to be more effective than their micron-sized counterparts due to their higher surface area and increased contact with the oxidizer. However, propellant processing becomes more difficult and ultimate mechanical properties can be negatively impacted as catalyst size is reduced. One proposed solution to these issues is to encapsulate a nano-sized catalyst inside the oxidizer. We have previously created composite particles with nano-sized iron oxide catalyst encapsulated in fine ammonium perchlorate (AP). In this paper we explore the effect of the composite particles on the burning rate and flame structure of an AP-based composite propellant. The propellant containing the composite nano-iron oxide/AP particles was compared to a baseline propellant without catalyst, a propellant formulated with micron-sized catalyst, and a propellant with nano-sized catalyst mixed directly. All catalyzed propellants contained the same catalyst mass fraction. High-speed (5 kHz) OH planar laser-induced fluorescence (PLIF) and high-speed surface imaging were used to examine differences in flame structure and coarse crystal burning characteristics and determine how the encapsulated catalyst influences the global burning rate. It was found that the burning rate of the propellant with the encapsulated catalyst was 90% higher than that of the baseline propellant, 44% higher than the propellant with micron-scale catalyst, and 15% higher than that of the propellant with nano-sized catalyst added directly. Our results indicate that the high global burning rate of the propellant with the encapsulated catalyst is due to an accelerating effect on the fine AP/binder matrix burning rate, assumed to be caused by the intimate contact between the fine AP and catalyst. At elevated pressures (4.4–7.1 atm) flame structure and burning surface morphology change as the catalyst size and location is changed. The coarse crystals are observed to protrude more from the burning surface as the catalyst size decreases. The microscale flame structure is observed to transition from jet-like to lifted arches as the pressure and catalyst size increase. [ABSTRACT FROM AUTHOR]

The burning characteristics and thermal-flow structures within laboratory-scale pool fires have previously been shown to be dependent on the pool’s bottom temperature and the pool wall material. Relatively unexplored are the effects of the thickness of liquid fuel in the pool without ullage. These effects are explored experimental here using a 90 mm diameter quartz pool burning methanol in a quiescent environment for a range of bottom boundary temperature ( i.e. , −4 °C to 50 °C). Three pools with fuel depth of 6, 12 and 18 mm were examined in terms of overall burning rate, flame height, and cross-sectional maps of mean temperature and in-plane 2-D velocity. It was shown that the burning rate and flame height for a specified pool bottom temperature increased with increasing pool depth, and shallower pools were more sensitive to pool bottom boundary temperature. Generally, the pool showed a distinct two-layer thermal structure with a near uniform upper layer, and a lower layer with a non-linear temperature gradient. The uniform layer was coincidental with a pair of standing counter-rotating vortices keeping this fluid well-mixed, while the lower layer had only a slow upward velocity associated with maintaining the liquid level. For the deepest pool, the thicknesses of the well-mixed and lower layers were essentially fixed, and the investigation showed that its velocity and non-dimensional temperature structures were independent of the bottom temperature. This independence was used to identify a regime of thick-pools , which allow the liquid to be modeled with relative simplicity. For shallow pools ( i.e. , thin-pools ), the thicknesses of these layers were dependent on the bottom temperature. Mechanisms for this behavior were proposed. Lastly, an energy analysis of the liquid beneath the well-mixed layer was able to explain the non-linear temperature profiles in that region as being a combination of vertical conduction and advection energy transfer. [ABSTRACT FROM AUTHOR]

The combustion-wave propagation of nickel-coated aluminum particles is studied theoretically for packing densities in the range of 10–100% of the theoretical maximum density. Emphasis is placed on the effect of packing density on the burning properties. The energy conservation equation is solved numerically and the burning rate is determined by tracking the position of the flame front. Atomic diffusion coefficients and reaction rate of isolated nickel-coated aluminum particles are input parameters to the model. The burning behaviors and combustion wave structures are dictated by the heat transfer from the flame zone to the unburned region. Five different models for the effective thermal conductivity of the mixture are employed. The impact of radiation heat transfer is also assessed. As a specific example, the case with a particle size of 79 μm is considered in detail. The burning rate remains nearly constant (<1 cm/s) up to a packing density of 60%, and then increases sharply toward the maximum value of 11.55 cm/s at a density of 100%. The Maxwell–Eucken–Bruggeman model of thermal conductivity offers the most accurate predictions of the burning rate for all loading densities. [ABSTRACT FROM AUTHOR]

Based on the classical volume-averaged equations, this work proposes a classification of porous media combustion in three distinct regimes for increasing interphase heat transfer, namely: decoupled, intermediate and hyperdiffusive. In the decoupled regime, flames behave as preheated free-flames. In the intermediate regime, large superadiabaticities are found. In the hyperdiffusive regime, flames are governed solely by an increase in thermal conductivity. The transition between these regimes is shown to be driven by two dimensionless parameters. The extent of the intermediate regime and the maximal superadiabaticity are proven to be related to the ratio between the diffusive and reactive length scales of the reference free-flame. Eventually, it is emphasized how the heat-recirculating system acts locally as a Lewis-changing device, whose effect harmonizes in the hyperdiffusive regime. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines the effects of various nanoparticle additives on the combustion behavior of nitromethane, using a pressure-based method recently demonstrated by the authors to measure the linear burning rates of liquid monopropellants and heterogeneous mixtures with high precision. The linear burning rates of these mixtures were measured in a constant-volume system at chamber pressures ranging from 3 to 14MPa, all without direct observation of the burning front. Nano-scale aluminum was used to increase the overall energy density of the mixture, fumed silica powder was used to increase the mixture thickness and encourage aluminum suspension, and nano-scale titania was also included based on its previous use as a burning rate modifier in solid propellants. The silica loading was varied from 1% to 3% by weight, aluminum was varied from 5% to 13.5% by weight, and titania was added at 1% by weight. The use of fumed silica yielded increased burning rates compared to those of neat nitromethane, and the pressure exponent of the burning rate curve shifted from lower to higher than the nitromethane baseline as more silica was added. This increased pressure sensitivity for mixtures containing 3% silica by weight was previously unobserved in similar studies by other groups and may be an effect of the higher specific surface area of the currently used silica. The subsequent addition of aluminum led to even faster burning rates and higher pressure exponents for all but one mixture. The addition of titania also led to elevated burning rates, with dramatically increased pressure sensitivity and rate inconsistency for chamber pressures above approximately 8MPa but a decreased pressure sensitivity for the same mixture below 8MPa. These changes in combustion behavior that accompanied titania were diminished by the presence of aluminum and completely negated in mixtures also containing fumed silica. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: Burn rate degradation of ammonium perchlorate based solid propellants can occur when moisture diffuses in and out of the material while exposed to fluctuating ambient humidity conditions. For high burn rate propellants with significant mass fractions of ammonium perchlorate particles less than 10μm, small changes in particle diameter can significantly alter the total oxidizer surface area resulting in propellant burning rate reductions. Ammonium perchlorate propellant samples are aged at various relative humidity and constant temperature. The samples are subsequently dried to equal moisture content and examined by SEM and optical microscopy. The propellant samples are combusted in a closed combustion bomb to measure the burning rate of the aged samples. The results show a clear correlation of the burning rate degradation and the level of humidity exposure. Evidence indicates that the degradation is a result of ammonium perchlorate crystal size growth and surface morphology changes reducing the available surface area. The changes are shown to be correlated with the moisture content of the aging environment. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: This study presents the experimental procedure and results of highly controlled pool fire tests, in a quiescent environment, designed to accurately measure the fuel burning rate and, consequently for sufficiently large pools, the thermal radiation flux back to the fuel surface. Steps were taken to minimize the effects of in-depth absorption of flame radiation, circulation within the liquid, and changes in fuel composition due to distillation of more volatile fuel components. With these precautions, focus is placed on gas phase phenomena controlling the heat release rate per unit pool area. The primary variables considered are: pool diameter, heat of gasification, flame sootiness as characterized by the inverse of the fuel smoke-point flame height, and, to a lesser extent, absorption of flame radiation by the fuel vapors just above the liquid surface. Results reported herein agree well with literature values for experiments conducted under similarly controlled conditions. A simple empirical formula is developed based primarily on heat of gasification and smoke point and is shown to correlate the mass burning rate within 9%, on average, of the experimental data. [Copyright &y& Elsevier]

Abstract: The current study outlines a general approach to construct a one-dimensional pyrolysis model based on milligram-scale and bench-scale test data collected systematically to isolate a specific process in each test. This approach is demonstrated by developing a model of burning for corrugated cardboard. Thermogravimetric analysis and differential scanning calorimetry were conducted on pulverized cardboard to determine the thermal degradation mechanism, the enthalpy of decomposition reactions, and the heat capacities of apparent species. Data collected in pyrolysis-combustion flow calorimetry tests were analyzed to assign a heat of combustion to the volatiles evolved from each reaction. Bench-scale tests were conducted with a cone calorimeter on samples in a horizontal orientation to observe the flaming combustion of this material at external heat fluxes ranging from 20 to 80kWm−2. Condensed phase temperature data was collected in these tests to measure and infer thermal transport properties and to characterize property changes associated with thermal degradation through an iterative inverse analysis. All the parameters determined through analysis of the milligram-scale and bench-scale tests were used to construct a one-dimensional pyrolysis model that predicted the average mass loss and heat release rates from cone calorimeter tests to within, on average, 17% and the times to ignition to within 2s. [Copyright &y& Elsevier]

Abstract: An experimental study of the flammability properties of PMMA at low pressures and oxygen concentrations was performed. The work was motivated by the importance of these effects on fire safety in the aviation industry. Measurements were obtained in a mass loss calorimeter inside a large 10m3 pressure vessel capable of reaching pressures as low as 0.1atm. The PMMA flammability was characterized by measuring the burning rate and the time to ignition of small test samples. These were ignited and burned under different external heat fluxes, total pressures and oxygen concentrations. The combined effects of pressure and oxygen concentration on the burning rate, combustion flow field, and ignition were evaluated. Results showed that at low pressure, the burning rate was less intense with a decrease in the mass loss rate. However, the reduction in pressure caused a shortened ignition delay time. Experimental measurements were compared with a simple analytical model showing good agreement. The results also show how pressure and oxygen concentration contributed to the heat transfer from the flame. The model revealed that a single function in oxygen and pressure could account for both flame radiative and convective effects. As a result, a power law fit was obtained for the relation of the combined pressure and oxygen effect on the burning rate. This correlation shows a good agreement with the measurements and predicts the burning rate behavior for the full range of pressure and oxygen tests. [Copyright &y& Elsevier]

Abstract: In this paper we report the burning rate characteristics of hydrogen peroxide and micron-aluminum propellants. Theoretical calculations show that the sea level specific impulse of this simple binary mixture is comparable to standard composite propellant. In addition, the aluminum particle size, hydrogen peroxide concentration, and mixture ratio can be adjusted over a flat peak performance regime to attain specific thrust profiles and durations. We measured the burning rates in a windowed pressure vessel at pressures ranging from 7 to 14MPa. Results show that mixture burning rates span from 0.5 to 4.5cm/s at 7MPa with power law burning rate pressure exponents ranging from 0.33 to 1.07. In this study, we focus a statistical analysis on the determination of the most influential variables affecting the burning rate and apply a thermal analysis to determine the combustion regimes of these mixtures. The statistical analysis provided a multivariate regression model for the logarithm of the burning rate with a correlation coefficient of 0.93. The model suggests aluminum diameter is the most important factor affecting the overall burning rate, and H2O2 concentration as the most influential variable on the burning rate pressure dependence. The burning rate dependence on theoretical combustion temperature shows two distinct combustion regimes attributed to kinetic and diffusion controlled combustion. A simple thermal analysis confirms the experimental burning rate pressure dependence observed for these two regimes. [Copyright &y& Elsevier]

Abstract: This paper presents semi-empirical investigations on the quasi-steady burning rates of laminar and turbulent fire whirls established over liquid fuel pools. The inflow boundary layer above the fuel surface consists of two regions: outer reactive region and inner non-reactive region. Based on the momentum boundary layer solutions with the applications of stagnant film model and Chilton–Colburn analogy, the burning rates are correlated with ambient circulation and pool size for laminar and turbulent fire whirls respectively. It is shown that in general pool fires the mass and heat transfers on the fuel surface are controlled by natural convection, while in fire whirls they are strongly enhanced by forced convection. Fuel evaporation rate in the outer region is relatively larger than that in the inner region. The large proportion of fuel evaporated from the outer region is mainly due to its larger area. The predictions agree well with the data from the present experiments and the literature. Furthermore, the flame height is confirmed to be proportional to the ambient circulation for both laminar and turbulent liquid fire whirls. [Copyright &y& Elsevier]

Abstract: A four-step reduced chemical-kinetic mechanism for syngas combustion is proposed for use under conditions of interest for gas-turbine operation. The mechanism builds upon our recently published three-step mechanism for H2–air combustion (Boivin et al., Proc. Comb. Inst. 33, 2010), which was derived from a 12-step skeletal mechanism by assuming O, OH, and H2O2 to be in chemical-kinetic steady-state and includes a correction to account for the failure of the O and OH steady-states during autoignition. The analysis begins by appropriately extending the number of elementary steps in the skeletal description to enable computation of the CO chemistry for mixtures with appreciable H2 content, giving a total of 16 elementary steps. It is seen that the formyl radical HCO, which appears as the only additional relevant intermediate in the extended chemical description, follows accurately a steady-state approximation, which can be used along with the steady-state approximations for O, OH, and H2O2 to derive the reduced description, involving the three global steps of our previous H2–air mechanism, 3H2+O2 ⇌ 2H2O+2H, 2H+M ⇌ H2+M, and H2+O2 ⇌ HO2+H, along with the additional step CO+ H2O ⇌ CO2+H2. Expressions are given for the rates of the four global reactions in terms of those of the elementary steps of the skeletal mechanism, with concentrations of the different steady-state species also given in explicit form. Comparisons of results of computations of laminar burning velocities and induction times with published experimental data for H2/CO/O2 mixtures with different diluents at atmospheric and elevated pressures and for varying equivalence ratios and initial temperatures indicate that the reduced description can be applied with reasonable accuracy in numerical studies of gas-turbine syngas combustion. [Copyright &y& Elsevier]

Abstract: The data set of our previous experiments in a wind tunnel is re-analyzed and interpreted in this paper in order to provide a more focused description of the manner the cross-flow affects the burning rate of gasoline pool fires. It was unexpectedly found that the burning rates (mass consumption rate per unit area) under a certain level of cross air flow decrease with pool size, which is opposite to that in quiescent air condition where the burning rates increase with pool size. The temperatures of the trailing, parallel-stream side rim walls were increased greatly, while those of the leading rim wall and the bottom changed relatively little when imposing a cross air flow. The burning rate of gasoline pool fire increased with cross air flow speed linearly in this range. The enhancement rate, defined as , was found to be mainly attributed to heat supplied from the trailing and parallel-stream side walls of the pool, due to the significant increase in their temperature difference with the adjacent fuel. A phenomenological model deduced the enhancement rate to vary linearly with (2L/W +1)/L. The experimental data was in good agreement with the theoretical analysis. [ABSTRACT FROM AUTHOR]

Abstract: An experimental investigation of the combustion characteristics of nanoaluminum (nAl), liquid water (H2O(l)), and hydrogen peroxide (H2O2) mixtures has been conducted. Linear and mass-burning rates as functions of pressure, equivalence ratio (Φ), and concentration of H2O2 in H2O(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 H2O2 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/cm2 s (0% H2O2) and 37.04 g/cm2 s (32% H2O2), 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% H2O2, 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% H2O2 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. [Copyright &y& Elsevier]

Abstract: The effects of a transverse air flow on the burning rate of square and rectangular methanol pools have been studied experimentally. The variables altered were the size and shape of the pool, and the air speed. The responding variable measured was a characteristic burning rate. Direct images of flame luminosity were collected to help interpret the burning rate data. The square pools ranged in size from to . The rectangular pools tested were the same area as a square pool, but with aspect ratios of 4:1. The air flow ranged in speed from quiescent to 5.5 m/s and was provided by a low-turbulence wind tunnel. The pools were held outside the tunnel boundary layers, so they would experience near-uniform upstream air flow conditions, and oriented perpendicular to the pool''s leading edge. The results of this study were that the quiescent burning rate per unit area was essentially constant for all pools tested. As the transverse air flow was introduced a variety of responses in burning rate were observed for different sizes and shapes of the pool. Depending on the size of the square pool, the burning rate showed a 2.5 times monotonic increase, invariance, or nonmonotonic response to increasing air speeds. The nonmonotonic burning rate response corresponded to the flame separating from the trailing edge of the pool. The use of pools of the same area, but different shape showed that the response of burning rate to air speed does not correlate to pool area, but supported an analysis based on the ratio of momentum to buoyancy forces that suggested that the characteristic dimension for combustion was the stream-wise length of the pool. [Copyright &y& Elsevier]

The radiation intensity at a given distance depends mainly on the radiative power and the flame''s size and shape. Considerable literature describing both experimental and theoretical studies of thermal radiation from flames is available. Even so, predicting the radiant power of large flames is still subject to considerable uncertainty, because some parameters associated with large turbulent diffusion flames cannot be determined accurately for a given fire. A series of outdoor large pool-fire experiments were performed using gasoline and diesel fuels lying above a layer of water. Five concentric circular pools made of reinforced concrete (1.5, 3, 4, 5, and 6 m in diameter) were used. The experiments were filmed with at least two video cameras registering visible light (VHS) and a thermographic camera (IR). In this study, thermographic images were used to determine the flames'' distribution of emissive power, the mean emissive power, and the flame''s irradiance. The contribution of each part of the flame to the total radiated energy was analyzed. A method is presented combining the IR images and the visible images; it offers further insight into the relationship between the heat emitted by the luminous part and the obscured, nonluminous, part of the flame. [Copyright &y& Elsevier]

A statistical (Monte Carlo) method for radiative heat transfer has been incorporated in CFD modeling of buoyant turbulent diffusion flames in stagnant air and in a cross-wind. The model and the computational tool have been developed and applied to simulate both burner flames with controlled fuel supply rate and in self-sustained pool fires with burning rates coupled with flame radiation. The gas–soot mixture was treated either as gray (using the effective absorption coefficient derived from total emissivity data or the Planck mean absorption coefficient) or as non-gray (using the weighed sum of gray gases model). The comparison of predicted radiative heat fluxes indicates applicability of the gray media assumption in modeling of thermal radiation in case of high soot content. The effect of turbulence-radiation interaction is approximately taken into account in calculation of radiation emission, which is corrected to allow for temperature self-correlation and absorption-temperature correlation. In modeling buoyant propane flames in still air above 0.3 m diameter burner, extensive comparison is presented of the predictions with the measurements of gas species concentrations, temperature, velocity and their turbulent fluctuations, and radiative heat fluxes obtained in flames with different heat release rates. Similar to previously published experimental data, the predicted burning rate of flames above the acetone pools exposed to flame radiation increases with the pool diameter and approaches a constant level for large pool sizes. The magnitude of predicted burning rates is shown to be in agreement with the reported measurements. Augmentation of burning rate of the pool fire in a cross-wind because of increased net radiative heat flux received by the fuel surface and non-monotonic dependence of burning rate on cross-wind velocity, subject to the pool diameter, is predicted. The statistical treatment of thermal radiation transfer has been found to be robust and computationally efficient. [Copyright &y& Elsevier]

Influences of the reaction zone thickening and extinctions on a heat release rate marker of extremely turbulent hydrogen-enriched methane-air flames are investigated using simultaneous planar laser-induced fluorescence of formaldehyde molecule and hydroxyl radical as well as separate stereoscopic particle image velocimetry techniques. The fuel-air equivalence ratio is set to 0.7, and the amount of hydrogen-enrichment varies from 0% to 70% by volume. Mean bulk flow velocities ranging from 5 to 35 m/s with Reynolds and Karlovitz numbers of 18 to 2729 and 0.1 to 76.0, respectively, are examined. It is shown that, by increasing the turbulence intensity, the preheat and reaction zone thicknesses can increase to values that are, respectively, 6.3 and 4.9 of the corresponding laminar flames, which were measured experimentally. Broadening of these zones for intensely turbulent hydrogen-enriched methane-air flames is shown experimentally for the first time. Broadening of the reaction zone suggests that the flamelet assumption used for development of the turbulent flame speed formulations may not hold. Thus, a new formulation, which does not utilize the flamelet assumption, is developed and used to calculate a heat release rate marker of the tested flames. It is shown that, at small turbulence intensities, the heat release rate marker values follow those of the local consumption speed, which is developed in the literature based on the flamelet assumption. However, at large turbulence intensities, the estimated heat release rate marker features large values, and the ratio of this parameter to the local consumption speed is consistent with the ratio of the global and local consumption speeds reported in the literature. It is shown that the ratio of the normalized heat release rate marker to the normalized local consumption speed is correlated with the broadening of reaction zone, suggesting that the disparity between the values of the heat release rate marker and the local consumption speed is linked to the reaction zone thickening. It is shown, although the flame thickening increases the heat release rate marker, extinctions decrease this parameter, which may contribute to the characterized bending behavior. It is argued that the amount of bending is related to the inverse of the Damköhler number. Using this, a mathematical formulation that allows for estimation of the heat release rate marker is developed. The predictions of the proposed formulation agree well with the normalized global consumption speeds of past experimental investigations. [ABSTRACT FROM AUTHOR]