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Graphical abstract Highlights • A gliding arc plasma fuel injector is proposed. • The ignition limit and lean blow-out limit were largely extended. • The drop size of kerosene spray decreases. • Several light hydrocarbons were generated. Abstract Flame extinction at high-altitude is one of the serious safety problems for aero-engines. The low pressure and temperature at high-altitude make it difficult to relight after extinction. Plasma-assisted combustion has been widely investigated in aerospace propulsion systems, while the mechanism is still not clear. In this study, a novel gliding arc plasma fuel injector using alternating current discharge is proposed to obtain reliable altitude relight performance and wider lean blow-out limits. The ignition and extinction experiments of gliding arc plasma fuel injector using 5 °C and −30 °C kerosene (RP-3) were conducted. A thin and string-like gliding arc plasma column was observed with repeating phases of breakdown between the fuel atomizer and venturi nozzle, extension with rotating airflow and column quenching. The optical emission spectroscopy of gliding arc plasma in gliding arc plasma fuel injector indicated the significant thermal effect caused by the arc column. Using gliding arc plasma fuel injector, the ignition limit was greatly extended, almost to the lean blow-out limits for the low temperature fuel. Low temperature and low injection pressure lead to much larger droplets and irregular injection cone. Influenced by heating and hydrodynamic effect of gliding arc plasma, the drop size at −30 °C kerosene spray decreases from around 150 μm to 30 μm, which is a main reason for ignition range extension. The equivalence ratios of lean blow-out were extended by up to 48% and 47% using 5 °C and −30 °C kerosene, respectively. Several light hydrocarbons, such as CH 4 (1.26 SLM), C 2 H 4 (1.31 SLM) and C 2 H 2 (2.30 SLM) with larger laminar flame propagation velocity, were detected as reforming products. Both heating and reforming effects contribute to the lean blow-out limit extension. The residence time of kerosene sprays in gliding arc plasma fuel injector is the determining factor for conversion capability. [ABSTRACT FROM AUTHOR]

Abstract The scope of this study is to improve combustion performance of a boron-based solid propellant ducted rocket (SPDR). A numerical approach for the internal reaction flow in SPDR was built, and an improved ignition and combustion model of boron particles was used. Many integrated physical processes of boron ignition and combustion, such as the balance of (BO) n on particle surface at ignition stage, the evaporation and boiling processes of boron at combustion stage, were considered. A new configuration of SPDR with swirling flow was proposed. Moreover, numerical and experimental investigations were conducted, and the effects of equivalence ratio and swirling flow on combustion performance were analysed. Validation of the developed numerical approach showed good agreement between the approach's results and available experimental data in open literature and also those in this study. Experimental results indicated that combustion efficiencies increase from 88% to 95% with swirling flow. [ABSTRACT FROM AUTHOR]

Abstract The skin-friction in a rocket-based combined-cycle engine operating from Mach 1.5 to 6.0 was analyzed in the present study. The friction proportion of different parts of the engine was investigated to offer a reference for the rearrangement of skin-friction reduction in the engine. The distribution and variation trend of the skin-friction in the flow path as well as its impacts on the engine performance were numerically compared. At three typical flight points, i.e. at 1.8Ma, 3.0Ma and 6.0Ma, the change of the skin-friction with attack angle was studied. A special focus was placed on the reduction of the skin-friction by using boundary layer combustion. It was modeled when the airstream flowed into the engine at the speed of 6 Ma. The method of hydrogen combustion in boundary layer has achieved 57.7% skin-friction reduction effect. Highlights • The average value of skin-friction at 6 Ma is almost 4 times higher than at 1.5 Ma. • The skin-friction is as high as 40.3% of the thrust at 6 Ma. • The higher flight speed, the higher proportion of the skin-friction in combustor. • After 3 Ma, the skin-friction gradually increases with the increasing attack angle. • Drag is reduced by 57.5% on a simplified inlet through boundary layer combustion. [ABSTRACT FROM AUTHOR]

In order to better understand how future candidate solid propellants may affect combustion characteristics in ramjet motor, the aluminum–magnesium fuel-rich propellant was tested on a sealed high-pressure laser ignition platform. The laser ignition experimental device was designed and built, then the ignition and combustion characteristics of aluminum–magnesium fuel-rich propellant under different pressures and oxygen concentrations in environment gas were studied. The comprehensive impacts of pressure and oxygen concentration on ignition delay time and burning rate were appraised. The results displayed that the ignition delay time was decreased with the increase of pressure and oxygen concentration. The effect of pressure on ignition delay time was weaker than oxygen concentration. The impact of oxygen concentration on ignition was very complex. When the oxygen concentration of environment gas was less than that of the oxidizing gas from the propellant pyrolysis products, the ignition gas phase reaction occurred in the dispersion region of the solid propellant pyrolysis product, the initial flame was close to the propellant surface. In contrast, when the oxygen concentration was larger than the pyrolysis oxidizing gas, the ignition gas phase reaction occurred in the diffusion zone of the propellant pyrolysis products and the environment gas, the initial flame appeared far from the propellant surface. The burning rate of aluminum–magnesium fuel-rich propellant increased with the increase of pressure and oxygen concentration, where their effect on burning rate is accord with B-number theory. Comparing the experimental measurements with an analytical model, the results showed good consistency. The function of oxygen concentration and pressure could explain the effect on burning rate and ignition delay time, because pressure and oxygen concentration were favorable for the heat transfer, radiation and convection from the flame. The function of the combined influences of pressure and oxygen concentration on burning rate was established by a power law fitting. The good fitting is consistent with the experimental measurements, and shows that the scaling is valid for a full range of pressure and oxygen concentration tests to predict burning rate. [ABSTRACT FROM AUTHOR]

This study proposes a modification to the temperature-composition pdf approach. Instead of equilibrium mass fractions, values of a time-varying homogeneous reacting system are used in the simplification of the joint-pdf into the multiplication of marginal pdfs. The approach takes into consideration of local time scales which are defined on the basis of energy balance with two competing transport mechanisms on the flame surface; these are the turbulent convective transport perpendicular to the flame and the diffusive flux tangent to the flame. The flame surface and flux directions are described by the gradient of mixture fraction and unitary tangent vector which is defined by the scalars. The new approach in combination with the ILDM chemistry is used in the numerical simulation of a transitional bluff-body flame. Despite the use of simple pdf models, the present formulation appears to be very successful in predicting the flow field, the temperature and progress variables and, except for the mixture fraction, show relatively good agreement with the experimental data. The profiles of the time scale show that the flame develops in relation to the distributions of progress variables in the T − ξ parameter space. Hence, the approach offers the possibility of local flame extinction. It also gives a rational explanation of the bimodal distributions of the reactive scalar pdfs in a turbulent flow with fluctuations of a wide spectrum of scales. [ABSTRACT FROM AUTHOR]

Numerical simulations were performed for the effect of injection and ignition timings on combustion and the generation of formaldehyde and unburned methanol emissions in the cylinder of a stratified charge direct injection spark ignition (DISI) methanol engine during cold start using AVL-fire, coupling the methanol chemical and kinetic reaction mechanisms. A non-uniform spray-line distribution nozzle was used to form stratified charge methanol-air mixtures during cold start. The simulation shows that injection and ignition timings have a significant effect on the concentration distribution of the methanol-air mixture, and hence the affect combustion and generation of formaldehyde and unburned methanol emissions. Optimized injection and ignition timings form an ideal stratified charge distribution. Formaldehyde and unburned methanol emissions decrease with retarding of the injection timing, but increase with retarding of the ignition timing. Formaldehyde and unburned methanol emissions at injection timing 41°crank angle before top dead center (CABTDC) were 48% and 82% higher than for 57°CABTDC, and those at ignition timing 8°CABTDC were 125% and 900% higher than for 20°CABTDC, respectively. Optimal injection and ignition timings provide the best compromise between the maximum cylinder pressure, maximum heat release rate, maximum cylinder temperature, and formaldehyde and unburned methanol emissions. Injection timing 45°CABTDC and ignition timing 14°CABTDC obtained the best compromise on cold start performance. [ABSTRACT FROM AUTHOR]

According to the requirements for neutron shielding and flame retardant properties of some nuclear devices, a new kind of polymer composite materials based on ethylene and vinyl acetate (EVA) polymer have been studied. EVA is the copolymer of ethylene and vinyl acetate, It can be used as materials for applications due to its flexibility, good processability, and low cost. Insulating EVA can be used for cable sheath, automotive sound damping and many other appication. Boron nitride (BN), zinc borate (ZB), magnesium hydroxide (MH) and EVA consisted the compounds with the properties of neutron shielding and flame retardant. With increasing of the contents of BN and ZB, the neutron shielding performance of materials increased up to 33.08%. With the increasing contents of MH and ZB as flame retardant, oxygen index of material have been improved. The elongation at break and tensile strength of material decreased with the increasing of filler powders. Sheet E was chosen and modified by electron beam accelerator in different doses. After modification by electron beam irradiation the sheets showed varying degrees of transformation in the OI, neutron shielding rate and mechanical properties. [ABSTRACT FROM AUTHOR]

The imbibition process into the pore structure of highly porous nanoparticle layers is investigated. The layers are fabricated with the Flame-Spray-Pyrolysis process and consist of aggregated $$\hbox {TiO}_{2}$$ nanoparticles. Measurements of the pore structure reveal that these layers have a very high porosity and a broad pore size distribution. Imbibition experiments show a deviation of the imbibition process from the classical capillary rising theory (Bell, Cameron, Lucas and Washburn). Therefore, a new capillary rising model is developed that accounts for the complex pore structure within layers of aggregated particles through the implementation of the fractal dimension of tortuosity. Comparison of the new model and the performed imbibition experiments enable the determination of the equivalent pore diameter for the imbibition process. The resulting pore diameter agrees well with the mean pore diameter of the nitrogen physisorption measurements. Hence, the new model enables the prediction of the imbibition process into highly porous layers of aggregated particles. [ABSTRACT FROM AUTHOR]

A cavity flame holder has been a very promising novel concept to be applied to the augmentor of a turbine-based combined cycle (TBCC) engine as it offers improvements in ignition performance, altitude relight, lean blowout limit, operating range, as well as combustion efficiency compared to conventional bluff body flame holders. The present paper describes the detailed designs of a cavity pilot augmentor and provides a laboratory-scale test rig. A series of experiments has been conducted to investigate the combustion characteristics of the test rig under atmospheric pressure. Experimental results show that for the bypass inlet temperature 343–473 K and inlet Mach number 0.25–0.40, ignition equivalence ratio and ignition pressure oscillation obtained is between 0.94–1.72 and 1.1–2.4% respectively, and lean blowout equivalence is between 0.42 and 0.92, indicating good ignition and lean blowout performances; as the total fuel-to-air ratio increases, the combustion efficiency decreases in the main at both afterburner operating mode and ramjet operating mode, while the increase of the bypass inlet Mach number or temperature is beneficial to high combustion efficiency, especially as inlet temperature over 423 K, and the combustion efficiency got in present experiment is between 70% and 95%. This paper demonstrates the feasibility for the cavity pilot flame holder to be applied to a realistic augmentor of TBCC engine preliminarily and provides reference for TBCC augmentor design. [ABSTRACT FROM AUTHOR]

Results of an experimental study of the effect of addition of CO to methane and to a propane-butane mixture on conditions of stabilization of the diffusion flame in air are reported. It is shown that the flame lift-off length substantially increases with increasing fraction of CO in the fuel; the main reason is the changes in the velocity and concentration in the region of the 'ignition' points; a secondary factor is an increase in the characteristic combustion time. Data on the influence of the CO admixture in the fuel in amounts greater than the fuel flow rate up to ≈1.5 times on the characteristic combustion time are obtained. [ABSTRACT FROM AUTHOR]

Abstract: The kinetics of oxidation, ignition, and combustion of Gas-to-Liquid (GtL) Fischer–Tropsch Synthetic kerosene as well as of a selected GtL-surrogate were studied. New experimental results were obtained using (i) a jet-stirred reactor – species profiles (10bar, constant mean residence time of 1s, temperature range 550–1150K, equivalence ratios φ= 0.5, 1, and 2), (ii) a shock tube – ignition delay time (≈16bar, temperature range 650–1400K, φ =0.5 and 1), and (iii) a burner – laminar burning velocity (atmospheric pressure, preheating temperature=473K, 1.0⩽ φ ⩽1.5). The concentrations of the reactants, stable intermediates, and final products were measured as a function of temperature in the jet-stirred reactor (JSR) using probe sampling followed by on-line Fourier Transformed Infra-Red spectrometry, and gas chromatography analyses (on-line and off-line). Ignition delay times behind reflected shock waves were determined by measuring time-dependent CH* emission at 431nm. Laminar flame speeds were obtained in a bunsen-type burner by applying the cone angle method. Comparison with the corresponding results for Jet A-1 showed comparable combustion properties. The GtL-fuel oxidation was modeled under these conditions using a detailed chemical kinetic reaction mechanism (8217 reactions vs. 2185 species) and a 3-component model fuel mixture composed of n-decane, iso-octane (2,2,4-trimethyl pentane), and n-propylcyclohexane. The model showed good agreement with concentration profiles obtained in a JSR at 10bar. In the high temperature regime, the model represents well the ignition delay times for the fuel air mixtures investigated; however, the calculated delays are longer than the measurements. It was observed that the ignition behavior of the surrogate fuel is mainly influenced by n-alkanes and not by the addition of iso-alkanes and cyclo-alkanes. The simulated laminar burning velocities were found in excellent agreement with the measurements. No deviation between burning velocity data for the GtL-surrogate and GtL was seen, within the uncertainty range. The presented data on ignition delay times and burning velocities agree with earlier results obtained for petrol-derived jet fuel. The suitability of both the current detailed reaction model and the selected GtL surrogate was demonstrated. Finally, our results support the use of the GtL fuel as an alternative jet fuel. [Copyright &y& Elsevier]

The article presents the bench test results of a fully instrumented, four cylinder, naturally aspirated, (60 kW) DI diesel engine running on the normal (class C) diesel fuel (DF) and aviation-turbine (JP-8) fuel. Analysis of changes in the autoignition delay, maximum in-cylinder pressure, performance efficiency of an engine and exhaust emissions caused by the variation of the cetane number of JP-8 fuel was provided. The series of engine tests were conducted running on the normal JP-8 fuel and JP-8 treated with 0.04vol%, 0.08vol%, 0.12vol%, 0.16vol%, and 0.24vol% of 2-ethylhexyl nitrate. Studies on operating characteristics of an engine were carried out for the fully loaded (100%) engine and the two ranges of speed, - 1400 rpm at which maximum torque occurs and rated 2200 rpm speed. Adding of 2-ethylhexyl nitrate to aviation-turbine fuel in the above proportions the cetane number (CN) of JP-8 fuel improved from 42.3 to 46.1, 47.6, 48.5, 49.4, and 49.8, respectively, enhancing ignition properties of the fuel to adapt it for using in ground-based military transport. The increase of CN from the reference value of 42.3 to optimum value of 48.5 suggested the brake specific fuel consumption lower 1.4%, both total unburned hydrocarbons (THCs) 7.5% and exhaust smoke 5.7% higher with almost unchangeable the NOx emissions behaviour and 11.9% lower CO emissions when running under a fully (100%) opened throttle at rated 2200 rpm speed. The brake thermal efficiency increased to maximum value of 0.309 (1.3%) for given operating conditions. Analysis of the results revealed that the improved cetane number can be considered as an effective but not the only measure to be applied for an intended use of JP-8 fuel in ground-based diesel engines. [ABSTRACT FROM AUTHOR]

Abstract: The present study aims at contributing to the development of a methodology to predict and improve the ignition performances of aircraft combustors. A model has been developed to investigate the early growth of a spherical ignition kernel in a two-phase flow mixture. It has been combined with a multiphysic code through two different approaches. The ignition kernel model is used to build the ignition probability map of a combustor. The output of the model can also be introduced as an initial condition in an unsteady simulation to test the flame propagation in the combustor. To validate both methods, RANS and LES simulations have been performed on an experimental combustion chamber, reproducing one sector of an industrial combustor. [Copyright &y& Elsevier]

Abstract: Ignition delay times of Jet-A/oxidizer and JP-8/oxidizer mixtures are measured using a heated rapid compression machine at compressed charge pressures corresponding to 7, 15, and 30bar, compressed temperatures ranging from 650 to 1100K, and equivalence ratios varying from 0.42 to 2.26. When using air as the oxidant, two oxidizer-to-fuel mass ratios of 13 and 19 are investigated. To achieve higher compressed temperatures for fuel lean mixtures (equivalence ratio of ∼0.42), argon dilution is also used and the corresponding oxidizer-to-fuel mass ratio is 84.9. For the conditions studied, experimental results show two-stage ignition characteristics for both Jet-A and JP-8. Variations of both the first-stage and overall ignition delays with compressed temperature, compressed pressure, and equivalence ratio are reported and correlated. It is noted that the negative temperature coefficient phenomenon becomes more prominent at relatively lower pressures. Furthermore, the first-stage-ignition delay is found to be less sensitive to changes in equivalence ratio and primarily dependent on temperature. [Copyright &y& Elsevier]