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Water is required to reach the ignition location accurately to ensure rapid fire extinguishing in the process of automatic fire suppression. In this paper, a water jet trajectory model was established based on the Moving Particle Semi-implicit (MPS) method, the experimental trajectory data were also collected through the experimental platform system and compared with the simulation trajectory data. Under low initial velocity, the results of simulation matched well with the experimental data while there were large errors under high velocity. It was found that air resistance was the main reason for deviation, the longer flight time caused by the high initial velocity and the bigger stressed area after the water jet break-up were main causes for the greater impact from air resistance. The binary function interpolation compensation method was used for error compensation and the jet angle and initial velocity were selected as error factors. The trajectory model was corrected based on existing simulation and experimental data and verified through unused data. The results showed that the revised trajectory model can accurately predict the position of the jet trajectory and provide data support for automatic fire suppression systems. [ABSTRACT FROM AUTHOR]

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 In this paper, a novel method is developed to analyze the thermal parameters of Nitrous Oxide Fuel Blend Ethylene and establish the saturated vapor pressure relationship with temperature from −80 to 30 °C based on Peng–Robinson equation of state, vapor/liquid phase equilibrium relation and Helmholtz equation. The experiments were conducted to validate the reliability of thermodynamic model. The self-pressurization flow model and sectionalized–centralized parameter gasification model of pipeline are established and validated. A chemical reaction mechanism with 129-species 900-reaction of Nitrous Oxide Fuel Blend Ethylene is established and the flame kernel parameters and laminar flame speed are obtained by steady-state, quasi-one-dimensional reacting flows simulations. Laminar flame speed of nitrous oxide fuel blend ethylene was measured and compared with calculated flame characteristics. [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 Ignition process, as the initiation of the entire discharge, plays an important role in ablative pulsed plasma thrusters. While spark plug exactly how initiate discharge is achieved is still under review. This study did some experiments with two kinds of propellant surfaces (normal or inclined) and without propellant to explain the ignition process. The experimental results showed: when the thruster discharge without propellant, it is essentially a surface flashover process on ceramics; when the propellant was loaded, the main discharge occurs after the initial conductive path composed of electrons emitted by spark plug forming. This study provides a reference for the high performance pulsed plasma thrusters. [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]

Ignition and flame stability in supersonic flow have always been the key problems of research in scramjet. In addition, ignition is difficult and the cavity flameholder is susceptible to support the flame stability under extreme conditions such as low equivalent ratio. In recent years, gliding arc plasma is recognized to expand ignition and extinction limit with lower energy consumption in the field of plasma assisted ignition due to its heating and chemical effects. In this paper, a gliding arc igniter has been designed and compared with the traditional spark plug in order to quantify the ignition ability. The igniter has the same size with the spark plug, using low-power AC gliding arc to carry out ethylene ignition test in Ma = 2.52 Ma supersonic flow. The average power of gliding arc discharge is 1199 W. A high-speed camera and CH ⁎ chemiluminescence were used to make combustion diagnosis. Founded in the same discharge period, the lean ignition limit of the gliding arc is lower than the ignition limit of the spark. The average expansion of ethylene ignition limit is 17%. The ignition process is that gliding arc continues to generate the initial flame kernels during the discharge period, but it is extinguished continuously due to the strong convection. Until generating an initial flame kernel which can successfully propagate the flame. The ignition process can be divided into four stages. It continues to generate new flame kernels in flame propagation process. Gliding arc reignites the fuel and generates the new flame kernels after forming a stable flame, appearing intermittent ignition in the cavity. The high equivalent ratio can make ignition delay time shorter, generating initial flame kernels more frequently. The heating effect of the gliding arc and reignition character make the thermal product and ethylene occur intermittent combustion more often in the cavity, increasing the area of the combustion reaction. Gliding arc plasma can achieve combustion enhancement during the flame stabilization process. The shear layer of flame thickness increased by the average of 2 mm on S-B-1 and G-B-1 conditions. Compared with the traditional spark ignition, gliding arc broaden the lean blow-off limit in different stages may be the significant reason for broadening lean ignition limit. It concludes that gliding arc makes the flame's ignition limit closer to the flame's blow-off limit. [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]

Several alternative synthetic fuels are in discussion as a replacement for conventional fuels like Jet A-1 to cope with limited supplies of crude oil as well as their emissions connected with its use such as the greenhouse gas CO 2 . One of the alternative fuels which have received high attention recently is farnesane (2,6,10-trimethyldodecane), a biofuel produced from sugar using a biotechnological process. In this paper, combustion characteristics of farnesane were investigated by measuring its ignition delay time using a shock tube at elevated pressure (16 bar) and two different stoichiometries ( φ  = 1.0 and φ  = 2.0) and the laminar burning velocity at atmospheric and elevated pressures (1, 3, and 6 bar). These results were compared to a conventional Jet A-1 fuel showing that farnesane has a similar combustion behavior. Furthermore, a reaction model was developed capable to predict the measured combustion properties. The calculation of the ignition delay times yields excellent results when compared to the measurements; the computations of the laminar flame speeds are in good agreement with the measurements. In addition, the reaction model was analyzed to get further insight into the main reaction steps of farnesane oxidation. [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]

This work investigates the ability of the one-dimensional turbulence model (ODT) to detect, in a predictive manner, occurrence of successful ignition or misfire in a reacting gas mixture subjected to turbulence. Since ODT is computationally very efficient, this significantly aids in the analysis of safety-relevant applications. ODT delivers fast predictions, while still capturing most relevant physicochemical processes controlling ignition. However, ODT contains some empirical parameters that must be set by comparison with reliable reference data. In order to determine these parameters and check the accuracy of resulting ODT predictions, they are compared in this work with reference data from direct numerical simulation (DNS). DNS is recognized as the most accurate numerical tool to investigate ignition in turbulent flows. However, it requires very high computational times, so that it cannot be used for practical safety predictions. It is demonstrated in this article that, thanks to validation and comparison with DNS, ODT realizations can be used to predict correctly the occurrence of ignition in turbulent premixed flames while saving more than 90% of the required computational time, memory and disk space. [ABSTRACT FROM AUTHOR]

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A numerical investigation is carried out to study ignition events of different premixed stoichiometric fuel/air mixtures by hot exhaust gas jets. The simulations are performed for premixed, stoichiometric hydrogen/air, ethylene/ air and propane/air mixtures in configurations relevant to safety applications. The ignition events of the different fuel/air mixtures by their corresponding exhaust gas jet are examined qualitatively analyzing processes and conditions leading to ignition. A stand-alone probability density function (PDF) method in connection with a projection method (PM) to calculate the mean pressure is used to model the turbulent flow. The transport equation for the joint velocity - turbulent frequency - scalar PDF is solved by a Monto Carlo/particle method. In order to reduce the computational costs concerning the chemical kinetics the reaction diffusion manifolds (REDIM) technique is used to get an appropriate reduced kinetic scheme. [ABSTRACT FROM AUTHOR]

This paper analytically and computationally examines the transient jet used to ignite combustible mixtures during Turbulent Jet Ignition (TJI). In TJI the ignition source, which originates in the prechamber, enters through a connecting nozzle into the main chamber as a transient high temperature jet of reacted mixture, reacting mixture and active radicals. A Computational Fluid Dynamics (CFD) model is developed for the flow field, density gradients, turbulence intensity, and temperature fields in both the prechamber and the main chamber. None of these quantities are currently measurable. The pressure traces computed using four (4) comprehensive chemical kinetic mechanisms (San Diego, Aramco, GRI, and, NUI) and one (1) reduced chemical kinetic mechanism are compared with the experimental pressure data. Results indicate that none of the mechanisms are in complete agreement, however they are in good agreement with the experimental burn rate, peak pressure and ignition delay predictions. Comparison is made of the simulations with high speed chemiluminescence images of combustion and measured pressure traces in a Rapid Compression Machine (RCM). The influences of nozzle size and mixture stoichiometry on jet penetration speed and combustion performance are investigated. Normalized transient results are presented that produce good agreement between the various classes of model predictions. A discussion is provided of a new correlation model for the transient TJI process. [ABSTRACT FROM AUTHOR]

Discharge reliability is typically neglected in low-ignition-cycle ablative pulsed plasma thrusters (APPTs). In this study, the discharge reliability of an APPT is assessed analytically and experimentally. The goals of this study are to better understand the ignition characteristics and to assess the accuracy of the analytical method. For each of six sets of operating conditions, 500 tests of a parallel-plate APPT with a coaxial semiconductor spark plug are conducted. The discharge voltage and current are measured with a high-voltage probe and a Rogowski coil, respectively, to determine whether the discharge is successful. Generally, the discharge success rate increases as the discharge voltage increases, and it decreases as the electrode gap and the number of ignitions increases. The theoretical analysis and the experimental results are reasonably consistent. This approach provides a reference for designing APPTs and improving their stability. [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]

The autoignition characteristics of n-heptane/hydrogen were investigated numerically. The effects of the temperature, fuel compositions and Exhaust gas recirculation (EGR) on the autoignition characteristics were evaluated. The numerical study on the ignition delay time was performed using the CHEMKIN-PRO software to calculate ignition delay time and predict the chemical species in the combustion process. The results revealed that ignition delay time decreased with an increase in the hydrogen fraction in the mixture in low temperature regime, but the opposite behavior observed in high temperature regime. The oxidation reaction of n-heptane in low temperature regime is limited with increasing mole fraction of hydrogen in blended fuel. Thus, cool flame which is characteristics of n-heptane is weakened. As consumption rate of n-heptane and temperature are decreased, total reactivity is decreased. The ignition delay time sharply decrease with EGR due to intermediate species in the exhaust gas. [ABSTRACT FROM AUTHOR]

Currently, there are used in power plants two types of burners to combustion the pulverized coal: jet burners and swirl burners. Most of the published works in the field of analysis of burners operating conditions are related to jet burners and are limited to initial region of the flame at the outlet of the burner. For jet burners an offset between the burner outlet and the flame beginning point could be analyzed. In the case of swirl burners with the pre-mixing diffuser chamber an offset between flame ignition point and burner outlet is usually not visible. Because already in the pre-mixing diffuser chamber of swirl burner occurs the ignition of coal-dust together with oxidant-gas cloud and thereby the formation of the flame. Therefore, the analysis of operating conditions for this types of burners requires a different approach. There is a lack of researches, in which will be analyzed changes of the flame length, that are caused by changes of design and operating parameters of swirl burner with the pre-mixing diffuser chamber. Therefore the aim of this study was to implement and verify the methodology for estimating the flame length, which will allow monitoring changes of operating conditions, that are caused by changes of design and operating parameters of swirl burner with the pre-mixing diffuser chamber. To carry out the analysis of the image of the coal flame, the two algorithms have been presented. This two algorithms, being based on the video images of the coal flame allows to specify and detect the end point of the flame and on that basis the length of the flame, as well as the fluctuation changes of flame length can be later determined. Both, the change thermal loads of coal burner, streams of secondary air, as well as the constructional change of the angle of swirler assemblies, resulted in distinct changes in the length of the pulverized coal flame. [ABSTRACT FROM AUTHOR]

Ignition enhancement characteristics of partially covered cavity in an ethylene-fueled scramjet combustor were investigated experimentally under the inflow conditions of Ma = 2.1, stagnation pressure P0 = 0.7 MPa, and stagnation temperature T0 = 846 K. It is concluded that under the influence of the baffle, the covered region will become a region with sufficient fuel during the stable combustion process and wall-pressure distribution will be increased due to this restricted region. Comparing with the uncovered cavity under the same equivalence ratio, partially covered cavity is more suitable for the formation and propagation of flame kernel, and ignition in the scramjet combustor could be intensified due to this cavity configuration. [ABSTRACT FROM AUTHOR]

Autoignition characteristics of laminar nonpremixed methane jet flames in high-temperature coflow air are studied numerically. Several flame configurations are investigated by varying the initial temperature and fuel mole fraction. At a relatively low initial temperature, a non-autoignited nozzle-attached flame is simulated at relatively low jet velocity. When the initial temperature is higher than that required for autoignition, two regimes are investigated: an autoignited lifted flame with tribrachial edge structure and an autoignited lifted flame with Mild combustion. The autoignited lifted flame with tribrachial edge exhibited three branches: lean and rich premixed flame wings and a trailing diffusion flame. Characteristics of kinetic structure for autoignited lifted flames are discussed based on the kinetic structures of homogeneous autoignition and flame propagation of stoichiometric mixture. Results showed that a transition from autoignition to flame propagation modes occurs for reasonably stoichiometric mixtures. The autoignited lifted flame with Mild combustion occurs when methane fuel is highly diluted with nitrogen. The kinetic structure analysis shows that the characteristics of Mild combustion can be treated as an autoignited lean premixed lifted flame. Transition behavior from Mild combustion to nozzle-attached flame was investigated by increasing the fuel mole fraction. As the maximum flame temperature increases with decreasing liftoff height, the kinetic structure showed a transition behavior from autoignition to flame propagation of a lean premixed flame. [ABSTRACT FROM AUTHOR]

The influences of fuel Lewis number LeF (ranging from 0.8 to 1.2) on localised forced ignition and early stages of combustion of stoichiometric and fuel-lean homogeneous mixtures have been analysed using simple chemistry threedimensional compressible direct numerical simulations for different values of root-mean-square velocity fluctuation and the energy deposition characteristics (i.e. characteristic width and the duration of energy deposition by the ignitor). The localised forced ignition is modelled using a source term in the energy transport equation, which deposits energy in a Gaussian manner from the centre of the ignitor over a stipulated period of time. The fuel Lewis number LeF has been found to have significant influences on the extent of burning of stoichiometric and fuel-lean homogeneous mixtures. It has been shown that the width of ignition energy deposition and the duration over which the ignition energy is deposited have significant influences on the success of ignition and subsequent flame propagation. An increase in the width of ignition energy deposition and the duration of energy deposition for a given amount of ignition energy have been found to have detrimental effects on the ignition event, which may ultimately lead to misfire. For a given value of u′ (LeF), the rate of heat transfer from the hot gas kernel increases with increasing LeF (u′), which in turn leads to a reduction in the extent of overall burning for both stoichiometric and fuel-lean homogeneous mixtures but the detrimental effects of high values of u′ on localised forced ignition are particularly prevalent for fuel-lean mixtures. Detailed physical explanations have been provided for the observed LeF, u′ and energy deposition characteristics effects. [ABSTRACT FROM AUTHOR]

The present study attempts to fix the location of thermal throat by employing transverse fuel injection downstream and adding a short constant-area section in the divergent part in a realistic scramjet combustor configuration in order to depress the pressure rise effects on the isolator. Ground tests using a direct-connect model along with pressure measurements and three-dimensional numerical simulation are also performed for the present investigations. The results indicate that the thermal throat is fixed in the constant-area section for the configuration with a short constant-area section in the divergent part. Numerical simulations also indicate that the constant-area section is helpful for ignition and flame holding. The configuration with a short constant-area section increase heat release in the combustor and decrease entropy production. Numerical simulation results imply that there could exist more than one thermal throat in dual-mode scramjet combustor, and the constant-area section in a divergent part could reduce the number of thermal throat. [ABSTRACT FROM AUTHOR]

The influence of addition of methane on the self-ignition of a hydrogen jet was studied experimentally for the pulse discharge into an open channel filled with air. During the pulse discharge from a high pressure chamber a shock wave is formed, which heats the surrounding air and a contact surface. The self-ignition of the jet occurred at the contact surface of the jet with air. A mixture of hydrogen with methane was preliminary prepared in a vessel of 40 L. The initial pressure of the mixture was varied from 3 to 15 MPa. During the discharge of the binary mixture ignition delays were measured relative to the moment of a breaking of the diaphragm. Ignition delays were determined by a photomultiplier tube. The delays were measured for different initial pressures and the methane addition. A molar concentration of the methane addition varied from 5% to 18%. Analysis of the effect of impurities on the thermodynamic parameters of the pulsed jets was carried out. [ABSTRACT FROM AUTHOR]

Freejet experiments have been conducted in a shock tunnel with a small airframe-integrated three-dimensional scramjet engine. The goal of the investigation was to examine the influence of airframe integration on the engine operation. The tests were conducted at a condition replicating flight at Mach 10.4 and a dynamic pressure of 48 kPa. The internal flowpath featured a rectangular-to-elliptical shape transition inlet designed for flight at Mach 12, an elliptical combustor with a constant area and diverging section, and a three-dimensional thrust nozzle. This flowpath was integrated with a forebody and streamlined external geometry similar to what would be expected on a flight vehicle. Gaseous hydrogen fuel was injected either through portholes on the inlet or a combination of inlet injection with a series of portholes behind a rearward-facing step at the combustor entrance. Experiments at a range of fuel equivalence ratios indicated that both fuel-injection schemes produced robust combustion without the use of ignition aids. Comparison with semifreejet tests of a three-times scale engine with a truncated forebody showed a slight reduction in the overall combustion generated pressure rise in the current experiments. These results indicate that airframe integration does not lead to significant degradation in the performance of this engine. [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]

Knock is the main obstacle toward increasing the compression ratio and using lower octane number fuels. In this paper, a small two-valve aircraft spark ignition engine, Rotax-914, was used as an example to investigate different methods to suppress engine knock. It is generally known that if the octane number is increased and the combustion period is shortened, the occurrence of knock will be suppressed. Thus, in this paper, different methods were introduced for two effects, increasing ignition delay time in end-gas and increasing flame speed. In the context, KIVA-3V code, as an advanced 3D engine combustion simulation code, was used for engine simulations and chemical kinetics investigations were also conducted using CHEMKIN. The results illustrated gas addition, such as hydrogen and natural gas addition, can be used to increase knock resistance of the Rotax-914 engine in some operating conditions. Replacing the traditional port injection method by direct injection strategy was another way investigated in this paper to suppress engine knock. Some traditional methods, such as adding exhaust gas recirculation (EGR) and increasing swirl ratio, also worked for this small spark ignition engine. [ABSTRACT FROM AUTHOR]

We studied a stable pilot flame system for a cannon type combustor of turbo-engines by adding both a frontchamber and a pilot flame chamber with an electric ignition system to the conventional two-stage system with two-stage combustion domains: a primary zone and a secondary zone. The electric ignition period was about 1 second with 2760V(theoretical) generated by Kochcroft-Welton circuit, and the fuel was butane gas. The inlet was 25m/sec and the pressure was about 400Pascal at the inlet of combustor.We set two cases for the outlet configuration: a converging nozzle and a turbine. The outlet temperature was 280C at the outlet velocity of 25m/sec, and the temperature was 230C at the outlet velocity of 12m/sec. A sufficiently large rotational speed and torque was obtained to rotate the compressor and fan of the turbine. Weanalyzed the ignition mechanism by solving two dimensional compressible Navier-Stokes equations and found that the role of the front chamberwas to maintain the pilot flame after the ignition shock. We confirmed experimentally that this system can ignite and preserve a stable pilot flame with quite high probability. [ABSTRACT FROM AUTHOR]

The supersonic combustion ramjet (scramjet) engine is expected to become the most efficient air breathing propulsion system in the hypersonic flight regime. The behavior of wall pressures propagation was examined using a direct-connect model scramjet experiment along with high-frequency pressure measurements. High-frequency pressure transducers are employed to record the history of pressure response during ignition process. A first order plus dead time (FOPDT) model is employed to model the S-shape step response curve. The time scale distributions of pressure propagation are reported in this paper. The characteristics of pressure propagation are closely related to the combustion mode of the scramjet. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model was developed to investigate the pilot injection process in homogeneous charge compression ignition engines using the Open-FOAM computational fluid dynamics package. In particular, the percentage of trapped fuel in the crevice before the main injection was studied. The model was validated by comparing the numerically determined in-cylinder pressures with experimental data available in the literature. The amount of trapped fuel in the crevice was determined for three pilot injections at crank angles of 180°, 120° and 80° before top dead centre and for various pilot injection percentages with different swirl ratios. The results showed that the percentage and the timing of a pilot injection directly and proportionally affect the contributions due to the fuel trapped in the crevice. It was also found that the swirl ratio is an important parameter in accumulation of the fuel in the crevice. [ABSTRACT FROM PUBLISHER]

The ignition transient phase in a model supersonic combustor with transverse fuel injection upstream of a cavity flame holder was experimentally investigated. Three types of ignition processes for scramjet startup were covered and compared: gaseous ethylene ignition, direct kerosene ignition by high-energy spark, and indirect ignition piloted by a preestablished ethylene flame. The flame propagation process and precombustion shock wave train dynamics were captured by using a high-speed camera and schlieren photography system. Three kinds of flame propagation failures were observed before a successful ignition result, after increasing the injection pressure to an ignitable pressure. The ignition limits of ethylene covered a much broader injection pressure range than ethylene when sparked directly in the cavity. The evolvement of the precombustion shock wave train for ethylene ignition indicated that the initiation of the ethylene flame was characterized by a subsonic flame expanding within the cavity. The enhanced mixing and evaporation process by the precombustion shock wave train provided another mechanism for piloted ignition, in addition to high-temperature heating of the ethylene flame. [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]

The influence of changes in fuel composition and heating value on the performance of an industrial gas turbine combustor was investigated. The combustor tested was a single cannular combustor for Siemens SGT-400 13.4MW dry low emission engine. Ignition, engine starting, emissions, combustion dynamics, and flash back through burner metal temperature monitoring were among the parameters investigated to evaluate the impact of fuel flexibility on combustor performance. Lean ignition and extinction limits were measured for three fuels with different heat values in term of Wobbe Index (WI): 25, 28.9, and 45 MJ/Sm³ (natural gas). The test results show that the air fuel ratio at lean ignition/extinction limits decreases and the margin between the two limits tends to be smaller as fuel heat value decreases. Engine start tests were also performed with a lower heating value fuel and results were found to be comparable to those for engine starting with natural gas. The combustor was further tested in a high pressure air facility at real engine operating conditions with different fuels covering WIs from 17.5 to 70 MJISm3. The variation in fuel composition and heating value was achieved in a gas mixing plant by blending natural gas with CO2, CO, N2, and H2 (for the fuel with WI lower than natural gas) and C3H8 (for the fuel with WI higher than natural gas). Test results show that a benefit in NOx reduction can be seen for the lower WI fuels without H2 presence in the fuel and there are no adverse impacts on combustor performance except for the requirement of higher fuel supply pressure, however, this can be easily resolved by minor modification through the fuel injection design. Test results for the H2 enriched and higher WI fuels show that NOx, combustion dynamics and flash back have been adversely affected and major change in burner design is required. For the H2 enriched fuel, the effect of CO and H2 on combustor performance was also investigated for the fuels having a fixed W1 of 29 MJ/Sm³. It is found that H2 dominates the adverse impact on combustor performance. The chemical kinetic study shows that H2 has significant effect on flame speed change and CO has significant effect on flame temperature change. Although the tests were performed on the Siemens SGT-400 combustion system, the results provide general guidance for the challenge of industrial gas turbine burner design for fuel flexibility. [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]

Two types of rotary valve were designed and fabricated for single-tube pulse detonation rocket engines. According to their driving components, they were named as gear-driven and cam-driven rotary valve, respectively. Preliminary experimental investigations were carried out to test their feasibility for supply of the single-tube pulse detonation rocket engine. Based on the gear-driven rotary valve, the single-tube pulse detonation rocket engine operated at 10 Hz stably. When operating frequency was increased to 15 Hz, inconsecutive detonations happened which was due to signal output block in the encoder. It was found that optimal ignition timing needed to be adjusted to obtain stable operation in the gear-driven rotary-valved pulse detonation rocket engine. Discussion on this was conducted. Operations of the cam-driven rotary-valved pulse detonation rocket engine were also performed. Fully developed and successive detonations at 40 Hz were successfully obtained although an explosion happened in initial tests. Experimental results showed that these two types of rotary valve were able to realize supply control of single-tube pulse detonation rocket engines effectively. [ABSTRACT FROM PUBLISHER]

Abstract: By using a plasma jet (PJ) torch with 1.5 kW input power as an igniter, successful ignition for liquid-kerosene fueled combustion experiment was conducted in a direct-connected supersonic test facility. The incoming flow has total temperature of 950 K and local Mach number of 1.8, corresponding to Mach 4 flight condition. In this study, several optical techniques, including high speed photography, high speed schlieren photography, and planar laser scattering (PLS) technique, were combined to study the ignition process, flame propagation, and mixing features of liquid kerosene fuel with air around the cavity. The effect of fuel injection position, injection pressure, and feedstock gas on ignition performance has been analyzed. The results indicate that local mixing is a critical factor for ignition. It is also shown that the PJ torch with N2 +H2 feedstock is superior to the PJ torch with N2 feedstock for the ignition of liquid-kerosene fuel. These results are valuable for the future optimization of kerosene-fueled scramjet engine when using a PJ torch as an igniter. [Copyright &y& Elsevier]

An unsteady flamelet progress variable (UFPV) model is evaluated for modeling autoignition and flame lift-off in diesel jets. Changes in injection pressure, orifice diameter, ambient temperature, density, and O2concentration are considered. In implementing the model in a Reynolds-averaged Navier–Stokes (RANS) code, a look-up table of reaction source terms is generated as a function of mixture fraction Z, stoichiometric scalar dissipation rate χstand progress variable Cstby solving the unsteady flamelet equations. It is assumed that the probability density functions (pdfs) of Z, χst, and Cstare statistically independent, and presumed functions are employed for the pdfs. Comparisons with experimental results show that the model is able to predict ignition delay and flame lift-off with reasonable accuracy in the RANS simulations. The quantitative agreement between computed and measured results depends on the definitions employed to quantify autoignition time and lift-off height, but, in general, the agreement is within 25%. [ABSTRACT FROM AUTHOR]

It is widely known that the use of biofuels is still limited by their particular physical–chemical properties, which are less advantageous than fossil fuels (diesel and gas petro fuels). This amounts to significant apparent differences related to the functional parameters and pollutant emissions (NOx) for compression–ignition (CI) engines fueled with biofuels. Elimination of these shortcomings can be achieved via a modern injection system, an electronically controlled high-pressure common rail system. This is why achieving a precise injection process is necessary, and injection parameters are directly influenced by the phenomenon of cavitation and the erosion process of the injector nozzle. This article presents a computer simulation study of the influence of various fuels (biofuels) on the effect of erosion (and intensity) on the performance of the injection process. The magnitude of the injector nozzle erosion process is different using diesel fuel compared to using biodiesel and pure vegetable oil, implying the need for greater attention to injection system self-development design programs and to the overall maintenance characteristics of CI engines fueled with biodiesel. There are also different patterns of velocity at the nozzle exit for each of the studied cases, differences that have a direct bearing on future fuel spray development inside the combustion chamber. [ABSTRACT FROM AUTHOR]