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The dynamic mode decomposition(DMD), based on the Koopman analysis, is a tool capable of spatiotemporal analysis of one-dimensional signals to three-dimensional CFD data. Outputs of the DMD consist of amplitudes, frequencies, decaying rates, and spatial modes. However, the effects of data resolution (size) and the quantitative analysis of the DMD are limited to one-dimensional signal data. In this study, the effects of the data resolution and quantitative analysis using scaling factors 2/√M on the DMD amplitudes and √M on the DMD spatial mode strengths are investigated, with M being the data size. Firstly, proofs of the scaling factors for one-dimensional and two-dimensional data are presented. Second, the effect of data size on amplitudes and spatial mode strengths and their scaled results are confirmed using one-dimensional artificial signal data, two-dimensional artificial signal field, two-dimensional vortex shedding simulation, and a two-dimensional pulsating flow experiment data with various data resolutions. The results show that the amplitude increase proportionally to the size of the data, and the spatial mode strength is inversely proportional to the size of the data in all cases. As a result of applying the scaling factors in one-dimensional artificial signal and two-dimensional artificial signal field data, the amplitudes and spatial modes contain the same values regardless of the change in resolutions. The scaled amplitudes and spatial mode strengths on vortex shedding simulation and two-dimensional laminar pulsating jet show good agreements with slight differences regardless of the resolution change. The proposed scaling factor can be applied to compare data quantitatively obtained with different resolutions.

Influences of the spark-plug mounting angle on the spark ignition of gasoline-air mixtures in high velocity flows were investigated in a combustion chamber connected at the end of a rapid compression machine. Gasoline-air mixture with an air excess ratio of 1.9 was combusted. A spark plug was mounted on the combustor wall at angles of −30°, −15°, 0°, 15°, and 30°. Flow velocity at the spark plug was varied from 5 to 20 m/s by adjusting the driving pressure of the piston. Time resolved spark-discharge voltage and current were measured with a voltage and a current probe, and the discharge energy was calculated using them. Behaviors of the discharge channel were observed optically with a high-speed video camera in addition to simultaneous measurements of OH* chemiluminescence from the flame kernel with the other high-speed video camera. The results showed that the averaged induction time of flame kernel initiation increased monotonously with an increase in the plug mounting angle from −30° to 30° in flows with velocities of 5 and 10 m/s. However, the increasing trend became less obvious with velocities of 15 and 20 m/s, involving with larger variation in induction time. Discharge duration became shorter with an increase in the plug mounting angle regardless of the same setup. Negative mounting angle resulted in frequent short-circuits/restrikes at the early phase. On the other hand, positive mounting angles resulted in a longer discharge channel length, higher energy deposition before the first restrike and longer induction time of the first restrike. Although the discharge channel at positive mounting angles elongates along with the flow, the early flame kernel was developed toward the center of the swirl. Thereafter, flame propagated outward from the central region, where the flow speed was lower than the downstream area. Negative mounting angle led to a more oval flame kernel shape and the propagation closer to combustion chamber wall, which showed a shorter induction time of flame kernel initiation in the cases of 5 and 10 m/s. However, fragments of the flame kernel tended to be formed due to frequent short-circuits with the velocities of 15 and 20 m/s. This resulted in the large variation of the induction time.

Self-sustained oblique detonation behind a spherical projectile formed in a non-uniform H2/O2/Ar mixture was numerically investigated. A hypersonic combustible mixture flow around a 4.76 mm diameter body was modeled to be flowing at 2500 m/s and 100 kPa. The concentration gradient was prescribed applying the Gaussian distribution to hydrogen concentration. Axisymmetric Euler equations including a detailed kinetics of 9 species and 27 elementary reactions were solved with an explicit 2nd-order time integration scheme combined with point implicit method for chemical reaction. Oblique detonation was always obtained when the mixture on the centerline was stoichiometric, as it is for a uniform mixture, and a broader range of equivalence ratio could sustain oblique detonation far from the sphere. Local detonation angle was revealed to reasonably match Chapman-Jouguet analytical solutions with a minor difference attributed to curvature, less reactive composition, and the concentration gradient. Also, a strongly fuel-rich region encountered decoupling of the shock-flame, in which an abrupt deflection of the shock front appeared. These decoupling phenomena can be attributed to a slower kinetics of a less reactive mixture. All of interesting findings in this study will also benefit understanding of various form of detonation in non-uniform mixture taking advantage of the analogy between them.

This study presents the effect of the microwave radiation control parameters (i.e. pulse width and pulse repetition frequency (PRF)) on laser-induced plasma (LIP) ignition in a constant volume combustion chamber (CVCC) of the n-butane-air mixture (ϕ = 0.7) under atmospheric conditions. The ignition behaviors were investigated using ignition success rate, high-speed imaging, net heat release, and plasma temperatures. The result shows that the ignition success rate (ISR) defined in this study gets increased with an increase in the pulse width. Moreover, ISR was found insensitive to PRF at higher pulse width. This implies that the combustion advancement occurred at a faster rate on the coupling of high PRF and longer pulse width microwave radiation with the LIP. The ignition delay evaluated based on the net heat release gets reduced; consequently, the ignition advances much faster with an increase in the pulse width as compared to the small pulse width. The direct images of early combustion revealed an expansion of the plasma when the coupling between microwave radiation and the LIP occurs. This expansion of the plasma is the generation of active radicals due to the direct collisions of electrons with ionized molecules via interaction between the electromagnetic field and free electrons. Besides, the combustion advancement mechanism was investigated using plasma emission spectroscopy. The plasma temperatures which are closely related to the chemical reaction rates were calculated by fitting the homo-nuclear N2(2+) system. The results show that the ignition success rate enhances with an increase in the rotational temperature. This might be due to much faster rotational-transnational energy transfer as compare to vibrational-transnational energy transfer. This implies that the bulk gas temperature increases quickly since the rotational temperature is approximately equal to the gas temperature.

Combustion instabilities were investigated experimentally for a hydrogen-rich combustion in a model afterburner installed at the end of a high-enthalpy wind tunnel. Air was supplied at 0.3 MPa and 950 K. The combustion instabilities were studied with the time-resolved measurements of a near-infrared (NIR) emission from water molecules over 780 nm using a high-speed video camera. Pressure was also measured in the combustor. The pressure and the NIR images were analyzed by data-driven approach, which include the fast Fourier transform (FFT), the wavelet transform, the dynamic mode decomposition (DMD) and the Gaussian process latent variable methods (GP-LVM). Thermoacoustic instability was observed under a rich condition, and the amplitude of the pressure oscillation was the maximum at the overall equivalence ratio of approximately 2.4 or 2.7 as a result of the FFT. The combustion dynamics were investigated in detail for an experimental run at the equivalence ratio of 2.4. A pressure spectrogram indicated a flame–vortex interaction with a Strouhal number of 0.5 (2300 Hz), thermoacoustic instability (560 Hz), and their transitions with the wavelet transform. For NIR images, the same tendency was also observed in the spectrogram of the modes obtained by the Gabor-filtered DMD, which could clearly resolve the high-order harmonic modes of the flame–vortex interaction and the thermoacoustic instability. Furthermore, NIR images were analyzed with GP-LVM to study the evolution of the combustion dynamics in a three-dimensional latent space. Recurrence plots with the Euclidean distance function were used to visualize the evolutions of the combustion dynamics. A limit cycle behavior of the flame–vortex interaction was clearly observed, whereas the limit cycle of the thermoacoustic instability showed more complicated behaviors. The transition behaviors of the instabilities were observed in the recurrence plots in detail, indicating that the flame–vortex interaction excited the fourth harmonic mode of the thermoacoustic instability, followed by the basic mode.

The combustion instabilities of supersonic combustion were investigated experimentally in a laboratory-scale scramjet combustor with a cavity flame holder. Ethylene was injected transversely from an orifice to the supersonic flow of Mach 2 with a stagnation temperature of 1900 K and a total pressure of 0.37 MPa. The dynamic pressure, CH* chemiluminescence and shadowgraph images were measured with a pressure sensor and a high-speed video camera. Dynamic pressure was analyzed by fast Fourier transform, and time-resolved CH* chemiluminescence images were modally decomposed by the sparsity-promoting dynamic mode decomposition (SP-DMD). The results indicated that two combustion instabilities were observed for cavity shear-layer stabilized combustion and the oscillation between jet-wake stabilized and cavity shear-layer ram combustions for the power spectral density (PSD) of pressure. In the case of the combustion instability of cavity shear-layer stabilized combustion, a dominant peak of approximately 128 Hz was observed for the PSD of pressure. This instability corresponded to an entire flame oscillation of the cavity shear-layer stabilized combustion, which was validated by the SP-DMD and a low rank reproduction with 10 modes. This was driven by a fuel injection oscillation in the injection orifice. In the case of oscillation between the jet-wake stabilized and the cavity shear-layer ram combustions, peaks around 1600 Hz were observed for the PSD of pressure. This mechanism was also explained by the SP-DMD modes and a low rank reproduction using within 10 modes. The DMD and shadowgraph images indicated that the vortex formed by a separation of the boundary layer induced a strong jet-wake flame, resulting in the temporal thermal choke followed by cavity shear-layer stabilized ram combustion. The data-driven approach with SP-DMD clarified the combustion instability mechanisms of the supersonic combustion in detail.

Droplet combustion behavior of degraded and non-degraded fatty acid methyl ester (FAME) were studied under microgravity condition. Accelerated oxidation tests were conducted for methyl laurate (LME) and methyl oleate (OME) as representatives of FAME, and the changes in viscosities and compositions were evaluated. The Rancimat method was employed for oxidizing fuels at 100 °C for 6 or 24 h. The viscosities and compositions were measured with an Ubbelohde viscometer and a gas chromatography-mass spectrometer (GC–MS), respectively. Results showed that various oxidation products were formed only for OME during the oxidation test, and the amounts increased with an increase in the oxidation time. In addition, viscosities of degraded OME were higher than those of non-degraded OME, which would be attributable to the formation of oxidation products. Subsequently, the droplet combustion experiment was performed at 0.1 MPa, 750 °C in CO2-rich atmosphere under microgravity. Droplet combustion behavior was analyzed in terms of evolutions of droplet diameter squared and the instantaneous burning rate. As a result, the instantaneous burning rate of LME maintained almost constant during its combustion period as well as n-decane, which indicates that the behavior of instantaneous burning rate was not influenced by the difference in fuel properties between hydrocarbon and unsaturated FAME. In contrast, the droplet of degraded OME exhibited puffing during the evaporation and combustion periods. This indicates that oxidation products with various volatilities were formed in the degraded fuels, which resulted in puffing. As oxidation proceeded, the release rate of liquid and vapor by puffing decreased owing to the increased viscosity, which causes puffing more frequently. Furthermore, the time of bubble formation increased with an increase in the initial droplet diameter owing to an increase of the heat capacity of the droplet. On the contrary, the time of bubble formation did not vary drastically with the oxidation time, which was probably owing to a limited change in the heat capacity during the oxidation test. This study addresses the droplet burning behavior of LMEs and OMEs with an emphasis on the effects of oxidation.

Combustion of electric wires is the most probable cause of fire in human space activities. Therefore, the fire performance of electric wires in microgravity conditions must be thoroughly analyzed. This study investigates the opposed-flow flame spread and its limits in electric wires preheated by external radiation, under both normal gravity and microgravity, to understand their fire performance when exposed to external heat sources in such gravity conditions. The experiments were performed on low-density polyethylene (LDPE)-insulated copper (Cu) wires having an outer diameter of 4 mm and differing in core diameter (2.5 and 0.7 mm, corresponding to insulation thicknesses of 0.75 and 1.65 mm, respectively). Both standard and black LDPE insulations were used to study the effect of radiation absorption on the wire preheating and subsequent flame spread. The comparison of the flame spread limits revealed that the wire with the thicker Cu core was less flammable under both normal gravity and microgravity; in particular, its flammability further decreased in the case of microgravity, in contrast with thinner electric wires (~ 1 mm outer diameter), which exhibited higher flammability in the same gravity condition. These results suggest that different mechanisms, for thicker and thinner wires, determining the critical conditions to sustain flame spread under microgravity. This study provides valuable information about the fire performance of electric wires in space gravity.

This work addressed laser-induced spark ignition (LSI) of dimethyl ether (DME)-air mixtures in a microgravity environment which was simulated in a laboratory-scale drop tower, to study fundamental ignition characteristics in microgravity. A laser-induced spark was adopted as a pilot source because LSI has potential benefits as compared to conventional electrical spark ignition: no wall effects, no heat-loss to electrodes, etc. Ignition characteristics, therefore, can be investigated without effects of ignition source via LSI. DME-air mixtures filled in a combustion chamber on a framework package were ignited during free-fall. The ignition energy and flame kernel development were investigated with varying the equivalence ratio and the laser pulse energy. The minimum ignition energy (MIE) was calculated statistically via the logistic regression method. Comparing MIE and flame kernel development between normal gravity and microgravity, MIE in microgravity was lower than that in normal gravity over tested equivalence ratios ranging from 0.575 to 0.675. The difference of MIE became significant with decreasing the equivalence ratio, the maximum value of which was 7.0 mJ at the lowest equivalence ratio. Flame kernel was deformed in normal gravity due to buoyant force. The flame kernel in the bottom region was often not able to develop against buoyant force. Ignition succeeded consequently when only the top of flame kernel developed into a self-sustaining flame to propagate throughout the entire chamber. This flame deformation was noticeable as the equivalence ratio decreased because the flame propagation speed also decreased. On the other hand, in microgravity, reduced buoyant force allowed the flame kernel to stably develop into a self-sustaining flame. This is the first time that both MIE and flame kernel development via LSI were found experimentally to be well affected by gravity.

This work studied the piloted ignition of electrical wires in both normal gravity and microgravity using the laser-induced spark. Unique experiments were conducted in the microgravity parabolic flight with laboratory wires under the oxygen concentration of 14–21% and external radiation of up to 15.9 kW/m2. The wire sample consists of a 2.5-mm thick core, using the solid copper (Cu) or the hollow stainless steel (SS) tube, and a 0.75-mm thick black polyethylene (PE) insulation. This is the first piloted-ignition experiment on solid fuel in microgravity with the laser-induced spark as the pilot. Experimental results show that regardless of the oxygen level, the ignition delay time is always smaller in microgravity than in normal gravity, indicating a higher fire risk in the microgravity space environment. As the heat flux and the oxygen concentration increase, auto-ignition is observed. Moreover, if the core is exposed to the external heating source, it can heat the insulation to promote the ignition, different from the heat sink found in past ignition research. This unique research provides valuable information about the fire risk of electrical wire in microgravity and future long-term space travel.

Supersonic combustion behaviors in a Mach 2.0 scramjet model combustor were experimentally investigated at stagnation temperatures of 1700–2300 K. An ethylene/methane fuel mixture was used, and the mole fraction of ethylene and the equivalence ratio were varied. Six typical combustion modes were classified based on high-speed imaging of CH* chemiluminescence at 50,000 fps, shadowgraph imaging at 4000 fps and pressure distributions. Ram mode combustion was observed, without thermal choke at the nozzle throat. Mode maps indicated that an increase in the ethylene concentration improved the supersonic combustion performance. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) of sequential images of CH* chemiluminescence were also conducted. POD and DMD mode spectra showed large peaks in the frequency range of 100–500 Hz for cavity shear-layer stabilized combustion. Although the power of the spectra decreased, peaks were also observed in the same frequency range for jet-wake stabilized and ram combustion. In the case of the ram combustion mode, the peak heights decreased. The FFT of several primary POD modes and the power spectra of DMD modes showed peaks with similar frequencies. Combustion modes could be classified from these spectra. The fundamental combustion frequencies were captured by the modal decomposition of the high-speed images of CH* chemiluminescence.

This work studied laser-induced spark ignition for dimethyl ether (DME)–air mixtures with low velocities ranging from 10 cm/s to 100 cm/s, with an emphasis on the minimum ignition energy (MIE) and flame kernel development. Part of ignition experiments were conducted in a microgravity environment that was simulated in a laboratory-scale drop tower to eliminate natural convective flow. MIE was significantly affected by flow velocity. MIE first decreased with increasing flow velocity but then increased with further increasing flow velocity. The role of the flowing mixture was to act as a fuel supplier in a low-velocity flow, and in contrast, to increase convective heat loss in a high-velocity flow. The competition between these roles determined MIE. The effect of flow direction on MIE was also investigated by inclining the combustion chamber. The minimum value of MIE was the same in all flow directions. Therefore, changing flow directions was merely intended to vary the flow velocity by adding or removing buoyant flow to or from the forced flow, which caused the MIE distribution to shift from side to side. The flame kernel hardly developed against a forced flow and was often locally quenched. Flame deformation was noticeable as flow velocity increased. Quenching is comparable to the heat loss from the flame kernel. Therefore, flame kernel development should have a significant effect on MIE. Flame stretch was also found at a high velocity. A high-velocity flow did not allow the flame kernel to stably develop into a self-sustaining flame. Thus, higher energy was required to form a large-volume of flame kernel to overcome both conductive heat loss and the effect of flame stretch. Accordingly, flame kernel behaviors were very much a concern regarding the mechanism via which to determine MIE.

Combustion rig tests of an RQL (rich burn quick quench lean burn) gas turbine combustor and of a single concentric lean-burn combustor were performed using kerosene (Jet A1), 100% hydro-treated ester and fatty acid (HEFA) alternative jet fuel and their blends. The results showed a clear tendency of suppression of non-volatile PM (nvPM) emission with an increase in the blending ratio of HEFA. Combustion test results were examined with the help of basic atmospheric and high-temperature and high-pressure testing. Blowout conditions and flame oscillations are also discussed.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2018-04-16, 資料番号: PA1810060000

Thermometry using two-band near-infrared (NIR) emissions less than 1.1 μm has been implemented for combustion gases formed on a flat flame burner. The gas temperature was calibrated using a SiC thin filament pyrometry (TFP) above 1500 K for H2/air and CH4/air mixtures. The emission spectra were measured with a spectrometer and a monochrome CMOS sensor with an image doubler and two bandpass filters. Two pairs of bandpass filters (850 nm/925 nm and 975 nm/925 nm) were tested to measure temperature. Results indicated that strong H2O emissions existed in the NIR region, and they became stronger with an increase in the temperature. The CO2 emission spectra in the NIR region were negligible compared to that of H2O. Calibration temperature lines were obtained as a function of the intensity ratio of the two-band emissions in the NIR region. Using a CMOS camera, the uncertainties of temperature were 59 K for 850 nm/925 nm and 145 K for 975 nm/925 nm. The resolutions were 0.7 K for 850 nm/925 nm and 1 K for 975 nm/925 nm. Using the calibration line, two-dimensional temperature measurements were performed for H2/air flames, CH4/air flames and Bunsen flames of 13A city gas. For the high temperature region above 1500 K, a quantitative two-dimensional temperature field was measured for lean and rich combustion. In addition, temperature measurement at high pressure was conducted for combustion gas of propane/air mixture with the equivalence ratio of 0.7 using a rapid compression machine. Temperature could be estimated using the calibration line obtained for the flat flame burner. We have demonstrated the performance and the possibility of thermometry two-band H2O emissions in the NIR region.

In electrical wires with insulations that burn and melt, the dripping of molten insulation can change the wire fire behavior, ignite nearby objects, and enhance the fire spread. Dripping is a result of gravity and depends on the insulation type of the wire and its orientation. In this work, the opposed flame spread over simulated electrical wires was studied with emphasis on the effect of the core and insulation type, and the melting and dripping of insulation. To facilitate the study, “laboratory” wires with polyethylene (PE) as insulation, were selected for the experiments. Horizontal and vertical wires of 8- and 9-mm diameter with soild copper (Cu) and hollow stainless steel (SS) cores and two types of PE insulations, low density and high density, were tested. The sizes of the laboratory wires were selected to facilitate the study of the effect of the type of insulation, the ratio of insulation to core thickness, or the thermal properties of the core, on the wire fire behaviors. Experimental results show a strong dependence of wire orientation on molten insulation dripping and flame spread. For horizontal wires, the flame spread is faster with Cu core than SS core because of a larger heat transfer ahead of the flame through the core. For vertical wires, the flame spread rate is dominated by the downward dripping of the molten insulation, but is comparatively not sensitive to the core material. Increasing the opposed flow speed, the flame gets closer to the wire which enhances the heating from Cu core and locally increases the flame spread. The effects of other parameters such as oxygen concentration, wire diameter, and insulation material are also discussed. This work provides important support to a larger project aimed at studying the fire behavior of electrical wires in a spacecraft environment. Without gravity, the dripping of molten material will not occur in a spacecraft, thus, characteristics of the flame spread process over a wire insulation material that melts during the spread of the flame will be drastically different on Earth or in a spacecraft.

An experimental study on dielectric barrier discharge (DBD) plasma-assisted ignition was performed through the use of a plug-type DBD electrode at various pressures p up to 500 kPa. The ignition behaviors of lean methane–air mixtures with high-frequency DBD were investigated near the lean limit in a constant volume chamber. Using a high-speed video camera, the plasma formations of the DBD were observed for air and a mixture of the equivalence ratio ϕ = 0.3. Plasma spectroscopy was also conducted with an echelle grating and imaging spectrometers. The time-averaged detailed plasma emission spectra from 400 nm to 920 nm and the transient spectrograms in the vicinity of the N2 second positive system (372–382 nm) for ϕ = 0.5 were measured. The minimum ignition energy (MIE), minimum discharge duration, and minimum amplitude of the applied voltage were obtained through logistic regression. The results indicated that a plasma emission lifetime became much longer than the discharge time with an application of the high-frequency sinusoidal voltage at high pressure. The longest plasma lifetime was observed at a discharge frequency of 1,180 kHz. The plasma lifetime showed the maximum at a specific pressure. The detailed spectroscopy also showed that continuum spectra at 500 kPa could be fitted to the Planck's curve with temperature above 3000 K at 500 kPa. The profiles of the MIE as a function of pressure exhibited the minimum in the vicinity of 100 kPa. The existence of the optimal frequency for high-efficiency ignition was shown. At the fixed frequency, an increase in the applied voltage amplitude resulted in a decrease in the MIE. In contrast, the effects were saturated over a certain threshold. At 500 kPa, time-resolved plasma spectroscopy showed a rapid local gas heating to the flame temperature over a specific frequency of the applied voltage. This indicated that there was the optimal frequency where the MIE was the minimum.

Dripping of polymer insulations in wire fire has a potential risk of igniting nearby objects and expanding the size of fire, but has not been well studied so far. In this experimental study, dripping behaviors during the flame spread over horizontal and vertical polyethylene (PE) insulated wires were investigated without external airflow. Two different wire dimensions – core/wire diameter of 3.5/8.0 and 5.5/9.0 mm – and three different PE insulations were tested. To identify effects of the core, wires with solid copper (Cu) core, hollow stainless steel (SS) core, and without core were tested, and both core and insulation temperatures were also measured during the flame spread. Experimental results showed that the high-conductance copper core acted as a heat source downstream to increase the flame-spread rate. However, in the upstream burning zone, the copper core also acted as a heat sink to cool the molten insulation and reduce its mobility. Thus, the copper core extended the residence time of molten insulation inside the flame to facilitate the burning while reducing the dripping. Moreover, for the downward flame spread, the heating by the dripping flow of hot molten insulation dominated over the heating by the core. The downward dripping flow is driven by gravity while limited by the viscous and surface tension forces. Therefore, the limited dripping flow along the cooler copper core reduced the downward flame spread. The trend of results was also found to be insensitive to the type of PE insulation. This is the first time that within a single flame, the simultaneous dual effect of the heat source and heat sink for the wire core was observed, and the influence of dripping on the flame spread over the wire was discovered.

The pintle injector is a promising candidate for propellant-injection systems for various rocket engines. However, fundamental studies focusing on the pintle injector are rather limited, and the effects of key design parameters on combustion behaviors of the injector are still not identified. Therefore, combustion tests of an ethanol/liquid-oxygen rocket-engine combustor with a planar pintle injector are conducted to investigate the effects of total momentum ratio and O/F on the combustion characteristics of the pintle injector. The total momentum ratio and O/F vary from 1.0 to 2.2 and from 0.32 to 3.65, respectively, whereas the combustion pressure is kept constant at approximately 0.4 MPa. The spray structures are observed with high-speed cameras. The characteristic exhaust velocity (C*) efficiency increases with the increase in O/F in the fuel-centered configuration, and the opposite tendency is observed in the oxidizer-centered configuration. The C* efficiency decreases with the increase in the total ...

Ignition behaviors of close dual-point laser breakdown spark ignition were investigated experimentally for methane/air mixtures at 0.1 and 1.0 MPa in a constant volume combustion vessel. Absorbed energy was measured from the difference between incident and transmitted laser rays using two joule meters as the ignition energy, and the behaviors of the initial flame kernel were observed with Schlieren photography using a high-speed video camera. First, the ignition behaviors of a single-point laser breakdown spark ignition were investigated. The results indicated that the effect of the focusing lens on the minimum ignition energy (MIE) was limited in terms of the absorbed energy. Although the MIE at 1.0 MPa was lower than that at 0.1 MPa near the stoichiometric equivalence relationship was reversed near the lean limit. For lean mixtures, local quenching of the initial flame kernel was clearly observed including third lobe region especially at 1.0 MPa. In the case of the close dual-point sparks for an equivalence ratio of 0.6, formation of a third lobe was suppressed. When the dual spark gap was large, two flame kernels were formed as observed in the case of the single spark. An optimal gap in which the absorbed energy was minimal for successful ignition existed depending on the pressure, although the magnitude of the associated energy was not so different from that in the case of the single spark. However, the growth rate of the initial flame kernel formed by the close dual sparks was considerably higher than that formed by the single spark, especially at 1.0 MPa. Enhancement of the flame kernel development due to the close dual spark was clearly observed.

We performed numerical simulation against oblique detonation formed in an inhomogeneous hydrogen–air mixture. Planar two-dimensional Navier–Stokes equations were solved with the chemical source terms calculated with a detailed chemical kinetics including 9 species and 27 elementary reactions. Fuel concentration gradients described by the Gaussian function were introduced into two different conditions: (A) a Mach 8 flow onto a 28.20° wedge with a static pressure of 8.50 kPa, and (B) a Mach 8 flow onto a 23.8° wedge with a static pressure of 34.0 kPa. Fuel-rich conditions on the centerline generated V-shaped deflagration front under the former condition, and V + Y Mach stem under the latter due to the strong concentration gradients near the surface. These shapes caused a couple of triple points to appear on the incident shock front. Transition to detonation occurred twice along the front in some cases. The apex of V-shape was always located in a similar position due to almost constant thermochemical properties of the incoming streamline. The threshold for emergence of these V-shapes can be reasonably attained by the magnitude of induction length gradient close to the wedge.

Combustion rig tests of an (RQL) rich burn quick quench lean burn gas turbine combustor and of a single concentric lean burn combustor were performed using kerosene (Jet A1), 100% hydro-treated ester and fatty acid (HEFA) alternative jet fuel and their blends. The results showed a clear tendency of suppression of non-volatile PM (nvPM) emission with an increase in blending ratio of HEFA for the RQL gas turbine combustor and for low-load conditions in the single concentric lean burn combustor. The O-ring sink test of the fuels was also performed that showed HEFA to have less effect on the strength and volume of the O-rings compared to kerosene. Chemical analysis of both the fuels showed differences in aromatic content, wherein aromatics in kerosene reached up to around 20% while HEFA contained no aromatics at all, which is considered to be the cause for differences in nvPM emission and the effect of fuels on O-rings. Storage stability was tested for both Jet A1 and HEFA that showed high stability.

This work studied flame spread over polyethylene (PE)-insulated wires at high pressure with an emphasis on the effect of ambient pressure. Laboratory simulated wires were used as a tested wire, which consisted of semi-transparent low-density PE (LDPE) tube as an insulation and copper (Cu) rod or stainless-steel (SS) tube as a core. Increasing ambient pressure produced much soot, leading to an intensively yellow luminous flame. Flame spread rate for Cu rod increased with ambient pressure, whereas that for SS tube decreased. A qualitative analysis via a simplified flame spread model shows that the flame heat flux increases with ambient pressure, but consequently the flame heating decreases due to the decreased flame heating length. Therefore, the flame spread rate for SS tube decreased with increasing ambient pressure. For Cu rod, however, the core heating is dominant enough to compensate the decreased flame heating, and thus the ambient pressure positively affected the flame spread. During the flame spread, melted insulations were often dripped down, but it reduced with increasing ambient pressure. For Cu rod, no dripping occurred when further increasing ambient pressure, which indicates that there is the dripping limit at a high-pressure region. These findings should contribute to electrical fire safety in elevated pressure environments.

A parametric numerical study was performed to study effects of the nonuniform premixing of a hydrogen–air mixture on the structures of oblique detonation and shock-induced combustion formed on the surface of a hypersonic spherical projectile. Axisymmetric two-dimensional Navier–Stokes equations were solved with a detailed chemical kinetic mechanism involving nine species. A smooth-front oblique detonation formed on the hypersonic projectile at a Mach number of 6.46 under a uniformly stoichiometric condition was taken as the completely premixed case. For nonuniform cases, hydrogen mole fraction was distributed at the inlet boundary based on the Gaussian function. As the nonuniformity was increased, the following occurred: 1) deformed oblique detonation, 2) oscillating shock-induced combustions with nonuniform corrugated structures, 3) steady shock-induced combustions with (and without) the minimum induction length located outside the centerline, and 4) nose-confined combustion. Analyses on distributions of...

Supersonic combustion ramjet technology using liquid hydrocarbon fuels is invaluable due to great benefits of the energy density per unit volume and handling. This study carried out experiments to investigate the effects of the components in thermally cracked n-dodecane on ignition and supersonic combustion behavior at supersonic speeds (Mach 2.0) and at stagnation temperatures between 1800 K and 2400 K in a supersonic model combustor with a cavity flame holder. Components of the thermally cracked n-dodecane at 1.0 MPa in a flow reactor were experimentally measured by using a gas chromatograph. Based on the measured components, surrogate fuels consisting of main components of cracked n-dodecane, hydrogen, methane, ethane and ethylene were defined, and fractions of the component was varied at less than 10%. Ignition and supersonic combustion behaviors were tested for single component and surrogate fuels. In all cases where supersonic combustions were achieved, ignitions were firstly observed in the boundary layer at the expanding cross section zone. The flames, then, propagated upstream and were held at the cavity. Two combustion modes, jet wake stabilized and cavity stabilized flames were observed based on the distribution of rise in pressure and OH chemiluminescence. In cases of pure fuel injections, ignitions for ethylene injections could be observed at a stagnation temperature of 1800 K. On the other hand, ignitions for methane and ethane injections could be observed above a stagnation temperature of 2200 K. Ignitions for surrogate fuel injections were also investigated. These ignition limits were also validated by a simple estimation of chemical reaction and residence times. It was also found that an increase in fuel temperature improved the ignition limits although the effect of the dynamic pressure ratio between the fuel injection and the main flow was rather limited in the present study., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1610082000

This study carried out experiments to investigate the combustion behavior of ethanol at Mach 2.0 and stagnation temperatures between 1800 and 2200 K using a supersonic combustor with a cavity flame holder. The distribution of droplet diameters in fuel sprays was measured by a Laser Diffraction Spray Analyzer to estimate the vaporization time of fuel in the combustor. CH chemiluminescences and Schlieren photographs were measured by a video camera to identify the combustion mode and to measure the penetration height of fuel in the supersonic flow. Both gaseous and liquid ethanol injections were tested to examine the effect of the fuel vaporization process. The minimum equivalence ratio of injected fuel to air in the main flow and the minimum stagnation temperature for successful combustion were derived based on Weibull distribution statistics. Two combustion modes, intensive and transient, were identified based on the distribution of a rise in pressure and the CH chemiluminescence emissions over the cavity. Supersonic combustion was observed above the stagnation temperature of 2200–2000 K for liquid and gaseous ethanol injections. Compared with previous research, the reactivity of ethanol was located between the levels of ethane and ethylene. The ignition limits of the stagnation temperature for gaseous and liquid ethanol injections were also estimated and validated by calculating fuel breakup, vaporization, residence times and chemical reaction time. A fuel penetration height over 10 mm resulted in no supersonic combustion for the equivalence ratio of 1.0, although this enhanced fuel and air mixing.

The burning characteristics of isolated fuel droplets for ethanol, 1-butanol and n -decane were experimentally examined in carbon dioxide-rich or argon-rich environments under microgravity. The ambience consisted of oxygen, nitrogen, carbon dioxide and argon. The concentration of oxygen was fixed to be 21% in volume percentage, and that of carbon dioxide or argon was varied from 0% to 60% in volume percentage. Detailed measurements of the droplet surface areas were conducted using a high speed video camera and the effective droplet diameter squared were calculated. The instantaneous burning rates were calculated from the evolutions of the droplet diameter squared. When the soot production was limited, the instantaneous burning rate increased with time. Variations of the instantaneous burning rate with the instantaneous droplet diameter squared were obtained. The results indicated that the instantaneous burning rate was correlated with the droplet diameter squared after the initial heating period and that was approximated by the first order linear function. From the function, an indicative burning rate and the dependence of the burning rate on the droplet diameter squared were obtained. The indicative burning rate increased with increase in the argon concentration and decreased with increase in the carbon dioxide concentration. They were well related to the thermal diffusivity of the ambient gas regardless of the gas components. The existence of other factors influencing the burning rate was inferred as well. On the other hand, the dependence of the instantaneous burning rate on the droplet diameter squared was attributed to the fuel properties.

A flame kernel initiation of methane/air combustible mixtures in the spark ignition process was investigated using a two-dimensional theoretical model including a detailed description of gas-phase chemical kinetics, shock capturing scheme and diffusive molecular transport. Interactions of chemical reactions and diffusive transports of radicals in the process of the flame kernel initiation were investigated. Although the model of plasma might be oversimplified, the qualitative behavior of OH for hydrogen/air mixture agreed well with experimental one. The influences of diffusive molecular transport and ignition energy on the flame kernel initiation were discussed. As a result, in the early stage of the flame kernel development for methane/air mixture, the hot gas expansion was dominated by a flow whichwas inducedby the blastwave and the thermal gas at the electrode gapwas self-sustainedwith anapplication of minimum ignition energy. The induction time of the flame kernel initiation strongly depended on the ignition energy and effects of preferential diffusion of lighter molecules in the early phase of the flame kernel development are outstanding especially in the case of low ignition energy near the minimum.

Combustion characteristics of liquid hydrocarbon fuels are studied in a model combustor of SCRAM jet engines. The Mach number and total pressure of main flow in the combustor are 2.0 and 0.38 MPa, respectively, and the total temperature is varied from 1800 to 2400 K. Five kinds of n-alkane fuels such as n-heptane, n-octane, n-decane, n-tridecane and n-hexadecane are employed in experiments. Fuels are injected with a carrier nitrogen gas perpendicular to the mail flow in the combustor and the self-ignition behavior is investigated. The results show that the liquid fuels with lower carbon number have better self-ignition performance. This suggests that physical properties of liquid fuels such as volatility have a dominant effect on the self-ignition. The flame-holding behavior is investigated with the addition of pilot hydrogen to carrier nitrogen gas. The critical equivalence ratio at which the stable combustion keeps after cut-off of the pilot hydrogen is obtained. The relationship between the critical equivalence ratio and carbon number of fuel shows that fuels with the carbon numbers from 8 to 10 have the best flame-holding performance among the tested fuels. These experimental results can be expressed qualitatively by the simplified analysis with the concept of physical and chemical induction times.

Flame is affected by an external electric field because it contains ions and electrons related to chemical reactions. On the other hand, the movement of ions and electrons affects the external electric field due to their charge. This paper reports the combustion experiments of ethanol droplets in vertical electric field with variable distance electrodes apparatus in order to discuss the change of the external electric field due to the existence of flame. From a one-dimensional steady-state analysis, if the electric field is changed spatially, its effect on combustion behavior is aligned with V 2 / L 3 and not V / L, where V is the applied voltage between electrodes, and L is distance between the electrodes. The droplet is burned between the two horizontal parallel electrodes. The flame deformation and the electric current are characterized by various electrode distances, and respectively, applied voltages. The vertical electric field induces a body force downwards on the flame. The flame deforms downward in the electric field because the electric body force counters the natural buoyancy. The relation between the applied voltage and electrode distance is investigated when the flame becomes vertically symmetrical and the results show that the deformation is the function of V / L 1.5 . This indicates that the change in the electric field should be considered to discuss the effect of an external electric field on combustion behavior. The experimental results are rearranged using e V 2 / L 3 where e is electric permittivity of air because its unit is N/m 3 and it considered to be the representative electric body force. Although its application is limited, qualitatively it can help to explain the experimental results of a droplet combustion. In addition, the degree of electron attachment to neutral molecules is discussed to interpret our experimental results.

Flames are affected by external electric fields because they contain ions and electrons. The movement of ions and electrons affects the external electric field owing to their charge. In this context, this paper discusses the change in the electric field on the basis of experimental results obtained at different electrode distances. Employing one-dimensional (1D) steady-state analysis, we assume that if the electric field is changed spatially, the effect of the electric field on combustion behavior is aligned with V2/L3, where V is the applied voltage between the electrodes and L is the distance between the electrodes. Because a flame deforms to a cathode owing to electric body force in an electric field, the change in the flame shape of burning ethanol droplets observed in a vertical DC electric field and the electric current during combustion are measured as flame characteristics. The results reveal that applied electric voltage exists where the flame becomes vertically symmetrical to balance the buoyancy due to the electric body force. The relationship between m and n of Vm/Ln for flame symmetry is around n/m = 1.5. On the basis of these results, all experimental results for different electrode distances are rearranged with eV2/L3, which is a representative electric body force, and it is proved that the use of parameter eV2/L is effective. These results indicate that the change in the electric field due to the existence of a flame should be considered when examining the effect of an external electric field on combustion behavior.