High-speed Schlieren Photography Research Articles

Vortex-Flame Interaction of Spark-Ignited Flame with Coherent Structure in a Plane Shear Flow. Simultaneous Schlieren Observations of Front-and Side Views of a Propagating Flame.

In order to elucidate the enhancement mechanisms of turbulent flame propagation, it is indispensable to examine and clarify the detailed process of vortex-flame interaction. This is because that it constitutes one of the key processes to promote flame propagation velocity. In this investigation a series of large scale organized eddies formed in the plane propane-air premixed shear flow are selected as an objective eddy-field. Two ingenious techniques are introduced and combined with each other ; one is the acoustic excitation of the shear layer, the other is the divided-double-beam high-speed schlieren photography. The former makes the organized eddy formation extremely regular and enables synchronized spark ignition with a single high-speed video camera, while the latter gives the chance to analyze two different directional views of a same propagating flame. In this paper a spark-ignited flame, which is initiated at the center of an organized eddy and propagates in the vortex tube, is optically observed and analyzed by using the proposed technique. According to the results, it is found that two kinds of effects of neighboring eddies on the enhancement processes of flame propagation.

Transitional behavior of a supersonic flow in a two-dimensional diffuser

Two-dimensional blow-down type supersonic wind tunnel was designed and built to investigate the transient behavior of the startup of a supersonic flow from rest. The contour of the divergent part of the nozzle was determined by the MOC calculation. The converging part of the nozzle, upstream of the throat was contoured to make the flow profile uniform at the throat. The flow characteristics of the steady supersonic condition were visualized using the highspeed schlieren photography. The Mach number was evaluated from the oblique shock wave angle on a sharp wedge with half angle of 5 degree. The measured Mach number was 2.4 and was slightly less than the value predicted by the design calculation. The initial transient behavior of the nozzle was recorded by a high-speed digital video camera with schlieren technique. The measured transition time from standstill to a steady supersonic flow was estimated by analyzing the serial images. Typical transition time was approximately O.1sec.

332 音響により励起されたコヒーレント渦構造をもつ平面せん断流における火花点火予混合火災の伝播方向に関する研究(大気環境保全技術(III))(大気・水保全技術)

In order to elucidate the enhancement mechanisms of turbulent flame propagation, it is indispensable to examine and clarify the detailed process of vortex-flame interaction.In this investigation a series of large scale organized eddies formed in the plane propane-air premixed shear flow are selected as an objective eddy field. Two devised techniques are introduced and combined with each other ; one is the acoustic excitation of the shear flow, the other is the simultaneous two-directional high-speed schlieren photography. The former makes the organized eddy formation extremely regular and enables synchronized ignition with a high-speed video camera. The latter gives the chance to analyze two different directional views of a same propagating flame. In this paper it is shown that the propagation velocity along the axis of the eddy is promoted by the vortex bursting and is greater than that in the radial direction by about 80 %.

Influence of molecular structure on the propagation of streamer discharge in dielectric liquids

Prebreakdown phenomena in dielectric liquids which have various molecular structure are observed in detail for point-to-plane electrode configuration under an impulse voltage condition. Experiments are carried out by using a high-speed schlieren photography and a current measurement system by LED simultaneously.

Visualization of Initiation Processes of Film Detonation.

The detonation wave caused by the combustion of a liquid fuel film coated on an inner tube wall has recently attracted considerable attention because of its possible role in explosion hazards in compressed oxygen pipelines. This type of detonation is known as "film detonation" and is classified as a heterogeneous detonation. The structure of a heterogeneous detonation wave is very complex because physical processes such as momentum, heat and mass exchanges between liquid film and the gas phase are coupled with a chemical reaction. Experiments are conducted to investigate the initiation process of film detonation using high-speed schlieren photography and direct photography. It is clearly observed that a secondary shock wave caused by the combustion of liquid fuel film plays a significant role in the transition of film detonation wave.

Effect of swirl on combustion in a short cylindrical chamber

Combustion of a swirling, stoichiometric, and homogeneous mixture of natural gas and air in a short cylindrical chamber has been studied experimentally and simulated numerically. Each mixture was given a steady-state swirling motion by a rotating roughened disc before being ignited at the center of the chamber. By using discs of differing roughness and by varying the disc speed, the intensities of swirl and turbulence could be varied independently so that the effects on combustion of mixture turbulence and swirl-induced buoyancy could be separately examined. Combustion rate and overall chamber heat transfer were inferred from chamber pressure-time records. High-speed schlieren photography showed the effect of swirl on the early flame kernel. With given swirling angular momentum, increased turbulence level always reduced burning duration and increased total heat transfer rate. With given turbulence level, increasing the swirl intensity from zero first decreased, then strongly increased, the burning duration. The swirling Reynolds number (based on chamber radius and peak tangential velocity) at which combustion duration was minimized was in the range 30,000–40,000. At high Reynolds number buoyancy forces appear to have a strongly inhibiting effect on flame propagation.

Performance of pulsed combustion jet at high pressures and temperatures

Ignition and combustion by Pulsed Combustion Jet (PCJ) at high pressures and temperatures were investigated using a compact rapid compression machine. The compression and stop motion of a compression piston in the rapid compression machine is controlled by the cam shape, and the cam is operated by a driving piston and compressed air. The combustion process was monitored by pressure measurements and high-speed schlieren photography. Lean limit of methane-air mixture was extended by PCJ, and the superiority of PCJ over standard spark ignition was demonstrated in enhancing the process of combustion in turbulent lean mixtures.

Hypersonic control flap effectiveness

SummaryThe effects of flap deflection, configuration incidence and leading edge bluntness on boundary layer separation and transition have been studied experimentally. A quasi two-dimensional flat plate equipped with a full span trailing edge control flap has been employed for these tests.The studies have been carried out in a hypersonic gun tunnel at M∞ = 8·2 and Re∞/cm = 9·0 × 104. The flap deflection angles studied were in the range 0° ≤ β ≤ 30°. The incidence range was from zero to α = 10° (positive α is nose down). Leading edge bluntness effects were simulated by the introduction of a hemi-cylindrical leading edge.The flow structure was studied using high speed Schlieren photography as well as surface pressure and heat transfer measurements. Liquid crystals were employed to study the threedimensionality of the flow structure for selected configurations. Analytical theories have been developed to estimate the flap pressure and heat transfer levels for the sharp and blunt configurations. These are compared against experimental measurements.

Flame front disturbance induced by a weak pressure wave

An experimental study has been conducted on the effect of unburned mixture properties on flame front disturbance induced by acceleration. Experiments were performed using a rectangular combustion chamber of 80×80×440 mm. The flame front disturbance was observed in two different directions by high-speed schlieren photography. Mixtures used are of three different concentrations (Φ=0.8, 1.0, and 1.25) of methane/air and two different concentrations (Φ=1.0 and 1.5) of propane/air. For the methane/air mixture of Φ=1.0, experiments were performed at three different initial pressures ( P i =50, 70, and 101 kPa). The observed disturbance was of a very fine structure of circular spikes, which penetrated into the burned gas. The scales of disturbance were measured and indicated to be in the range of 1.7–4.0 mm. The circular spike shape is a typical structure induced by accelerating the flame front where the density changes steeply. However, the shape of the disturbance observed for a rich propane/air mixture was not of circular spikes but of a net of ridges. For the rich propane/air mixture, the effect by preferential diffusion was remarkable and the disturbance of a different structure was generated. For the methane/air mixtures, the scale was the smallest at Φ=1.0 and larger at Φ=0.8 and 1.25. The scale for the propane/air mixture of Φ=1.0 was slightly larger than that in the methane/air mixture of Φ=1.0. The scale became larger with the decrease of the intial pressure. The measured scales were compared with the preheat zone thicknesses of corresponding flames. It was shown that the scale is closely related with the flame thickness. The scale of disturbance is found to be about 15 times as large as the preheat zone thickness.

Fundamental Operating Mechanism for a Twin Valveless Pulse Combustor

Abstract Ensemble averaged heat release rates and pressures during pulsating combustion have been measured to better understand the operating characteristics for a twin valveless pulse combustor with two aerodynamic valves. An OH chemiluminescence intensity detection system is applied to determine the periodic variations in heat release. Also, the combusting flows have been visualized using high-speed schlieren photography. Based on the well-known Rayleigh criterion, the varying equivalence ratio and mass flux effects on combustion stability are discussed. Results show that the phase relationship between the heat release rate and the pressure significantly affects the pulse combustion stability and operating frequency. Of particular interest is that the pulsating frequency decreases with increasing equivalence ratio. In addition, the phase difference does not depend on the mass flux, but on the equivalence ratio. This fact indicates that the fuel-air mixing rate is a determinant factor in the phase relationship. The importance of fluid dynamic mixing in pulsating combustion is successfully illustrated by the schlieren observations.

Transition Processes from Deflagration to Detonation Waves. Effects of Obstacles.

Transition processes from deflagration to detonation waves in stoichiometric oxyhydrogen mixtures diluted with nitrogen were observed using high-speed Schlieren photography as well as pressure and ionization current measurements. In this study, effects of obstacles on the transition processes were investigated. The obstacles were multigutter shaped and were installed near an ignition plug on an inner wall of a rectangular tube. Width and depth of the gutter were selected as parameters while composition of the mixture was fixed. It was revealed that there exist optimum values of these parameters for enhancing the transition to detonation.

Effects of Swirl on Early Flame Development and Late Combustion Characteristic in a High-Speed Single-Shot Visualized SI Engine.

The effects of swirl on early flame development and late combustion characteristic were investigated using a high-speed single-shot visualized SI engine. LDV measurements were performed to obtain a better understanding of the flow field in this combustion chamber. Spark plugs were located at half-radius (R/2) and center of bore. High-speed Schlieren photographs at 20 000 frames/sec were taken to visualize the detailed formation and development of the flame kernel with cylinder pressure measurements. This study showed that high swirl favorably affected combustion-related performances in terms of the maximum cylinder pressure and flame growth rate regardless of spark position. However, at R/2 ignition the low swirl showed desirable effects at low engine speed, and degraded performance as engine speed increased than without swirl. There were distinct signs of slow-down in flame growth during the period when the flame front expanded from 2.5 mm in diameter until it reached 5.0 mm apparently due to the presence of ground electrode. There seemed to be a heat-transfer effect on the flame expansion speed which was evidenced in the high swirl case by the slowdown of the late flame front presumably caused by relatively large heat loss from burned gas to wall compared with low- or no-swirl cases.

気体デトネーション開始過程の可視化

The feature and mechanism of flame acceleration and transition to detonation are not fully understood. In particular, the occurrence of “explosions in explosion” in the transition processes is very important phenomena which is worth studying carefully. The present study aims to observe this phenomena and to elucidate the transition processes by using the high-speed Schlieren photography.

Local flame front disturbance development under acceleration

The development of a local flame front disturbance in an accelerating gas flow was explored using high-speed schlieren photography. The movement of the local flame front was examined in detail, and, based on these results, the flow field around the disturbed flame front was inferred. From this, it was confirmed that in an accelerating flow field, the unburned gas penetrates into the burned gas region at a velocity about 30 times higher than the laminar burning velocity. Flame front acceleration generates intense shear flow layers across disturbed flame fronts.

The structure of fast turbulent flames in very rough, obstacle-filled channels

Flames propagating in very rough confined tubes or channels are known to acheve very high speeds of propagation that can approach the speed of sound of the burnt products. The mechanisms underlying the propagation of such flames are not understood. The present paper reports the results of a detailed photographic study of such high speed flames. The experiments are carried out in rectangular channels equipped with arrays of periodically spaced obstacles to generate controlled roughness. Observations are via high speed schlieren photography. The mechanisms responsible for the high speed propagation are identified as those which cause intense turbulization of the flame. These include shock-flame interaction, Rayleigh-Taylor instabilities in an accelerating flow and auto-ignition in large recirculating eddies in the wake of obstacles.

Experimental investigation of soot and NO x reduction by impinging spray combustion in a closed vessel

Experiments have been conducted to obtain essential data on how dry soot and NO x emmissions are influenced by the impinging fuel spray injection in a closed vessel. The effect of the impinging injection on these emmissions is investigated at conditions of 623 to 673 K in initial temperature and 0.2 MPa in initial pressure and compared with that of the single injection. The combustion behavior of single and impinging fuel sprays, ignited by a small nichrome wire, are observed by taking high speed schlieren photographs at a conditions of 300-K initial temperature and 0.1-MPa initial pressure. The results show that it is possible to realize very strong turbulence and EGR effects in the combustion region by using the impinging type of fuel spray, and this method is fairly useful for reducing the emissions of dry soot and NO x simultaneously achieving the high-load condition. It is also suggested that this combustion technique makes it possible to predict the conditions under which the low-soot and low-NO x emission is compatible with the high-load operation.

Combustion of Lean Flammable Mixtures in an Intense Radiation Field

A combustion process showing a slow temperature rise was realized in an intense radiation field by burning a lean combustible mixture in a refractory-lined cylindrical combustion chamber in which a permeable board was placed at the upstream and downstream ends, respectively. It was found by the measurement of the temperature and composition of gas along with the ion current and by high-speed schlieren photography that a laminar flame was attached to the lower part of the red-hot refractory surface and that it spread toward the top center, even if the mixture strength was below the lean flammable limit. The flame becomes convex toward the unburnt mixture due to some instability resulting in a kind of cellular structure. The flickering of the flame, due partly to the wavy motion of the flame base and partly to the cellular structure, is the cause of the slow temperature rise.

Preinjection—A Measure to Influence Exhaust Quality and Noise in Diesel Engines

The combustion noise generated by DI diesel engines can be clearly reduced during both steady-state and transient operation in applying a pilot injection. After optimization, a slight increase in fuel consumption is found in the upper load range. The pilot injection also tends to reduce the NOx emissions. An increase in black smoke emissions is considered to be the main drawback with pilot injection. High-speed Schlieren photographs of injection and combustion phenomena within a pressurized chamber show that the higher black smoke emissions may be due to the combustion of the main injection quantity that occurs in a mixture that is insufficiently prepared and with nearly no delay due to the pilot injection. On the basis of these findings, it is concluded that a high degree of atomization and rapid vaporization of the main injection quantity must be accomplished. To achieve these goals better, a separate injection nozzle for the pilot quantity is preferred to an injection system with a single injection nozzle, providing both the pilot and the main quantity. Therefore, rather simple injection systems with a separate pilot injector can be developed that provide a constant pilot quantity and controlled pilot injection time over the entire engine map.

Propagation mechanism of quasi-detonations

The propagation mechanism of quasi-detonations in very rough tubes is studied using high speed schlieren photography. Stoichiometric mixtures of H2, C2H4 and C3H8 in oxygen at an initial pressure range 10≤po≤160 torr are investigated in a 61.8×61.8 mm by 1.5 m long channel with two-dimensional obstacles with a height of 25.4 mm and for various obstacle spacings. The results indicate that shock reflections (transition from regular to Mach reflections) from the walls lead to re-initiation. The obstacle spacing is found to represent an effective reaction zone length (or cell length) of the quasi-detonation. At the critical condition of transition from the choking to the quasi-detonation regime, this effective reaction zone length is found to be about twice the normal cell length of the mixture in accordance with Shchelkin's stability criterion for a perturbed wave. The minimum open dimension of the channel is found to be of the order of a cell size λ for transition to the quasi-detonation regime in agreement with the previous results of Peraldi17 and Gu8 for rough tubes. Photographic observations of the propagation mechanism in the choking regime reveal the absence of ignition via shock reflection. The placement of wire screens to damp the shock reflections at the channel walls suppresses the transition to quasi-detonations indicating the essential role of shock reflections. It is not clear whether the adiabatic heating or the turbulent vortex mixing associated with the shear layer wall jet by the Mach stem near the wall is the responsible mechanism for re-initiation.

The straining of premixed turbulent flames

The paper discusses the meaning to be attached to the Lagrangian strain rate of a material surface element, under conditions of turbulent burning of a gaseous premixture. It is important to resolve this question in order to utilise strained laminar flame data in turbulent modelling. It is postulated that material line elements of the flame surface always tend towards the axis of maximum principal strain. As a consequence, Lagrangian pdfs for flame strain rate can be derived from assumed Eulerian pdfs of strain rate and these are presented. Partial quenching of premixed C3H8-air flames at different levels of turbulence, in a fanstirred bomb, was observed by high speed schlieren photography. The proportion of the total flame surface area that was supporting propagation was measured. This was equated to the probability that for propagation, the strain rate was less than a limiting quenching strain rate, evaluated from pdfs derived as described above. Experiments were at three equivalence ratios and it was found that the quenching strain rate was appreciably higher than that observed by Law et al3 for laminar flames of the same composition, and about three times that predicted theoretically by Stahl et al5. Reasons for these discrepancies are discussed.