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The response of laminar methane/air counterflow premixed flames under sinusoidal equivalence ratio oscillation was investigated numerically. The timescales of the oscillation were chosen to be sufficiently longer than the flame timescale so that the flame responds quasi-steadily. The response of periodically stratified flame (SF) with a detailed reaction mechanism exhibited the “back-support” effect, in that the consumption speed Sc response deviated increasingly from Sc of steady homogeneous flames (HFs) at higher oscillation frequencies. It was shown that even when the imposed oscillation timescale is much longer than the flame timescale, the flame response can still be delayed under a sufficiently large equivalence ratio gradient. Subsequently, the above results were compared with those obtained with a global four-step mechanism that omits back-diffusion radicals into the reaction zone. As a result, SFs with the global mechanism displayed a much smaller back-support effect in both lean and rich mixtures. Further analysis with modified diffusion coefficients revealed the dominant roles of H2 and radical species diffusion in inducing the back-support effect. Contrary to the previous findings, variations in burned gas temperature were found to play a negligible role in modifying Sc. Additionally, the hysteresis of the back-support effect under periodical stratification was found to be more prominent on the richer side because of the presence of a larger H2 pool.

A novel hybrid material, which is an enzyme/inorganic nanosheet complex coated by molecularly imprinted polymer (MIP), was developed, and applied to colorimetric quercetin assay. First, an enzyme/inorganic nanosheet complex was prepared from horseradish peroxidase (HRP) enzyme and titanate nanosheet (TiOx), using electrostatic interactions between them in acetate buffer. In the next place, dopamine self-polymerization was performed in the presence of HRP/TiOx complex with quercetin as a template, to prepare MIP membrane onto the HRP/TiOx complex. After washing process, a new hybrid material, MIP-coated HRP/TiOx complex (MIP-HT) was obtained. MIP-HT adsorbed quercetin efficiently, compared with NIP-HT that is an HRP/TiOx complex coated with non-imprinted polydopamine. MIP-HT showed enzymatic activity for an oxidation reaction of guaiacol, which is a chromogenic substrate of HRP, whereas the enzymatic activity of NIP-HT was significantly suppressed. The amount of brown product, formed by the color reaction, reduced owing to the presence of quercetin in sample solution, and a good liner relationship was observed between the concentration of quercetin and the increment of absorbance at 470 nm. The investigation using several biomolecules indicates that MIP-HT has the ability to detect quercetin and its analogues with selectivity. Therefore, MIP-HT shows great promise as a new and attractive material for use in colorimetric assay of quercetin or quercetin analogues.

The dynamics of a laminar premixed stagnating flame subjected to simultaneous velocity and equivalence ratio oscillations are investigated experimentally. A flame of premixed methane and air is formed in a wall-stagnating flow. The oscillations of velocity and equivalence ratio are imposed independently by two sets of dual cylinder–piston oscillators. To impose the equivalence ratio oscillation, methane/air mixtures with equivalence ratios of $$\phi =1.0$$ and $$0.4$$ are supplied to different cylinder–piston units. The pistons move in anti-phase and create a sinusoidal equivalence ratio oscillation while keeping the volume flow rate constant. To impose the velocity oscillation, a mixture with $$\phi =0.7$$ is supplied to the single unit of another oscillator. The phase difference between the two oscillations is set by changing the relative position of the pulley teeth of the two oscillators. The oscillator frequency is varied between 2 and 20 Hz, meaning that the oscillation wavelengths are much longer than the flame thickness. When frequencies of the velocity and equivalence ratio fluctuations are in a quasi-steady state condition, the flame motion is affected independently by velocity oscillation and equivalence ratio oscillation and the linear-superposition assumption can be applied. When the velocity and equivalence ratio oscillation frequencies are increased and exceed the transition frequency from the quasi-steady state to unsteady state, the flame motion gradually deviates from linear-superposition assumption. This is due to the variations in the unsteady molecular diffusion and the back-support effect.

The effect of the non-uniform profile of scalar variables, such a fuel at the upstream and temperature at the downstream of the flame zone was discussed theoretically to elucidate; (1) the deviation of motion from the steady state case and (2) the hysteresis of premixed flames response to the equivalence ratio oscillations seen in an experimental and numerical works. One-dimensional integral model for the non-uniform scalar variable profile with low frequency equivalence ratio oscillation has been developed. Here, the wavelength of the oscillation is assumed to be larger than the nominal flame thickness. Through the integral analysis, we obtained the relation of the flame propagation speed for steady and unsteady cases depending on the non-uniform scalar profile at the upstream and downstream of the flame zone. Hysteresis of the flame propagation speed is found due to the transport of fuel and heat by the non-uniform scalar profile at the upstream and downstream of the flame zone. This result qualitatively agreed with the numerical results of a response of the stagnation laminar CH4/air premixed flames for a low equivalence ratio oscillation frequency.

Luminescent europium-doped layered titanates (Eu-TiOx) were synthesized and complexed with horseradish peroxidase (HRP) and glucose oxidase (GOx). The emission of a resultant Eu-TiOx/HRP/GOx complex decreased upon the addition of glucose in the presence of guaiacol. The emission decrease was dependent on the concentrations of glucose, and the detection limit for glucose was 3.1 μM. The proposed system would be promising as a new detection method for glucose.

A complex composed of enzyme (horseradish peroxidase, HRP), layered titanate (TiOx), and magnetic beads (MB) was prepared, and the enzymatic activity of the complex and the repeatable use for the enzyme in the complex were investigated. The HRP/TiOx/MB complex was formed in acetate buffer (pH 4) by electrostatic interaction among the components where HRP and MB have positive charge, whereas TiOx is negatively charged. The resultant complex was sufficiently stable in an aqueous solution, and HRP immobilized on the layered titanate in the complex kept a certain enzymatic activity in which typical enzymatic relationship was observed between substrate concentration and initial reaction rate. By giving a magnetic field using commercial magnets, the HRP/TiOx/MB complex was easily and rapidly recovered, and the recovered complex was able to be used for the next enzymatic reactions. The enzyme/inorganic nanosheet/MB complex proposed in this study is expected to be applied as a new tool in the bioengineering and nanobiotechnological fields.

The performance of high purity hydrogen production from methanol for a compact steam reformer with a hydrogen purification membrane was investigated experimentally. A 77 wt.% Pd/23 wt.% Ag membrane with 25µm thickness and CuO/ZnO/ Al2 O3 catalyst were used. Heating was performed by a Bunsen type burner using City Gas 13A. The methanol reforming and purification of H2 were investigated at different reference catalyst zone temperatures (589-689K), pressures at the retentate side (0.2-0.5MPa), steam to methanol(S/C) ratios (0.8-1.6) and reactant flow rates (1.7×10-4 to 4.4×10-4 mol/s). The results show that at high reference temperature, high pressure and certain points of the reactant flow rate, the maximum hydrogen permeation rate is obtained when the S/C ratio is around 1. The modified Sieverts’ equation which considers the decrease in H2 concentration at the membrane surface, was proposed. The experimental result was lower than the permeation rate estimated by the modified Sieverts’ equation, which is probably caused by the adsorption of non-H2 species during permeation. It is further demonstrated that the modified Sieverts’ equation is able to estimate a more reasonable hydrogen permeation rate in comparison to the estimation by the ordinary Sieverts’ equation. In addition, it is shown that the compact methanol steam reformer with a Pd/Ag membrane is able to produce high purity hydrogen with very low CO concentration, which fulfills the Polymer Electrolyte Fuel Cell (PEFC) requirement (

The dynamic behavior of the near-field region in a coaxial variable property jet has been experimentally investigated under a swirling flow produced by rotating cylindrical inner and outer tubes, focusing on how the swirl of the outer jet affects the formation of a stagnation point in the swirling inner jet. The inner and outer jets rotate in the same direction. Air, CO2, or He is issued from the inner tube as a variable property jet, and air is issued from the outer tube in this work. In the case of a CO2 jet (a high-density, low-viscosity gas jet), a stagnation point flow is more easily formed than in the case of an air jet, and the stagnation point location is significantly lower than in that of the air jet. When the swirl of the outer jet is introduced, a stagnation point flow is more easily formed than in the case of a nonswirling outer jet, and the stagnation point location is much lower than in the case of a nonswirling outer jet. In the case of a He jet (a low-density and high-viscosity gas jet), the inner jet does not have a stagnation point flow, and its overall behavior remains nearly unchanged even under high swirl numbers of the inner and outer jets. These results clearly show that the density and viscosity differences between the inner and outer jets have a significant impact on the dynamic behavior of the near-field region in the coaxial swirling jet. The significant lowering of the stagnation point location can be physically explained by considering the theoretical equation obtained in this work.

Several ion channel forming cyclic peptides have been reported over the past two decades and various ion conducting mechanisms have been proposed. In this article, we report on amino acid substitut...

The near-field instability of variable property jets of air, CO2, and He, issued into the ambient air, has been investigated experimentally within normal gravity and microgravity fields. The density ratio to the ambient air is unity for air jets, more than unity (1.53) for CO2 jets, and less than unity (0.14) for He jets, respectively. The ratio of kinematic viscosity to the ambient air is unity for air jets, less than unity for CO2 jets (0.53), and more than unity for He jets (7.75), respectively. The jet velocity is varied from 0.4 to 1.8 m/s and then the jet Reynolds number varies from 60 for Helium jet to 2,000 for CO2 jet, while the Richardson number varies from negative to positive values. The motion of the jet is visualized using a laser tomographic method and recorded by a high-speed digital video camera with 250 frames/s. The result shows that the instability of the jet is intensified when Re > 800 while it is weakened at Re

Nuclear lamins are a type of intermediate filament (IF) proteins. They have a characteristic tripartite domain structure with a alpha-helical rod domain flanked by non-alpha-helical N-terminal head and C-terminal tail domains. While the head domain has been shown to be important for the formation of head-to-tail polymers that are critical assembly intermediates for lamin IFs, essential structural elements in this domain have remained obscure. As a first step to remedy this, a series of mouse lamin A mutants in which the head domain (30 amino acid residues) was deleted stepwise from the N-terminus at intervals of 10 residues were bacterially expressed. The assembly properties in vitro of the purified recombinant proteins were explored by electron microscopy. We observed that while a lamin A mutant lacking N-terminal 10 residues formed head-to-tail polymers, a mutant lacking N-terminal 20 residues or the whole head domain (30 residues) showed significantly decreased potency to form head-to-tail polymers. These results suggest that the last 20 residues (from Arg-11 to Gln-30) of the head domain of mouse lamin A contain essential structures for the formation of head-to-tail polymers. The last 20 residues of the head domain include several conserved residues between A- and B-type lamins and also the phosphorylation site for cdc2 kinase, which affects lamin IF organization in vivo and in vitro. Our results provide clues to the molecular mechanism by which the head domain plays a crucial role in lamin polymerization.

The empirical correlation ( St * 2 / Ri = 0.00028 Re * 2 / 3 : St * is the reduced Strouhal number, Ri the Richardson number, Re * the reduced Reynolds number) proposed by Kostiuk and Cheng [1995. The coupling of conical wrinkled laminar flames with gravity. Combustion and Flame 103, 27–40] for predicting buoyancy-induced flame flickering frequency, has been experimentally investigated under the swirling flow conditions produced by a rotating Bunsen burner, focusing on how the flame flickering frequency changes with increasing swirl number S . Under low swirling conditions up to S ≈ 0.1 , the flame flickering frequency f t did not change and monotonically increased with increasing the bulk flow velocity from the burner tube U . The trend of the data is similar to those obtained by Durox et al. [1995. Some effects of gravity on the behavior of premixed flames. Combustion and Flame 82, 66–74] and Kostiuk and Cheng (1995) and could be fitted well with the empirical correlation Kostiuk and Cheng (1995). However, under high swirling conditions ( S > 1 ), f t significantly decreased up to ≈ 80 % of that with non-swirling condition, and became insensitive to U . As a result, the data under S > 1 could not be correlated by the empirical equation. From results obtained by laser doppler velocimetry (LDV), this inconsistency is due to flow divergence shown by the change in the velocity distribution between the burner exit and the flame tip with burner rotation. A minimum value of the centerline velocity u j , min between the burner exit and the flame tip fulfills an important role in controlling the flame flickering frequency. The use of this parameter allows the empirical correlation to be extended to the high swirling case by means of a modified empirical correlation St * 2 / Ri = 0.00028 ( Re * exp ( - 0.64 S 1.78 ) ) 2 / 3 .

The combustion behavior of a methane hydrate sphere under normal gravity is experimentally investigated. The initial diameter of the sphere is 20 mm. Variation in temperature at the center of the sphere (T c) is measured with a K-type thermocouple at ignition temperatures (T c,i) from 193 to 253 K at 20 K intervals. Variation in the near-surface temperature of the sphere (T s) is measured at ignition temperatures (T s,i) from 233 to 263 K at 10 K intervals. Two combustion phases are observed. When the hydrate is ignited, a stable flame envelope is formed around the sphere (phase 1). In phase 1, the surface of the sphere is dry. After a few seconds, water formed by dissociation of the methane hydrate appears on the surface and methane bubbles are formed by methane ejected from inside the sphere (phase 2), thus destabilizing the flame and causing local extinction. Methane bubbles move down along the surface and merge into a large methane bubble at the bottom of the sphere. This bubble bursts, releasing methane to form a temporary flame, and the water drops from the hydrate sphere. Water on the surface is cooled by the hydrate inside, and an ice shell confines the methane gas that dissociated inside the sphere. Because the dissociation occurs continuously inside the hydrate, the inner pressure gradually increases and at some instant, the ice cracks and methane gas is ejected from the cracks, which results in a micro-explosion with a flame. In phase 1, the surface temperature is below the freezing point of water, and so the surface remains dry and a stable flame envelope is formed; in phase 2, the surface temperature is above the freezing point, and so water appears on the surface. When the temperature at the center of the sphere is lower (193, 213, or 233 K), some methane hydrate remains even after flame extinction because heat transfer from the flame decreases in phase 2 as a result of local extinction. The diameter of the sphere decreases during combustion in accordance with the d-square law, which indicates that the heat of dissociation is supplied by the flame and methane is supplied by the dissociation of methane hydrate in the sphere, as in single-droplet combustion.

An orbital instability of the unsteady flame tip motion with burner rotation and the possibility of the prediction of the flame tip motion are investigated from the viewpoint of nonlinear dynamics. A rich methane–air mixture of equivalence ratio 1.43 is used. The mean exit velocity from the burner tube, U , is varied from 0.6 to 1.3 m/s, keeping the swirl number S = 1.14 constant. The variation in the flame tip motion is quantitatively evaluated by calculating the mean value of a parallel trajectory value Γ . At U ⩽ 0.7 m / s , the value of Γ is estimated to be approximately zero, indicating that periodic motion occurs. As U increases, the trajectory of the attractor becomes complicated and Γ gradually increases. The value of Γ approaches the value of the Fourier-transformed surrogate data with further increase in U . This suggests that the flame tip motion varies from periodic to chaotic due to the influence of phase randomness with increasing U . The short-term forward prediction method, that is based on the orbit of the attractor, first was used. Then the short-term forward prediction method was modified to successfully extend the prediction term. The results suggest that the modified short-term forward prediction method proposed in the present study provides a valid method for predicting the motion of unsteady flames.

Reactive miscible viscous fingering occurs when a reactive and miscible less-viscous liquid displaces a more-viscous liquid. Effects of bulk finger-growth velocity on reaction characteristics of miscible viscous fingering with a chemical reaction were studied analytically by using a convection–diffusion–reaction model. The model assumes the existence of a distinct interface between both liquids, assumes the existence of a two-dimensional, steady, stagnated flow field in the less-viscous liquid, and assumes an infinite chemical reaction rate. The model was then used to determine the reaction characteristics, such as the location of the reaction surface and the profile of the product, as functions of the velocity and initial reactant concentrations. The results reveal that the effects of the velocity on the reaction characteristics can be divided into low-, intermediate-, and high-velocity regions. In the low-velocity region, the reaction characteristics strongly depend on the reactant concentrations. In the intermediate-velocity region, the dependence of the reaction characteristics on the reactant concentrations decreases with increasing velocity. In the high-velocity region, the reaction characteristics are nearly independent of the reactant concentrations. Experiments confirm the existence of these three velocity regions predicted by the model.

Effects of burner rotation on the shapes and dynamics of premixed Bunsen flames have been investigated experimentally in normal gravity and in microgravity. Mixtures of CH{sub 4}-air and C{sub 3}H{sub 8}-air are issued from the burner tube with mean flow velocity U = 0.6 m/s. The burner tube is rotated up to 1400 rpm (swirl number S = 1.58). An oscillating flame with large amplitude is formed between a conical-shape flame and a plateau flame under the condition of Lewis number Le > 1 mixtures (rich CH{sub 4}-air and lean C{sub 3}H{sub 8}-air mixtures). In contrast, for Le = 1 mixtures (lean CH{sub 4}-air and rich C{sub 3}H{sub 8}-air), asymmetric, eccentric flame or tilted flame is formed under the same swirl number range. Under microgravity condition, the oscillating flames are not formed, indicating that the oscillation is driven by buoyancy-induced instability associated with the unstable interface between the hot products and the ambient air. The flame tip flickering frequency {nu} is insensitive to burner rotation for S 0.11, {nu} decreases linearly with increasing S. As S exceeds 0.11, a minimum value of axial mean velocity along the center line uj,m due to flow divergence ismore » found and it has a linear relationship with {nu}. This result shows that uj,m has direct control of the oscillation frequency. When S approaches unity, the flame oscillation amplitude increases by a factor of 5, compared to the flickering amplitude of a conical-shape flame. This is accompanied by a hysteresis variation in the flame curvature from positive to negative and the thermo-diffusive zone thickness varying from small to large. With S > 1.3, the plateau flame has the same small flickering amplitudes as with S = 0. These results show that the competing centrifugal and buoyancy forces, and the non-unity Lewis number effect, play important roles in amplifying the flame-tip oscillation.« less

The association of 10 different tannins and related polyphenols with gramicidin S, a cyclic peptide having a rigid β-turn structure, has been examined using 1H-NMR spectroscopy. In the presence of pentagalloylglucose and epigallocatechin-3-O-gallate, the proton signals due to proline and the adjacent phenylalanine moieties selectively shifted to up field, suggesting a regioselective association with the β-turn structure. The association was also supported by the observation of intermolecular nuclear Overhauser effects between epigallocatechin-3-O-gallate and the peptide. In contrast, ellagitannins, biogenetically derived from pentagalloylglucose, showed small and non-selective chemical shift changes, suggesting that interaction with these tannins is relatively weak. The hydrophobicity of the tannin molecules and the steric hindrance of the interaction site are thought to be important in the association., Chemical & Pharmaceutical Bulletin v.50(2) p.258-262, 2002

Stabilizing mechanism of a methane-air edge flame that is a partially premixed flame, is investigated experimentally using the axisymmetric wall jet burner. Measurements of stability limits, flame locations and velocity fields with PIV, and visualization of themal boundary layer around of the edge flame have been performed. As a result, it is found that Peclet number Pe, which is expressed as Pe= Ut× pe/D, where Ut is tangential velocity toward the edge flame, Pe is curvature radius of thermal boundary layer of the edge flame and D is mass diffusivity, shows constant value at stability limits. The Pe physically means the ratio between partially premixing time to form flammable mixing layer and residential time of a fluid element in partially premixing region. Therefore, the result obviously indicates that the edge flame's stability is determined due to the balance between the partially premixing time and the residential time in partially premixed region. Thus, it is clear that a partially premixing, which is absent in a pure premixed flame, is important for stabilizing mechanism of the edge flame in the wall jet.

The relation between blowoff characteristics of the edge flame in a methane-air diffusion flame and its flame structure has been investigated by using our original burner. The burner can form an edge flame without premixed flame, as a hole, in the stagnation region of an axisymmetric impinging jet. Varying the hole diameter, blowoff limits and maximum flame temperature were measured for an edge flame's blowoff character, and also, for an edge flame's structure, temperature profile and flame location were measured and a thermal boundary layer around the edge flame was visualized with laser tomographic technique. It is found that all the edge flames in the stagnation flow have a critical stagnation velocity gradient, beyond which the flame can never be existed. The critical stagnation velocity gradient that represents the overall reaction rate in the edge flame zone decreases as the hole diameter is increased. The increase in a hole diameter leads to change of the edge flame's structure and to addition of two heat loss factors to the edge flame. One heat loss factor is the edge flame-wall interaction, which occurs due to decrease in the edge flame's location. Another is penetration of cold flow into a hole in the flame, which occurs due to decrease in the overlapping range of thermal boundary layer of the edge flame in the hole. These additional heat losses occur at lower stagnation velocity gradient and have stronger influence on the edge flame as a hole diameter becomes larger. Consequently the overall reaction rate in the edge flame zone is reduced by the increase in a hole diameter. Finally it is clarified that the edge flame shows qualitatively same extinction as a pure diffusion flame, when the flame zone of the edge flame lies spatially as the boundary that divides between oxidizer side and fuel side in spite of existence of a partially premixture of fuel and oxidizer ahead of the reaction zone in the edge flame region.

Responses of stagnating laminar methane/air premixed flames under fuel concentration oscillation, i.c., equivalence ratio oscillation, were numerically investigated using a one-step overall reaction mechanism and a four-step reaction mechanism that included CO and H2 formation. The flame motion was numerically investigated for three different oscillation cases namely: lean, rich and lean-rich crossover case. Methane/air mixtures with sinusoidal equivalence ratio oscillations were issued from the burner exit with uniform 1.0 m/s velocity profiles. In the steady state condition, the one-step overall reaction mechanism and the four-step reaction mechanism had nearly the same characteristics in the lean region, while in the rich region variations in characteristics such as flame location and flame displacement speed for the four-step model were much more significant than those for one-step model. When the equivalence ratio was oscillated, the flame location oscillated. The amplitude of the flame location oscillation did not change with the equivalence ratio oscillation when the frequency of equivalence ratio oscillation was less than 8Hz, while it decreased monotonically when it exceeded 8 Hz. Here 8 Hz corresponds to a Strouhal number (St) of unity. Thus, this result indicates that the flame was in a quasi-steady state when St1.0. The variation in flame location and the flame displacement speed did not follow those for the steady state condition and made a limit cycles. This was due to the back support effect. The cycles were significantly inclined at higher frequencies. In the lean condition, the limit cycle was inclined similarly for both the one-step and four-step reaction mechanisms. In the rich condition, however, the limit cycle for the four-step reaction was more inclined than that for the one-step reaction. These results show that the formation of CO and H2 played an important role in the rich condition.

The flow field and coherent structure of the flow over a backstep with slot-gas-injection were investigated. The mean flow, turbulence properties and coherent structure characteristics are significantly influenced by the increase of injection momentum near the step ( l f /H=2). The mean flow field is characterized by recirculation zone splitting, with mass transfer of injected gas toward each part depending on the specific momentum ratio of injection. More mass entrainment, turbulence structures alteration in the shear layer and the hydrodynamic disturbances from the injection flow suppress the coherent structure velocity, size and detachment. These results provide basis information for achieving the balance of competing factors between geometrical and fluid dynamical parameters of injection to establish an efficient design of a combustor from the perspective of enhancing the gas–air mixing process.

Transition processes from the edge flame without premixed flame to the edge flame with premixed flame, the triple flame, of methane-air edge flame are experimentally investigated by using the novel burner system. The burner system forms a diffusion flame in an axisymmetric wall jet. The measurements of the stability limits and the flame locations, and the observation of the flame shapes have been performed. The burner can make three types of the stable edge flame. First one is the edge flame without premixed flame, whose location is determined by the wall interaction with the edge flame, that is, the thermal factor. Second one is the edge flame with premixed flame, whose location is still determined by the thermal factor. Third one is the edge flame with premixed flame, and the location is determined by a balance between the propagation rate of the edge flame and the local velocity of unburnt mixture, that is, the fluid dynamical factor. The change of the dominant factor for flame location is due to the increase in the partially premixing region ahead of the edge flame. The increase in the partially premixing region makes it possible to form premixed flame and the first edge flame changes into the second one. Further increase in the partially premixing region lead to an increase in a total amount of the heat release around the edge flame region and the flow field ahead of the edge flame diverges. Finally, the edge flame is lifted up by the flow divergence and the dominant factor to determine the edge flame location changes from the thermal factor to the fluid dynamical factor.

An experimental investigation of turbulent counterflow nonpremixed flames has been undertaken in order to clarify the interaction between the properties of the nonpremixed flames and the characteristics of the turbulent counterflow field. In particular, to distinguish between the effects of turbulence caused by the air and fuel streams, the turbulent characteristics of each flow in an opposed jet flow were controlled individually. From the visualization by laser tomographic technique, it was found that the width of the diffusion region along the centerline regarded as a macroscopic parameter of the local structure of nonpremixed flames was not changed by the flow turbulence, and was determined by the mean flow condition characterized by the bulk velocity gradient, while whole diffusion regions spatially showed the typical wrinkled motion within the turbulent counterflowing stream. On the other hand, the mixture fraction fluctuations which were estimated by measurements of the behavior of the flame and the diffusion region, depended mainly on turbulence and were not affected by the bulk velocity gradient. The mean scalar dissipation rate χturb due to the turbulence, estimated by combining the turbulent strain rate of the air side stream and the rms of mixture fraction fluctuation, increased with an increase in the turbulent strain rate of the air side stream, that is, with a decrease in the turbulent Damkohler number, Da. However, it is known that in a counterflow field the strain caused by the mean flow is also effective for properties such as the transport phenomena. Then, the total scalar dissipation rate χtotal, which is derived from the turbulence and the mean flow velocity gradient, was suggested as the characteristic quantity of nonpremixed flames formed in counterflow geometry. The total scalar dissipation rate of flames at extinction showed almost constant value regardless of the initial turbulent conditions. The present results agree with the laminar flamelet concept.

The response of the flame displacement speeds of stagnating flat premixed flames to the periodical fluctuation of stretch is investigated experimentally, regarding two mixtures with different Lewis numbers: lean C 3 H 8 /air and lean CH 4 /air in the range of the Strouhal number 2.3 to 4.8 (24 to 51 Hz). The stagnation wall is oscillated sinusoidally along the stagnation streamline. The oscillation of the stagnation wall induces the periodic fluctuations of flow velocity and flame stretch, and hence the fluctuations of flame displacement speed. The amplitude of the flame stretch fluctuation increases with increasing frequency of the wall in the Strouhal number greater than unity owing to the increase in the amplitude of the oscillatory wall velocity, when the stagnation wall is oscillated along the stagnation streamline at constant amplitude. The displacement speed is measured as the propagating velocity of the flame relative to the unburned gas velocity at the cold edge of the flame by using the history of flow velocity fluctuation. The response of the flame displacement speed is discussed from two viewpoints: its amplitude response and phase response. The significant dependence of the flame displacement speed on the flame stretch has been shown by the amplitude response. It is shown that the sinusoidal fluctuations of flame displacement speeds for both mixtures follow the sinusoidal fluctuation of flame stretch with a phase delay from about 20° to 90° in the present frequency range. Hence, the displacement speeds for both mixtures, although these mixtures have the contrary ratio of thermal to mass diffusivity against unity, show the same increasing dependence on unsteady stretch because the flow divergence affects the displacement speed with unsteady stretch as well as steady stretch.

This structure of turbulent flow in an annulus with strong inner cylinder wall heating has been studied in terms of velocity and temperature with wall temperatures up to 707 °C and a Reynolds number of 48,000. With increase in wall heating, the turbulence very close to the wall was suppressed due to an increase in the kinematic viscosity. In the inner region, the intermittent mixing became intensive and the turbulent intensity increased whereas, in the outer region, the turbulence was suppressed since intermittent mixing was no longer effective. The results show that the thermal structure can be considered in terms of a passive scalar for wall temperatures less then around 200 °C, except in the leading region of the heated area.

The flow field of the turbulent counterflowing stream with and without nonpremixed flames, and the behavior of diffusion region of turbulent nonpremixed flames have been investigated experimentally. The turbulent characteristics of each side flow in a counterflowing stream would be regarded as keeping their initial turbulent characteristics from one burner exit to near the stagnation region. And the mutual interferences of turbulence of both flows are slight in the present nonreactive turbulent counterflow field. The trends of velocity fields at every turbulent condition agree well with the flow field of nonreactive turbulent counterflowing stream and it is considered that the mutual interferences of both air side stream turbulence and fuel side stream turbulence seem to be slight in a turbulent counterflowing stream with nonpremixed flames. The behavior of diffusion region including the nonpremixed flame is mostly affected by the turbulence of the air stream. And it indicate that the presence of a certain close relation between the macroscopic behavior of diffusion region caused by the turbulent effect and extinction phenomena of turbulent nonpremixed flames formed in a counterflowing stream.

The velocity variation induced by the sinusoidal wall oscillation is measured along the stagnating streamline and its amplitude is estimated. The mechanisms of the flow velocity oscillation are discussed in terms of the amplitude of the flow velocity oscillation when the angular frequency of the wall oscillation ω, the average distance between the nozzle and the wall y^-ω, and the flow velocity at the nozzle exit U0 are varied. The characteristics of the flow velocity oscillation are classified into three regions by a Strouhal number S=ωy^-ω/U0. When S 6, it is dominated by the change of the wall velocity. In between, 2

A stability limit of the diffusion flame with fuel injection from a porous wall in a laminar flat plate boundary layer is measured as functions of fuel (CH4) concentration of CH4/N2 injectant mixture (x) and its injection velocity (v). The free stream velocity (U∞) is set as 0.6 m/s. The thermal condition at the wall is controlled by setting temperature at the upstream end of the porous wall as a reference temperature. When v>20 mm/s, the flame becomes unstable with the separation of leading flame edge with decreasing x. The value of x at the stability limit is constant without regard to v as long as the wall temperatrure is kept constant. As the wall temperature is decreased the value of x increases. The separation is supposed to take place as a result of the limit of the reaction rate. When v

Velocity statistics along the stagnation line of an axi-symmetric wall stagnating turbulent flow are studied experimentally. A low turbulence, uniform air flow from a nozzle type air supply with an exit diameter of 50 mm stagnates at a wall located 50 mm downstream. A flow velocity is set to 3 m/s, 10 mm downstream from the exit of the air supply. Instantaneous values of streamwise and radial velocities are measured by laser-Doppler velocimetry. The turbulence level in the air flow is changed by use of turbulence generator. When the turbulence generator is not installed in the air supply, the mean velocity profile in the streamwise direction fits well with that of a laminar viscous flow with the rms value of velocity fluctuations low near the wall. With the turbulence generator installed, a significant turbulence structure appears near the wall. When the wall is approached, the rms value of velocity fluctuations in the streamwise direction decreases monotonically while the profile of the rms value in the radial direction reaches a maximum near the wall. The increase in the rms value of velocity fluctuations in the radial direction near the wall is attributed to the bi-modal histogram of the fluctuating velocity in the radial direction. Near the wall, the instantaneous stagnation streamline fluctuates and the probability of the mean location of the stagnation point reaches a maximum not at the stagnation line but on a circle around the stagnation line, resulting in the bi-modal histogram. Turbulence statistics, the rms value of velocity fluctuation and the turbulent kinetic energy, can be normalized successfully by similarity parameters based on the strain rate and the reference turbulent kinetic energy introduced by Champion and Libby.