Yojiro Ishino

Interactions between nonsymmetrical wake structure and turbulent diffusion flames behind a rear-facing semicircular cylinder

Abstract Effects of the nonsymmetrical wake structure on turbulent diffusion flames, stabilized behind a rear-facing semicircular cylinder between two divided, vertically upward flowing airstreams, are experimentally investigated. Interactions between two vortex sheets in cold flows are also examined. Detailed optical observations and measurements of velocity and temperature are made on nine flames, while varying the ratio of the lower airstream velocity, U 2 , to the higher, U 1 , from U 2 U 1 = 1.0 to 0.083. Cold flows behind the semicircular rod are classified into three patterns depending on the velocity ratio: wake-type flow ( U 2 U 1 ≧ 0.5 ), transition flow showing strong interaction between two vortex sheets ( 0.5 ≧ U 2 U 1 ≧ 0.15 ), and free shear flow type ( U 2 U 1 ≦ 0.15 ). Flames also exhibit a sudden transition from the bluffbody-stabilized wake flame to the plane diffusion flame, indicating good agreement with transition in the cold flow. A comparison of the vortex characteristics in the cold flow with those in the reacting flow suggests that the flame-flow interaction originates mainly from the combined effects of the suppression of the Kelvin-Helmholtz instability due to the positive temperature dependence of the kinematic viscosity, the isotropic expansion and acceleration due to the exothermic reaction, and the strong pressure fluctuation associated with turbulent combustion.

Flow and heat transfer characteristics in pulsating pipe flows (Effects of pulsation on internal heat transfer in a circular pipe flow)

Effects of pulsation on flow and heat transfer characteristics are experimentally examined in the pulsating pipe flows having sinusoidal velocity fluctuations around a nonzero mean. By systematically varying three pulsation parameters (the amplitude, frequency, and mean velocity), time-averaged and fluctuating temperature profiles are measured under the heating condition of constant wall temperature using saturated vapor. The mean Nusselt number, Nup, is calculated, and compared with that in ordinary turbulent pipe flows without pulsation. The results show that Nup, decreases initially as the pulsation amplitude increases, then recovers gradually, and finally becomes much greater than the original value. In pulsating pipe flows with a nonzero mean velocity, therefore, pulsation cannot always promote heat transfer, but sometimes suppresses it, depending mainly on the pulsation amplitude and mean velocity. It is also found that these heat transfer characteristics of a pulsating pipe flow are controlled by the transition of flow patterns with pulsation amplitude from a fully turbulent flow to a conditionally turbulent flow via a transitional flow.

Simultaneous and two-directional highspeed schlieren observation of effects of ignition points on vortex-flame interaction

To observe and analyze detailed processes of the vortex-flame interaction in the two-dimensional coherent structure, a premixed flame is generated by a spark ignition either at the center of an organized eddy or at the midpoint between two adjacent eddies, and the initial propagation stage of the spark-ignited flame is optically observed using the simultaneous and two-directional high speed schlieren photography. The tangential velocity of organized eddy and the equivalence ratio of the shear flow are varied as two main parameters. The results obtained and analyzed show that there exists another type of vortex-flame interaction in addition to the vortex bursting, and that it is due to the eddying motion peculiar to the coherent structure in the plane shear flow. The vortex-flame interaction is named here the vortex boosting. It is concluded therefore that in the ordinary turbulent premixed flames these two fundamental vortex-flame interactions get tangled with each other in complicated manners to augment the propagation velocity. An empirical expression which takes into account of effects of both vortex and chemical properties is finally proposed.

Optical Analysis of Pulse Combustion Using Shadowgraph and Planar CH-LIF Imaging Technique

Planar imaging of laser-induced fluorescence of CH radical is made to examine combustion processes in a valveless pulse combustor. An excimer-pumped dye laser tuned to a wavelength of 387 nm is used to excite the R 1 (N=6) line of (0,0) band of the B 2 Σ - -X 2 Π system of CH radical, and an image-intensified CCD camera system is used to detect the (0,1) band emission at around 435 nm. According to the CH-LIF images, it is found that the progress in combustion during a pulsation period is expressed by the enlargement and breakup of the earlobe-shaped flame front along the outline of a pair of large-scale eddies of fresh mixture.

Acoustic excitation of dual diffusion flames with coherent structure behind a rear-facing semicircular cylinder

Abstract In this paper two types of dual diffusion flames formed behind a rear-facing semicircular fuel nozzle are experimentally investigated; one is the plane diffusion flame consisting of an ordinary laminar diffusion flame on the lower-velocity side of the fuel nozzle and a lifted flame on the higher-velocity side, the other the bluffbody stabilized wake flame composed of two lifted flames on both sides of the fuel nozzle. The CAPLA(computer-aided phase-locked averaging) method is combined with the acoustic excitation to examine not only effects of the acoustic excitation on the flame structure, but also phase characteristics of the formation processes of organized eddies and eddy-flames. The acoustic excitation at a constant frequency of 300 Hz is applied by a flat loudspeaker flush-mounted on the higher-velocity side wall of the combustion tunnel. According to flash shadowgraphs of the flames the acoustic excitation is found to make the outline of eddy-flames clear with increasing the excitation int...

3D Printing of Instantaneous Turbulent Flame Shapes, Experimentally Captured by 3D-Computer Tomography and Multi-Directional Schlieren Photography

Non-scanning 3D-CT(Computer Tomography) technique employing a multi-directional quantitative schlieren photographic system(top-left picture) with flash light source, has been performed to obtain instantaneous density distributions of high-speed turbulent flames(for reference, the target flame of 8 m/s exit velocity is indicated in the right-top picture). For simultaneous schlieren photography, the custom-made 20-directional schlieren camera was constructed and used. The target turbulent flame is high-speed flames, anchored on the burner of a nozzle exit of 4.2 mm diameter. The image set of 20 directional schlieren images are processed by MLEM CT-algorithm to obtain the 3D reconstruction of instantaneous density distribution. The solid models(bottom picture) of threshold density level of 0.7 kg/m3 are 3D-printed as 4 times large size for detail observations. The average exit velocity of the propane-air mixture of equivalence ratio of 1.1 is set to be 10, 8, 6 and 4 m/s (models from left to right in the bottom picture). The solid models show the complicated shape of the high speed turbulent flames. The flame structure of higher speed flame has fine scale corrugations. This corresponds to the “corrugated flamelets regime” of the Borghi & Peters diagram well.

Microscopic Approaches to Decomposition and Burning Processes of a Micro Plastic Resin Particle

From a fundamental and microscopic viewpoint to elucidate the possibility and availability of thermal recycling of wasted plastic resin, a series of heating processes of melting, thermal decomposition and burning of a spherical micro plastic resin particle having a diameter of about 200 μm are observed, when it is suddenly exposed to hot oxidizing combustion gas. Three ingenious devices are introduced; the first is a high-speed microscopic direct and schlieren system, the second is a pre-mixed mini-burner for abrupt heating, which is equipped with a pair of spark gaps at its exit and is discharged synchronously with the starting signal of high-speed camera, and the third is a single mini-puff generator, which enables to extinguish instantly all flames around the micro particle at an arbitrary assigned time after the spark ignition. Polyethylene terephthalate and polyethylene are used as two typical plastic resins. In this paper the dependency of internal and external appearances of residual plastic embers on the heating time and the initial plastic composition is optically analyzed, along with appearances of internal micro bubbling, micro jets and micro diffusion flames during abrupt heating. Based on temporal variations of the surface area of a micro plastic particle, the burning rate constant is also evaluated and compared with well-known volatile liquid fuels.Copyright

An Investigation on Thermal Recycling of Recycled Plastic Resin (Numerical Prediction of Unburnt Rate in PET-Resin Powder Combustion)

To burn PET-resin powder as an alternative fuel and to realize the effective thermal recycling of a great deal of recycled PET-resin, a series of experimental investigations has been made on the physical aspect of PET-powder combustion in the industrial burner. The results showed that a large amount of PET-powder of up to about 80% is exhausted without burn-up in the open atmospheric operation, whereas it is perfectly consumed in the high temperature in-furnace operation. Understanding of the relationship between the heating time and the unburnt rate of PET-powder is therefore necessary for getting an important knowledge to reduce the amount of unburnt PET-powder. In this paper, the behavior of particle diameter is first modeled according to those experimentally measured, a particle-size histogram of PET-powder is transformed to a particle-number distribution by stepping the particle diameter at 0.01 μm intervals, and finally a numerical prediction of the unburnt rate in the PET-powder combustion is attempted by introducing the parcel approximation. The results give good quantitative agreement between the unburnt rates of PET-powder measured experimentally and those predicted numerically.

An Investigation on Thermal Recycling of Recycled Plastic Resin (Burning Characteristics of PET-Resin Powder in High Temperature Oxidizing Atmosphere Formed by Annular Burner)

Energy recycling of recycled plastic-resin is focused in this investigation. Polyethylene terephthalate resin powder is employed as an auxiliary fuel, whereas high temperature oxidizing atmosphere is generated downstream of the annular burner. Temperature and O2 concentration fields downstream of the annular burner without PET-powder supply are first examined by varying the slit-jet and nozzle-jet velocities with both equivalence ratios kept constant at 1.0. PET-powder is then introduced into the high temperature oxidizing region by varying the slit-jet velocity, the nozzle-jet velocity and the median diameter of PET-powder. Variations of temperature and O2 concentration fields with PET-powder combustion are discussed qualitatively. According to the results, the dependency of the PET-powder unburnt rate on the properties of high temperature oxidizing atmosphere is examined.

Optical Observation and Discrete Vortex Analysis of Vortex-Flame Interaction in a Plane Premixed Shear Flow

ABSTRACT To obtain practical schemes of the vortex-flame interaction, a series of organized eddies formed in the plane shear flow is employed in this investigation, instead of a single vortex ring or a single vortex tube. The plane shear layer is first formed between two parallel uniform propane-air mixture streams. After an electric spark is discharged at the midpoint between two neighboring organized eddies, a premixed flame is initiated and is observed using the simultaneously two-directional and high-speed schlieren system. The outline of propagating flame after the spark ignition is simulated combining the discrete vortex method with the Huygens’ Principle. According to detailed comparison of the results simulated with those optically observed, it is found that the simulated flame profiles after the midpoint ignition agree well with those of schlieren images, and that the lateral flame propagation velocity depends almost linearly on the vortex tangential velocity and the mixture equivalence ratio.