Norio Ohiwa

Structure and blow-off mechanism of rod-stabilized premixed flame

Abstract In order to determine the key feature in the blow-off and to reexamine the combustion mechanism, an experimental investigation was performed on propaneair flames stabilized by cylindrical rod bluff-bodies when the number and diameter of rods were changed under a constant blockage ratio of 0.4. Measurements were made of pressure gradient fluctuations along the rod surface and the rms values of ion current on the wake axis. The development of a flame generated at the attachment point was also pursued by using high-speed schlieren photography and ionization probes. The results showed that the instantaneous flame structure is composed of a series of small scale eddy-flames and large scale lump-flames containing a small definite number of eddy-flames. The attached flame causes a locally intense fluctuation of the surface pressure gradient at the attachment point probably due to flame-flow interaction. The lump-flames originate in the strongly stretched eddy-flames because of the maximum velocity gradient across a boundary layer at that point, and the surface pressure gradient fluctuation governs the process of lump-flame formation. It is also shown that the recirculation zone length varies with the range of L R d = 1.5–6.0 , which agrees well with that of the lump-flame formation point Lcd = 1.4–5.8. From these results a local extinction of the excessively stretched weak eddy-flames at the end of the recirculation zone was found to trigger the blow-off of the rod-stabilized flame. This enabled a qualitative modeling of the mechanisms of the lump-flame formation and flame stabilization behind the rods.

Ignition and burning process in a divided chamber bomb

Abstract Ignition and burning mechanisms of the main chamber mixture by a torch jet were experimentally investigated using a divided chamber bomb. The effects of the nozzle diameter and volume ratio on the structure of the torch jet, ignition process, and subsequent burning process in the main chamber were minutely examined, in both uniform and stratified charges, by measurements of ion current, light emission, OH-emission (306.4 nm), initial torch jet velocity, and main chamber pressure histories and by schlieren photography. The structure of the torch jet was greatly influenced by the nozzle diameter and volume ratio independently of the main chamber mixture ratio. According to the physical and chemical characteristics obtained for it, the structure of the torch jet could be classified into four types, and the ignition and burning processes in the main chamber could also be classified into four patterns depending on the torch jet structure: pattern I, chemical chain ignition and well-dispersed burning; pattern II, composite ignition and well-dispersed burning followed by wrinkled laminar burning; pattern III, flame kernel torch ignition and wrinkled laminar burning; and pattern IV, flame front torch ignition and wrinkled laminar burning. Combustion characteristics such as main chamber pressure and net burning time in the main chamber also showed their own peculiar features. Examination of the lean flammability limit gave a possibility of lean burning outside of the normal flammability limit by using divided chamber systems. From these results combustion pattern II was found to be most favorable for lean burning.

Noise characteristics of turbulent diffusion flames with coherent structure

Abstract In order to elucidate the interrelationship between flame structure and noise characteristics and to obtain those properties which are easily measurable and understandable, and are strongly correlated with the acoustic pressure perturbation, three kinds of flames with different structure are first formed in a plane shear layer by varying the flow and turbulence conditions: a flame within which organized eddies are clearly observed, a flame which has a rather complicated appearance and a higher eddy-formation frequency than the first due to the increased mean convection velocity, and a flame without any organized eddy, which is stabilized under the same flow conditions as the first flame, but has increased turbulence levels. Simultaneous measurements and FFT-analyses of the fluctuations of sound pressure, ion current, temperature, and CH-emission are made on these three flames. The results are shown concerning not only clear properties of the correlations between flame structure and sound intensit...

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.

Improvement of the nonlinear eddy diffusivity model for rotational turbulent heat transfer at various rotating axes

This paper presents an improved turbulent heat transfer model for analysis of a turbulent heat transfer in a rotational wall-bounded shear flow. Since a turbulent heat transfer with rotation has often been encountered in physical or engineering problems, a turbulence model should be reconstructed to adequately predict a turbulent heat transfer with rotation. In order to predict turbulent heat transfer with rotation, models of both the velocity and thermal fields are needed. In the velocity field, the modified nonlinear eddy diffusivity for a momentum model (NLEDMM) was proposed to accurately predict rotational wall-bounded turbulent shear flows with arbitrary rotating axes. As for a thermal field, the turbulence model was only improved to predict a heated channel turbulent flow with spanwise rotation. Thus, a turbulence model, which can be applied in heated rotational turbulent shear flows with arbitrary rotating axes, should be modified. In order to improve the turbulent heat transfer model, the adequate data, which are used for evaluation of turbulence model performance are arranged by conducting direct numerical simulations in this study. Since there are few reports on streamwise and wall-normal rotating channel flows with heat transfer, DNSs are carried out under conditions of streamwise and wall-normal rotating axes. In the present study, based on our DNS databases of streamwise and wall-normal rotating channel flows, existing nonlinear two-equation heat-transfer turbulence models were evaluated. Using the results of this evaluation, we then improved the modeled expression of turbulent heat flux for heat transfer in flows with arbitrary rotating axes, i.e. a nonlinear eddy diffusivity for the heat model (NLEDHM) is proposed.

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...

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