Instability pulsating

This section emphasizes on flame quenching by stretch, as well as highlights and separately discusses the four aspects of counterflow premixed flame extinction limits, including (1) effect of nonequidiffusion, (2) influence of different boundary conditions, (3) effect of pulsating instability, and (4) relahonship of the fundamental limit of flammability. 

The inset of Figure 6.3.7 shows the flame response for rich hydrogen/air mixture of =7.Q. Since the Lewis number of this mixture is sufficiently greater than unity, it is susceptible to diffusional-thermal pulsating instability. Four flames, denoted by Flames 1-lV along the... 

Third, since the pulsating instability observed for the planar unstretched flame was expected to be promoted by positive stretch, pulsation may develop beyond a critical stretch rate smaller than the static extinction limit. As pulsating extinction occurred at a smaller stretch rate than the steady extinction limit, the flame extinguished in the pulsating, instead of the steadily propagating mode, and the flammable range was accordingly narrowed. 

Sung, C.J., Makino, A., and Law, C.K., On stretch-affected pulsating instability in rich hydrogen/air flames Asymptotic analysis and computation. Combust. Flame, 128, 422, 2002. 

Thermosyphon reboilers can suffer from flow instabilities if too high a heat flux is used. The liquid and vapour flow in the tubes is not smooth but tends to pulsate, and at high heat fluxes the pulsations can become large enough to cause vapour locking. A good practice is to install a flow restriction in the inlet line, a valve or orifice plate, so that the flow resistance can be adjusted should vapour locking occur in operation. 

Multiple Draft Tubes. Studies in the past always concentrate on beds with a single draft tube. A literature survey failed to uncover any reference on operation of multiple draft tubes. Even in the area of conventional spouted beds, the references on multiple spouted beds are rare. Foong et al. (1975) reported that the multiple spouted bed was inherently unstable due to pulsation and regression of the spouts. Similar instability was also... 

In the Galaxy, we know 93 (3 Cephei (Stankov Handler 2004) and about 100 SPB-type stars (De Cat et al. 2004). They fall within the instability strips predicted by the theory. The K-mechanism driving pulsations in (3 Cephei and SPB stars strongly depends on the abundance of the iron-group ions in the driving zone at temperatures around 2 x 105 K (Dziembowski Pamyatnykh 1993, Dziembowski et al. 1993). Theoretical models predict that pulsations of (3 Cephei and SPB-type vanish for Z = 0.01 and Z = 0.006, respectively (Pamyatnykh 1999). 

Fig. 5.19. Evolutionary track in the HR diagram of an AGB model of total mass 0.6 Mq, initial composition (Y, Z) = (0.25, 0.001 Z /20). Heavy dots marked 2 to 11 indicate the start of a series of thermal pulses (see Fig. 5.20), which lead to excursions along the steep diagonal lines. Numbers along the horizontal and descending track indicate times in years relative to the moment when an ionized planetary nebula appears and (in parentheses) the mass of the envelope in units of Mq. R = 0.0285 indicates a line of constant radius (R in solar units) corresponding to the white-dwarf sequence. Shaded areas represent earlier evolutionary stages for stars with initial masses 3,5 and 7 Mq and the steep broken line marks the high-temperature boundary of the instability strip in which stars pulsate in their fundamental mode. The y-axis gives log L/Lq. Adapted from Iben and Renzini (1983).

Pulsation in a spray is generated by hydrodynamic instabilities and waves on liquid surfaces, even for continuous supply of liquid and air to the atomizer. Dense clusters of droplets are projected into spray chamber at frequencies very similar to those of the liquid surface waves. The clusters interact with small-scale turbulent structures of the air in the core of the spray, and with large-scale structures of the air in the shear and entrainment layers of outer regions of the spray. The phenomenon of cluster formation accounts for the observation of many flame surfaces rather than a single flame in spray combustion. Each flame surrounds a cluster of droplets, and ignition and combustion appear to occur in configurations of flames surrounding droplet clusters rather than individual droplets. 

Recently, a study[265] has been conducted on the formation of droplets by the capillary wave instability of a spherical liquid shell with pulsating cavity. 

At Re = 130, a weak long-period oscillation appears in the tip of the wake (T2). Its amplitude increases with Re, but the flow behind the attached wake remains laminar to Re above 200. The amplitude of oscillation at the tip reaches 10% of the sphere diameter at Re = 270 (GIO). At about this Re, large vortices, associated with pulsations of the fluid circulating in the wake, periodically form and move downstream (S6). Vortex shedding appears to result from flow instability, originating in the free surface layer and moving downstream to affect the position of the wake tip (Rll, R12, S6). 

Figure 11-19. Ifemporal instabilities of the Ag/AgI interface under anodic load and galvanostatic conditions. T= 260 °C. This plot represents a) the periodic voltage drop across the interface and h) the change in coordinate of the pulsating receding interface [J. Janek, S. Majoni (1994)).

Ya.B. s more recent papers have been devoted to the study of nonlinear problems. In 1966 Ya.B. turned his attention to the stabilizing effect of accelerated motion through a hot mixture of a boundary of intersection of two flame fronts, convex in the direction of propagation, and proposed an approximate model of a steady cellular flame. G. I. Sivashinsky, on the basis of this work, proposed a nonlinear model equation of thermodiffusional instability which describes the development of perturbations of a bent flame in time and, together with J. M. Michelson, studied its solution near the stability boundary Le = Lecrit. It was shown numerically that the flat flame is transformed into a three-dimensional cellular one with a non-steady, chaotically pulsating structure. The formation of a two-dimensional cellular structure was also the subject of a numerical investigation by A. P. Aldushin, S. G. Kasparyan and K. G. Shkadinskii, who obtained steady flames in a wider parameter interval. 

Ya.B., together with R. M. Zaidel, showed23 that nonlinear perturbations may also cause one-dimensional instability, i.e., lead to a pulsating detonation wave even without its being bent. Numerical calculations of the process of establishing the structure of a plane detonation wave also indicated one-dimensional instability.24,25... 

In general, gas and liquid flows pulsate, with a particular perforation passing both gas and liquid intermittently, but seldom simultaneously. In large-diameter (>2.5-m, or 8-ft) dual-flow trays, the pulsations sometimes develop into sloshing, instability, and vibrations. The Ripple Tray is a proprietary variation in which the tray floor is corrugated to minimize this instability. 

This device is filled partially with working fluid. The flow instabilities inside of this device are produced due to the heat input in one part of it and heat output from the another part by heating multi-channels (H = 2mm, L = 5 mm) at one end and simultaneously cooling the other end thus resulting in pulsating fluid. This heat input and output stimulates a heat transfer, as a combination of sensible and latent heat portions. The flow instabilities are a superposition of various underlying effects. 

The mechanism of the instability that has been described here suggests that intrinsic instability of gasless combustion evolves to an inherently pulsating mode of propagation rather than to a sustained explosion or... 

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