Atom twinning

FIG 27-25ii Common types of atomizers twin-fluid atomizers. (From Lefeh-V7 e, Atomization and Sprays, Hemisphere New Yof k, 1989. Reproduced with peimission. All rights reseroed. )... 

Twin-Fluid Air-Blast Atomizer. Twin-fluid atomizers can be divided into internal and external mixing systems. Atomization occurs by passing a high-velocity gas stream over a liquid sheet or by mixing in the form of a Y jet. The gas stream is usually air although steam has been used to improve the injection characteristics of heavy viscous fuels. The air stream is usually derived from the main air flow to the combustor, thus utihzing a portion of the combustor pressure drop. 

The size, shape, and length of the flames obtained from the two types of atomizers (twin-fluid air blast and Sonicore) show some fundamental differences between the two atomizers. A relatively large droplet size from the twin-fluid atomizer together with low concentrations of oxygen and temperature within the spray allow combustion to occur essentially at the spray boundary. 

In chemical terms, ethanol is a primary alcohol (Fignre 2.1), i.e. its carbon 1 is tetrahedrally hybridized sp, and carries two hydrogen atoms twinned with the hydroxyl radical (Fignre 2.1). Alcohol fnnctions shonld not be confnsed with enol fnnctions... 

This is a Nukiyama-Tanasawa type distribution. The parameter a > 1, guarantees that the number distribution vanishes at small D, as opposed to the expression derived by Cousin et al. [19] which corresponds to a = 1. Unfortunately this introduces a third parameter that needs to be determined. Lecompte and Dumouchel [21] suggest that there may be a tmique pair q and a that can represent all drop size distributions for a specific atomization process (ultrasonic atomization, twin-fluid atomization, etc.). This could transform the MEF into a pseudo-predictive method. 

TbQH mimics the structure of ZrBr when one considers only the TbQ framework. Tbe octahedra are condensed via edges and form layers which are surrounded by Q atoms above the octahedral faces as in the M6Xg cluster. The Tb atoms are close packed, and neutron diffraction investigations on the deuterated compound TbQHo.s show that the tetrahedral voids within the Tb atom twin layers are occupied by D atoms (Fig. 5-37a). [187] The compound has a homogeneity range of TbQDo.67-i.oo which arises from the D atoms partial occupancy of the void. 

Hashimoto H et ai 980 Direot observations of the arrangement of atoms around staoking faults and twins in gols orystals and the movement of atoms aooompanying their formation and disappearanoe Japan. J. Appi. Phys. 19 LI... 

Table 7.1 presents us with something of a dilemma. We would obviously desire to explore i much of the phase space as possible but this may be compromised by the need for a sma time step. One possible approach is to use a multiple time step method. The underlyir rationale is that certain interactions evolve more rapidly with rime than other interaction The twin-range method (Section 6.7.1) is a crude type of multiple time step approach, i that interactions involving atoms between the lower and upper cutoff distance remai constant and change only when the neighbour list is updated. However, this approac can lead to an accumulation of numerical errors in calculated properties. A more soph sticated approach is to approximate the forces due to these atoms using a Taylor seri< expansion [Streett et al. 1978] ... 

Twin-fluid atomizer Twisted pair cable Twitchell splitting Twitchell s reagents Two-film theory... 

The principal parameters affecting the size of droplets produced by twin-fluid atomizers have also been discussed (34). These parameters include Hquid viscosity, surface tension, initial jet diameter (or film thickness), air density, relative velocity, and air—Hquid ratio. However, these parameters may have an insignificant effect on droplet size if atomization occurs very rapidly near the atomizer exit. 

Most studies indicate that air velocity has a profound influence on mean droplet size in twin-fluid atomizers. Generally, the droplet size is inversely proportional to the atomizing air velocity. However, the relative velocity between the Hquid and air stream is more important than the absolute air velocity. 

Both effects can produce coarser atomization. However, the influence of Hquid viscosity on atomization appears to diminish for high Reynolds or Weber numbers. Liquid surface tension appears to be the only parameter independent of the mode of atomization. Mean droplet size increases with increasing surface tension in twin-fluid atomizers (34). is proportional to CJ, where the exponent n varies between 0.25 and 0.5. At high values of Weber number, however, drop size is nearly proportional to surface tension. 

Fig. 9. (a) Pressure atomizers (b) rotary atomizer and (c) twin-fluid atomizers (89,90). 

Atomizers for large boiler burners are usually of the swid pressure jet or internally mixed twin-fluid types, producing hoUow conical sprays. Less common are the externally mixed twin-fluid types (89,90). 

FIG, 27-27 Y-jet twin-fluid atomizer. (From Lefehv70, Atomization and Sprays, Hemisphe7e, New Yo7 k, 1989. Rep7oduced with pe7mission. All 7 ights 7 ese7 oed.)... 

When Li metal is cold-worked it transforms from body-centred cubic to cubic close-packed in which each atom is surrounded by 12 others in twinned cuboctahedral coordination below 78 K the stable crystalline modification is hexagonal dose-packed in which each lithium atom has 12 nearest neighbours in the form of a cuboctahedron. This very high coordination... 

However, it is not yet clear why the ener es of the SISF and the twin boundary increase with increasing A1 concentration. To find a clue to the problem, it would be needed to make out the effects of the short-range ordering of A1 atoms in excess of the stoichiometric composition of the HAl phase on the energies of planar faults and the stmcture of dislocation cores in the Al-rich HAl phase. 

The core structure of the 1/2 [112] dislocation is shown in Fig. 4. This core is spread into two adjacent (111) plames amd the superlattice extrinsic stacking fault (SESF) is formed within the core. Such faults have, indeed, been observed earlier by electron microscopy (Hug, et al. 1986) and the recent HREM observation by Inkson amd Humphreys (1995) can be interpreted as the dissociation shown in Fig. 4. This fault represents a microtwin, two atomic layers wide, amd it may serve as a nucleus for twinning. Application of the corresponding external shear stress, indeed, led at high enough stresses to the growth of the twin in the [111] direction. 

Atomic structure of the ordered twin with APB tsrpe displacement... 

Fig. 7. Maps of the electronic charge density in the (110) planes In the ordered twin with (111) APB type displacement. The hatched areas correspond to the charge density higher than 0.03 electrons per cubic Bohr. The charge density differences between two successive contours of the constant charge density are 0.005 electrons per cubic Bohr. Atoms in the two successive (1 10) planes are denoted as Til, All, and T12, A12, respectively, (a) Structure calculated using the Finnis-Sinclair type potential, (b) Structure calculated using the full-potential LMTO method.

Similarly, in studies of lamellar interfaces the calculations using the central-force potentials predict correctly the order of energies for different interfaces but their ratios cannot be determined since the energy of the ordered twin is unphysically low, similarly as that of the SISF. Notwithstcinding, the situation is more complex in the case of interfaces. It has been demonstrated that the atomic structure of an ordered twin with APB type displacement is not predicted correctly in the framework of central-forces and that it is the formation of strong Ti-Ti covalent bonds across the interface which dominates the structure. This character of bonding in TiAl is likely to be even more important in more complex interfaces and it cannot be excluded that it affects directly dislocation cores. 

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