Breakup mode

From a liquid film such as a water film, the diameter of a drop formed under the action of gravity is calculated to be 9 mm with the above equation. Similarly to the liquid dripping mode, the liquid film breakup mode governed by the dripping mechanism is also typified by large droplets and low liquid flow rates. 

Figure 33. (a) Axisymmetric and (b) Non-axisymmetric Rayleigh-type breakup mode of round liquid jets in coaxial air flow. (Reprinted with permission from Ref. 210.)... 

Figure 3.5. Fiber-type breakup mode of round liquid jets in coaxial air flow.

Figure 3.8. Liquid sheet/film breakup modes Successive stages in the idealized breakup of (a) a sheet with a thick rim, (b) a wavy sheet, and (c) a perforated sheet.

B) Flat Liquid Sheets into Air Streams Mechanical and Aerodynamic Disintegration. In air streams (with an air flow), a liquid sheet issuing from the 2-D nozzle will form a quasi-2-D expanding spray. The breakup modes are divided into two groups (1) mechanical mode due to the action of liquid injection pressure, and (2) aerodynamic mode due to the action of air friction. 

The mechanical breakup mode occurs around the rims of the sheet where the air-liquid relative velocity is low, forming relatively large droplets. At low relative velocities, aerodynamic forces are much smaller than surface tension and inertia forces. Thus, the breakup of the liquid rims is purely mechanical and follows the Rayleigh mechanism for liquid column/jet breakup. For the same air pressure, the droplets detached from the rims become smaller as the liquid flow rate is increased. 

The aerodynamic breakup mode occurs in the liquid sheet between the rims. In aerodynamic breakup, the perforation and wave... 

Farago and Chigier 2l() found that at similar aerodynamic Weber numbers, the disintegration modes of a thin liquid sheet in air streams are similar to those of a round liquid jet in a coaxial air stream (Table 3.2). At high aerodynamic Weber numbers, Membrane-Type or Fiber-Type breakup mode may set in. 

Basic Breakup Modes. Starting from Lenard s investigation of large free-falling drops in still air,12671 drop/droplet breakup has been a subject of extensive theoretical and experimental studies[268] 12851 for a century. Various experimental methods have been developed and used to study droplet breakup, including free fall in towers and stairwells, suspension in vertical wind tunnels keeping droplets stationary, and in shock tubes with supersonic velocities, etc. These theoretical and experimental studies revealed that droplet breakup under the action of aerodynamic forces may occur in various modes, depending on the flow pattern around the droplet, and the physical properties of the gas and liquid involved, i.e., density, viscosity, and interfacial tension. 

The first mode may occur when a droplet is subjected to aerodynamic pressures or viscous stresses in a parallel or rotating flow. A droplet may experience the second type of breakup when exposed to a plane hyperbolic or Couette flow. The third type of breakup may occur when a droplet is in irregular flow patterns. In addition, the actual breakup modes also depend on whether a droplet is subjected to steady acceleration, or suddenly exposed to a high-velocity gas stream.[2701[2751... 

Figure 3.12. Schematic showing Liquid Jet-Ligament Breakup mode (left) and Liquid Film Sheet Breakup mode (right) in two-fluid atomization of melts.

Al-Roub et all421 identified three basic modes of liquid breakup during droplet impingement onto a liquid film (1) rim breakup, (2) cluster breakup, and (3) column breakup. The rim breakup mode involves the breakup and ejection of one or a few small droplets at the outer edge of the film, while the cluster breakup mode involves the breakup of liquid into clusters of many small droplets at the outer edge of the film. In the column breakup mode, liquid breaks up into one or a few droplets from a column of liquid at the center of the spreading droplet as a result of the surface waves reflecting back to their source. The diameter and number of the... 

Indirectly related to the cell models of this section is the work of Davis and Brenner (1981) on the rheological and shear stability properties of three-phase systems, which consist of an emulsion formed from two immiscible liquid phases (one, a discrete phase wholly dispersed in the other continuous phase) together with a third, solid, particulate phase dispersed within the interior of the discontinuous liquid phase. An elementary analysis of droplet breakup modes that arise during the shear of such three-phase systems reveals that the destabilizing presence of the solid particles may allow the technological production of smaller size emulsion droplets than could otherwise be produced (at the same shear rate). 

Critical Flvk Breakup mode Contact time ... 

Ganan-Calvo showed that the breakup mode is axisymmetric and that the resulting droplets are monodispersed when the We number has a value below 40, with We defined as... 

Keywords Bag breakup Breakup mode Breakup time Catastrophic breakup Fragments Fragment size distribution Initiation time Multimode breakup Newtonian drops Non-Newtonian drops Ohnesorge number (Oh) Secondary atomization Secondary breakup Sheet-thinning breakup Total breakup time Vibrational breakup Weber number (We)... 

The exact manner in which drops fragment is a function of We. This is typically represented by a breakup morphology figure as shown in Fig. 6.1, where each row represents a different breakup mode. In the literature, various breakup modes are identified and a wide variety of nomenclature is used. The morphology shown here is a modified version of that proposed in [2],... 

Although the transition between breakup modes is actually a continuous function of We, experimentation and modeling is simplified by assuming the breakup modes occur in the distinct ranges of We shown in Table 6.1. The value of We demarcating breakup modes is typically referred to as a transitional We. 

As noted by [7], in many high-pressure spray applications, the drop phase approaches the thermodynamic critical point where Oh increases rapidly. At elevated Oh, the observed breakup modes remain the same, but experiments have shown an increase in the transitional We and breakup times. 

In the pages to follow, each of the breakup modes are discussed in detail. Results and conclusions are presented to aid the designer of spray systems. 

In some instances, oscillation may lead to breakup into a few large fragments. This is referred to as vibrational breakup. As noted by [1], this breakup mode does not always occur, proceeds much more slowly than the other modes, and does not lead to small final fragment sizes. As a result, most authors ignore vibrational breakup and consider bag breakup to be the first mode of secondary atomizatirm. 

Multimode breakup occurs at values of We between those of bag- and sheetthinning and resembles a combination of the two breakup modes. Bag formation accompanied by the presence of a core drop results in the formation of a long ligament in the center of the bag, which is referred to as a stamen or plume [1,16], The third image in the second row of Fig. 6.1 illustrates the bag/plume structure. 

To date, no definitive explanation exists for why bag breakup occurs at low levels of aerodynamic forces and sheet-thinning breakup occurs at higher levels. Some have proposed that unstable surface waves dictate the breakup modes. However, as discussed in [4], this explanation fails to fully explain aU of the modes and is not supported by recent numerical simulations. Other possibilities may include a competition between internal flow in the deforming drop and surface tension [4], or strong backflow in the wake at high We which prevents bag growth [12]. More research is warranted. 

Catastrophic breakup has only been observed in shock tube experiments where extremely high initial relative velocities are possible. In [7], it is noted that such high velocities are not expected in typical dense sprays. Therefore, the practical applications of this breakup mode are limited. 

Regardless of breakup mode, the final product of secondary atomization is a collection of fragments with some size distribution, knowledge of which is cmcial for determining subsequent evaporation rates and for characterizing target interactiOTis. 

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