Spray pulsation

All these three modes can be divided into two groups (i) Pulsating Jet Disruption as the normal sub-mode of atomization, and (it) Super-Pulsating Jet Disruption that involves an extremely high periodic change between low and high density regions in the spray. 

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. 

Yu, K., K. J. Wilson, T. P. Parr, R. A. Smith, and K.C. Schadow. 1996. Characterization of pulsating spray droplets and their interaction with vortical structure. AIAA Paper No. 96-0083. 

In order to get a defined spraying condition the pulsation-free feeding of the liquid product is very important. Therefore the unit is equipped with a servo motor driven metering pump which allows to accurately adjust and control the liquid product feed and operates without any pulsations. 

For some reactions listed in Table 1-4A, the fixed-bed reactor is operated under cocurrent-upflow conditions. Unlike the trickle-flow condition, this type of operation is normally characterized by bubble-flow (at low liquid and gas rates) and pulsating-flow (at high gas flow rates) conditions. Normally, the bubble-flow conditions are used. In the SYNTHOIL coal-liquefaction process, both pulsating-and spray-flow conditions are used, so that the solid reactant (coal) does not plug the reactor. In bubble flow, the gas is the dispersed phase and the liquid Ls a continuous phase. In pulsating flow, pulses of gas and liquid pass through the reactor. In the spray-flow regime, the gas is a continuous phase and the liquid is a dispersed phase. 

The applicability of the proposed macromixing models have been generally restricted to the bubble- and trickle-flow conditions. Their usefulness in correlating RTD in pulsating- and spray-flow regimes needs to be investigated. 

An alternative to spray drying of solids that are suspended in a liquid using high pressure single and two phase or rotary nozzles (see Section 7.4.3) is the break-up of a low pressure stream of slurry in gas dynamic atomization. In this process, the fluid is pumped to an orifice where it is released into a pulsating flow of hot gas (Fig. 7.81) and atomized. 

Fig. 33.7. In this figure the atomizing gas enters fi om the top while the liquid enters firom a circumferential slot. As both fluids reach the core opening, the liquid is pushed toward the nozzle exit by the gas pressure. At an arbitrary time (tj), the liquid flow is redirected by the gas pressure and a thin film is formed at the nozzle wall. The hquid partially blocks the gas flow, building a pressure. As the pressure builds to a critical value, a hquid chunk is removed. This process causes an oscillatory spray formation. The frequency of this oscillation depends on the liquid and gas flow rates. The frequency increases with increasing the velocity of the liquid or the gas. Two separate variables are important for the pulsation (a) shear stresses at the liquid/gas interface, and (b) fluid momentum.

The cone-jet mode at the spray capillary tip described and illustrated in Figures 1.1 and 1.3a is only one of the many possible ES modes. For a qualitative description of this and other modes, see Cloupeau [13a-c]. More recent studies by Vertes and coworkers [15] using fast time-lapse imaging of the Taylor cone provide details on the evolution of the Taylor cone into a cone jet and pulsations of the jet. These pulsations lead to spray current oscillations. The current oscillations are easy to determine with conventional equipment and can be used as a guide for finding conditions that stabilize the jet and improve signal-to-noise ratios of the mass spectra. The cone-jet mode is the most used and best characterized mode in the electrospray literature [12, 13]. 

---