Cavitation nozzle

Vortex breakers create turbulence, reduce the open area of nozzles, and thus lead to premature nozzle cavitation limits. 

Aleiferis, P. G., Serras-Pereira, J., Augoye, A., Davies, T. J., Cracknell, R. F., Richardson, D. (2010). Effect of fuel temperature on in-nozzle cavitation and spray formation of liquid hydrocarbons and alcohols from a real-size optical injector for direct-injection spark-ignition engines. International Journal of Heat and Mass Transfer, 53,4588 1606. 

Suction pressure is the pressure at the pump s suction nozzle as measured on a gauge. The suction pressure is probably the most important pressure inside the pump. All the pump s production is based on the suction pressure. The pump takes suction pressure and converts it into discharge pressure. If the suction pressure is inadequate, it leads to cavitation. Because of this, all pumps need a gauge at the suction nozzle to measure the pressure entering the pump. 

When cavitation occurs in a pump, its efficiency is reduced. It ean akso cause sudden surges in flow and pressure at the discharge nozzle. The calculation of the NPSITr (the pump s minimum required energy) and the NPSITa (the system s available energy), is based on an understanding of the lic]uid s absolute vapor pressure. 

Bottom draw nozzle too small. Pump cavitation problem. Raising tower 10 feet did not help. Flooded the bottom of the tower. Design error in original plant. 

In summary, the lowest pressure that can be reached at point D in Fig. 11.6 is the pressure at point A. When these two pressures are equal, we say that the draw-off nozzle is limited by cavitation. If we were to lower the pressure downstream of point D, say, by opening a control valve, the increase in flow would be zero. 

I once tried to increase the flow of jet fuel from a crude distillation column by opening the draw-off, flow-control valve. Opening the valve from 30 to 100 percent did not increase the flow of jet fuel at all. This is a sure sign of nozzle exit loss—or cavitation limits. To prove my point, I increased the level of liquid in the draw-off sump from 2 to 4 ft. Since flow is proportional to velocity and head is proportional to (veloc-... 

It is positively my experience that the most common reason for pumps cavitation is partial plugging of draw nozzles. This problem is illustrated in Fig. 25.5. This is the side draw-off from a fractionator. Slowly opening the pump s discharge control valve increases flow up to a point. Beyond this point, the pump s discharge pressure and discharge flow become erratically low. It is obvious, then, that the pump is cavitating. 

Well, dear reader, it no longer exists. Figure 25.6 illustrates the true situation. Let s say we are pumping 110 GPM from the pump discharge. But only 109 GPM can drain through the draw-off nozzle. We would then slowly lower the water level in the suction line. The water level would creep down, as would the pump s suction pressure. When the water level in the suction line dropped to 14 ft, the pump would cavitate or slip. The flow rate from the pump would drop, and the water level in the suction line to the pump would partially refill. The pump s... 

Many draw nozzles, especially those in the bottom of vessels, plug because of the presence of vortex breakers. Many designers routinely add complex vortex breakers to prevent cavitation in pumps. But vortex breakers are needed only in nozzles operating with high velocities and low liquid levels. Corrosion products, debris, and products of chemical degradation can more easily foul and restrict nozzles equipped with vortex breakers. 

Liquid carbon dioxide (purity 99, 95 Vol %) was undercooled (W2) to avoid cavitation in the membran pump (P). After the compression to pre-expansion pressure, the fluid is heated to the extraction temperature (W3). The supercritical fluid loaded with anthracene leaves the extractor (V = 0,6 1). With a additional heat exchanger (W4), the solution is heated to pre-expansion temperature. In the separation vessel, the supercritical solution is expanded through a nozzle. The expanded gas will be condensed (Wl) and recompressed or let off. After the experiment, the separation vessel is opened and the particles were collected. The particle size is measured by laser diffraction spectroscopy (Malvern Master Sizer X). 

The velocity of the liquid inside the nozzle (v,) is much higher than the velocity outside it (v ), and therefore the pressure inside the gap pg must be much lower than the external pressure v . When the velocity becomes sufficiently high, Pg can become even lower than the vapor pressure of water, and bubbles of water vapor will be formed. These bubbles will collapse violently when the liquid is out of the small nozzle, creating a large amount of local turbulence. This is called cavitation. Cavitation should be kept limited, as it may form radicals, which may compromise product quality. 

Gas bubbles are relevant to various aspects of the atomization and sprays. In flashing process or flash atomization, bubbles are formed inside the liquid which significantly alter the atomization process (see Chap. 10). Also in effervescent atomizers, high-pressure air is injected inside a liquid and disperses as small bubbles. In addition, bubbles are formed in cavitating nozzles, which significantly alter the atomization process. Gas bubbles go through volume oscillations in addition to shape oscillation discussed in the previous section. In this section, dynamic evolution and stability of a spherical bubble undergoing volume oscillation is discussed. 

A commonly used primary atomization model for liquid jets has been developed by Huh et al. [1], The model considers the effects of both infinitesimal wave growth on the jet surface and jet turbulence including cavitation dynamics. Initial perturbations on the jet surface are induced by the turbulent fluctuations in the jet, originating from the shear stress along the nozzle wall and possible cavitation effects. This approach overcomes the inherent difficulty of wave growth models, where the exponential wave growth rate becomes zero at zero perturbation amplitude. 

The first spray phenomenon that needs to be modeled is the atomization process, that is, the disintegration of the bulk liquid into tiny droplets. The atomization process can be separated into inner-nozzle and outer-nozzle effects. The forces that govern the inner-nozzle atomization include cavitation-induced and turbulence-induced disturbances of the liquid. Once the liquid exits the nozzle, it interacts with the gaseous environment that induces disturbances on the liquid-gas interface caused by aerodynamic and inertial forces. Also, when the liquid exits the nozzle, it experiences a discontinuity in the boundary condition, namely, from the fixed boundary of the nozzle orifice to a free surface boundary. This abrupt change in the boundary condition leads to disturbances of the liquid that influence the atomization process. In general, the atomization of a bulk liquid is a very complex process and is still the subject of intensive research. 

Pump manufacturers have established guidelines to ensure each pump they supply is not exposed to conditions that result in cavitation. The design standard is called NPSHR or net positive suction head required. The NPSHR takes into account any potential head losses that might occur between the pump s suction nozzle and impeller thereby ensuring the liquid does not drop below its vapour pressure (bubble point). The NPSH is a measure of the proximity of a liquid to its vapour pressure, and must exceed the pump manufacturer s pump NPSHR. There are two process variables that can be adjusted, in case the available NPSH is less than the NPSHR raise the static head and lower friction losses. Conversely, the NPSHR can be reduced by using a larger, slower speed pump, a double suction impeller, a larger impeller inlet area, an oversized pump and a secondary impeller placed ahead of the primary impeller. 

Recent developments in microelectrome-chanical systems (MEMS) have enabled the integration and fabrication of numerous micro components such as pumps, valves, and nozzles into complex high-speed microfluidic machines. These systems possess geometrical dimensions in the range 1-1,000 pm, which are 10-10 -10-10" times less than conventimial machines, and operate at liquid flow speeds up to 300 m/s. It has been confirmed that microfluidic systems, like their large-scale counterparts, are susceptible to the deleterious effects of cavitation when appropriate hydrodynamic conditions develop. Cavitation damage in micro-orifices has been reported by Mishra and Peles [2], Small pits on the silicon surface have been detected after only 7-8 h of operation under cavitating flow. 

Droplets from Free Surfaces It is also possible to dispense microdroplets from free surfaces, either formed on a sessile droplet or along the meniscus formed at a nozzle. In the latter case, the critical difference with the previous section is that the primary droplet formed from the nozzle is much smaller than the diameter of the nozzle, eliminating the direct influence of the nozzle s size oti the size of the droplet. The study of the formation of droplets from free surfaces has a long and controversial history, which has been suggested to be due to either capillary wave breakup, cavitation, cavitation-induced capDlaiy waves which subsequently break up to form droplets, or even cavitation-screened capillary wave breakup, where a cavitation layer beneath the surface acts to suppress atomization. It is likely that over the wide range of operating... 

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