Cavitation flow systems

Cavitation erosion With increasing ship speeds, the development of high-speed hydraulic equipment, and the variety of modem fluid-flow applications to which metal materials are being subjected, the problem of cavitation erosion becomes ever more important. Erosion may occur in either internal-flow systems, such as piping, pumps, and turbines, or in external ones like ships propellers (36). 

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. 

In connection with studies on cavitation and pipe flow systems in chemical engineering it is desirable to have mathematical expressions for the pressure pulse speed and the damping of propagating pressure waves in Two-Phase flow mixtures. The propagation of pressure waves is one of the characteristic phenomena in Two-Phase flow. It is well known that pressure waves propa-... 

For flow systems, the strength and size of the cavitating zone should be optimized so that it matches the flow rate into the system. Care should be taken to find the best frequency for both homogeneous and heterogeneous reactions. A particular problem with heterogeneous systems is that it is necessary to ensure... 

The vibrating device functions on the principle of magnetostriction or piezoelectricity. A transducer supplies vibrations at a frequency on the order of 1 kHz, with an amplitude of 20 to 100 /fm typically. During each oscillation, cavitation bubbles appear as the sample is pulled upwards, and these then implode as the sample is pushed back downwards. This experimental arrangement stands out by its simplicity it does not require any heavy equipment and it requires only small volumes of solution. In addition, it is well suited for electrochemical experiments. On the other hand, because the entire surface is exposed to intense cavitation, the application of the results to other flow systems may pose problems. 

From a performance viewpoint, cavitation in cryogenic fluids, such as liquid oxygen, has produced very erratic operation of propulsion systems. Tests of complex flow systems have indicated once cavitation bubbles have been formed there is very little reduction or recondensation of them before they reach the combustion chamber, A comparison of the physical characteristics of the cryogenic fluids with water indicates some of the reasons for the reduced recondensation of cavitation bubbles in these liquids. Recent performance tests of valves and pumps adapted for use with liquid oxygen indicate a large amount of research and development work must be conducted before the performance of this equipment with the cryogenic fluids is comparable to that of similar equipment with water. 

Cavitation occurs wherever irregular water flow takes place, particularly if high pressures and vacuum are involved in the flow systems. Cavitation is a serious problem in desalination plants. In multi-stage flash distillation units, cavitation occurs in ... 

Occurrences in Practice. In practice, cavitation can occur in any liquid in which the pressure fluctuates either because of flow patterns ot vilxations in the system. If, in a particular location in a liquid flow system, the local pressure falls below the vapor pressure of the liquid, then cavities can be nucleated, grow to a stable size, and be transported downstream with the flow. When they reach a higher-pressure region, they become unstable and collqise, usually violently. This form of cavitation commonly occurs in hyclt foils, pipelines, hydraulic punqis, and valves. The pressures produced by the coll tse can cause localized deformation and/or removal of matoial (erosion) from the surface of any solid in the vicinity of the cavities. 

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. 

Do not confuse NPSH vdth suction head, as suction head refers to pressure above atmospheric [17]. If this consideration of NPSH is ignored the pump may well be inoperative in the system, or it may be on the border-line and become troublesome or cavitating. The significance of NPSH is to ensure sufficient head of liquid at the entrance of the pump impeller to overcome the internal flow losses of the pump. This allows the pump impeller to operate wfith a full bite of liquid essentially free of flashing bubbles of vapor due to boiling action of the fluid. 

Cavitation of a centrifugal pump, or any pump, develops when there is insufficient NPSH for the liquid to flow into the inlet of the pump, allowing flashing or bubble formation in the suction system and entrance to the pump. Each pump design or family of dimensional features related to the inlet and impeller eye area and entrance pattern requires a specific minimum value of NPSH to operate satisfactorily without flashing, cavitating, and loss of suction flowt... 

Provided that the system is initially clean and fitted with efficient air filters, metal edge-strainers of 0.005-inch spacing appear to be adequate, although clearances of vane pumps may be below 0.001-inch. It should be remembered that an excessive pressure drop, due to a clogged full-flow fine filter, could do more harm to pumps by cavitation than dirty oil. 

Cavitation in pipe systems is possible wherever there are changes in section or flow direction such as expansions, bends and branches. However, serious erosion problems are normally only associated with components within which flow is severely constricted and consequently accelerated. If pumps are excluded then, in most systems, this situation applies to devices used... 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

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