Threshold velocity, water

Impediments to water flow resulting from inadequate equipment design or lodgement of foreign objects in the tubes can exercise a dramatic effect on the erosion-corrosion process. Much of this influence is linked to the creation of turbulence and the simple increase in fluid velocity past obstructions. The importance of these factors is quickly recognized when the phenomenon of threshold velocity is considered. 

Figure 4. Evolution of the air entrainment threshold velocity U as a fnnction of the static contact angle 0(, of the impacting spheres on a water-air interface. The sphere diameters are 25.4 mm ( ), 20 mm (v), 15 mm (O) and 7 mm (A). The dashed line, in the hydrophobic domain, is the theoretical evolution predicted by expression (6).

Vickers, G. W. and Johnson, W., The Development of an Impression and the Threshold Velocity for Erosion Damage in Alpha-Brass and Perspex Due to Repeated Water Jet Impact," Journal Institute of Mechanical Science, Vol. 14, No. 11, November 1972, pp. 765-777. 

Cooling Wa.ter. The primary rehabihty concern is that water chemistry must be maintained in a low fouling, noncorroding regime. In addition, water flow velocity must be maintained above a certain threshold (ca 0.5 m/s in tubeside flow) to avoid fouling and corrosion. 

A sharp velocity decrease is seen for the water-base muds. Assuming a threshold detection of 500 ft/s, the alarm could be given for 0.5% of free gas or 1.1 to 1.4% of total gas (dissolved and free). 

The oil-base muds having no free gas behave differently and the 500-ft/s threshold is not reached before approximately 5% of gas is dissolved. Then the velocity decrease is almost as fast as with the water-base mud. 

Colorless gas characteristic odor of rotten eggs odor threshold Ippm sweetish taste fumes in air flammable gas, bums with a pale blue flame refractive index at 589.3nm, 1.000644 at 0°C and 1 atm density 1.539 g/L at 0°C critical temperature 100.4°C critical pressure 88.9 atm liquefies at -60.7°C solidifies at -85.5°C velocity of sound 289 m/sec in H2S gas slightly soluble in water (0.4% at 20° C) pH of a saturated aqueous solution 4.5 slightly acidic diffusivity in water at 16°C, 1.77x10 cm /sec soluble in carbon disulfide, methanol, acetone very soluble in N-methylpyrrolidinone and alka-nolamines (salt formation occurs salt dissociates on heating) liquid H2S dissolves sulfur and SO2. 

In this section, we present the results obtained for water droplets impacting a fiber of radius 350 pm. Physical properties of water are given in Table 1. Threshold radiuses of the droplets in different impact velocities are obtained and compared with those exhibited by Lorenceau et al. [1], All obtained results are shown in Figure 1, which demonstrates the threshold radiuses in different impact velocities. 

Figure 1. Threshold radiuses of the water droplets in different impact velocities...

If the skin friction exceeds the critical threshold for resuspension, sedimentary material is I ifted off from the bottom and is transported into the water body. Grainy particles may also be moved by the so-called bed-load transport that occurs already at a lower threshold. Deposition results from the settling of the sediment particles, if the shear stress falls below a certain limit. The critical thresholds, the settling velocities, and the erosion and deposition rates are material constants derived from experiments (Soulsby, 1997). 

Low Pressure Flame Apparatus. All experiments were performed on flat, premixed low pressure, 2.7 kPa (20 Torr) C2H2/O2 flames stabilized on a water cooled burner. This 8.6 cm diam burner was constructed of approximately 900, 0.12 cm i.d. stainless steel tubes microbrazed into two stainless steel perforated plates to form a water jacket around the tubes. Gases were metered using calibrated critical flow orifices. The burner was installed in a low pressure vessel pumped by a 140 L s" (300 CFM) mechanical vacuum pump. Unburned gas velocities (298 K and 2.7 kPa) in all cases were 50 cm s . Equivalence ratios from = 1.5 to 4.0 were studied with most emphasis on a sooting ( ) = 3.0 flame. Visible soot emission became apparent at a soot threshold of <() = 2.4 to... 

The beach profile (Figure 2.9) is the product of the oscillatory onshore and offshore motions of its constituent materials affected by the waves and the wave-induced currents. Granular particles on the seabed can be dislodged and suspended in the water column by a combination of drag and lift forces. These forces on the particles are exerted by bott-tom shear stresses developed by either the wave velocity exceeding a threshold value or the occurrence of turbulence. Once the particles are in the water column, they are kept in suspension longer than in subaerially wind-blown conditions because of a combination... 

If the activation threshold Ajyw vanishes or is very low, just about every encotmter will lead to a reaction. Hence, it is not the height of this potential threshold but the frequency of collisions that then determines the conversion rate. In this case, the concentration of the transition complex can remain far below its equilibrium value because continued supply is stalled, while decomposition continues taking place. Reactions of this kind are said to be dijfusion-controlled (or diffusion-limited) because their collision frequency is dependent upon the diffusion rate (diffusion velocity) of the partners involved. Bimolecular reactions in water and similarly viscous liquids are of this type if the activatimi threshold sinks tmder the third or fourth rung of our potential ladder, meaning that Ajyw <20 kG (see Fig. 18.2). Because diffusion in solid substances proceeds incomparably slowly, almost all the bimolecular reactions in such an environment are diffusion-controlled. 

Figure 6. Zoomed-in chronophotographs of the impact region, when a hydrophobic sphere (static contact angle 6>q 115°) is falling on an air-water interface at different impact velocities compared with the air entrainment threshold f/ (a) U = 2.4 m/s < f/ and (b) U = 5.0 m/s > f/. The thin liquid film that develops and rises along the sphere in both cases either gathers at the pole to encapsulate the sphere (low velocity), or is ejected from the sphere thus creating an air cavity behind it (high velocity).

Recently, terms flow-assisted corrosion and flow-accelerated corrosion (FAC) have been used to describe the erosion (or thinning) of carbon steel in nuclear and fossil power plants where there is no threshold solution velocity. FAC is a complex phenomenon that is a function of many parameters of water chemistry, material composition and hydrodynamics. FAC involves the electrochemical aspects of general corrosion plus the effects of mass transfer and momentum transfer. 

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