Velocity, critical

When a liquid flows in a channel the flow is laminar for very small velocities it remains laminar as the flow velocity is increased but, at a certain, fairly definite, mean flow velocity in the channel, turbulence sets in. The mean flow velocity at which this happens is called the critical velocity When turbulence occurs the definition of coefficient of viscosity is no longer applicable 17 is not defined under turbulent conditions. 

If d refers to the diameter of a cylindrical channel, turbulent flow occurs for Re 2000 and laminar flow for Re 1000. When Re lies between 1000 and 2000 the flow is usually laminar provided that there is no undue disturbance at the entrance of the channel. 

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Countercurrent Separation and Elutriation. The process known as elutriation in cell separation is a refined method for separation of cells having close mass densities. Cells can be separated by making use of differences in the critical velocity of cells. If the mass densities of two cells are identical, but the sizes are different, then the larger particle has a higher critical velocity than the smaller one. 

When this inward drag force, is exceeded by the plasma, the particle moves inward with the plasma. The inward velocity the plasma needs to exceed in order to drag the particle inward is called the critical velocity, U, of the particle ... 

The special design of the Latham bowl allows for a specific blood cell separation known as SURGE. This technique makes use of the principle of critical velocity. The Latham bowl is filled until the huffy coat, ie, layer of platelets and white cells, moves in front of the bowl optics. At this point the machine starts to recirculate plasma through the bowl at increasing rates. The smallest particles, ie, platelets, ate the first to leave the bowl. Their high number causes the effluent line to turn foggy. The optical density of the fluid in the effluent line is monitored by the line sensor. A special algorithm then determines when to open and close the appropriate valves, as well as the optimum recirculation rate. 

Fluid-Elastic Coupling Fluid flowing over tubes causes them to vibrate with a whirling motion. The mechanism of fluid-elastic coupling occurs when a critical velocity is exceeded and the vibration then becomes self-excited and grows in amplitude. This mechanism frequently occurs in process heat exchangers which suffer vibration damage. 

Fluidized This is an expanded condition in which the sohds particles are supported by drag forces caused by the gas phase passing through the interstices among the particles at some critical velocity. It is an unstable condition in that the superficial gas velocity upward is less than the terminal setting velocity of the solids particles the gas... 

This phenomenon is caused by self-excitation of the blade and is aero-elastic. It must be distinguished from classic flutter, since classic flutter is a coupled torsional-flexural vibration that occurs when the freestream velocity over a wing or airfoil section reaches a certain critical velocity. Stall flutter, on the other hand, is a phenomenon that occurs due to the stalling of the flow around a blade. 

In the above equation, is the critical velocity (m/s), K is the ratio of specific heats (Cp/C ) at inlet conditions, P is the pressure in the restriction at critical flow conditions (KPa, absolute - Note that this term is known as the critical flow pressure ), and p, is the density of the fluid at the critical flow temperature and pressure (kg/m ). 

If the pressure Pj downstream of the restriction is less than the critical flow pressure, then the maximum obtainable flow which occurs at critical velocity is a function of P, and P but is unaffected by Pj. If Pj is greater than P , however, then the flow is termed "subcritical," and the rate is a function of P, and Pj. There are thus two equations for sizing PR valves in vapor service, depending on whether the flow is critical or subcritical. 

The maximum vapor load on the drum is based on the largest release from safety valves discharging as a result of a single contingency. Vapor velocities in the drum are based on 100% of critical velocity (refer back to Equation 1). However, a velocity of 175% of critical is permitted when one is applying the 1.5... 

The vapor space is sized to avoid water entraimnent in the flare gas. As a rule, vapor velocities in the drum should not exceed 150 % of critical. This however can be increased to 230 % critical velocity) when considering... 

For this safety criterion, we consider the fact that as the velocity decreases with increasing distance from the surface of the tank, it will reach some critical velocity, at which the induced movement of air will be insufficient to overcome the effects of crossdrafts or the buoyancy velocity At this point, we must ensure that the concentration is at, or below, some critical allowable concentration, Qfj,. The values of the critical concentration and velocity will depend (tn particular circumstances, but it is worth noting that must be at least equal to I g in order to overcome the effects of buoyancy, and the appropriate value will depend on the crossdrafts, which typically vary between 0.05 m to 0.5 in s F For the sake of providing examples, we have chosen to be the maximum of the buoyancy velocity and the typical cross-draft velocity. For the critical concentration we have chosen two values, C = 0.05 and C = 0.10. The actual value used by a designer would depend on the toxicity of the contaminant in question. 

Figures 10.75 and 10.76 show the initial kinematic momentum required to meet this criterion as a function of the buoyancy velocity and the length of the tank, for different values of the allowable concentration Qj. , and critical veloc-ity As we would expect, the required momentum increases both as the length of the tank increases and as the buoyancy of the contaminant increases.

The sonic or critical velocity (speed of sound in the fluid) is the maximum velocity which a compressible fluid can attain in a pipe [3]. 

In general, the sonic or critical velocity is attained for an outlet or downstream pressure equal to or less than one half the upstream or inlet absolute pressure condition of a system. The discharge through an orifice or nozzle is usually a limiting condition for the flow through the end of a pipe. The usual pressure drop equations do not hold at the sonic velocity, as in an orifice. Conditions or systems exhausting to atmosphere (or vacuum) from medium to high pressures should be examined for critical flow, otherwise the calculated pressure drop may be in error. 

Figure 2-48. Critical velocity characteristics depend on whether slurry is heterogeneous or homogeneous. By permission, Deramme-laere, R. H. and Wasp, E. J., Fluid Flow, Slurry Systems and Pipelines, Encyclopedia of Chemical Processing and Design, J. McKetta, Ed., M. Dekker, vol. 22,1985 [25].

Vj = Sonic (critical) velocity in compressible fluid, ft/sec or speed of sound, ft/sec... 

The critical velocities for the Bingham plastic and Power law fluids can be calculated as follows ... 

In the case of pipe flow, for practical purposes, the corresponding critical velocities may be calculated using Equation 4-110 and 4-111, but letting d = 0. 

The critical velocity, which when exceeded may result in erosion corrosion, can be calculated by the equation presented in API RP 14E, which is [199]... 

Craig, B., Critical Velocity examined for effects of erosion-corrosion, Oil and Gas Journal, May 27, 1985. 

If flow becomes turbulent, the corrosion rate increases even more rapidly. In practice, most engineering materials have a critical velocity above which the corrosion rate is unacceptably high. This does not correspond with the laminar-to-turbulence transition. Surface roughness is an important consideration. 

Table 1.30 Critical velocity of copper alloys on seawater (from reference )...

Alloy Composition Critical velocity in 25 mm dia. tube ms Critical shear stress Nm ... 

Small variations in solution composition may also affect the value of any critical velocity. In laboratory tests using recirculating artificial sea-water the presence of dissolved copper from copper alloy test-pieces has been shown to affect the value of the critical velocity for such materials . 

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