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Introduction
The term “critical velocity” can refer to
- The relative velocity between plasma and a neutral gas at which the neutral gas begins to ionize is known as the critical ionization velocity.
- At the throat of a rocket, the speed of sound (otherwise known as throat velocity)
- Landau A superfluid’s critical velocity is equal to the bandgap width divided by the fermi momentum.
- Transitional velocity of a liquid from subcritical to supercritical flow.
- A fast spinning star’s break-up velocity
- The rate at which leukocytes in a blood artery transition from rolling to freely flowing.
Body
Definition of critical velocity
The critical velocity of a fluid is the speed at which a streamlined liquid flow becomes turbulent. The streamlines are straight parallel lines when the fluid velocity in the pipe is low. As the fluid’s velocity increases, the streamline remains straight and parallel to the pipe wall. When the velocity hits a certain level, patterns begin to emerge. The streamlines will be distributed throughout the pipe due to the critical velocity.
Critical velocity correlations determine the pace at which liquid loading occurs when well rates fall. It has nothing to do with the liquid output. It is based on the rate or velocity at which liquid droplets will be carried up (for some commonly used correlations), and when they can no longer be expected to travel up, liquid loading is forecast. Turner and Coleman are two well-known models, but there are a slew of others to choose from.
Critical velocity formula:
Vcrit = Nrµ/Dρ
Vcrit is the critical velocity,
Nr is the Reynolds number,
µ is the coefficient of viscosity (i.e., the resistance to flow) for a given liquid,
D is the inner diameter of the pipe,
ρ is the density of the given liquid in this equation.
Critical Velocity Vc = Kη/rρ
where, K is Reynold’s number, η= liquid viscosity coefficient, r is capillary tube radius, and ρ = liquid density
Dimensional Formula for Critical Velocity:
Critical Velocity is measured in meters per second in SI units.
Hydrodynamics: In physics, hydrodynamics of fluid dynamics explains how fluids like liquids and gasses flow. It has a wide range of uses, including assessing forces and momentum on airplanes, weather forecasting, and so on.
For example, suppose you have a two-meter-long length of pipe with an inner diameter of 0.03 meters and you want to know the critical velocity of water traveling through it at 0.25 meters per second, which is denoted by V. Although its value changes with temperature, we shall use 0.00000114 meters-squared per second in this example. Water has a density of one kilogramme per cubic meter.
If we don’t have Reynold’s number, we can compute it using the formula Nr = ρVD/µ. A Reynold’s number of less than 2,320 denotes laminar flow, whereas a Reynold’s number of more than 4,000 denotes turbulent flow.
The Reynold’s number after entering in the values is 6,579. The flow is termed turbulent because it exceeds 4,000.
Critical Velocity of Liquid:
The critical velocity of a liquid is the maximum speed at which it can maintain a streamline flow; above that speed, the flow becomes unsteady or turbulent.
Turbulence is created by the rotating or whirling effect of a liquid or air flow. Under turbulent conditions, neither speed nor direction can be maintained at any point in the fluid.
The critical velocity of a liquid flow is the speed at which the flow becomes streamlined (laminar) and turbulent above it. Vc is the symbol for it, and it is determined by: Coefficient of liquid viscosity (η) Liquid density is a measurement of how dense a liquid is, the tube’s radius.
The viscosity of the fluid, the size of the internal pipe diameter, the internal roughness of the inner surface of the pipe, the change in elevation between the pipe’s ends, and the length of the pipe along which the fluid travels all affect overall head loss in a pipe.
Valves and fittings on a pipe contribute to overall head loss, but they must be estimated independently from pipe wall friction loss using a method of modeling pipe fitting losses with k factors.
Conclusion:
The flow velocity at which a fluid switches from laminar to turbulent flow is referred to as critical velocity. The critical velocity is affected by the fluid’s density and viscosity, as well as the hydraulic diameter, which is simply the diameter in a circular pipe.
The dimensionless Reynolds Number, Re = ρVD/µ is used to describe these characteristics, where ρ is density, V is velocity, D is hydraulic diameter, and µ is dynamic viscosity. Re 2300 is normally laminar in a closed pipe, and Re > 2300 is generally turbulent.
However, the transition zone is not absolute, and flow conditions surrounding the critical velocity may fluctuate between laminar and turbulent flow on the fly. Because the friction, pressure drop, and other characteristics of these flow states are so dissimilar, it is not suggested to design systems with flow velocities at or near critical.
The rate of gas required to keep fluids entrained in the gas stream and hoisted to the surface is known as critical velocity. The higher the line pressure, the greater the required flow rate. The higher the flow rate required, the larger the pipe or tube size. Density and volume of the fluids to be raised are further factors that “greater flow rates” can generally overcome. The pattern emerges: a higher gas flow rate is required to keep a well from being overloaded. However, when wells age and lose bottom hole pressure, the gas-to-fluid ratio becomes out of balance, increasing the likelihood of loading concerns. When additional wells are brought online, or nearby wells begin to adopt various production tactics that temporarily raise line pressures, the capacity of a well to remain unloaded is dramatically impacted.
