critical flow Critical flow occurs when the flow velocity in a channel equals the wave velocity generated by a disturbance or obstruction. In this condition the Froude number (Fr) = 1. When Fr is greater than 1 waves cannot be generated upstream and the flow is said to be supercritical, rapid, or shooting.
Subcritical flow is dominated by gravitational forces and behaves in a slow or stable way. It is defined as having a Froude number less than one. Supercritical flow has a Froude number greater than one. Critical flow is the transition or control flow that possesses the minimum possible energy for that flowrate.
In the context of corrosion or errosion, critical flow rate is the maximum flow rate that avoids damage to the pipe from corrosion or erosion. In the context of liquid unloading, critical flow rate is the minimum flow rate to produce liquids from a well.
A wellhead choke controls the surface pressure and production rate from a well. Under critical flow conditions, the flow rate is a function of the upstream or tubing pressure only. For this condition to occur, the downstream pressure must be approximately 0.55 or less of the tubing pressure.
Calculations for a Trapezoidal Channel For a channel with a trapezoidal cross section, the critical flow condition is given by Fr = Vc/[g(A/B)c]1/2 = 1, where Ac = yc(b + zyc) and Bc = b + zyc2, where z is the trapezoidal channel side slope (H:V = z:1).
Choked flow is a fluid dynamic condition associated with the venturi effect. When a flowing fluid at a given pressure and temperature passes through a constriction (such as the throat of a convergent-divergent nozzle or a valve in a pipe) into a lower pressure environment the fluid velocity increases.
The Froude number is a measurement of bulk flow characteristics such as waves, sand bedforms, flow/depth interactions at a cross section or between boulders. The denominator represents the speed of a small wave on the water surface relative to the speed of the water, called wave celerity.
Definitions. Critical Flow: The variation of specific energy with depth at a constant discharge shows a minimum in the specific energy at a depth called critical depth at which the Froude number has a value of one. Critical depth is also the depth of maximum discharge, when the specific energy is held constant.
A supercritical flow is a flow whose velocity is larger than the wave velocity. The flow at which depth of the channel is less than critical depth, velocity of flow is greater than critical velocity and slope of the channel is also greater than the critical slope is known as supercritical flow.
Choked flow occurs when the downstream pressure is less than the critical pressure or the pressure ratio is less than the critical ratio. This is shown in equation 1 and repeats your initial question. Once you know the flow will be choked, you can then use the remaining equations.
Gas flow through nozzles – sonic chokes Sponsored Links. The maximum gas flow through a nozzle is determined by critical pressure. critical pressure ratio is the pressure ratio where the flow is accelerated to a velocity equal to the local velocity of sound in the fluid.
: the greatest velocity with which a fluid can flow through a given conduit without becoming turbulent.
R = Universal Gas Flow Constant (1545 ft•lbf/(lb•mol)(°R)) divided by M.W. As an example, let’ss assume that we have dry air flowing at 100 lb/min, 200°F and 24.7 psia. We will assume a molecular weight (M.W.) of 28.964 lb/lb•mol. Since we have low pressure, we will assume that compressibility is 1.0.
A choke valve is sometimes installed in the carburetor of internal combustion engines. Its purpose is to restrict the flow of air, thereby enriching the fuel-air mixture while starting the engine.
Wellhead chokes are used to limit production rates for regulations, protect surface equipment from slugging, avoid sand problems due to high drawdown, and control flow rate to avoid water or gas coning. Two types of wellhead chokes are used. They are (1) positive (fixed) chokes and (2) adjustable chokes.