壅塞流(临界流、阻塞流、窒息流、Choked Flow)简介英文版

Type of valve Cage-trim globes Unbalanced Balanced tables. Butterfly fishtail Conventional Ball Vee-ball, modified-ball, etc. Full-area ball Conventional globe single and double port(full port) single and double port(reduced port) Three way Angle Flow tends to open (standard body) Flow tends to close (standard body) Flow tends to close (venturi outlet) Camflex Flow tends to close FL 0.8 0.7 0.43 0.55 0.4 0.3 0.75 0.65 0.75
Restriction & Choked Flow
Restriction and Choked Flow

Restriction
Restriction is mainly used to achieve controlled or restricted flow of process medium. Such as orifice, venturi tube, control valve, etc.

As a rule of thumb, the K is 1.4 for most di-atomic gases. When K is 1.4, the Critical Pressure Ratio is 0.53, applicable to a wide range of 5 common gases approximately.

We can obtain Pmax from the following relationships:
where, FL is the valve recovery factor, can be found in the below FF is the liquid critical pressure ratio factor. P1 is the upstream pressure P2 is the downstream pressure Pv is the vapor pressure at inlet temperature Pc is the thermodynamic critical pressure

Negative effect
Choked Flow

Choked Flow in Gases
Critical Pressure Ratio Mass Flow Rate


Choked Flow in Liquids
Valve Max Allowable Pressure Drop Cavitation and flashing

When the gas velocity is choked, the equation for the mass flow rate is:
F is mass flow rate C is discharge coefficient A is discharge hole cross-sectional area P is absolute upstream pressure of the gas M is the molar mass of the gas K is Cp/Cv, the adiabatic index R is the molar gas constant T is the absolute temperature
Choked Flow in Gases -Mass Flow Rate

Note that the limited parameter in choked flow is the gas velocity, and mass flow can be increased with increased upstream pressure (increased fluid density).

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Choked Flow in Gases


Choked flow is a limiting condition that when gas passing the restriction in pipe the flow will not increase with a further decrease in the downstream pressure while upstream condition is fixed. Choked flow occurs when gas velocities across a restriction in the flow path become SONIC i.e. at a Mach number of 1. Choked flow is independent of restriction downstream conditions. Any further drop in the downstream pressure will not result in an increase in gas flow.
Choked Flow in Liquids
When the liquid flows through the restriction, there is a point downstream of the restriction called Vena Contracta. It is a place where the diameter of the stream is the least, and fluid velocity is at its maximum, pressure is at its minimum. If the liquid pressure at vena contracta is below that of the liquid’s vapor pressure at the prevailing liquid temperature, vapor cavities will form. When the vapor bubble formation in the restriction prevents the flow from increasing any further, then the liquid-vapor mixture flow reaches the limit condition, also called choked flow. An increase in pressure drop by decreasing the downstream pressure does not result in more 7 flow.

Restriction and Choked flow in Chemical Engineering

Positive application
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One application of choked flow is in cracking reactors. Maintaining equal flow through parallel reactor tubes is essential to ensure even residence time in the cracking reactor. Installing a restriction orifice at the entrance of each reactor tube achieves choked flow. Provided that choked flow is maintained, flowrates are not affected by any variation in reactor tube pressure drop.

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The mass flow rate is primarily dependent on the cross-sectional area A and the upstream pressure P, and only weakly dependent on the temperature T. The rate does not depend on the downstream pressure at all. All other terms are constants or depend only on the composition of the material in the flow.
K is theGas ratio of specific heats at constantPpressure to constant volume, Cp/Cv, also called the K = CP / C V D / PU adiabatic index.
Dry air Nitrogen Oxygen Hydrogen Carbon monoxide Methane Ammonia Chlorine Sulfur dioxide Carbon dioxide Superheated steam Saturated steam Propane Butane 1.400 1.404 1.400 1.410 1.404 1.307 1.310 1.355 1.290 1.300 1.300 1.131 1.131 1.096 0.53 0.53 0.53 0.53 0.53 0.54 0.54 0.54 0.55 0.55 0.55 0.58 0.58 0.59

Choked Flow in Liquids -Valve Max Allowable Pressure Drop