壅塞流(临界流、阻塞流、窒息流、Choked Flow)简介英文版教学文案.ppt
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1、壅塞流(临界流、阻塞流、窒息流、Choked Flow)简介英文版Restriction and Choked FlowRestrictionRestriction is mainly used to achieve controlled or restricted flow of process medium.Such as orifice,venturi tube,control valve,etc.Choked flowA flowing fluid can reach a limiting condition when passing through a restriction,tha
2、t the mass flow will not increase with a further decrease in the downstream pressure while at a given upstream pressure and temperature.This is called Choked Flow.Restriction and Choked flow in Chemical EngineeringqPositive applicationOne application of choked flow is in cracking reactors.Maintainin
3、g equal flow through parallel reactor tubes is essential to ensure even residence time in the cracking reactor.Installing a restriction orifice at theentrance of each reactor tube achieves choked flow.Provided that choked flow is maintained,flowrates are not affected by any variation in reactor tube
4、 pressure drop.qNegative effectIt is usually desirable to avoid choking conditions in a liquid system,because it often accompanies cavitation,two-phase flow which can damage equipment.2Choked Flow in Gases-Mass Flow RateNote that the limited parameter in choked flow is the gas velocity,and mass flow
5、 can be increased with increased upstream pressure(increased fluid density).When the gas velocity is choked,the equation for the mass flow rate is:F is mass flow rateC is discharge coefficientA is discharge hole cross-sectional areaP is absolute upstream pressure of the gasM is the molar mass of the
6、 gasK is Cp/Cv,the adiabatic indexR is the molar gas constantT is the absolute temperatureThe 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
7、other terms are constants or depend only on the composition of the material in the flow.6Choked Flow in LiquidsWhen 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 velo
8、city is at its maximum,pressure is at its minimum.If the liquid pressure at vena contracta is below that of the liquids 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 t
9、he 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 flow.7Choked Flow in Liquids-Valve Max Allowable Pressure DropIf choked flow occurs in a valve?pCalculate the max allowable pres
10、sure drop(Pmax)of this valvepCompare with the actual service pressure differential(P=P1 P2)pIf Pmax is less than P,choked flow will exist under the service conditions specified.We can obtain Pmax from the following relationships:where,FL is the valve recovery factor,can be found in the below tables.
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