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.

11、FF is the liquid critical pressure ratio factor.P1 is the upstream pressureP2 is the downstream pressurePv is the vapor pressure at inlet temperaturePc is the thermodynamic critical pressure Type of valve FLCage-trim globesUnbalanced0.8Balanced0.7Butterflyfishtail0.43Conventional0.55BallVee-ball,mod

12、ified-ball,etc.0.4Full-area ball0.3Conventional globesingle and double port(full port)0.75single and double port(reduced port)0.65Three way0.75AngleFlow tends to open(standard body)0.85Flow tends to close(standard body)0.5Flow tends to close(venturi outlet)0.2CamflexFlow tends to close0.72Flow tends

13、 to open0.46Split body0.88Choked Flow in Liquids-Cavitation and FlashingOnce it is known that choked flow conditions will develop within the specified valve design (Pmax is calculated to be less than P),a further distinction can be made to determine whether the choked flow is caused by cavitation or

14、 flashing.CavitationIf downstream pressure recovery is sufficient to raise the outlet pressure above the vapor pressure of the liquid,the bubbles will collapse,or implode,producing cavitation.Collapsing of the vapor bubbles releases energy and produces a noise similar to what one would expect if gra

15、vel were flowing through the valve.If the bubbles collapse in close proximity to solid surfaces in the valve,the energy released will gradually tear away the material leaving a rough,cinderlike surface.Cavitation damage may extend to the adjacent downstream pipeline,if that is where pressure recover

16、y occurs and the bubbles collapse.FlashingIf pressure at the valve outlet remains below the vapor pressure of the liquid,the bubbles will remain in the downstream system and the process is said to have flashed.Flashing can produce serious erosion damage to the valve trim parts and is characterized b

17、y a smooth,polished appearance of the eroded surface.Flashing damage is normally greatest at the point of highest velocity,which is usually at or near the seat line of the valve plug and seat ring.Pv9Choked Flow in Liquids-Eliminate or minimize the damage of CavitationSelect the type of the valve th

18、at can split the total pressure drop across the valve into smaller portions,such as using multiplestage trims.Select the valve with hard materials.Change the system by raising pressure P2,such as moving the valve to a location that has more static head.10SummaryChoked Flow in GasesqGas velocity acro

19、ss restriction in the flow path become SONIC i.e.at a Mach number of 1qAs a rule of thumb,the Critical Pressure Ratio 0.53 is applicable to a wide range of common gases.qThe mass flow rate is primarily dependent on the cross-sectional area A and the upstream pressure P,and only weakly dependent on t

20、he temperature T.The rate does not depend on the downstream pressure at all.Choked Flow in LiquidsqIf the liquid pressure at vena contracta is below that of the liquids vapor pressure at the prevailing liquid temperature,vapor cavities will form,then may cause choked flow.qIf Pmax is less than P,choked flow will exist in a valve under the service conditions specified.qCavitation,flashing,two-phase flow can cause severe damage to valve elements,and associated pipe.11结束语结束语谢谢大家聆听！谢谢大家聆听！12