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Local Restriction (G)

Fixed restriction in flow area

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Description

The Local Restriction (G) block models the pressure drop due to a temporary reduction in flow area, such as a valve or an orifice, in a gas network. Choking occurs when the restriction reaches the sonic condition.

Ports A and B represent the restriction inlet and outlet. The restriction area, specified as a block parameter, remains constant during simulation. The restriction is adiabatic. It does not exchange heat with the environment.

The restriction consists of a contraction followed by a sudden expansion in flow area. The gas accelerates during the contraction, causing the pressure to drop. The gas separates from the wall during the sudden expansion, causing the pressure to recover only partially due to the loss of momentum.

Local Restriction Schematic

Caution

Gas flow through this block can choke. If a Mass Flow Rate Source (G) block or a Controlled Mass Flow Rate Source (G) block connected to the Local Restriction (G) specifies a greater mass flow rate than the possible choked mass flow rate, you get a simulation error. For more information, see Choked Flow.

Mass Balance

The mass balance equation is:

m˙A+m˙B=0

where m˙A and m˙B are mass flow rates at ports A and B, respectively. Flow rate associated with a port is positive when it flows into the block.

Energy Balance

The energy balance equation is:

ΦA+ΦB=0

where ΦA and ΦB are energy flow rates at ports A and B, respectively.

The block is assumed adiabatic. Therefore, there is no change in specific total enthalpy between port A, port B, and the restriction:

hA+wA22=hR+wR22hB+wB22=hR+wR22

where h is the specific enthalpy at port A, port B, or restriction R, as indicated by the subscript.

The ideal flow velocities at port A, port B, and the restriction are:

wA=m˙idealρASwB=m˙idealρBSwR=m˙idealρRSR

where:

  • S is the cross-sectional area at ports A and B.

  • SR is the cross-sectional area at the restriction.

  • ρ is the density of gas volume at port A, port B, or restriction R, as indicated by the subscript.

The theoretical mass flow rate without nonideal effects is:

m˙ideal=m˙ACd

where Cd is the discharge coefficient.

Momentum Balance

The pressure difference between ports A and B is based on a momentum balance for flow area contraction between the inlet and the restriction plus a momentum balance for sudden flow area expansion between the restriction and the outlet.

For flow from port A to port B:

ΔpAB=ρRwR|wR|(1+r2(1rρRρA)r(1rρRρB))

where r is the area ratio, r = SR/S.

For flow from port B to port A:

ΔpBA=ρRwR|wR|(1+r2(1rρRρB)r(1rρRρA))

The pressure differences in the two preceding equations are proportional to the square of the flow rate. This is the typical behavior for turbulent flow. However, for laminar flow, the pressure difference becomes linear with respect to flow rate. The laminar approximation for the pressure difference is:

Δplam=ρRΔptransition2(1r)

The threshold for transition from turbulent flow to laminar flow is defined as Δptransition = pavg(1 — Blam), where Blam is the pressure ratio at the transition between laminar and turbulent regimes (Laminar flow pressure ratio parameter value) and pavg = (pA + pB)/2.

The pressure at the restriction is based on a momentum balance for flow area contraction between the inlet and the restriction.

For flow from port A to port B:

pRAB=pAρRwR|wR|1+r2(1rρRρA)

For flow from port B to port A:

pRBA=pB+ρRwR|wR|1+r2(1rρRρB)

For laminar flow, the pressure at the restriction is approximately

pRlam=pavgρRwR21r22

The block uses a cubic polynomial in terms of (pApB) to smoothly blend the pressure difference and the restriction pressure between the turbulent regime and the laminar regime:

  • When ΔptransitionpApB

    then pApB = ΔpAB

    and pR = pRAB

  • When 0 ≤ pApB < Δptransition

    then pApB is smoothly blended between ΔpAB and Δplam

    and pR is smoothly blended between pRAB and pRlam

  • When Δptransition < pApB ≤ 0

    then pApB is smoothly blended between ΔpBA and Δplam

    and pR is smoothly blended between pRBA and pRlam

  • When pApB ≤ –Δptransition

    then pApB = ΔpBA

    and pR = pRBA

Choked Flow

When the flow through the restriction becomes choked, further changes to the flow are dependent on the upstream conditions and are independent of the downstream conditions.

If A.p is the Across variable at port A and pBchoked is the hypothetical pressure at port B, assuming choked flow from port A to port B, then

A.ppBchoked=ρRaR2(1+r2(1rρRρA)r(1rρRρB))

where a is speed of sound.

If B.p is the Across variable at port B and pAchoked is the hypothetical pressure at port A, assuming choked flow from port B to port A, then

B.ppAchoked=ρRaR2(1+r2(1rρRρB)r(1rρRρA))

The actual pressures at ports A and B, pA and pB, respectively, depend on whether choking has occurred.

For flow from port A to port B, pA = A.p and

pB={B.p,if B.ppBchokedpBchoked,if B.p<pBchoked 

For flow from port B to port A, pB = B.p and

pA={A.p,if A.ppAchokedpAchoked,if A.p<pAchoked 

Variables

Use the Variables tab in the block dialog box (or the Variables section in the block Property Inspector) to set the priority and initial target values for the block variables prior to simulation. For more information, see Set Priority and Initial Target for Block Variables and Initial Conditions for Blocks with Finite Gas Volume.

Assumptions and Limitations

  • The restriction is adiabatic. It does not exchange heat with the environment.

  • This block does not model supersonic flow.

Ports

Conserving

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Gas conserving port associated with the inlet or outlet of the local restriction. This block has no intrinsic directionality.

Gas conserving port associated with the inlet or outlet of the local restriction. This block has no intrinsic directionality.

Parameters

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Area normal to flow path at the restriction.

Area normal to flow path at ports A and B. This area is assumed the same for the two ports.

Ratio of actual mass flow rate to the theoretical mass flow rate through the restriction. The discharge coefficient is an empirical parameter that accounts for nonideal effects.

Pressure ratio at which the gas flow transitions between laminar and turbulent regimes. The pressure loss is linear with respect to mass flow rate in the laminar regime and quadratic with respect to mass flow rate in the turbulent regime.

Introduced in R2016b

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