Documentation

Pipe (2P)

Rigid conduit for fluid flow in two-phase fluid systems

Library

Two-Phase Fluid/Elements

Description

The Pipe (2P) block models the flow dynamics of a two-phase fluid inside a rigid pipe. The dynamic compressibility and thermal capacity of the fluid are assumed non-negligible. The two-phase fluid conserving ports A and B represent the pipe inlets. The thermal conserving port H represents the pipe wall, through which heat transfer with the pipe surroundings occurs.

Fluid Inertia

The block provides an option to model fluid inertia, the resistance to sudden changes in mass flow rate. By default, fluid inertia modeling is turned off. This setting is appropriate when the pressure forces driving the flow far exceed the inertial forces acting on the flow.

The default setting reduces computational costs and is recommended for most models. However, fluid inertia can become important if the mass flow rate changes rapidly. In such cases, turning fluid inertia modeling on can help improve simulation accuracy.

Energy Balance

Energy conservation in the pipe is observed through the equation:

Mu˙I+(m˙A+m˙B)uI=ϕA+ϕB+QH,

where:

  • M is the fluid mass inside the pipe.

  • uI is the specific internal energy of the fluid inside the pipe.

  • ϕA is the energy flow rate into the pipe through port A.

  • ϕB is the energy flow rate into the pipe through port B.

  • QH is the heat flow rate into the pipe through the pipe wall, represented by port H.

Heat Flow Rate

Heat transfer between the pipe wall and the internal fluid volume is modeled as a convective process, with the heat flow rate computed as:

QH=hAcoef+hBcoef2Ssurf(THTI),

where:

  • hAcoef is the heat transfer coefficient for the half pipe adjacent to port A.

  • hBcoef is the heat transfer coefficient for the half pipe adjacent to port B.

  • SSurf is the pipe surface area.

  • TH is the pipe wall temperature.

  • TI is the temperature of the fluid in the pipe.

The heat transfer coefficient in the half pipe adjacent to port A is computed as

hAcoef=NuAkIDh,

while the heat transfer coefficient in the half pipe adjacent to port B is computed as

hBcoef=NuBkIDh,

where:

  • NuA is the Nusselt number in the half pipe adjacent to port A.

  • NuB is the Nusselt number in the half pipe adjacent to port B.

  • kI is the thermal conductivity of the fluid inside the pipe.

Nusselt Number

In laminar flows, the Nusselt number is assumed constant and equal to the value specified in the block dialog box. The laminar flow Nusselt number applies when the Reynolds number is smaller than the value entered for the Laminar flow upper Reynolds number limit parameter.

The turbulent flow Nusselt number applies when the Reynolds number is greater than the value entered for the Laminar flow upper Reynolds number limit parameter. In the transitional region between laminar and turbulent flow, a cubic polynomial function blends the two Nusselt numbers. This blending ensures a smooth transition between flow regimes.

In turbulent flows, the Nusselt number in the half pipe adjacent to port A is

NuAturbulent=fA8(ReA1000)PrI1+12.7fA8(PrI2/31),

while in the half pipe adjacent to port B it is

NuBturbulent=fA8(ReB1000)PrI1+12.7fB8(PrI2/31),

where:

  • PrI is the Prandtl number inside the pipe.

Mass Balance

Mass conservation in the pipe is observed through the equation:

[(ρp)up˙I+(ρu)pu˙I]V=m˙A+m˙B+ϵM,

where:

  • ρ is the fluid density.

  • pI is the pressure inside the pipe.

  • V is the volume of fluid in the pipe.

  • m˙A is the mass flow rate into the pipe through port A.

  • m˙B is the mass flow rate into the pipe through port B.

  • M is a correction term that accounts for the smoothing of the density partial derivatives across phase transition boundaries.

The block blends the density partial derivatives of the various domains using a cubic polynomial function. At a vapor quality of 0–0.1, this function blends the derivatives of the subcooled liquid and two-phase mixture domains. At a vapor quality of 0.9–1, it blends those of the two-phase mixture and superheated vapor domains. The correction term in the mass conservation equation,

ϵM=MV/vIτ,

is added to correct for the numerical errors introduced by the cubic polynomial function, with:

  • M as the fluid mass in the pipe, computed from the equation:

    M˙=m˙A+m˙B,

  • vI as the specific volume of the fluid in the pipe.

  • τ as the phase-change time constant—the characteristic duration of a phase-change event. This constant ensures that phase changes do not occur instantaneously, effectively introducing a time lag whenever they occur.

Momentum Balance

The momentum balance equations are defined separately for each half pipe section. In the half pipe adjacent to port A:

pApI=m˙AS|m˙AS(νIνA)|+Fvisc,A+IA,

where:

  • pA is the pressure at port A.

  • S is the cross-sectional area of the pipe.

  • νA is the specific volume of the fluid at port A.

  • Fvisc,A is the viscous friction force in the half pipe adjacent to port A.

  • IA is the fluid inertia at port A:

    IA=m¨AL2S

    The parameter L is the pipe length.

In the half pipe adjacent to port B:

pBpI=m˙BS|m˙AS(νIνB)|+Fvisc,B+IB,

where:

  • pB is the pressure at port B.

  • νB is the specific volume of the fluid at port B.

  • Fvisc,B is the viscous friction force in the half pipe adjacent to port B.

  • IB is the fluid inertia at port B:

    IB=m¨BL2S

The fluid inertia terms, IA and IB, are zero when the Fluid inertia parameter is set to Off. The calculation of the viscous friction forces, Fvisc,A and Fvisc,B depends on the flow regime, laminar or turbulent.

Viscous Friction Force in Laminar Flows

In the laminar regime—that is, when the Reynolds number is smaller than the Laminar flow upper Reynolds number limit value specified in the block dialog box—the viscous friction force in the half pipe adjacent to port A is

Fvisc,Alaminar=fshapeLeffνIm˙A4Dh2S,

while in the half pipe adjacent to port B it is

Fvisc,Blaminar=fshapeLeffνIm˙B4Dh2S,

where:

  • fshape is the pipe shape factor.

  • Leff is the effective pipe length—the sum of the pipe length and the aggregate equivalent length of local resistances.

  • Dh is the hydraulic diameter of the pipe.

Viscous Friction Force in Turbulent Flows

In the turbulent regime—that is, when the Reynolds number is greater than the Turbulent flow lower Reynolds number limit value specified in the block dialog box—the viscous friction force in the half pipe adjacent to port A is

Fvisc,Aturbulent=m˙A|m˙A|fALeffνI4DHS2,

while in the half pipe adjacent to port B it is

Fvisc,Bturbulent=m˙B|m˙B|fBLeffνI4DHS2,

where:

  • fA is the Darcy friction factor for turbulent flow in the half pipe adjacent to port A.

  • fB is the Darcy friction factor for turbulent flow in the half pipe adjacent to port B.

The Darcy friction factor for turbulent flow in the half pipe adjacent to port A follows from the Haaland equation as

fA=1{1.8log10[6.9ReA+(ϵr3.7)1.11]}2,

and in the half pipe adjacent to port B as

fB=1{1.8log10[6.9ReB+(ϵr3.7)1.11]}2,

where:

  • r is the relative roughness of the pipe.

  • ReA is the Reynolds number in the half pipe adjacent to port A,

    ReA=|m˙A|DhvISνI.

  • ReB is the Reynolds number in the half pipe adjacent to port B,

    ReB=|m˙B|DhvISνI.

A cubic polynomial function is used to blend the friction losses in the transition region between laminar flow and turbulent flow.

Block Source Code

The block dialog box does not have a Source code link. To view the source code for the various block configurations, open the following files in the MATLAB® editor:

Assumptions and Limitations

  • The pipe wall is rigid.

  • The flow is fully developed.

  • The effect of gravity is negligible.

  • Heat transfer is calculated with respect to the temperature of the fluid volume in the pipe. To model temperature gradient due to heat transfer along a long pipe, connect multiple Pipe (2P) blocks in series.

Parameters

Geometry

Pipe length

Distance between the pipe inlet and outlet. The default value is 5 m.

Cross-sectional area

Internal pipe area normal to the direction of flow. This area is constant along the length of the pipe. The default value is 0.01 m^2.

Hydraulic diameter

Diameter of an equivalent pipe with a circular cross section. In a cylindrical pipe, the hydraulic diameter is the same as its actual diameter. The default value is 0.1 m.

Friction and Heat Transfer

Aggregate equivalent length of local resistances

Pressure loss due to local resistances such as bends, inlets, and fittings, expressed as the equivalent length of these resistances. The default value is 0.1 m.

Internal surface absolute roughness

Average height of all surface defects on the internal surface of the pipe. This parameter enables the calculation of the friction factor in the turbulent flow regime. The default value is 1.5e-5m.

Laminar flow upper Reynolds number limit

Largest value of the Reynolds number corresponding to fully developed laminar flow. As the Reynolds number rises above this limit, the flow gradually transitions from laminar to turbulent. The default value is 2000.

Turbulent flow lower Reynolds number limit

Smallest value of the Reynolds number corresponding to fully developed turbulent flow. As the Reynolds number falls below this limit, flow gradually transitions from turbulent to laminar. The default value is 4000.

Shape factor for laminar flow viscous friction

Semi-empirical parameter encoding the effect of pipe geometry on the viscous friction losses incurred in the laminar regime. The appropriate value to use depends on the cross-sectional shape of the pipe.

Typical values include 56 for a square cross section, 62 for a rectangular cross section, and 96 for a concentric annulus cross section [1]. The default value, corresponding to a circular cross section, is 64.

Nusselt number for laminar flow heat transfer

Proportionality constant between convective and conductive heat transfer in the laminar regime. This parameter enables the calculation of convective heat transfer in laminar flows. Its value changes with the pipe cross-sectional area and thermal boundary conditions, e.g., constant temperature or constant heat flux at the pipe wall. The default value, corresponding to a circular pipe cross section, is 3.66.

Effects and Initial Conditions

Fluid inertia

Option to model fluid inertia, the resistance of the fluid to rapid acceleration. The default is Off.

Initial fluid regime

Fluid regime at the start of simulation. The fluid can be a subcooled liquid, a two-phase mixture, or a superheated vapor. The default setting is Subcooled liquid.

Initial fluid pressure

Pressure in the pipe at the start of simulation. The default value is 0.101325 MPa.

Initial fluid temperature

Temperature in the pipe at the start of simulation. This parameter appears only when Initial fluid regime is set to Subcooled liquid or Superheated vapor. The default value is 293.15 K.

Phase change time constant

Characteristic duration of a phase-change event. This constant introduces a time lag into the transition between phases. The default value is 0.1 s.

Ports

The block has two two-phase fluid conserving ports, A and B. Port H is a thermal conserving port representing the pipe wall through which heat exchange occurs.

References

[1] White, F.M., Viscous Fluid Flow, McGraw-Hill, 1991

Introduced in R2015b

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