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:

$M {\dot{u}}_{I} + ({\dot{m}}_{A} + {\dot{m}}_{B}) u_{I} = ϕ_{A} + ϕ_{B} + Q_{H},$

where:

M is the fluid mass inside the pipe.
u_I is the specific internal energy of the fluid inside the pipe.
ϕ_A is the energy flow rate into the pipe through port A.
ϕ_B is the energy flow rate into the pipe through port B.
Q_H 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:

$Q_{H} = h_{coeff} S_{surf} (T_{H} - T_{I}),$

where:

h_coeff is the average heat transfer coefficient in the pipe.
S_Surf is the pipe surface area.
T_H is the pipe wall temperature.
T_I is the temperature of the fluid in the pipe.

The calculation of the heat transfer coefficient depends on the fluid phase. In the subcooled liquid and superheated vapor phases, the coefficient is:

$h^{*}_{coeff} = \frac{k^{*}_{I} {Nu}^{*}}{D_{h}},$

where the asterisk denotes a value specific to the phase considered (liquid or vapor) and:

Nu is the average Nusselt number in the pipe.
k_I is the average thermal conductivity in the pipe.
D_h is the hydraulic diameter of the pipe (that which a cross section of general shape would have if it were made circular).

In a two-phase mixture, the same coefficient is:

$h^{M}_{coeff} = \frac{k^{M}_{I,SL} {Nu}^{M}}{D_{h}},$

where the subscript M denotes a value specific to the two-phase mixture and the SL subscript indicates a value obtained for the saturated liquid.

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 the liquid and vapor phases, the Nusselt number for turbulent flow follows from the Gnielinski correlation:

${Nu}^{*} = \frac{\frac{f}{8} ({Re}^{*} - 1000) {Pr}^{*}_{I}}{1 + 12.7 \sqrt{\frac{f}{8}} ({Pr}^{*}_{I}^{2 / 3} - 1)},$

where, as before, the asterisk denotes the phase considered and:

f is the friction factor of the pipe.
Re is the Reynolds number.
Pr_I is the Prandtl number.

The friction factor is calculated as:

$f = {- 1.8 {log}_{10} [\frac{6.9}{{Re}^{*}} + {(\frac{ϵ_{r}}{3.7})}^{1.11}]}^{-2},$

where ε_r is the roughness of the pipe. The Reynolds number is calculated as:

${Re}^{*} = \frac{| {\dot{m}}_{Avg} | D_{h} v_{I}^{*}}{S ν_{I}^{*}},$

where the subscript Avg denotes an average value between the ports and:

S is the cross-sectional area of the pipe.
v_I is the specific volume.
ν_I is the kinematic viscosity.

In the two-phase mixture, the Nusselt number for turbulent flow follows from the Cavallini and Zecchin correlation:

${Nu}^{M} = 0.05 {[(1 - x_{I} + x_{I} \sqrt{\frac{v_{SV}}{v_{SL}}}) {Re}_{SL}]}^{0.8} {Pr}_{SL}^{0.33},$

where the subscript SL denotes a value for saturated liquid, the SV subscript a value for saturated vapor, and:

x_I is the vapor quality.
v is the specific volume.

The Reynolds number of the saturated liquid is calculated as:

${Re}_{SL} = \frac{| {\dot{m}}_{Avg} | D_{h} v_{SL}}{S ν_{SL}},$

Mass Balance

Mass conservation in the pipe is observed through the equation:

$[{(\frac{\partial ρ}{\partial p})}_{u} {\dot{p}}_{I} + {(\frac{\partial ρ}{\partial u})}_{p} {\dot{u}}_{I}] V = {\dot{m}}_{A} + {\dot{m}}_{B} + ϵ_{M},$

where:

ρ is the fluid density.
p_I is the pressure inside the pipe.
V is the volume of fluid in the pipe.
${\dot{m}}_{A}$ is the mass flow rate into the pipe through port A.
${\dot{m}}_{B}$ 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,

$ϵ_{M} = \frac{M - V / v_{I}}{τ},$

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:

$\dot{M} = {\dot{m}}_{A} + {\dot{m}}_{B},$
v_I 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:

$p_{A} - p_{I} = \frac{{\dot{m}}_{A}}{S} | \frac{{\dot{m}}_{A}}{S} (ν_{I} - ν_{A}) | + F_{v i s c, A} + I_{A},$

where:

p_A is the pressure at port A.
S is the cross-sectional area of the pipe.
ν_A is the specific volume of the fluid at port A.
F_visc,A is the viscous friction force in the half pipe adjacent to port A.
I_A is the fluid inertia at port A:

$I_{A} = {\ddot{m}}_{A} \frac{L}{2 S}$
The parameter L is the pipe length.

In the half pipe adjacent to port B:

$p_{B} - p_{I} = \frac{{\dot{m}}_{B}}{S} | \frac{{\dot{m}}_{B}}{S} (ν_{I} - ν_{B}) | + F_{v i s c, B} + I_{B},$

where:

p_B is the pressure at port B.
ν_B is the specific volume of the fluid at port B.
F_visc,B is the viscous friction force in the half pipe adjacent to port B.
I_B is the fluid inertia at port B:

$I_{B} = {\ddot{m}}_{B} \frac{L}{2 S}$

The fluid inertia terms, I_A and I_B, are zero when the Fluid inertia parameter is set to Off. The calculation of the viscous friction forces, F_visc,A and F_visc,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

$F_{v i s c, A}^{l a m i n a r} = \frac{f_{s h a p e} L_{e f f} ν_{I} {\dot{m}}_{A}}{4 D_{h}^{2} S},$

while in the half pipe adjacent to port B it is

$F_{v i s c, B}^{l a m i n a r} = \frac{f_{s h a p e} L_{e f f} ν_{I} {\dot{m}}_{B}}{4 D_{h}^{2} S},$

where:

f_shape is the pipe shape factor.
L_eff is the effective pipe length—the sum of the pipe length and the aggregate equivalent length of local resistances.
D_h 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

$F_{v i s c, A}^{t u r b u l e n t} = \frac{{\dot{m}}_{A} | {\dot{m}}_{A} | f_{A} L_{e f f} ν_{I}}{4 D_{H} S^{2}},$

while in the half pipe adjacent to port B it is

$F_{v i s c, B}^{t u r b u l e n t} = \frac{{\dot{m}}_{B} | {\dot{m}}_{B} | f_{B} L_{e f f} ν_{I}}{4 D_{H} S^{2}},$

where:

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

$f_{A} = \frac{1}{{- 1.8 \log_{10} [\frac{6.9}{R e_{A}} + {(\frac{ϵ_{r}}{3.7})}^{1.11}]}^{2}},$

and in the half pipe adjacent to port B as

$f_{B} = \frac{1}{{- 1.8 \log_{10} [\frac{6.9}{R e_{B}} + {(\frac{ϵ_{r}}{3.7})}^{1.11}]}^{2}},$

where:

∊_r is the relative roughness of the pipe.
Re_A is the Reynolds number in the half pipe adjacent to port A,

$R e_{A} = \frac{| {\dot{m}}_{A} | D_{h} v_{I}}{S ν_{I}} .$
Re_B is the Reynolds number in the half pipe adjacent to port B,

$R e_{B} = \frac{| {\dot{m}}_{B} | D_{h} v_{I}}{S ν_{I}} .$

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

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 energy specification

Thermodynamic variable in terms of which to define the initial conditions of the component. The default setting is Temperature.

The value for the Initial fluid energy specification parameter limits the available initial states for the two-phase fluid. When Initial fluid energy specification is:

Temperature — Specify an initial state that is a subcooled liquid or superheated vapor. You cannot specify a liquid-vapor mixture because the temperature is constant across the liquid-vapor mixture region.
Vapor quality — Specify an initial state that is a liquid-vapor mixture. You cannot specify a subcooled liquid or a superheated vapor because the liquid mass fraction is 0 and 1, respectively, across the whole region. Additionally, the block limits the pressure to below the critical pressure.
Vapor void fraction — Specify an initial state that is a liquid-vapor mixture. You cannot specify a subcooled liquid or a superheated vapor because the liquid mass fraction is 0 and 1, respectively, across the whole region. Additionally, the block limits the pressure to below the critical pressure.
Specific enthalpy — Specify the specific enthalpy of the fluid. The block does not limit the initial state.
Specific internal energy — Specify the specific internal energy of the fluid. The block does not limit the initial state.

Initial pressure

Pressure in the chamber at the start of simulation, specified against absolute zero. The default value is 0.101325 MPa.

Initial temperature

Temperature in the chamber at the start of simulation, specified against absolute zero. This parameter is active when the Initial fluid energy specification option is set to Temperature. The default value is 293.15 K.

Initial vapor quality

Mass fraction of vapor in the chamber at the start of simulation. This parameter is active when the Initial fluid energy specification option is set to Vapor quality. The default value is 0.5.

Initial vapor void fraction

Volume fraction of vapor in the chamber at the start of simulation. This parameter is active when the Initial fluid energy specification option is set to Vapor void fraction. The default value is 0.5.

Initial specific enthalpy

Specific enthalpy of the fluid in the chamber at the start of simulation. This parameter is active when the Initial fluid energy specification option is set to Specific enthalpy. The default value is 1500 kJ/kg.

Initial specific internal energy

Specific internal energy of the fluid in the chamber at the start of simulation. This parameter is active when the Initial fluid energy specification option is set to Specific internal energy. The default value is 1500 kJ/kg.

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.