# T-Junction (IL)

Three-way junction in an isothermal liquid system

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• Simscape / Fluids / Isothermal Liquid / Pipes & Fittings

## Description

The T-Junction (IL) block models a three-way pipe junction with a branch line at port C connected at a 90o angle to the main pipe line, between ports A and B. You can specify a custom or standard junction type. When Three-way junction type is set to Custom, you can specify the loss coefficients of each pipe segment for converging and diverging flows. The standard model applies industry-standard loss coefficients to the momentum equations.

### Flow Direction

The flow is converging when the branch flow, the flow through port C, merges into the main flow. The flow is diverging when the branch flow splits from the main flow. The flow direction between A and I, the point where the branch meets the main, and B and I must be consistent for all loss coefficients to be applied. If they are not, as shown in the last two diagrams in the figure below, the losses in the junction are approximated with the main branch loss coefficient for converging or diverging flows.

Flow Scenarios

### Standard T-Junction

When Three-way junction type is set to Standard, the pipe loss coefficients, Kmain and Kside, and the pipe friction factor, fT, are calculated according to Crane [1]:

${K}_{main}=20{f}_{T,main},$

${K}_{side}=60{f}_{T,side}.$

In contrast to the custom junction type, the standard junction loss coefficient is the same for both converging and diverging flows. KA, KB, and KC are then calculated in the same manner as custom junctions.

Friction Factor per Nominal Pipe Diameter

### Custom T-Junction

When Three-way junction type is set to Custom, the pipe loss coefficient at each port, K, is calculated based on the user-defined loss parameters for converging and diverging flow and mass flow rate at each port. The coefficients are defined generally for positive and negative flows:

${K}_{A}={m}_{A}^{+}\left({\stackrel{˙}{m}}_{B}^{+}{\stackrel{˙}{m}}_{C}^{-}\frac{{K}_{main,conv}}{2}+{\stackrel{˙}{m}}_{B}^{-}{\stackrel{˙}{m}}_{C}^{+}{K}_{main,conv}\right)+{m}_{A}^{-}\left({\stackrel{˙}{m}}_{B}^{+}{\stackrel{˙}{m}}_{C}^{-}{K}_{main,div}+{\stackrel{˙}{m}}_{B}^{-}{\stackrel{˙}{m}}_{C}^{+}\frac{{K}_{main,div}}{2}\right),$

where

• Kmain,conv is the Main branch converging loss coefficient.

• Kmain,div is the Main branch diverging loss coefficient.

${K}_{B}={m}_{A}^{+}\left({\stackrel{˙}{m}}_{B}^{+}{\stackrel{˙}{m}}_{C}^{-}\frac{{K}_{main,conv}}{2}+{\stackrel{˙}{m}}_{B}^{-}{\stackrel{˙}{m}}_{C}^{-}{K}_{main,div}\right)+{m}_{A}^{-}\left({\stackrel{˙}{m}}_{B}^{+}{\stackrel{˙}{m}}_{C}^{+}{K}_{main,conv}+{\stackrel{˙}{m}}_{B}^{-}{\stackrel{˙}{m}}_{C}^{+}\frac{{K}_{main,div}}{2}\right).$

${K}_{C}=\left({m}_{A}^{+}{\stackrel{˙}{m}}_{B}^{-}+{\stackrel{˙}{m}}_{A}^{-}{\stackrel{˙}{m}}_{B}^{+}\right)\left({\stackrel{˙}{m}}_{C}^{+}{K}_{side,conv}+{\stackrel{˙}{m}}_{C}^{-}{K}_{side,conv}\right),$

where:

• Kside,conv is the Side branch converging loss coefficient.

• Kside,div is the Side branch diverging loss coefficient.

The positive mass flow direction at each port, when the flow direction is from A to B, from A to C, and from C to B, is defined as:

${\stackrel{˙}{m}}_{port}^{+}=\frac{1+\mathrm{tanh}\left(\frac{4{\stackrel{˙}{m}}_{port}}{{\stackrel{˙}{m}}_{thresh}}\right)}{2}.$

The negative mass flow direction is defined as:

${\stackrel{˙}{m}}_{port}^{-}=\frac{1-\mathrm{tanh}\left(\frac{4{\stackrel{˙}{m}}_{port}}{{\stackrel{˙}{m}}_{thresh}}\right)}{2}.$

The mass flow rate threshold, which is the point at which the flow in the pipe begins to reverse direction, is calculated as:

${\stackrel{˙}{m}}_{thresh}={\mathrm{Re}}_{c}\upsilon \overline{\rho }\sqrt{\frac{\pi }{4}{A}_{\mathrm{min}}},$

where:

• Rec is the Critical Reynolds number, beyond which the transitional flow regime begins.

• ν is the fluid viscosity.

• $\overline{\rho }$ is the average fluid density.

• Amin is the smallest cross-sectional area in the pipe junction.

### Momentum Balance

Mass is conserved in the pipe segment:

${\stackrel{˙}{m}}_{A}+{\stackrel{˙}{m}}_{B}+{\stackrel{˙}{m}}_{C}=0.$

Flow through the pipe junction is calculated from momentum conservation equations between ports A, B, and C:

${p}_{A}-{p}_{I}=\frac{{K}_{A}}{2\overline{\rho }{A}_{{}_{main}}^{2}}{\stackrel{˙}{m}}_{A}\sqrt{{\stackrel{˙}{m}}_{A}^{2}+{\stackrel{˙}{m}}_{thresh}^{2}}$

${p}_{B}-{p}_{I}=\frac{{K}_{B}}{2\overline{\rho }{A}_{{}_{main}}^{2}}{\stackrel{˙}{m}}_{B}\sqrt{{\stackrel{˙}{m}}_{B}^{2}+{\stackrel{˙}{m}}_{thresh}^{2}}$

${p}_{C}-{p}_{I}=\frac{{K}_{C}}{2\overline{\rho }{A}_{{}_{side}}^{2}}{\stackrel{˙}{m}}_{C}\sqrt{{\stackrel{˙}{m}}_{C}^{2}+{\stackrel{˙}{m}}_{thresh}^{2}}$

where Amain is the Main branch area (A-B) and Aside is the Side branch area (A-C, B-C).

## Ports

### Conserving

expand all

Liquid entry or exit port.

Liquid entry or exit port.

Liquid entry or exit port.

## Parameters

expand all

Area of connecting pipe between ports and B.

Area of connecting pipe between ports and C and between ports B and C.

The junction loss coefficient type. Set this parameter to Custom to specify individual diverging and converging loss coefficients for each flow path segment.

Loss coefficient for pressure loss calculations between ports A and B for converging flow.

#### Dependencies

To enable this parameter, set Three-way junction type to Custom.

Loss coefficient for pressure loss calculations between ports A and B for diverging flow.

#### Dependencies

To enable this parameter, set Three-way junction type to Custom.

Loss coefficient for pressure loss calculations between port C and the main line for converging flow.

#### Dependencies

To enable this parameter, set Three-way junction type to Custom.

Loss coefficient for pressure loss calculations between port C and the main line for diverging flow.

#### Dependencies

To enable this parameter, set Three-way junction type to Custom.

Upper Reynolds number limit for laminar flow through the junction.

## References

[1] Crane Co. Flow of Fluids Through Valves, Fittings, and Pipe TP-410. Crane Co., 1981.