T-Junction (TL)

Three-way junction in a thermal liquid system

• Library:

• Simscape / Fluids / Isothermal Liquid / Pipes & Fittings

• 

Description

The T-Junction (TL) block represents a three-way pipe junction with a branch line at port C connected at a 90° 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

The block uses mode charts to determine each loss coefficient for a given flow configuration. This table describes the conditions and coefficients for each operational mode.

Flow Scenario	ṁA	ṁB	ṁC	KA	KB	KC
Converging to node B	>ṁthresh	<-ṁthresh	>ṁthresh	Kmain,conv	0	Kmain,conv
Converging to node A	<-ṁthresh	>ṁthresh	>ṁthresh	0	Kmain,conv	Kside,conv
Diverging from node A	>ṁthresh	<-ṁthresh	<-ṁthresh	0	Kmain,div	Kside,div
Diverging from node B	<-ṁthresh	>ṁthresh	<-ṁthresh	Kmain,div	0	Kside,div
Converging to node C (branch)	>ṁthresh	>ṁthresh	<-ṁthresh	(Kmain,conv + Kbranch,conv)/2	(Kmain,conv + Kbranch,conv)/2	0
Diverging from node C (branch)	<-ṁthresh	<-ṁthresh	>ṁthresh	(Kmain,div + Kbranch,div)/2	(Kmain,div + Kbranch,div)/2	0
Stagnant	–	–	–	1 or last valid	1 or last valid	1 or last valid

The flow is stagnant when the mass flow rate conditions do not match any defined flow scenario. The mass flow rate threshold, which is the point at which the flow in the pipe begins to reverse direction, is calculated as:

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

where:

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

• ν is the fluid viscosity.

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

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

Standard T-Junction

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

$$K_{main,div}=K_{main,conv}=20f_{T,main},$$

$$K_{side,div}=K_{side,conv}=60f_{T,side}.$$

In contrast to the custom junction type, the standard junction loss coefficient is the same for both converging and diverging flows. K_A, K_B, and K_C 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. You must specify K_main,conv, K_main,div,K_side,conv, and K_side,div as the Main branch converging loss coefficient, Main branch diverging loss coefficient, Side branch converging loss coefficient, and Side branch diverging loss coefficient parameters, respectively.

Mass and Momentum Balance

The block conserves mass in the junction such that

$${\dot{m}}_A+{\dot{m}}_B+{\dot{m}}_C=0.$$

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

$$\begin{array}{l}{p}_A-p_I=I_A+\frac{K_A}{2\overline{\rho }{A}_{{}_{main}}^2}{\dot{m}}_A\sqrt{{\dot{m}}_A^2+{\dot{m}}_{thresh}^2}\\ p_B-p_I=I_B+\frac{K_B}{2\overline{\rho }{A}_{{}_{main}}^2}{\dot{m}}_B\sqrt{{\dot{m}}_B^2+{\dot{m}}_{thresh}^2}\\ p_C-p_I=I_C+\frac{K_C}{2\overline{\rho }{A}_{{}_{side}}^2}{\dot{m}}_C\sqrt{{\dot{m}}_C^2+{\dot{m}}_{thresh}^2}\end{array}$$

where I represents the fluid inertia, and

$$\begin{array}{l}{I}_A={\ddot{m}}_A\frac{\sqrt{\pi \cdot A_{side}}}{A_{main}}\\ I_B={\ddot{m}}_B\frac{\sqrt{\pi \cdot A_{side}}}{A_{main}}\\ I_C={\ddot{m}}_C\frac{\sqrt{\pi \cdot A_{main}}}{A_{side}}\end{array}$$

A_main is the Main branch area (A-B) parameter and A_side is the Side branch area (A-C, B-C) parameter.

Energy Balance

The block balances energy such that

$${\varphi }_A+{\varphi }_B+{\varphi }_C=0,$$

where:

• ϕ_A is the energy flow rate at port A.

• ϕ_B is the energy flow rate at port B.

• ϕ_C is the energy flow rate at port C.

Variables

To set the priority and initial target values for the block variables prior to simulation, use the Initial Targets section in the block dialog box or Property Inspector. For more information, see Set Priority and Initial Target for Block Variables.

Nominal values provide a way to specify the expected magnitude of a variable in a model. Using system scaling based on nominal values increases the simulation robustness. Nominal values can come from different sources, one of which is the Nominal Values section in the block dialog box or Property Inspector. For more information, see Modify Nominal Values for a Block Variable.

Ports

Conserving

A — Liquid port

thermal liquid

Thermal liquid conserving port associated with the liquid entrance or exit of the junction.

B — Liquid port

thermal liquid

Thermal liquid conserving port associated with the liquid entrance or exit of the junction.

C — Liquid port

thermal liquid

Thermal liquid conserving port associated with the liquid entrance or exit of the junction.

Parameters

Main branch area (A-B) — Cross-sectional area between ports A and B

0.01 m^2 (default) | positive scalar

Area of connecting pipe between ports A and B.

Side branch area (A-C, B-C) — Cross-sectional area of the side branch

0.01 m^2 (default) | positive scalar

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

Three-way junction type — Junction loss coefficient type

Standard (default) | Custom

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

Main branch converging loss coefficient — Loss coefficient between A and B for custom junctions

1.12 (default) | positive scalar

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.

Main branch diverging loss coefficient — Loss coefficient between A and B for custom junctions

1.12 (default) | positive scalar

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.

Side branch converging loss coefficient — Branch loss coefficient for custom junctions

1.12 (default) | positive scalar

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.

Side branch diverging loss coefficient — Branch loss coefficient for custom junctions

1.12 (default) | positive scalar

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.

Critical Reynolds number — Upper Reynolds number limit for laminar flow

150 (default) | positive scalar

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.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2022a

R2022b: Conversion to Mode Charts

The block uses mode charts to handle the different flow configurations. Results during flow scenario transitions may be different than in previous releases. Results during flow scenario transitions are not steady state and are not necessarily accurate.