Mixing T-Junction: Thermal Mixing Simulation

What is a Mixing T-Junction?

A mixing T-junction (or mixing tee) brings two fluid streams together at a 90° angle, one entering horizontally, one vertically, and forces them to mix in a common downstream channel.

In HVAC and thermal management applications, the goal is to blend a hot stream and a cold stream into a target comfort temperature. The challenge is that mixing quality is not guaranteed, poor velocity ratios or short channel lengths can leave stratified or unmixed zones at the outlet, delivering inconsistent temperatures.

Two parameters primarily control mixing effectiveness:

Channel length, more length means more time and turbulence for the streams to homogenise
Momentum ratio (MR), the ratio of hot-inlet velocity to cold-inlet velocity. Higher MR means the hot stream dominates and penetrates further into the cold stream, creating stronger turbulent mixing

Mixing effectiveness is quantified by the standard deviation of temperature at the outlet. A low σ means the outlet temperature is uniform, good mixing. A high σ means stratification remains, poor mixing.

Physical Setup & CAD Model

The mixing tee geometry was built in SolidWorks and imported into ANSYS Fluent. The configuration:

Hot inlet (horizontal): warm airstream entering at 25 °C
Cold inlet (vertical, 90°): chilled airstream entering at 10 °C
Outlet: downstream channel where mixed air exits and temperature uniformity is measured

Mixing T-junction CAD model, horizontal hot inlet, vertical cold inlet, downstream outlet channel — SolidWorks CAD model of the mixing tee. Hot air enters horizontally (left), cold air enters vertically (bottom), and mixed air exits through the downstream channel (right).

Boundary Conditions & Assumptions

Parameter	Value
Hot inlet temperature	25 °C
Cold inlet temperature	10 °C
Hot inlet velocity (Cases 1, 2, 3)	3 m/s
Momentum ratio, Cases 1 & 2	MR = 2 (V_hot / V_cold = 2)
Momentum ratio, Case 3	MR = 4
Tee length, Cases 1 & 3	Short
Tee length, Case 2	Long
Solver	Steady-state, pressure-based
Turbulence model	k-ε

Mixing effectiveness metric: Standard deviation (σ) of temperature at the outlet cross-section. Lower σ = better mixing.

Case 1, Short Tee, MR = 2

Hot inlet velocity: 3 m/s · Cold inlet derived from MR = 2 · Short downstream channel

Case 1 residual convergence plot — Residual plot, Case 1. All residuals converge to acceptable levels, confirming solution stability.

Case 1, standard deviation of temperature at outlet — Standard deviation of outlet temperature, Case 1 (short tee, MR 2). Baseline mixing performance.

Case 1, temperature distribution at outlet — Temperature distribution across the outlet cross-section. Visible stratification between the hot and cold regions.

Case 1, temperature on longitudinal cut plane — Temperature contour on the longitudinal cut plane, shows the mixing front as the two streams interact downstream of the junction.

Case 1, velocity vectors on cut plane — Velocity vectors at the junction, the cold inlet jet deflects the hot stream, creating a shear layer that drives turbulent mixing.

Case 2, Long Tee, MR = 2

Same velocity conditions as Case 1; downstream channel length increased to allow more mixing distance.

Case 2 residual convergence plot — Residual plot, Case 2. Converges cleanly; longer channel adds computational cells but does not destabilise the solver.

Case 2, standard deviation of temperature at outlet — Standard deviation, Case 2 (long tee, MR 2). Lower σ than Case 1, confirming that longer mixing length improves uniformity.

Case 2, temperature distribution at outlet — Outlet temperature distribution, more uniform than Case 1 across the cross-section.

Case 2, temperature on longitudinal cut plane — Temperature cut plane, the longer channel gives the mixing front more distance to flatten out, reducing the hot/cold stratification visible in Case 1.

Case 3, Short Tee, MR = 4

Same short geometry as Case 1; hot inlet velocity doubled to raise momentum ratio to 4. Cold inlet velocity adjusted accordingly.

Case 3 residual convergence plot — Residual plot, Case 3. Higher momentum ratio increases turbulence intensity but convergence remains stable.

Case 3, standard deviation of temperature at outlet — Standard deviation, Case 3 (short tee, MR 4). Lower σ than Case 1 (same geometry, MR 2), momentum ratio alone improves mixing on the short tee.

Case 3, temperature distribution at outlet — Outlet temperature, more uniform than Case 1 despite identical geometry, driven by the stronger penetration of the hot stream at higher MR.

Case 3, temperature on longitudinal cut plane — Temperature cut plane, higher momentum ratio creates a stronger shear layer at the junction, accelerating cross-stream heat transfer.

Case 3, velocity vectors on cut plane — Velocity vectors, the dominant hot stream at MR 4 penetrates further into the cold stream region, increasing turbulent mixing intensity near the junction.

Results & Engineering Conclusions

Key findings

Channel length reduces standard deviation. The long tee (Case 2) achieved lower σ than the short tee (Case 1) at identical MR = 2. More mixing distance allows turbulence to flatten the temperature gradient before the outlet.

Higher momentum ratio improves mixing on a short tee. Case 3 (MR = 4, short) outperforms Case 1 (MR = 2, short), the stronger hot-stream penetration creates a more energetic shear layer, accelerating cross-stream mixing within the same physical length.

Both levers are independent and additive. Length and momentum ratio address mixing through different physical mechanisms, length gives the flow time to homogenise, momentum ratio increases the turbulent energy available at the junction. In a real system, either can be used depending on whether geometric constraints or flow rate control is more accessible.

Case	Tee Length	Momentum Ratio	Outlet σ (°C)	Mixing Quality
1	Short	2	Baseline (highest)	Poorest
2	Long	2	~0.5	Good, length helps
3	Short	4	Lower than Case 1	Good, MR helps