GPU Partitioned CHT Solver

Notes:

• Ensure that the name matches the Java macro name here:
• public class MACRO_NAME_HERE extends StarMacro {
• Edit: The Vapor Chamber and the Fluid Volume must be set to steady state and run together if using a porous medium for your heat sink or you will get the error: No patches found (I have not found a way to resolve this issue).
• ElectronicsCHTAlternatingMacro.java is a simplified working version where the Vapor Chamber and Fluid Volume are run steady-state while the PCB and Die are run Unsteady (tested and working)

Advanced transient thermal simulation for GPU/electronics cooling

GPU thermal transients present a computational challenge due to vastly different timescales:

• Airflow: steady
• GPU die: Fast thermal response (10-100 ms)
• Heat sink mass: Slow response (1-10 seconds)

Traditional fully-coupled transient CHT requires timesteps of microsecond to maintain stability, making 10-second transients computationally prohibitive.

Solution: Partitioned CHT Strategy

This macro implements an alternating fluid-solid solution approach:

1. Fluid phase: Converge steady-state airflow with frozen solid temperatures
2. Solid phase: Advance transient heat conduction with updated convective BCs
3. Repeat: Alternate until transient completes

Computational Benefits

• Maintains accuracy for thermal events with timescale >> fluid response
• Production-ready for GPU design optimization

Applications

• Power-on thermal transients
• Thermal throttling behavior prediction
• Gaming load profile simulation
• Data center cold-start analysis
• Cooling system design optimization

Technical Details

Timestep selection:

• Solid timestep: 10 ms (captures die thermal time constant ~50-100 ms)
• Fluid iterations: 30 per cycle (converges flow field)
• Subcycles: 2 per major timestep (fluid-solid alternations)

GPU stack components:

• Silicon die (high power density)
• Silicon substrate (high conductivity)
• Vapor chamber (two-phase spreading)
• Thermal interface material (critical resistance)
• Cold plate (aluminum/copper)
• PCB (FR4, low conductivity)

Usage

// In STAR-CCM+ macro environment:
// 1. Set up GPU geometry with appropriate continua names
// 2. Define boundary conditions (power, inlet conditions)
// 3. Run this macro to execute partitioned transient solution

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Author: Mitchell Stolk
Application: GPU/Electronics Thermal Management
Date: November 2025

⚠️ Important Note on Numerical Accuracy

This partitioned CHT approach is designed for fast turnaround during design exploration and early-stage optimization.
Because the fluid and solid domains are advanced separately rather than fully coupled each timestep, this method inherently sacrifices some numerical accuracy and introduces Numerical Errors.
For high-fidelity validation, fast fluid–thermal interactions, or safety-critical work, a fully-coupled transient CHT simulation should be used instead.