A Single-loop Pulsating Heat Pipe CFD simulation is a powerful way to study advanced cooling for electronics. Unlike normal pipes that just move water, a Pulsating Heat Pipe (PHP) acts like a living system. It uses the heat from a computer chip to boil liquid into vapor bubbles. These bubbles expand and push “slugs” of liquid toward a cold zone, where they shrink back down. This constant growing and shrinking creates a heartbeat-like pumping action without any mechanical pumps. A Pulsating Heat Pipe ANSYS Fluent analysis allows engineers to see this complex “dance” of bubbles and liquid inside the closed tube.

This report details a Single-loop Pulsating Heat Pipe Fluent study based on the research by Kang et al [1]. The simulation solves the difficult physics of Evaporation condensation fluent processes. We use the Volume of Fluid (VOF) model to track the moving boundaries between the liquid water and the vapor. Crucially, this study uses User-Defined Functions (UDFs) to make the simulation realistic. These special codes tell ANSYS Fluent that the boiling temperature changes as the pressure changes inside the pipe. A VOF fluent simulation like this is essential for optimizing heat exchangers in satellites and high-power batteries. For those interested in the physics of phase change, our detailed mass transfer tutorials are available at https://cfdland.com/product-category/engineering/mass-transfer-cfd-simulation/

• Reference [1]: Kang, Zhanxiao, Dahua Shou, and Jintu Fan. “Numerical study of a novel Single-loop pulsating heat pipe with separating walls within the flow channel.” Applied Thermal Engineering196 (2021): 117246.

Figure 1: A schematic of the novel OPHP (Outer Pulsating Heat Pipe) design showing the separating wall located closer to the outer side of the channel to enhance circulation.

Simulation Process: Fluent UDF Setup & Phase Change Configuration

The simulation process for this Single-loop Pulsating Heat Pipe CFD project began by creating a 2D geometry that mimics the experimental setup. The loop consists of a bottom heating section (evaporator), a middle adiabatic section, and a top cooling section (condenser). The engineers used ANSYS Meshing to build a clean, structured grid with 6,040 cells. This specific mesh type with orthogonal cells is very important because it helps the VOF model track the sharp interface between water and vapor accurately without numerical diffusion, or blurring, of the bubble edges.

Inside ANSYS Fluent, the physics were set up to capture the transient nature of the pulsating flow. The Volume of Fluid (VOF) multiphase model was activated to handle the interaction between the primary phase (liquid water) and the secondary phase (water vapor). To simulate the phase change, the Lee evaporation-condensation model was used. This model calculates how much mass turns into vapor based on the temperature difference. However, standard settings are not enough for a PHP. The engineers wrote custom User-Defined Functions (UDFs) in C language. One UDF updates the water viscosity as it gets hot, and another UDF calculates the exact saturation temperature based on the local pressure using the Antoine equation. This ensures that boiling happens at the correct temperature (which changes from 25°C to 46°C as pressure rises) rather than a fixed value. This rigorous setup allows the Pulsating Heat Pipe CFD model to predict the real thermal performance of the device.

Figure 2: The structured computational grid generated for the Single-loop Pulsating Heat Pipe CFD, showing the fine mesh used to capture the thin liquid films and phase change interfaces.

Post-processing: Thermal & Hydrodynamic Performance Analysis

The simulation results provide a deep look into the chaotic but effective operation of the heat pipe. We will analyze the thermal startup and the two-phase flow dynamics to understand why this device works well. The temperature contours in Figure 3 tell the story of the system “waking up.”

• The Ignition Phase: At t = 1 second, the heat is confined strictly to the bottom U-turn (Evaporator). We see a sharp red zone (329 K), while the top remains cold (298 K). This large temperature difference is the “engine” that starts the motion.
• The Propagation Phase: By t = 3 and 5 seconds, we see green and yellow zones moving up the left channel. This proves that the vapor expansion is successfully pushing hot fluids towards the condenser. The heat is no longer stuck at the bottom; it is being transported.
• The Quasi-Steady State: At t = 10 seconds, the temperature distribution stabilizes. The bottom stays hot, and the top stays cold, but the middle sections show a mix of temperatures. This confirms the PHP has reached a stable operating mode where it continuously moves heat from the source to the sink.

Figure 3: Temperature contours from ANSYS Fluent showing heat transfer and temperature distribution in the Single-loop Pulsating Heat Pipe at t = 1, 3, 5, and 10 seconds during transient CFD simulation.

The Volume Fraction contours in Figure 4 are the most critical evidence of the “pulsating” mechanism: Slug Flow Formation: At t = 3 seconds, we observe distinct red blocks (Liquid, Volume Fraction ≈ 1.0) separated by blue gaps (Vapor, Volume Fraction ≈ 0.0). This is the classic slug-plug flow regime. The simulation proves that the Lee model is correctly generating vapor bubbles that separate the liquid into moving “trains.”

Secondly, Circulation: At t = 5 and 10 seconds, the pattern becomes complex. We see liquid on one side and vapor on the other. This indicates that the fluid is circulating. The vapor pushes up one side, condenses at the top, and falls back down the other side.

Figure 4: Liquid volume fraction contours from VOF model showing two-phase flow patterns with liquid plugs (red) and vapor bubbles (blue) in the Pulsating Heat Pipe CFD simulation at different time steps.

This Single-loop Pulsating Heat Pipe CFD study converts complex physics into actionable design rules:

1. the “Fill Ratio”: The simulation used a 70% fill ratio. The VOF results show this provides enough liquid to absorb heat but enough vapor space to allow movement. If the pipe were 100% full, it would not pulsate.
2. It Optimizes Cooling Speed: The transient analysis shows the system needs 10 seconds to become fully effective. Manufacturers can use this data to program cooling fans to turn on at the right moment.