Passive Ventilation by PCM

Passive Ventilation by PCM, ANSYS Fluent CFD Simulation Tutorial

Description

Phase change materials (PCMs) can store thermal energy and release it back to the environment as needed, enabling both cooling, heating, and thermal storage through their transition between solid and liquid states. When ambient temperature rises, the material transitions from solid to liquid, storing the latent energy of melting in the process; when temperature later drops, the material transitions back to solid, releasing that stored latent energy of solidification. Since this heat transfer occurs without any mechanical device, PCM-based systems fall within the broader category of passive ventilation.

This passive ventilation system operates on the buoyancy effect, where temperature-driven density differences drive natural convection. This project models such a system in two stages: first, examining natural convection heat transfer in a simple room with one wall subjected to a constant heat flux (representing warm ambient air and incoming solar radiation), and second, applying a PCM panel to that same heated wall to evaluate its effect on the resulting heat transfer into the room.

The geometry for both stages was designed in Design Modeler, with the first stage modeling a simple room and the second adding the PCM panel. Both models were meshed in ANSYS Meshing using a structured grid, totaling 228,448 elements for the first stage and 241,560 for the second.

Methodology

This problem was solved as unsteady, time-dependent CFD using a pressure-based solver. Since the PCM's operating mechanism relies on continuous phase change between solid and liquid, the Solidification and Melting model was applied, requiring three defining parameters: the solidus temperature (the maximum temperature at which the material remains fully solid), the liquidus temperature (the minimum temperature at which it becomes fully liquid), and the pure solvent's latent heat of melting.

Since natural convection — driven by the buoyancy effect — also plays a central role in this problem, air density was not treated as constant. Instead, the incompressible ideal gas model was used, applying the ideal gas relationship between density, pressure, and temperature; since density in this model depends on operating pressure rather than local relative pressure, assuming constant pressure effectively makes density a function of temperature alone — precisely what's needed to capture the buoyancy-driven flow.

Conclusion

Results include a plot of the room's average temperature over time, comparing the with-PCM and without-PCM cases, along with 2D and 3D temperature contours and velocity vectors for both scenarios.

In the first simulation (without PCM), room temperature rises steadily over time as the wall's heat flux drives heat transfer into the space. In the second simulation (with the PCM panel), temperature rises with a noticeably shallower slope — even though the same heat flux is applied, the PCM layer positioned between the heat source and the room interior absorbs much of that incoming heat as latent heat during its phase change, allowing significantly less heat to actually penetrate into the room. This confirms the PCM panel's effectiveness at moderating and delaying heat transfer in a passive ventilation context, directly reducing the room's temperature rise without any active mechanical cooling system.