Heat transfer and pressure drop studies on a PCM-heat exchanger module for free cooling applications

Free cooling/Night ventilation is the process of storing the cool energy available in the night time ambient air in a storage device. During the day time the cool energy is retrieved from the storage device in order to cool the building using mechanical ventilation system. The modular heat exchanger developed in this work is a shell and tube type with phase change materials in the shell portion of the module and passage for the flow of air through the tubes. The modules of the modular heat exchanger are stacked one over other with air spacers in between each module. This modular heat exchanger arrangement is suitable for free cooling application where the diurnal temperature variation is low. Transient and steady state CFD modeling is carried out for a single module and two air spacers. Conjugate heat transfer analysis is carried out for the fluid and PCM of heat exchanger module. The latent heat value of the PCM is modeled using apparent heat capacity method with suitable profile approximated from the experimental results. The CFD results are validated with the experimental results. The steady state CFD analysis is useful to determine the pressure drop across the module and the spacers and to know the flow and temperature variation of heat transfer fluid in the module so as to select the geometrical and flow parameters for a given surface temperature and inlet condition. The transient analysis results are useful to determine the PCM solidification characteristics and to verify the suitability of the selected geometrical dimensions. The air spacers provided between the module increases the retention time of the air for better heat transfer and its effect is more pronounced at the lower velocities and decreases as the frontal velocity increases and its effect is negligible above the frontal velocity of 2 m/s.