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Decentralized heat recovery ventilation (HRV) systems are assumed as simple solutions to obtain a healthy and comfortable indoor environment. A wall or window mounted compact version of decentralized HRV systems (mono unit) are used for small scale, mostly residential applications. A fan and a heat exchanger are the critical components of this compact system. The flow capacity of these units are down to 10 m3/h, where efficiencies over 90% are commonly declared by the manufacturers. On the other hand, spherical packed beds (SPD) are widely used in the heat transfer applications such as; chemical reactors, grain driers, nuclear reactors, thermal storage in buildings and in solar thermal power plants, due to operational convenience. These systems are operated under steady flow conditions, unlike decentralized HRV systems which are designed for cyclic operation. In this study, heat recovery performance of a spherical packed bed heat exchanger for a decentralized HRV system is investigated. A one dimensional mathematical model for a SPD is obtained and an in-house computer code is developed to solve the transient heat transfer inside the packed bed under cyclic operation conditions. Well known convenient correlations were used for pressure drop calculations. A number of bed and sphere diameters were studied in a wide range. Various flow time and number of cycles were studied for the hot and cold flow to understand the SPD performance for HRV applications. This novel application also has the potential for regenerative heat recovery systems.

The contact angle of droplets attracts attention as one of the relevant thermophysical properties describing the wettability behaviour of the fluids. The contact angle depends on the surface characteristics such as surface type and roughness as well as on the liquid type and surrounding atmosphere. This study aims to correct the error in the coefficient of the theoretical model developed for droplet shape prediction by Vafaei and Podowski [1]. The corrected model is also rearranged by non-dimensional numbers. The contact angle and the shape of water droplets for different volumes and surface types are predicted by the rearranged model and validated by experimental results. Contact angles have been over-estimated compared to experimental results because of measurement errors in geometrical parameters. It is found that the contact angle model is too sensitive to geometrical parameters. Moreover, the contact angle is found to be independent of the volume.