Matteo Bortolato

Minichannel condensation in downward, upward and horizontal configuration

An experimental investigation of condensation of R134a inside a single square cross section minichannel when varying the channel orientation is presented. Local heat transfer coefficients are measured in horizontal, vertical downflow and vertical upflow configurations. In the literature the number of local heat transfer coefficient values measured during condensation inside non-circular minichannels is rather limited and the effect of channel orientation during condensation is not much investigated. Some studies have been performed in inclined smooth tubes of larger diameters, where it was shown that the heat transfer coefficient is strongly affected by the liquid and vapour distributions. But minichannels may display a different behaviour because of the relative importance of shear stress, gravity and surface tension. The action of these forces may depend on operating conditions and orientation. In the present study, the channel is obtained from a copper rod and has a square cross section with 1.18 mm side length. Each corner has a curvature radius equal to 0.15 mm, which leads to a hydraulic diameter equal to 1.23 mm. Tests have been performed with R134a at 40°C saturation temperature, at mass velocity ranging between 100 and 790 kg m−2 s−1. From the experimental results, the effect of the channel inclination when varying mass velocity and vapour quality is investigated.

Solar Thermal Collectors with Low and High Concentration

This chapter describes the performance analysis of different concentrating technologies through experimental and numerical modeling activities. Two solar thermal systems with different designs and, accordingly, different concentration ratios have been studied. The first solar device is a stationary compound parabolic concentrator (CPC) collector: it is provided with truncated or full CPC reflectors and evacuated tubes. Each evacuated tube is composed of an outer glass envelope and a glass absorber with selective coating in thermal contact, via absorber fin, with a U-shaped channel for the liquid flow. The second system is a parabolic trough concentrator (PTC) with two-axis solar tracking: the primary optics consists of a segment of parabolic cylinder which concentrates the direct normal irradiance (DNI) on a linear receiver. In this system, two types of flat receivers have been tested. One receiver has been designed for thermal energy extraction, and it consists of a canalized roll-bond plate provided with a semi-selective coating. The other receiver has been designed for cogeneration of electricity and heat (CPVT), and it is equipped with triple-junction photovoltaic cells, which are actively cooled by an aluminum roll-bond heat exchanger. The performance of the described collectors has been experimentally characterized at the Solar Energy Conversion Laboratory of the University of Padova (45.4°N, 11.9°E), Italy. The collectors have also been mathematically modeled, and the numerical data have been validated against the experimental measurements.