A flat plate solar collector with dual-function was proposed, which was modified from the conventional liquid solar collector. Three working modes were presented, named (A) air heating, (B) water heating and (C) air-water compound heating. Mathematical models for air and water heating modes were developed to investigate the effect of mass flow rate on the thermal performance. An experimental setup was established in order to test the actual performance of collector. The experimental results show that the average collector efficiency in mode A and B can achieve 51.3 % and 51.4 % while the mass flow rate of fluid is 0.024 kg/s and 0.13 kg/s, respectively. The maximum temperature rise of air and water reach 60.4 ℃ and 59.8 ℃ in the respective two modes. The average of total collector efficiency based on the mode C can reach 73.4 %, higher than that of mode A or B. The mass flow rate is a major factor to affect the efficiency, outlet temperature of fluid and heat transfer effectiveness. When the water flow rate exceeds 0.10 kg/s, the heat removal factor increases insignificantly. To balance the efficiency and outlet temperature of fluid, the air flow rate between 0.02 kg/s and 0.025 kg/s is recommended in mode A and the water flow rate between 0.06 kg/s and 0.08 kg/s is recommended in mode B.
Air entrainment at the intake of a bottom outlet often gives rise to air pockets in its conduit and formation of geysers. The outlet in question comprises a bulkhead gate, gate shaft, horizontal conduit, and exit. Operations show that it suffers from appreciable flow fluctuations and blowouts in the tailwater, which leads to gate operation restrictions. For the purpose of understanding the hydraulic phenomenon, both prototype discharge tests and three-dimensional computational fluid dynamics (CFD) modeling of two-phase flows are performed. The operational focus of the facility are small and large gate openings. The CFD results reveal that, with air entrained in the gate shaft, continual breakup and coalescence of air bubbles in the conduit typify the flow. At small openings below 1 meter, the air–water flow is characterized by either distinct blowouts of regular frequency or continuous air release. In terms of geyser behaviors inclusive of frequency, the agreement is good between field and numerical studies. At large openings, the gate becomes fully submerged, and the flow is discharged without air entrainment and blowouts. The paper showcases the air–water flow features in a typical bottom outlet layout in Sweden, which is intended to serve as an illustration of the study procedure for other similar outlets.
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