| Horizontal double-rowed all-glass vacuum tube solar collectors have been widely used in solar water heaters due to their excellent thermal performance, easy installation and transportation, maintenance convenience, and low costs. In this paper, an experimental and numerical study was carried out to investigate the influence of the inclination of the tubes and the inlet mass flow rate on the internal flow and temperature fields in a horizontal double-rowed all-glass vacuum tube solar collector. More specifically, the following studies were conducted and the results were obtained from this study:1. There would be a small angle of the vacuum tubes with respect to the horizontal direction due to the inevitable installation deviation. In this paper, this small angle was defined as the vacuum tube inclination. An experimental study was carried out to examine the influence of this inclination on the instantaneous efficiency of the horizontal double-rowed vacuum tube solar collector system and the internal flow within the vacuum tube with three inclinations, that is, 0o, 1o, and 3o. The experimental results showed that when the inclination was at 3o the internal heat transfer was the fastest, with the highest circulation flow rate and the largest instantaneous efficiency, whereas when the inclination was at 0o the internal heat transfer was the slowest, with the smallest circulation flow rate and the lowest instantaneous efficiency. However, the heat losses from the collectors were very close under these three different inclinations.2. The commercial CFD package Fluent was used to carry out three-dimensional numerical simulations of the flow and heat transfer in the horizontal double-rowed vacuum tube solar collectors under the three inclinations of 0o, 1o, and 3o. Five mass flow rates were selected from the experimental results, which corresponds to the five typical operation conditions of the collectors, to be used to study the internal flow and heat transfer within the tubes and the collectors. These five mass flowrates were 0.00083 kg/s, 0.0083 kg/s, 0.0166 kg/s, 0.05 kg/s and 0.15 kg/s, with the total collector area of 7.8 m2.3. The results showed that with the increase of the collector mass flow rate, the low-temperature region at the sealed end of the tube increased initially but then decreased and the average temperature within the tube decreased initially but then increased. When the mass flow rate was 0.0166 kg/s, the average collector Nusselt number was the maximum, and the low-temperature region was the largest, with the lowest average temperature and the strongest internal convective heat transfer. When the mass flow rate was 0.00083 kg/s, the average collector Nusselt number was the minimum and the average temperature was the highest due to the small flow rate. When the mass flow rate was 0.15 kg/s, vortexes appeared at the open end of the vacuum tube, inhibiting the exit of the hot water from the tube.4. When the mass flow rate was the same, the average collector Nusselt number was the largest at the inclination of 3o, the convective heat transfer was more thoroughly, and the collector equilibrium temperature was the lowest. On the contrary, when the inclination was 1o, the average collector Nusselt number was the smallest and the collector equilibrium temperature was the largest. When the mass flow rate was 0.00084 kg/s and at the inclination of 0o and 1o, there was hot water stayed in the vacuum tube and the collector could not attain the equilibrium temperature within 60 minutes.5. With the increase of the mass flow rate, the collector thermal efficiency increased initially but then decreased. At the inclination of 3o, the thermal efficiency was the highest when the mass flow rate was 0.016 kg/s. It is found that whether the heat in the vacuum tube can be flowed out in time so that the average temperature within the tube is lowered will be the key factor influencing the collector thermal efficiency. |
PDF Full Text Request