Research On Solar Air Collectors Having Dimple-shape Artificial Roughness On Absorber Plate

Due to its good environmental friendliness, high collection efficiency, simple structure and low cost, solar air collectors are widely used in many areas. Therefore, the research on solar air collector is of great significance.Numerical simulation study has been carried out to investigate the flow state and heat transfer on the absorb plate with hemispherical protrusion or dimple, then an experimental set up has been designed, fabricated and tested outdoor to analysis the thermal performance of the solar collector. Then we have established the theoretical model for bottom channel solar air collector, top to bottom channel solar air collector, bottom to top channel solar air collector, parallel pass double duct solar air collector and opposite-double duct solar air collector, the difference of thermal performance among them are analyzed at the same time. Besides, the best configurations of the solar air collector with the five different types are calculated based on the MATLAB software, which can provide reference for improving the performance of solar air collector. The main research works are as follows:1) For the absorb plate with hemispherical protrusions, When the value of Re and P/e are fixed, the Nu was increases as the H decreases; the friction factor (f) increases as the H decreases; PF decreases as the Re increases under H<60mm and the peak of PF locates at where the Re is around 7000 under H≥60mm. Correlations for Nusselt number and friction factor have been developed as follows:2) For the absorb plate with hemispherical dimples, When the value of Re and P/e are fixed, the heat transfer was enhanced as the e/D (H) increases; the friction factor increases as the e/D (H) increases; the PF increases as the e/D increases. PF decreases as the Re increases under H=20-40mm and PF first increases then decreases as the Re increases under H=60-100mm. When the value of Re and e/D is fixed, Nu increases as the P/e increase under Re<7000-9000 and decreases as the P/e increase under Re>7000-9000; the friction factor increases as the relative spacing (P/e) decreases; PF increases as the P/e increases. Correlations for Nusselt number and friction factor have been developed as follows:3) When both the heights of top and bottom channels are 50mm, the outlet temperature increases as the intensity of radiation enhance, decreases as the mass flow rate increases; the thermal efficiency change little as the intensity of radiation enhance, increases as the mass flow rate increases; the hydraulic thermal efficiency change little as the intensity of radiation enhance, however, increase slightly as the intensity of radiation enhance for the top to bottom channel and bottom to top channel solar air collector and the hydraulic thermal efficiency first increases then decreases as the mass flow rate increases.4) When both the heights of top and bottom channels are 50mm, the outlet temperature, the thermal efficiency and the hydraulic thermal efficiency decreases slightly as relative spacing increases and increases slightly as relative height increases.5) When the total height of channels is 120mm,the performance of parallel pass double duct solar air collector and opposite-double duct solar air collector change little as the height of bottom channel change; for bottom channel solar air collector, the outlet temperature, the thermal efficiency and the hydraulic thermal efficiency decreases as the height of bottom channel increases; for top to bottom channel and bottom to top channel solar air collector, the outlet temperature and the thermal efficiency decreases slightly as the height of bottom channel increases, the hydraulic thermal efficiency decreases rapidly as the difference of height between top and bottom channels increases.6) The experimental result shows that the performance of bottom channel solar air collector with hemispherical protrusions on absorb plate is better than that with smooth flat plate, and the outlet temperature is higher 1.8-4.6℃ and the thermal efficiency is higher 12.3%-15.3%; the temperature of outlet is increases as the intensity of radiation enhance; the thermal efficiency of the collector is lower in the morning and afternoon during the whole day.