| Solar collectors with horizontal double-row all-glass vacuum tubes widely used in solar water heating systems. However, the research work on the internal fluid dynamics and heat transfer in the collectors is scarce. In order to enhance the thermal efficiency of solar collectors with horizontal double-row all-glass vacuum tubes and to use them more effectively, this thesis studies the most common solar collectors with horizontal double-row all-glass vacuum tubes available on the market. More specifically:Firstly, for the research object——solar collectors with horizontal double-row all-glass vacuum tubes, this thesis studies its structure, thermal properties, the core components vacuum tubes theoretically. And concluded that the energy equilibrium equation of collector, solar radiation, thermal energy losses, useful energy gains, instantaneous efficiency and the optical coefficient of the tube.Because of the limitation of processing technology of solar collectors with horizontal double-row all-glass vacuum tubes, L=1500mm and L=1800mm collectors with same diameter 58mm was test under different installation angle a to receive their temperature distribution and instantaneous efficiency. All experiments are using TYD-BL test system for thermal performance of solar collector in National Solar Water Supervision And Inspection Center Of Quality (Kunming). The experimental results show that the increase of the installation angle a improves the thermal efficiency of L=1500mm and L=1800mm collectors by reducing their heat loss coefficients. They also show that the thermal performance of L=1800mm collector is better than that of L=1500mm collector.Based on the geometric dimensions of the most common 50-tube solar collectors with horizontal double-row all-glass vacuum tubes available on the market, but taking into account of the capacity of the computing facilities available and the time restriction, thesis simplifies the collector geometrical model to be modelled by one half of a solar collector with 5 horizontal double-row of all-glass vacuum tubes. The simplified geometric model is drawed using the 3D software Creo and then converted into unstructured grids using the ICEM software for grid division. Continuity equation and N-B equation of the ANSYS Fluent CFD package is then used to control the calculate methods. The collector internal working medium uses the Boussinesq model. Solar load model, the solver, material property, discrete items, relaxation factor, residual, initialization, and so on is taking into account to make the numerical simulation results more believable.Numerical simulations with Fluent are carried out for three different tube lengths(L=1500mm, L=1800mm and L=2100mm), three different diameters (Φ 47/38mm,Φ58/47mm and Φ70/58mm) and different installation angles a. For all the numerical simulations, the inlet flow is 0.013kg/s, the inlet water temperature is 293K which is equal to the ambient temperature, and the installation angle of 35°. All these simulations are set to take palce from 13pm to 14pm on November 21st in Kunming (25°N,102°N). The numerical simulation results show that among the three different lengths, the tube length of L=1500mm produces the smallest flow rate of hot water at the end of the tube, but its inlet velocity is relatively uniform and the shear layer effect is small. Although the L= 1800mm and L=2100mm tubes have the almost overlapped hot water exiting velocity profiles, both of them are affected significantly by the shear layer, resulting in very small inlet velocity, which becomes large only until the bottom of the vacuum tube opening. The largest inlet velocity occurs at the midpoint of the nozzle below, with the largest inlet velocity of L=1800mm tube is slight larger than that of L=2100mm tube. Among the three different tube diameters, the influence of the shear layer on the inlet velocity becomes more and more serious with the increase of the tube diameter. With the increase of the tube diameter, the amount of water in the tube increases, leading to stronger temperature stratification in the tube. In the actual installation of solar collectors with horizontal double-row all-glass vacuum tubes, defining the angle between the axis of the tube and the vertical line as the installation angle a. The value of the installation angle a is small, about 2°. This thesis studies on the collector when its installation angle a is 1°,3°, 5°and 7°. When the installation angle a is increased from 1° to 7°, the circulation flow rate on the radial cross section of the tube becomes larger and the flow circulates more strongly, which is more beneficial for the heat exchange and the removal of heat. Nevertheless, when the installation angle a is inverted, in other words, the installation angle a is negative, more heat will be stranded in the closed end of the tube with the increase of a, which has a negative bearing on the heat exchange in the tube. Hence, it is recommended a positive value of the installation angle a is ensured for the collectors installed and the inverted installation angle a should be avoided.Finally, when the inlet flow is 0.013kg/s, correlation between the installation angle a and the the circulation flow rate for solar collectors with horizontal double-row all-glass vacuum tubes with length L=1800mm and diameters 058mm is developed. For the installation angle a varies between 0° and 7°, the correlation ism= 0.00143+4.02291×10-4a. |
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