Simulation Of Flow Heat Transfer Characteristics And Geometric Dimension Optimization Of Plate Collector

Flat-plate solar collector is the key structure for a solar water heating system,which directly dictates the stability, reliability and thermal performance of the whole solar water heating system. Flat-plate solar collector has attracted more and more attention and interests due to its advantages, such as high water production rate, no tube bursting, pressure-bearing, resistance to high temperature, no fouling, high thermal efficiency, more suitable for circulatory systems, easiness to be integrated with buildings, and low maintenance costs in its life-span. In this thesis, the flow dynamics, temperature distribution and thermal performance of flat-plate solar collector under different geometrical parameters(spacing between the glass cover and the absorber plate, absorber plate thickness, distance between risers, diameter of risers)are analysed through numerical simulation using ANSYS Fluent and experiments.More specifically,Firstly, the heat and mass transfer mechanisms of flat-plate solar collector is analyzed theoretically, in particular overall heat loss coefficient, the efficiency factor,the collector heat removal factor and the collector flow factor. Several common methods of evaluation have been achieved for the efficiency of flat-plate solar collector.Secondly, a measure of collector performance is the collection efficiency intercept in the respect of numerical simulation. After a comparison analysis of the thermal performance of flat plate collectors with different values of the spacing between the glass cover and the absorber plate, it is found that a larger spacing will lead to a smaller convective heat transfer coefficient between the absorber plate and the glass cover; when the spacing is more than 30 mm, further increase in the spacing does not result in significant change of the convective heat transfer coefficient between the absorber plate and the glass cover is not noticeable; hence, a spacing of30 mm is recommended for achieving a small convective heat transfer coefficient between the absorber plate and the glass cover and a high thermal efficiency. Theresults also show that the thermal performance of flat plate collector increases with the decrease of the distance between risers, the increase of the diameter of the riser,and the increase of the absorber plate thickness. In order to optimize the geometrical dimensions of flat plate collector, for the same material coasts, the gain in improving the collector thermal efficiency by increasing the diameter of risers is not very obvious when compared to that by increasing the absorber plate thickness and by reducing the distance between the risers;When the equivalent-price plate thickness is in the range of 1.6mm~1.73 mm and the distance between risers is in the range of96mm~120mm, the improvement in the collector thermal efficiency through increasing the absorber plate thickness is equal to that through decreasing the distance between risers. When the equivalent-price plate thickness Z is smaller than 1.6mm or larger than 1.73 mm, the improvement in the collector thermal efficiency through reducing the distance between risers is significantly better than that through increasing the absorber plate thickness; hence, in the course of optimization of the flat-plate solar collector geometrical dimensions, the priority should be given to reducing the distance between risers, then to increasing the absorber plate thickness,and then to increasing the diameter of the risers.Finally, the flat-plate solar collector efficiency obtained from the experimental tesing is 0.73. It is found that the absorber temperature decreases along the flow direction in the headers with temperature differences of 4~9 ℃ but increases along the flow direction in the risers with temperature differences of 13~18 ℃.Through a comparative study, this thesis analyses the thermal efficiency of flat-plate collectors with variations in geometrical dimensions, and produces the optimized geometrical dimensions. The results are of significant importance to provide guides for the optimal design and manufacturing of flat-plate collectors.