Study On The Characteristics Of Temperature Rising Of The Step-wise Evacuated Tubular Solar Air Collector

Because evacuated tubular solar collector has a vacuum, it has a much better thermal insulation characteristic than flat-plate solar collector. Using water as heat transfer fluid, the vacuum tube is easy to frost, scale and be corrosion, while solar air collector doesn’t have these problems. At present, most of the studies about the evacuated tubular solar air collector focus on parallel form structure. There is almost no study on the collector with tandem type, especially about the distribution of temperature and the pressure drop. Compared with the parallel vacuum tube air collector, the step-wise evacuated tubular solar air collector can heat air to a higher temperature, which rises the thermal collect efficiency, and has a wider application foreground.An experimental system was performed to observe the pressure drop, the characteristics of temperature rising, and thermal efficiency in the step-wise evacuated tubular solar air collector. Furthermore, the hot air got from this collector was applied to evaporate the brine. A three-dimensional transient mathematical modeling has been developed to investigate the fluid flow, heat transfer, and pressure drop in the step-wise evacuated tubular solar air collector by finite volume method. The step-wise temperature rising rule was investigated with different flow rate of the air by fluid-solid coupling method. A numerical correlation of fluid resistance was set to analyze the resistance characteristics of the collector. Based on the results of the simulation, the feed tube diameter was optimized.The results indicate that most air would directly flow into the feed tube after air flow in the inner tube, the other part of air would form a vortex in the end of the inner tube. Then the air flow through the feed tube, at the end of it, a part of the air shock to the end of the vacuum tube, where forming the flow stagnation phenomenon. After flowing out from the vacuum tube, most air move as cylindrical disturbed flow in the outer tube. Almost all the air would flow out of the collector from the up part of the outer tube.The optimized feed tube size is which the inner diameter is 28mm and the outer diameter is 34mm by simulation and theoretical calculation. The pressure drop of the collector by simulation and theoretical calculation are 834Pa and 762Pa at 12m3/h. The optimized feed tube diameter would reduce the pressure drop as about 3% comparing to the traditional one.The collector system is composed of 6 collector units. The temperature of the air can rise 15℃ by one unit and rise to above 100℃ by the whole system at 12m3/h. The highest instantaneous thermal efficiency would be 97%.It has an excellent result when the collector applies in brine evaporation. The result shows that it can rise the Baume degrees of brine from 25 degree to 33.2 degree in seven days, and from 31.5 degree to 33 degree in two days.