Experimental Investigation Of Heat Transfer Characteristics Of A Natural Circulation Solar Steam Generation System

Trough solar thermal technology is a hotspot of current research,120-200℃medium-high temperature steam produced by this technology can be widely used in industry. Trough system use parabolic mirror to focus sunlight to the outer wall of the collector tube, heat working fluid within the collector tube. The wall of collector tube will be in the state of partially heating when system working properly, the portion face the mirror has higher temperature than the back of the collector tube. In order to improve the photo thermal conversion efficiency and operational stability of the system, it is very important to make a deeply study of the flow and heat transfer characteristics of the collector system in local heating state.A natural circulation separated heat pipe steam generating system applied to the trough solar collector was designed, with the heat flux of collector tube varies from3.7kW/m2to7.5kW/m2, and the exhaust steam pressure varies from0.15MPa to0.35MPa. According to characteristics of our separate type heat pipe, we re-definition the filling ratio of heat pipe, and study the features and variation of the temperature distribution of the heat pipe loop experimentally, from aspects of heating way of the collector tube (whole heating, bottom heating, top heating), heating temperature of the collector tube (500℃,550℃), and the filling ratio of heat pipe (60%,120%) at stable exhausting stage.The overall temperature of the heat pipe circuit increased, with the improving of the pressure of the exhaust steam and heating temperature. With the filling ratio of the heat pipe increases, countercurrent vapor at the inlet of condensing water and liquid reflux of the rising pipe enhanced, the cyclical fluctuations of temperature of the circuit of the heat pipe occurs. We get following conclusions under the0.2MPa stable steam exhausting conditions:(1) Under60%filling ratio,500℃heating temperature:The average heat transfer coefficient of the collector tube (hj) of the bottom heating method increased13.20%, and the system thermal resistance (Rpipe) decreased11.41%while compare to that of whole heating method; the hj of the top heating method increased7.98%, and the Rpipe decreased8.49%while compared to that of whole heating method.(2) Under60%filling ratio, when heating temperature varies from500℃to550℃:the hj increased13.53%, and the Rpipe decreased12.27%under whole heating method; the hj increased15.25%, and the Rpipe decreased14.37%under bottom heating method; and the hj increased18.54%, and the Rpipe decreased15.42%under top heating method.(3) Under500℃heating temperature, when filling ratio varies from60%to120%:the hj increased25.80%, and the Rpipe decreased17.74%under whole heating method; the hj increased20.40%, and the Rpipe decreased14.82%under bottom heating method; and the hj increased32.64%, and the Rpipe decreased18.58%under top heating method.Under the constant temperature heating conditions of this paper, the collector tube under bottom heating method has a higher hj, lower Rpipe, and better natural circulation heat transfer characteristics of the separate type heat pipe system, while the evaporator section is placed horizontally.