内置旋转扭带换热管的传热强化机理

HEAT TRANSFER ENHANCEMENT MECHANIMS OF HEAT EXCHANGER TUBES WITH ROTATING TWISTED TAPE INSERT

针对换热设备的低效率和污垢沉积问题,研究开发具有强化传热和污垢在线清洗双重功效的旋转清洗扭带技术。提出旋转扭带强化传热的机理包括:①换热管当量直径减少效应强化。②近管壁区域流速加大效应强化。③螺旋线流动流速加大效应强化。④二次流流速增大效应强化。对旋转扭带的这些强化传热机理进行理论分析,建立湍流工况下强化传热的努塞尔数预测关联式。研究结果表明:当扭率大于等于10时,当量直径减小效应和近管壁区域流速加大效应是传热强化的主要控制机理;当扭率小于10时,螺旋线流动流速加大效应是强化传热主要控制机理。二次流流速加大效应对强化传热的贡献与其他三种机理相比相对较弱,只有在扭率小于等于1时,对强化传热的贡献份额才比较大。对内置旋转扭带换热管的努塞尔数预测值与试验值进行了比较,两者吻合较好。

The question of the low efficiency and fouling deposit for heat exchangers is a worldwide difficult problem all the time. The technology of a self-rotating cleaning twisted tape is dveloped, which has double functions of heat transfer enhancement and online cleaning fouling. Some heat transfer enhancement mechanisms of the rotating twisted tape are proposed, it can be attributed to the following four effects： ① The reduction of the equivalent diameter. ② The velocity increase near the tube wall due to the blockage of the twisted tape. ③ The velocity increase due to the helical flow following the twisted tape. ④ The velocity increase due to the second flow. Based on the theory analysis of the four heat transfer enhancement mechanisms, the prediction correlations of the heat transfer Nusselt number are put forward respectively. The results show that, for y≥10, the reduction effect of the equivalent diameter and the velocity increase effect near the tube wall area are the main contributing factor of heat transfer enhancement, for y〈10, the helical flow effect is the main contributing factor of heat transfer enhancement. The contribution to heat transfer enhancement of the second flow motion is smaller than other three ones, only for y≤1, it becomes the main contributing factor. The prediction results of the heat transfer Nusselt number for the tube with a self-rotating twisted tape insert are compared with the experimental results. They show reasonably good agreement.