超临界变压运行直流锅炉内螺纹管螺旋管圈水冷壁的传热特性研究

AN INVESTIGATION ON HEAT TRANSFER CHARACTERISTICS OF SPIRAL WALL WITH INTERNAL RIB IN SUPERCRITICAL SLIDING-PRESSURE OPERATION ONCE-THROUGH BOILER

在压力9～28MPa,质量流速600～1200kg/(m2s),内壁热负荷200～500kW/m2的工况范围内,研究了φ38.1×7.5mm倾斜上升内螺纹管(倾角α=19.5°)中水的传热特性.试验结果表明:在亚临界压力区,内螺纹管传热强化作用明显,有效地抑制了膜态沸腾的发生,但在近临界压力区此传热强化作用有所减弱.超临界压力区拟临界温度附近,内螺纹管内壁面与流体之间的温差较之前有所增加,但是此增幅远没有亚临界压力区发生传热后的壁温飞升幅度大.随着系统压力接近临界压力,拟临界点附近管壁与工质的温差显著增加.在超临界压力区,不同的质量流速与热负荷比例下,在大比热区内螺纹管内流体传热可能被强化也可能被恶化.在超临界压力下,由于螺旋内槽的旋流作用减弱了自然对流的影响,倾斜上升内螺纹管内壁温度的周向分布比较均匀.在高焓值区内螺纹管的周向最大温差只有10℃左右.文中提出了在考虑大比热区工质物性剧烈变化对传热影响的情况下,倾斜上升内螺纹管项部内壁传热系数的试验关联式.

Within the range of pressures from 9 to 28MPa, mass velocities from 600 to 1200kg/（m^2s）, and heat fluxes from 200 to 500kW/m^2, experiments have been performed to investigate the heat transfer to water in the inclined upward internally ribbed tube with the inclined angle of 19.5 degree, the maximum outer diameter of 38.1mm, and the thickness of 7.5mm. Based on the experiments, it was found that heat transfer enhancement of the internally ribbed tube could postpone departure from nucleate boiling at the sub-critical pressure. However, the heat transfer enhancement decreased near the critical pressure. At supercritical pressure, the temperature difference between the wall and fluid increased near the pseudo-critical temperature, but the increasing of wall temperature was less than that of departure from nucleate boiling at sub-critical pressure. When pressure was close to the critical pressure, the temperature difference between the wall and fluid greatly increased near the pseudo-critical temperature. Heat transfer to supercritical water in the inclined upward internally ribbed tube was enhanced or deteriorated near the pseudo-critical temperature with the variety of ratio between the mass velocity and the heat flux. Due to the rotational flow of the internal groove reducing the effect of nature convection, the internal wall temperature of the inclined upward internally ribbed tube uniformly distributed along the circumference. The maximum internal wall temperature difference of the inclined upward internally ribbed tube along the circumference was only 10 degree Centigrade when the fluid enthalpy exceeds 2000kJ/kg. Considering the effect of acute variety of the fluid property on heat transfer, the correlation of heat transfer coefficient on the top of the inclined upward internally ribbed tube have been provided.