Experiment-based visualization of characteristics of secondary flow phenomenon in horizontal heating tubes

• Published in: International journal of heat and mass transfer 2020-03, Vol.149, p.119249, Article 119249
• Main Authors: Wu, Chao, Li, Hui-xiong, Zhang, Qian
• Format: Article
• Language: English
• Subjects: Acceleration; Buoyancy; Convection; Convective heat transfer; Cooling systems; Cross-sections; Deionization; Deterioration; Flow characteristics; Flow velocity; Fluid dynamics; Fluid flow; Gravitational effects; Heat exchangers; Heat transfer; Heating; Laminar mixing; Mass flow; Mixed convection; Nuclear reactors; Quartz; Secondary flow; Stratification; Tubes; Velocity distribution; Visualization measurement; Vortices; Working fluids
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2019.119249

•Quantitative experimental study on secondary flow field in the cross-section of tube.•Three areas with different secondary flow characteristics are formed by three factors.•Two areas with opposite secondary flow direction are divided by a horizontal line.•Horizontal line position and secondary flow vortices dominate flow field structure.•Heat flux and mass flow velocity significantly affect secondary flow characteristics. In recent years, the characteristics of mixed convective heat transfer and heat transfer deterioration, which are seriously affected by secondary flow in the horizontal tube, have received increasing attention in the field of new heat exchange system development, such as the supercritical water heat exchangers and nuclear reactor cooling system. To gain insight into the mechanisms of secondary flow in horizontal heated tubes, experiment-based visualization of the secondary flow field in the cross-section is performed in thermally developing, laminar and transitional mixed convection. Deionized water is used as the working fluid, and the test section is a circular quartz glass tube with an internal diameter of 25.8 mm and length of 300 mm. Transparent conductive oxide heating films, which are used to uniformly heat the working fluid, are deposited on the outer surface of the quartz glass tube by the ion deposition method. The operating heat flux is varied from 1.15 kW/m2 to 9.24 kW/m2, the mass flow velocity is varied from 15 kg/(m2∙s) to 35 kg/(m2∙s), and the pressure is varied from 0.1 to 1.8 MPa. The experimental results indicate that fluid rises up along the tube wall under the effect of buoyancy in the vicinity of the tube wall. In the central region of the cross-section, an approximately horizontal flow boundary line is formed, which divides the center region into two parts that have significantly different secondary flow characteristics. Above the flow boundary line, the fluid slowly moves upward under the effect of thermal acceleration; below the flow boundary line, the fluid accelerates to descend under the effect of gravity. With increasing heat flux, both the flow stratification phenomena and buoyancy effect are enhanced, which leads to higher secondary flow velocity in the vicinity of the tube wall and downward motion of the flow boundary line and secondary flow vortices. A higher mass flow velocity weakens the flow stratification phenomena; thus, the flow boundary line and secondary flow vortices move upward, and the secondar