Characteristics of Transient Heat Transfer and Wetting Phenomena During Laminar Jet Quenching on Rotating Cylinder

Abstract

Hot strip leaving from finishing roll is quenched from about 850 to 500°Cby array of laminar jets on cooling table (Run-Out-Table). Cooling temperature control of the hot strip is important to obtain better mechanical strength and grain size. However, the cooling process includes unstable transition boiling region as well as moving boundary problem due to relative movement between the hot strip and the jets. To improve quality of hot strip deeper understanding about laminar jet quenching process on the moving hot surface should be elucidated. In this study, single laminar jet quenching tests on a rotating hollow hot cylinder mounted horizontally have been conducted to understand characteristics of transient heat transfer and wetting phenomena. The experiments were done for 18-8 stainless steel (SUS304) hollow cylinder (O.D. 136mm, I.D. 116mm, W 150mm) under rotational speed ranged from 15 to 60rpm, cooling water temperature ranged from 10 to 60°C (corresponding degree of subcooling 40-90K) and flow rate ranged from 6 to 23 L/min. Surface temperature and surface heat flux on the rotating cylinder were estimated from two sheath thermocouples embedded at two depths from the outer surface with one dimensional inverse heat conduction analysis. Visual observations over the top surface were done with two normal speed video cameras which were synchronized with rotating cylinder temperature recording. The observation results showed that unstable wetted area was gradually growing from the stagnation area and finally the wetted area became continuous around the circumference of the cylinder. It was found that the growth and shape of the wetted area were strongly affected by the liquid temperature and the liquid flow rate. Wetted front which is the boundary between wetted and dry areas repeated advance and recession. Since the wetting area propagated faster in the rotational direction, the shape of the wetted area was asymmetry on the hot surface. The liquid film flow was completely splashed out due to violent nucleate boiling on the wetted front line. The characteristics of local transient heat transfer were evaluated with boiling curves taken around the stagnation point. The boiling curves indicated shift to much higher wall superheat as compared with a steady state pool boiling curve and those were categorized into three regions such as 1) single phase heat transfer, boiling heat transfer and 3) transition boiling by inspecting gradient of the boiling curves and the observation of the boiling situations. The transition boiling region disappeared and the single phase heat transfer became dominant for higher subcooling and higher flow rate conditions.