Fluid dynamics around flat-end cylindrical quench probes under isothermal and non-isothermal conditions

The most common methodology to describe the complex phenomena occurring during quenching operations is based on measuring cooling curves inside metal test probes instrumented with thermocouples. There are many variants regarding specimen shape, although the preferred shape is a flat-end cylindrical probe. However, mathematical and physical modeling studies, carried out under isothermal conditions, regarding the hydrodynamic behavior of the quenchant around flat-end cylindrical probes suggest that this type of probe should not be recommended for studies quenching. In this paper, we study fluid dynamics around an instrumented flat-end cylindrical probe, both under isothermal (unheated probe) and non-isothermal conditions (probe heated to 900 ºC and cooled in water at 60 ºC). For this purpose, water at 60 ° C was used as cooling fluid; three free-stream flow velocities (0.2, 0.4 and 0.6 m/s) were studied. Visualization techniques using a high-speed camera and optical filters were applied to characterize the flow around the probe and, at the same time, thermal histories were measured within the probe. The results show that fluid dynamics under non-isothermal conditions is very different from that under isothermal conditions; therefore, it is not advisable to directly correlate fluid dynamics around metal probes characterized under isothermal conditions with cooling curves or the evolution of the wetting front during an actual quench.

Keywords: quenching, cooling curves, flat-end cylindrical probe, wetting front, fluid dynamics.