Effect of fluid-probe interaction on heat extraction in laboratory-scale forced convective quench tests

Quenching is conducted with agitation, which produces a non-uniform velocity field around the components. To assess the effect of quenchant velocity on cooling curves in laboratory-scale tests, special devices have been developed to produce one-dimensional fluid flow parallel to the probe. However, the international standards for these tests recommend probes that are flat-end cylinders which may promote non-uniform flow around them.

In this work we report the evolution of heat extraction from probes of three different geometries: flat-end, conical-end and hemispherical-end cylinders, characterized through video-recordings and cooling curves. The experimental set up assures one-dimensional fully developed flow, parallel to the probe. The probes were machined from AISI 3034 stainless steel and instrumented with four thermocouples (one at its geometric center and the others near the surface). The initial test temperature was 850 °C; water was flowing at 0.2 and 0.6 m/s, at 60 °C.

From the video recordings the flat-end probe favors a distorted vapor film near the probe end which was barely seen in the hemispherical-end probe and absent in the conical-end probe. This observation could be explained through the results of a previous paper in which an isothermal CFD model was developed to compute the velocity and dynamic pressure fields around the probes at room temperature. The model showed a region of low pressure near the base of the flat-end probe which is consistent in shape and position with the vapor film observed at the beginning of the quench of this probe. Because of the distorted vapor film, the cooling curves showed more scatter from test to test when the flat-end probe was used. The probe geometry had little influence on the wetting front velocity for water flowing at 0.2 m/s, but a higher wetting front velocity was observed for the flat-end probe with water at 0.6 m/s.