EFFECT OF THE GEOMETRY OF LABORATORY-SCALE PROBES ON HEAT EXTRACTION DURING FILM BOILING

In this work, we studied the effect of the geometry of a laboratory-scale probe (conical-end vs. hemispherical-end cylindrical probe) and the fluid dynamics around its surface as it relates to heat extraction, particularly during the film boiling stage.

Laboratory-scale hollow probes were heated, using an internal electrical resistance, to 900 °C and rapidly cooled in water at 60 °C, flowing parallel to the probe lateral surface at 0.15 m/s. Streamlines in the fluid around the probe surface were determined capturing high-speed images generated using tracer particles seeded in the quenchant.

It was found that the conical-end probe favors the early rupture of vapor film, increases the wetting front velocity and produces a more symmetric wetting front.

To understand the experimental results, a mathematical model of fluid flow in the quenchant - assuming isothermal conditions - was developed. The numerical results show a localized gradient of dynamic pressure at the intersection of the hemisphere and the cylindrical surface and a stagnation zone directly below the pole of the hemispherical probe. On the other hand, the conical-end probe produces a very smooth transition between regions of uniform dynamic pressure distribution and no stagnation point occurs. These findings explain the experimental results.