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Experiments on Pool Boiling of Water from Downward-Facing Hemispheres

Mohamed S. El-Genk, Cheng Gao

Nuclear Technology

Volume 125 / Number 1 / January 1999 / Pages 52-69


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Quenching experiments were conducted to investigate pool boiling of saturated water on downward-facing aluminum and 303e stainless steel hemispheres. Test sections had an outer diameter of 0.152 m and a wall thickness of 0.020 m. Destabilization of film boiling and wetting of the stainless steel surface occurred earlier than with aluminum (15 s versus 92 s), at 20 K higher wall superheat and ~10% higher minimum film boiling heat flux qmin. Wetting of the stainless steel surface occurred first near the edge of the test section and then gradually propagated azimuthally inward, followed by the maximum heat flux (MHF) front. Conversely, wetting of the aluminum surface occurred first at the lowermost position ( = 0 deg) and then propagated azimuthally outward. The azimuthal propagation of the MHF front on the stainless steel surface (~5 deg/s for 60 deg < < 90 deg decreasing to ~1.8 deg/s for 10 deg < < 60 deg and then increasing slightly to ~2 deg/s for 0 deg < < 60 deg) was much slower than on aluminum (~22.5 deg/s on average). The MHF front traversed the entire stainless steel boiling surface in ~40 s versus only 4 s for aluminum. When MHF for the latter first occurred at = 0 deg, the radial and near-boiling surface azimuthal temperature gradients were 4 K/mm and 0.6 K/deg, respectively, compared to 12 to 15 K/mm and 1.5 K/deg for stainless steel. For both surfaces, the MHF and qmin values displayed parabolic dependencies on azimuthal angle. The local MHF at = 0 deg was 0.81 and 0.40 MW/m2 for aluminum and stainless steel, respectively, decreasing with increased azimuthal angle to minimums of 0.47 and 0.256 MW/m2 at = 45 deg. Beyond 45 deg, the local MHF increased with increased azimuthal angle to 0.76 and 0.47 MW/m2, respectively, near the edge of the surface ( = 80 deg). However, the wall superheats corresponding to the MHF (30 K for aluminum and 80 K for stainless steel) and the qmin (125 K for aluminum and 145 K for stainless steel) were independent of azimuthal angle.

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