Bubble formation and the critical heat flux to transit to boiling state for
cyclic heat inputs have been investigated using a platinum - 0.5at% cobalt
(Pt-Co) wire and a silver sheathed Bi-2223 tape, in order to evaluate the
cooling stability for superconducting AC power apparatus. Sinusoidal cyclic
heat inputs ranging from 2 to 150 Hz were applied to a horizontal thin wir
e immersed in a pool of saturated liquid nitrogen under normal atmospheric
pressure. A high-speed video camera with a frame speed of 1000 frames/s has
been used to observe bubble formation around the wire. The bubble frequenc
y and the critical heat flux to transit to boiling state have been measured
for each frequency. As a result, it has been found that the bubble frequen
cy and the critical heat flux have depended on the heating frequency. The s
ame experiment has been performed on a silver sheathed Bi-2223 tape, howeve
r, the mechanism of bubble formation Is different. The dependence of the cr
itical heat flux on the heating frequency has been also observed.