Pressure oscillations induced by forced convection heat transfer to liquid hydrogen have been probed using a preliminary apparatus which cooled from ambient to LH temperatures during each run. Acoustic modes due to Helmholtz and open-open pipe resonances were observed at sub and supercritical pressures. These oscillations did not develop when the inlet hydrogen was superheated. Pressure fluctuations in the form of sawteeth, negative pulses, and beats were also observed above and below the critical pressure. A mode which primarily occurred, above the critical pressure and one associated with subcritical two phase flow were also observed. Experimental evidence also indicated that Heat transfer was substantially better in two phase flow than in supercritical flow at low flow rates, that pressure drop increased after the apparatus had cooled near LH_ temperatures at moderate flow rates, and that greater axial temperature gradients were obtained in the slower cool downs.
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