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For many industrial, medical and space technologies, cryogenic fluids play indispensable roles. An integral part of the cryogenic
transport processes is the chilldown of the system components during initial applications. In this paper, we report experimental
results for a chilldown process that is involved with the unsteady two-phase vapor-liquid flow and boiling heat transfer of
the cryogen coupled with the transient heat conduction inside pipe walls. We have provided fundamental understanding on the
physics of the two-phase flow and boiling heat transfer during cryogenic quenching through experimental observation, measurement
and analysis. Based on the temperature measurement of the tube wall, the terrestrial cryogenic chilldown process is divided
into three stages of film boiling, nucleate boiling and single-phase convection that bears a close similarity to the conventional
pool boiling process. In earth gravity, cooling rate is non-uniform circumferentially due to a stratified flow pattern that
gives rise to more cooling on the bottom wall by liquid filaments. In microgravity, there is no stratified flow and the absence
of the gravitational force sends liquid filaments to the central core and replaces them by low thermal conductivity vapor
that significantly reduces the heat transfer from the wall. Thus, the chilldown process is axisymmetric, but longer in microgravity.
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