Transient heat transfer in liquid nitrogen

Abstract

With the advent of the new high Tc superconductors, liquid nitrogen will be one of the preferred cryogens used to cool these materials. Consequently, a more thorough understanding of the heat transfer characteristics of liquid nitrogen is required. In our investigations we examine the transient heating characteristics of liquid nitrogen to states of nucleate and film boiling under different liquid flow conditions. Using a platinum hot wire technique, it is verified that there is a premature transition to film boiling in the transient case at power levels significantly lower than under steady state nucleate boiling conditions. It is also shown that the premature transition can be reduced or eliminated depending on the flow velocity. Similar results are obtained with no forced liquid flow when a train of constant power step pulses is used. For certain ranges of power levels and repetition rates, increasing the pulse rate is analogous to increasing the flow rate.