Liquid nitrogen injection into water: Pressure build-up and heat transfer

摘要

This paper is concerned about the expansion of a small amount of liquid nitrogen injected into a relatively large pool of water and the heat transfer behaviour during the process. Both the transient pressure and temperature profiles are experimentally measured and analysed. The results show that the pressure and the rate of pressure rise increase approximately linearly with increasing injection pressure and reach, respectively, to 284 kPa and 500 kPa/s at a liquid nitrogen injection velocity of approx 0.85 m/s. The temperature varies little during the injection process due to relatively small amount of liquid nitrogen injected. A comparison of the experimental results with related work on surface boiling of cryogen suggests that the heat transfer of direct mixing be much stronger than boiling on smooth surfaces and flow boiling through smooth pipes, but comparable to the boiling on very rough surfaces and flow boiling in pipes with porous inserts. A comparison with the results generated by injecting a small amount of water into liquid cryogens shows that a higher pressure increase rate could be achieved if operating conditions are optimised to induce fragmentation. Implications of the results to cryogenic engine work output and ways to improve the performance of cryogenic engines are also discussed.