PHASE CHANGE HEAT TRANSFER OF LIQUID NITROGEN INJECTION INSIDE A HEAT TRANSFER FLUID

Phase change heat transfer and flow characteristics of cryogenic fluids are of high importance to a number of industries. Rapid mixing of cryogen and water has been proposed to simulate the large scale complicated fuel-coolant interaction (FCI) in nuclear industry, and to provide rapid pressure build up and a heat source inside an engine cylinder to drive a piston. However very few experiments is directly relevant to this process. This paper reports an experimental investigation of pressure build-up and phase change heat transfer of injecting small amount of liquid nitrogen into a relatively large pool of water under ambient temperatures. Some experiments have also been performed for direct injection of liquid nitrogen into ambient air that serves as a comparison. Both transient pressure and temperature signals are acquired through a high-speed data acquisition system. The heat transfer coefficient during the injection process is estimated from pressure rise curve and compared with other boiling heat transfer correlations. The results show that a high pressure rise gradient up to 500 kPa/s can be achieved under the condition of this work. Both the gauge pressure and pressure increase gradient are found to increase linearly with the injection pressures, with higher values obtained for direct injection into water. The temperature varies little during the injection due to the relative small amount of liquid nitrogen. The comparison of experimental results with cryogen boiling experiment suggests that the heat transfer coefficient during direct mixing be much higher than the standard boiling heat transfer coefficient. The work provides useful data and suggestions for investigating the rapid phase change heat transfer process.