Comparison of Particle Image Velocimetry and Planar Laser-Induced Fluorescence Experimental Measurements and Numerical Simulation of Underwater Thermal Jet Characteristics

During the underwater movement of a submarine, cooling water at a specific temperature is discharged into the surrounding water through nuclear reactor secondary loop circulation, creating a thermal jet. Thermal jets are characterized by initial velocity and temperature properties that allow for complete mixing with the surrounding water through a combination of mixing and heat transfer processes. This paper aims to investigate the movement and diffusion of underwater thermal jets, specifically examining the temperature stratification of the ambient water, the initial velocity of the jet, and the effect of temperature on the velocity field and temperature field of the underwater thermal jet. This study utilizes particle velocity measurements and the laser-induced fluorescence method to measure the velocity field and temperature field of the thermal jet, as well as simulation methods to validate conclusions. The experimental and simulation conditions in this paper are mainly categorized into two types: uniform water body and thermally-stratified water body. Upon analysis and comparison of the experimental and simulation results, it has been observed that an increase in jet velocity will hinder the upward diffusion of jet temperature, decrease the floating height of the jet, and slow down the rate at which the jet temperature decays. Furthermore, as the difference between the jet temperature and the ambient water temperature increases, the upward diffusion of the jet temperature becomes predominant, resulting in a 40–50% increase in its floating rate. It is evident that the stratification conditions of the background environment have a significant impact on the jet temperature diffusion. When the jet temperature diffuses to the thermally-stratified interface of water in the tank, it ceases to float due to density differences; consequently, its temperature cannot diffuse further towards or reach the water surface.