Research On Phase Change Process And Dynamic Performances Of An Underwater Thermal Glider

The underwater glider propelled by ocean thermal energy has the property of long endurance, simple structure, low cost and low noise. It was widely applied in ocean environment monitoring and military detection. Recently it has become the research focus of underwater engineering. However, the related research is still at the stages of prototype design and field test. The thermal glider changes its net buoyancy by a special power system which can convert ocean thermal energy into mechanical energy and drive the glider moving. The working characteristics of the power system will influence the energy harvesting efficiency from the ocean thermocline while the phase change energy storage system of the power system is a key part for the glider to realize the long-term and non-noise ocean monitoring. The phase change process and the influencing factors of the power system were investigated in this thesis. And the dynamic performance of the underwater glider and the influence of the operating parameters on the whole performance of the underwater glider were discussed when it is operating in the ocean.Firstly, the numerical simulation and experimental study method was applied in this thesis to study the rule of the volumetric change rate for the solid-liquid phase change material. The influencing factors of the volumetric change rate were analyzed. The numerical model established in this thesis is suitable for studying the volumetric change rate of any other types of solid-liquid phase change material. And the experimental method can prevent the solid phase change material adhereing to the cylinder wall which cause measuring difficulty for the volumetric change rate during solidification process.According to the physical model of the power device of the thermal glider, a numerical model of the phase change heat transfer was established based on enthalpy method. The numerical equation was solved by decoupling the liquid fraction with the temperature field. The influencing factor of the heat transfer rate for the energy-storage power system was analyzed. The simulation results showed that the heat transfer rate of the power device can be enhanced improved by improving the properties of the phase change material, increasing the temperature difference between the devices and the surrounding, decreasing the diameter of the container or improving the gliding speed.The thermal glider changes the outer bladder volume by changing the volume of the energy-storage material during phase change process. Accordingly it changes the net buoyancy of the whole vehicle and gliding up and down in the ocean. The rule and effects of volume change for the energy-storage material decide the working performance of the power device and the gliding attitude of the glider. Therdfore the volume change rule of the energy-storage material was experimentally studied and numerical analyzed in this thesis.Based on the numerical model of the energy-storage device of the power system and according to the profilel of liquid fraction field, the numerical model of the volumetric change rate of the phase change material was deduced. By experimental verification and numerical simulation, the rule of the volumetric change rate during phase change process was discussed. The volume changes fast at the initial stage of the phase change process. The value of the volumetric change rate of the phase change material is related to the phase change rate. It is also related to the mass fraction of the liquid phase change material. All the factors affecting the phase change rate will influence the volumetric change rate.The volumetric expansion rate is less than theoretical value under an external pressure. While a high pressure situation is taken into consideration, the numerical model should be modified by adding a function calculating density varying with pressure to ensure that the model operates properly.For a certain volume of phase change material, the output power can be improved by improving Ste number (Ste ? c p?T /L), Bi number( Bi ? hl /?) and reducing the radius of the cylinder container. According to the curve of the volumetric change rate, the optimal time and volumetric change rate of the energy storage material can be determined. It can improve the comprehensive performance of the power device when the glider is operating with the optimal volumetric change rate.Secondly, the phase change process of the power system was studied when the glider was operating within the ocean thermocline by numerical simulation method, attending the results of the volumetric change rate, the time of staying at the cold and warm water layer, and the valve timing control method of the power system. Several methods to improve the output work of the glider were attained by studying the gliding depth optimization, the cycle time optimization and the variation of the output work.The working process was analyzed and optimized when the thermal glider working within the ocean thermocline near the equator. After optimization the total time of a cycle decreases 30% and the gliding depth decreases 40%. The cycle time and depth can be decreased when the phase change material is partly melted when the volumetric expansion rate is less than 100%. The rest of the material can be set aside for the compensation of the energy loss caused by friction.Thirdly, the working process of the glider was studied in this thesis when the glider was operating within the shallow-water ocean thermocline and the deep-water ocean thermoclien. And the effects of the thickness and the strength of the thermocline on the gliding performance were attained.For the shallow-water ocean thermocline, the upper and lower bound of the thermocline influence the working process. The higher the warm water temperature and the lower the cold temperature, the faster the phase change rate, and the shorter the glider stays on both the warm and the cold water layer.For the deep-water ocean thermocline, the intensity and the upper bound temperature of the thermocline influence the working process. The stronger the intensity of the thermocline, the lower the gliding depth is and the shorter the cycle time needs. The lower the temperature of the top layer of the thermocline, the longer the glider stays in the warm water layer.Lastly, the relationship among the volumetric change rate, gliding speed and the attack angle was studied in this thesis by the method of combining working performance of the power system with the hydrodynamic performance of the glider. The balancing relationship between the output work of the power system and the gliding drag was analyzed and the methods to improve the comprehensive performance of the glider were put forward.The hydrodynamic parameters of the glider will influence the gliding attitude and the design of the power system. When the glider operate at different speed and glide angle, the net buoyancy needed increases with the attack angle. So is the volumetric change rate of the phase change material. For a certain ocean thermocline, the gliding depth, glide angle, gliding speed and the total cycle time increase accordingly. The design of the glider should be based on specific task and the gliding path should be optimized according to the parameters such as the gliding speed, distance, depth and load etc.The working process and the influencing factors of the power device of the thermal glider were presented in this thesis. Several methods to improve the output work of the power device were put forword. According to the actually measured temperature profile of the ocean thermocline the underwater working process can be simulated. By the simulation results, the time of the thermal glider staying in the warm water and cold water can be suitable selected. So the open-close timing of the valve in the power system can be pricesly controlled. The gliding attitude can be promptly adjusted to ensure the glider operateing stably in the ocean for the long endurance. The factors influencing the performance of the whole glider were also analyzed by combining the hydrodynamic parameters and the working characteristics of the power device. It provides theoretical fundament for suitably selecting the volumetric change rate of the phase change material, improving the performance and optimal design of the whole glide.