Theoretical Analysis And Experimental Research Of The Thermal Engine For Underwater Gliders

Traditional underwater gliders mostly use batteries as energy source.Limited energy on board becomes the main constraint for further development and application of underwater gliders.Through harvesting ocean thermal energy as its supplementary energy,using the environmental energy for underwater gliders can improve its cruising range.This thesis takes the thermal engine for underwater glider application as the research object.Focusing on energy collection and effective usage of the thermal engine,theoretical analysis and experimental research are conducted.The research work provides important theoretical guidance for the structure design and parameter configuration of the thermal engine.This thesis firstly establishes a numerical method for phase change problem under the framework of phase-change heat transfer theory.Under the framework of effective specific heat method,a numerical method for the phase change process of the thermal engine is established by applying the finite difference method.The established method is validated by comparing with other theoretical numerical method and with experimental results in lab.For the features of the thermal engine,the effect of ambient pressure on the physical properties of PCM is included in the numerical method.Secondly,by analyzing the working environment and technical requirements of used PCM,a thermal engine for underwater gliders is designed.On that basis,energy balance equations are established,and performance evaluation indexes are obtained and analyzed.By considering the effect of compressibility by air and structure parameters on the phase change process,a non-linear model for the phase-change energy storage of thermal engine is established.Then the effects of air compressibility and structure parameters on stored energy are analyzed comprehensively.At last,a method of parameter configuration of the thermal engine for underwater gliders is proposed and validated by sea trials.For the traditional working mode,drifting time on sea surface and net buoyancy are optimized;for the proposed working mode,numerical simulations are conducted in the warm water layer and cold water layer respectively.Main conclusions of the thesis are as follows: requirement for the working sea area is that the surface water temperature should be higher than the melting temperature while the deep sea water temperature should be lower.Technical requirements for used PCM include suitable melting temperature,large density difference of two phases and large bulk modulus of liquid phase.Dissolved air and residual air in the system can decrease stored energy greatly.During the melting process,existed air can decrease the melting rate.Through configuring reasonably the working depth,pitch angle and drifting time,the phase change process and gliding motion can be matched.The working parameters of thermal underwater glider should be stable,avoiding too frequent adjusting during its work.Though using thermal engine as the main energy source and taking the hydraulic pump as auxiliary for buoyancy adjusting,this hybrid buoyancy adjusting strategy can improve the adaptability of the thermal underwater glider to environment remarkably.The research work of the thesis improves the accuracy of the numerical method for the phase change process of thermal engine.Dynamic analysis provides important theoretical basis for improving stored energy of each cycle;and the proposed working mode provides reference for the working parameter configuration of thermal underwater gliders.