New Coaxial Borehole Heat Exchanger Enhances Heat Transfer Mechanism Research And Structural Optimization

As a renewable energy source,geothermal resources have the characteristics of wide distribution and abundant resources.Coaxial borehole geothermal energy mining technology is an effective way for medium and deep geothermal energy mining and is widely used.However,the current research on coaxial borehole heat exchangers mainly focuses on the structure,operation method,formation parameter change and production application of heat exchangers,and there are few studies on improving the heat recovery capacity of coaxial borehole heat exchangers through structural optimization design and enhanced heat transfer research.Therefore,In this paper,by analyzing the enhanced heat transfer theory,three new coaxial borehole heat exchanger structures are proposed,and the enhanced heat transfer mechanism is studied by numerical simulation technology,and the influence of process parameters and structural parameters on the heat transfer performance of the new coaxial borehole heat exchanger is analyzed,and the heat exchanger structure is optimized by genetic algorithm to form a complete set of heat exchanger design optimization process scheme.The main work of the thesis is as follows:Firstly,the research status of coaxial borehole heat exchanger and enhanced heat transfer is expounded,the heat recovery process of coaxial borehole heat exchanger is analyzed,and based on the theory of enhanced heat transfer,three new coaxial borehole heat exchanger structures with vortex generators and their enhanced heat transfer evaluation indicators are proposed,and the grid independent analysis is carried out by establishing a three-dimensional finite element fluid-structure interaction analysis model,and the reliability of the model is verified by empirical formula.Secondly,in order to obtain a new structure with good mining effect,the influence of vortex generator on the heat transfer performance of coaxial borehole heat exchanger was studied based on the analysis model and finite element simulation calculation method.The turbulent flow energy,temperature field,velocity field and field synergy angle of the three heat exchangers under different inlet flow velocities were compared and analyzed,and the influence of vortex generators on the enhanced heat transfer parameters was revealed,and a new coaxial borehole heat exchanger with spiral fin(TVG)with better heat exchange performance was obtained,and its PEC value reached 1.1,the production temperature of the smooth tube heat exchanger was increased by 24.06%,and the heat recovery power was increased by 11.93%,which was more suitable for geothermal mining.Thirdly,the heat transfer performance of TVG coaxial borehole heat exchanger with good heat exchange performance is studied,and the influence of process parameters and structural parameters on heat transfer performance is analyzed by finite element analysis method,and the influence law of process parameters and structural parameters on the heat transfer performance of the new coaxial borehole heat exchanger is obtained,which provides a theoretical basis for structural design and process parameter design.Finally,in order to study the structural optimization of coaxial borehole heat exchanger,a multi-parameter optimization method of orthogonal test matrix analysis method,BP neural network algorithm and genetic algorithm is proposed,a multi-parameter optimization model is established,the main influencing factors of the new coaxial borehole heat exchanger and the primary and secondary order of each factor are explored,and the optimal solution and optimal target value of the parameters are obtained in the global range: the wing height is 20 mm,the pitch is 153.6mm,the number of fins is 3,the number of vortex generators is 3,The distance from the bottom of the well was 668.3mm,and the optimal target result was predicted: PEC=1.4.A rapid design method for efficient coaxial borehole heat exchangers was obtained.The research results can not only lay a theoretical foundation for the performance evaluation and optimal design of coaxial borehole heat exchanger,but also have great significance for improving the mining efficiency of geothermal energy.