Study On Liquid-solid Flow And Heat Transfer Enhancement At The Bottom Of Medium-deep Geothermal Wells

The medium-deep geothermal energy refers to the energy stored in the rocky soil thousands of meters underground.It has the characteristics of high temperature and high heat flux density.At present,it has become the key development direction of the 13 th Five-Year Plan for the Development and Utilization of Geothermal Energy.As one of the main forms of medium-deep geothermal wells,casing heat exchangers often cause blockage problems due to the deposition of solid particles.This situation is more serious in the middle and deep geothermal wells with a depth of two or three kilometers.To solve this problem,an open-hole casing structure is proposed.At the same time,due to the high temperature of the middle and deep geothermal bottom,the enhanced bottom heat transfer can effectively improve the heat transfer performance of the casing structure.At present,the research on the above problems has not been found yet.Therefore,this paper studies the liquid-solid two-phase flow characteristics and enhanced heat transfer in the open-hole casing.Firstly,a flow visualization experimental device for open-hole casing heat exchanger was established.The flow law was studied by colored tracer method.The effects of operating parameters and physical parameters on fluid flow structure and residence time were analyzed.The results show that there is a recirculation zone near the opening,and the fluid near the recirculation zone is more likely to flow into the inner casing.For the open-hole casing structure distributed longitudinally on both sides,the recirculation zone in the pipe is also arranged longitudinally.When the inlet flow is reduced and the outer casing diameter is increased,the flow rate in the pipe is reduced.The range of the recirculation zone will also decrease.The study of fluid flow law can provide guidance for liquid-solid two-phase flow and enhanced heat transfer research.Secondly,a three-dimensional physical model was established based on the middle-deep geothermal wells in the actual project.The liquid-solid two-phase flow characteristics in the open-hole casing were studied by DPM and double-Eulerian methods respectively.The effects of incident position,particle size and inflow velocity on the residence time and deposition rate of the particles were mainly studied.Studies have shown that the closer the incident position is to the small hole,the easier the particles are carried out of the casing by the fluid.When the incident position changes along the diameter,the deposition rate of the particles is basically unchanged.Increasing the inlet velocity can reduce the deposition rate of the particles.At a speed of 0.8 m/s,the deposition rate is only 13%.The larger the particle size,the easier it is to deposit at the bottom of the well.When the particle size is 5 mm,the deposition rate can reach 74%.When the deposition of particles at the bottom of the well is higher than the bottom of the inner casing,the higher the deposition height,the easier it is to block the casing structure.Since the incidentposition,particle size and inflow velocity affect the deposition of the particles,the opening structure needs to be optimized in order to reduce the deposition rate of the particles.Finally,a three-dimensional physical model with a radius of three meters underground is established.The heat transfer model is established by coupling the casing heat exchanger with the geothermal permeability.The results show that:(1)Considering the problem of solid phase particle deposition and enhanced heat transfer,the open-cell structure on both sides is optimized to spiral opening.The heat transfer performance of the spiral opening method is better than that of the two sides,and the heat exchange per unit length can be increased by 25.6%.The outer sleeve thread groove structure has better heat transfer performance than the smooth sleeve.The heat exchange per unit length can be increased by 46.9%.Properly increasing the diameter of the outer casing is conducive to improving the heat transfer performance of the geothermal well.(2)Increasing the inlet flow rate can increase the heat exchange amount.However,excessive flow rate results in insufficient heat transfer and increased flow resistance.In order to ensure a large temperature difference between the inlet and outlet and does not affect the overall heat exchange rate,a flow rate of 0.6-0.8 m/s is usually employed.Increasing the inlet temperature will reduce the heat transfer efficiency of the geothermal well.The thermal conductivity of cementing cement and the specific heat of geotechnical cement are two important indicators for the environment in which the buried management wants to lay.Increasing their parameters can improve the heat transfer performance of the system.