| As a basic requirement for the development of human society,the demand for energy is continuing to increase along with the economic prosperity of human society and the development of the population.In today’s world energy structure,traditional fossil energy still occupies a major position,but the excessive exploitation of traditional fossil energy,which generates environmental pollution,is unsustainable.And for the future of human society,the use of clean,renewable energy is an important turning point in the history of human energy.Geothermal energy resources,characterized by cleanliness,renewability and huge reserves,can provide energy on a sustainable basis.Among them,the quality of middle and deep geothermal energy is higher and the development is simpler than that of deep geothermal energy,and our country requires that the principle of"taking heat but not water"be adopted in the development of geothermal energy.Therefore,the middle and deep geothermal energy resources can be extracted through coaxial geothermal heat exchangers in a sealed manner.Due to the closed operation,the heat transfer performance cannot be improved by means of dry-heat-rock development such as hydraulic fracturing.In the closed operation of the base if there is a need to strengthen its heat transfer effect,can only be used to strengthen the heat transfer technology for its transformation.This paper focuses on the efficient development and utilisation of medium-depth coaxial geothermal heat exchanger in the development of medium-depth geothermal energy under the principle of"taking heat but not water"as the research background,proposes a new type of medium-depth coaxial geothermal heat exchanger structure,and carries out the research of medium-depth coaxial geothermal heat exchanger heat transfer performance and enhancement mechanism.Firstly,the coaxial geothermal heat exchanger project in the middle and deep layers of Songyuan was used as a prototype,and the indoor experimental platform of coaxial geothermal heat exchanger was designed and fabricated based on the similarity theory to clarify the similarity multiplier.Indoor experiments on convective heat transfer in coaxial geothermal heat exchangers with spiral finned inner tubes were carried out to determine the changes in heat transfer performance,pipe pressure drop and power consumption.The experimental results show that the temperature difference between the inlet and outlet of the coaxial geothermal heat exchanger assembled with different helical finned inner tubes and the heat exchanged per unit time are greater than that of smooth inner tubes under static condition,and its increase is 13.2-32.1%;with the increase of the width of the helical fins,the greater the pressure drop is,and its increase is 2.67-14.7 times as much as that of the smooth inner tubes;and the experimental power consumption of the pump is increased with the increase of the width of the helical fins,and its magnitude is 0.2-9.5%.The power consumption of the experimental pump increases with the increase of the width of the spiral fins,and the range is 0.2-9.5%.When rotating clockwise,each spiral finned tube heat transfer with the increase in rotational speed and reduce,but reduce the magnitude of small;clockwise rotation,due to the favorable fluid through the pipeline pressure drop with the increase in rotational speed and reduce the average reduction in the magnitude of about 10%;clockwise rotation speed increases will reduce the power consumption of the pump,but the faster the rotation of the motor power required to lead to the experimental increase in total power consumption.Secondly,based on the results of indoor experiments and CFD numerical simulation,the heat transfer performance and flow characteristics of coaxial geothermal heat exchangers with different geometrical parameters of helical fins and different rotating conditions are investigated,so as to obtain the friction criterion equations describing the heat transfer and flow of medium-depth coaxial heat exchangers with inner tubes assembled with helical fins in the static and rotating conditions,and to reveal the mechanism for enhancement of the heat transfer of helical finned inner tubes.Numerical simulation results for static conditions show that the use of spiral finned inner tubes can change the configuration,velocity field and thermal field of the fluid flow inside the coaxial geothermal heat exchanger,improve the mixing degree between the near-wall fluid and the internal fluid,and promote the heat exchange efficiency between the fluid and the wall.Compared with the smooth inner tube,the Nu and f of the coaxial geothermal heat exchanger were significantly improved by using spiral fins in the ranges of 53-211%and 108-1033%,respectively.In addition,increasing the width ratio a and thickness ratio b as well as decreasing the pitch ratio S can lead to an improvement of the coaxial geothermal heat exchanger’s heat transmission property,but too low a pitch ratio reduces the overall enhancement effect.The established criterion equations of heat transmission and flowing friction for coaxial geothermal heat exchangers with spiral finned inner tubes for static conditions are the descriptions for thermal and flowing friction properties in medium-depth coaxial geothermal heat exchangers under deep（>1000m）geothermal conditions in Songyuan heating project.And the numerical simulation results of different rotational conditions show that clockwise rotation decreases Nu,while counterclockwise rotation increases Nu,but the effect of both increases and decreases are small.The friction coefficient f of each width H is always greater than the static friction coefficient f when rotating clockwise,and the friction coefficient f of each width H increases with the increase of clockwise rotation speed.While the friction coefficient f of each width H is always lower than the static friction coefficient f when rotating counterclockwise,and the friction coefficient f of each width decreases with the increase of counterclockwise rotation speed.The PEC of each width H decreases with the increase of clockwise rotation speed and increases with the increase of counterclockwise rotation speed,and there is a maximum PEC=2.097 when H=11mm and rpm=120.The heat transfer and flow friction criterion equations for coaxial geothermal heat exchangers with spiral finned inner tubes were also developed for clockwise and counterclockwise rotation,respectively.Then,based on the numerical simulation results,according to the 2nd law of pyrodynamics,the entropy production,thermodynamic irreversibility and exergy analysis of coaxial geothermal heat exchanger with spiral finned inner tubes are carried out.The findings reveal that the spiral finned inner tube under static condition effectively changes the fluid flow pattern and promotes the fluid mixing resulting in the heat transfer entropy yield significantly lower than that of the smooth inner tube,the lowest is 67.9%lower than that of the smooth inner tube,but the friction entropy yield has been significantly improved,the highest is 10.33 times higher than that of the smooth inner tube,and according to the value of Be,it is known that 99%of the friction irreversibility is achieved when Re>30,000.While in the rotating condition,the heat transfer irreversibility of different widths H decreases with increasing counterclockwise rotational speed and increases with clockwise rotational speed,indicating that counterclockwise rotation,which reduces the fluid temperature gradient inside the coaxial geothermal heat exchanger,promotes the mixing of fluids,but the effect is not obvious;and frictional entropy production rate increases with clockwise rotational speed and decreases with counterclockwise rotational speed.Increasing the frictional entropy production rate with Re both will provide the main entropy production rate.Increasing the width H and thickness of the spiral fins and decreasing the pitch P both increase the entropy generation rate,but changes in width and thickness have a small effect on entropy,while changes in pitch P have a huge effect on entropy.In addition,the change in entropy produced by different widths H under different rotational conditions has a greater effect on entropy than the change in width itself,and increases with increasing clockwise rotational speed and decreases with increasing counterclockwise rotational speed.Based on the heat transfer improvement numberthat synthesizes the first and second laws of thermodynamics,the direction of energy-saving improvement in the geometric parameters of spiral fins should be to increase the width H,thickness W and pitch P.The effect of rotating condition onis significantly higher than the effect on Nu.Therefore,considering the changes of*and heat transfer improvement numberat the same time,counterclockwise high-speed rotation should be chosen as much as possible when rotation is needed.The change rule of fire loss in each working condition of coaxial geothermal heat exchanger is consistent with the total entropy production rate.Finally,the heat transfer effect of coaxial geothermal heat exchanger with spiral finned inner tube under the influence of geotechnical heat storage is also considered.Through the establishment of coaxial geothermal heat exchanger geotechnical thermal storage model for heat transfer performance analysis,the results show that the use of spiral finned tube coaxial geothermal heat exchanger heat transfer effect is higher than the smooth tube coaxial geothermal heat exchanger,with a pitch of 200mm number of3 spiral finned tube coaxial geothermal heat exchanger,for example,the cumulative heat extraction is 13.4%higher than that of the smooth tube.Meanwhile,the outlet temperature and cumulative heat extraction both increase with the increase in the number of helical fins and the increase in the pitch,indicating that increasing the length of the presence of helical fins in the annular basin can effectively improve the heat transfer effect.And when the flow rate increases it will lead to a decrease in the outlet temperature and a decrease in the product of the difference between the inlet and outlet temperatures and the flow rate,resulting in a decrease in the cumulative heat extraction. |
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