Optimization Of Heat Transfer Performance Of Medium-deep Casing Type Buried Pipe In Severe Cold Region

Energy shortage and global warming due to carbon emissions are common problems faced by all countries in the world.Renewable energy application is an important way to reduce energy consumption and lower carbon emissions in buildings.Medium-deep geothermal energy is a stable,large storage capacity,clean and efficient renewable energy source,which has received wide attention in the field of heating.Casing type buried pipe is an important way to extract medium-deep geothermal energy in cold areas.Optimization research on the influence factors of casing type buried pipe heat transfer for different regional conditions is of great significance to the development and application of medium-deep geothermal energy.In this paper,based on the climatic and geothermal resource conditions in Daqing area,the numerical heat transfer model of medium-deep casing-type buried pipe heat exchanger is established considering the heat extraction and transformation method of abandoned oil wells,and the model is verified with the experimental measured data for the heat transfer study of buried pipes.Using the established model,the influence law of insulation backfill on the heat transfer performance of cased buried pipe is studied,and the concept of optimal insulation backfill depth is proposed.Using the numerical model established in this paper,orthogonal experiments were designed to study the effects of five heat transfer influencing factors,such as inlet flow rate（A）,inner pipe thermal conductivity（B）,inlet temperature（C）,ground temperature gradient（D）and buried pipe depth（E）,on the heat extraction performance of cased buried pipe and the optimal insulation backfill depth.It was found that for the optimal insulation backfill depth,the influence of each factor was ranked as C>D>B>A>E.Among them,the influence of inlet temperature,inner tube thermal conductivity and ground temperature gradient was significant.For the buried pipe heat extraction performance,the influence of each factor is ranked as D>E>A>B>C,among which inlet flow rate,internal pipe thermal conductivity,ground temperature gradient and buried pipe depth have significant influence,and the optimal combination of factors for heat extraction performance is A₅B₁C₁D₄E₄.Multiple regression analysis is conducted to fit the regression equation for the factors affecting buried pipe heat extraction performance and optimal insulation backfill depth,and the regression equation can be within the range of factor levels The regression equation can predict the change trend of buried pipe heat extraction performance and optimal insulation backfill depth within the level of factors.The numerical model of buried pipe established in this paper is adjusted for long-term heat transfer simulation,and the influence of inlet temperature,flow rate,thermal conductivity of inner pipe,ground temperature gradient,buried pipe depth,insulation backfill depth and insulation backfill thermal conductivity on the long-term heat transfer performance and thermal impact radius of buried pipe is investigated after 5 years of simulated heat transfer duration.Four groups of heat transfer conditions were selected to simulate the change of buried pipe heat extraction performance and soil thermal influence radius after 30 years of heat transfer.It is found that the long-term heat extraction performance of buried pipe is better with insulation backfill than without insulation backfill;the soil temperature can be recovered in stages under the intermittent operation condition,which is beneficial to the long-term stable heat extraction of buried pipe.