Research On Heat Transfer Of Different Nanofluids In Sandbox Buried Tubes

The ground source heat pump（GSHP）system is an energy-saving and environmental friendly system that utilizes renewable energy-shallow geothermal energy,but the initial investment of the GSHP system is too high,especially the cost of the buried tube heat exchanger,affecting its popularization and application.Nanofluid,as a new type of heat transfer medium with high thermal conductivity,embodies good heat transfer enhancement characteristics in the field of shell and plate heat exchangers.The buried tube heat exchanger is the core component of the GSHP system,the convective heat exchange strength of the medium in the tube affects the heat exchange performance of the buried tube heat exchanger.In this paper,nanofluids are combined with buried pipes,which are used to replace water as heat exchange medium in the pipe,in order to improve the heat transfer performance of buried pipes,and to provide the possibility of reducing the drilling depth or length of buried pipe heat exchangers.In this paper,the research is carried out through experiments and numerical simulations.Firstly,a serpentine horizontal buried pipe sandbox experiment platform is built,which contains upper and lower double-layer independent buried pipes to run nanofluid and water respectively,the heat transfer experiment of the two media in the buried tube is carried out at the same time,and the difference in the heat transfer performance of the buried tube is compared.According to the experimental data of constant heating power under the same condition,it is found that the adopted Cu O-water nanofluid gets greater temperature difference between the inlet and outlet of the buried pipe than water,reflecting better heat transfer characteristics.The ratioηof the buried tube heat transfer capacity to the pumping power is used as the evaluation index of the comprehensive heat transfer performance of the buried tube,the experimental data shows that the heat transfer value of the Cu O-water nanofluid with a mass fraction of 3%and a particle size of 30 nm through the buried tube is far exceeding water,the increase rate is close to 35%.The viscosity of the nanofluid is greater than that of water,the operating resistance of the system increases,the pumping power of the nanofluid side increases by approximately 16.4%compared to water.However,the increase rate of the heat transfer value of the nanofluid exceeds the pumping power increase ratio,under the experimental conditions,η_{Cu O} increased by nearly 16%compared toη_water,the use of Cuo-water nanofluid effectively improved the heat transfer performance of horizontal serpentine buried pipes.At the same time,the temperature change of the nanofluid buried tube wall was studied,and it was found that the addition of nanoparticles strengthened the destruction of the secondary flow in the U-shaped bend on the wall boundary layer,and the centrifugal force of the bent pipe strengthened the brownian motion of the nanoparticles.As a result,the convective heat transfer of the nanofluid in the U-shaped pipe section is significantly enhanced,the wall temperature of the U-shaped pipe section has increased but the overall temperature of the tube wall has shown a downward trend.Then a horizontal serpentine buried pipe coupled soil heat transfer model is established,the nanofluid is treated as a single-phase flow for simplified process considering the low concentration of nanofluid used in the experiment,the accuracy of the model is verified by experimental data from the aspects of buried pipe inlet and outlet temperature,sandbox soil temperature,and pipe wall temperature.Combined with the verified numerical model,the simulation conditions are controlled to run continuously for 2 hours,and the effects of nanofluid flow rate,concentration,and inlet temperature on the heat transfer performance of the buried tube are studied respectively.The results show that the flow rate factor has little effect on the heat transfer of the buried nanofluid tube,although the heat transfer rate of the buried nanofluid tube will decrease with the decrease of the flow rate,the pump power required by the buried tube is more affected by the flow rate,the pump power decreases with the decrease of the flow rate far exceeds the reduction rate of the heat exchange of the buried tube,so the comprehensive heat transfer performance of the nanofluid buried tube increases as the flow rate decreases.With the increase of nanofluid concentration,the heat transfer of buried pipes has been improved,at the same time,the viscosity of the nanofluid and the flow resistance increases,since the pump power is more sensitive to the viscosity of the fluid,the pump power increases with the increase of the fluid concentration,which is greater than the increase in the heat transfer of the nanofluid buried tube,resulting in a decrease in the heat transfer performance of the nanofluid buried tube.As the inlet temperature increases,the heat transfer value of the nanofluid buried tube is significantly improved and the pump power is reduced,which in turn leads to a significant increase in the heat transfer performance of the buried tube.Based on factor analysis,the heat transfer performance of five kinds of nanofluids with different nanoparticles in buried pipes was studied,including Cu-water nanofluids,Cu O-water nanofluids,Al--water nanofluids,Al₂O₃-water nanoparticles fluid,Si O₂-water nanofluid.the analysis shows that the comprehensive heat transfer performance coefficient of different types of nanofluids in buried pipes:₄2)>₇)>₇23)>>.Cu-water nanofluid has the largest heat transfer value but also has the largest pressure drop at the inlet and outlet of buried pipe,the heat transfer value of the buried tube of Cu-water nanofluid is lower than that of the rest nanofluids in terms of pumping power increase,resulting in poor overall heat transfer performance of Cu-water nanofluid;the heat transfer effect of Al-water nanofluid and Si O₂-water nanofluid through buried pipes is similar,and the comprehensive heat transfer performance of Si O₂-water nanofluid is the best among five nanofluids.In this paper,combined with experimental research and numerical simulation,the heat transfer performance of Cu O-water nanofluid and water in a horizontal serpentine buried pipe is compared at the same time.The study provides a reference for the subsequent research of nanofluids combined with buried pipes.