| The ground source heat pump system is a type of environmentally friendly and efficient building service system.It absorbs the shallow geothermal energy to supply cold and heat to the building,which achieves through the ground heat exchanger(GHE)system.However,the initial cost and energy efficiency of this type of system is still limited by the heat transfer efficiency of GHEs.Nanofluid,as a new type of heat transfer fluid,can enhance the heat transfer of GHEs,while its higher viscosity will also limit the improvement of system energy efficiency.Therefore,it is necessary to study the heat transfer of nanofluid-enhanced GHEs to optimize its performance.In this paper,based on the research gap of nanofluid-enhanced GHEs,experimental and numerical investigation were conducted.A GHE experimental system using nanofluid and pure water as heat transfer fluid was set up,and the differences in performance between the two fluids were compared.Compared to pure water,nanofluid had a 39.84% higher heat transfer rate,16.6% higher pumping power consumption and20.2% higher energy efficiency.Meanwhile,the thermal performance of the nanofluid in the elbow segment and the straight tube segment was compared.The result showed that the thermal performance of the nanofluid in the elbow segment was higher than that in the straight tube segment.In the long straight tube segment,the heat transfer enhancement of the nanofluid gradually decreased along the axis direction.In this paper,a discrete phase model of nanofluid flow in a GHE was established based on experiments,and it was validated using the previous correlation.The discrepancy did not exceed 6.3%.The radial velocity of the nanoparticles in the elbow and the straight tube segment was also analyzed using the numerical model,and the difference of nanofluid heat transfer inside the elbow and the straight tube segment was explained.Furthermore,the previous literature showed that the nanofluids have an insignificant effect(less than 5%)on the energy efficiency of GHEs.This paper shows the reason and solutions for this phenomenon.In addition,a single phase numerical model was established based on the experimental system,and it was validated using experimental data.The discrepancy was not more than 4.47%.The effects of nanoparticle size and sphericity on the performance of GHEs were studied using this model.The 5 nm and 50 nm particles had a performance efficiency coefficient below 1 and therefore,it is not recommended to use these two sizes of particles for nanofluids in GHEs.Moreover,40 nm particles had the highest performance efficiency coefficient and therefore,40 nm was the optimal particle size.Meanwhile,the spherical particles had a 7.5% lower pressure drop,0.19% higher heat transfer rate and 8.55% higher energy efficiency than that of rod-shaped particles.The finding proved that nanofluids can enhance the heat transfer of GHEs.The measures to enhance the heat transfer ability of nanofluids in GHEs were also found.Meanwhile,the effects of nanoparticle size and sphericity on the heat transfer of GHEs were studied.The results will contribute to the design of GHE systems using nanofluids. |
