纳米流体地热循环换热实验研究

Experimental study of recirculating heat transfer in geothermal wells with nanofluids

提升换热介质的换热性能是高效开采地热资源的有效手段之一。添加纳米级金属氧化物可有效提升流体的换热能力,而纳米颗粒种类、质量分数、粒径、分散剂质量分数等物性参数以及流速对纳米流体换热性能具有重要影响。采用球形纳米CuO和Al_(2)O_(3)(粒径20~50 nm)作为换热介质,十二烷基苯磺酸钠(SDBS)作为分散剂配制纳米流体,利用自主搭建的纳米流体基础换热实验装置进行室内换热实验,优选纳米流体参数。此外,通过自主搭建循环流动换热实验装置,以湖北英山某水热型地热井中地热水作为热源,讨论了在现场实际热源边界条件下,流速对纳米流体和去离子水的换热性能影响规律。结果表明:(1)CuO纳米流体换热性能优于Al_(2)O_(3)纳米流体;(2)纳米流体的换热性能与纳米颗粒质量分数呈负相关关系,CuO质量分数为1%时纳米流体升温效率最高,在150 s内温度可由25℃上升到79.2℃,同时间内比去离子水高4.1℃,同时,随着纳米颗粒质量分数的增加,纳米流体与热源界面的润湿性减小;(3)纳米流体换热性能随着纳米颗粒粒径增加呈现先增加后减小的趋势,在纳米颗粒粒径为40 nm时纳米流体换热性能最佳;(4)纳米流体的换热性能与分散剂质量分数呈负相关关系,当分散剂质量分数为1%时换热性能最佳;(5)层流状态下纳米流体的换热性能与流速呈负相关关系;在湍流状态下纳米颗粒运动状态逐渐剧烈,有利于纳米流体传热。研究成果可为纳米流体应用于地热换热从而提升地热系统的换热效率提供依据,并为纳米流体参数以及流速参数的选择提供理论依据。

[Objective]Enhancing the heat transfer performance of heat transfer media is an effective means of efficiently exploiting geothermal resources.Numerous studies have shown that the addition of nanoscale metals or metal oxides to fluids can effectively improve the heat transfer capacity of the fluid.The physical parameters that can impact the heat transfer performance of nanofluids are type,mass fraction,size of the nanoparticle,dispersant mass fraction.Additionally,the flow rate can have an important effect on the heat transfer performance of nanofluids.[Methods]In this study,spherical nano-CuO and spherical nano-Al_(2)O_(3) were used as nanomaterials for configuring nanofluids.The particle size of nanomaterials ranges from 20 nm to 50 nm.Sodium dodecylbenzene sulfonate was selected as the dispersant for configuring the nanofluids.Basic heat transfer experiments are performed on nanofluids by utilizing a self-constructed basic heat transfer experimental setup.The physical parameters of the nanofluids were also optimized.In addition,a self-designed experimental setup for recirculating heat exchange was established.This experimental system uses geothermal water from hydrothermal geothermal wells as the heat source.The experimental system was also utilized for field testing in a hydrothermal-type geothermal well in Yingshan County,Hubei Province.The preferred nanofluid and deionized water from the basic heat transfer experiments were subjected to on-site circulating heat transfer experiments.Comparison of the circulating heat transfer performance of nanofluids and water under actual heat source conditions in the field.The effect of the flow rate on the heat transfer performance of nanofluids and water under real heat source boundary conditions in the field is also discussed.[Results]The results show that(1)the heat transfer performance of CuO nanofluids is better than that of Al_(2)O_(3) nanofluids.(2)There is a negative correlation between the heat transfer performance of nanofluids and the nanoparticle mass fraction.(3)The nanofluid warming efficiency was highest at a 1%mass fraction of CuO nanoparticles.The nanofluid temperature increased from 25℃to 79.2℃in 150 s.The nanofluid temperature increased by 4.1℃more than that of deionized water in the same amount of time.Moreover,the wettability of the nanofluid-heat source interface decreases with increasing nanoparticle mass fraction.The heat transfer performance of nanofluids increases and then decreases with increasing particle size.The best heat transfer performance of the nanofluid was achieved when the nanoparticle size was 40 nm.(4)The heat transfer performance of nanofluids is negatively correlated with the dispersant mass fraction.The best heat transfer performance of the nanofluid was achieved when the dispersant mass fraction was 1%.(5)The heat transfer performance of the nanofluid is negatively correlated with the flow rate when the fluid is in laminar flow.The motion of nanoparticles is progressively more intense when the fluid is in a turbulent state.This phenomenon can effectively enhance the heat transfer performance of nanofluids.[Conclusion]The research results can provide a basis for the application of nanofluids in geothermal heat transfer to improve the heat transfer efficiency of geothermal systems.It also provides theoretical references for the selection of nanofluid parameters as well as fluid flow rate parameters applied to geothermal heat transfer.