土壤蓄热-放热过程中地埋管周围土壤温度特性模拟
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摘要
为探索内蒙中部地区地源热泵蓄热-放热过程中地埋管周围土壤温度变化特性,以垂直U型地埋管周围土壤为研究对象,基于有限元分析法建立了二维非稳态传热物理数学模型。在与试验结果进行验证的基础上,对土壤蓄热、放热和蓄热-放热耦合过程进行模拟研究。分析了热作用半径、单位管长换热量和土壤温度随热泵运行时间及运行模式的变化规律;单一条件下的蓄热、放热以及蓄热-放热耦合模式下土壤热平衡问题,探讨了流体入口流速、温度、土壤类型和热泵运行模式等因素对土壤温度场的影响。研究结果表明:热作用半径随蓄热时间的增加而增大且逐渐趋于平缓,热泵运行25和28 d后,热作用半径分别为3.3和3.4 m;流体入口温度对热作用半径及单位管长换热量影响较大但流体流速影响较小,流体入口温度和速度分别为40、60℃和0.6、1.2 m/s时,对应热作用半径分别为3.7、4.5和3.5、3.6 m。合理的间歇运行模式对换热量及埋管周围土壤温度的恢复均有改善;土壤导热系数越大土壤温度恢复时间与效果越佳,土壤导热系数为3.1 W/(m?K)时恢复后温度为9.3℃(土壤初温9.5℃)。此外,蓄热-放热耦合模式下换热量不等对土壤热平衡具有较大影响。试验验证表明,所建模型具有一定的准确性其相对最大误差为5.35%。
Ground source heat pump(GSHP)has been very popular for space heating and cooling due to its high energy efficiency and low operating cost and thus it is still a very important research subject.Ground heat exchanger is a key component of GSHP.The comprehensive understanding about the heat transfer characteristic of the ground heat exchangers and the soil temperature distribution around the ground heat exchangers is crucial to the performance of GSHP and a large number of researches were carried out,because the operating conditions of GSHP were related closely to the soil temperature field around the ground heat exchangers.Besides,the heat imbalance of GSHP has been become a serious problem,because the amount of heat extracted from and rejected to the soil is usually not equal.It is an especially obvious problem for heating-dominated buildings in the cold and severely cold regions.So,the temperature recovery ability of soil has attracted wide attention.This paper presents the study of the temperature variation characteristics of soil around the ground heat exchangers in the process of heat storage and release of GSHP in the center of Inner Mongolia,China.Based on the finite element method,two-dimensional physical and mathematical models of transient heat transfer were established for the soil around the vertical U-tube ground heat exchanger.The heat storage,heat release and the coupling process were studied on the basis of experimental verification.The variation laws of thermal influencing radius,heat exchange of unit pipe and soil temperature with the operation time and operation mode of GSHP were revealed.The soil heat equilibrium problems in the heat storage,heat release and the coupling process were discussed.The influence of fluid inlet velocity,inlet temperature,soil types and GSHP operation model on the soil temperature field were explored.The results indicated that the thermal influencing radius increased with the increase of operation time and became gentle eventually.With the 25 and 28 days running-time of GSHP,thermal influencing radius is 3.3 and 3.4 m,respectively.The fluid inlet temperature has a great influence on the thermal influencing radius and heat flux of unit pipe,while the fluid inlet velocity has a small impact on these.The fluid inlet temperature and inlet velocity are40,60℃and 0.6,1.2 m/s,respectively.And the corresponding thermal influencing radius is 3.7,4.5 and 3.5,3.6 m.The proper intermittent operation mode could improve the heat transfer rate and the temperature recovery ability of soil around the ground heat exchanger.The recovery time and recovery effect of soil temperature were better with the increase of the thermal conductivity of soil.With the running time of 84 h(heat rejection of 12 hour and recovery of 72 hour),the soil temperature is 9.3℃with a thermal conductivity of 3.1 W/(m?K)(the initial temperature of soil is 9.5℃).In addition,the unequal heat transfer had a great influence on the soil heat balance in the coupling mode of heat storage and release.The experimental validation indicated that enough accuracy could be achieved using the model developed in this study with a maximum difference of 5.35%.
Ground source heat pump(GSHP)has been very popular for space heating and cooling due to its high energy efficiency and low operating cost and thus it is still a very important research subject.Ground heat exchanger is a key component of GSHP.The comprehensive understanding about the heat transfer characteristic of the ground heat exchangers and the soil temperature distribution around the ground heat exchangers is crucial to the performance of GSHP and a large number of researches were carried out,because the operating conditions of GSHP were related closely to the soil temperature field around the ground heat exchangers.Besides,the heat imbalance of GSHP has been become a serious problem,because the amount of heat extracted from and rejected to the soil is usually not equal.It is an especially obvious problem for heating-dominated buildings in the cold and severely cold regions.So,the temperature recovery ability of soil has attracted wide attention.This paper presents the study of the temperature variation characteristics of soil around the ground heat exchangers in the process of heat storage and release of GSHP in the center of Inner Mongolia,China.Based on the finite element method,two-dimensional physical and mathematical models of transient heat transfer were established for the soil around the vertical U-tube ground heat exchanger.The heat storage,heat release and the coupling process were studied on the basis of experimental verification.The variation laws of thermal influencing radius,heat exchange of unit pipe and soil temperature with the operation time and operation mode of GSHP were revealed.The soil heat equilibrium problems in the heat storage,heat release and the coupling process were discussed.The influence of fluid inlet velocity,inlet temperature,soil types and GSHP operation model on the soil temperature field were explored.The results indicated that the thermal influencing radius increased with the increase of operation time and became gentle eventually.With the 25 and 28 days running-time of GSHP,thermal influencing radius is 3.3 and 3.4 m,respectively.The fluid inlet temperature has a great influence on the thermal influencing radius and heat flux of unit pipe,while the fluid inlet velocity has a small impact on these.The fluid inlet temperature and inlet velocity are40,60℃and 0.6,1.2 m/s,respectively.And the corresponding thermal influencing radius is 3.7,4.5 and 3.5,3.6 m.The proper intermittent operation mode could improve the heat transfer rate and the temperature recovery ability of soil around the ground heat exchanger.The recovery time and recovery effect of soil temperature were better with the increase of the thermal conductivity of soil.With the running time of 84 h(heat rejection of 12 hour and recovery of 72 hour),the soil temperature is 9.3℃with a thermal conductivity of 3.1 W/(m?K)(the initial temperature of soil is 9.5℃).In addition,the unequal heat transfer had a great influence on the soil heat balance in the coupling mode of heat storage and release.The experimental validation indicated that enough accuracy could be achieved using the model developed in this study with a maximum difference of 5.35%.
引文
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[10]Bottarelli M,Bortoloni M,Su Y,et al.Numerical analysis of a novel ground heat exchanger coupled with phase change materials[J].Applied Thermal Engineering,2014,88:369-375.
[11]Yang W,Kong L,Chen Y.Numerical evaluation on the effects of soil freezing on underground temperature variations of soil around ground heat exchangers[J].Applied Thermal Engineering,2015,75:259-269.
[12]Lee C K,Lam H N.Computer simulation of borehole ground heat exchangers for geothermal heat pump systems[J].Renewable Energy,2008,33(6):1286-1296.
[13]Han,Chanjuan,Bill,et al.Sensitivity analysis of a vertical geothermal heat pump system[J].Applied Energy,2016,170:148-160.
[14]Carli M D,Fiorenzato S,Zarrella A.Performance of heat pumps with direct expansion in vertical ground heat exchangers in heating mode[J].Energy Conversion&Management,2015,95:120-130.
[15]Liang P,Qi D,Li K,et al.Simulation study on the thermal performance of vertical U-tube heat exchangers for ground source heat pump system[J].Applied Thermal Engineering,2015,79:202-213.
[16]张长兴,郭占军,刘玉峰,等.土壤源热泵系统运行特性的快速预测方法[J].农业工程学报,2012,28(24):173-178.Zhang Changxing,Guo Zhanjun,Liu Yufeng,et al.A fast forecast method for operation characteristics of ground-coupled heat pump system[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE),2012,28(24):173-178.(in Chinese with English abstract)
[17]张长兴,郭占军,刘玉峰,等.模式搜索算法在地埋管换热器热阻确定中的应用[J].农业工程学报,2013,29(21):182-187.Zhang Changxing,Guo Zhanjun,Liu Yufeng,et al.Application of pattern search algorithm for determining heat resistance of ground heat exchanger[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE),2013,29(21):182-187.(in Chinese with English abstract)
[18]杨卫波,施明恒,陈振乾.非连续运行工况下垂直地埋管换热器的换热特性[J].东南大学学报:自然科学版,2013,43(2):328-333.Yang Weibo,Shi Mingheng,Chen Zhenqian.Heat exchange characteristics of vertical U-tube ground heat exchange with dis continuous operation condition[J].Journal of Southeast University:Natural Science Edition,2013,43(2):328-333.(in Chinese with English abstract)
[19]杨昌智,黄兵.U型管换热性能影响因素研究[J].湖南大学学报:自然科学版,2009,36(12):44-48.Yang Changzhi,Huang Bing.Study on influence factor of heat transfer performance of the U-tube[J].Journal of Hunan University:Natural Sciences,2009,36(12):44-48.(in Chinese with English abstract)
[20]李新国,汪洪军,赵军,等.不同回填材料对U型垂直埋管换热性能的影响[J].太阳能学报,2003,24(6):810-813.Li Xinguo,Wang Hongjun,Zhao Jun,et al.Experimental study on the heat exchange performance of U-vertical underground heat exchanger by different back filler materials[J].Acta Energiae Solaris Sinica,2003,24(6):810-813.(in Chinese with English abstract)
[21]李新国,陈志豪,赵军,等.桩埋管与井埋管实验与数值模拟[J].天津大学学报,2005,38(8):679-683.Li Xinguo,Chen Zhihao,Zhao Jun,et al.Experiment and numerical simulation on U-vertical ground coupled heat exchange with sandstone and cement backfills[J].Journal of Tianjin University:Science and Technology,2005,38(8):679-683.(in Chinese with English abstract)
[22]张琳琳,赵蕾,杨柳.管群间歇散热的土壤温度响应与恢复特性[J].浙江大学学报:工学版,2016,50(2):299-305Zhang Linlin,Zhao Lei,Yang Liu.Soil temperature response and recovery characteristics of intermittent heat emission in multi-boreholes[J].Journal of Zhejiang University:Engineering Science,2016,50(2):299-305.(in Chinese with English abstract)
[23]张琳琳,赵蕾,杨柳.渗流作用下垂直埋管换热器钻孔内外耦合传热计算与分析[J].化工学报,2015,66(4):1290-1300.Zhang Linlin,Zhao Lei,Yang Liu.Analyses on heat transfer of borehole heat exchanger in soil with groundwater advection using coupled heat transfer model[J].CIESC Journal,2015,66(4):1290-1300.(in Chinese with English abstract)
[24]Vijay K Mishra,Subhash C Mishra,Dipankar N.Basu.Combined mode conduction and radiation heat transfer in a porous medium and estimation of the optical properties of the porous matrix[J].Speculum,2015,67(10):308-310.
[25]Benazza A,Blanco E,Aichouba M,et al.Numerical Investigation of Horizontal Ground Coupled Heat Exchanger[J].Energy Procedia,2011,6(6):29-35.
[26]Gu Yian,O’Neal Denni L.Development of equivalent diameter expression for vertical U-tubes used in ground-coupled heat pumps[J].ASHRAE Transactions,1998,104(2):347-355.
[27]陈杰,等.MATLAB宝典[M].北京:电子工业出版社,2011.
[28]林瑞泰.多孔介质传热传质引论[M].北京:北京科技出版社,1995.
[29]李人宪.有限体积法基础[M].北京:国防工业出版社,2008
[30]陶文铨.数值传热学[M].第2版.西安:西安交通大学出版社,2001.
[31]高蓬辉,纪绍斌,周国庆,等.地层储放能过程中温度场演化规律的试验研究[J].太阳能学报,2013,34(11):1916-1923.Gao Penghui,Ji Shaobin,Zhou Guoqing,et al.Experimental research on temperature of underground soil in the process of storage and release[J].Acta Energiae Solaris Sinica,2013,34(11):1916-1923.(in Chinese with English abstract)
[32]张永志,顾洁.内蒙古中部地区地源热泵单位井深换热量设计的研究[J].建筑节能,2012,40(256):24-25,32.Zhang Yongzhi,Gu Jie.Designing of average quantity of heat per meter for ground source heat pump at central Inner Mongolia[J].Building Energy Efficiency,2012,40(256):24-25,32.(in Chinese with English abstract)
[2]余鑫,王如竹,翟晓强.竖直埋管地源热泵系统研究进展[J]暖通空调,2010,40(2):1-9.Yu Xin,Wang Ruzgu,Zhai Xiaoqiang.Progresses in research of vertical pipe ground-surce heat pump systems[J].Heating Ventilating&Air Conditioning,2010,40(2):1-9.(in Chinese with English abstract)
[3]Yang W,Zhou J,Xu W,et al.Current status of ground-source heat pumps in China[J].Energy Policy,2010,38(1):323-332.
[4]王俊清,袁艳平,曹晓玲等.基于混合解换热模型的地源热泵系统井群热干扰特性[J].农业工程学报,2016,32(10):194-200.Wang Junqing,Yuan Yanping,Cao Xiaoling,et al.Thermal interference characteristics of wells in ground source heat pump system based on analytical and numerical calculation of mixed solution[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE),2016,32(10):194-200.(in Chinese with English abstract)
[5]Chen J,Xia L,Li B,et al.Simulation and experimental analysis of optimal buried depth of the vertical U-tube ground heat exchanger for a ground-coupled heat pump system[J].Renewable Energy,2015,73:46-54.
[6]Ma W W,Li M,Li P,et al.New quasi-3D model for heat transfer in U-shaped GHEs(ground heat exchangers):Effective overall thermal resistance[J].Energy,2015,90:578-587.
[7]Li S,Dong K,Wang J,et al.Long term coupled simulation for ground source heat pump and underground heat exchangers[J].Energy&Buildings,2015,106:13-22.
[8]白莉,王有镗,高青,等.地下换热管土结构冻胀变形模拟[J].农业工程学报,2016,32(18):118-124.Bai Li,Wang Youtang,Gao Qing,et al.Simulation on underground pipe-soil heat exchange structure deformation due to frost heave[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE),2016,32(18):118-124.(in Chinese with English abstract)
[9]Dai L H,Shang Y,Li X L,et al.Analysis on the transient heat transfer process inside and outside the borehole for a vertical U-tube ground heat exchanger under short-term heat storage[J].Renewable Energy,2015,87:1121-1129.
[10]Bottarelli M,Bortoloni M,Su Y,et al.Numerical analysis of a novel ground heat exchanger coupled with phase change materials[J].Applied Thermal Engineering,2014,88:369-375.
[11]Yang W,Kong L,Chen Y.Numerical evaluation on the effects of soil freezing on underground temperature variations of soil around ground heat exchangers[J].Applied Thermal Engineering,2015,75:259-269.
[12]Lee C K,Lam H N.Computer simulation of borehole ground heat exchangers for geothermal heat pump systems[J].Renewable Energy,2008,33(6):1286-1296.
[13]Han,Chanjuan,Bill,et al.Sensitivity analysis of a vertical geothermal heat pump system[J].Applied Energy,2016,170:148-160.
[14]Carli M D,Fiorenzato S,Zarrella A.Performance of heat pumps with direct expansion in vertical ground heat exchangers in heating mode[J].Energy Conversion&Management,2015,95:120-130.
[15]Liang P,Qi D,Li K,et al.Simulation study on the thermal performance of vertical U-tube heat exchangers for ground source heat pump system[J].Applied Thermal Engineering,2015,79:202-213.
[16]张长兴,郭占军,刘玉峰,等.土壤源热泵系统运行特性的快速预测方法[J].农业工程学报,2012,28(24):173-178.Zhang Changxing,Guo Zhanjun,Liu Yufeng,et al.A fast forecast method for operation characteristics of ground-coupled heat pump system[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE),2012,28(24):173-178.(in Chinese with English abstract)
[17]张长兴,郭占军,刘玉峰,等.模式搜索算法在地埋管换热器热阻确定中的应用[J].农业工程学报,2013,29(21):182-187.Zhang Changxing,Guo Zhanjun,Liu Yufeng,et al.Application of pattern search algorithm for determining heat resistance of ground heat exchanger[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE),2013,29(21):182-187.(in Chinese with English abstract)
[18]杨卫波,施明恒,陈振乾.非连续运行工况下垂直地埋管换热器的换热特性[J].东南大学学报:自然科学版,2013,43(2):328-333.Yang Weibo,Shi Mingheng,Chen Zhenqian.Heat exchange characteristics of vertical U-tube ground heat exchange with dis continuous operation condition[J].Journal of Southeast University:Natural Science Edition,2013,43(2):328-333.(in Chinese with English abstract)
[19]杨昌智,黄兵.U型管换热性能影响因素研究[J].湖南大学学报:自然科学版,2009,36(12):44-48.Yang Changzhi,Huang Bing.Study on influence factor of heat transfer performance of the U-tube[J].Journal of Hunan University:Natural Sciences,2009,36(12):44-48.(in Chinese with English abstract)
[20]李新国,汪洪军,赵军,等.不同回填材料对U型垂直埋管换热性能的影响[J].太阳能学报,2003,24(6):810-813.Li Xinguo,Wang Hongjun,Zhao Jun,et al.Experimental study on the heat exchange performance of U-vertical underground heat exchanger by different back filler materials[J].Acta Energiae Solaris Sinica,2003,24(6):810-813.(in Chinese with English abstract)
[21]李新国,陈志豪,赵军,等.桩埋管与井埋管实验与数值模拟[J].天津大学学报,2005,38(8):679-683.Li Xinguo,Chen Zhihao,Zhao Jun,et al.Experiment and numerical simulation on U-vertical ground coupled heat exchange with sandstone and cement backfills[J].Journal of Tianjin University:Science and Technology,2005,38(8):679-683.(in Chinese with English abstract)
[22]张琳琳,赵蕾,杨柳.管群间歇散热的土壤温度响应与恢复特性[J].浙江大学学报:工学版,2016,50(2):299-305Zhang Linlin,Zhao Lei,Yang Liu.Soil temperature response and recovery characteristics of intermittent heat emission in multi-boreholes[J].Journal of Zhejiang University:Engineering Science,2016,50(2):299-305.(in Chinese with English abstract)
[23]张琳琳,赵蕾,杨柳.渗流作用下垂直埋管换热器钻孔内外耦合传热计算与分析[J].化工学报,2015,66(4):1290-1300.Zhang Linlin,Zhao Lei,Yang Liu.Analyses on heat transfer of borehole heat exchanger in soil with groundwater advection using coupled heat transfer model[J].CIESC Journal,2015,66(4):1290-1300.(in Chinese with English abstract)
[24]Vijay K Mishra,Subhash C Mishra,Dipankar N.Basu.Combined mode conduction and radiation heat transfer in a porous medium and estimation of the optical properties of the porous matrix[J].Speculum,2015,67(10):308-310.
[25]Benazza A,Blanco E,Aichouba M,et al.Numerical Investigation of Horizontal Ground Coupled Heat Exchanger[J].Energy Procedia,2011,6(6):29-35.
[26]Gu Yian,O’Neal Denni L.Development of equivalent diameter expression for vertical U-tubes used in ground-coupled heat pumps[J].ASHRAE Transactions,1998,104(2):347-355.
[27]陈杰,等.MATLAB宝典[M].北京:电子工业出版社,2011.
[28]林瑞泰.多孔介质传热传质引论[M].北京:北京科技出版社,1995.
[29]李人宪.有限体积法基础[M].北京:国防工业出版社,2008
[30]陶文铨.数值传热学[M].第2版.西安:西安交通大学出版社,2001.
[31]高蓬辉,纪绍斌,周国庆,等.地层储放能过程中温度场演化规律的试验研究[J].太阳能学报,2013,34(11):1916-1923.Gao Penghui,Ji Shaobin,Zhou Guoqing,et al.Experimental research on temperature of underground soil in the process of storage and release[J].Acta Energiae Solaris Sinica,2013,34(11):1916-1923.(in Chinese with English abstract)
[32]张永志,顾洁.内蒙古中部地区地源热泵单位井深换热量设计的研究[J].建筑节能,2012,40(256):24-25,32.Zhang Yongzhi,Gu Jie.Designing of average quantity of heat per meter for ground source heat pump at central Inner Mongolia[J].Building Energy Efficiency,2012,40(256):24-25,32.(in Chinese with English abstract)