夏季间歇运行工况下相变温度对相变回填地埋管换热器传热性能的影响

为探究相变温度对相变材料回填地埋管换热器传热性能的影响,建立管内流体换热、回填区域相变换热及土壤换热的三维耦合传热数值模型,利用焓-多孔介质模型对相变区域相变问题进行处理,研究夏季间歇运行工况下不同相变温度回填材料对埋管换热器传热性能的影响。结果表明:添加PCM,可有效提高换热量,短期内缓解埋管周围热积聚,利用相变温度18℃的PCM回填,单位井深换热量至少比普通材料回填提高49.54%;在间歇运行初期,换热量随相变温度的升高逐渐减小,低相变温度的PCM可明显改善埋管换热量,但随着时间的进行,较高相变温度PCM回填对换热器换热量的改善效果优于前期低相变温度。此外,在运行期间,不同相变温度的PCM表现出不同的熔化、凝固特性,当PCM的熔化、凝固过程交替进行时,可减缓土壤温度在运行期间内波动幅度。

STUDY ON THERMAL PERFORMANCE OF GROUND HEAT EXCHANGER BACKFILLED WITH PHASE CHANGE MATERIAL UNDER INTERMITTENT OPERATION IN SUMMER

In order to investigate the phase transition temperature on the heat transfer performance of the backfill ground heat exchanger with phase change material, the three-dimensional numerical model of coupled heat transfer with fluid in pipe heat exchanger,backfilling region phase change heat transfer and soil heat transfer was established. By using enthalpy porous medium model to deal with the phase change problem in phase transition region,heat transfer performance of buried tube heat exchanger with different phase change temperature backfill materials is studied under intermittent operation in summer. The results show that buried tube heat exchanger adding PCM can effectively improve the heat transfer performance, which can relief heat accumulation around the pipe in the short term. By using the phase transformation temperature of 18 ℃ PCM backfill,the heat transfer in unit well depth is at least 49.54% higher than that of ordinary backfill material. At the beginning of the intermittent operation,the heat transfer decreases gradually with increasing temperature of phase change temperatures. The lower phase transition temperature of the PCM can obviously improve the heat exchange. But with operation time being longer,the higher transition temperature of PCM backfill heat exchanger is better than previous lower temperature. In addition,during the period of running,different phase transition temperature of the PCM show different melting and solidification characteristics. When melting and solidification alternates,buried tube heat exchanger with PCM can slow down the soil temperature fluctuation during the operation period.