Research On The Double-Spiral Heat Exchanger In Ground Heat PUMP System

As an important form of the efficient use of energy in building, Ground source heat pump system is a new energy technology that could only consume little power and translate the low-quality energy in the soil into high-quality energy. Due to the characters of the geothermal energy are clean, cheap and renewable, the ground source heat pump system as an energy saving way of "green air conditioning", is approved by unanimous praise of domestic and foreign experts in today’s increasingly serious energy crisis.The ground heat exchanger is most important part of the ground source heat pump system and has decisive impact to the overall operating efficiency of heat transfer performance of ground source heat pump system. Improve heat transfer efficiency of the heat exchanger and reduce the drilling cost have always been the focus people study. According to the current in the horizontal pipe or building foundation piles buried in a single spiral to increase the heat transfer area inspired, dual-coil pipe heat exchanger in vertical well is invented. Longer can be placed in the same bulk depth under the conditions of the tube spacing maintain constant, and the effect of heat transfer may be better. In contrast, assume that the total amount of heat keeps constant, with the help of the double spiral tube heat exchanger, the depth of the bulk could be decreased and then the expense on the drilling would be lower. The heat transfer characteristics of dual spiral heat exchanger in vertical bulk are studied in this article and the main substances are presented below:First, with the help of the Ground source heat pump exchanger bench in Southwest Jiaotong University,6different working conditions are tested and then, the same working conditions are simulated, having examined between the experimental data and simulation result, a conclusion is made that the simulation result basically matches the test data and the simplified boundary conditions are acceptable and reasonable.Second, several double spiral and single spiral heat exchanger model are made and by the use of the boundary conditions mentioned above, the time-dependent situation of spiral tube for heat exchanger can be simulated, and by designing the simulation, some factors (spiral distance, inlet velocity, inlet temperature, tube distance) are accounted in the model and case to quantify the influence to the total thermal quality.Different model such as u-pipe, single spiral pipe and double spiral pipe are created and each model is simulated in different working conditions. By the results, different spiral distance and pipe distance are compared for temperature distribution, heat exchanger rate, total amount of heat exchange and so on. Some conclusions are stated below:1. When the depth of bulk keeps the same, the total amount of heat exchange in spiral pipe is superior to that in u-pipe heat exchanger. Comparing to pipe length in u-pipe3.15m,500mm spiral distance with double spiral heat exchanger’s length is3.34m and250mm spiral distance with double spiral heat exchanger’s length is3.47m. According to the simulation results, when the running duration is4hrs and on the condition of that inlet temperature and velocity keep alike, total amount of heat exchanger separately increase to about5%and10%.2. In some specific and proper spiral distance range, less spiral distance means higher total amount heat exchange. A suitable spiral could maximize the pipe length, strength the phenomenon convective heat transfer the and avoid the appearance of thermal short-circuit, and then improve the overall heat exchange rate.500mm spiral distance vs250mm spiral distance, pipe length are3.334m and3.471m, relevantly, the total amount of heat exchange in each working condition increase6%.3. Tube distance is a crucial factor to the model of heat exchanger and the further the tube distance is, the higher the efficiency of the heat exchanger is. This article establish2model with same spiral distance500mm and different tube distance:100mm,150mm and having compared the simulation results in each working conditions, further pipe distance implies more total amount of heat exchange:on average on the amount of heat exchange, the case with150mm tube distance is15%more than the case with100mm tube distance.4. Basically, the single spiral tube is similar to the u-pipe; the only difference is the way the outlet pipe goes. On the aspect of the length of pipe, although u-pipe’s length is3.16m and single spiral pipe is3.34m long, the pipe distance play a greater role in heat exchange due to u-pipe’s pipe distance is100mm and tube distance of the single spiral pipe is50mm. At all working conditions, the amount of heat exchange of u-pipe is3%to4%more than single spiral pipe.5. The special laying way of single spiral pipe limits the max distance of the pipe distance to the ridus of the bulk. Comparing to the double spiral pipe, even if the spiral distance (500mm) and the length of the pipe (3.14m&3.33m) of the two models are close, but in all condition the total amount of heat transfer in single spiral pipe are only80%to the double spiral model as a result of the thermal bright caused by the less pipe distance.6. Increasing the tube spacing and reduce the spiral distance appropriately could enhance the efficiency of the heat exchanger, but in the long-term heat transfer conditions, the effect of decrease the spiral distance will be more pronounced than the increase in tube spacing.