| Geothermal energy of the surface soil has the characteristics of environmental, energy-efficient and renewable on the base of scientific use of energy. In recent years, more and more people pay attention to ground source heat pump system because of its energy efficient. Pump energy consumption accounted for more than30%of the total energy consumption of air conditioning systems in traditional central air conditioning systems, especially chillers. The large part of the underground water circulation system is increased in the ground source heat pump system, which makes the water system energy consumption increased again. As an energy-saving technology, ground source heat pump system should give priority to energy saving in case of no significant increase investment. Improvement the efficiency of the unit could only partially reduce HVAC energy consumption, while the running of the pump is a long-term, and long-term cumulative energy consumption will be high. Only if we study the entire air-conditioning system, can we find the main energy consumption impaction.The design of the heat exchanger in the ground source heat pump system is determined by the load and import and export water temperature in pipe, and to determine how much water flow in pipe needs to bear the load, the depth of well, the number, diameter of pipes and the flow rate in pipe. While, these are selected in accordance with the majority of engineering experience, not follow a clear selection method.This article aims to change the diameter of the U-pipe to reduce the hydraulic loss of circulating water in single U-pipe and thus reduce pump energy consumption. We use software GAMBIT to build three-dimensional modeling of the heat exchanger, and use commercial CFD software FLUENT to analysis heat transfer performance and hydraulic losses of different diameter single U-pipe under the same flow rate and flow rate in summer with simulation and computational methods, then compared the results with the existing engineering formula results, and proved its feasibility and effectiveness. We conclude that, in both cases, Increase of the pipe diameter has little effect on convective heat transfer thermal resistance of the ground water circulating, while it has some influence on the thermal resistance of the backfill material. Ground heat exchange increases with increasing diameter, especially in the case of the same flow rate, and the rate of change is from4.8to8.6percent. Because increased diameter will make the temperature difference between the import and export of ground heat decreased by32.6to38.1percent, In this case, only by increasing the pipe diameter to improve the heat transfer performance is undesirable. Flow loss of circulating water in U tube is the loss of resistance along the way, and loss of local resistance in elbow accounts less than2%of all flow loss. Increased diameter will make all flow loss of ground heat circulating water reduced in both cases. The ratio of loss resistance along the way is inversely proportional to the4.75power of ratio of inner diameters in the same flow, while inversely proportional to the1.25power of ratio of inner diameters in the same flow rate.We selected the PE single U-pipe which has nominal outside diameter of20mm,25mm,32mm,40mm and50mm within the existing specification as an object of study, and gave the comprehensive analysis of the heat transfer performance and hydraulic losses. By using the programming software VB showing the relationship of each factor with diameter size, we determine the optimization scheme of diameter selection of U tube. |
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