Study on heat transfer of multi-hole vertical U-tube heat exchanger

introduction

The ground source heat pump technology known as the 21st century “green air conditioning technology” has attracted more and more attention. Most researchers have focused on the heat transfer performance of single-hole borehole heat exchangers [1, 2] and have obtained some useful conclusions. However, in actual engineering, the borehole heat exchanger system is usually composed of multi-hole boreholes [3, 4]. If the previous heat transfer model is used to simulate it, the results of the single borehole simulation are simple. Superimposed to simulate the multi-drilling temperature field, it can not truly and accurately reflect the temperature change of the soil in the entire borehole area.

Taking the multi-drilling system as the research object, the heat transfer theory and numerical method are used to simulate the soil temperature field, and the simulation results can reflect the actual situation more realistically and intuitively. Usually, the site where the pipe is placed and the depth of the hole are limited during the construction of the system, so the choice of the layout and spacing of the hole is crucial. Generally, there are two types of underground heat exchangers for ground source heat pump systems, which are sequential and cross-aligned. In this paper, the above two typical layouts are studied to analyze the influence of system running time and drilling spacing and arrangement on soil temperature field.

1 Heat transfer model and meshing

(1) The model is established based on the following assumptions: 1 Ignore the influence of surface temperature fluctuation on soil temperature, and consider that the initial temperature of the soil is uniform and constant, and equal to the temperature of the boundary soil; 2 that the contact between the buried pipe and the backfill is good, Ignore the contact thermal resistance; 3 the wall thickness is neglected compared with the external soil, that is, the thermal resistance of the pipe wall is not considered; 4 the effect of water migration on heat transfer is not considered.

(2) Geometry: The geometry of the model includes antifreeze, U-tube, backfill material and soil in the U-tube. At the same time, the left side of each U-shaped buried pipe is the inlet pipe, and the right side is the outlet pipe. The cross section of the borehole is considered to be circular. The cross section of the soil in the simulated range is considered to be square, the calculated area is 10 m × 10 m, and the four holes are arranged in the area, and the hole spacing is the same.

(3) Control equation



(6) Meshing

The principle of meshing is to densely divide the grid where the temperature and velocity fields change drastically, and loosely divide the grid where the temperature and velocity fields change slowly. Since the temperature of the underground heat exchanger changes greatly in the radial direction during the heat transfer process, the mesh around the U-tube is locally encrypted in the horizontal direction. The three-node unit is used to divide the backfill and soil around the U-tube, as shown in Figure 1 for the drilling sequence, and Figure 2 for the cross-row arrangement.



   　  (7) Simulation parameters

In the cooling mode of the heat pump, the inlet and outlet temperatures of the antifreeze are taken as 37 ° C and 33 ° C respectively (the maximum temperature difference between the inlet and outlet liquid is about 5 ° C [5] ). The distribution of the soil temperature field around the heat exchanger at a depth of 2 m was studied. The simulated calculation parameters are shown in Table 1 [6,7]. 2 The influence of system running time on soil temperature field has shown that the 400 h simulation operation of the system can reflect the trend of soil temperature field well [4]. Therefore, the distance between the drilling centers in the 720 h is 4 m. The change in soil temperature field.



It can be seen from Fig. 3 to Fig. 6 that within 24 h of the system operation, the thermal action radius of each borehole (the horizontal distance from the center of the heat exchanger to the interface where the soil temperature is 15 °C) is 2.04 m, indicating that the boreholes are mutually The temperature field is basically unaffected, and the temperature fields of the four heat exchangers are the same. After the system was operated for 24 hours, thermal interference began to appear between the boreholes. As the running time increased, the thermal interference phenomenon became stronger. Figure 7 shows the temperature distribution of the soil temperature in the X direction at different operating times Y = 2 m.





Table 2 shows the temperature changes of the soil when there is thermal interference between the boreholes. Table 3 shows the temperature changes of the soil when the boreholes are not thermally disturbed. The two sets of data are comparable. Referring to Figure 5, Table 1 and Table 2, it is found that the temperature distribution in each borehole is hardly affected by the thermal interference between the boreholes. The temperature inside the borehole is much higher than the surrounding soil temperature, and the thermal interference mainly occurs in Between the boreholes, the effect is greatest between the two boreholes, and the temperature rise (3.78 °C) is 1.73 °C higher than without thermal interference (2.05 °C) (although the maximum temperature difference is not in the middle of the two boreholes).



3 Influence of drilling layout on soil temperature field

After the simulation system was operated for 480 h, the distance between the center of the drilling was 4 m, 5 m and 6 m, the soil temperature distribution during the drilling sequence, and the distance between the drilling centers was 4 m, and the soil temperature when the holes were crossed Distribution. The other parameters are the same as above.

It can be seen from Fig. 8 to Fig. 11 that, during the same running time, when the drilling order is arranged, the temperature in the middle part of the simulated soil area gradually decreases as the distance between the center of the drilling hole increases. Fig. 12 is a graph showing the temperature distribution of the soil temperature in the X direction when Y = 0.



First, for the case where the drilling order is arranged, as the distance between the centers of the holes increases, the soil temperature at each point gradually decreases, and the temperature curve also tends to be gentle. This means that when the operating conditions of the heat pump unit are the same, the larger the drilling distance, the corresponding amount of heat can be discharged into the soil, which is more beneficial to the full utilization of the soil energy near the borehole.





Secondly, when the drilling distance is 4 m, it is not difficult to find the comparison of the drilling sequence and the cross-arrangement curve. In the case of sequential arrangement, the soil temperature distribution curve fluctuates greatly, and the temperature fluctuation between the four drilling holes is obvious. The minimum temperature is 16.55 ° C, the highest is 17.54 ° C, and the average soil temperature in the X direction is 16.84 ° C. In the case of cross-drilling, the soil temperature distribution curve is relatively flat and the temperature distribution is uniform, especially the temperature fluctuation between the boreholes is particularly small, and the average soil temperature in the X direction is 17.43 °C. This shows that the way in which the drilling sequence is arranged is very unbalanced in the utilization of soil energy. The amount of cold in the area near the borehole is taken out in large quantities, while the area between the boreholes is used less. The situation of cross-arrangement is different, and the energy utilization of the soil near the borehole is very uniform. The above analysis shows that the form of borehole cross-arrangement is better than the sequential arrangement from the perspective of soil energy utilization balance.

4 Conclusion

The simulation of 720 h operation of four vertical U-shaped buried heat exchangers shows that after 24 hours of operation, thermal interference begins to appear between the boreholes. With the increase of running time, the thermal interference phenomenon is stronger. The temperature distribution in each borehole is hardly affected by the thermal interference between the boreholes. The temperature inside the borehole is much higher than the surrounding soil temperature, and the thermal interference mainly occurs between the boreholes, which has the greatest influence among the two boreholes; The larger the hole spacing, the corresponding more heat can be discharged into the soil, which is more beneficial to the full utilization of the soil energy near the borehole. In terms of the balance of soil energy utilization, the form of the cross-arrangement of the drill holes is superior to the form of the sequential arrangement.