The applied boundary conditions to the simulation models are presented in Table 3 and are based on values close to those measured during DTRT 1.
This becomes even more evident at the lowest volumetric flow rates during DTRT 5 and DTRT 6 (last 120 h).
The latter is evident, for example, when comparing the temperature profiles of DTRT 4, where the fluid in the central pipe almost does not change, with DTRT 6, where the fluid loses about 0.
In DTRT 2 to DTRT 6, the flows in each peripheral pipe meet at the bottom, and a slight jump in temperature is observed, meaning that the flow from the measurement pipe (pipe 1) has a slightly lower temperature than the rest of the peripheral pipes.
The Reynolds number in the peripheral channels during all DTRTs varied between 1300 and about 400, being highest during DTRT 1 and lowest during DTRT 6.
Figure 15 presents the average fluid temperatures in each section during the heat recovery period between DTRT 1 and DTRT 2, showing how the temperatures in the borehole approach the original undisturbed ground temperatures in each section.
During DTRT 1, where the volumetric flow rate is highest and the heat is first injected through the central pipe, the local thermal resistances along the depth tend to be uniform at a value of 0.
The supplied power during DTRT 2 was about half of the one supplied in all other tests (see Figure 3), decreasing the vulnerability of [R.
As inferred from Figures 9 to 14, the mean temperature along the borehole was lowest during DTRT 2, followed by DTRT 1, and it increased continuously between DTRT 3 and DTRT 6.
Comparing the results from DTRT 2 with DTRT 3 (both tests carried out with the same volumetric flow rate), the effects of a larger heat injection rate and temperature level on the borehole thermal resistance are evident.
The DTRT method based on temperature measurements with fiber-optic cables placed inside BHE pipes has been proposed for testing coaxial BHEs.
For the central pipe, the lateral position of the fiber optic cable is not known either, but the flow pattern in the pipe is turbulent in all DTRTs, except for DTRT 6.