The resulting average [R.sub.b] during all DTRTs (except for test 2), ranges between 0.035 and 0.040 Km/W (0.06 and 0.07[degrees]F ft h/Btu), being significantly lower than usually found in u-pipe BHEs.
Six DTRTs are carried out and variations of the thermal conductivity of the rock are also identified.
Thermal power and temperature versus depth profiles along the multi-pipe BHE show that the distribution of the injected power into the ground is uneven during all DTRTs. The relative difference from test to test becomes larger as lower volumetric flow rates were tested.
The resulting range of experimental average borehole thermal resistance from all DTRTs was 0.035 to 0.040 Km/W (0.06 to 0.07[degrees]F ft h/Btu), which is about half of the value typically obtained for U-pipe BHEs.
Error bars for the thermal conductivity of the rock and borehole thermal resistance during all DTRTs are presented together with the results.
The flow is laminar with the flow rates used (the maximum Reynolds number during the DTRTs was 1300), and heat transport in the circulating fluid occurs then solely by conduction (any free convection may safely be disregarded).
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.
The DTRTs were made with the flow going upwards in the peripheral pipes (DTRT 1) and vice versa in the following tests (DTRTs 2-6), which were made with different heat injection and flow rates.
In DTRT 1 this is a warm part in the loop, while in the other DTRTs it is a cool part.
However, since it merely presents an average thermal conductivity of the surrounding ground and an average borehole thermal resistance, its results can be enhanced by measuring temperatures at different depths in the borehole during the TRT, i.e., carrying out a DTRT as suggested by Acuna et al.
The injected power during DTRT 2 is about 17.0 W/m (17.7 Btu/h ft), while during the rest of the tests, it is between 35.0 and 40.0 W/m (36.4 and 41.6 Btu/h ft).
However, since the DTRT analyses based on the line source model are dependent on temperature differences, even a systematic error due to a baseline offset of [+ or -] 0.1 K ([+ or -] 0.18[degrees]F) on the absolute temperature values has a negligible effect on the results.