Experimental study on heat transfer characteristics of helix ground heat exchanger coil under dynamic load

Abstract

In order to study the thermal behavior of helix ground heat exchanger coil when subjected to different thermal load distributions, a full scale helix ground heat exchanger coil system was constructed in this paper. On basis of the system, two kinds of simulated thermal loads were subjected to the full scale helix ground heat exchanger coil. To obtain the actual thermal performance, the temperature response and transient effective heat transfer coefficient were analyzed under the two kinds of simulated thermal loads. The results indicate that under the cyclic dynamic load, the temperature of the inlet and outlet of the helix ground heat exchanger coil also changes periodically. And from the overall trend, the inlet and outlet water temperature of the exchanger increase gradually. In the constant flow rate condition, when the load rate of the helix ground heat exchanger coil is low, the effective heat transfer coefficient is small, thus the load rate of the exchanger should be as high as possible. When the system is under part load, the number of sections of the running helix ground heat exchanger coil should be reduced in order to guarantee a high load value. The accumulated heat rejection from the heat exchanger to ground will affect the effective heat transfer coefficient to a certain extent. The effective heat transfer coefficient under dynamic load condition can be up to 67.4% higher than fixed load condition under the effect of accumulated heat rejection. It is also preliminarily found that rainfall infiltration can effectively strengthen the heat transfer effect of the helix ground heat exchanger coil by 18.4%, which is almost equal to the increase of the effective heat transfer coefficient after the night-time recovery period during intermittent operation.