[S24-2] Methods and instruments of experimental geothermics - application and recent evolution II

[S24-2-01] The structure of free thermal convection flows in water filled borehole inferred from a laboratory experiment

^○Dmitry Demezhko, Bogdan Hatskevich, Mansur Mindubaev (Institute of Geophysics, UB RAS, Yekaterinburg, Russia)

One of the main methods of geothermics is temperature measurements in water filled boreholes. Development of modern equipment provides high accuracy, stability, spatial and temporal resolution of temperature measurements and significantly expands the geological applications of borehole temperature data. However, the capabilities of these applications often cannot be fully realized due to the effect of free thermal convection (FTC) of the liquid in the wells. The non-stationary character of FTC leads to temperature variations, which represent a significant source of errors in conducting high-precision temperature measurements.
A new experimental lab method for studying the structure of FTC flows in conditions approached to the borehole's one has been developed. The methods based on infrared thermography of temperature anomalies appearing on the outer walls of the vertical water-filled ceramic pipe, where conditions of FTC are maintained. Upward flow of air from the toroidal heater produces temperature gradient on the outer wall of the pipe. The experiments have shown that when the critical Rayleigh number is slightly exceeded (Ra = 290 - 1000) the flows of thermal convection constitute a system consisting of 2 - 4 helical jets, rotating around a vertical axis. At inner pipe's diameter of 20 mm the helical pitch varies from 140 to 300 mm. Angular velocity of rotation increases from 4 E-3 to 25 E-3 rad/s while Rayleigh number grows from 290 to 1000.