NATURAL THERMAL CONVECTION IN A VERTICAL WATER-FILLED CYLINDER: INFRARED THERMOGRAPHY INVESTIGATION
Temperature logging furnishes the essential part of geothermal data. Its applications are progressively expanding owing to advanced temperature loggers and data acquisition systems that ensure precise and stable measurements at high spatial and temporal resolution. However, it may be hard to achieve the full effect of the available logging facilities because of noisy temperature oscillations produced by natural convection in water-filled boreholes. A new laboratory method is suggested to study the structure of convection flows and their thermal effect by infrared thermography in conditions close to those of real temperature logging. Thermographic cameras image temperature anomalies on the outer wall of a water-filled pipe, which are imprints of the convection processes in the water column. The temperature gradient on the pipe wall maintains flow of warm air ascending from a toroidal heater. It is shown experimentally, using a pipe 20 mm in inner diameter, that convection of fluid in the pipe forms a helical system rotating around a vertical axis at Rayleight numbers in the range of 280 to 2800. As the Rayleigh numbers increase from 280 to 2800, the helical pitch decreases from 270 to 130 mm, while the angular velocity increases from 0.7×10-2 to 3.4×10-2 rad/s. The experiment confirms the theoretically predicted dependence of the standard deviation of temperature fluctuations on the temperature gradient and inner radius of the logged borehole: σ T = 3 G·r .