Investigation of thermal convection in water columns using particle image velocimetry

Abstract

Particle image velocity measurements were applied on thermally driven convection at low Rayleigh numbers. In a model experiment using a water column heated from bottom and cooled from above, the velocity field was studied at different vertical temperature gradients. In the testing facility with high aspect ratio (about 19) representing a 1-m-long column with 5 cm diameter, occurrence of free convection was verified for destabilizing temperature gradients of 0.1-2 K/m. The PIV results revealed that significant flow exists already at low vertical temperature gradients. The velocity of the stable large-scale circulations increased linearly with temperature gradient (\1 K/m) from 8 9 10 -5 to 1 9 10 -3 m/s. At higher temperature gradients (1-2 K/m), a transition from quasistationary into time-dependent flow was observed, where convection cells changed position, number, and form temporarily. The motivation of this research was to gain more insight into density-driven convection in boreholes and groundwater monitoring wells.

Thermal-plume fibre optic tracking (T-POT) test for flow velocity measurement in groundwater boreholes

“…For the T-POT experiments here in a borehole with radius 0.059 m, with T / z at most 0.5 • C m −1 during the heating phase, substituting values of 2 × 10 −4 • C −1 , 9.81 m s −2 , 0.14 × 10 −6 m 2 s −1 , and 1 × 10 −6 m 2 s −1 for β, g, κ, and ν respectively gives a Ra of 85 000. Scaling the results of Berthold and Resagk (2012), who imaged flow velocities due to free convection in a vertical cylinder, using this Rayleigh number, suggests that in the absence of any forced convection, free convection due to T-POT heating would give rise to flow velocities of the order of 2 cm s −1 . This is similar to the velocity that would be expected under ambient flow conditions.…”

Thermal-Plume fibre Optic Tracking (T-POT) test for flow velocity measurement in groundwater boreholes

“….81 m s −2 , 0.14 × 10 −6 m 2 s −1 , and 1 × 10 −6 m 2 s −1 for β, g, κ and ν respectively gives a Ra of 85 000. Scaling the results of Berthold and Resagk (2012), who imaged flow velocities due to free convection in a vertical cylinder, using this Rayleigh number, suggests that in the absence of any forced convection, free convection due to T-POT heating would give rise to flow velocities of the order of 2 cm s −1 . This is similar to the velocity that would be expected under ambient flow conditions.…”

Sustainable Intensive Thermal Use of the Shallow Subsurface—A Critical View on the Status Quo

“…Groundwater temperature data is often retrieved from groundwater quality monitoring wells of varying diameter and varying filter screen depth and length by using electric contact meters for reference date measurements or automated continuous temperature logging devices. In this regard, the potential effects of well construction and design, which include free and forced convection within groundwater monitoring wells (see Berthold and Resagk ), and choice of measuring device and strategy upon measured groundwater temperatures need to be critically reviewed. This is especially important considering the given legislation that in many cases allows only a very limited range of induced maximum temperature difference when using shallow geothermal energy (see Hähnlein et al ).…”