A study of heat transfer in liquid bridges near onset of instability

Abstract

The appearance and development of thermoconvective oscillatory flows are investigated experimentally with numerical support in a liquid bridge of aspect ratio Γ = 1.2 filled with 5- and 10-cSt silicone oils and subjected to a temperature difference ΔT. The experiments with high Prandtl number fluids were carried out under terrestrial conditions for a wide range of volumes of liquid bridges in open air and with shielding. For the experiments in open air, the temperatures of the gas and liquid close to the interface remain practically constant along most of the interface. Moving across the interface, i.e., from the liquid to the gas, a temperature ‘‘jump’’ is found with  0.4ΔT ≈ 9 K over a distance of less than 1 mm for slender liquid bridges. This tendency remains valid for fat liquid bridges. This sharp temperature drop represents additional disturbances in destabilizing the system. The flow is steady and axisymmetric at suffciently low values of ΔT. The magnitude of ΔT at which the flow becomes oscillatory depends on the relative volume of the liquid and on the external conditions.

To better understand the influence of the thermal conditions of the surroundings on transition to oscillations, the experiments were carried out under two different conditions: in an open-air environment and in shielded liquid bridges. In this way, different heat transfer possibilities at the free surface are realized. Complementary numerical calculations were also performed to support our explanations of the experimental observations.